Central and Lateral Bicyclo[1.1.0]butane Bond Cleavage with Subsequent Wagner-Meerwein Rearrangements or Carbene Formation in the 185-nm Photolysis of Tricyclo[3.1.0.02,6]hexane, Tricyclo[4.1.0.02,7]heptane, and Tricyclo[5.1.0.02,8]octane

Article abstract of DOI:10.1021/ja00012a027

The 185-nm photochemistry of tricyclop[3.1.0.0^2,6]hexane, tricyclo[4.1.0.0^2,7] heptane, [1,7-d₂]tricyclo[4.1.0.0^2,7]heptane, tricyclo[5.1.0.0^2,8]octane, and [1-d]tricyclo[5.1.0.0^2,8]octane was investigated. Tricyclo[5.1.0.0^2,8]octane yields bicyclo[4.2.0]oct-7-ene, tricyclo[4.1.0.0^2,7]heptane yields 85% bicyclo[3.2.0]hept-6-ene and 15% 3-methylenecyclohexene, and tricyclo[3.1.0.0^2,6]hexane yields 39% 3-methylenecyclopentene, 15% 1,3-cyclohexadiene, 26% trans-1,3,5-hexatriene, and 20% cis-1,3,5-hexatriene. From the deuterium-labeling studies, it is concluded that, in the case of the tricyclooctane, the central bicyclobutane bonds cleave in the primary step to give radical cationic or zwitterionic species that undergo a Wagner-Meerwein rearrangement. Also, in the case of tricycloheptane, this is the dominating pathway but lateral C-C bond cleavage with subsequent carbene and product formation takes place to the extent of ca. 15%. For tricyclohexane, this pathway becomes the major route. Our photomechanistic observations are in good agreement with earlier theoretical investigations on the relative energetic ordering of the bicyclobutane HOMOs, in that the product composition reflects this.