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DOI: 10.1039/A702585H (Paper) Reference Section for: J. Chem. Soc., Perkin Trans. 2, 1997, 1633-1640

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

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Jarrett R. Black, Colin Eaborn, Philip M. Garrity and Duncan A. R. Happer

Abstract

In the reactions of the isotopically labelled (Me3Si)3CSi(CD3)2I, 1-D, with alcohols ROH (R = Me, Et, Pri or But[hair space]) 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)2[upper bond 1 start]CSiMe2-Me-S[hair space] [upper bond 1 end]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 But. 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.

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