The aryl-tin bond.

A survey has been made of the literature concerning the cleavage of tin-carbon bonds. Twenty-three tricyclohexyl- and trimethyl- phenylstannanes, and p-methoxyphenyltriphenyltin have been prepared, and their cleavages by aqueous-ethanolic perchloric acid at 50° or by iodine in carbon tetrachloride have been studied. First-order rate constants have been determined. The acid-catalysed cleavage reactions, which can be represented by the equation Aryl-SnR3 + MOH H Aryl-H + R3SnOM (M = Et or H). were followed spectrophotometrically, and first-order rate constants were determined. The +E effects of substituents appear to operate less effectively in this reaction than in most electrophilic aromatic substitutions, including the analogous cleavages of aryl-silicon and aryl-germanium bonds. Although the effects of substituents cannot be related directly to the electrophilic substituent constants o+, or to the Hammett o substituent constants, they can be interpreted in terms of a combination of both sets of constants. The effects of substituents are greater in the tricyclohexyl series than in the trimethyl series, but smaller than in the triphenyl series. Third-order rate constants have been measured for the cleavage of the above compounds by iodine in carbon tetrachloride: Aryl-SnR3 + I3 → Aryl-I + R3SnI. There is a second-order dependence on the iodine concentration, and a first-order dependence on the concentration of the stannane over the range M/200 - M/1500. The + E effects of substituents operate less effectively than in the acid-catalysed cleavage of the same tin compounds, although the spread of relative rates is slightly greater in the iodine reaction. The cleavages of tricyclohexylphenyltin compounds are more rapid than the cleavages of the analogous trimethylphenyltin compounds, and the spread of relative rates is greater in the trimethyl series - the reverse of the situation in the acid-catalysed cleavage of these compounds. Detailed mechanisms ere suggested for both cleavage reactions. There is no overwhelming evidence for a mechanism fundamentally different from those commonly proposed for the well known aromatic substitutions in which Aryl-H bonds are broken by electrophilic attack at carbon, but it is suggested that the small effects of substituents in the destannylation reactions may be a result of some nucleophilic attack at the metal atom in the rate-determining step.