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 N- phenylmaleimide 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.
Highlights
Benzocyclobutenols are considered as masked equivalents of o-xylylenols/dienols due to the fact that their thermolysis leads to the latter in a facile manner.[1]
We have shown recently that presence of an electron withdrawing group leads to stabilization of the dienols.[6]. In these cases, the initially formed photoenols 8, which are devoid of any steric congestion, are expected to be more stable to permit bimolecular thermal as well as other photochemical reactions to occur efficiently to account for the observed intractable polymeric material; it should be noted that Meador and co-workers have utilized the photoenolization of diketone 3 in the synthesis of polymers.10a The fact that a variety of monoketones undergo photocyclization to benzocyclobutenols, as shown by Wagner and coworkers,3a suggests that the location of a second electron withdrawing group as in ketones 3 and 4 modifies their reactivity
We have examined the photoreactivity of dicarbonyl compounds containing o-methyl groups to understand the factors that favor i) diphotocyclization to synthetically useful bisbenzocyclobutenols and ii) 2-fold tandem photoenolization and Diels-Alder cycloaddition reactions
Summary
Benzocyclobutenols are considered as masked equivalents of o-xylylenols/dienols due to the fact that their thermolysis leads to the latter in a facile manner.[1]. The diaryl ketones 1a and 2a, and aryl alkyl ketones 1b,c and 2b,c afford excellent yields (71-91%) of bis-benzocyclobutenols (BCBs); the fact that the latter were formed in a stepwise manner was established from 1H NMR monitoring, from which, it was not possible to determine the relative ratios of mono- and diphotocyclization products with increasing duration of irradiation, see Supplementary Information.
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