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.

Application of the Yukawa–Tsuno equation to the desilylation data yields a value for σ⁺_4-SH of –0·365; a value for ƒ₄^MeO of 1270 is argued to be more accurate than the literature value of 1520.

Metallation of diphenyl sulphide produces a mixture of ortho- and meta-derivatives, not merely the former as stated in the literature, and this parallels a recent report on metallation of thiophenetole. The reported rearrangement of 4-LiC₆H₄·S·SiMe₃ to 4-SiMe₃·C₆H₄·SLi is shown to be almost certainly an intermolecular reaction.