Deciphering a 20-Year-Old Conundrum: The Mechanisms of Reduction by the Water/Amine/SmI2 Mixture

The reaction of SmI2 with the substrates 3‐methyl‐2‐butanone, benzyl chloride, p‐cyanobenzyl chloride, and anthracene were studied in the presence of water and an amine. In all cases, the water content versus rate profile shows a maximum at around 0.2 M H2O. The rate versus amine content profile shows in all cases, except for benzyl chloride, saturation behavior, which is typical of a change in the identity of the rate‐determining step. The mechanism that is in agreement with the observed data is that electron transfer occurs in the first step. With substrates that are not very electrophilic, the intermediate radical anions lose the added electron back to samarium(III) relatively quickly and the reaction cannot progress efficiently. However, in a mixture of water/amine, the amine deprotonates a molecule of water coordinated to samarium(III). The negatively charged hydroxide, which is coordinated to samarium(III), reduces its electrophilicity, and therefore, lowers the rate of back electron transfer, which allows the reaction to progress. In the case of benzyl chloride, in which electron transfer is rate determining, deprotonation by the amine is coupled to the electron‐transfer step. Secret behind the magic: The mechanisms of the reactions of SmI2 with the substrates 3‐methyl‐2‐butanone, benzyl chloride, p‐cyanobenzyl chloride, and anthracene are revealed (see scheme). In reversible electron‐transfer reactions, the role of the amine is to lengthen the lifetime of the radical anion. In dissociative electron‐transfer reactions, electron transfer is coupled to proton transfer.