Photochemical and ESR spectral evidence for a stereoselective rearrangement of radical cations derived from azoalkanes and bicyclopentanes

Abstract

Studies of photosynthesized electron transfer (PET) reactions of anti/syn-5-methylbicyclo[2.1.0]pentane (1a,b) and syn/anti-7-methyl-2-3-diazabicyclo[2.2.1]hept-2-ene (2a,b) have revealed a remarkable stereochemical memory effect. Thus, 1a and 2a furnished only 1-methylcyclopentene (3b). The pathway responsible for the stereoselective olefin formation can be assigned to a radical cation rearrangement. This was established on the basis of direct ESR evidence showing that the 1,3-diyl radical cations 1a,b{sup .+} detected initially at 80-90 K following the radiolytic oxidation of 1a,b in CF{sub 3}CCI{sub 3} rearranged stereoselectively into the olefin radical cations 3a,b{sup .+} at 105 K. The ESR results further establish that, in the puckered conformations of 1a,b{sup .+}, the psuedo-axial substituent on the methylene bridge is almost perfect coplanar alignment with the radical cation 2p orbital lobes, thus facilitating is stereoselective migration. Also, in agreement with (PET) results, matrix ESR studies demonstrated that, on radiolytic oxidation of the azoalkanes 2a,b, the olefin radical cation 3a,b{sub .+} were formed with high selectivity, although no precursor radical cations were detected in this case. 29 refs., 5 figs., 2 tabs.