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

Abstract

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 H2 O. 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.

Pushing SmI2 Reactions towards the Limit‐ Entropy Driven Reduction of a Benzene Ring

“…Since water is known to be a good complexing agent to SmI 2 and may compete with the ligands attached to the benzene ring, we have optimized the reaction conditions for BNO and found that the maximum yield is obtained with 0.2 M of water, similar to the value previously found for the water/amine/SmI 2 reactions . Thus, under these conditions, the binding of SmI 2 to BNO is not hampered by the presence of water at this concentration.…”

“…We showed that, under these conditions, the rate of the back electron transfer is significantly slowed down by deprotonating a water molecule attached to the Sm +3 . This generation of a hydroxide anion attached to the Sm +3 reduces the effective charge on the samarium and significantly decreases its ability to pull the electron back from the radical anion . As a result, the forward reaction is considerably enhanced.…”

Aza versus Oxophilicity of SmI2: A Break of a Paradigm

“…UV/Visible spectra study in the presence of Et 3 N (0.05 M–0.5 m ) shows no change in the shape of SmI 2 spectrum (Figure S12). It known that under similar condition, water significantly changes the shape of SmI 2 spectrum . The enthalpy of complexation for water and Et 3 N measured through ITC experiments was found to be comparable (−2.5 kcal mol −1 for water and −3.3 kcal mol −1 for Et 3 N).…”

Task-Dependent Coordination Levels of SmI2

“…Rate of reactions were monitored by following the disappearance of the SmI 2 -additive complex absorbance. Reduction of anthracene, 3-methyl-2-butanone, and cyclohexanone resulted in 9,10-dihydroanthracene, 3-methyl-2-butanol, and cyclohexanol, respectively. The reduction of benzyl chloride provided a mixture of toluene and 1,2-diphenylethane …”