Efficient photocycloaddition of phenanthroquinones with simple olefins

A series of substituted phenathroquinones (PQ's) was prepared, their phosph orescence spectra and decay rate constants, cyclic voltammograms, and photo reaction in the presence of tetramethylethylene (TME) were investigated. Wi th 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 irr adiation. The pattern of the reaction was effected weakly in random fashion by the substituent at the 3-position of PQ's, The photocycloaddition, as e xemplified by PQ itself, is so efficient that it occurred even by room ligh t and during optical spectroscopic recordings; its limiting quantum yield i n benzene is established to be unity with 450 nm irradiation. It owes the u nusual efficiency to a long lifetime of the PQ triplet excited state with t he n-*pi configuration, and to the successful cyclization in every diffusio n controlled collision as implicated by TME quenching of PQ phosphorescence intensity in CCL4 solution. The triplet excited state reaction was support ed by competitive quenching of the dioxene formation by triplet quenchers a s well as by oxygen. These PQ's do not fluoresce in solution, but show phos phorescence in solid solution with lifetimes of about 10ms and in CCl4 solu tion with lifetimes in the order of 100 mus in the room temperature range. PQ's with 3-cycano, 3-chloro, 3-methoxy, and without substitutions photocyc lized to cyclohexene to give the corresponding dioxenes with small stereoch emical scrambles, that was interpreted that the extent of electron transfer in the 1,6-diradical stage is small. Similar photocycloadditions to isobut ene 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. (C ) 2001 Published by Elsevier Science B.V.