Electrophilic substitution in annulenes. Part 4. Transmission of substituent effects in 1,6-methano[10]annulene, determined via protiodesilylation: evidence for substantial C(1)–C(6) transannular orbital interaction

Abstract

The rates of acid-catalysed desilylation of a series of 7-substituted 2-trimethylsilyl-1,6-methano[10]annulenes have been measured spectrophotometrically at 50 °C using a mixture of aqueous perchloric acid and methanol (2:5 v/v). The substituent effects are greatly reduced as compared with their effects in benzene, the resonance component being adversely affected in particular. This provides further evidence that the structure for 1,6-methano[10]annulene is best described in terms of homonaphthalene, because in the latter conjugative effects are relayed only weakly between the 2- and the 7-position. The trimethylsilyl substituent deactivates slightly in the annulene wheras it activates slightly in benzene; this activation was confirmed by measuring the rate of protiodegermylation of 4-tri-methylsilylphenyltriethylgermane. The difference between the effect of the trimethylsilyl substituent in the annulene from that in benzene may reflect the greater bond localisation in the annulene such that opportunity for (p→d)π electron withdrawal by silicon is greater in the ground state.