Photochemical rearrangement of dibenzo[1,4]dioxins proceeds through reactive spirocyclohexadienone and biphenylquinone intermediates.

Photochemical studies on a range of model dibenzo[1,4]dioxins were performed in aqueous and organic solutions. The compounds were found to undergo a photochemically initiated aryl-ether bond homolysis that yields reactive 2-spiro-6'-cyclohexa-2',4'-dien-1'-one and subsequent 2,2'-biphenylquinone intermediates. Under steady-state irradiation, the 2,2'-biphenylquinones were observed to participate in excited state hydrogen abstraction from the organic solvent to give the corresponding 2,2'-dihydroxybiphenyls. In the absence of continued irradiation, 2,2'-biphenylquinones with electron donating substituents thermally rearrange to the corresponding oxepino[2,3-b]benzofurans, whereas the unsubstituted 2,2'-biphenylquinone and its derivatives with electron withdrawing groups thermally rearrange to the corresponding 1-hydroxydibenzofurans.