Organic Photochemistry with 6.7-eV Photons. The Divergent Photobehavior of exo- and endo-7-Methyl-2-norcarene

Abstract

The photochemistry of exo- and endo-7-methylbicyclo[4.1.0]hept-2-ene (7-methyl-2-norcarene) upon direct and toluene-sensitized photolysis has been investigated. Direct photolysis of the two isomers in hydrocarbon solution with monochromatic 214-nm light leads to widely divergent photobehavior. Ring opening, yielding cis-5-methyl-l,3,6-heptatriene (via a formal [2π + 2w + 2σ] process), and exo-endo isomerization are major decomposition pathways in both cases, accounting for 30-45% of the observed products. The formation of 7-methylnorbornene and 4-methylbicyclo[3.2.0]hept-2-ene is observed to proceed stereospecifically in each case, while the formation of cyclopropyl ring opening/hydrogen migration products appears to depend strongly on the orientation of the methyl group in the starting isomer. Common biradical intermediates are thus believed not to be involved in the direct photolyses of 2 and 3. Toluene-sensitized photolysis of the two isomers leads to formation of exo-4-methylbicyclo[3.2.0]hept-2-ene (the photochemical vinylcyclopropane rearrangement) and exo-endo isomerization. These results are rationalized in terms of the expected behavior of two commnn biradical intermediates, which are formed with different efficiencies from the isomeric 2 and 3 upon triplet sensitization. © 1983, American Chemical Society. All rights reserved.