Efficient photocycloaddition of phenanthroquinones with simple olefins

A series of substituted phenanthroquinones (PQ’s) was prepared, their phosphorescence spectra and decay rate constants, cyclic voltammograms, and photoreaction in the presence of tetramethylethylene (TME) were investigated. With 450 nm irradiation their triplet excited state cyclized with TME to give the corresponding dioxenes, 2′,2′,3′,3′-tetramethyl-1′,4′-dioxa-1′,2′,3′,4′-tetrahydrotriphenylene, cleanly and much faster than that with 300 nm irradiation. The pattern of the reaction was effected weakly in random fashion by the substituent at the 3-position of PQ’s. The photocycloaddition, as exemplified by PQ itself, is so efficient that it occurred even by room light and during optical spectroscopic recordings; its limiting quantum yield in benzene is established to be unity with 450 nm irradiation. It owes the unusual efficiency to a long lifetime of the PQ triplet excited state with the n – ∗ π configuration, and to the successful cyclization in every diffusion controlled collision as implicated by TME quenching of PQ phosphorescence intensity in CCl 4 solution. The triplet excited state reaction was supported by competitive quenching of the dioxene formation by triplet quenchers as well as by oxygen. These PQ’s do not fluoresce in solution, but show phosphorescence in solid solution with lifetimes of about 10 ms and in CCl 4 solution with lifetimes in the order of 100 μs in the room temperature range. PQ’s with 3-cycano, 3-chloro, 3-methoxy, and without substitutions photocyclized to cyclohexene to give the corresponding dioxenes with small stereochemical scrambles, that was interpreted that the extent of electron transfer in the 1,6-diradical stage is small. Similar photocycloadditions to isobutene gave a 1:1 mixture of two regio-isomers, this was taken as evidence for a direct radical attack on olefins to give 1,6-diradical intermediates.