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

In the reactions of the isotopically labelled (Me3Si)(3)CSi(CD3)(2)I, 1-D, with alcohols ROH (R = Me, Et, Pr-1 or Bu') in the presence of silver salts AgX (X = ClO4, O3SCF3, O2CCF3, NO3 or BF4) the ratio of the rearranged product (Me3Si)(2)C[Si(CD3)(2)Me]SiMe2OR to unrearranged (Me3Si)(3)CSi(CD3)(2)OR always falls below the value of 1:1 expected for capture of an intermediate methyl-bridged cation [(Me3Si)(2)CSiMe2-Me-Si(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 Pr-1, respectively, at room temperature and the corresponding figures at the reflux temperature are 29, 40, 41 and 44 (all +/-8) for Me, Et, Pr-1 and Bu', 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)(3)CSi(CD3)(2)](+) which can sometimes be captured before conversion into II, The alkoxide products are accompanied by some (Me3Si)(3)CSi(CD3)(2)X and (Me3Si)(2)C[Si(CD3)(2)Me]SiMe2X (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.