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.
In dioxane or ether singlet excited phenanthrene (PN) reacts with acetylacetonatoboron difluoride (AABF2) irreversibly to form a fluorescent exciplex with a well-defined isosbestic point in the low range of [AABF2] < 0.03 M. This exciplex is shown to be the precursor to the observed product of cis-9-acetyl-9,10-dihydro-10-(2-oxopropyl)phenanthrene by quantitative quenching analysis of the product and fluorescence. In contrast, the excitation of AABF2 in the presence of PN gives neither exciplex emission nor any products. The exciplex was studied on the basis of its static and timeresolved fluorescence properties to give the dipole moment of 11.2 D and its reaction pattern. The exciplex undergoes self-quenching by AABF2, and the quantum yield carries dependency on the square of its concentrations that would impose a nonlinear relation in the double reciprocal plot of 1/ vs 1/ [AABF2]. The plot started to curve upward at [AABF2] as low as 0.02 M owing to simultaneous complication from the competing and unproductive absorption by AABF2, particularly at a high concentration range. Experimentally, the isosbestic point is unfocused at [AABF2] ^0.025 M to indicate a small effect of self-quenching. In acetonitrile singlet excited PN preferentially undergoes electron transfer to give the phenanthrene cation radical without causing the cycloaddition.
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