ALLENYL(VINYL)METHANE PHOTOCHEMISTRY - PHOTOCHEMISTRY OF 5-[2-(1,2-PROPADIENYL)-SUBSTITUTED ALKYLIDENE]-2,2-DIMETHYL-1,3-DIOXANE-4,6-DIONES

The Photochemistry of 5-[2-(1,2-propadienyl)-substituted alkylidene]-2 ,2-dimethyl-1,3-dioxane-4,6-diones has been studied. These acylals for med an intramolecular charge-transfer (CT) complex between the 1,2-pro padienyl and vinyl moieties in acetonitrile and in hexane, the direct photolyses of which gave (1-alkenylidene)cyclopropanes as the main pro ducts, and the intramolecular [2+2] cycloadducts, 3-alkylidene-2',2', yclo[2.1.0]pentane-2,5'-[1,3]dioxane]-4',6'-diones as minor products. The (1-alkenylidene)cyclopropanes were produced from the enol intermed iate via a preferred 1,5-hydrogen transfer of a diradical, which was f ormed via an intramolecular CT excited state. On the other hand, the a cetone-sensitized photolyses afforded mainly the intramolecular [2+2] cycloadducts. It is considered that the lowest triplet energies of the se acylals in acetonitrile were estimated to lie in the range of 250-2 90 kJ mol(-1), and that these energies were near to the values of the reported triplet energies of alpha,beta-unsaturated esters, because th e allene underwent a [2+2] cycloaddition to the triplet vinyl moiety, excited by triplet sensitization.