Electrophilic aromatic substitution. 25. Acid-catalyzed hydrogen exchange of 9-tritiated polymethylphenanthrenes: effect of ring distortion on aromatic reactivity and substituent effects

Rate coefficients have been measured for detritiation of the 9-position of some strained and unstrained polymethylphenanthrenes by anhydrous trifluoroacetic acid at 70/sup 0/C. These lead to partial rate factors (in parentheses) as follows: 2,7-dimethylphenanthrene (12,950), 3,6-dimethylphenanthrene (59,800), 3,4,5,6-tetramethylphenanthrene (356,000), 4,5-dimethylphenanthrene (38,300), 2,4,5,8-tetramethylphenanthrene (230,000). The reactivities of the former two (unstrained) compounds are in excellent agreement with those predicted from the effects of monomethyl substituents in phenanthrene. By contrast the reactivities of the latter two compounds are approximately threefold greater than predicted due to steric interaction of the 4- and 5-methyl substituents; these produce nonplanarity of the aromatic rings and hence increased reactivity through loss of ground-state resonance. The strained 3,4,5,6-tetramethylphenanthrene is by contrast less reactive than predicted, this being attributable to substantial reduction, through distortion, of the conjugative interaction between the 3- and 9-positions (which is normally much greater than the other substituent site interactions). 1 figure, 1 table.