Reductions with SmI2: mechanistic probe for distinguishing between two operational modes of proton transfer

The reduction of 1,1-diaryl-2,2-dicyanoethylenes with SmI2 in THF was studied in the presence of four proton donors: H2O, MeOH, i-PrOH and trifluoroethanol ( TFE). The kinetic order for the. first two is nearly unity at low proton donor concentrations and approaches four at high concentrations, whereas, for i-PrOH and TFE, the log-log plot is linear with a slope smaller than one. Detailed analysis shows that a curved log-log plot such as for H2O and MeOH is indicative of a major contribution by protonation within an ion pair of the radical anion and Sm+3 complexed to a variable number of proton donor molecules, whereas a linear plot is a result of protonation from the bulk solution.