Stereoselective coupling of prochiral radicals with a chiral C-2-Symmetricnitroxide

The coupling reaction between a chiral C-2-symmetric nitroxide, trans-2,5-d imethyl-2,5-diphenylpyrrolidin-1-oxyl (DPPO; 1), and a series of stabilized secondary prochiral radicals was studied to determine the factors that aff ect stereoselectivity. Both steric and electronic perturbations on the sele ctivity by the substituents of the prochiral radical were observed. The eff ects of temperature, solvent polarity, and solvent viscosity were examined. High selectivity for reactions carried out in solvents of low viscosity pr ovides evidence for the formation of an encounter complex on the reaction p ath. Ab initio calculations on simplified model systems predict the C-O-N a ngle of attack to be greater than 110 degrees at a carbon-oxygen bond-formi ng distance of approximately 2.2 Angstrom, although no transition state was found.