Electrophilic aromatic substitution. Part 36. Protiodetritiation of some annelated meta-cyclophanes: effect of ring-buckling on reactivity, and the first example of electrophilic substitution through a hole

Abstract

A series of 1,3-brigded naphthalenes in which the alkyl bridge contains ten, eight, or seven methylene groups have been prepared with tritium located at either the 2- or the 4-position. Their rates of detritiation in anhydrous trifluoroacetic acid at 70 °C have been measured. The partial rate factors for exchange at the 4-position increase as the size of the bridge decreases viz. 1.28 × 10⁷([CH₂]₁₀) 2.71 × 10⁷([CH₂]₈), 2.97 × 10⁷([CH₂]₇); cf. 1.15 × 10⁷ for 1,3-dimethylnaphthalene, the corresponding σ⁺ values being –0.812, –0.849, –0.854, and –0.807. This is consistent with increased loss of ground-state aromaticity as the aromatic ring becomes more buckled. At the 2-positions the partial rate factors are 2.37 × 10⁵([CH₂]₁₀), 2.92 × 10⁵([CH₂]₈), and 1.50 × 10⁵([CH₂]₇); cf. 2.82 × 10⁵ for 1,3-dimethylnaphthalene, the corresponding σ⁺ values being –0.614, –0.625, –0.592, and –0.623. The results here reflect the opposing effects of increased reactivity due to loss of ground-state aromaticity, and steric hindrance to exchange as the bridge is made smaller. Because of the symmetry of the reaction pathway for hydrogen exchange, reaction at the 2-positions involves attack of the electrophile at one face of the loop formed between the alkyl chain and the aromatic ring, and departure of the leaving group (triton) from the opposite face, i.e. substitution occurs through a ‘hole’. This is the first example of electrophilic substitution at an enclosed site. The fact that steric effects in both reagent and product in hydrogen exchange are almost identical is postulated as a primary reason for the extremely low steric hindrance generally observed in the reaction. The effect of fusing a bicyclic substituent onto an aromatic ring has been determined for the first time. For hexahydrophenanthrylene and hexahydroaceanthrylene (naphthalenes with bicyclic substituents attached to the 1-, 2-, and 3-positions) the average partial rate factor for detritiation of the ‘4’-positions is 5.7 × 10⁷, and σ⁺ is –0.886. This is greater than calculated on the basis of additivity of the alkyl group substituent effects and is attributed to steric facilitation of hyperconjugation. The recent suggestion (based on structure counts) that the exalted reactivities of helicenes (and by implication of other aromatic systems containing bent benzene rings) is not due to loss of ground-state stability is shown to be invalid.