Photochemical Generation of Highly Destabilized Vinyl Cations:The Effects of alpha-and beta-Trifluoromethyl versus a-and beta-Methyl Substituents

摘要

The photochemical reactions in methanol of the vinylic halides 1-4,halostyrenes with a methyl or a trifluoromethyl substituent at the alpha-or beta-position,have been investigated quantitatively.Next to E/Z isomerization,the reactions are formation of vinyl radicals,leading to reductive dehalo-genation 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 beta-proton loss from the primarily generated alpha-CH_3 and alpha-CF_3 vinyl cations,or from the alpha-CH_3 vinyl cation formed from the beta-CH_3 vinyl cation via a 1,2-phenyl shift.The beta-CF_3 vinyl cation reacts with methanol yielding nucleophilic substitution products,no migration of the phenyl ring producing the alpha-CF_3 vinyl cation occurs.The alpha-CF_3 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,alpha-