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 photoinduced electron-transfer Cope rearrangements of the 4-methylphenyl derivatives are concurrent with the formation of trans- or endo,cis-1,4-bis(4-methylphenyl)-2,3-dimethylbicyclo[2.2.0]hexane in a Cope photostationary mixture. Observed stereospecificity of the Cope rearrangement and the formation of the bicyclo[2.2.0]hexane derivatives demonstrate the intermediacies of both the chair and boat 1,4-diaryl-1,2-dimethylcyclohexane-1,4-diyl and cation radical intermediates in a Cope rearrangement cycle. Photoreactions of trans- 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 rearrangement cycle. By photoacoustic analysis, a cation radical cyclization-diradical cleavage mechanism is proposed for the photoinduced electron-transfer Cope rearrangement of the title dienes.
Ikeda
Hiroshi
H
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan and Department of Chemistry, University of Rochester, Rochester, New York 14627.
Takasaki
Toshihiko
T
Takahashi
Yasutake
Y
Konno
Akinori
A
Matsumoto
Masao
M
Hoshi
Yosuke
Y
Aoki
Takashi
T
Suzuki
Takanori
T
Goodman
Joshua L.
JL
Miyashi
Tsutomu
T