Solvolyses of some 13C- or 14C-labeled triarylvinyl bromides in aqueous acetic acid

Abstract

The presence of different amounts of added LiClO4 in the solvolysis of triphenyl[2-14C]vinyl bromide (1-Br-2-14C) or trianisyl[2-14C]vinyl bromide (2-Br-2-14C) in 70% HOAc did not materially affect the extent of scrambling of the label arising from 1,2-aryl shifts in the triarylvinyl cation or kr, the rate constant for the 1,2-aryl shift. Solvolysis of a 2:3 mixture of (E)- and (Z)-1,2-diphenyl-2-tolylvinyl bromide ((E,Z)-3-Br) in 70% HOAc gave an 85:15 mixture of 1,2-diphenyl-2-tolylethanone (5) and 2,2-diphenyl-1-tolylethanone (6), the latter product, 6, being derived from a 1,2-phenyl shift in the 1,2-diphenyl-2-tolylvinyl cation to the more stable 2,2-diphenyl-1-tolylvinyl cation. With (E,Z)-3-Br-2-13C as substrate and analysis by gc–ms, about 26–27% scrambling of the 13C label was found in both products, 5-1,2-13C and 6-1,2-13C. Solvolyses of (E,Z)-3-Br-2-13C in 50% and 70% HOAc showed that the extent of scrambling in the major products, 5-1,2-13C, was lower in 50% HOAc than in 70% HOAc, similar to a trend previously observed for solvolyses of 1-Br-2-14C. In contrast, solvolyses of 2-Br-2-14C in 50, 70, or 90% HOAc gave essentially the same extent of scrambling. Mechanistic implications of these results are discussed.