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

Abstract

(Chemical Equation Presented) 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 (τ1/2 = minutes to days at 20°C) to dihydropyrenes 2, dinitrile 1b shows no tendency to convert thermally to 2b at room temperature (τ1/2 > 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°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°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. © 2008 American Chemical Society.