Studies of Carbenium Ion on Unsaturated Carbon. II. Exclusive Formation of Benzofurans via Vinyl Cations in the Solvolytic Reactions of 1-Aryl-2,2-bis(o-methoxyphenyl)vinyl Halides

Abstract

Abstract The reactions of 1-aryl-2,2-bis(o-methoxyphenyl)vinyl halides (4)–(7) in 80% aqueous ethanol (80% EtOH), acetic acid (AcOH), or 70% aqueous N,N-dimethylformamide gave benzofurans (8)–(10), respectively. The kinetic results i.e., 1) first-order reaction rates, 2) large α-substituent effect (ρ=−4.12 against σ+), 3) large leaving group effect (kBt⁄kCl=85 in 80% EtOH and 28 in AcOH) and 4) the solvent effect (m=0.53), are consistent with the formation of the intermediate vinyl cations (3) which are captured exclusively by the β-o-methoxyl group. In the reaction of vinyl bromide (7), the formation of 7–9% of benzofuran (11) results from the β-o-methoxyphenyl migration in the vinyl cation (17) followed by cyclization. The exclusive intramolecular cyclization of the vinyl cations is discussed in terms of the proximity effect of the β-o-methoxyl group.