The proportion of 1,3-migration of a methyl group in the reactions of the iodide (Me3Si)3CSi(CD3)2I with silver salts in alcohols. Mechanistic implications

Abstract

In the reactions of the isotopically labelled (Me₃Si)₃CSi(CD₃)₂I, 1-D, with alcohols ROH (R = Me, Et, Prⁱ or Bu^t) in the presence of silver salts AgX (X = ClO₄, O₃SCF₃, O₂CCF₃, NO₃ or BF₄) the ratio of the rearranged product (Me₃Si)₂C[Si(CD₃)₂Me]SiMe ₂OR to unrearranged (Me₃Si)₃CSi(CD₃)₂OR always falls below the value of 1∶1 expected for capture of an intermediate methyl-bridged cation [(Me₃Si)₂CSiMe₂-Me-S i(CD₃)₂]⁺, II, by nucleophilic attack of the alcohol at either end of the bridge. For example, in the reaction with AgClO₄ the percentages of rearranged product are 26, 22 and 37 (all values ±8) for R = Me, Et and Prⁱ, respectively, at room temperature and the corresponding figures at the reflux temperature are 29, 40, 41 and 44 (all ±8) for Me, Et, Prⁱ and Bu^t. Thus the reaction cannot proceed exclusively by direct formation of a fully free cation II as assumed in the simplest picture. It is concluded that ad hoc modification of this simple picture, for example, by postulating some pre-association of an alcohol molecule at the Si–I bond in 1-D, is preferable to possible alternative mechanisms, such as one involving initial formation of an unbridged cation [(Me₃Si)₃CSi(CD₃)₂] which can sometimes be captured before conversion into II. The alkoxide products are accompanied by some (Me₃Si)₃CSi(CD₃)₂X and (Me₃Si)₂C[Si(CD₃)₂Me]SiMe ₂X (from AgBF₄, X = F in both cases) and the corresponding hydroxides, the proportion of rearranged product always being significantly below 50%. In the reactions with AgClO₄ or AgO₃SCF₃ the silicon hydroxides are mainly produced by hydrolysis of initially formed perchlorates or trifluoromethanesulfonates by traces of water in the solvents, but for the other silver salts they arise virtually wholly by capture of the intermediate cation by the water.