Organic Photochemistry with 6.7-eV Photons: The Photochemistry of Bicyclo[4.1.0]hept-2-ene (2-Norcarene)

Abstract

The photochemistry of bicyclo[4.1.0]hept-2-ene (2-norcarene) upon direct and triplet-sensitized photolysis has been investigated. The results differ significantly from those of previous studies on related systems. Direct photolysis with monochromatic radiation in the region 185-230 nm leads to a variety of products, of which cis-1,3,6-heptatriene, the major product, is shown by deuterium labeling studies to result from formal electrocyclic ring opening in a manner analogous to the 1,3-cyclohexadiene/1,3,5-hexatriene interconversion. The evidence suggests that this process may occur via a concerted mechanism in the singlet state. The other products arise from activation of the two cyclopropane bonds that are in conjugation with the olefinic group. The product distribution is independent of both solvent and excitation wavelength and is unchanged by the addition of naphthalene. Toluene-sensitized photolysis results in the formation of cis-1,3,6-heptatriene and bicyclo[3,2.0]hept-2-ene, which can be rationalized as resulting from decay of a common biradical intermediate. © 1983, American Chemical Society. All rights reserved.