Surface organometallic chemistry on metals: controlled hydrogenolysis of Me4Sn, Me3SnR, Me2SnR2, MeSnBu3 and SnBu4 (R = methyl, n-butyl, tert-butyl, neopentyl, cyclohexyl) onto metallic rhodium supported on silica

Abstract

The controlled hydrogenolysis of Me_xSnR_4 − x (0 ≤ x ≤ 4; R = methyl, n-butyl, tert-butyl, neopentyl, cyclohexyl) onto Rh/SiO₂ is followed by quantitative and qualitative analysis of evolved gases. Only MeH and RH are detected in the evolved gases. There is hydrogenolysis of the Sn–C bonds without any C–C bond hydrogenolysis, leading to formation of grafted organometallic fragments. Using various organotin compounds, Me_xSnR_4 − x, it has been possible to determine the regioselectivity of the hydrogenolysis of the Sn–C bonds. The initial selectivity is inversely proportional to the steric bulk of the alkyl group: tBu < Np < Bu. The formation of a five-coordinate tetraalkyl tin on the surface, Rh–SnMe_xR_4 − x (symmetry D_3h), in which the bulkiest group, e.g., R, is away from the surface could explain these results. This surface five-coordinate tin species could eliminate an alkyl group, generally a methyl group, thus decreasing the steric bulk around the tin, into the equatorial plane of D_3h, via a concerted hydrogen transfer-elimination mechanism to give Rh–SnMe_x − 1R_4 − x. Then, in the successive steps of the hydrogenolysis, the bulkiest group, R, would be eliminated.