Proton-coupled electron transfer in the reduction of carbonyls using SmI₂-H₂O: Implications for the reductive coupling of acyl-type ketyl radicals with SmI₂-H₂O

Samarium diiodide-water (SmI₂-H₂O) reagents have emerged as some of the most practical systems enabling reduction and reductive cyclizations of ketyl radicals. Recently, this reaction manifold has been extended to acyl-type radicals generated from cyclic polar carboxylic acid derivatives. However, the relationship between the fundamental electron- and proton-transfer steps in the generation of ketyl-type radicals with SmI₂-H₂O remains unclear. An intriguing scenario involves an initial proton-coupled electron transfer (PCET) mechanism from SmI₂-H₂O to the carbonyl group. Herein, we calculate with high accuracy bond dissociation free energies (BDFE) for the O-H bond in ketyl radicals in 14 cyclic and acyclic ketone, ester, imide and amide substrates and in anthracene relevant to reductions with SmI₂-H₂O and quantitatively assess the feasibility of concerted PCET in the reduction of carbonyl groups using SmI₂-H₂O. Reduction potentials of all substrates have been calculated. The data argue against concerted PCET from SmI₂-H₂O to carbonyl substrates.