Sustainable Ester Reduction by Aqueous SmI₂/Amine: A Bimetallic Proton-Coupled Electron Transfer Mechanism from Density Functional Theory Calculations

The mechanisms by which cyclic and acyclic aliphatic esters are reduced into their corresponding diols by SmI₂ in water or water/amine solutions have been investigated computationally using density functional theory (DFT). Based on the calculations, the reduction is proposed to be initiated by a two-electron, two-metal reduction of the substrate, resulting in the formation of a bimetallic Sm(III)(substrate²⁻)Sm(III) complex. The reaction pathway then involves the protonation of the oxygen atom of the doubly reduced ketyl intermediate. For activated ester-lactone reduction, the lowest-energy pathway reveals the important role of a specific first-hydration-shell water molecule, which acts as the proton source and facilitates the overall transformation. For aliphatic esters in binary water/amine solution, the amine enables a proton cascade required to fulfill the reduction. As a consequence, the role played by the amine towards the reduction of aliphatic esters is attributed primarily to a specific relay in a proton cascade, in addition to, rather than instead of, any concurrent effect on the reduction potential of SmI₂. The results establish a unified bimetallic proton-coupled electron transfer (PCET) mechanism for SmI₂-mediated ester reduction in aqueous and water/amine media, which may extend to other divalent lanthanides, broadening the range of aqueous-compatible single-electron reductants for green synthesis.