Photoaddition of α-diketones to olefins. Stereospecificity of the addition reaction

Abstract

Phenanthraquinone (PQ) photolytically adds to cis- and trans-alkenes to give a mixture of cis- and trans-1,4-dioxenes and two reduction products. The ratios of cis- to trans- 1,4-dioxenes remain the same irrespective of the photoaddition to cis- or trans-alkenes and are 1:1 for 2-butenes, 6:4 for 3-hexenes, and 8:2 for cyclohexene. A triplet excited phenanthraquinone is proposed as the reactive species which initiates the addition leading to the intermediate diradical 10. Depending on the nature of addenda the diradical 10 may be stabilized by a charge transfer structure in different conformations leading to the observed stereochemistry of the adducts. The PQ fails to add to fumaronitrile photolytically or thermally and thermally adds to 2-butenes only inefficiently. Biacetyl undergoes photoreduction readily in the presence of a large excess of cyclohexene.