Annelated Dimethyldihydropyrenes. Electrophilic Substitution and Valence Isomerization to Metacyclophanedienes

The reversible valence isomerization of cyclophanedienes to dihydropyrenes is discussed for a number of [a]- and [e]-annelated examples, 5 → 6 and 7 → 8, and is related to AM1 and MM2+Pi calculations. Only in [e]-fused systems is the isomerization easily seen to be reversible. Electrophilic substitution of dihydropyrenes is discussed. Nitration of the highly annelated 20 gives a mixture of 8- and 7-nitro derivatives, while NBS/DMF bromination of the benzo[a]dihydropyrene 3 gives mostly 12-bromo derivative.