SmI₂(H₂O)_n Reduction of Electron Rich Enamines by Proton-Coupled Electron Transfer

Abstract

Samarium diiodide in the presence of water and THF (SmI2(H2O)n) has in recent years become a versatile and useful reagent, mainly for reducing carbonyl-type substrates. This work reports the reduction of several enamines by SmI2(H2O)n. Mechanistic experiments implicate a concerted proton-coupled electron transfer (PCET) pathway, based on various evidence against initial outer-sphere electron transfer, proton transfer or substrate coordination. A thermochemical analysis indicates that the C-H bond formed in the rate determining step has a bond dissociation free energy (BDFE) of ~32 kcal mol−1. The O–H BDFE of the samarium aquo ion is estimated to be 26 kcal mol−1, which is among the weakest known X–H bonds of stable reagents. Thus SmI2(H2O)n should be able to form very weak C-H bonds. The reduction of these highly electron rich substrates by SmI2(H2O)n shows that this reagent is a very strong hydrogen atom donor as well as an outer sphere reductant.

Computational and Synthetic Investigation of Cationic Rearrangement in the Putative Biosynthesis of Justicane Triterpenoids

“…Thiophenol may act as a hydrogen atom transfer agent to enhance the diastereoselectivity of thermodynamic ketone reductions. After initial proton-coupled electron transfer, [19] it is possible that the polarizability of the S–H bond facilitates H-atom abstraction by the samarium-complexed ketyl radical, as opposed to proton transfer to the samarium alkoxide from other proton donors. To our delight, we found that the conditions of our diketone formation and subsequent reduction were amendable to a one-pot transformation, resulting in the requisite syn -diol in 77% yield.…”

A fresh approach to synthesizing ammonia from air and water

“…Ashida et al use this source of hydrogen atoms with a molybdenum catalyst to fix nitrogen. Considerable coordinationinduced bond weakening has previously been measured in SmI 2 -water mixtures, and used to make carbon-hydrogen bonds 11,12 .…”

Interconversion of Phosphinyl Radical and Phosphinidene Complexes by Proton Coupled Electron Transfer

“…[9] Thes carcity of catalytic PR-transfer might in part be attributed to alack of suitable oxidizing PR-transfer reagents, such as analogs of iminoiodane (ArI=NR) or azide (RN 3 ) nitrene sources.Primary phosphines are attractive precursors from athermochemical point of view due to lower intrinsic P À Hc ompared to N À Hb ond dissociation (free) energies (BD(F)Es;S cheme 1). [12] Amide ligand N À Hb ond activation by proton coupled electron transfer (PCET) is an active field of research. [12] Amide ligand N À Hb ond activation by proton coupled electron transfer (PCET) is an active field of research.…”