Anodic Oxidation of Methyl α-Dimethylsilyldihydrocinnamate. A Novel Silicon γ-Aryl Effect

Article abstract of DOI:10.1021/jo030264e

The anodic oxidation of methyl 3-phenyl-2-dimethylsilylpropionate occurs at a potential almost 1 V positive of that required to oxidize other α-silyl esters. Semiempirical and ab initio calculations on the model compound 1-phenyl-2-trimethylsilylethane indicate that electron removal from these two compounds is highly stereoelectronically dependent. Both molecules exist almost exclusively in a conformation in which the phenyl group and silicon atom are anti and the side chain is perpendicular to the aromatic ring. This conformation has a higher energy HOMO orbital and lower computed ionization potential than the only other significantly populated conformation of 1-phenyl-2-trimethylsilylethane. Finally, the ab initio calculations show that in the cation radical of this model compound the ipso carbon of the aromatic ring and the side chain carbon bound to silicon draw significantly closer together than in the neutral species; an electrostatic potential map of the cation radical shows that the ipso carbon bears the highest degree of positive charge of any of the benzenoid carbons. We interpret these data, taken together, as an indication that this cation radical is stabilized by overlap of the rear lobe of the carbon-silicon bond with the p-orbital of the ipso carbon.

Full text of DOI:10.1021/jo030264e

An od ic Oxid a tion of Meth yl r-Dim eth ylsilyld ih yd r ocin n a m a te.
A Novel Silicon γ-Ar yl Effect
Christos Kaimakliotis and Albert J . Fry*
Chemistry Department, Wesleyan University, Middletown, Connecticut 06459
afry@wesleyan.edu
Received August 15, 2003
The anodic oxidation of methyl 3-phenyl-2-dimethylsilylpropionate occurs at a potential almost 1
V positive of that required to oxidize other R-silyl esters. Semiempirical and ab initio calculations
on the model compound 1-phenyl-2-trimethylsilylethane indicate that electron removal from these
two compounds is highly stereoelectronically dependent. Both molecules exist almost exclusively
in a conformation in which the phenyl group and silicon atom are anti and the side chain is
perpendicular to the aromatic ring. This conformation has a higher energy HOMO orbital and
lower computed ionization potential than the only other significantly populated conformation of
1-phenyl-2-trimethylsilylethane. Finally, the ab initio calculations show that in the cation radical
of this model compound the ipso carbon of the aromatic ring and the side chain carbon bound to
silicon draw significantly closer together than in the neutral species; an electrostatic potential map
of the cation radical shows that the ipso carbon bears the highest degree of positive charge of any
of the benzenoid carbons. We interpret these data, taken together, as an indication that this cation
radical is stabilized by overlap of the rear lobe of the carbon-silicon bond with the p-orbital of the
ipso carbon.
In tr od u ction
it is believed that they enjoy a degree of π-stabilization
by the carbonyl group,9 the electronegative inductive
effect of the carbonyl group still tends to destabilize such
cations. Controlled anodic production of 2c might permit
carrying out a variety of trapping reactions not possible
under solvolytic conditions.
Yoshida1-3 and others4-6 have shown that anodic oxi-
dation of benzylic silanes (1a ) and those bearing car-
bocation-stabilizing atoms at the R-position (1b) affords
the corresponding cationic species (2) (eq 1). We have
To implement an anodic strategy to the preparation
of species 2c, one requires an R-silyl carbonyl compound
(1c). We chose to examine silyl esters. Silylation of ester
enolates generally affords the corresponding enol silyl
esters by kinetically controlled O-silylation.10,11 Although
a few examples of formation of C-silylated products have
been reported,12,13a the only generally useful method for
C-silylation until recently involved methyldiphenylsilyl
chloride as the silylating agent.13b We felt that the
presence of phenyl groups attached to silicon in these
species would create ambiguities in the nature of the
reaction intermediates. Following up an earlier report by
Miura,14 we recently showed15 that ester enolates are
C-silylated in excellent yields with dimethylchlorosilane.
become interested in the possibility of producing less
stable carbocations by anodic oxidation of the corre-
sponding silanes. In particular, we were interested in the
possibility of generating a carbocation R to a carbonyl
group (2c) in this manner. Although species of type 2c
have previously been prepared by solvolytic routes7,8 and
* Address correspondence to this author. Phone: 860-685-2622.
Fax: 860-685-2211.
(1) Yoshida, J .-i.; Murata, T.; Isoe S. Tetrahedron Lett. 1986, 27,
3373.
(2) Yoshida, J .-i.; Itoh, M.; Isoe, S. J . Chem. Soc., Chem. Commun.
1993, 547.
(3) Yoshida J .-i.; Ishichi, Y.; Nishiwaki, K.; Shiozawa, S.; Isoe, S.
Tetrahedron Lett. 1992, 33, 2599.
(8) Be´gue´, J .-P.; Charpentier-Morize, M. Acc. Chem. Res. 1980, 13,
207.
(9) Dixon, D. A.; Eades, R. A.; Frey, R.; Gassman, P. G.; Hendewerk,
M. L.; Paddon-Row: M. N.; Houk, K. N. J . Am. Chem. Soc. 1984, 106,
3885.
(10) Chan, T.-H. In Comprehensive Organic Synthesis; Heathcock,
C. H., Ed.; Elsevier: Oxford, UK, 1991; Vol. 2, p 604.
(11) Sheldrake, G. N. In Comprehensive Organic Functional Group
Transformations; Kirby, G. W., Ed.; Elsevier: Oxford, UK, 1995; Vol.
4, p 831.
(4) Koizumi, T.; Fuchigami, T.; Nonaka, T. Bull. Chem. Soc. J pn.
1989, 62, 219.
(5) Koizumi, T.; Fuchigami, T.; Nonaka, T. Electrochim. Acta 1988,
33, 1635.
(6) Porter, J . M.; Xuan, X.; Blackman, B.; Hsu, D.; Fry, A. J .
Tetrahedron Lett. 1997, 38, 7147.
(12) Ainsworth, C.; Chen, F.; Kuo, Y.-N. J . Organomet. Chem. 1972,
46, 59.
(13) (a) Rathke, M. W.; Sullivan, D. L. Synth. Commun. 1973, 3,
67. (b) Larson, G. L.; Maldonado, V. C. d.; Fuentes, L. M.; Torres, L.
E. J . Org. Chem. 1988, 53, 633.
(7) Creary, X. Acc. Chem. Res. 1985, 18, 3-8.
10.1021/jo030264e CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/21/2003
J . Org. Chem. 2003, 68, 9893-9898
9893

Kaimakliotis and Fry
of 4. As we will see below, 4 is in fact an excellent model
for electron removal from 3d .
P r ep a r a tive Sca le Electr olysis. Controlled potential
electrolysis of 3d in TFA/0.1 M TBATFB at a carbon cloth
anode in a divided cell at room temperature afforded a
1:2 mixture of methyl 3-trifluoroacetoxydihydrocinna-
mate (5) and methyl dihydrocinnamate (6). The ratio of
5 to 6 was 1:1 and 2:1 when electrolysis was carried out
at 0 and -20 °C, respectively. The same results were
obtained employing constant current electrolysis. While
5 is a true anodic product, a control experiment estab-
lished that 6 is produced by direct reaction between 3d
and TFA; this reaction apparently proceeds more slowly
at lower temperature, improving the efficiency of the
anodic conversion of 3d to 5.
The structure of 5 was established by synthesis of an
authentic sample. A commercial sample of methyl 3-hy-
droxydihydrocinnamate was dissolved in trifluoroacetic
anhydride at room temperature for 10 min to afford 5
cleanly. An alternate structure originally considered for
the electrolysis product was methyl 2-trifluoroacetoxy-
dihydrocinnamate (8). However, an authentic sample of
8 prepared in 85% yield by silver-assisted SN2 reaction
between methyl 2-bromodihydrocinnamate (7) and silver
trifluoroacetate (THF, room temperature, 30 min) exhib-
ited different spectroscopic properties from the electroly-
sis product.
F IGURE 1. (a) Dashed line: voltammogram of methyl
3-phenyl-2-dimethylsilylpropionate (3d ) in trifluoroacetic acid
containing 0.1 M tetrabutylammonium tetrafluoroborate at
100 mV s-1 vs Ag/0.1 M AgNO3. (b) Solid line: background.
The four 2-dimethylsilyl esters 3a -d were prepared and
characterized by a series of 1H and 13C NMR spectroscopic
experiments and molecular mechanics calculations. The
unusual anodic behavior of 3d will be the focus of this
paper.
Resu lts
Cyclic Volta m m etr y. Silyl esters 3a -d were exam-
ined by cyclic voltammetry at 100 mV s-1 at a glassy
carbon electrode in trifluoroacetic acid (TFA) containing
0.1 M tetrabutylammonium tetrafluoroborate (TBATFB),
with potentials measured relative to Ag/0.1 M AgNO3
reference.16 TFA was used because its anodic limit is
more positive than that of more commonly used solvents
such as acetonitrile; no voltammetric waves were observ-
able in these media. Even in TFA, esters 3a -c exhibited
no voltammetric peak before the solvent breakdown
potential of roughly +3.3 V. However, ester 3d exhibited
a broad anodic peak at -2.38 V (Figure 1). No reverse
wave is observable on the reverse scan, indicating that
the initial electron transfer generates a species that
undergoes rapid chemical reaction. For comparison pur-
poses, the oxidation peak potential of the model com-
pound 1-phenyl-2-trimethylsilylethane17 (4), prepared by
trimethylsilylation of the corresponding Grignard re-
agent, was found to be +1.88 V. The difference between
the oxidation potentials of 3d and 4 is about what one
would expect for the inductive effect of the additional
carbomethoxy group in 3d upon the oxidation potential
Con for m a tion a l Ch a r a cter iza tion of Mod el Com -
p ou n d 4. The unique ease of oxidation of ester 3d
appears to be associated with the â-phenyl group. Be-
cause the electron-transfer process was presumed to be
sensitive to stereoelectronic factors,18 the conformational
energy surface of the conformationally less complex
model substance 4 was explored by a series of AM1
semiempirical calculations.19 As will be seen, this sub-
stance is a good conformational and electronic model for
ester 3d . Silane 4 possesses two rotatable bonds whose
orientation should significantly affect the energy of the
system: Φ, the C4-C3-C2-C1 dihedral angle, and Ψ, the
C3-C2-C1-Si dihedral angle. Angles Φ and Ψ were rotated
in 30° increments from 0° to 360° and the AM1 energy
of each combination was computed with full geometry
optimization, requiring (because of the symmetry of the
system) 91 separate AM1 computations. The resulting
surface was found to contain only three low-lying
(14) Miura, K.; Sato, H.; Tamaki, K.; Ito, H.; Hosomi, A. Tetrahedron
Lett. 1998, 39, 2585.
(15) Kaimakliotis, C.; Fry, A. J . J . Organomet. Chem. 2003, 671,
126-130.
(16) This reference electrode is +0.35 V vs sce.
(17) Negishi, E.-i.; Swanson, D. R.; Rousset, C. J . J . Org. Chem.
1990, 55, 5406.
(18) Yoshida, J . In Electrochemistry V; Steckhan, E., Ed.; Springer-
Verlag: New York, 1994; Vol. 170, p 39.
9894 J . Org. Chem., Vol. 68, No. 26, 2003

Oxidation of Methyl R-Dimethylsilyldihydrocinnamate
TABLE 1. Com p u ted En er gies of Neu tr a l
1-P h en yl-2-tr im eth ylsilyleth a n e a n d Its Ca tion Ra d ica l
a s a F u n ction of Con for m a tion
conformationa
neutralb
cation radicalb
IP (eV)c,d
A
B/C
EA - EB/C
-719.551144
-719.549208
0.001936
-719.262265
-719.253461
0.008804
7.861
8.048
0.187f
e
a
Ab initio density functional calculations at the B3LYP/6-31G*
b
level with full geometry optimization. Energy of species in
hartrees (1 H ) 627.5 kcal mol -1). c Ionization potential ) Eneutral
d
-1
- Ecation
.
1 eV ) 0.0367 H ) 23.06 kcal mol
.
e Difference in
energy between conformation A and conformation B/C (in har-
trees). f Difference in ionization potential between conformation
A and conformation B/C (in eV).
F IGUR E 2. Electrostatic potential map of 1-phenyl-2-tri-
methylsilylethane (4) computed at the DFT/6-31G* level. Color
scheme: red, regions of high negative charge; yellow, low
negative charge; blue, regions of high positive charge; green,
low positive charge.
minima: A, Φ ) 90°, Ψ ) 180°; B, Φ ) 120°, Ψ ) 90°;
and C, Φ ) 300°, Ψ ) 90°. No other minima were found.
Conformation A, in which the phenyl and trimethylsilyl
groups are anti, is lower energy than B and C, which
are enantiomers and of course of equal energy. Explora-
tion of the potential energy surface in the vicinity of the
minima with a finer mesh grid showed that while the
global minimum A is indeed at Φ ) 90°, Ψ ) 180°, the
other two minima are actually centered at Φ ) 62°, Ψ )
105° and 285°, respectively. While relative AM1 energies
are meaningful, in general their absolute values are far
less so; the energies of these conformations were therefore
recomputed by density functional ab initio theory at the
6-31G* level with full geometry optimization, using the
B3LYP functional (Table 1).19 Conformation A is still the
global minimum and has the same geometry (Φ ) 90°,
Ψ ) 180°) at this computational level. However, confor-
mation B/C optimizes to a new geometry (Φ ) 69°, Ψ )
67°), starting from the AM1 minimum (Φ ) 62°, Ψ )
105°) when all geometry constraints are removed. From
the data of Table 1, conformation A can be computed to
be 1.21 kcal lower in energy than B or C. With use of
this value, a Maxwell-Boltzmann calculation then in-
dicates that 4 exists about 88% in the anti conformation
A. An extensive conformational energy search had shown
earlier that in its global minimum energy conformation
silyl ester 3d has exactly the same anti backbone
geometry (Φ ) 90°, Ψ ) 180°) as A.15 Therefore, conclu-
sions reached from examination of the stereoelectronic
dependence of the electron-transfer properties of 4 should
be directly transferable to 3d .
F IGURE 3. Electrostatic potential map of the cation radical
of 1-phenyl-2-trimethylsilylethane (4) computed at the DFT/
6-31G* level. Color scheme: red, regions of high negative
charge; yellow, low negative charge; blue, regions of high
positive charge; green, low positive charge.
the cation radical (113.0° vs 100.7°). From the data of
Table 1, one may further conclude that the A conforma-
tion of the cation radical is considerably more stable
(0.008804 H, or 5.5 kcal) relative to that of B/C than the
neutral species (1.2 kcal), suggesting the existence of a
special electronic effect in the cation. Another useful
conclusion may be reached by plotting the HOMO energy
of 4 as a function of Ψ while maintaining Φ fixed at 90°
(Figure 4). It may be noted that while the total energy of
the system decreases, the HOMO energy steadily in-
creases as Ψ increases from 0° (benzene ring and silicon
eclipsed) to 180°.
The B3LYP/3-21G computations were then repeated
for the corresponding radical cations of A and B/C (Table
1). Since the difference between the energy of the neutral
and radical cation is the computed ionization potential
(IP), the IP of the A conformation of 4 is seen to be 0.187
eV less than that of B/C. It is also instructive to examine
the differences between the electron distribution in
neutral 4 and in its corresponding radical cation (Figures
2 and 3, which represent electrostatic potential energy
maps for the two species). Whereas the carbon atoms of
the benzene ring are uniformly negative in neutral 4, the
ring becomes highly positive in the corresponding cation
radical; in particular, the ipso carbon bears a distinctly
higher positive charge than the other ring carbons. A final
point of significance is the fact that at the B3LYP/6-31G*
level, the distance between C1 and C3 of the anti
conformation is shorter by 0.20 Å (2.346 Å vs 2.550 Å) in
the radical cation, associated with a decrease of 12.3° in
the C1-C2-C3 angle in going from the neutral species to
Discu ssion
Volta m m etr y. While the oxidation potential of the
silyl ester 3d is approximately +2.4 V (Figure 1), those
of esters 3a -c are greater than +3.3 V, the decomposi-
tion potential of our solvent system. This enormous
difference of at least 0.9 V points to a powerful effect of
the aryl group on the initial electron-transfer process to
stabilize the initial radical cation by upward of 20 kcal
mol-1
.
Th e An od ic γ-Effect. The very large effect of the
phenyl group on the oxidation potential of 3d relative to
the other silyl esters 3a -c demands explanation. We
applied a variety of computational methods to under-
stand the nature of this effect. Primary among these were
quantum mechanical calculations on model compound 4.
Silane 4 is a reasonable model for 3d for the following
reasons: (a) the actual electrophore (oxidizable unit) of
3d appears to be its â-phenylethylsilane substructure and
J . Org. Chem, Vol. 68, No. 26, 2003 9895

Kaimakliotis and Fry
that the increase in HOMO energy arises by interaction
between the filled carbon-silicon σ-bond and the nearby
π-system. We argue here for a similar interaction, but
involving the rear lobe of the C-Si σ bond. There is
precedent, both experimental and theoretical, for this
hypothesis. Shiner and co-workers have reported a silicon
γ-effect upon solvolysis of alkyl sulfonates, in which
sulfonates bearing a trialkylsilyl group at the γ-position
ionize substantially faster that those lacking this sub-
stituent.21-23 This neighboring group effect exhibits a
pronounced stereoelectronic preference, which is opposite
to that expected in the absence of participation by
silicon: the equatorial cis-brosylate 10 ionizes 89 times
faster than the axial trans-isomer 11 in 2,2,2-trifluoro-
ethanol,21 whereas equatorial sulfonates ionize 3-4 times
slower than equatorial sulfonates.24 High-level ab initio
computations showed that stabilization of the incipient
positive charge in such ionizations is maximal when the
developing p-orbital is aligned collinearly with the rear
lobe of the carbon-silicon bond.23
F IGURE 4. (a) Black triangles: HOMO energy of 1-phenyl-
2-trimethylsilylethane (4) computed at the AM1 level as a
function of Ψ, the C3-C2-C1-Si dihedral angle. (b) Open
diamonds: total energy of 4 computed at the AM1 level as a
function of Ψ, the C3-C2-C1-Si dihedral angle. The dihedral
angle Φ is fixed at 90° throughout.
(b) the phenyl and silyl groups were shown to be anti (Ψ
) 180°) and the side chain is perpendicular to the
benzene ring (Φ ) 90°) in both compounds. AM1 calcula-
tions were used to explore the conformational surface of
4. This was not feasible with 3d because of its larger
number of rotatable bonds, but its conformational prop-
erties had been explored in detail previously by molecular
mechanics.15 Taken together, the molecular mechanics,
AM1, and ab initio computations point to two clear
conclusions: 4 and 3d exist, at a conservative estimate,
close to 90% in the anti conformation and this conforma-
tion has an ionization potential that is lower by about
4.3 kcal (0.187 eV) than that of the other low-energy
conformation. We then ask what property of the anti
conformation accounts for the relative ease with which
it gives up an electron. A significant part of the answer
to this question can be seen in Figure 4: the HOMO
orbital of 4 is highest in energy when the silicon atom is
anti to the benzene ring, i.e., in the molecule’s most
highly populated conformation. This conclusion in itself
simply begs the question. Rather, it is more appropriate
to ask what it is specifically about the anti conformation
(Ψ ) 180°) that drives up the energy of the HOMO and
thus allows it to give up an electron more readily than
for any other value of Ψ. We believe that interaction
between the highest filled orbital of the benzene ring (via
C3, the ipso carbon) and the rear lobe of the carbon-
silicon bond is the cause of the increase in the HOMO
energy. This is illustrated in structure 9. We refer to this
phenomenon as the “anodic γ-aryl effect” although similar
situations should arise with other appropriately oriented
π and silicon groups, even when the groups are separated
by a longer chain or where a π-system other than benzene
is located at the γ-position.
Mech a n ism of F or m a tion of Tr iflu or oa cetoxy
Ester 5. Formation of rearranged ester 5 is excellent
evidence for initial generation of a cation (14) R to the
ester carbonyl group. By analogy to mechanisms previ-
ously postulated for the anodic oxidation of silanes of type
1a and 1b,1,2,6,18,25 we suggest the following sequence of
events for the anodic conversion of 3d to 5. Initial
removal of an electron from 3d by the electrode takes
place from conformation A to afford a radical cation (3d +•
)
in which most of the charge is carried by the benzene
ring, particularly by the ipso carbon (C3); electron density
is supplied by the carbon-silicon bond via overlap of its
rear lobe with the π-system. Nucleophilic attack upon the
silicon atom of the radical cation by solvent (probably by
an addition-elimination mechanism) would afford radi-
cal 13; anodic oxidation of 13 at the very positive
electrode potential being employed would then afford 14.
Rearrangement of 14 to 15 by hydride migration and
(19) Spartan ‘02 for Macintosh; Wave Function, Inc.: Irvine, CA.
(20) Bernardi, F.; Bottoni, A.; Garavelli, M. Quant. Struct.-Act.
Relat. 2002, 21, 128.
(21) Shiner, V. J ., J r.; Ensinger, M. W.; Kriz, G. S.; Halley, K. A. J .
Org. Chem. 1990, 55, 653-661.
Effects analogous to this have been reported previ-
ously. Yoshida has observed that in silanes in which one
carbon atom separates the π-system and the silicon atom,
e.g., benzyl and allyl silanes, the HOMO energy is highest
and ionization potential lowest when the carbon-silicon
bond is perpendicular to the plane of the π-system, i.e.,
parallel to the p-orbitals of the π-system.18 It is believed
(22) (a) Ensinger, M. W.; Shiner, V. J ., J r. Stud. Org. Chem.
(Amsterdam) 1987, 31, 41-58. (b) Adcock, W.; Coope, J .; Shiner, V.
J ., J r.; Trout, N. A. J . Org. Chem. 1990, 55, 1411-1412.
(23) Davidson, E. R.; V. J . Shiner, J . J . Am. Chem. Soc. 1986, 108,
3135-3137.
(24) Winstein, S.; Holness, N. J . J . Am. Chem. Soc. 1955, 77, 5562.
9896 J . Org. Chem., Vol. 68, No. 26, 2003

Oxidation of Methyl R-Dimethylsilyldihydrocinnamate
SCHEME 1
final reaction with solvent would then afford 5. Rear-
rangement moves the positive charge further away from
the carbonyl group and should be particularly thermo-
dynamically favorable in the case of 14, since the result-
ing cation 15 is benzylic. We ruled out an alternative
pathway from 14 to 5 involving proton loss to afford
methyl cinnamate followed by conjugate addition of TFA
to the latter by a control experiment in which a TFA
solution of methyl cinnamate was found to be unchanged
under the reaction conditions. Similar rearrangements
of carbocations R to carbonyl groups are well prece-
dented.26,27 More than thermodynamics may be involved,
however. Rearrangement of homoenolate 16 to 17 by
proton migration in the opposite sense likewise should
clearly be highly exothermic, yet we have shown that 16
can be generated and captured by a variety of electro-
philes without any observable formation of 17.28 As we
have pointed out,28 conversion of 16 into 17 is forbidden
on orbital symmetry grounds, whereas rearrangement of
14 to 15 is orbital symmetry allowed.
which the carbon-silicon bond has the same orientation
with respect to a remote π-system, no matter what the
length of the chain connecting them.18,23
We finally return to the initial aim of this study, to
produce a cation R to an ester carbonyl group (2c). Tech-
nically, by generating cation 14 we have achieved this
aim. However, the requirement for the γ-phenyl group
for ready oxidation indicates that the role of the ester
group is minimal in this anode process. The significance
of the present work lies in its elucidation of the novel
role of the aryl group in stabilizing the initial cation
radical. Recent ab initio computations on the silyl esters
3a -c have shown, though, that the corresponding cation
radicals do derive stabilization by interaction between
the ester group and the silyl group.29 It should be possible
to oxidize 3a -c in less readily oxidized solvents, e.g.,
nitromethane.30,31 We also note that the very phenyl
group that stabilizes the initial cationic intermediate
stabilizes the transition state for rearrangement of 14
by hydride migration. The present work could be elabo-
rated in a variety of ways for synthetic applications. Re-
placement of silicon by germanium or tin should permit
generation of such cations at lower potentials, opening
the possibility of trapping by a variety of added nucleo-
philes, and incorporation of an appropriate nucleophilic
group such as a carbon-carbon double bond into the silyl
ester precursor may allow intramolecular trapping before
rearrangement can occur.
Con clu sion s
The remote phenyl group is intimately involved in the
initial removal of an electron from the phenyl group of
3d , resulting in a dramatic lowering of its oxidation
potential compared to that of the non-aryl esters 3a -c.
Ab initio computational studies have been carried out on
the phenethylsilane 4, which we consider to be an
excellent model for 3d because they share the same
essential geometry. The computations have shown that
the effect has a strong stereoelectronic component, re-
quiring that the phenyl and silyl groups be anti (Ψ )
180°) and the side chain be perpendicular to the benzene
ring (Φ ) 90°). In this orientation the ipso carbon of the
ring is ideally situated to interact with silicon through
the rear lobe of the carbon-silicon sp3 bond. This critical
geometry is exhibited by 3d as well as 4. We presume
that such stabilization can be achieved in any silane in
Exp er im en ta l Section
Meth yl 3-Tr iflu or oacetoxydih ydr ocin n am ate (5). Meth-
yl 3-hydroxydihydrocinnamate (Aldrich) (0.36 g, 2 mmol) was
dissolved in trifluoroacetic anhydride (0.63 g, 3 mmol). Acy-
1
lation was complete after 10 min as determined by H NMR
and GC-MS. Concentration and distillation in vacuo afforded
5 (0.55 g, 100%). Samples obtained by preparative gas chro-
matography were used for spectroscopic experiments and
1
submitted for elemental analysis; H NMR (CDCl3) δ 7.4 (s,
5H), 6.34 (dd, J ) 7.0 and 4.5 Hz, 1H), 3.7 (s, 3H), 3.13 (dd, J
) 10 and 7.0 Hz, 1H), 2.85 (dd, J ) 10 and 4.5 Hz, 1H); COSY
(25) Taylor, K.; Miura, K.; Akinfaderin, F.; Fry, A. J . J . Electrochem.
Soc. 2003, 150, D85.
(26) Creary, X.; Geiger, C. C. J . Am. Chem. Soc. 1982, 104, 4151.
(27) Koga, K.; Wu, C. C.; Yamada, S.-i. Tetrahedron Lett. 1971, 12,
2283.
(29) Kaimakliotis, C. Unpublished computations.
(30) Chiba, K.; Sonoyamo, J .; Tada, M. J . Chem. Soc., Perkin Trans.
1 1996, 1435.
(28) DiMauro, E.; Fry, A. J . Tetrahedron Lett. 1999, 40, 7945.
(31) Mann, C. K. Electroanal. Chem. 1969, 3, 57.
J . Org. Chem, Vol. 68, No. 26, 2003 9897

Kaimakliotis and Fry
spectra substantiated the expected AA′X coupling pattern of
the protons at δ 6.34, 3.13, and 2.85; MS m/z (%) 276 (32), 216
(69), 203 (28), 131 (40), 105 (100). A satisfactory microanalysis
could not be obtained because of the tendency of the material
to both hydrolyze and undergo elimination upon standing.
Meth yl 2-Br om od ih yd r ocin n a m a te (7).32 The following
procedure was considered more convenient than the literature
Filtration and evaporation afforded 8 as a clear liquid (0.70 g,
85%) whose spectrum showed no starting material or other
products: 1H NMR (CDCl3) δ 7.42 (m, 5Η), 5.43 (dd, J ) 7.0
and 4.5 Hz, 1H), 3.68 (s, 3H), 3.11 (dd, J ) 10 and 7.0 Hz,
1H), and 2.83 (dd, J ) 10 and 4.5 Hz, 1H); COSY spectrum
was consistent with the presence of an AA′X system; MS m/e
(%) 276 (2), 217 (9), 163 (10), 162 (100), 131 (15), 91 (19), and
77 (44). A satisfactory microanalysis could not be obtained
because of the tendency of the material to hydrolyze upon
standing. A mixture of high molecular weight products was
produced when the reaction was carried out at 60°; similar
behavior was observed for the reaction between methyl 2-bro-
mopropionate and silver trifluoroacetate.
procedure involving bromination of benzylmalonic ester.32
A
solution of 6 (3.28 g, 20 mmol) in 20 mL of dry THF under
nitrogen at -78 °C was treated with lithium diisopropylamide
monotetrahydrofuran complex (22 mmol). The resulting solu-
tion was added dropwise over a period of 10 min to 600 mL of
a 0.04 M solution of bromine in THF at -78 °C and allowed
to warm to room temperature over 4 h while being stirred.
The mixture was then treated with concentrated hydrochloric
acid and extracted with water containing sufficient sodium
thiosulfate to discharge the bromine color. The solution was
dried over MgSO4 and distilled in vacuo to afford 7 (4.63 g,
95%): 1H NMR (CDCl3) δ 7.25 (m), 4.41 (dd, J ) 9.0 and 7.0
Hz, 1H), 3.71 (s, 3H), 3.46 (dd, J ) 14 and 9.0 Hz, 1H), and
3.23 (dd, J ) 14 and 7.0 Hz, 1H).
Ack n ow led gm en t. Financial support of this work
by the National Science Foundation under grant No.
CHE-01-00727 is gratefully acknowledged. C.K. received
a summer research grant from an award to Wesleyan
University under the Undergraduate Biological Sciences
Education Program of the Howard Hughes Medical
Institute.
Meth yl 2-Tr iflu or oa cetoxyd ih yd r ocin n a m a te (8). Bro-
mo ester 7 (0.73 g, 3 mmol) and silver trifluoroacetate (0.88 g,
4 mmol) were stirred in anhydrous THF at room temperature.
A gray precipitate of AgBr began forming within minutes and
reaction was complete after 30 min (1H NMR and GC-MS).
Su p p or tin g In for m a tion Ava ila ble: General experimen-
1
tal procedures, NMR spectra of compounds 5, 7, and 8, and
table of AM1 energy for 4 as a function of rotation around
angles Φ and Ψ. This material is available free of charge via
the Internet at http://pubs.acs.org.
(32) Reinheckel, H.; Iovchev, A.; Spasov, S. L. Monatsh. Chem. 1965,
96, 1185.
J O030264E
9898 J . Org. Chem., Vol. 68, No. 26, 2003