The quantitative electrophilic reactivity of annulenes. Part 3. Partial rate factors for protiodetritiation and protiodesilylation of 1,6 : 8,13-propane-1,3-diyldene[14]annulene

Abstract

We have prepared 1,6 : 8,13-propane-1,3-diylidene[14]annulene with tritium and trimethylsilyl substituents at the 2-position. Rates of protiodetritiation and protiodesilylation have been measured, leading to partial rate factors, under standard conditions, of 2.0 × 10⁸(trifluoroacetic acid at 70 °C) and 8.34 × 10⁴(2 : 5 v/v perchloric acid–methanol at 50 °C), respectively. The σ⁺-value is –0.96. In both reactions the [14]annulene is more reactive than 1,6-methano[10]annulene at the corresponding 2-position, for which the partial rate factors are 1.03 × 10⁷ and 9.27 × 10³, respectively, with σ⁺–0.80. The reactivity difference towards exchange is 4.4-fold greater than that which exists between the corresponding 1-positions of anthracene and naphthalene, as predicted by topological resonance energy calculations. These calcualtions also predict the observed greater reactivity of the annulenes compared with the corresponding polyacenes. The detritiation and desilylation data predict a σ-value for the [14]annulene of –0.65, cf.–0.435 for the [10]annulene. The difference σ⁺–σ is therefore 0.31 for the [14]annulene, compared with 0.32 for the [10]annulenes, indicating that the [14]annulene is marginally less polarisable. Lithiation of the [14]annulene occurs preferentially at the 2-position.