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*C6H,*MMe, compounds (M = Si or Sn) in 1 :6 v/v waterdimethyl sulphoxide containing potassium hydroxide. In a plot of the logarithm of the rate constants versus the Hammett a-constants the points for some para-substituents deviate markedly from the line defined by those for the me&-substituents. There is a common pattern to these deviations and the much larger ones previously observed in cleavages of X*CBH4*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-MMe, or C-H bond under the attackof 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'C6ki4'-CIC-GeEt, bonds. In 1 :6 H,O-Me,SO a t 75', phenyltrirnethyl-silane and -stannane are cleaved a t identical rates.Aryltrimethylgermanes are much less readily cleaved.

Multiparameter Extensions of the Hammett Equation

“…II [33][34][35][36][37][38][39][40] The (4.14) where ap has replaced r+. Norman and Radda saw an analogy here to an equation of their own,18 viz., (4.15), where fp is the partial rate-factor for substitution at the para-position of a monosubstituted benzene, UG is a measure of ground-state electron density at the para-position, ( …”