α-nitration of ketones via enol silyl ethers. Radical cations as reactive intermediates in thermal and photochemical processes

Article abstract of DOI:10.1021/jo9515687

Highly colored (red) solutions of various enol silyl ethers and tetranitromethane (TNM) are readily bleached to afford good yields of α-nitro ketones in the dark at room temperature or below. Spectral analysis show the red colors to be associated with the intermolecular 1:1 electron donor-acceptor (EDA) complexes between the enol silyl ether and TNM. The formation of similar vividly colored EDA complexes with other electron acceptors (such as chloranil, tetracyanobenzene, tetracyanoquinodimethane, etc.) readily establish enol silyl ethers to be excellent electron donors. The deliberate irradiation of the diagnostic (red) charge-transfer absorption bands of the EDA complexes of enol silyl ethers and TNM at -40 °C affords directly the same α-nitro ketones, under conditions in which the thermal reaction is too slow to compete. A common pathway is discussed in which the electron transfer from the enol silyl ether (ESE) to TNM results in the radical ion triad [ESE^?+, NO₂^?, C(NO₂)₃^-]. A subsequent fast homolytic coupling of the cation radical of the enol silyl ether with NO₂^? leads to the α-nitro ketones. The use of time-resolved spectroscopy and the disparate behavior of theisomeric enol silyl ethers of α- and β-tetralones as well as of 2-methylcyclohexanone strongly support cation radicals (ESE^?+) as the critical intermediate in thermal and photoinduced electron-transfer as described in Schemes 1 and 2, respectively.

Full text of DOI:10.1021/jo9515687

J . Org. Chem. 1996, 61, 627-639
627
r-Nitr a tion of Keton es via En ol Silyl Eth er s. Ra d ica l Ca tion s a s
Rea ctive In ter m ed ia tes in Th er m a l a n d P h otoch em ica l P r ocesses
Rajendra Rathore and J ay K. Kochi*
Department of Chemistry, University of Houston, Houston, Texas 77204-5641
Received September 26, 1995X
Highly colored (red) solutions of various enol silyl ethers and tetranitromethane (TNM) are readily
bleached to afford good yields of R-nitro ketones in the dark at room temperature or below. Spectral
analysis show the red colors to be associated with the intermolecular 1:1 electron donor-acceptor
(EDA) complexes between the enol silyl ether and TNM. The formation of similar vividly colored
EDA complexes with other electron acceptors (such as chloranil, tetracyanobenzene, tetracyano-
quinodimethane, etc.) readily establish enol silyl ethers to be excellent electron donors. The
deliberate irradiation of the diagnostic (red) charge-transfer absorption bands of the EDA complexes
of enol silyl ethers and TNM at -40 °C affords directly the same R-nitro ketones, under conditions
in which the thermal reaction is too slow to compete. A common pathway is discussed in which
the electron transfer from the enol silyl ether (ESE) to TNM results in the radical ion triad [ESE•+
,
•
NO2 , C(NO2)3-]. A subsequent fast homolytic coupling of the cation radical of the enol silyl ether
•
with NO2 leads to the R-nitro ketones. The use of time-resolved spectroscopy and the disparate
behavior of the isomeric enol silyl ethers of R- and â-tetralones as well as of 2-methylcyclohexanone
strongly support cation radicals (ESE•+) as the critical intermediate in thermal and photoinduced
electron-transfer as described in Schemes 1 and 2, respectively.
In tr od u ction
the keto-enol tautomerization effectively converts a
(relatively electron-poor) carbonyl acceptor into a (cor-
respondingly electron-rich) olefinic donor;13 such an um-
polung allows the conventional nucleophilic additions at
the carbonyl center (e.g. Grignard reaction) to be readily
replaced by electrophilic substitution at the R-carbon (e.g.
bromination) of ketones, aldehydes, esters, etc.14 Espe-
cially noteworthy in this regard are the facile R-substitu-
tions of enol silyl ethers with N-bromosuccinimide,15,16
nitronium17 and diazonium18 salts, chloranil19 and DDQ,20
silver oxide,21 lead tetraacetate,22 vanadium oxytrichlo-
ride,23 ceric ammonium nitrate,24 etc. The wide diversity
of such reagents (that includes cationic and neutral
electrophiles as well as strong and weak 1-electron
oxidants) raises the question as to whether a unified
mechanistic formulation is applicable to enol silyl ethers
as electron donors in general.25
Various R-nitro ketones are widely utilized as synthetic
intermediates1-3 since they are excellent precursors for
carbon to carbon bond-forming reactions owing to the
powerful activation of the R-hydrogen atom by the
combined effect of the nitro and carbonyl groups.4,5
Furthermore, the facile cleavage of the C-C bond be-
tween the carbonyl and the nitro-substituted carbon atom
in R-nitro ketones can be exploited in the synthesis of
ring-enlarged cycloalkanones.6,7 Although general meth-
ods for the efficient preparation of R-nitro ketones are
desirable, the most direct route involving the electrophilic
nitration of ketones (with nitric acid) suffers from a
variety of oxidative byproducts.8 Alternatively, the con-
version of ketones to their enolates,9 enol acetates,10 and
enol ethers,11 coupled with milder nitrating agents such
as alkyl nitrates, nitryl chloride, and acyl nitrates, etc.,
does facilitate the R-nitration of ketones, but their use
has generally met with limited success.12 Interestingly,
(11) (a) Bachman, G. B.; Hokama, T. J . Org. Chem. 1960, 25, 178.
(b) Shavarts, I. S.; Yarovenko, V. N.; Krayushkin, M. M.; Novikov, S.
S.; Sevostyanova, V. V. Izv. Akad. Nauk SSSR, Ser. Khim. 1976, 1674.
(c) Dampawan, P.; Zajac, W. W., J r. J . Org. Chem. 1982, 47, 1176.
(12) See Fischer, R. H.; Weitz, H. M. Synthesis 1980, 261.
(13) Schmittel, M. Topics Curr. Chem. 1994, 169, 183 and references
cited therein.
(14) March, J . Advanced Organic Chemistry, 4th ed.; Wiley: New
York, 1992.
(15) Reuss, R. H.; Hassner, A. J . Org. Chem. 1974, 39, 1785.
(16) Reaction of enol silyl ether with other halogenating agents, see
(a) Hambly, G. F.; Chan, T. H. Tetrahedron Lett. 1986, 27, 2563. (b)
Sha, C.-K.; Young, J .-J .; J ean, T.-S. J . Org. Chem. 1987, 52, 3919.
(17) See Shavarts et al. in ref 11b.
X Abstract published in Advance ACS Abstracts, J anuary 1, 1996.
(1) (a) Munz, R.; Simchen, G. Liebigs Ann. 1979, 628. (b) Cookson,
R. C.; Ray, P. S. Tetrahedron Lett. 1982, 22, 3521. (c) Ono, N.; Miyake,
H.; Kaji, A. J . Chem. Soc., Chem. Commun. 1983, 875.
(2) (a) Ono, N. Miyake, H.; Kaji, A. J . Org. Chem. 1984, 49, 4997.
(b) Ono, A.; Hamamoto, I.; Kaji, A. J . Org. Chem. 1986, 51, 2832. (c)
Ballini, R.; Petrini, M. Synth. Commun. 1986, 1781. (d) Rosini, G.;
Ballini, R.; Marotta, E. Tetrahedron 1989, 45, 5935.
(3) (a) Ballini, R.; Palestini, C. Tetrahedron Lett. 1994, 31, 5731.
(b) Ballini, R.; Bartoli, G.; Gariboldi, P. V.; Marcantoni, E.; Petrini,
M. J . Org. Chem. 1993, 58, 3368. (c) Warsinsky, R.; Steckhan, E. J .
Chem. Soc., Perkin Trans. 1 1994, 2027.
(18) Sakakura, T.; Hara, M.; Tanaka, M. J . Chem. Soc., Perkin
Trans. 1 1994, 283.
(19) Fukuzumi, S.; Fujita, M.; Matsubayashi, G.; Otera, J . Chem.
Lett. 1993, 1451.
(4) (a) Kostova, K.; Hesse, M. Helv. Chim. Acta 1984, 67, 1713. (b)
Denmark, S. E.; Sternberg, J . A.; Lueoend, R. J . Org. Chem. 1988, 53,
1251.
(5) Ono, N.; Miyake, H.; Fujii, M. Tetrahedron Lett. 1983, 24, 3477.
(6) Bienz, S.; and Hesse, M. Helv. Chim. Acta 1987, 70, 2146.
(7) For other examples of ring enlargement via R-nitro ketones, see
(a) Benkert, E.; Hesse, M. Helv. Chim. Acta 1987, 70, 2166 and (b)
Stach, H.; Hesse, M. Helv. Chim. Acta 1986, 69, 85.
(8) Fischer, R. H.; Weitz, H. M. Liebigs Ann. 1979, 612.
(9) (a) Cushman, M.; Mathew, J . Synthesis 1982, 397 and references
therein. (b) Elfehail, F. E.; Zajac, W. W., J r. J . Org. Chem. 1981, 46,
5151.
(20) (a) Bhattacharya, A.; DiMichele, L. M.; Dolling, U. H.; Grabow-
ski, E. J . J .; Grenda, V. J . J . Org. Chem. 1989, 54, 6118. (b) Fleming,
I.; Paterson, I. Synthesis 1979, 736.
(21) Ito, Y.; Konoike, T.; Saegusa, T. J . Am. Chem. Soc. 1975, 97,
649.
(22) Moriarty, R. M.; Penmasta, R.; Prakash, I. Tetrahedron Lett.
1987, 28, 873.
(23) Fujii, T.; Hirao, T.; Ohshiro, Y. Tetrahedron Lett. 1992, 33, 5823.
(24) (a) Baciocchi, E.; Casu, A.; Ruzziconi, R. Tetrahedron Lett. 1989,
30, 3707. (b) Paolobelli, A. B.; Gioacchini, F.; Ruzziconi, R. Tetrahedron
Lett. 1993, 34, 6333.
(10) (a) Dampawan, P.; Zajac, W. W., J r. Synthesis 1983, 545. (b)
Ozbal, H.; Zajac, W. W., J r. J . Org. Chem. 1981, 46, 3082.
0022-3263/96/1961-0627$12.00/0 © 1996 American Chemical Society

628 J . Org. Chem., Vol. 61, No. 2, 1996
Rathore and Kochi
methide was confirmed by comparison with an authentic
sample28 using the HPLC method (see Experimental
Section). Furthermore, HPLC analysis was used to
monitor the progress of the nitration reaction and
establish the 1:1 molar stoichiometry between 2-nitro-
R-tetralone 1a and trinitromethide, i.e.
[The trimethylsilyl derivative of trinitromethide as pre-
sented in eq 1 was isolated as a highly labile yellow solid.
However, an attempt to obtain a single crystal suitable
for X-ray crystallography resulted in a violent explosion
and further studies were suspended.] With this simple
procedure, the enol silyl ethers derived from various types
of ketones listed in Table 1 were converted to the
corresponding R-nitro ketones in fair to excellent yields.
It is noteworthy that every R-nitro ketone was ac-
companied by 1 equiv of trinitromethide to confirm the
general stoichiometry in eq 1.
Effect of Solven t. The efficiency of the nitration
reaction was rather insensitive to solvent variation, since
comparable yields of R-nitro ketones were obtainable in
either acetonitrile, dichloromethane, or n-pentane. For
example, when a solution of R-tetralone enol silyl ether
1a in n-pentane was mixed with 1 equiv of tetrani-
tromethane at room temperature, the solution immedi-
ately took on the characteristic red coloration. Upon
standing shortly, the solution progressively bleached as
the yellow crystals of 2-nitro-R-tetralones 1a were readily
precipitated in quantitative yields (see Experimental
Section).
F igu r e 1. Typical bleaching of the red coloration obtained
from 0.2 M enol silyl ether 1a and 0.22 M TNM in dichlo-
romethane at the indicated times, and the UV-vis spectrum
of TNM alone (- - -).
In this report, we report a facile method for the
preparation of a variety of R-nitro ketones in good to
excellent yields by the thermal as well as the photo-
chemical nitration of readily available enol silyl ethers
with tetranitromethane.26 We focus in this study on (a)
the structural effects of enol silyl ethers on their reactiv-
ity with such a mild nitrating agent as tetranitromethane27
and (b) the spectroscopic identification of the accompany-
ing color changes and the reactive intermediates that
play a critical role in the R-nitration of enol silyl ethers.
Resu lts
Effect of Keton e Str u ctu r e. The nitration of the
enol silyl ethers in Table 1 qualitatively occurred at
varying rates and efficiencies that largely depended on
the number and location of various substituents. How-
ever in every case, a transient reddish coloration which
changed to yellow with time, was a common, unmistak-
able feature of this transformation. Indeed, such a color
change provided a convenient visual indicator for the
progress of the nitration reaction, as illustrated by the
spectral changes in Figure 1. Thus the dark-red colors
derived from R-tetralone enol silyl ethers (1a -c) and
TNM were rapidly discharged within minutes to afford
the corresponding 2-nitro-R-tetralones (1a -c) in es-
sentially quantitative yields. Also, the enol silyl ethers
of the acyclic 1-phenylketones (such as acetophenone and
its analogs 2a -d ) afforded excellent yields of 2-nitro-1-
phenylketones (2a -d ) with concomitant bleaching of the
red color over the course of 5-6 h (see Table 1). In a
similar vein, the cycloalkanone enol silyl ethers (3a -e)
afforded good yields of 2-nitrocycloalkanones in relatively
slow reactions, as indicated by the gradual bleaching of
the color over a 16-h period. [Note that the isolation of
the labile R-nitrocyclopentanone was hampered due to
interference from its facile ring cleavage.29] On the other
hand, the enol silyl ethers of acyclic ketones 4a and 4b
afforded only poor yields of R-nitro ketones in signifi-
cantly slower nitrations with TNM (see Table 1).
The time for the visual color change (required to obtain
the isolated yields of R-nitro ketones listed in Table 1)
provided a qualitative measure of the reactivities of
Th er m a l r-Nit r a t ion of Ket on es via E n ol Silyl
Eth er s. When a solution of the enol trimethylsilyl (TMS)
ether of R-tetralone 1a in anhydrous dichloromethane
was mixed with 1 equiv of tetranitromethane (TNM)
under an argon atmosphere at room temperature, the
solution immediately took on a bright red coloration.
Upon standing in the dark, the solution progressively
bleached to a yellow solution over the course of ∼10 min,
as shown by the corresponding spectral changes in Figure
1. In a simple workup procedure, the reaction mixture
was extracted repeatedly with water until the washings
were colorless. The dichloromethane layer was dried over
anhydrous magnesium sulfate and evaporated in vacuo
to afford the crystalline 2-nitro-R-tetralone 1a in es-
sentially quantitative yield. In order to identify the
accompanying trinitromethyl moiety in the reaction
mixture, the yellow-colored aqueous washings were
examined by UV-vis spectroscopy and by HPLC analy-
sis. Spectroscopic analysis of the dark-yellow aqueous
extracts (after dilution by water) indicated the presence
of 1 equiv of trinitromethide [C(NO2)3-: λmax ) 350 nm,
ꢀmax ) 14000 M-1 cm-1].28 The identity of the trinitro-
(25) (a) Fukuzumi, S.; Fujita, M. Otera, J .; Fujita, Y. J . Am. Chem.
Soc. 1992, 114, 10271. (b) Gassman, P. G.; Bottorff, K. J . J . Org. Chem.
1988, 53, 1097.
(26) For a preliminary report, see Rathore, R.; Lin, Z.; Kochi, J . K.
Tetrahedron Lett. 1993, 34, 1859.
(27) (a) Altukhov, K. V.; Perekalin, V. V. Russ. Chem. Rev. 1976,
45, 1052. (b) Kochi, J . K. In Comprehensive Organic Syntheses, Vol
VII; Trost, B. M., Ed.; Pergamon: New York, 1991; p 849.
(28) Go¨bl, M.; Asmus, K.-D. J . Chem. Soc., Perkin Trans. 2 1984,
691. Owing to the extensive dissociation in solution, nitroform and
trinitromethide are hereinafter referred to interchangeably.
(29) Feuer, H.; Pivawer, P. M. J . Org. Chem. 1966, 31, 3152.

R-Nitration of Ketones via Enol Silyl Ethers
J . Org. Chem., Vol. 61, No. 2, 1996 629
Ta ble 1. Th er m a l Nitr a tion of En ol Silyl Eth er s w ith
Tetr a n itr om eth a n e
reacted at least 1 order of magnitude faster than either
derivative 1a or 1c. In contrast, the nitro-substituted
acetophenone derivative 2e reacted much slower than the
parent acetophenone derivative 2a . The effect of ring
strain was shown in the enol silyl ether derivatives of
norbornanone (5) and camphor (6), both of which resulted
in the corresponding R-nitro ketones via the rapid
bleaching of the color even at -30 °C. It is noteworthy
that nitrations with TNM were not regiospecific, since
the kinetic and thermodynamic enol silyl ethers of
methylcyclohexanone 3c and 3d , respectively, yielded the
same 2-methyl-2-nitrocyclohexanone, i.e.
The efficiency of the nitration reaction prompted us to
examine the enol silyl ether 9 (with a tethered olefinic
group) derived from citronellal.30 Treatment of a solution
of 9 in dichloromethane with tetranitromethane instan-
taneously led to the characteristic red coloration, which
on standing (in the dark) progressively bleached to a
yellow colored solution. Aqueous workup afforded a pair
of diastereomeric R-nitro ketones 10 in excellent yields;
but no cyclized product 11 was detected (<0.1%), i.e.
Com p a r a tive Rea ctivity of r- a n d â-Tetr a lon e
En ol Silyl Eth er s. Although the characteristic red
coloration of R-tetralone enol silyl ether 1a and TNM in
dichloromethane rapidly bleached on standing to afford
a quantitative yield of 2-nitro-R-tetralone, the similar red
coloration of the isomeric â-tetralone enol silyl ether 7
and TNM persisted at 25 °C for hours without any
noticeable change. After standing in the dark for ∼12
h, the enol silyl ether 7 was recovered unchanged; upon
aqueous workup and spectral analysis the aqueous layer
showed the presence of very little nitroform (<0.05 equiv,
∼5%, see Experimental Section). Such a dramatic dif-
ference in the reactivity between a pair of isomeric enol
silyl ethers with tetranitromethane was not restricted
only to R- and â-tetralone derivatives 1a and 7. A similar
difference in the reactivity was observed with other
isomeric pairs of acyclic enol silyl ethers derived from
1-phenyl ketone versus 2-phenyl ketone, such as 8a
compared to 2b, or 8b compared to 2d . Thus, 1-phenyl
ketone enol silyl ethers 2b and 2d afforded excellent
yields of the corresponding R-nitro ketones, whereas both
of the isomeric 2-phenyl ketone enol silyl ethers 8a and
8b did not react under the same conditions (see Table
1).
P h otoch em ica l Nitr a tion of En ol Silyl Eth er s.
The persistence of the colored solutions of enol silyl ethers
(especially those derived from the relatively unreactive
cycloalkanones 3a -e and the acyclic ketones 4a -b) in
(30) (a) Similar cyclizations have been effected with ceric ammonium
nitrate (CAN) as the 1-electron oxidant. (b) Snider, B. B, Kwon, T. J .
Org. Chem. 1992, 57, 2399.
various enol silyl ethers towards TNM. On this basis,
the methoxy-substituted R-tetralone enol silyl ether 1b

630 J . Org. Chem., Vol. 61, No. 2, 1996
Rathore and Kochi
In each case, the photoactivated transformation of enol
silyl ethers with TNM were carried out at low temper-
atures (typically <-40 °C) at which the thermal reactions
(vide supra) were too slow to compete. For example, the
irradiation of an orange-red dichloromethane solution of
cyclohexanone enol silyl ether 3a and TNM in a dry ice/
acetonitrile bath (under an argon atmosphere) resulted
in the continuous bleach of the color over the course of 1
h, to finally yield a yellow solution. An identical solution
of enol ether 3a and TNM was also prepared but kept
protected from light alongside the irradiated solution at
the same temperature and for the same irradiation
period. The UV-vis analysis of this (dark) control
showed no spectral change. The simultaneous workup
of the (dark) control and the photolysate by removal of
the solvent and excess TNM in vacuo, followed by the
aqueous treatment of the residues (after redissolution in
dichloromethane) was followed by quantitative GC and
HPLC analysis using the internal standard method (see
Experimental Section). Although the enol ether 3a from
the unirradiated solution remained wholly intact, HPLC
analysis of the photolysate indicated that R-nitrocyclo-
hexanone (3a ) was formed in 73% yield. The spectral
properties and GC-MS behavior of R-nitrocyclohexanone
(3a ) coincided with those of an authentic sample prepared
independently.10a The subsequent spectrophotometric
analysis of the aqueous extracts from the photolysate
revealed that 1 equiv of nitroform accompanied the
formation of R-nitrocyclohexanone (see Figure 2). By
contrast, the aqueous extracts from the (dark) control
showed the presence of only traces of nitroform. Accord-
ingly, the stoichiometry for the photochemical nitration
of enol silyl ether of cyclohexanone with TNM was
described as
F igu r e 2. Spectral comparison of the trinitromethide con-
centration in the aqueous extract of the photolysate (s) and
the dark control (‚‚‚‚) after the photonitration with λ > 415
nm of 0.2 M enol silyl ether 3a and 0.22 M TNM in dichlo-
romethane at -40 °C.
Ta ble 2. P h otoch em ica l Nitr a tion of En ol Silyl Eth er s
w ith Tetr a n itr om eth a n e in Dich lor om eth a n e a t -40 °C
The photochemical nitrations of other silyl enol ethers
with TNM at low temperatures (-40 °C) are included in
Table 2. It is noteworthy that irradiation for a rather
short duration (1-2 h) was generally sufficient to obtain
good yields of R-nitro ketones, that otherwise required
much longer periods for preparation in the dark at 25
°C (compare Tables 1 and 2). Furthermore, the dark-
red solution of the reactive R-tetralone enol silyl ether
1a and TNM was bleached within 20 min even at -78
°C, and the analysis of photolysate showed the formation
of 2-nitro-R-tetralone 1a in >98% yield. [By comparison,
the (dark) control at this temperature showed only a 17%
yield of R-nitro ketone 1a , see Experimental Section]. On
the other hand, the intensely colored solution of the
isomeric â-tetralone enol silyl ether 7 and TNM upon
irradiation at 25 °C bleached rather sluggishly to afford
a pale brown solution. Aqueous workup, followed by GC
and GC-MS analysis showed that a complex mixture of
products was formed. Notably, enol silyl ether 7 was
completely consumed. Although no â-nitro ketone was
detected in the crude reaction mixture (see Experimental
Section) spectral analysis of the aqueous washings
showed that substantial amounts of nitroform (0.45
equiv) was produced.
the dark, in comparison to those solutions that were
exposed to room light, prompted us to examine the effect
of deliberate irradiation. Thus, photochemical studies
were carried out with filtered actinic radiation from a
medium pressure 500-W mercury lamp equipped with a
sharp Pyrex cutoff filter that allowed only light with λ >
415 nm to be transmitted. [Note that under these
conditions, neither tetranitromethane nor enol silyl
ethers were excited since they only absorb below 400 nm.]
Sp ectr a l Ch a r a cter iza tion of th e Tr a n sien t Col-
or s Obser ved d u r in g th e Nitr a tion of En ol Silyl
Eth er s w ith Tetr a n itr om eth a n e. The addition of
TNM to cyclohexanone enol silyl ether (3a ) resulted in

R-Nitration of Ketones via Enol Silyl Ethers
J . Org. Chem., Vol. 61, No. 2, 1996 631
F igu r e 3. Charge-transfer spectra of the EDA complexes of enol silyl ethers with TNM. (a) Spectral changes accompanying the
incremental addition of cyclohexanone enol silyl ether 3a (0.02-0.2 M) to 2.0 M TNM (- - -) in dichloromethane at -15 °C. Inset:
Benesi-Hildebrand plot. (b) Comparative charge-transfer absorption spectra obtained from the addition of 0.2 M enol silyl ethers
3a , 7, 1a , 3d , and 2b to 0.2 M TNM (‚‚‚‚) as indicated.
an instantaneous coloration which varied from deep
yellow to wine red depending on the concentration of each
component. For example, the absorption spectrum of
TNM in dichloromethane solution underwent the suc-
cessive changes shown in Figure 3a upon the incremental
addition of enol ether 3a at -15 °C. When the absor-
bance change was measured at 450 nm (A450) as a
function of the enol silyl ether concentration [3a ] accord-
ing to the Benesi-Hildebrand treatment,31 i.e.
complex in eq 6, we examined the formation of analogous
charge-transfer complexes of enol silyl ethers with other
commonly used π-acceptors, such as chloranil (CA),
tetracyanobenzene (TCNB), and tetracyanoquinodimeth-
ane (TCNQ). Fortunately, these acceptors led to well-
resolved absorption bands with enol silyl ethers (as
electron donors) sufficient to identify the transient colors
as follows.
Electr on Don or -Accep tor Com p lexes of En ol
Silyl Eth er s w ith π-Accep tor s. The visual indication
of enol silyl ethers as electron donors derives from the
vivid charge-transfer colors that were observed when enol
silyl ethers were exposed to various electron acceptors.33
For example, an intense orange coloration developed
immediately when chloranil (CA) was added to a solution
of cyclohexanone enol silyl ether 1a in dichloromethane
under an argon atmosphere at 25 °C. Figure 4a shows
the progressive growth of the spectral absorbance with
incremental additions of the ether 1a . Spectrophotomet-
ric analysis by the Benesi and Hildebrand method in eq
5, under conditions in which [CA] . [1a ], yielded a linear
plot (see inset of Figure 4a) with a typical correlation
coefficient of >0.99. From the slope and intercept, the
values of the formation constant and extinction coefficient
[TNM]
A450
1
ꢀ450
1
1
[3a ]
)
+
(5)
K
EDAꢀ450
the linear correlation shown in the inset of Figure 3a
yielded the formation constant of K ) 0.24 M-1 and the
extinction coefficient of ꢀ450 ) 19 M-1 cm-1. Such values
of K and ꢀ are indeed typical for the spontaneous
formation of electron donor-acceptor (EDA) complexes
of tetranitromethane when the TNM acceptor was ex-
posed to various types of electron donors (including
arenes, alkenes, etc.),32 e.g.
were obtained as KCT ) 1.1 M-1 and ꢀ460 ) 416 M-1 cm-1
,
respectively.34 The other substituted enol silyl ethers
listed in Table 3 and chloranil similarly afforded brightly
colored solutions of varying shades of red, blue, and
green. The quantitative effect of the color changes is
illustrated in Figure 4b by the systematic spectral shifts
of the well-defined new absorption bands of chloranil with
the enol silyl ethers of cyclohexanone, methylcyclohex-
anone, and â-tetralone. [Note that neither chloranil nor
the enol silyl ethers absorbed beyond λ ) 400 nm.] These
well-resolved, but featureless absorption bands were
Similarly, the other substituted enol silyl ethers in
Table 1 afforded red solutions of varying intensity when
mixed with TNM. In each case, the colors attendant
upon the exposure of the various enol silyl ethers to TNM
were associated with the appearance of new broad
absorption bands (see Figure 3b), since neither TNM nor
the enol silyl ethers absorbed in the visible region above
400 nm. Unfortunately, the quantitative treatment of
these new (tailing) absorptions was hampered owing to
the absence of distinct spectral maxima (see Figure 3b).
Accordingly, in order to establish the viability of the EDA
(33) (a) Rathore, R.; Kochi, J . K. Tetrahedron Lett. 1994, 35, 8577.
(b) Bockman, T. M.; Perrier, S.; Kochi, J . K. J . Chem. Soc., Perkin
Trans. 2 1993, 595.
(31) (a) Benesi, H. A.; Hildebrand, J . H. J . Am. Chem. Soc. 1949,
71, 2703. (b) Foster, R. Mol. Complexes 1974, 2, 107.
(32) Sankararaman, S.; Haney, W. A.; Kochi, J . K. J . Am. Chem.
Soc. 1987, 109, 5235 and 7824.
(34) The values of the formation constants KEDA of enol silyl ether/
chloranil complexes were uniformly small (∼1-3 M-1),33a characteristic
of typical π-π electron donor-acceptor complexes; see also Foster, R.
F. in ref 41.

632 J . Org. Chem., Vol. 61, No. 2, 1996
Rathore and Kochi
F igu r e 4. Charge-transfer absorption spectra of enol silyl ether complexes with π-acceptors. (a) Spectral changes accompanying
the incremental addition of cyclohexanone enol silyl ether 3a (0.08-0.69 M) to 0.003 M chloranil in dichloromethane at 25 °C.
Inset: Benesi-Hildebrand plot. (b) Charge-transfer absorption spectra of chloranil complexes of enol silyl ethers 3a , 3d , and 7
(as indicated) in dichloromethane at 25 °C. (c) Comparative charge-transfer spectra of the EDA complexes of enol silyl ether 1a
with tetracyanobenzene, 2,6-dichlorobenzoquinone, chloranil, and tetracyanoquinodimethane (as indicated) in dichloromethane
at 25 °C.
characteristic of weak intermolecular EDA complexes in
which the colors derived from the charge-transfer transi-
tion (hνCT), for example as depicted in eq 7.35
modified procedure36 was used to evaluate the charge-
transfer energies. Thus the relative charge-transfer
energies were calculated from the wavelengths λCT′ at
which all TNM complexes exhibited the same arbitrary
absorbance of 0.5 (under identical conditions of enol silyl
ether and TNM concentrations.) As shown in Figure 5c,
the relative charge-transfer energies (hνCT′) of the various
enol silyl ether complexes with TNM determined in this
manner followed an excellent linear correlation with hνCT
of the corresponding chloranil complexes (r ) 0.99 for a
slope of 1.13).
Sp ectr a l Obser va tion of th e En ol Silyl Eth er
Ca tion Ra d ica l a s th e Rea ctive In ter m ed ia te by
Tim e-Resolved Sp ectr oscop y. The temporal events
following the photoexcitation of enol silyl ether complexes
with TNM were directly examined by transient picosec-
ond as well as nanosecond laser-flash spectroscopy.
Thus, the laser pulse at λexc ) 532 nm (corresponding to
the second harmonic of a Q-switched Nd3+: YAG laser)
was well suited for the selective excitation of the charge-
transfer bands derived from various enol silyl ethers and
tetranitromethane (see Figure 3). The transient absorp-
tion spectrum in Figure 6a represents the spectral
transient obtained upon the application of a 25-ps laser
pulse at λexc ) 532 nm to the red solution of â-tetralone
enol silyl ether 7 and TNM in dichloromethane at -78
°C. The transient absorption band with a maximum at
530 nm did not change over a period of 4 ns after laser
excitation. In order to inquire into the fate of the spectral
transient with time, we excited the same sample with a
longer 10-ns laser pulse. Indeed, the 532-nm excitation
with a 10-ns laser pulse produced the same transient (see
inset), which followed second-order decay kinetics on the
microsecond time scale with kDEC ) 1.9 × 105 s-1 in
dichloromethane.
In order to establish the generality of such intermo-
lecular EDA complexes, we examined various enol silyl
ethers with several electron acceptors of different accep-
tor strengths, such as tetracyanobenzene (TCNB), 2,6-
dichlorobenzoquinone (DCBQ), chloranil (CA), and tet-
racyanoquinodimetane (TCNQ). For example, when a
colorless solution of R-tetralone enol silyl ether in dichlo-
romethane was mixed with either TCNB, DCBQ, CA, or
TCNQ, the colorless solution immediately changed to
orange (λmax ) 438 nm), red (λmax ) 486 nm), purple (λmax
) 538 nm), or green (λmax ) 628 nm), respectively, as
illustrated in Figure 4c. The absorption maxima λCT
obtained for the EDA complexes of various substituted
enol silyl ethers and these electron acceptors in dichlo-
romethane solutions are compiled in Table 3.
To gain further insight into the nature of the colored
complexes of representative enol silyl ethers with various
electron acceptors, the energies of the new absorption
maxima (hν/λCT) of the enol silyl ether complexes of
chloranil were plotted against those of the corresponding
TCNB and TCNQ complexes. The excellent linear cor-
relations with the slopes of 1.27 and 1.44 (r > 0.99) are
illustrated in Figures 5a and 5b, respectively.
Identification of the transient absorption spectrum
shown in Figure 6a was independently carried out by the
method of Gschwind and Hasselbach37 in which triplet
chloranil (3CA) was used to effect electron transfer from
(36) For example, see Wei, C.-H.; Bockman, T. M.; Kochi, J . K. J .
Organomet. Chem. 1992, 428, 85.
(37) Gschwind, R.; Hasselbach, E. Helv. Chim. Acta 1979, 62, 941
see also Kawai, K.; Yamamoto, N.; Tsubomura, H. Bull. Chem. Soc.
J pn. 1969, 42, 369.
Since the enol silyl ether complexes with TNM did not
exhibit well-defined absorption maxima (vide supra), a
(35) See Bockman et al. in ref 33b.

R-Nitration of Ketones via Enol Silyl Ethers
J . Org. Chem., Vol. 61, No. 2, 1996 633
F igu r e 5. Correlations of the charge-transfer energies (hνCT) of the enol silyl ether complexes of chloranil with those of (a)
tetracyanobenzene, (b) tetracyanoquinodimethane, and (c) tetranitromethane with the same series of enol silyl ethers (identified
by numbers in Table 1).
Ta ble 3. EDA Com p lex F or m a tion w ith En ol Silyl Eth er s
a n d Va r iou s Electr on Accep tor s in Dich lor om eth a n e a t
enol silyl ether 7, i.e.
25 °C
The transient spectrum of the enol silyl ether cation
radical 7•+ is shown in Figure 6b by the absorption band
at 520-30 nm38 together with that of chloranil anion
radical at 440-450 nm.37 We estimate the extinction
coefficient of 7•+ to be ∼5000 M-1 cm-1 based on the value
of ꢀ ) 9600 M-1 cm-1 for chloranil anion radical.39
The 25-ps laser pulse at λexc ) 532 nm was also used
in the photoexcitation of the EDA complexes of tetrani-
tromethane with the enol silyl ethers of R-tetralone and
cyclohexanone. Even under optimum conditions, how-
ever, we were able to observe at best only a weak,
nondescript absorbance in the spectral region from 460-
600 nm. Clearly, the lifetimes of the spectral transients
derived from these enol silyl ethers were shorter than
the 25-ps pulsewidth of the laser.
Discu ssion
The facile nitration of ketones with tetranitromethane
(TNM) in Tables 1 and 2 is illustrative of the synthetic
utility of enol silyl ethers in facilitating the R-substitution
of carbonyl derivatives.40 Moreover, the simple experi-
mental procedures, the mild reaction conditions, and the
trivial workup enable this to be the method of choice for
R-nitro ketone isolation. The fact that various enol silyl
ethers (1a , 2b, 3a , 3b) can be nitrated with tetrani-
tromethane in the dark at room temperature (see Table
1) or equally well with the aid of actinic promotion at
very low temperatures (see Table 2) is unique, and
(38) (a) A similar spectral transient was observed when either
tetracyanobenzene or maleic anhydride excited singlet was quenched
with enol silyl ether 7 (unpublished results, S. Hubig). (b) Compare
also the spectral transient produced from diphenylacetaldehyde enol
methyl ether: Mattay, J .; Vondenhof, M. Topics Curr. Chem. 1991, 159,
221.
(39) (a) Andre, J . J .; Weill, G. Mol. Phys. 1968, 15, 97. (b) Kobashi,
H.; Funabashi, M.; Kondo, T.; Morita, T.; Okada, T.; Mataga, N. Bull.
Chem. Soc. J pn. 1984, 57, 3557. (c) Iida, Y. Bull. Chem. Soc. J pn. 1970,
43, 2772. (d) Bonneau, R.; Carmichael, I.; Hug, G. L. Pure Appl. Chem.
1991, 63, 289.
(40) The method is also potentially applicable to other carbonyl
compounds such as aldehydes, esters, etc. For example, see Fukuzumi
particularly noteworthy from a mechanistic viewpoint.
et al. in ref 25.
The striking parallel between the thermal and photo-

634 J . Org. Chem., Vol. 61, No. 2, 1996
Rathore and Kochi
F igu r e 6. (a) Spectral transient obtained at 25-ps following the 532-nm excitation of the EDA complex from 0.1 M â-tetralone
enol silyl ether 7 and 0.1 M TNM in dichloromethane (at -78 °C). Inset: Temporal evolution of the same spectral transient (from
the sample above) measured at the indicated times following the 532-nm excitation with a 10-ns laser pulse. (b) Transient absorption
spectrum of the cation radical of â-tetralone enol silyl ether (7•+) and the chloranil anion radical (CA•-) obtained from the
photoexcitation of the chloranil complex of 7 with a 10-ns laser pulse at 532 nm according to the method of Gschwind and
Hasselbach.37
chemical nitrations extends to the isomeric enol silyl
ethers of tetralone, in which the R-isomer is quite reactive
and yields R-nitrotetralone in essentially quantitative
yields in both procedures, whereas the â-isomer is
unreactive and/or yields a complex mixture of products
in which â-nitrotetralone is absent. Since the dramatic
color changes accompanying the treatment of enol silyl
ethers with TNM is an unmistakable feature of the
nitrations, let us first consider how they bear critically
on all mechanistic questions in the following way.
P r e-Equ ilibr iu m F or m a tion of Ch a r ge-Tr a n sfer
Com p lexes. The development of a strong (orange to
dark red) coloration immediately upon the mixing of enol
silyl ethers with tetranitromethane (TNM) invariably
precedes the actual production of the R-nitro ketones.
Spectroscopic studies reveal the transient colors to be due
to new broad charge-transfer absorption bands (Figure
3) resulting from the instantaneous formation of electron
donor-acceptor (EDA) complexes between the enol silyl
ethers and TNM according to eq 6. The correlations in
Figure 5 show that the colored EDA complexes of various
enol silyl ethers with tetranitromethane (as the common
electron acceptor) are similar to those formed with other
well-known electron acceptors, such as chloranil (CA),
tetracyanobenzene (TCNB), and tetracyanoquinodimeth-
ane (TCNQ).41 All of these complexes are typical of
intermolecular associations of the type described by
Mulliken for aromatic electron donors.42 Indeed, the
linear variation of the charge-transfer energies (hνCT) for
the EDA complexes with the ionization potentials of the
enol silyl ethers (see IP in Table 3) confirms the Mulliken
prediction42 that hνCT ) IP - EA - ω, under conditions
in which the electron affinity (EA) of the acceptor and
the electrostatic interaction term (ω) of the charge-
transfer ion pair in eq 7 are invariant when a related
series of EDA complexes are considered. The linear
correlations obtained between the energies of the charge-
transfer bands (hνCT) of chloranil complexes with those
of the corresponding tetracyanobenzene and tetracyano-
quinodimethane complexes (see Figure 5, parts a and b,
respectively) show that enol silyl ethers form a family of
analogous 1:1 electron donor-acceptor complexes with
various π-acceptors. Furthermore, the distinctive colors
associated with charge-transfer transitions (hνCT) provide
the quantitative guide for assessing the electron donor
properties of enol silyl ethers (see Table 3).33a Since
charge-transfer energies of chloranil complexes are lin-
early correlated (r ) 0.99) with those of the corresponding
tetranitromethane complexes with a slope close to unity
(as shown in Figure 5c), those factors relevant to the
charge-transfer interactions of both acceptors with the
enol silyl ethers are more or less equivalentsdespite the
marked difference in the LUMOs of chloranil and tet-
ranitromethane.43
Ch a r ge-Tr a n sfer P h ot on it r a t ion of E n ol Silyl
Eth er . According to Mulliken theory,42 the direct pho-
toactivation of intermolecular EDA complexes by the
selective irradiation of the charge-transfer band (hνCT
)
leads to a photoinduced electron transfer.44 As applied
to the EDA complexes of enol silyl ethers and TNM
examined in this study, such a photoactivation is tanta-
mount to the spontaneous formation of the ion-radical
pair in Scheme 1.45
The experimental results of the photochemical nitra-
tion of enol silyl ethers with TNM indeed coincide with
the mechanistic formulation in Scheme 1. Thus, Figure
(43) Chloranil is an extensively delocalized planar π-acceptor,
whereas tetranitromethane is a rather localized tetrahedral acceptor
in which only one NO2 group is (presumably) involved in the charge-
transfer interaction. An X-ray crystallographic determination of a TNM
charge-transfer complex would establish this point.
(44) Hilinski, E. F.; Masnovi, J . M.; Kochi, J . K.; Rentzepis, P. M.
J . Am. Chem. Soc. 1984, 106, 8071.
(41) Foster, R. F. Organic Charge-Transfer Complexes; Academic:
New York, 1969.
(42) (a) Mulliken, R. S. J . Am. Chem. Soc. 1952, 74, 811. (b)
Mulliken, R. S.; Person, W. B. Molecular Complexes: A Lecture and
Reprint Volume; Wiley: New York, 1969.

R-Nitration of Ketones via Enol Silyl Ethers
J . Org. Chem., Vol. 61, No. 2, 1996 635
Sch em e 1
molytic coupling of an enol silyl ether radical cation with
NO2• is so rapid that it even precludes competition from
the facile intramolecular capture of the radical cation by
a tethered olefinic group in the citronellal derivative 9•+
as previously identified by Snider and Kwon,30b i.e.
,
3 shows that the exposure of enol silyl ether to TNM leads
immediately to the colored EDA complex in eq 9. The
photoactivation of the EDA complex by the specific
irradiation of the charge-transfer band (under the condi-
tions where thermal reaction is too slow to compete)
results in the photoinduced electron transfer in eq 10.
The rapid fragmentation of the tetranitromethane anion
radical leads to the ion radical triad shown in eq 11.46
As such, this mechanistic formulation is related to
previous studies of the photoinduced electron transfer of
tetranitromethane with arene donors (ArH), in which the
The absence of cyclized products in the photochemical
nitration of enol silyl ether 9 (see Table 2) is thus
consistent with the time-resolved spectroscopic results,
which establish short lifetimes of the enol silyl ether
cation radicals of generally less than 10-7 s. Only the
most stable cation radical such as the highly conjugated
7•+ is sufficiently persistent to record its transient
spectrum on the ps/ns timescale (see Figure 6).
Com m en ts on th e Th er m a l Nitr a tion of En ol Silyl
Eth er s w ith Tetr a n itr om eth a n e. As useful as time-
resolved spectroscopy is for the detection and identifica-
tion of highly reactive intermediates in establishing the
mechanism of the photochemical nitration according to
Scheme 1, no comparable technique is available to probe
the same types of intermediates in the corresponding
thermal nitration described in Table 1.50 Let us therefore
resort to an indirect (inferential) approach to relate the
mechanism of the photochemical nitration in Scheme 1
to the corresponding thermal process. First, the strik-
ingly similar color changes that accompanying the pho-
tochemical and thermal nitration of enol silyl ethers with
tetranitromethane indicates that the pre-equilibrium
formation of the EDA complex described in eq 9 is
common to both processes. Next, it is singularly note-
worthy that the thermal nitration (in the dark) at 25 °C
and photonitration at -40 °C leads to the same R-nitro
ketones in Tables 1 and 2, respectively. One can conclude
from these observations that intermediates leading to
thermal nitration are similar to, if not the same as, those
derived by photonitration. Accordingly, the differences
in the qualitative rates of thermal nitration are best
reconciled on the basis of donor strength of enol silyl
ethers toward TNM as a weak oxidant27 in a thermal
electron transfer (∆), as described in Scheme 2, eq 14.
corresponding radical ion triad [ArH•+, C(NO2)3-, NO2 ]
•
was directly produced in the course of charge-transfer
activation.47 The observation of the enol silyl ether
radical cation by time-resolved spectroscopy in Figure 6
thus confirms the formulation in Scheme 1. Further-
more, the excellent material balance obtained in charge-
transfer nitrations (Table 2) demands that the radical
ion triad in eq 11 proceeds quantitatively to the nitration
products by the following stoichiometry
•- 32,44
The short life time of less than 3 ps for C(NO2)4
,
indicates that the triad in eq 11 is initially trapped within
the solvent cage, since this time scale precludes any
competition from diffusional separation.47 Thus, a fast
homolytic coupling of radical cation of enol silyl ether
•
with NO2 , followed by the loss of the cationic trimeth-
Sch em e 2
ylsilyl moiety accounts for the R-nitro ketone formed in
eq 12 (see the pathway indicated by the dashed arrows).48
The failure to spectrally observe the radical cations of
the reactive R-tetralone and cyclohexanone derivatives
1a and 3a , respectively, in marked contrast to the
unreactive â-tetralone enol silyl ether 7, strongly suggests
that the radical pair annihilation in eq 12 is fast and it
effectively supersedes cage escape.49 Indeed, the ho-
According to Scheme 2, the pre-equilibrium formation
of the EDA complex of the enol silyl ether and TNM (eq
9) is followed by the rate-limiting electron transfer to
afford the ion radical triad in eq 14.51 As such, the
reactivity of the various enol silyl ethers in thermal
nitrations is directly related to their donor strengths. For
example, the tetralone derivatives, being among the best
(45) (a) Compare Masnovi, J . M.; Kochi, J . K.; Hilinski, E. F.;
Rentzepis, P. N. J . Am. Chem. Soc. 1986, 108, 1126. (b) Note that the
direct photoexcitation of the EDA complex by irradiation of the charge-
transfer band allows the nitration in high yields of such relatively poor
donors as the enol silyl ethers 3a , 3b, and 4a even at low temperatures.
(46) (a) Rabani, J .; Mulac, W. A.; Matheson, M. S. J . Phys. Chem.
1965, 69, 53. (b) Chaudhri, S. A., Asmus, K.-D. Ibid. 1972, 76, 26.
(47) See Bockman, T. M.; Kochi, J . K. J . Phys. Org. Chem. 1994, 7,
325.
(48) (a) Compare the TMS transfer in the related (chloranil) charge-
transfer process, as described by Bockman, T. M. et al. in ref 33b. (b)
Although the alternative sequence (involving TMS transfer in the ion
pair, followed by homolytic coupling) cannot be rigorously discounted,
it is disfavored since it does not readily account for the difference in
nitro ketone yield between the isomeric 1a and 7.
•
(49) The analogous kinetic behavior of NO2 toward reactive aro-
matic radical cations is described in ref 47.
(50) With thermal activations, the steady-state concentrations of
reactive intermediates are generally so low as to make their detection
experimentally difficult to observe.

636 J . Org. Chem., Vol. 61, No. 2, 1996
Rathore and Kochi
electron donors, do indeed react the fastest with TNM
(see Table 1) in comparison to the acyclic derivatives
(4a ,b), which are poor electron donors and react the
slowest. The alternative electrophilic mechanism for the
thermal nitration of enol silyl ethers with TNM requires
the intimate approach of the nucleophilic enol silyl ether
to a NO2 group on the quaternary carbon center of TNM,
which is sterically very demanding and thus does not
readily account for the observed reactivity. Furthermore,
the observed lack of regioselectivity in the nitration of
the isomeric enol silyl ethers (3c and 3d ) of 2-methylcy-
clohexanone with TNM that leads to the same 2-nitro-
2-methylcyclohexanone in eq 2 is not readily reconciled
with an electrophilic mechanism. On the other hand, the
cation radical of the kinetic enol silyl ether 3c•+ is readily
converted to the cation radical of the thermodynamic
isomer 3d •+ by a 1,3-prototropic shift in the course of the
photoinduced electron transfer with chloranil,52 i.e.
in their charge-transfer R-arylations with cationic aro-
matic diazonium acceptors.18
Exp er im en ta l Section
Ma ter ia ls. Tetrachloro-p-benzoquinone (chloranil), 2,6-
dichloro-p-benzoquinone, 1,2,4,5-tetracyanobenzene (TCNB),
and tetracyanoquinodimethane (TCNQ) were commercially
available (Aldrich), and they were purified by recrystallization
prior to use. Chlorotrimethylsilane (TMSCl, Aldrich), sodium
iodide (Aldrich), cyclohexanone, cyclopentanone, cyclohep-
tanone, 2-methylcyclohexanone, propiophenone, R-tetralone,
â-tetralone, diphenylacetone, 3-heptanone, pinacolone, nor-
bornanone, and camphor (Aldrich) were used as received.
Dihydrochalcone was prepared according to the procedure in
Organic Syntheses.54 Dichloromethane (Mallinckrodt analyti-
cal reagent) was repeatedly stirred with fresh aliquots of concd
sulfuric acid (∼20% by volume) until the acid layer remained
clear. After separation, the dichloromethane was washed
successively with water, aqueous sodium bicarbonate, water,
and aqueous sodium chloride, and it was then dried over
anhydrous calcium chloride. The dichloromethane was dis-
tilled twice from P2O5 under an argon atmosphere and stored
in a Schlenk tube equipped with a Teflon valve fitted with
Viton O-rings. Acetonitrile (Fischer) was stirred with KMnO4
for 24 h, and the mixture was refluxed until the liquid was
colorless. The MnO2 was removed by filtration. The aceto-
nitrile was distilled from P2O5 under an argon atmosphere and
then refluxed over CaH2 for 6 h. After distillation from the
CaH2, the acetonitrile was stored in a Schlenk flask under an
argon atmosphere. The n-pentane was distilled twice from
P2O5 and stored in a Schlenk flask under an argon atmosphere.
Tetranitromethane was prepared as described in Organic
Syntheses55 and purified by steam distillation.
In str u m en ta tion . The time-resolved spectra were ob-
tained with a flash photolysis apparatus described earlier.32
The steady-state photolyses were carried out with the focussed
beam from a 500-W high pressure mercury lamp. The beam
was passed through a water filter and the appropriate glass
cutoff filter (Corning CS series) to remove infrared and
ultraviolet light.
P r ep a r a tion of En ol Silyl Eth er s. The enol silyl ethers
used in this study were prepared from commercially available
ketones using the procedure of Cazeau et al.,56 in which a
mixture of the ketone (25 mmol) and pre-dried sodium iodide
(4.50 g, 30 mmol) in dry acetonitrile (30 mL) was stirred under
an argon atmosphere. To the resulting solution, triethylamine
(3.04 g, 30 mmol) was added, followed by chlorotrimethylsilane
(3.26 g, 30 mmol). The reaction mixture was stirred for 0.5 h,
and it was successively treated with prechilled (∼0 °C) pentane
(50 mL) and saturated aqueous ammonium chloride (50 mL).
The aqueous phase was further extracted with pentane (2 ×
25 mL). The combined pentane extract was washed with ice-
water (2 × 50 mL), followed by saturated ammonium chloride
(50 mL), and then dried over anhydrous magnesium sulfate.
Removal of solvent in vacuo afforded the crude enol silyl ether
as a colorless oil, which was further purified by fractional
distillation under reduced pressure. The various enol silyl
Such an efficient photoisomerization of the cation
radical 3c•+ to 3d •+ accounts for the isomerization of enol
silyl ethers 3c to 3d in the course of the thermal nitration
with TNM, and it explains the lack of regioselectivity in
eq 2 (vide supra). Other evidence that the cation radicals
are critical intermediates in the thermal nitration of enol
silyl ethers can be deduced from the distinctive behavior
of the R- and â-tetralone derivatives 1a and 7. Thus, the
cation radical of â-tetralone enol silyl ether (7•+) is
stabilized owing to extensive π-delocalization, which
accords with its unusually long lifetime in the time-
resolved spectroscopic experiments (Figure 6a) in marked
contrast to the short lifetime of the cross conjugated and
more reactive R-isomer (1a •+).53 We thus attribute the
disparate yields of the respective nitrotetralones (Table
1) to derive from the relative rate difference in homolytic
coupling in eq 12, i.e.
It is noteworthy that a similar disparate behavior of R-
and â-tetralone enol silyl ethers has been recently noted
(52) (a) See the Experimental Section for the photochemical genera-
tion of 3c•+ and its rearrangement to the isomeric 3d •+ by the procedure
of Gschwind and Hasselbach in ref 37. (b) The facile 1,3-hydrogen shift
in the prototypical propene radical cation has been predicted by Clark,
T. J . Am. Chem. Soc. 1987, 109, 6838. For the analogous conversion
of 1-butene radical cation to the 2-butene isomer, see Roth, H. D. Topics
Curr. Chem. 1992, 163, 168.
(51) (a) The thermal electron-transfer step (eq 14) is presented as a
dissociative electron attachment to tetranitromethane. The alternative
stepwise electron transfer (i.e. eq 10 followed by eq 11) is expected to
be too slow owing to a driving force that is estimated to be endergonic
by at least +∆GET ) 25-34 kcal mol-1, based on the reduction potential
for TNM of E°red e 0.0 V vs SCE27b and oxidation potentials for various
enol silyl ethers of E°ox g 1.1 - 1.5 V vs SCE (calculated from the
values of the irreversible anodic (CV) peak potentials Ep in ref 33a
plus the correction according to: Andrieux, C. P.; LeGorande, A.;
Save´ant, J .-M. J . Am. Chem. Soc. 1992, 114, 6892). (b) On the other
hand, the dissociative electron attachment as presented in eq 14 is
more favorable than the stepwise mechanism, since it will lead to an
enhancement in the driving force for electron transfer (-∆GET) owing
to the simultaneous (concerted) formation of the stable fragments
(53) The unusually long lifetime of 7•+ allows sufficient time for the
diffusive separation of the ion radical triad in Scheme 1; and it accounts
for the observed second-order decay kinetics (compare Figure 6a).
Otherwise the formulation in eq 12 predicts first-order decay kinetics
of the spectral transients in the photochemical nitrations with TNM.
(54) Adams, R.; Kern, J . W.; Shriner, R. L. Org. Synth. 1941, 1, 101.
(55) Available from Aldrich Chemical Co. or readily prepared
according to Liang, P. Org. Synth. 1955, 3, 803. Wa r n in g: Although
we have never encountered difficulties, TNM is potentially hazardous.
(56) Cazeau, P.; Duboudin, F.; Moulines, F.; Babot, O.; Dunogues,
J . Tetrahedron 1987, 43, 2075.
-
C(NO2)3 and NO2•. [For a discussion of this important point, see
Save´ant, J .-M. J . Am. Chem. Soc. 1987, 109, 6788.]

R-Nitration of Ketones via Enol Silyl Ethers
J . Org. Chem., Vol. 61, No. 2, 1996 637
ethers were characterized by GC-MS analysis, and the NMR
spectra were compared with those previously reported.57
P r ep a r a t ion of 3,7-Dim et h yl-1-p h en yl-6-oct en -1-on e
(9).30 A 500-mL flask was charged with a solution of phenyl-
magnesium bromide (40 mL, 3 M, Aldrich) and anhydrous
diethyl ether (100 mL), and it was cooled to 0 °C. A solution
of citronellal (15.4 g, 100 mmol) in ether (100 mL) was added
dropwise under an argon atmosphere. The resulting mixture
was stirred for 1 h at 0 °C and then for an additional 6 h at
room temperature. The usual workup afforded the crude
alcohol which was directly oxidized with pyridinium chloro-
chromate (PCC) as follows. A solution of the alcohol (20 g, 86
mmol) in dichloromethane (200 mL) was added dropwise to a
stirred mixture of PCC (32 g, 150 mmol) and Celite (100 g) in
dichloromethane at room temperature. The resulting suspen-
sion was stirred for 2 h, and the solvent was removed in vacuo.
The dark solid residue was dissolved in ether (500 mL), and
the ethereal solution was filtered through a short pad of silica
gel. Removal of the solvent in vacuo and distillation afforded
the desired ketone as a colorless oil (16.1 g, 70 mmol, 81%):
IR (KBr) 2964, 2922, 2877, 2855, 1688 (vs), 1598, 1582, 1448,
1376, 1366, 1287, 1214, 1003, 752, 691 cm-1; 1H NMR (CDCl3)
δ 0.90 (d, J ) 6.6 Hz, 3H), 1.17-1.42 (m, 2H), 1.53 (s, 3H),
1.61 (s, 3H), 1.97 (sym m, 2H), 2.13 (sym m, 1H), 2.67 (dd, J
) 15.6, 8.1 Hz, 1H), 2.89 (dd, J ) 15.6, 5.4 Hz), 5.04 (tm, J )
6.9 Hz, 1H), 7.36 (t, 2H), 7.45 (t, 1H), 7.88 (d, 2H); 13C NMR
(CDCl3) δ 17.41, 19.68, 25.31, 25.47, 29.17, 36.92, 45.55, 124.20,
127.82, 128.25, 131.05, 132.55, 137.12, 199.82; GC-MS m/z 230,
M+, 230 calcd for C16H22O.
221 calcd for C11H11NO4. 2-Nitr o-2-m eth yl-r-tetr a lon e (1c):
oil; IR (KBr) 3070, 3031, 2996, 2945, 2879, 2852, 1697 (vs),
1603, 1548 (vs), 1456, 1388, 1347, 1334, 1309, 1231, 981, 853,
806, 797, 750, 731 cm-1; 1H NMR (CDCl3) δ 1.73 (s, 3H), 2.26
(sym m, 1H), 2.83-2.97 (m, 3H), 7.18 (d, J ) 7.5 Hz, 1H), 7.25
(t, J ) 7.5 Hz, 1H), 7.44 (td, J ) 7.2, 0.9 Hz, 1H), 7.94 (d, J )
7.2 Hz, 1H); 13C NMR (CDCl3) δ 20.32, 25.11, 34.38, 91.37,
127.03, 128.73, 128.08, 134.31, 142.14, 188.81; GC-MS m/z 205
M+, 205 calcd for C11H11NO3. Anal. Calcd for C11H11NO3: C,
64.38; H, 5.40; N, 6.83. Found: C, 64.04; H, 5.35; N, 6.88.
r-Nitr oa cetop h en on e (2a ):58 mp 102-103 °C (lit. mp7a 103-
104 °C); IR (KBr) 3074, 3016, 2959, 1696 (s), 1598, 1557 (vs),
1450, 1384, 1335, 1229, 1204, 999, 761, 687, 663 cm-1; 1H NMR
(CDCl3) δ 5.91 (s, 2H), 7.50 (t, 2H), 7.65 (t, 1H), 7.83 (d, 2H);
13C NMR (CDCl3) δ 81.36, 128.14, 129.20, 132.00, 135.03,
185.92; GC-MS m/z 165, M+, 165 calcd for C8H7NO3. 2-Nitr o-
1-p h en yl-1-p r op a n on e (2b):58 oil; IR (KBr) 3070, 3004, 2944,
1696 (vs), 1563 (vs), 1450, 1390, 1324, 1284, 1224, 1178, 1072,
1
1012, 965, 873, 786, 699, 660 cm-1; H NMR (CDCl3) δ 1.74
(d, 6.9 Hz, 3H), 6.19 (q, 6.9 Hz, 1H), 7.41 (t, 2H), 7.58 (sym m,
1H), 7.87 (d, 2H); 13C NMR (CDCl3) δ 15.91, 84.98, 129.08,
128.66, 133.28, 134.65, 190.12; GC-MS m/z 179 M+, 179 calcd
for C9H9NO3. 2-Nitr o-1-p h en yl-1-h ep ta n on e (2c):58 oil; IR
(KBr) 2957, 2931, 2864, 1689, 1563, 1450, 1370, 1277, 1231,
1
972, 693 cm-1; H NMR (CDCl3) δ 0.88 (t, 3H), 1.32 (sym m,
6H), 2.12 (sym m, 1H), 2.34 (sym m, 1H), 6.11 (dd, 1H), 7.38-
7.56 (m, 3H), 7.95 (sym m, 2H); 13C NMR (CDCl3) δ 36.13.68,
22.10, 25.53, 30.43, 30.94, 89.85, 128.42, 129.06, 134.55,
137.00, 189.12; GC-MS m/z 235 M+, 235 calcd for C13H17NO3.
2-Nitr o-1,3-d ip h en yl-1-p r op a n on e (2d ): oil; IR (KBr) 2969,
2951, 2832, 1697 (vs), 1560 (vs), 1494, 1455, 1370, 1259, 755,
Th er m a l (Da r k ) Nitr a tion of En ol Silyl Eth er s w ith
Tetr a n itr om eth a n e. Gen er a l P r oced u r e for th e P r ep a -
r a tion of r-Nitr ok eton es in Dich lor om eth a n e. A 50-mL
flask was charged with a solution of enol silyl ether 1a (2.18
g, 10 mmol) in anhydrous dichloromethane (40 mL), and a
solution of TNM (2.11 g, 11 mmol) in dichloromethane (10 mL)
was added under an argon atmosphere at room temperature.
The solution immediately developed a characteristic deep red
coloration, and it was stirred at room temperature (in dark)
for the specified period denoted in Table 1. The resultant
yellow reaction mixture was diluted with dichloromethane (50
mL), and it was repeatedly washed with water until the
washings were colorless. The dichloromethane layer was dried
over anhydrous magnesium sulfate and then filtered. Removal
of the solvent in vacuo afforded the desired 2-nitro-R-tetralone
(1a ) in essentially quantitative yields (1.88 g, 99%). An a lysis
of Aqu eou s La yer . The highly colored aqueous layer was
transferred to a 5-L volumetric flask and diluted with distilled
water. A 10-mL aliquot of the aqueous layer was further
diluted to 200 mL. An aliquot was transferred to a 1-cm
quartz cuvette and the UV-vis spectrum recorded. The
1
696 cm-1; H NMR (CDCl3) δ 3.55 (dd, J ) 14.7, 5.4 Hz, 1H),
3.73 (dd, J ) 14.7, 9.0 Hz, 1H), 6.38 (dd, J ) 9.0, 5.4 Hz, 1H),
7.25-7.42 (m, 5H), 7.57 (t, 2H), 7.72 (t, 1H), 8.01 (d, 2H); 13C
NMR (CDCl3) δ 36.37, 90.45, 127.67, 128.86, 128.97, 129.15,
130.13, 134.48, 134.73, 188.37; GC-MS m/z 255 M+, 255 calcd
for C15H13NO3. Anal. Calcd for C15H13NO3: C, 70.58; H, 5.13;
N, 5.49. Found: C, 70.23; H, 5.08; N, 5.41. 2-Nitr o-1-(4-
n itr op h en yl)-1-eth a n on e (2e):58 mp 149-151 °C (lit. mp58
150-151 °C); IR (KBr) 3117, 3083, 1716 (vs), 1669, 1603, 1569,
1523 (vs), 1417, 1390, 1344, 1323, 1277, 1218, 1105, 1012, 859,
793, 719 cm-1; 1H NMR (CDCl3) δ 6.62 (s, 2H),7.64 (d, J ) 8.6
Hz, 2H), 8.11 (d, J ) 8.6 Hz, 2H); GC-MS m/z 210 M+, 210
calcd for C8H6N2O5. 2-Nitr ocycloh exa n on e (3a ):10a mp 38-
39 °C (lit. mp10a 39.5-40.5 °C); IR (KBr) 2952, 2928, 2853, 1720
(vs), 1552 (vs), 1448, 1420, 1389, 1334, 1298, 1240, 1110, 1087,
784, 648 cm-1; 1H NMR (CDCl3) δ 1.48-2.56 (m, 8H), 5.19 (dd,
J ) 13.0, 5.2 Hz, 1H); 13C NMR (CDCl3) δ 22.15, 26.15, 31.30,
40.39, 91.76, 199.03; GC-MS m/z 143, M+, 143 calcd for C6H9-
NO3. 2-Nitr o-4-ter t-bu tylcycloh exa n on e (3b):10a mp 86-
88 °C (lit. mp10a 87-90 °C); IR (KBr) 2957, 2911, 2871, 1723
(vs), 1556 (vs), 1463, 1417, 1384, 1337, 1297, 1271, 1238, 1145,
absorbance A350 ) 1.39 (ꢀ350 ) 14000 M-1 cm-1
)
28 corresponded
to a yield of 9.93 mmol (99%) of nitroform. The crude R-nitro
ketones prepared from the general procedure were further
purified by crystallization, distillation, or column chromatog-
raphy. The spectral data for various R-nitro ketones are
summarized below: 2-Nitr o-r-tetr a lon e (1a ):7a mp 71-72 °C
(lit. mp7a 72-73 °C); IR (KBr) 3069, 2964, 2943, 1702 (vs),
1598, 1560 (vs), 1456, 1434, 1379, 1355, 1308, 1264, 1232,
1010, 920, 789, 764, 745, 723 cm-1; 1H NMR (CDCl3) δ 2.61-
2.87 (m, 2H), 3.12 (sym m, 2H), 5.43 (dd, J ) 12.6, 4.8 Hz,
1H), 7.30 (sym m, 2H), 7.54 (t, 1H), 7.99 (d, 1H); 13C NMR
(CDCl3) δ 26.53, 28.10, 89.73, 127.40, 128.15, 128.88, 130.18,
134.93, 142.75, 186.49; GC-MS m/z 191 M+, 191 calcd for C10H9-
NO3. 2-Nitr o-6-m eth oxy-r-tetr a lon e (1b):7a mp 139-140
°C (lit. mp7a 138-139 °C); IR (KBr) 3010, 2957, 2924, 2845,
1689 (s), 1603 (vs), 1556 (vs), 1496, 1457, 1350, 1311, 1251
(vs), 1111, 1025, 919, 879, 852, 819, 746 cm-1; 1H NMR (CDCl3)
δ 2.63 (sym m, 1H), 2.81 (sym m, 1H), 3.08 (dd, J ) 8.1, 4.5
Hz, 2H), 3.85 (s, 3H), 5.38 (dd, J ) 12.0, 4.5 Hz, 1H), 6.70 (d,
J ) 2.4 Hz, 1H), 6.86 (dd, J ) 8.7, 2.4 Hz, 1H), 8.01 (d, J )
8.7 Hz); 13C NMR (CDCl3) δ 26.92, 28.25, 55.64, 89.59, 112.68,
114.24, 123.62, 130.91, 145.32, 164.77; GC-MS m/z 221, M+,
1091, 786, 733, 653 cm-1 1H NMR (CDCl3) δ 0.94 (s, 9H),
;
1.51-2.66 (m, 7H), 5.28 (dd, J ) 13.2, 5.4 Hz, 1H); 13C NMR
(CDCl3) δ 27.52, 32.68, 33.03, 40.00, 45.06, 61.44, 92.07, 219.2;
GC-MS m/z 199, M+, 199 calcd for C10H17NO3. 2-Nitr o-2-
m eth ylcycloh exa n on e (3d ):10a mp 33-35 °C (lit. mp10a 34-
35 °C); IR (KBr) 2951, 2877, 1731 (vs), 1645, 1612, 1550 (vs),
1453, 1431, 1388, 1356, 1337, 1294, 1266, 1125, 1088, 853, 822,
800 cm-1; 1H NMR (CDCl3) δ 1.62 (s, 3H), 1.65-1.85 (m, 4H),
1.98 (sym m, 1H), 2.54 (sym m, 2H), 2.83 (sym m, 1H); 13C
NMR (CDCl3) δ 21.52, 22.38, 26.58, 38.36, 39.20, 93.90, 200.70;
GC-MS m/z 157, 157 calcd for C7H11NO3. 2-Nitr ocycloh ep -
ta n on e (3e):59 mp 37-38 °C (lit. mp59 38-38.5 °C); IR (KBr)
2934, 2861, 1721, 1696, 1557, 1458, 1376, 1343, 1311, 1204,
1147, 1073, 925, 843, 728 cm-1; 1H NMR (CDCl3) δ 5.36 (dd, J
) 3.7, 9.4 Hz, 1H), 1.28-2.55 (m, 10H); 13C NMR (CDCl3) δ
23.0, 26.09, 28.56, 41.13, 43.38, 93.64, 210.44; GC-MS m/z 157,
M+, 157 calcd for C7H11NO3. 3-Nitr oh ep ta n -4-on e (4a ):1a oil;
IR (KBr) 2910, 2822, 1718 (vs), 1692, 1550, 1448, 1366, 1317,
1202, 1150, 1060, 905, 809, 750 cm-1; 1H NMR (CDCl3) δ 0.83
(58) Ashwell, M. A.; J ackson, R. F. W. Synthesis 1988, 229, and
references therein.
(59) Feuer, H.; Hall, A. M.; Golden, S.; Reitz, R. L. J . Org. Chem.
1968, 33, 3622.
(57) (a) House, H. O.; Czuba, L. J .; Gall, M.; Olmstead, D. J . Org.
Chem. 1969, 34, 2324. (b) For reviews, see Brownbridge, P. Synthesis
1983, 1 and 85, and references therein.

638 J . Org. Chem., Vol. 61, No. 2, 1996
Rathore and Kochi
(t, 3H), 0.90 (t, 3H), 1.42-1.51 (m, 4H), 2.30 (t, 2H), 4.99 (dd,
J ) 5.4, 9.9 H2, 1H); 13C NMR (CDCl3) δ 13.04, 16.40, 17.79,
21.03, 41.05, 95.34, 198.98; GC-MS m/z 159, M+, calcd for
C7H13NO3. 1-Nitr o-3,3-d im eth ylbu ta n -2-on e (4b):1a oil; IR
(KBr) 2968, 2924, 2869, 1721, 1629, 1558 (vs), 1466, 1364,
1325, 1275, 1172, 1053, 262, 683 cm-1; 1H NMR (CDCl3) δ 0.98
(s, 9H), 4.82 (s, 2H); 13C NMR (CDCl3) δ 22.35, 26.12, 46.90,
92.12, 201.27; GC-MS m/z 145, M+, 145 calcd for C6H11NO3.
3-Nitr on or bor n a n on e (5): oil; IR (KBr) 2956, 2920, 2876,
1736 (vs), 1565 (vs), 1424, 1439, 1392, 1297, 1068, 815, 760
mixture of nitro ketones 9 (vide supra) together with the traces
of corresponding ketone and enol silyl ether. The 13C NMR
spectrum of the crude reaction mixture did not show a peak
(∼90 ppm) for the quaternary carbon bearing a nitro group.
Since this NMR resonance is characteristic of tertiary nitroal-
kanes, we inferred that the cyclized product 11 in eq 3 was
not formed. The flash chromatography of the residue using a
hexane-ether (3:1) mixture as an eluent, afforded the stere-
oisomeric mixture of nitro ketone 9 as a pale yellow oil (214
mg, 78%). Th er m a l Nitr a tion of 2-Meth ylycloh exa n on e
En ol Silyl Eth er 3c w ith TNM. According to general
procedure, a solution of the kinetic enol silyl ether 3c (1.84 g,
10 mmol) in dichloromethane (50 mL) was mixed with TNM
(2.11 g, 11 mmol) under an argon atmosphere at room
temperature. The solution immediately developed a charac-
teristic orange coloration, and it was stirred (in dark) for 16 h
at room temperature. The resultant yellow reaction mixture
was diluted with dichloromethane (20 mL), and it was washed
repeatedly with water (4 × 25 mL). The dichloromethane layer
was dried over anhydrous magnesium sulfate and evaporated.
The GC and GC-MS analysis of the crude residue showed the
presence of 2-methyl-2-nitrocyclohexanone 3d as a major
product together with the traces of the corresponding (hydro-
lyzed) ketone and the unreacted kinetic enol silyl ether 3c.
Distillation of the crude oil afforded the pure 2-methyl-2-
nitrocyclohexanone 3d (1.1 g, 70%). [Note that the nitro
ketone 3d obtained above from the kinetic enol silyl ether 3c
was identical to an authentic sample, prepared from the
reaction of the thermodynamic enol silyl ether 3d and TNM.]
In a similar experiment, a mixture of enol silyl ether 3c (184
mg, 1 mmol) and TNM (211 mg, 1.1 mmol) in dichloromethane
was treated at room temperature. After 3 h, the solvent and
excess TNM were removed under high vacuum, and the crude
residue was redissolved in dichloromethane (20 mL). It was
washed with distilled water (3 × 20 mL) and dried over
anhydrous magnesium sulfate. The GC and GC-MS analysis
(internal standard method) of the dichloromethane layer
showed that the reaction mixture consisted of nitro ketone 3d
(0.12 mmol) and the unreacted enol silyl ether 3c (0.82 mmol)
together with the traces of the 2-methylcyclohexanone (<1%).
[It is noteworthy that no isomerization of the kinetic enol silyl
ether 3c to the thermodynamic enol silyl ether 3d was
observed during the course of reaction, as evidenced by the
recovery of only the kinetic enol silyl ether 3c.] In another
experiment, a solution of enol silyl ether 3c (184 mg, 1 mmol)
and 6-methyl-2-nitrocyclohexanone (157 mg, 1 mmol; prepared
by the method of Dampawan and Zajac11c) in dichloromethane
(5 mL) was mixed with TNM (211 mg, 1.1 mmol). The pale
orange solution was stirred at room temperature for 16 h. The
standard aqueous workup, as described above, afforded an oily
residue. The GC and GC-MS analysis (internal standard
method) showed the residue consisted of 2-methyl-2-nitrocy-
clohexanone (0.73 mmol) and unchanged 6-methyl-2-nitrocy-
clohexanone (0.96 mmol) together with a small amount of
starting ketone.
1
cm-1; H NMR (CDCl3) δ 1.05-2.50 (m, 6H), 2.59 (br s, 1H),
3.01 (br s, 1H) 5.15 (br s, 1H) 13C NMR (CDCl3) δ 21.46, 24,-
81, 34.59, 40.70, 47.47, 89.26, 201.92; GC-MS m/z 155, m+,
155 calcd for C7H9NO3. Anal. Calcd for C7H9NO3: C, 54.19;
H, 5.85; N, 9.03. Found: C, 54.39; H, 5.89; N, 8.91. 3-Nitr o-
ca m p h or (6):11b mp 102-103 °C (lit. mp11b 103 °C); 1H NMR
(CDCl3) δ 0.88 (s, 3H), 0.97 (s, 3H), 1.05 (s, 3H), 1.22-198 (m,
4H), 2.72 (t, J ) 4.5 Hz, 1H), 5.04 (d, 2H); 13C NMR (CDCl3)
δ 9.35, 18.73, 19.67, 20.74, 21.58, 29.45, 48.69, 88.95, 178.09;
GC-MS m/z 197 M+, 197 calcd for C10H15NO3. 2-Nitr o-3,7-
d im eth yl-1-p h en yl-6-octen -1-on e (9). As a mixture of ste-
reoisomers: oil; IR (KBr) 2971, 2931, 2858, 1696 (s), 1598,
1
1581, 1557 (vs), 1450, 1364, 1291, 1218, 680 cm-1; H NMR
(CDCl3) δ 0.95 (d, J ) 6.9 Hz, 3H), 1.07 (d, J ) 6.6 Hz, 3H),
1.14-1.50 (m, 2H), 1.56 (s, 3H), 1.64 (br s, 3H), 1.90-2.13 (m,
2H), 2.70 (sym m, 1H), 5.00 (sym m, 1H), 5.95 (dd, J ) 8.1,
5.7 Hz, 1H), 7.43-7.98 (m, 5H); 13C NMR (CDCl3) δ 15.34,
15.43, 17.65, 24.74, 25.12, 25.65, 32.50, 32.70, 34.51, 34.64,
93.26, 93.34, 123.06, 128.69, 128.70, 129.16, 132.56, 132.72,
134.60, 134.80, 135.02, 188.44, 188.59; GC-MS m/z 229 [M+
46], 229 calcd for [C16H21NO3 - NO2]. Anal. Calcd for C16H21
NO3: C, 69.79; H, 7.69; N, 5.09. Found: C, 69.38; H, 7.46; N,
5.18.
-
-
In Aceton itr ile. A solution of enol silyl ether 1a (2.18 g,
10 mmol) in dry acetonitrile (50 mL) was treated with TNM
(2.16 g, 10 mmol) at 25 °C. The red solution was stirred for
15 min and diluted with 100 mL of dichloromethane. The
yellow solution was washed with water (3 × 50 mL) and dried
over anhydrous magnesium sulfate. Evaporation of the sol-
vent afforded the 2-nitro-R-tetralone 1a in essentially quan-
titative yield (1.90 g, 99%). UV-vis spectral analysis indicated
that the aqueous layer contained 1 equiv of nitroform (∼10
mmol). In n -P en ta n e. Tetranitromethane (2.16 g, 11 mmol)
was added to a solution of enol silyl ether 1a (2.18 g, 10 mmol)
in anhydrous n-pentane (50 mL) under an argon atmosphere
at 25 °C. The characteristic dark-red colored solution bleached
within minutes (∼10 min) as the yellow crystals of 2-nitro-R-
tetralone 1a separated. The standard aqueous workup af-
forded 2-nitro-R-tetralone in 98% yield (1.87 g), and the
aqueous layer showed the presence of 1 equiv of nitroform.
Rea ction of â-Tetr a lon e En ol Silyl Eth er 7 w ith TNM. A
dichloromethane solution of 7 (10 mL, 0.2 M) and TNM (430
mg, 2 mmol) was mixed (in the dark) at 25 °C under an argon
atmosphere to produce the characteristic dark-red solution.
The red solution was stirred for 12 h, but no spectral change
was apparent. The solvent and TNM were removed in vacuo,
and the crude residue was dissolved in dichloromethane (25
mL) and washed repeatedly with distilled water (3 × 50 mL).
The dichloromethane layer was dried over anhydrous magne-
sium sulfate and evaporated in vacuo. The GC and GC-MS
analysis showed that the enol silyl ether 7 was recovered intact
(200 mg, 91%). The UV-vis spectral analysis of the aqueous
layer showed the presence of traces of nitroform (0.05 mmol,
5%). The treatment of enol silyl ethers 8a and 8b with TNM
was carried out in a similar manner. Th er m a l (Da r k )
Nitr a tion of En ol Silyl Eth er 9. A solution of enol silyl ether
9 (302 mg, 1 mmol) in dichloromethane (5 mL) was mixed with
TNM (211 mg, 1.1 mmol) under an argon atmosphere at room
temperature. The solution immediately developed a charac-
teristic deep red coloration, and it was stirred (in dark) for 8
h at room temperature. The resultant yellow reaction mixture
was diluted with dichloromethane (20 mL) and was washed
with water (4 × 25 mL). The dichloromethane layer was dried
over anhydrous magnesium sulfate and filtered. Removal of
the solvent in vacuo afforded the a oily residue which upon
GC and GC-MS analysis showed the presence of stereoisomeric
P h oton itr a tion of En ol Silyl Eth er s w ith TNM. Gen -
er a l P r oced u r e. In a typical experiment, a solution contain-
ing 0.2 M enol silyl ether and 0.22 M TNM in dichloromethane
(10 mL) under an argon atmosphere was irradiated with a
focussed beam that was passed through an IR water filter and
a 415-nm cutoff filter from a 500-W mercury lamp. The
irradiation was performed in a Pyrex tube immersed in a
Dewar flask filled with a mixture of dry ice and acetonitrile
(-40 °C). As the reaction progressed, the color changed from
red-orange to yellow. After photolysis, the solvent and excess
TNM were removed in vacuo at approximately -40 °C. The
crude residue was redissolved in dichloromethane (20 mL) and
washed with distilled water (3 × 50 mL). The dichloromethane
extract was dried over anhydrous magnesium sulfate, and it
was analyzed by GC, GC-MS, and NMR spectroscopy using
internal standard method. The highly colored aqueous layer
was transferred to a 5-L volumetric flask and diluted with
distilled water. A 10-mL aliquot of the aqueous layer was
further diluted to 50 mL, and an aliquot was transferred to a
1-cm quartz cuvette. UV-vis spectral analysis indicated the
absorbance of A350 ) 1.09 corresponded to a yield of 1.95 mmol

R-Nitration of Ketones via Enol Silyl Ethers
J . Org. Chem., Vol. 61, No. 2, 1996 639
(97%) of nitroform. In a (dark) control experiment, an identical
mixture of 0.2 M enol silyl ether and 0.22 M tetranitromethane
in dichloromethane (10 mL) and kept alongside the irradiated
solution (after it was tightly wrapped in aluminum foil to
protect it from adventitious light.) Simultaneous workup of
the photolysate and the dark control, followed by GC and GC-
MS analysis, indicated that cyclohexanone enol silyl ether in
the dark control was intact. (The aqueous extract was almost
colorless). P h oton itr a tion of En ol Silyl Eth er 9. According
to general procedure, a solution containing 0.2 M enol silyl
ether 9 and 0.22 M TNM in dichloromethane (3 mL), under
an argon atmosphere, was irradiated with a focused beam from
a 500-W mercury lamp at -40 °C. As the reaction proceeded,
the solution changed from red-orange to yellow (0.8 h). After
photolysis, the solvent and excess TNM were removed in vacuo
and the crude residue was redissolved in dichloromethane (50
mL) and washed with distilled water (3 × 50 mL). The
dichloromethane layer was dried over anhydrous magnesium
sulfate and evaporated. The GC and GC-MS analysis of the
crude residue showed the presence of stereoisomeric mixture
of nitro ketones 9 (vide supra) together with the traces of
corresponding ketone and enol silyl ether. The 13C NMR
spectral analysis of the crude reaction mixture as described
above did not show the presence of cyclized product 11.
Chromatographic purification as above afforded the pure nitro
ketone 9 (119 mg, 72%). Note that an identical mixture of
0.2 M enol silyl ether 9 and 0.22 M tetranitromethane in
dichloromethane (3 mL) in dark (-40 °C) did not show any
perceptible color change and GC-MS analysis indicated that
enol silyl ether 9 remained unchanged.
decanted, and it was washed with water and dried over
anhydrous magnesium sulfate. Removal of the solvent af-
forded a crude residue (420 mg), which upon spectral (GC,
NMR) analysis consisted of a 98:2 molar mixture of the
thermodynamic enol silyl ether 3d and the kinetic enol silyl
ether 3c. A similar isomerization of 3c to 3d was observed
when tetracyanobenzene (TCNB) was employed as the sensi-
tizer.
Tim e-Resolved Sp ectr oscop y of th e EDA Com p lexes
of En ol Silyl Eth er s w ith Tetr a n itr om eth a n e. Typically,
in a 1-cm square quartz cuvette equipped with a Schlenk
adapter was placed a solution of tetranitromethane with the
aid of a hypodermic syringe, under an argon atmosphere. The
cuvette was cooled in an dry ice/acetone bath (-78 °C), and
the enol silyl ether was added. The UV-vis absorption
spectrum of the enol silyl ether complex with TNM was
recorded before and after laser flash photolysis to ensure that
only minimal diminution of the CT absorbance occurred during
the transient spectroscopic experiment. Time-resolved differ-
ence absorption spectra on the picosecond time scale were
obtained by utilizing a 532 nm (second harmonic) laser pulse
from a Quantel YG 501-C mode-locked Nd3+: YAG laser as
the excitation source.32 This wavelength corresponded to the
excitation of only the charge-transfer band of the EDA complex
(see Figure 3b). The analyzing beam in the form of a white
light continuum was produced by passing the 1064-nm fun-
damental through a solution consisting of a 1:1 mixture (v/v)
of D2O and H2O) in a 10-cm cuvette. Temporal measurements
were made by varying the optical delay between the excitation
beam and the analyzing beam. The spectrograph was cali-
brated using a low-pressure mercury lamp. Spectra were
measured at each time setting by averaging data from 300
individual pulses. Time-resolved spectra on the nanosecond
time scale were obtained using the second harmonic (532 nm)
of a Quantel YG 580-10 Q-switched Nd3+; YAG laser with a
10-ns pulse as the excitation source. The output of a 150 W
Xenon arc lamp focused on the sample at a 90° angle to the
excitation beam served as the probe light. The probe beam
emerging from the sample was focused on the entrance slit of
an Oriel Model 77250 monochromator equipped with a Model
77298 (500 nm blaze) grating in the spectral range between
300 and 800 nm. The monochromatic light was detected by a
Hamamatsu model R-928 photomultiplier tube. The timing
sequence of the excitation and probing of the sample was
controlled by a Kinetics Instruments sequence generator and
laser controller. Data acquisition and digitization were per-
formed with a Tektronix Model 7104 oscilloscope in conjunction
with a Tektronix Model C101 video camera and DCS-01
softword. The ns-spectral decays were processed with the
ASYST (2.0) scientific software package. The transient ab-
sorption spectrum of the cation radical of enol silyl ether 7 in
Figure 6b was generated from a dichloromethane solution of
chloranil (0.008 M) and 7 (0.03 M) under an argon atmosphere
at 25 °C, using the 355-nm (10-ns) laser pulse.
UV-vis Sp ect r a l Mea su r em en t s of E DA Com p lex
F or m a tion . Typically, a 0.22 M solution of tetranitromethane
was placed in a 1-cm quartz cuvette (UV cell equipped with a
side arm and Schlenk adapter) with the aid of a hypodermic
syringe, under an argon atmosphere. The cuvette was cooled
in an ice/acetone bath (-15 °C), and an aliquot of enol silyl
ether (0.2 mmol) was carefully placed into the side arm with
the aid of a Teflon cannula. After the cuvette was thermally
equilibrated in the cooling bath, the solutions were rapidly
mixed. The UV-vis absorption spectrum was then recorded
immediately (see Figure 3b). All experiments were carried out
in duplicate to ensure spectral reproducibility. The determi-
nation of the formation constant (K) of the EDA complex of
cyclohexanone enol silyl ether 1a and TNM was carried out
as follows. A 2-mL aliquot of a 2.0 M solution of TNM in
dichloromethane was placed in a cuvette, which was cooled in
an ice/acetone bath (-15 °C). Enol silyl ether 1a was added
incrementally (total concentration ) 2.0 - 20.0 × 10-4 mol)
under a positive pressure of argon, and the UV-vis absorption
spectra were recorded (see Figure 3a). The absorbance change
was measured at λ ) 450 nm, the wavelength at which both
TNM and 1a were completely transparent. From a plot of
-1
[TNM]/ACT versus [1a ]-1, the slope was estimated as ꢀCTKEDA
and the intercept as ꢀCT-1. The average formation constant,
KEDA, was 0.24 M-1 and ꢀCT ) 19 M-1 cm-1 at 450 nm. The
linear fit obtained by the least squares method generally had
a correlation coefficient of >0.99.
Ack n ow led gm en t. We thank S. M. Hu¨big for kindly
performing the time-resolved spectroscopic experiments,
Z. Lin for carrying out some preliminary experiments,
and the National Science Foundation, R. A. Welch
Foundation, and the Texas Advanced Research Project
for financial support.
P h otoisom er iza tion of th e Kin etic En ol Silyl Eth er 3c
w ith Ch lor a n il. A Pyrex tube containing a dichloromethane
solution of enol silyl ether 3c (0.1 M, 25 mL) and a catalytic
amount of chloranil (24 mg, 0.1 mmol) under an argon
atmosphere was irradiated for 2 h with a 500-W mercury
lamp.37 The solvent was evaporated in vacuo and the residue
then treated with pentane. The colorless pentane layer was
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