Deuterium kinetic isotope effects as redox mechanistic criterions

This account article focuses on deuterium kinetic isotope effects (KIEs) used as criterions to elucidate redox mechanisms including proton-, hydrogen- and hydride-transfer reactions. Hydrogen atom transfer (HAT) is composed of two elementary steps: electron transfer (ET) and proton transfer (PT), while hydride transfer is composed of three elementary steps: ET, PT, and ET. Large tunneling effects are often observed for proton-coupled electron-transfer (PCET) reactions of metal-oxygen complexes in which ET occurs to the metal center and PT occurs simultaneously to the ligand, exhibiting large KIEs. Whether HAT proceeds via sequential ET/PT, PT/ET, or concerted PCET (cPCET) depending on the redox properties of hydrogen donors and acceptors to exhibit different KIEs. Whether hydride transfer also proceeds via sequential ET/PT/ET, PT/ET/ET, or cPCET/ET depending on the redox properties of hydride donors and acceptors to exhibit different KIEs. Temperature dependence of KIEs for aldehyde deformylation reactions has enabled to distinguish two reaction pathways: one is a HAT and the other is a nucleophilic addition. The change of the mechanism from cPCET to sequential ET/PT is made possible by binding acids to the hydrogen and hydride acceptors when no KIE is observed. Inverse KIEs are also discussed for acid (or deuteron)-promoted ET reactions.

Automated summary

This article focuses on deuterium kinetic isotope effects (KIEs) as criteria for elucidating redox mechanisms, including proton transfer, hydrogen transfer, and hydride transfer reactions. Different reaction pathways and mechanisms are discussed based on the redox properties of donors and acceptors, exhibiting unique KIEs. Temperature dependency of KIEs in aldehyde deformylation reactions distinguishes between two reaction pathways, shedding light on the mechanisms involved.