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.