Electrophilic aromatic substitution. Part VI. Protodesilylation and protodetritiation of some oxygen- and sulphur-containing aromatic compounds and of diphenylmethane. Evidence for hyperconjugation

Abstract

Partial rate factors for protodesilylation of some aromatic compounds Me3Si·C6H4X in methanol–perchloric acid at 50 °C have been determined as: (X =)2-OH, 3720; 2-SH, 4·42; 4-SH, 11·3; 2-SMe, 18·4; 4-SMe, 65·2; 2-CH2Ph, 3·75, and for detritiation of the 2-position of thioanisole in trifluoroacetic acid at 70 °C, 21,500. The relative reactivity of thioanisole and thiophenol is in the inductive order, i.e., SMe activates more than SH in contrast to the results for the oxygen and carbon analogues and this is interpreted as constituting impressive evidence for hyperconjugation in the latter compounds. The ortho:para ratio decreases along the series CH2Y, OY, SY, (where Y = H, Me, or Ph) and ZMe, ZH, ZPh (where Z = CH2, O, or S) apparently through the increasing importance of the inductive effect at the ortho-position; the previously observed low reactivity of the 2-position of diphenyl sulphide in protodesilylation is confirmed and shown not to be anomalous.