Stereospecificity in the geminate recombination of 1,3-diphenylpropyl radical pairs

1,3,4,6-Tetraphenylhexanes (3m) and (3r), formed by the geminate recombination of 1,3-diphenylpropyl (1) radical pairs generated at 90 deg from diastereomerically pure meso- and rac-1,1',3,3'-tetraphenylazopropane (2m) and (2r), are obtained with substantial retention (up to 46% d.e.) of precursor stereochemistry. The stable nitroxyls 1,1,3,3-tetramethylisoindolin-2-yloxyl (4) and 2,2,6,6-tetramethylpiperidin-l-yloxyl (5) were used as scavengers to isolate the geminate reaction (30-35% cage effect). A slight selectivity (6-8% d.e.) in favour of meso-1,3,4,6-tetraphenylhexane (3m) is found in the encounter reaction of (1). The nature of the molecular motions associated with the partial loss of stereochemistry during the reaction (2)→(1)→(3) is discussed. N.m.r. spectroscopy, particularly 13C, was used for correlating product and precursor stereochemistries and for diastereomer identification. The structural assignments were confirmed by single-crystal X-ray diffraction analysis of (3r). 31 refs., 2 figs., 4 tabs