Electrophilic aromatic substitution. Part 28. Rate factors for protiodetritiation of all positions of hexahelicene (benzo[c]naphtho-[1,2g]phenanthrene): the reactivity of bent benzene rings

Abstract

The first study of the quantitative electrophilic aromatic reactivity of hexahelicene has been made through preparation of each of the eight specifically tritium-labelled isomers, and measurement of their rates of protiodetritiation in trifluoroacetic acid (TFA)–chloroform (9 : 1 v/v) at 100°. Comparison with rates of detritiation of [9-³H]-phenanthrene in this medium and in TFA at both 100 and 70° leads to the following partial rate factors for detritiation in TFA at 70°(positions in parentheses): 10 250(1); 4 400(2); 905 (3); 8 770 (4); 25 000 (5); 6 530 (6); 21 600 (7); 18 200 (8); the corresponding σ⁺-values are: –0.46; –0.415; –0.34; –0.45; –0.505; –0.435; –0.495; –0.485. The positional reactivity order: 5 > 7 > 8 > 1 > 4 > 6 > 2 > 3 is in very good agreement with that predicted by Hückel localization energies viz.: 5 > 7 > 4 > 8 > 6 > 1 > 2 > 3; as in the case of tetra- and penta-helicene, and chrysene, the calculations underestimate the reactivity of the most central positions of the molecule. With the exception of the 3- and 6-positions, all positions are 2–3 times more reactive than the corresponding positions in pentahelicene, whereas the calculations predict they should be the same [the average localization energy per position is predicted to be precisely constant (–2.366β) for tetra-, penta-, and hexa-helicene]. The contrasting and regular increase in reactivity along this series demonstrates the effect of the increased distortion of the aromatic rings so that the reaction ground states are destabilized. The increased reactivity does not arise from desolvation of the ground states for reaction of the (less soluble) higher homologues because: (i) coronene, which is planar, shows normal reactivity even though it is much less soluble than hexahelicene and (ii) the 1-, 3-, 4-, and 5-positions at the planar end of benzo[a]naphtho[1,2-h]anthracene (which is less soluble than the isomeric hexahelicene) are much less reactive (f₁ 3 810; f₃ 270; f₄ 1 190; f₅ 11 100; σ⁺₁–0.41; σ⁺₃–0.28; σ⁺₄–0.35; σ⁺₅ 0.46) than the corresponding positions in hexahelicene in contrast to localization energy predictions. The 1- and 2-positions of hexahelicene do not appear to be sterically hindered towards exchange (similar results were obtained for the 1-positions of tetra- and penta-helicene) reflecting the moleclar distortion which maintains a minimum of 3 Å between C-1 and -16; if anything, the 1-position is rather more reactive relative to the other positions than might have been expected though no clear reason for this is evident. The hydrogen exchange data show that the 7-isomer should accompany formation of the 5-isomer in nitration, bromination, and acetylation of hexahelicene, rather than the 8-isomer recently reported. A number of annelation rules for electrophilic aromatic substitution are formulated.