Vinylic cations from solvolysis. Part XI. Solvolysis of triarylvinyl arenesulphonates and bromides

Abstract

Triarylvinyl arenesulphonates and bromides, Ar22CC(Ar1)X (X = Br or O·SO2Ar3), were solvolysed in 70% acetone, and tris-p-methoxyphenylvinyl tosylate (II–OTs) and brosylate were solvolysed in acetic acid. The products were 2,2-diarylacetophenones and tris-p-methoxyphenylvinyl acetate, respectively. The solvolysis of (II–OTs) shows common ion rate depression and the intermediate vinyl cation is captured by bromide and tosylate ions. For change in Ar3 of the leaving group, the Hammett ρ value is 1·91 at 50 and 1·67 at 75°. For 60–90% aqueous acetone a Winstein–Grunwald mY plot is linear with m= 0·42. The reaction rate is sensitive to the α-activating substituent with kα-p-MeO·C6H4/kα-Ph= 630, and much less sensitive to the nature of the β-aryl groups. Solvent isotope effects are small. It is suggested that the reaction has an SN1 mechanism with an intermediate ‘selective’ vinyl cation. The solvent effect and the kOBs : kOTs and the kOTs : kBr reactivity ratios are discussed and it is concluded that the solvolysis is anchimerically and nucleophilically unassisted.