PHOTOCHEMICAL AND ESR SPECTRAL EVIDENCE FOR A STEREOSELECTIVE REARRANGEMENT OF RADICAL CATIONS DERIVED FROM AZOALKANES AND BICYCLOPENTANES

Studies of the photosensitized 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 (3a) as the rearrangement product, while their isomers 1b and 2b afforded predominantly 3-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.+ detected initially at 80-90 K following the radiolytic oxidation of 1a,b in CF3CCl3 rearranged stereoselectively into the olefin radical cations 3ab.+ at 105 K. The ESR results further establish that, in the puckered conformations of 1a,b.+, the pseudo-axial substituent on the methylene bridge is in almost perfect coplanar alignment with the radical cation 2p orbital lobes, thus facilitating its stereoselective migration. Also, in agreement with PET results, matrix ESR studies demonstrated that, on radiolytic oxidation of the azoalkanes 2a,b, the olefin radical cations 3a,b.+ were formed with high selectivity, although no precursor radical cations were detected in this case.