Aromatic reactivity. Part LV. Solvent isotope effects in the base cleavage of aryltrimethylstannanes in methanol: evidence for electrophilic assistance by the solvent

Abstract

Values of the product isotope-effects, corresponding to the product ratios XC₆H₅/XC₆H₄D obtained in 1 : 1 MeOH–MeOD mixtures for cleavage of X·C₆H₄·SnMe₃ compounds by sodium methoxide in methanol at 21°, have been found to be as follows: (X =)p-OMe, 5·2; p-Me, 5·1; H, 4·9; m-OMe, 4·9; p-CI, 4·5; m-CF₃, 4·3; p-NO₂, 3·8. For the compounds with X =p-OMe and m-CF₃ values of 2·45 and 2·00 for the overall solvent effects, (k_MeOH/k_MeOD)S, have been determined by measurement of the separate rates at 50° in MeOH and in MeOD. It is concluded that proton transfer from the solvent to the carbon atom of the Sn–aryl bond is involved in the rat-determining step, which is probably the conversion of the Sn^V intermediate (MeO)Me₃Sn^–·C₆H₄·X into the Wheland-intermediate (MeO)Me₃Sn^–·C₆H₅^–·X.