Reprinted from the Journal of the American Chemical Society, 117, 7826-7827 (1995).

©American Chemical Society, 1995.

Orbital Symmetry and the Photochemical Ring Opening of Cyclobutene

William J. Leigh,* J. Alberto Postigo, and Paul C. Venneri

Contribution from the Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1

Abstract: The photochemistry of two bicyclic alkylcyclobutenes has been investigated in hydrocarbon solution using monochromatic 214-nm excitation. Direct irradiation of cis-4,4,6,7-tetramethylbicyclo[3.2.0]-hept-1(5)-ene leads to the formation of three isomeric 1,2-bis(ethylidene)-3,3-dimethylcyclopentanes in the ratio EE:EZ:ZE = 5.5:1.3:1.0 at 1-5% conversion. The EZ:ZE diene ratio obtained from irradiation of the cyclobutene is identical within experimental error to the EZ:ZE ratio obtained from direct (254-nm) irradiation of the EE-diene. Similarly, direct irradiation of cis-6,7-dimethylbicyclo[3.2.0]hept-1(5)-ene affords E,E- and E,Z-1,2-bis(ethylidene)cyclopentane in a ratio (EE/EZ = 2.0) which is identical (within somewhat larger error limits) to that calculated from the ratio of the quantum yields for non-productive decay and EE->EZ isomerization for direct (254-nm) photolysis of the E,E-diene. The results are consistent with a ring opening mechanism involving fully orbital-symmetry controlled disrotatory ring opening to yield the E,E-diene in a singlet electronic excited state (i.e., adiabatically), which ultimately leads to a mixture of formally-allowed and -forbidden dienes due to torsional relaxation of the initially formed, excited E,E-diene.