Photoinduced electron-transfer cope rearrangements of 3,6-diaryl-2,6-octadienes and 2,5-diaryl-3,4-dimethyl-1,5-hexadienes: Stereospecificity and an unexpected formation of the bicyclo[2.2.0]hexane derivatives

Under the 9,10-dicyanoanthracene-sensitized photoinduced electron-transfer conditions, (Z,Z)-, (E,E)-, (E,Z)-3,6-diaryl-2,6-octadiene and (d,l),(meso) -2,5-diaryl-3,4-dimethyl-1,5-hexadiene stereospecifically undergo the Cope rearrangement to give a Cope photostationary mixture. Remarkably, the photo induced electron-transfer Cope rearrangements of the 4-methylphenyl derivat ives are concurrent with the formation of trans- or endo,cis-1,4-bis(4-meth ylphenyl)-2,3-dimethylbicyclo[2.2.0]hexane in a Cope photostationary mixtur e. Observed stereospecificity of the Cope rearrangement and the formation o f the bicyclo[2.2.0]hexane derivatives demonstrate the intermediacies of bo th the chair and boat 1,4-diaryl-1,2-dimethylcyclohexane-1,4-diyl and catio n radical intermediates in a Cope rearrangement cycle. Photoreactions of tr ans- and exo,cis-1,4-diaryl-5,6-dimethyl-2,3-diazabicyclo[2.2.2]oct-2-enes further support the interventions of the diyl intermediates in the Cope rea rrangement cycle. By photoacoustic analysis, a cation radical cyclization-d iradical cleavage mechanism is proposed for the photoinduced electron-trans fer Cope rearrangement-of the title dienes.