Reactivity and mechanism in some substitution reactions of benzyltrialkyltins.

Reactions between benzyltrialkyltins, benzyl-SnR3, and mercury(II) salts have been studied; the stoichiometry can be represented by equation (1; R =. Bu[n], Pr[n], Et and X=Cl, OAc), or equation (2; R = Me and X = Cl, OAc). C[6]H[5]CH[2]SnR[3] + HgX2 → C[6]H[5]CH[2]HgX + R[3]SnX (1) C[6]H[5]CH[2]SnMe[3] + HgX[2] → MeHgX + C[6]H[5]CH[2]SnMe[2]X (2) Kinetic studies showed that both reactions (1) and (2) follow second-order kinetics, first order in each reactant, and it is suggested that these reactions are simple electrophilic substitutions. Iodination of benzyltrialkyltins in methanol gave R3SnI and 100% benzyl iodide (when R = Bu[n], Pr[n] and Et); with the compound benzyl-SnMe3 the product was 70% benzyl iodide and 30% methyl iodide. In the corresponding bromination and chlorination, benzylmethyl ether, a novel and unusual product, was obtained in quite high yield [50% yield in bromination of benzyl-SnR[3] when R = Bu[n], Pr[n] and Et, and 75% yield for the similar chlorination reaction]. In the case of benzyl-SnMe3, the methyl group is cleaved as well as the benzyl group. Kinetic studies on the iodination and bromination reaction showed that both followed simple second-order kinetics, first order in each reactant. Free radical inhibitors did not affect either the rate constants or the products. Comparison of rate constants for the reactions studied in this work with previous data enabled the reactivity of the benzyl-tin bond towards electrophilic reagents to be obtained for the first time. Relative to Me = 1000, results are: [figures]. It is suggested that the bromination and chlorination must proceed (at least in part) via a reactive intermediate; the latter can collapse with involvement of a solvent molecule to yield the benzylmethyl ether. The benzyl halide might be formed either by a normal electrophilic substitution or by rearrangement of the intermediate. From the reactivity sequence obtained, it appears that the benzyl iodide produced in the iodination reaction is formed by the same type of mechanism as is the benzyl bromide