Photochemical Generation of Highly Destabilized Vinyl Cations:  The Effects of α- and β-Trifluoromethyl versus α- and β-Methyl Substituents

The photochemical reactions in methanol of the vinylic halides 1−4, halostyrenes with a methyl or a trifluoromethyl substituent at the α- or β-position, have been investigated quantitatively. Next to E/Z isomerization, the reactions are formation of vinyl radicals, leading to reductive dehalogenation products, and formation of vinyl cations, leading to elimination, nucleophilic substitution, and rearrangement products. The vinyl cations are parts of tight ion pairs with halide as the counterion. The elimination products are the result of β-proton loss from the primarily generated α-CH3 and α-CF3 vinyl cations, or from the α-CH3 vinyl cation formed from the β-CH3 vinyl cation via a 1,2-phenyl shift. The β-CF3 vinyl cation reacts with methanol yielding nucleophilic substitution products, no migration of the phenyl ring producing the α-CF3 vinyl cation occurs. The α-CF3 vinyl cation, which is the most destabilized vinyl cation generated thus far, gives a 1,2-fluorine shift in competition with proton loss. The experimentally derived order of stabilization of the vinyl cations photogenerated in this study, α-CF3 < β-CF3 < β-CH3 < α-CH3, is corroborated by quantum chemical calculations, provided the effect of solvent is taken into account.