Electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part IX. The effect of the trimethylsilyl substituent

Abstract

The three isomeric 1-(trimethylsilylphenyl)ethyl acetates have been prepared and their rates of pyrolysis together with that of 1-phenylethyl acetate measured between 340·6 and 392·9°. The trimethylsilyl substituent activates in all positions according to the sequence o > m > p as theoretically predicted and found in protodesilylation, but contrary to the activation order observed in nitration, and the deactivation order found in solvolysis of 1-aryl-1-methylethyl chlorides. The variability of the substituent effect in these and some other reactions is attributed mainly to varying demands for resonance stabilisation of the transition states together with some solvent–substituent interactions. The variation in the meta-substituent effect coupled with the high activation by the ortho-substituent indicates that the trimethylsilyl group supplies electrons inductomerically. In the light of the present results the reactivity pattern in nitration of phenyltrimethylsilane is seen to be affected by steric hindrance. Values of σ⁺ are –0·16 (m-SiMe₃) and –0·09 (p-SiMe₃).