Steady state photolysis of bridgehead disubstituted dibenzobarrelenes and thermal isomerization of their photoproducts

Abstract

The photorearrangements of a few bridgehead disubstituted dibenzobarrelenes 5a-c and thermal transformations of their primary photoproducts 8a-c are described. Irradiation of 9-acetyl-10-methyl-substituted dibenzobarrelene 5a in benzene gave a mixture of the regioselective 8b-acetyl substituted dibenzosemibullvalene 8a (74%), involving a di-π-methane rearrangement and a novel polycyclic ketone 11 (7%), through δ-hydrogen abstraction. Irradiation of the disubstituted dibenzobarrelenes 5b and 5c, under identical conditions, gave the corresponding dibenzosemibullvalene derivatives 8b (85%) and 8c (81%), respectively. Interestingly, compound 8a underwent a facile reaction with methanol to give the acetal derivative 15. Thermolysis of the semibullvalene derivative 8a at 180-200°C for 2 h gave a dibenzopentalenofuran 12a, in 92% yield. Similarly, thermal transformations of 8b and 8c, under analogous conditions, gave the corresponding pentalenofurans 12b and 12c, respectively, in nearly quantitative yields. The kinetics of the thermal isomerization reactions were studied and the activation energies of 8a-c to 12a-c have been found to be 21.04, 23.42 and 24.74 kcal mol^-1, respectively. The structures of 5a, 8c, 11 and 15 were established unambiguously through X-ray crystallographic analysis.