Proton-coupled electron transfer in the reduction of carbonyls using SmI 2 -H 2 O: implications for the reductive coupling of acyl-type ketyl radicals with SmI 2 -H 2 O.

Samarium diiodide-water (SmI2 -H2 O) 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 -H2 O remains unclear. An intriguing scenario involves an initial proton-coupled electron transfer (PCET) mechanism from SmI2 -H2 O 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 -H2 O and quantitatively assess the feasibility of concerted PCET in the reduction of carbonyl groups using SmI2 -H2 O. Reduction potentials of all substrates have been calculated. The data argue against concerted PCET from SmI2 -H2 O to carbonyl substrates.