Aromatic reactivity. Part XLVI. Interpretation of the substituent effects in base-catalysed cleavage of aryltrimethyl-silanes and -stannanes

Abstract

Rates of cleavage have been measured for some X·C₆H₄·MMe₃ compounds (M = Si or Sn) in 1 : 6 v/v water–dimethyl sulphoxide containing potassium hydroxide. In a plot of the logarithm of the rate constants versus the Hammett σ-constants the points for some para-substituents deviate markedly from the line defined by those for the meta-substituents. There is a common pattern to these deviations and the much larger ones previously observed in cleavages of X·C₆H₄·SnMe₃ compounds in aqueous methanolic alkali and in hydrogen exchange of X·C₆H₅ compounds in liquid ammonia containing potassium amide, and it is suggested that in all cases there is transfer of hydrogen from the solvent to the aromatic compound in an electrophilic attack at carbon concerted with the breaking of the C–MMe₃ or C–H bond under the attack of the base. Substituents with electron-releasing resonance effects are thought to assist this electrophilic attack to produce the unusual substituent effects observed. The interpretation can be extended to account for the substituent effects previously observed in base cleavage of X·C₆H₄·-C⋮C–GeEt₃ bonds. In 1 : 6 H₂O–Me₂SO at 75°, phenyltrimethyl-silane and -stannane are cleaved at identical rates. Aryltrimethylgermanes are much less readily cleaved.