Substitution at saturated carbon. Part XVI. Steric effects in the SE2 reaction

Abstract

Rate constants are reported for the substitution of tetra-alkyltins, R₄Sn, by mercury(II) acetate in methanol and in t-butyl alcohol. The observed steric sequence of reactivity in methanol, R = Me > Et > Prⁿ > Buⁿ > Buⁱ > C₅H₁₁^neo > Prⁱ, is suggested to be characteristic of the S_E2(open) mechanism with retention of configuration at the site of substitution. Since this steric sequence differs from those in the S_N2 reaction and from that observed by Jensen and Davis for the S_E2(open) mechanism, with inversion of configuration, it is now possible to distinguish between the mechanisms S_E2(open)Ret and S_E2(open)Inv by use of these different steric sequences of reactivity. On this basis, stereochemical assignments for a number of S_E2(open) reactions have been made. For example it is concluded that the substitution of alkylcobaloximes by mercury(II) in water proceeds by the S_E2(open)Ret mechanism, whereas the iodo-demetallation of tetra-alkyltins in methanol takes place by the S_E2(open)Inv mechanism. The substitution of tetra-alkyltins by mercury(II) acetate in t-butyl alcohol is suggested to take place through a transition state that is intermediate between an ‘open’ and a fully ‘cyclic’ one.