Generation of Highly Destabilized Vinyl Cations
oxygen was removed by bubbling through Argon during the
irradiations and prior to them for either 20 (λexc ) 254 nm) or
5 min (λexc ) 248 nm).
the reaction mixture was stirred at room temperature for 1 h.
The solvent was evaporated. The residue was dissolved in 100
mL of t-BuOH together with 32 mmol of t-BuOK (3.6 g) and
refluxed for 1 h. H2O (100 mL) was added and the mixture
was extracted 3 times with 75 mL of diethyl ether. The
combined ether layers were washed with H2O and brine and
dried over MgSO4. Evaporation and column chromatography
with silica/petroleum ether 40/60 afforded 4Z-Br in an overall
yield of 43% (12 mmol, 3.1 g) with an E/Z ratio of 85:15. This
stereoisomeric mixture46 was used as such for the irradiations.
19F NMR δ -63.43 (d, JHF ) 1.2 Hz). 1H NMR δ 7.36 (m, 6H).
MS m/z (rel intensity) 252, 250 (M+, 100, 95), 171 (M+ - Br,
30), 151 (M+ - HBrF, 40), 102 (M+ - CF3Br, 50). High-
resolution MS m/z 249.9648 (C9H6F3Br requires 249.9605). UV
λmax ) 266 nm (ꢀ ) 6.6 103 L/mol‚cm).
(E)-1-Chloro-3,3,3-trifluoro-1-phenylpropene (4E-Cl):
To a solution, cooled to -78 °C, of 23 mmol of (Ph)3PCH2ClI47
(10.1 g) in THF was added a solution of t-BuOK in THF (23
mmol, 23 mL of a 1 M solution) dropwise. The solution was
stirred at room temperature for 15 min, after which 23 mmol
of R,R,R-trifluoroacetophenone (4.0 g) was added in one portion.
The reaction mixture was stirred for half an hour. The solvent
was evaporated, petroleum ether 40/60 was added, the mixture
was filtered, and the filtrate was dried over MgSO4 and
concentrated. The product was purified by column chroma-
tography (silica/petroleum ether:diethyl ether 10:1), yielding
9 mmol of 4E-Cl (38%, 1.8 g) in an E/Z ratio of 87:13. This
stereoisomeric mixture48 was used as such for the irradiations.
1H NMR δ 6.59 (s, 1H), δ 7.38 (m, 5H). MS m/z (rel intensity)
208, 206 (M+, 30, 100), 171 (M+ - Cl, 40), 151 (M+ - HFCl,
40), 102 (M+ - CF3Cl, 80). High-resolution MS m/z 206.0117
(C9H6F3Cl requires 206.0110). UV λmax ) 265 nm (ꢀ ) 5.8 103
L/mol‚cm).
Identification of Products. All photoproducts were iden-
tified by GLC co-injection and GC-MS analysis of reference
compounds. In all cases the E/Z-isomeric products of the
starting material were obtained during the synthesis of the
starting materials. Products 5(E), 10, 11, and 19 are com-
mercially available. Product 18 is an intermediate in the
synthesis of 4E-Br. All other reference compounds were
synthesized independently, mainly according to literature
procedures. The methods of syntheses and the spectral listings
for 3E, 4Z-Br, 4Z-Cl, 5Z, 6, 7, 14E, 14Z, 15, 16, and 18 are
reported in the Supporting Information.
3,3,3-Trifluoro-1-methoxy-2-phenylpropene (20E and
20Z): To 4.7 mmol of 3,3,3-trifluoro-2-phenylpropene (18) (0.80
g), obtained via a Wittig reaction as described in the synthesis
of 4E-Br, in 30 mL of CH2Cl2 was slowly added 5.1 mmol of
BH3‚Et2O (5.1 mL of a 1 M solution in diethyl ether) under N2
via a syringe. The reaction mixture was stirred at 0 °C for 2
h, after which the diethyl ether was evaporated. The reaction
mixture was slowly added to a solution of 100 mmol of PCC
(21.8 g) in 50 mL of CH2Cl2. After the initial vigorous reaction
had subsided, the reaction mixture was refluxed for 2 h.
Diethyl ether (50 mL) was added, the mixture was filtered,
and the filtrate was concentrated. The thus obtained 3,3,3-
trifluoro-2-phenylpropionaldehyde was used without purifica-
tion in the next step. HC(OCH3)3 (4.2 mmol, 0.45 g), 3.5 mmol
of 3,3,3-trifluoro-2-phenylpropionaldehyde (0.65 g), and a
catalytic amount of p-toluenesulfonic acid were dissolved in
methanol. The reaction was distilled until no more HCOOCH3
was collected. The resulting mixture was dried over MgSO4
and concentrated. The products were purified and separated
by column chromatography with silica/petroleum ether:diethyl
The formation of the photoproducts was studied as a
function of time. This was done by removing aliquots (0.5 mL
for irradiations at λ ) 254 nm and 50 µL for irradiations at λ
) 248 nm) at regular time intervals. The samples were
analyzed at least in duplicate on a gas chromatograph
equipped with a flame ionization detector and monitored by
using a internal standard (decane or dodecane). Due to the
low conversion during the irradiations the disappearance of
starting material could not be reliably measured. All irradia-
tions were performed in triplicate. The quantum yields were
calculated from the kinetic data obtained in the irradiations
by using least-squares treatment and the intensity of light at
the wavelength used. The intensity of the light employed was
determined by using a chemical actinometer: Actino Chromo
1R (248/334) from Photon Technology International.
Generation of Vinyl Radicals. Vinyl radicals 1• were
thermally generated from 1 mM 1Z:1E (60:40) with 2 mM
n-Bu3SnH in refluxing toluene under an argon atmosphere and
a trace of 2,2′-azobis(2-methylpropionitrile) (AIBN) as initia-
tor.23 Oxygen was removed from the solvent prior to addition
of the reagents by refluxing the toluene overnight under argon.
According to GC and GC-MS at about 95% conversion the only
products were the methylstyrenes 5E and 5Z. No products
resulting from loss of HBr (i.e. 6 or 7) were formed.
Quantum Chemical Calculations. The computations
were performed with the Gaussian 03 program, version B3.44
Calculations of the compounds under study were performed
with the B3LYP functional method45 as implemented in G03
and the Møller-Plesset fourth-order perturbation theory. The
total energies were corrected with zero-point energies, obtained
at the level of optimization.
Synthesis of Starting Materials. The syntheses and
spectroscopic characterizations of 1E, 1Z, 2E, 2Z, 3Z, and 3dZ
are reported in the Supporting Information.
(E)-1-Bromo-3,3,3-trifluoro-2-phenylpropene (4E-Br):
CH3I (50 mmol, 7.1 g) was added slowly to a chilled solution
of 50 mmol of Ph3P (13.1 g) in THF. The reaction mixture was
stirred for 1 h and filtered, and the residue was washed with
THF and dried overnight in a vacuum stove, yielding
(Ph)3PCH3I in 98% yield (49 mmol, 19.8 g). Of this product 29
mmol was dissolved in THF. To this solution, cooled to -78
°C, a solution of 29 mmol t-BuOK (3.2 g) in t-BuOH was added
dropwise. After the solution was stirred for 15 min, 29 mmol
of R,R,R-trifluoroacetophenone (5.0 g) was added in one portion.
The reaction mixture was stirred for a half hour and warmed
to room temperature. The solvent was evaporated and petro-
leum ether 40/60 was added, the mixture was filtered, and the
filtrate was dried over MgSO4 and concentrated. A 29-mmol
sample of the product, 3,3,3-trifluoro-2-phenylpropene (18) (5.0
g), was dissolved in 30 mL of CCl4 at 0 °C and 32 mmol of Br2
(5.1 g) was added dropwise. After the addition was complete,
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Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo,
C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
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Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.;
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Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challa-
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(46) For previous characterization of 4E/Z-Br see: Harada, T.;
Katsuhira, T.; Hara, D.; Kotani, Y.; Maejima, K.; Kaji, R.; Oku, A. J.
Org. Chem. 1993, 58, 4897-4907.
(47) Miyano, S.; Izumi, Y.; Fujii, K.; Ohno, Y.; Hashimoto, H. Bull.
Chem. Soc. Jpn. 1978, 52, 1197-1202.
(48) For previous characterization of 4E/Z-Cl see: Nader, B. S.;
Cordova, J. A.; Reese, K. E.; Powell, C. L. J. Org. Chem. 1994, 59,
2898-2901.
(45) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J.
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J. Org. Chem, Vol. 70, No. 1, 2005 189