Electrophilic aromatic reactivity. Part 24. Protiodetritiation of pentahelicene (dibenzo[c,g]phenanthrene)

Abstract

All seven tritium-labelled dibenzo[c,g]phenanthrenes have been prepared and their rates of detritiation by anhydrous trifluoroacetic acid at 70 °C measured; these lead to the partial rate factors (positions in parentheses): 3 500 (1), 2 405 (2), 930 (3), 6 560 (4), 18 400 (5), 6 930 (6), and 10 200 (7) and corresponding σ⁺-values: –0.40, –0.385, –0.34, –0.435, –0.49, –0.44, and –0.46. The positional reactivity order in this, the first study of electrophilic substitution in pentahelicene, is: 5 > 7 > 6 > 4 > 1 > 2 > 3 in good agreement with that predicted by localisation energies calculated by the Hückel method viz.: 5 > 4 > 6 > 7 > 1 > 2 > 3. Each position is more reactive than the corresponding position in benzo[c]phenanthrene (tetrahelicene) by a factor of 2–3, a similar amount to that by which the positions in the latter exceeded the reactivities of the corresponding positions in phenanthrene. By contrast the localisation energies predict that the corresponding positions in all three molecules should be of closely similar reactivity. The data therefore indicate that loss of conjugation in penta- and tetrahelicene through non-planarity of the rings destabilises the ground state and produces enhanced reactivity. Position 7 in pentahelicene and position 6 in tetrahelicene show the most significant exaltation in reactivity. The reason for this is not yet clear, but the implication is that maximum distortion of the π-framework occurs at these, the central positions in each molecule.

The reactivity of the 1-position exceeds that of the 1-position in tetrahelicene, by an amount which is similar to the differences between the reactivities of the other pairs of corresponding positions. This indicates that exchange at the 1-position is unaffected by steric hindrance and this is consistent with X-ray studies which show that the hydrogens on the terminal rings are the same distance apart as the corresponding hydrogens in tetrahelicene; in the latter, exchange at the 1-position is virtually unhindered.