Reduction of activated olefins by SmI2. Detouring the classical Birch mechanism and a negative order in SmI2

The reaction of an excess of 1,1-diaryl-2,2-dicyanoethylenes (1) with SmI2 is biphasic for olefin with at least one available para position. The first phase is completed in less than 0.5 s with the second phase extending over a few hundred seconds. This phase is second order with respect to the radical anion, which is formed in the dead-time of the mixing in the stopped flow spectrophotometer and is overall of -1 order in the initial concentration of SmI2. In this phase, a dimer is formed between two radical anions with the formation of a C-C bond between a benzylic and a para position. The second phase is enhanced by proton donors and shows an H/D kinetic isotope effect with MeOH. Minute amounts of ethylene glycol accelerated the reaction to such an extent that the second phase is "absorbed" into the first, rendering it rate determining. In this phase, the dianionic dimer disproportionates after protonation to furnish the neutral species and the anion, which after second protonation provides the reduced product. When the two para positions are occupied by substituents, the reaction takes place by the traditional Birch reduction sequence of electron-proton-electron-proton-transfer steps. It is shown that the detour mechanism, coupling followed by disproportionation, should be typical of olefin but not of carbonyl reduction. This difference stems from the dissimilarity in protonation rate on carbon and oxygen.