The photoaddition of 1,3-diketonatoboron difluorides with benzene derivatives

The boron difluoride complexes of 2-acetylcyclohexanone, 2-acetylcyclopentanone, and acetylacetone (abbreviated as ACHBF2, ACPBF2, and AABF2) were irradiated in the presence of benzene to give the 1:1 adducts as the primary photoadducts; for certain BF2 complexes, toluene, chlorobenzene, benzonitrile, and methyl benzoate were also used as substrates. The 1,5-diketone photoadducts were assumed to form by a [2+2] photocycloaddition followed by cyclobutane opening and hydrolysis to give 1,2 adducts. They undergo a variety of secondary thermal reactions, probably acid catalyzed, to give enol ethers, enol acetates, acetophenones, and ketonylacetophenones. The efficiency of these secondary reactions determines the final products. Photoaddition with a monosubstituted benzene preferentially occurs at the 3,4 bond without regioselectivity. Under oxygen, ACHBF2 photolytically reacts with benzene to give a secondary oxidation product of a 10-membered cyclic alkylphe-none, which is proven by X-ray crystallographic analysis to have the benzene ring and carbonyl group in orthogonal orientation. It is shown that the singlet excited state ACHBF2 initiates the photoaddition, probably through the formation of the benzene exciplex, which could be detected by its emission. While the Stern–Volmer rates are small, the quantum yield of photoaddition products is as high as 0.12–0.19 under limiting conditions. Key words: [2+2] photocycloaddition, non-planar alkanophenone, macrocyclic alkanophenone, boron difluoride complexes, photoaddition to benzenes.