THE PROPORTION OF 1,3-MIGRATION OF A METHYL-GROUP IN THE REACTIONS OFTHE 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 si lver 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 fo r capture of an intermediate methyl-bridged cation [(Me3Si)(2)CSiMe2-M e-Si(CD3)(2)](+), II, by nucleophilic attack of the alcohol at either end of the bridge. For example, in the reaction with AgClO4, the perce ntages of rearranged product are 26, 22 and 37 (all values +/-8) for R = Me, Et and Pr-1, respectively, at room temperature and the correspo nding 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 exclusiv ely by direct formation of a fully free cation II as assumed in the si mplest picture, It is concluded that ad hoc modification of this simpl e picture, for example, by postulating some pre-association of an alco hol molecule at the Si-I bond in 1-D, is preferable to possible altern ative mechanisms, such as one involving initial formation of an unbrid ged cation [(Me3Si)(3)CSi(CD3)(2)](+) which can sometimes be captured before conversion into II, The alkoxide products are accompanied by so me (Me3Si)(3)CSi(CD3)(2)X and (Me3Si)(2)C[Si(CD3)(2)Me]SiMe2X (from Ag BF4, X = F in both cases) and the corresponding hydroxides, the propor tion of rearranged product always being significantly below 50%, In th e reactions with AgClO4 or AgO3SCF3 the silicon hydroxides are mainly produced by hydrolysis of initially formed perchlorates or trifluorome thanesulfonates by traces of water in the solvents, but for the other silver salts they arise virtually wholly by capture of the intermediat e cation by the water.