Comparative study of the pyrolysis, photoinduced electron transfer (PET), and laser-jet and 185-nm photochemistry of alkyl-substituted bicyclic azoalkanes

The gas-phase pyrolysis, photoinduced electron transfer (PET), and laser-jet and 185-nm photochemistry of 2,3-diazabicyclo[2.2.1]hept-2-ene (1a), syn-7-methyl-2,3-diazabicyclo[2.2.1]hept-2-ene (syn-1b), anti-7-methyl-2,3-diazabicyclo[2.2.1]hept-2-ene (anti-1b), 1,4-dimethyl-2,3-diazabicyclo[2.2.1]hept-2-ene (1c), 7,7-dimethyl-2,3-diazabicyclo[2.2.1]hept-2-ene (1d), and syn-7-(1-methylethyl)-2,3-diazabicyclo[2.2.1]hept-2-ene (syn-1e) were investigated and the results of their product studies compared with one another. Pyrolysis and conventional direct and benzophenone-sensitized 350-nm photolysis of the azoalkanes 1 yielded the bicyclo[2.1.0]pentanes 2 and negligible amounts of cyclopentenes 3. PET and benzophenone-sensitized laser-jet and 185-nm photolysis of the azoalkanes 1 led to significant quantities of cyclopentene derivatives 3, a behavior that is attributed to radical cation-type 1,3-cyclopentadiyl intermediates, which subsequently suffer hydrogen or alkyl migration. The polar character of the radical cation D.+ is clearly demonstrated by the Wagner-Meerwein rearrangement into 2,3-dimethylcyclopentene (3'd) in the PET and 185-run photolysis of azoalkane Id. When the corresponding 1,3-cyclopentadiyl D.. was generated in the pyrolysis of 5,5-dimethylbicyclo[2.1.0]pentane (2d), the 3,3-dimethyl-1,4-pentadiene (4d) was obtained as the exclusive reaction product; instead of methyl migration, fragmentation into the 1,4-diene took place. The PET chemistry of the stereochemically labeled azoalkanes syn- and anti-1b revealed that the radical cations D.+ have a puckered geometry, because a stereochemical memory effect was observed for the cyclopentene products 3. Specifically, the pseudoaxial substituent at the stereolabeled center migrates with preference, which speaks for a coplanar arrangement for the rearrangement in D''. The common and distinct mechanistic features of the denitrogenation processes of the various thermal and photochemical activation modes will be discussed.