A mechanistic study on the SHi reaction at tin atoms in a radical cascade reaction

Abstract

A kinetic study on radical cascade reactions of 1,6-enyne compounds was undertaken. The efficiency of the reaction depended on the presence of an ester group at the alkene unit, which clearly suggests that the addition of a tin radical was accelerated by the α,β-unsaturated system. Stannolane formation progressed very quickly after a short induction period and completed within 12 min in the presence of Bu₃SnH at 10⁻² M concentration at 110 °C. Product ratios between stannolane and exo-methylene compound depended on the concentration of Bu₃SnH; a linear relationship was observed between the ratios of the two and the concentration of Bu₃SnH. These results clearly indicate that the S_Hi process is irreversible. The slope of the graph allowed us to estimate the k_s value, the S_Hi reaction rate at the tin atom, as 4.23 × 10⁸ s⁻¹ at 303 K. The use of Bu₂PhSnH for the reaction suggests that the S_Hi process partially progresses by the frontside attack of the vinyl radical, which generates a methylene radical that undergoes neophyl rearrangement to give methylene piperidine in a cis-selective manner.