Suppressing the thermal metacyclophanediene to dihydropyrene isomerization: synthesis and rearrangement of 8,16-dicyano[2.2]metacyclophane-1,9-diene and evidence supporting the proposed biradicaloid mechanism.

Synthesis of 8,16-dicyano-anti-[2.2]metacyclophane-1,9-diene, 1b, was achieved in five steps from 1,3-bis(bromomethyl)benzonitrile. Unlike most metacyclophanedienes which easily thermally isomerize (tau 1/2 = minutes to days at 20 degrees C) to dihydropyrenes 2, dinitrile 1b shows no tendency to convert thermally to 2b at room temperature (tau 1/2 > 30 years), consistent with predictions based on calculation of activation barriers. Irradiation of cyclophanediene 1b with UV light readily forms the dinitriledihydropyrene 2b, which unexpectedly shows a much more facile (50 degrees C) 1,5-sigmatropic rearrangement of the internal nitrile groups to give dihydropyrenes 9b and then 10b than is the case for internal methyl substituents, 2a, which forms 9a at temperatures above 190 degrees C. Synthesis of the 2-formyl derivative 1c and the 2-naphthoyl derivative 1d are also described. These substituents were predicted to lower the activation barrier for the thermal closing reaction to the corresponding dihydropyrenes, and experimental evidence supports these calculations.