A Cathodic Hydrogen Atom Transfer Strategy for Electrochemical Carboxylation of Dibenzylic C(sp 3 )H Bonds With CO 2

Hydrogen atom, proton and electron transfer self-exchange and cross-reaction rates have been determined for reactions of Os(IV) and Os(III) aniline and anilide complexes. Addition of an H-atom to the Os(IV) anilide TpOs(NHPh)Cl(2) (Os(IV)NHPh) gives the Os(III) aniline complex TpOs(NH(2)Ph)Cl(2) (Os(III)NH(2)Ph) with a new 66 kcal mol(-1) N-H bond. Concerted transfer of H* between Os(IV)NHPh and Os(III)NH(2)Ph is remarkably slow in MeCN-d(3), with k(ex)(H*) = (3 +/- 2) x 10(-3) M(-1) s(-1) at 298 K. This hydrogen atom transfer (HAT) reaction could also be termed proton-coupled electron transfer (PCET). Related to this HAT process are two proton transfer (PT) and two electron transfer (ET) self-exchange reactions, for instance, the ET reactions Os(IV)NHPh + Os(III)NHPh(-) and Os(IV)NH(2)Ph(+) + Os(III)NH(2)Ph. All four of these PT and ET reactions are much faster (k = 10(3)-10(5) M(-1) s(-1)) than HAT self-exchange. This is the first system where all five relevant self-exchange rates related to an HAT or PCET reaction have been measured. The slowness of concerted transfer of H* between Os(IV)NHPh and Os(III)NH(2)Ph is suggested to result not from a large intrinsic barrier but rather from a large work term for formation of the precursor complex to H* transfer and/or from significantly nonadiabatic reaction dynamics. The energetics for precursor complex formation is related to the strength of the hydrogen bond between reactants. To probe this effect further, HAT cross-reactions have been performed with sterically hindered aniline/anilide complexes and nitroxyl radical species. Positioning steric bulk near the active site retards both H* and H(+) transfer. Net H* transfer is catalyzed by trace acids and bases in both self-exchange and cross reactions, by stepwise mechanisms utilizing the fast ET and PT reactions.

There have been many examples of the accelerating effects of acids in electron transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. Herein, we report a contrasting effect of acids in the ET, OAT, and HAT reactions of a nickel(III) complex, [NiIII(PaPy3*)]2+ (1) in acetone/CH3CN (v/v 19:1). 1 was synthesized by reacting [NiII(PaPy3*)]+ (2) with magic blue or iodosylbenzene in the absence or presence of triflic acid (HOTf), respectively. Sulfoxidation of thioanisole by 1 and H2O occurred in the presence of HOTf, and the reaction rate increased proportionally with increasing concentration of HOTf ([HOTf]). The rate of ET from diacetylferrocene to 1 also increased linearly with increasing [HOTf]. In contrast, HAT from 9,10-dihydroanthracene (DHA) to 1 slowed down with increasing [HOTf], exhibiting an inversely proportional relation to [HOTf]. The accelerating effect of HOTf in the ET and OAT reactions was ascribed to the binding of H+ to the PaPy3* ligand of 2; the one-electron reduction potential (Ered) of 1 was positively shifted with increasing [HOTf]. Such a positive shift in the Ered value resulted in accelerating the ET and OAT reactions that proceeded via the rate-determining ET step. On the other hand, the decelerating effect of HOTf on HAT from DHA to 1 resulted from the inhibition of proton transfer from DHA•+ to 2 due to the binding of H+ to the PaPy3* ligand of 2. The ET reactions of 1 in the absence and presence of HOTf were well analyzed in light of the Marcus theory of ET in comparison with the HAT reactions.

Photochemical hydrogen atom transfer (HT) and electron transfer (ET) reactions of 1,4-anthraquinone (1,4-AQ) have been studied in acetonitrile at 295 K by means of CIDEP (chemically induced dynamic electron polarization) techniques and laser flash photolysis. It was shown on the basis of CIDEP measurements that both HT and ET reactions in the excited state of 1,4-AQ originated from the triplet state. Quantitative investigations on the photochemical reactions of 1,4-AQ were carried out by laser photolysis at 355 nm. The HT reaction from 4-phenylphenol (PhPhOH) to the 3(π,π*) state of 1,4-AQ (31,4-AQ*) proceeded rapidly with a rate constant (kHT) of 5.3×109 dm3 mol−1 s−1, where the efficiencies for HT (ϕHT) and induced quenching (IQ, ϕIQ) were obtained to be 0.57 and 0.43, respectively. Similar HT reactions were also observed for 31,4-AQ* with phenol (kHT=2.3×109 dm3 mol−1 s−1, ϕHT=0.49, ϕIQ=0.51) and 2,6-di-tert-butylphenol (kHT=1.9×109 dm3 mol−1 s−1, ϕHT=0.47, ϕIQ=0.53). The observed rapid HT reactions were shown to be due not to hydrogen atom abstractions but to protic hydrogen atom transfer reactions. The ET reaction from 1,2,4,5-tetramethoxybenzene (TMB) to 31,4-AQ* took place with a rate constant (kET) as high as 8.5×109 dm3 mol−1 s−1, which was close to the diffusion-controlled rate constant with efficiencies of ET (ϕET=0.77) and IQ (ϕIQ=0.23). These fast reactions may proceed via triplet exciplexes between 31,4-AQ* and phenols (or TMB) with charge transfer character.

"Enthalpy-Entropy Compensation Effect" (EECE) is ubiquitous in chemical reactions; however, such an EECE has been rarely explored in biomimetic oxidation reactions. In this study, six manganese(IV)-oxo complexes bearing electron-rich and -deficient porphyrins are synthesized and investigated in various oxidation reactions, such as hydrogen atom transfer (HAT), oxygen atom transfer (OAT), and electron-transfer (ET) reactions. First, all of the six Mn(IV)-oxo porphyrins are highly reactive in the HAT, OAT, and ET reactions. Interestingly, we have observed a reversed reactivity in the HAT and OAT reactions by the electron-rich and -deficient Mn(IV)-oxo porphyrins, depending on reaction temperatures, but not in the ET reactions; the electron-rich Mn(IV)-oxo porphyrins are more reactive than the electron-deficient Mn(IV)-oxo porphyrins at high temperature (e.g., 0 °C), whereas at low temperature (e.g., -60 °C), the electron-deficient Mn(IV)-oxo porphyrins are more reactive than the electron-rich Mn(IV)-oxo porphyrins. Such a reversed reactivity between the electron-rich and -deficient Mn(IV)-oxo porphyrins depending on reaction temperatures is rationalized with EECE; that is, the lower is the activation enthalpy, the more negative is the activation entropy, and vice versa. Interestingly, a unified linear correlation between the activation enthalpies and the activation entropies is observed in the HAT and OAT reactions of the Mn(IV)-oxo porphyrins. Moreover, from the previously reported HAT reactions of nonheme Fe(IV)-oxo complexes, a linear correlation between the activation enthalpies and the activation entropies is also observed. To the best of our knowledge, we report the first detailed mechanistic study of EECE in the oxidation reactions by synthetic high-valent metal-oxo complexes.

Synergistic CO2 Mediation and Photocatalysis for α-Alkylation of Primary Aliphatic Amines

Photoredox catalysis has continued to expand at a rapid pace in recent decades both in the diversity 20 of the transformations and the abundance of the catalytic scenarios. By utilizing simple and mild 21 reaction conditions, photocatalysis renders an intuitive strategy to construct valuable molecules in a 22 green fashion. In this field, functionalization of unsaturated and inert bonds represents one of the most 23 powerful synthetic tools to build up architecturally complex molecules. methods. In addition, the ion-pair effect was observed to have a crucial influence on the efficiency, as 42 the iridium complex bearing chloride anion exhibited superior advantage than hexafluorophosphate 43 anion. 44In the terms of sp 3 C-H bond functionalization, 1,5-hydrogen atom transfer (HAT) has demonstrated 45 to be an efficient protocol to generate alkyl radical and proceed further transformations. Yi and co-46 workers described a photocatalytic chemodivergent approach to 1-pyrrolines and 1-tetralones via 47 intermolecular radical addition and switchable distal sp 3 C-H bond functionalization from alkyl 48 bromides and vinyl azides. Changing of the photocatalyst could chemoselectively control the 49 construction of C-N bond or C-C bond, which was probably resulted from the different pathways in 50 the initial stage, quenching of the excited photocatalyst by a reductive or oxidative route. Manipulating 51 the reaction conditions led to different catalytic scenarios and delivered chemodivergent synthesis. 52 Organic photocatalysts have been extensively explored and utilized in the photocatalysis, due to that 53 the reactions could occur under metal-free and environment-benign reaction conditions. Hajra and co-54 workers (Gupta et al., 2023) have made a comprehensive summarization of the employment of organic 55 photocatalysts in difunctionalization of alkenes and their mechanism pathways, highlighting the green 56 and sustainable strategies to construct complex molecules. By using this strategy, Wei and co-workers 57 developed a difunctionalization of alkenes to provide 3-(arylmethyl)chroman-4-ones via phosphine-58 mediated C-O bond activation and visible light-mediated C-C bond cleavage. This metal-and oxidant-59 free reaction system represents a reliable and scalable method to achieve acylation from readily 60 available carboxylic acids. In addition, further functionalization of the products could give rise to 61 diverse core structures which are of great interest in pharmaceutical chemistry. 62 Caballero and co-workers reported a systematically study of the photophysical properties of 63 depsipeptides and peptoids which were synthesized via a multicomponent reaction. By careful varying 64 the electron property of the substituents, different photophysical properties of the products were 65 observed. In addition, some promising photoprotectors and fluorescent probes were discovered in their 66 exploration. Theoretical calculations were in agreement with the experimental UV radiation spectra. 67In conclusion, this Research Topic focuses on recent advances of functionalization of inert or 68 unsaturated bonds via photoredox, presenting fruitful synthetic methodologies and mechanism research 69 achievements in this field. Many interesting discoveries such as Ion-pair effect, chemodivergent 70 synthesis, and hydrogen atom-transfer would largely increase the complexity of the reaction design, 71 and furnish more diverse and complex valuable molecules in organic synthesis. 72 1The authors declare that the research was conducted in the absence of any commercial or financial 74 relationships that could be construed as a potential conflict of interest. 75Author Contributions 76 YL conceived and wrote the manuscript. All authors provided comments and discussed the 77 contents, and approved this Editorial for publication. 78 3

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.

Review: 180 refs.

Coupled electron and proton transfer reactions play a key role in the mechanisms of biological energy transduction.1–3 Such reactions are also fundamental for artificial energy-related systems such as fuel cells, chemical sensors, and other electrochemical devices. Biological examples include, among others, cytochrome c oxidase,4,5 bc1 complex,6,7 and photosynthetic reaction centers.8,9 In such systems, electrons tunnel between redox cofactors of an enzyme, while the coupled protons are transferred either across a single hydrogen bond or between protonatable groups along special proton-conducting channels. In this paper general theories and models of coupled electron transfer/proton transfer (ET/PT) reactions are discussed. Pure electron transfer reactions in proteins have been thoroughly studied in the past, both experimentally10–17 and theoretically.18–25 The coupled reactions are relatively new and currently are gaining attention in the field.6,8,26–43 Two types of coupled reactions can be distinguished. In concerted electron and proton transfer reactions (denoted PCET in Refs. 29,30,43–45, although this term is also used more generally), both the ET and PT transitions occur in one step. Such concerted processes occur in reactions in which proton transfer is typically limited to one hydrogen bond; however, examples with multiple hydrogen bond rearrangements are also known.46 In sequential reactions, the transitions occur in two steps: ET/PT or PT/ET. Typically each individual step is uphill in energy, while the coupled reaction is downhill. A sequential reaction can proceed along two parallel channels: ET then PT (EP) or PT then ET (PE). In each channel the reaction involves two sequential steps: uphill activation, and then downhill reaction to the final product state. The lifetime of the activated complex is limited by the back reaction. The general formula for the rate of such reactions can be easily developed. In the context of bioenergetics issues, however, it is interesting to analyze all of the possible cases separately because each corresponds to a different mechanism: for example, an electron can go first and pull out a proton; alternatively, a proton can go first and pull out an electron; or an electron can jump back and forth between donor and acceptor and gradually pull out a proton. In enzymes involving coupled proton and electron transport, the exact mechanism of the reaction is of prime interest. First we will consider a simple four-state model of reactions where the proton moves across a single hydrogen bond; both concerted and sequential reactions will be treated. Then we will consider models for long-distance proton transfer, also denoted proton transport or proton translocation. Typically, electron transfer coupled to proton translocation in proteins involves an electron tunneling over a long distance between two redox cofactors, coupled to a proton moving along a proton conducting channel in a classical, diffusion-like random walk fashion. Again, separately the electron and proton transfer reactions are typically uphill, while the coupled reaction is downhill in energy. The schematics of this process is shown in Fig. 1. The kinetics of such reactions can be much different from those involving proton transfer across a single hydrogen bond. In this paper, we will discuss the specifics of such long-distance proton-coupled reactions. Fig. 1 Schematics of the electron transfer reaction coupled to proton translocation. In the reaction, an electron is tunneling over a long distance between two redox cofactors, O and R, and a coupled proton is transferred over a proton conducting channel. The ... Following the review of theoretical concepts, a few applications will be discussed. First the phenoxyl/phenol and benzyl/toluene self-exchange reactions will be examined. The phenoxyl/phenol reaction involves electronically nonadiabatic proton transfer and corresponds to a proton-coupled electron transfer (PCET) mechanism, whereas the benzyl/toluene reaction involves electronically adiabatic proton transfer and corresponds to a hydrogen atom transfer (HAT) mechanism. Comparison of these two systems provides insight into fundamental aspects of electron-proton interactions in these types of systems. Next a series of theoretical calculations on experimentally studied PCET reactions in solution and enzymes will be summarized, along with general predictions concerning the dependence of rates and kinetic isotope effects (the ratio of the rate constants for hydrogen and deuterium transfer) on system properties such as temperature and driving force. The final application that will be discussed is cytochrome c oxidase (CcO). CcO is the terminal component of the electron transport chain of the respiratory system in mitochondria and is one of the key enzymes responsible for energy generation in cells. The intricate correlation between the electron and proton transport via electrostatic interactions, as well as the kinetics of the coupled transitions, appear to be the basis of the pumping mechanism in this enzyme.

Visible-light photoredox-catalyzed selective carboxylation of C(sp3)−F bonds with CO2

Chapter 26 - Methodology for the Measurement of Antioxidant Capacity of Coffee: A Validated Platform Composed of Three Complementary Antioxidant Assays

ConspectusMolecular photoelectrocatalysis, which combines the merits of photocatalysis and organic electrosynthesis, including their green attributes and capacity to offer novel reactivity and selectivity, represents an emerging field in organic chemistry that addresses the growing demands for environmental sustainability and synthetic efficiency. This synergistic approach permits access to a wider range of redox potentials, facilitates redox transformations under gentler electrode potentials, and decreases the use of external harsh redox reagents. Despite these potential advantages, this area did not receive significant attention until 2019, when we and others reported the first examples of modern molecular photoelectrocatalysis. These studies showcased the immense synthetic potential of this hybrid strategy, which not only inherits the strengths of its parent fields but also unlocks unprecedented reactivity and selectivity, enabling challenging transformations under mild conditions while minimizing the reliance on external stoichiometric harsh oxidants or reductants.In this Account, we present our efforts to develop photoelectrocatalytic strategies that leverage homogeneous catalysts to facilitate diverse radical reactions. By integrating electrocatalysis with key photoinduced processes such as single electron transfer (SET), ligand-to-metal charge transfer (LMCT), and hydrogen atom transfer (HAT), we have established photoelectrocatalytic methods to transform substrates such as organotrifluoroborates, arenes, carboxylic acids, and alkanes into reactive radical intermediates. These intermediates subsequently engage in heteroarene C-H functionalization reactions. Importantly, under these photoelectrochemical conditions with homogeneous catalysts, reactive radical intermediates generated in the bulk solution readily participate in efficient radical reactions without undergoing further overoxidation into carbocations, a common challenge in conventional electrochemical systems.By further integration of photoelectrocatalysis with asymmetric catalysis, we have developed photoelectrochemical asymmetric catalysis (PEAC), which proves to be efficient in the enantioselective synthesis of chiral nitriles. This approach involves two relay catalytic cycles: the initial photoelectrocatalytic process engenders benzylic radicals from precursors such as alkyl arenes, benzylic carboxylic acids, and aryl alkenes, and these C-radicals are then subjected to enantioselective cyanation in a subsequent copper-electrocatalytic cycle.Within the realm of oxidative photoelectrochemical transformations, the anode serves as a crucial component for recycling or generating the photocatalyst, while the cathode promotes proton reduction. This dual functionality enables oxidative transformations via H2 evolution, eliminating the reliance on external chemical oxidants. Furthermore, the adaptability of electrochemical systems, achieved through precise manipulation of electric current or potential, ensures meticulous control over the generation and turnover of multiple catalytic species of diverse electrochemical properties. This unique tunability allows for exceptional control over the catalytic process. As a result, despite being a relatively nascent field, molecular photoelectrocatalysis has become instrumental in enabling numerous challenging transformations that were once difficult or required harsh conditions.

Lignin, a three-dimensional aromatic polymer formed by the cross-linking of arylpropanol units via C-C/C-O bonds, faces the critical scientific challenge of achieving selective cleavage of these bonds to enable its valorization and the directional synthesis of renewable aromatic compounds. Photocatalysis offers a novel strategy for targeted bond dissociation through precise regulation of electron transfer pathways and radical generation modes. This review systematically elucidates the mechanisms underlying photocatalytic C-C and C-O bond cleavage in lignin systems. For C-C bond cleavage, three primary pathways include: generation of oxygen-centered radicals via ligand-to-metal charge transfer (LMCT) or proton-coupled electron transfer (PCET) processes, inducing β-C-C bond cleavage; generation of carbon-centered radicals via hydrogen atom transfer (HAT) or single-electron transfer (SET) processes, followed by C-C bond cleavage via oxygen participation. For C-O bond cleavage, the main pathways are: a stepwise oxidation-reduction mechanism driven by HAT or SET; generation of carbon-centered radicals via HAT or SET, inducing β-C-O bond cleavage; or activation of lignin models or auxiliary reagents via SET to form reactive radicals inducing C-O bond cleavage. These pathways universally rely on photocatalytically generated radicals (e.g., oxygen/carbon-centered radicals), which redistribute electrons and significantly weaken β-C-C and β-C-O bonds. Based on these insights, we propose feasible strategies for efficient native lignin depolymerization through catalyst electronic structure optimization and reaction microenvironment modulation, providing a theoretical framework for the photoelectrocatalytic valorization of lignin resources.

The functionalization of the ubiquitous carbon-hydrogen (C-H) bonds in organic molecules via transition-metal catalysis represents an ideal molecular transformation for the efficient synthesis of pharmaceutical and agrochemical compounds, fulfilling the principles of both atom economy and step economy. However, sp3 C-H bonds in readily available hydrocarbon feedstocks are inert, and conventional single-catalyst activation strategies have fundamental limitations, including the need for high temperatures and the introduction of directing groups. To address these challenges, we developed a reaction design based on a synergistic catalyst system that integrates multiple catalysts with orthogonal functions, enabling a previously unprecedented and broadly applicable strategy for sp3 C-H bond functionalization via the catalytic generation of organometallic species. Specifically, a ternary catalyst system comprising a hydrogen atom transfer (HAT) catalyst that can promote C-H bond cleavage under mild conditions, a metal catalyst that enables diverse and stereoselective transformations, and a photoredox catalyst that mediates the electron-transfer process was established. This system enabled sp3 C-H bond functionalization under mild conditions with high functional-group compatibility. Through the identification of uniquely effective catalyst systems, this strategy enables novel catalytic nucleophilic addition reactions and acceptorless dehydrogenation directly from simple hydrocarbon substrates.

A comparison of hydride, hydrogen atom, and proton-coupled electron transfer reactions is presented. Herein, hydride and hydrogen atom transfer refer to reactions in which the electrons and protons transfer between the same donor and acceptor, while proton-coupled electron transfer (PCET) refers to reactions in which the electrons and protons transfer between different centers. Within these definitions, hydride and hydrogen atom transfer reactions are typically electronically adiabatic, hence evolving on a single electronic surface. In contrast, PCET reactions are often electronically nonadiabatic since the electron transfers a longer distance through a proton transfer interface. For all three types of reactions, solute reorganization is important, particularly the hydrogen donor--acceptor mode. Solvent reorganization is critical for hydride transfer and PCET, which involve significant solute charge redistribution, but not for hydrogen atom transfer. Theoretical descriptions and simulation methodology for all three types of reactions are presented, as well as experimentally relevant applications to hydride transfer in enzymes and PCET in solution.