Photochemistry of 1,2-diketones. Part 5. Formation of dihydrodioxines, oxetanes, allylic ethers, and 1,2-adducts from alicyclic 1,2-diketones and alkenes

Abstract

The photoaddition of five alicyclic non-enolizing 1,2-diketones (1a–e), bipivaloyl, and biacetyl to various simple alkenes has been investigated. The dihydroxioxine formation dominates over the formation of oxetane, α-keto allylic ether, and 1,2-adduct (α-hydroxyketone) for the three cyclic 1,2-diketones (1b–d) that have a syn-periplanar (lowest) excited 1,2-diketo configuration. The four-membered ring diketone (1a) photo-fragments prior to its addition to the alkene. Biacetyl with an anti-periplanar excited 1,2-diketo configuration does not form dihydrodioxines. No information on the geometry of the excited 1,2-diketo configuration could be obtained with (1e) and (2), as the rate of α-cleavage [with (1e)] or intramolecular hydrogen abstraction of the anti-periplanar excited diketo moiety [with (1e) and (2)] greatly exceeds the rate of reaction with the alkene. Thirteen dihydrodioxines, twelve oxetanes, four allylic ethers, and fourteen 1,2-adducts have been identified. The formation of the dihydrodioxine and oxetane form ation has been discussed with special emphasis on (i) the nature of the reacting excited state (the reacting state is the lowest triplet n–π* state of the 1,2-diketone), (ii) the intermediacy of exciplexes and 1,4-diradicals, and (iii) the stereospecificity of the dihydrodioxine and oxetane formation.