Electron-Transfer-Photosensitized Conjugate Alkylation
J . Org. Chem., Vol. 63, No. 12, 1998 4033
Meth yl 3-m eth ylh ep ta n oa te (3b): 1H NMR (CDCl
These undergo conjugate addition to activated esters and
nitriles, and the selectivity of such reaction parallels that
found when radicals are produced by the classical atom-
transfer method, particularly when a triplet sensitizer
3
) δ 0.9
(
(
1
m, 6H), 1.1-1.6 (m, 7H), 2.15 (dd, 1H, J ) 5, 14 Hz), 2.35
dd, 1H, J ) 7, 14 Hz), 3.7 (s, 3H); 13C NMR (CDCl
) δ 13.91,
9.60, 22.64, 28.98, 30.18, 36.26, 41.53, 51.18, 173.68.
Meth yl 3,4,4-Tr im eth ylpen tan oate (5b): 1H NMR (CDCl
3
3
)
(an aromatic ester) is used. Different from the thermal
δ 0.85 (m, 12 H), 1.8 (m, 1H), 2.0 (dd, 1H, J ) 10.5, 14 Hz),
redox method, which is limited to good donorssusually
enolssthe PET reaction can be applied to weak donors
such as tetralkylstannanes or dioxolanes and further, the
adduct radical is reduced and not oxidized as in the
thermal method. In common with the thermal oxidative
method, e.g., Mn(III), this is not a chain reaction. It has
the advantage, however, that the sensitizer is used in a
catalytic amount (the stoichiometric reagent is light),
provided that it has a moderate to good turnover number.
Furthermore, the reaction is carried out in neat organic
solvent, avoiding the problems associated with the use
of an inorganic oxidant. As for the alkylation of acety-
lenedicarboxylate, this reaction involves assisted frag-
mentation concerted with radical addition. Despite some
limitations, these alkylations introduce a new facet to
PET-sensitized reactions, a field of growing synthetic
interest. From the photochemical point of view, this work
supplies a quantitative evaluation of the generation of
free radical ions, demonstrating that this is the key factor
that makes advantageous the use of a triplet sensitizer
or of a secondary donor with a singlet sensitizer.
3
3
.7 (s, 3H); 13C NMR (CDCl
7.20, 39.84, 51.25, 174.83.
3
) δ 14.86, 26.98 (3 Me), 29.30,
1
Dim eth yl 2-ter t-bu tylm a lea te ((Z-7): H NMR (CDCl
3
)
13
δ 1.2 (s, 9H), 3.7 (s, 3H), 3.85 (s, 3H), 5.85 (s, 1H); C NMR
CDCl ) δ 28.58, 35.38, 51.53, 51.80, 115.59, 160.60, 165.49,
68.82; NOE (6.9%) effect between vinylic and tert-butyl
(
1
3
hydrogens.
Dim eth yl 2-ter t-bu tylfu m a r a te ((E-7): 1H NMR (CDCl3)
δ 1.3 (s, 9H), 3.75 (s, 6H), 6.2 (s, 1H); 13C NMR (CDCl
) δ 28.27,
3
35.20, 51.67, 51.73, 122.64, 151.63, 167.09, 169.06; no signifi-
cant NOE effect between vinylic and tert-butyl hydrogens.
Dim eth yl 2-isop r op ylm a lea te (10): 1H NMR (CDCl
) δ
.15 (d, 6H), 2.65 (m, 1H), 3.7 (s, 3H), 3.85 (s, 3H), 5.8 (s, 1H);
3
1
1
3
C NMR (CDCl
3
) δ 20.49, 32.75, 51.70, 52.10, 116.77, 156.85,
1
65.55, 169.20; NOE effect (37%) between the vinylic and
methine hydrogens.
Qu a n tu m Yield Mea su r em en ts. Quantum yields were
measured in the above multilamp apparatus, using benzophe-
none-benzhydrol or ferrioxalate actinometry, and the product
yield was determined by GC. Conversions were limited to
25%.
F lu or escen ce Mea su r em en ts. Fluorescence emission
was measured by means of an Aminco Bowman MPF spec-
trofluorimeter by using argon-flushed solutions in spectropho-
tometric 1 cm cuvettes. Linear Stern-Volmer plots were
obtained in all cases.
Exp er im en ta l Section
The aromatic compounds used as photosensitizers and the
donors (stannanes, dioxolanes, and acids) were either com-
mercial samples or were sensitized according to published
procedures. HPLC-grade acetonitrile for photochemical reac-
F la sh P h otolysis. A kinetic apparatus supplied by Applied
Photophysics was used. In the experiments with TMPM, the
fourth harmonic of a Lumonics HY200 Nd:YAG laser was used.
The monitor system, arranged in a cross-beam configuration,
consisted in a 275W Xe lamp, a monochromator, and a five-
stage photomultiplier. The signals were captured by a Hewlett-
Packard 54510A digitizing oscilloscope, and the data were
processed on a computer system using a software developed
by Dr. C. Long (Dublin City University). The third harmonic
of the laser was used in the experiments with DCA.
tions was refluxed over CaH
reaction vessel.
2
and distilled directly in the
Meth yl Hep ta n oa te (3a ). A solution of methyl acrylate
484 µL, 0.1 M), tetrabutylstannane (882 µL, 0.05 M), and
(
TMPM (168 mg, 0.01 M) in 54 mL of MeCN was subdivided
in three serum-capped quartz tubes, deaerated by flushing
with argon, and then irradiated in a multilamp apparatus
fitted with six 15 W phosphor-coated lamps (center of emission,
3
20 nm). After 15 h, the solvent was evaporated and the raw
photolyzate chromatographed on silica gel, eluting with a
cyclohexanes-ethyl acetate 9:1 mixture to yield the title
compound (105 mg, 27%). The same product was obtained in
Ack n ow led gm en t. Assistance by Dr. M. Freccero
with flash photolysis measurements is appreciated.
Partial support of this work by Consiglio Nazionale delle
Ricerche, Rome, is gratefully acknowledged.
2
6% yield by 10 h of irradiation in the presence of DCN (48
mg, 0.005 M) and BP (921 mg, 0.1 M).
Analogously obtained were products 3b, 3c,d ,3a 5a ,17 5b,18
1
6
J O980093R
3
19
20
5
c,d , 7, and 10 (see Table 1). The products are known
but in some cases have not been fully characterized previously.
Additional data are reported below.
(
(
(
17) Yeung, D. W. K.; Warkentin, J . Can. J . Chem. 1976, 54, 1345.
18) Consiglio, G. Organometallics 1988, 7, 778.
19) Heck, R. F. J . Am. Chem. Soc. 1972, 94, 2712.
(16) Munch-Petersen, J . J . Org. Chem. 1957, 22, 157.
(20) Wada, F.; Matsuda, T. Bull. Chem. Soc. J pn. 1973, 46, 510.