A Continuum of Proton-Coupled Electron Transfer Reactivity

Abstract

CONSPECTUS: Proton-coupled electron transfer (PCET) covers a wide range of reactions involving the transfer(s) of electrons and protons. The best-known PCET reaction, hydrogen atom transfer (HAT), has been studied in detail for more than a century. HAT is generally described as the concerted transfer of a hydrogen atom (H• ≡ H+ + e−) from one group to another, Y + H−X → Y−H + X, but a strict definition of HAT has been difficult to establish. Distinctions are more challenging when the transfer of “H•” involves e− and H+ that transfer to/from spatially distinct sites or even completely separate reagents (multiple-site concerted proton−electron transfer, MS-CPET). MS-CPET reactivity is increasingly proposed in biological and synthetic contexts, and some reactions typically described as HAT more resemble MS-CPET. Despite that HAT and MS-CPET reactions “look different,” we argue here that these reactions lie on a reactivity continuum, and that they are governed by many of the same key parameters. This Account walks the reader across this PCET reactivity continuum, using a series of studies to show the strong similarities of reactions that move protons and electrons in seemingly different ways. To prepare for our stroll, we describe the thermochemical and kinetic frameworks for HAT and MS-CPET. The driving force for a solution HAT reaction is most easily discussed as the difference in the bond dissociation free energies (BDFEs) of the reactants and products. BDFEs can be analyzed as sums of electron and proton transfer steps and can therefore be obtained from pKa and E° values. Even though MS-CPET reactions do not make and break H−X bonds in the same way as HAT, the same thermochemical description can be used with the introduction of an effective BDFE (BDFEeff). The BDFEeff of a reductant/acid pair is the free energy of that pair to form H•, which can be obtained from pKa and E° values in an analogous fashion to a standard BDFE. When the PCET thermochemistry is known, HAT and PCET rate constants can be understood and often predicted using linear free energy relationships (the Brønsted catalysis law) and Marcus theory type approaches. After this background, we walk the reader through a continuum of PCET reactivity. Our journey begins with a study of metalmediated HAT from hydrocarbon substrates to a metal-oxo complex and travels to the MS-CPET end of the reactivity spectrum, involving the transfer of H+ and e− from the hydroxylamine TEMPOH to two completely separate molecules. These examples, and those in between, are all analyzed within the same thermodynamic and kinetic framework. A description of the first examples of MS-CPET with C−H bonds uses the same framework and highlights the importance of hydrogen bonding and preorganization. The examples and analyses show that the reactions along the PCET continuum are more similar than they are different, and that attempts to divide these reactions into subcategories can obscure much of the essential chemistry. We hope that developing the many common features of these reac...

Triplet state promoted reaction of SO2with H2O by competition between proton coupled electron transfer (pcet) and hydrogen atom transfer (hat) processes

“…The pcet reactions cover a wide range of chemical processes that have attracted increasing interest over the last decades because they play a key role in biological processes, photocatalysis and solar energy conversion. 3,5,10,12 The mechanism discussed in this work for the reaction of sulfur dioxide with water to yield HOSO and OH radicals belongs to a subcategory of bimolecular pcet processes that involve prior photoexcitation followed by intersystem crossing to a triplet state. We have shown that in this reaction pathway a very low activation barrier needs to be overcome, in contrast to the high stability of SO 2 in its ground electronic state.…”

“…Hydrogen atom transfer originated by radicals is one of the most fundamental processes for several areas of chemistry, from biological processes, to chemical synthesis, materials science, hydrocarbon combustion or atmospheric chemistry. [1][2][3][4][5][6][7][8][9][10][11][12] In the context of the chemistry of the troposphere, the oxidation processes involving abstraction of hydrogen atoms by radicals are among the most important reactions and many of these processes are well known. 7,13 Very recently, it has been shown that the triplet excited state of SO 2 can react with water producing hydroxyl radicals and is the most oxidizing species in the Earth's atmosphere, 14 and it has also been predicted that this reaction is enhanced by several orders of magnitude at the air-water (airclouds) interface.…”

A photochemical and theoretical study of the triplet reactivity of furano- and pyrano-1,4-naphthoquionones towards tyrosine and tryptophan derivatives

“…The similarity of the values for the quenching rate constants of triplet naphthoquinones by NATrpME and indole and by NATyrME and phenol as well as the formation of the radical pair naphthoquinone semiquinone/indolyl (or phenoxyl) radicals suggests that similar reaction mechanisms may be operating in both quenching processes. A series of previous experimental and theoretical studies from our groups, 58,71,[84][85][86][87] have clearly indicated that a proton coupled electron transfer (PCET) mechanism [88][89][90][91][92][93][94][95][96] operates in the hydrogen atom transfer reaction for both phenols and indole.…”

“…Thus hydrogen abstraction from phenol by the triplet quinone is best characterized as an asynchronous PCET mechanism. [88][89][90][91][92][93][94][95][96] (b) Hydrogen abstraction from indole. A similar analysis of hydrogen abstraction from indole by T 1 -NQF was investigated using a single molecular geometry (01À2) in order to compare the reactivity of indole and phenol (Fig.…”

Merging Regiodivergent Catalysis with Atom‐Economical Radical Arylation

“…[7] R-2' ' may be considered ar adical s-complex after the radical addition of R-2 to the arene (Scheme 2). [8] The overall process is an atom-economical catalytic radical reaction and proceeds under much milder conditions than typical Friedel-Crafts alkylations [9] that require strong electrophilic activation. [8] The overall process is an atom-economical catalytic radical reaction and proceeds under much milder conditions than typical Friedel-Crafts alkylations [9] that require strong electrophilic activation.…”