The effect of proton donors on the facial stereoselectivity in SmI2 reduction of norcamphor.

The endo/exo product ratio in the reactions of SmI(2) with norcamphor in the presence of various proton donors was determined. The effect of MeOH, EtOH, trifluoroethanol (TFE), ethylene glycol (EG), and water was investigated at various concentrations of these proton donors. At low concentrations of EtOH, TFE, and EG, an endo/exo ratio near unit was found. This ratio increased as the concentration of the proton donor increased. However, MeOH and water gave a U-type curve, in a plot of the endo/exo ratio vs proton donor concentration. The difference between the two groups of proton donors was shown not to result from differences in their acidities or polarity effects. It is suggested that at low MeOH and water concentrations, the second electron transfer takes place from the dimer of SmI(2) rather than from the monomer. This bulky electron donor approaches the radical anion preferentially from the exo direction giving rise to the high endo/exo ratio at the left arm of the U-shaped curve. Comparison of kinetic and product H/D isotope effects shows that protonation on carbon, the step that locks the stereochemistry, is a post rate determining step.