Stereoselective coupling of prochiral radicals with a chiral C2-Symmetric nitroxide

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