Photochemical reactions of 1,2-diketones with silyl enol ethers

Results arising from the current study demonstrate that UV irradiation-induced photoaddition reactions of the 1,2-diketones, acenaphthenequinone, 9,10-phenanthrenequinone, and benzil, with silyl enol ethers derived from propionaldehyde and isobutyraldehyde take place to form a variety of adducts. Product formation in these cases takes place mainly via two mechanistic pathways, one initiated by single electron transfer (SET) and the other by [2 + 2]-cycloaddition. In addition, observations show that the competition between these pathways depends on the nature of the substrates. An exploration of the photoreactions of acenaphthenequinone revealed that photoinduced SET is the predominant process occurring when the dimethyl-substituted silyl enol ether serves as a reactant, while cycloaddition and other excited state reactions become competitive with SET when the mono-methyl substituted silyl enol ether is used as a substrate. In the case of 9,10-phenanthrenequinone, photoreactions take place by [4 + 2]-cycloaddition routes regardless of the nature of the silyl enol ether employed. Finally, irradiation of benzil in the presence of both silyl enol ethers leads to exclusive or predominant formation of products arising by [2 + 2]-cycloaddition and photoinduced benzoyl radical forming alpha-cleavage.