Photochemistry of dicarbonyl-substituted benzenes: influence of steric and electronic factors in the cyclization and Diels-Alder trapping reactions of photoenols

Abstract

We have examined the photochemistry of a broad set of diketones 1-6 containing o-methyl substituents. Whereas sterically-congested diketones 1 and 2 are found to undergo diphotocyclization to afford high yields of bis-benzocyclobutenols (BCBs), photolysis of 3, 4 and 6, which are devoid of steric congestion, is found to lead to an intractable material; in these cases, the initially formed photoenols appear to derive stabilization from the second electron withdrawing group (electronic effect) such that other competing reactions become promoted. Quite remarkably, it is these ketones that undergo very efficient tandem photoenolization-Diels- Alder cycloaddition reactions in the presence of an excellent dienophile such as Nphenylmaleimide to yield novel Diels-Alder cycloadducts in near quantitative yields. Photolysis of sterically-encumbered ketones 1, 2 and 5 in the presence of N-phenylmaleimide is found to result in the suppression of the formation of benzocyclobutenols with virtually insignificant formation of the product; the short-lived photoenols appear to revert rapidly to the precursor ketones. Thus, we have unraveled a subtle role of steric and electronic factors in the diphotocyclization and 2-fold tandem photoenolization-Diels-Alder cycloaddition reactions of diketones.