Reprinted from the Canadian Journal of Chemistry, 74, 951-964 (1996).
© National Research Council of Canada, 1996.
William J. Leigh,* J. Alberto Postigo, and K. C. Zheng
Contribution from the Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1
Abstract: The photochemistry of the cis- and trans-isomers of a series of dimethylbicyclo[n.2.0]-alk-(n+2)-enes (n=2-5) in pentane solution is described. Photolysis of these compounds using monochromatic 193- or 214-nm light sources results in ring-opening to yield the corresponding 1,2-bis(1-ethylidene)cycloalkanes (C4-C7) in high chemical and quantum yields. In all cases, the reaction proceeds with a high (70-90%) degree of disrotatory stereoselectivity. Quantum yields for direct cis,trans-photoisomerization of the isomeric E,E- and E,Z-1,2-bis(1-ethylidene)-cycloalkanes have also been determined. The product distributions from photolysis of the cyclobutenes are wavelength dependent, but for 214-nm excitation, the isomeric diene distributions obtained from cyclobutene ring opening agree fairly closely with those calculated assuming that the process involves purely disrotatory ring opening to yield a single diene isomer in the lowest excited singlet state. The results are consistent with an orbital-symmetry controlled, adiabatic mechanism for ring opening.