Electrophilic aromatic substitution. Part 22. The effect of methyl substituents on detritiation of the 9-position of phenanthrene; tritium migration during exchange

Abstract

Rates of protiodetritiation in trifluoroacetic acid at 70 °C have been measured for 9-tritiated phenanthrenes containing a methyl substituent in each of the positions 1–8. The activating effects of these substituents are as follows (positions in parentheses): 4.24(1); 2.77(2); 17.4(3); 2.96(4); 2.37(5); 3.16(6); 3.05(7); 2.67(8). The methyl substituent effects (with the exception of 6-methyl) closely parallel those obtained for the corresponding interactions in naphthalene, such differences as do exist being predicted by empirical calculations which simply consider the change in bond orders on going from ground state to transition state; calculations of the effect of a CH₂^– substituent on localization energies are also satisfactory models. The reactivity of the 6-methyl compound is anomalously high and is believed to be due to migration of tritium across the 9,10-bond accompanying the exchange, thereby giving a small amount of the more reactive 3-methyl[9-³H]phenanthrene. Likewise, detritiation of 3-methyl[9-³H]phenanthrene gave a curved kinetic plot indicating the presence of an isomer, less reactive towards exchange, i.e. from tritium migration to give 6-methyl[9-³H]phenanthrene. This, the first example of a 1,2-migration of hydrogen in non-super acid media is calculated to occur to the extent of 6–10% during the time of the kinetic runs. σ⁺-Constants for all but the 3-methyl compound are closely similar and in the range –(0.415–0.44); the high reactivity of the 3-methyl compound for which σ⁺ is –0.51 derives from the strong conjugative interaction leading to a p-quinonoid-like transition state.