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 k_{α-p-MeO·C6H4}/k_α-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 S_N1 mechanism with an intermediate ‘selective’ vinyl cation. The solvent effect and the k_OBs : k_OTs and the k_OTs : k_Br reactivity ratios are discussed and it is concluded that the solvolysis is anchimerically and nucleophilically unassisted.