Electrophilic aromatic substitution. Part 29. Protiodetritiation of 4H-cyclopenta[def]phenanthrene: the substituent activation factor Af

Abstract

Rate coefficients have been determined for detritiation of each position of 4H-cyclopenta[def]phenanthrene (I) in trifluoroacetic acid at 70 °C, leading to the following partial rate factors (positions in parentheses): 27 050 (1), 5 680 (2), 14 000 (3), and 6 950 (8); the corresponding σ⁺ values are –0.5065, –0.43, –0.475, and –0.44. The reactivity of (I) shows a combination of the properties of fluorene and phenanthrene, from each of which it is derived, and the relative positional reactivities are correctly predicted by Hückel localization energies. The σ⁺ values predict accurately the isomer distribution in nitration (in acetic anhydride), but rather less satisfactorily the distributions in acetylation and molecular bromination due to the high polarizability of (I) which may be due in part to strain. Strain reduces steric hindrance at the 1- and 8-positions of (I) compared with the corresponding positions in phenanthrene. The reactivity of the 8-position of planar (I) is substantially less than that of the corresponding 9-position of non-planar 4,5-dimethylphenanthrene, supporting the view that the high reactivity of the latter derives from loss of ground-state conjugation. The effect of the methylene substituent in (I) compared with the methyl substituent in benzene and naphthalene demonstrates the over-riding importance of bond fixation in polycyclic aromatics; these substituent effects are predicted by easily calculated substituent activation factors (A_f).