A new strategy to efficiently cleave and form C-H bonds using proton-coupled electron transfer

A molecular-level mechanistic framework for interfacial proton-coupled electron transfer kinetics

Electrochemical proton-coupled electron transfer (PCET) reactions can proceed via an outer-sphere electron transfer to solution (OS-PCET) or through an inner-sphere mechanism by interfacial polarization of surface-bound active sites (I-PCET). Although OS-PCET has been extensively studied with molecular insight, the inherent heterogeneity of surfaces impedes molecular-level understanding of I-PCET. Herein we employ graphite-conjugated carboxylic acids (GC-COOH) as molecularly well-defined hosts of I-PCET to isolate the intrinsic kinetics of I-PCET. We measure I-PCET rates across the entire pH range, uncovering a V-shaped pH-dependence that lacks the pH-independent regions characteristic of OS-PCET. Accordingly, we develop a mechanistic model for I-PCET that invokes concerted PCET involving hydronium/water or water/hydroxide donor/acceptor pairs, capturing the entire dataset with only four adjustable parameters. We find that I-PCET is fourfold faster with hydronium/water than water/hydroxide, while both reactions display similarly high charge transfer coefficients, indicating late proton transfer transition states. These studies highlight the key mechanistic distinctions between I-PCET and OS-PCET, providing a framework for understanding and modelling more complex multistep I-PCET reactions critical to energy conversion and catalysis.

Luminescence and Excited-State Reactivity in a Heteroleptic Tricyanido Fe(III) Complex

Harnessing sunlight via photosensitizing molecules is key for novel optical applications and solar-to-chemical energy conversion. Exploiting abundant metals such as iron is attractive but becomes challenging due to typically fast nonradiative relaxation processes. In this work, we report on the luminescence and excited-state reactivity of the heteroleptic [FeIII(pzTp)(CN)3]- complex (pzTp = tetrakis(pyrazolyl)borate), which incorporates a σ-donating trispyrazolyl chelate ligand and three monodentate σ-donating and π-accepting cyanide ligands. Contrary to the nonemissive [Fe(CN)6]3-, a broad emission band centered at 600 nm at room temperature has been recorded for the heteroleptic analogue attributed to the radiative deactivation from a 2LMCT excited state with a luminescence quantum yield of 0.02% and a lifetime of 80 ps in chloroform at room temperature. Bimolecular reactivity of the 2LMCT excited state was successfully applied to different alcohol photo-oxidation, identifying a cyanide-H bonding as a key reaction intermediate. Finally, this research demonstrated the exciting potential of [Fe(pzTp)(CN)3]- as a photo-oxidant, paving the way for further exploration and development of emissive Fe-based photosensitizers competent for photochemical transformations.

Reconciling Imbalanced and Nonadiabatic Reactivity in Transition Metal-Oxo-Mediated Concerted Proton Electron Transfer (CPET)

Recently, there have been several experimental demonstrations of how the rates of concerted proton electron transfer (CPET) are affected by stepwise thermodynamic parameters of only proton (ΔG°PT) or electron (ΔG°ET) transfer. Semiclassical structure-activity relationships have been invoked to rationalize these linear free energy relationships, but it is not clear how they would manifest in a nonadiabatic reaction. Using density functional theory calculations, we demonstrate how a decrease in ΔG°PT can lead to transition state imbalance in a nonadiabatic framework. We then use these calculations to anchor a theoretical model that reproduces experimental trends with ΔG°PT and ΔG°ET. Our results reconcile predictions from semiclassical transition state theory with models that treat proton transfer quantum mechanically in CPET reactivity, make new predictions about the importance of basicity for uphill CPET reactions, and suggest similar treatments may be possible for other nonadiabatic reactions.

Electron-Transfer-Enabled Concerted Nucleophilic Fluorination of Azaarenes: Selective C-H Fluorination of Quinolines

Direct C-H fluorination is an efficient strategy to construct aromatic C-F bonds, but the cleavage of specific C-H bonds in the presence of other functional groups and the high barrier of C-F bond formation make the transformation challenging. Progress for the electrophilic fluorination of arenes has been reported, but a similar transformation for electron-deficient azaarenes has remained elusive due to the high energy of the corresponding Wheland intermediates. Nucleophilic fluorination of electron-deficient azaarenes is difficult owing to the identity of the Meisenheimer intermediate after fluoride attack, from which fluoride elimination to regenerate the substrate is favored over hydride elimination to form the product. Herein, we report a new concept for C-H nucleophilic fluorination without the formation of azaarene Meisenheimer intermediates through a chain process with an asynchronous concerted F--e--H+ transfer. The concerted nucleophilic aromatic substitution strategy allows for the first successful nucleophilic oxidative fluorination of quinolines.

Titanium Salalen Catalyzed Enantioselective Benzylic Hydroxylation

The titanium complex of the cis-1,2-diaminocyclohexane (cis-DACH) derived Berkessel-salalen ligand is a highly efficient and enantioselective catalyst for the asymmetric epoxidation of terminal olefins with hydrogen peroxide ("Berkessel-Katsuki catalyst"). We herein report that this epoxidation catalyst also effects the highly enantioselective hydroxylation of benzylic C-H bonds with hydrogen peroxide. Mechanism-based ligand optimization identified a novel nitro-salalen Ti-catalyst of the highest efficiency ever reported for asymmetric catalytic benzylic hydroxylation, with enantioselectivities of up to 98 % ee, while overoxidation to ketone is marginal. The novel nitro-salalen Ti-catalyst also shows enhanced epoxidation efficiency, as evidenced by e.g. the conversion of 1-decene to its epoxide in 90 % yield with 94 % ee, at a catalyst loading of 0.1 mol-% only.

Enroute sustainability: metal free C-H bond functionalisation

The term "C-H functionalisation" incorporates C-H activation followed by its transformation. In a single line, this can be defined as the conversion of carbon-hydrogen bonds into carbon-carbon or carbon-heteroatom bonds. The catalytic functionalisation of C-H bonds using transition metals has emerged as an atom-economical technique to engender new bonds without activated precursors which can be considered as a major drawback while attempting large-scale synthesis. Replacing the transition-metal-catalysed approach with a metal-free strategy significantly offers an alternative route that is not only inexpensive but also environmentally benign to functionalize C-H bonds. Recently metal free synthetic approaches have been flourishing to functionalize C-H bonds, motivated by the search for greener, cost-effective, and non-toxic catalysts. In this review, we will highlight the comprehensive and up-to-date discussion on recent examples of ground-breaking research on green and sustainable metal-free C-H bond functionalisation.

Inexpensive and bench stable diarylmethylium tetrafluoroborates as organocatalysts in the light mediated hydrosulfonylation of unactivated alkenes

In this paper, we present the synthetic potential of diarylmethylium tetrafluoroborates as catalysts for the visible light promoted hydrosulfonylation of unactivated alkenes. For the first time, these salts, which are bench stable and easily preparable on a multi-gram scale, were employed as organocatalysts. Interestingly, a catalyst loading of only 1 mol% allowed sulfone products to be efficiently obtained from good-to-excellent yields with high functional-group tolerance and scalability up to 15 mmol of alkene. The mechanistic study, both experimental and computational, presented here, revealed an alternative mechanism for the formation of the key sulfonyl radical. Indeed, the photoactive species was proved not to be the diarylcarbenium salt itself, but two intermediates, a stable S-C adduct and an ion couple, that were formed after its interaction with sodium benzenesulfinate. Upon absorbing light, the ion couple could reach an excited state with a charge-transfer character which gave the fundamental sulfonyl radical. A PCET (proton-coupled electron transfer) closes the catalytic cycle reforming the diarylcarbenium salt.

A Proton-Coupled Electron Transfer Strategy to the Redox-Neutral Photocatalytic CO2 Fixation

Herein, we report our study on the design and development of a novel photocarboxylation method. We have used an organic photoredox catalyst (PC, 4CzIPN) and differently substituted dihydropyridines (DHPs) in combination with an organic base (1,5,7-triazabicyclodec-5-ene, TBD) to access a proton-coupled electron transfer (PCET) based manifold. In depth mechanistic investigations merging experimental analysis (NMR, IR, cyclic voltammetry) and density-functional theory (DFT) calculations reveal the key activity of a H-bonding complex between the DHP and the base. The thermodynamic and kinetic benefits of the PCET mechanism allowed the implementation of a redox-neutral fixation process leading to synthetically relevant carboxylic acids (18 examples with isolated yields up to 75%) under very mild reaction conditions. Finally, diverse product manipulations were performed to demonstrate the synthetic versatility of the obtained products.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Electrochemical C(sp3)-H Lactonization of 2-Alkylbenzoic Acids toward Phthalides

Herein, we report direct electrochemical C(sp³)-H lactonization of 2-alkylbenzoic acids toward phthalides. The reaction provides a wide substrate scope of 2-alkylbenzoic acids bearing primary to tertiary C(sp³)-H bonds by utilizing a graphite anode, dichloromethane (DCM) solvent, hexafluoroisopropanol (HFIP) cosolvent, and n-Bu₄NClO₄ electrolyte. Our synthetic approach offers a simple, intuitive, and atom-economical protocol to synthesize various phthalides (25 examples, up to 92% yield) and obtain other 5- and 6-membered lactones (10 examples, up to 83% yield).

Gas-Phase Mechanism of O.- /Ni2+ -Mediated Methane Conversion to Formaldehyde

The gas-phase reaction of NiAl₂ O₄ ⁺ with CH₄ is studied by mass spectrometry in combination with vibrational action spectroscopy and density functional theory (DFT). Two product ions, NiAl₂ O₄ H⁺ and NiAl₂ O₃ H₂ ⁺ , are identified in the mass spectra. The DFT calculations predict that the global minimum-energy isomer of NiAl₂ O₄ ⁺ contains Ni in the +II oxidation state and features a terminal Al-O^.- oxygen radical site. They show that methane can react along two competing pathways leading to formation of either a methyl radical (CH₃ ⋅) or formaldehyde (CH₂ O). Both reactions are initiated by hydrogen atom transfer from methane to the terminal O^.- site, followed by either CH₃ ⋅ loss or CH₃ ⋅ migration to an O^2- site next to the Ni²⁺ center. The CH₃ ⋅ attaches as CH₃ ⁺ to O^2- and its unpaired electron is transferred to the Ni-center reducing it to Ni⁺ . The proposed mechanism is experimentally confirmed by vibrational spectroscopy of the reactant and two different product ions.

Proton-Coupled Electron-Transfer Reduction of Dioxygen: The Importance of Precursor Complex Formation between Electron Donor and Proton Donor

The proton-coupled electron transfer (PCET) reaction has drawn extensive attention for its widespread occurrence in chemistry, biology, and materials science. The mechanistic studies via model systems such as tyrosine and phenol oxidation have gradually deepened the understanding of PCET reactions, which was widely accepted and applied to bond activation and transformation. However, direct PCET activation of nonpolar bonds such as the C-H bond, O₂, and N₂ has yet to be explored. Herein, we report that the interaction between electron donor and proton donor could overcome the barrier of direct O₂ activation via a concerted electron-proton transfer mechanism. This work provides a new strategy for developing direct PCET activation of nonpolar bonds.

Intriguing Chloride: Involvement of Chloride Ions in Proton Transfers

The proton transfer from carbon to a chloride ion and the proton transfer to a molecule of water promoted by chloride ions in the acid-catalyzed formation of hydroxamic acids from aldehydes and substituted nitrosobenzenes in mixed solvents have been proposed based on experimental and theoretical investigations. The formation of uncommon contact ion pairs consisting of the nitrosocarbinolic cation intermediate and a chloride anion, followed by the proton transfer from a C-H moiety of the cation intermediate, has been proposed. The influence of chloride on the proton transfer to a water molecule of the solvent-separated nitrosocarbinolic-cation–chloride ion pair was investigated too. The insights are based on the obtained kinetic and other evidence with regard to (1) influences of chloride anions on the observed reaction rates and primary kinetic isotope effects (PKIE) in the reaction; (2) the observed variation of the PKIE-s and rates of the reaction when perchlorate anions are present along with the chloride ions; and (3) the consideration of a model of the nitrosocarbinolic-cation-intermediate—chloride ion pair and transition structure for the proposed proton transfers based on the ab initio calculations.

Structural and Thermodynamic Effects on the Kinetics of C-H Oxidation by Multisite Proton-Coupled Electron Transfer in Fluorenyl Benzoates

Our recent experimental and theoretical investigations have shown that fluorene C-H bonds can be activated through a mechanism in which the proton and electron are transferred from the C-H bond to a separate base and oxidant in a concerted, elementary step. This multisite proton-coupled electron transfer (MS-PCET) mechanism for C-H bond activation was shown to be facilitated by shorter proton donor-acceptor distances. With the goal of intentionally modulating this donor-acceptor distance, we have now studied C-H MS-PCET in the 3-methyl-substituted fluorenyl benzoate (2-Flr-3-Me-BzO^-). This derivative was readily oxidized by ferrocenium oxidants by initial C-H MS-PCET, with rate constants that were 6-21 times larger than those for 2-Flr-BzO^- with the same oxidants. Structural comparisons by X-ray crystallography and by computations showed that addition of the 3-methyl group caused the expected steric compression; however, the relevant C···O^- proton donor-acceptor distance was longer, due to a twist of the carboxylate group. The structural changes induced by the 3-Me group increased the basicity of the carboxylate, weakened the C-H bond, and reduced the reorganization energy for C-H bond cleavage. Thus, the rate enhancement for 2-Flr-3-Me-BzO^- was due to effects on the thermochemistry and kinetic barrier, rather than from compression of the C···O^- proton donor-acceptor distance. These results highlight both the challenges of controlling molecules on the 0.1 Å length scale and the variety of parameters that affect PCET rate constants.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Strategies for switching the mechanism of proton-coupled electron transfer reactions illustrated by mechanistic zone diagrams

The mechanism by which proton-coupled electron transfer (PCET) occurs is of fundamental importance and has great consequences for applications, e.g. in catalysis. However, determination and tuning of the PCET mechanism is often non-trivial. Here, we apply mechanistic zone diagrams to illustrate the competition between concerted and stepwise PCET-mechanisms in the oxidation of 4-methoxyphenol by Ru(bpy)₃³⁺-derivatives in the presence of substituted pyridine bases. These diagrams show the dominating mechanism as a function of driving force for electron and proton transfer (ΔG⁰_ET and ΔG⁰_PT) respectively [Tyburski et al., J. Am. Chem. Soc., 2021, 143, 560]. Within this framework, we demonstrate strategies for mechanistic tuning, namely balancing of ΔG⁰_ET and ΔG⁰_PT, steric hindrance of the proton-transfer coordinate, and isotope substitution. Sterically hindered pyridine bases gave larger reorganization energy for concerted PCET, resulting in a shift towards a step-wise electron first-mechanism in the zone diagrams. For cases when sufficiently strong oxidants are used, substitution of protons for deuterons leads to a switch from concerted electron–proton transfer (CEPT) to an electron transfer limited (ETPT_lim) mechanism. We thereby, for the first time, provide direct experimental evidence, that the vibronic coupling strength affects the switching point between CEPT and ETPT_lim, i.e. at what driving force one or the other mechanism starts dominating. Implications for solar fuel catalysis are discussed.

The mechanism by which proton-coupled electron transfer (PCET) occurs is of fundamental importance and has great consequences for applications, e.g. in catalysis.

Free Energies of Proton-Coupled Electron Transfer Reagents and Their Applications

We present an update and revision to our 2010 review on the topic of proton-coupled electron transfer (PCET) reagent thermochemistry. Over the past decade, the data and thermochemical formalisms presented in that review have been of value to multiple fields. Concurrently, there have been advances in the thermochemical cycles and experimental methods used to measure these values. This Review (i) summarizes those advancements, (ii) corrects systematic errors in our prior review that shifted many of the absolute values in the tabulated data, (iii) provides updated tables of thermochemical values, and (iv) discusses new conclusions and opportunities from the assembled data and associated techniques. We advocate for updated thermochemical cycles that provide greater clarity and reduce experimental barriers to the calculation and measurement of Gibbs free energies for the conversion of X to XH_n in PCET reactions. In particular, we demonstrate the utility and generality of reporting potentials of hydrogenation, E°(V vs H₂), in almost any solvent and how these values are connected to more widely reported bond dissociation free energies (BDFEs). The tabulated data demonstrate that E°(V vs H₂) and BDFEs are generally insensitive to the nature of the solvent and, in some cases, even to the phase (gas versus solution). This Review also presents introductions to several emerging fields in PCET thermochemistry to give readers windows into the diversity of research being performed. Some of the next frontiers in this rapidly growing field are coordination-induced bond weakening, PCET in novel solvent environments, and reactions at material interfaces.

C-H oxidation in fluorenyl benzoates does not proceed through a stepwise pathway: revisiting asynchronous proton-coupled electron transfer

2-Fluorenyl benzoates were recently shown to undergo C–H bond oxidation through intramolecular proton transfer coupled with electron transfer to an external oxidant. Kinetic analysis revealed unusual rate-driving force relationships. Our analysis indicated a mechanism of multi-site concerted proton–electron transfer (MS-CPET) for all of these reactions. More recently, an alternative interpretation of the kinetic data was proposed to explain the unusual rate-driving force relationships, invoking a crossover from CPET to a stepwise mechanism with an initial intramolecular proton transfer (PT) (Costentin, Savéant, Chem. Sci., 2020, 11, 1006). Here, we show that this proposed alternative pathway is untenable based on prior and new experimental assessments of the intramolecular PT equilibrium constant and rates. Measurement of the fluorenyl 9-C–H pK_a, H/D exchange experiments, and kinetic modelling with COPASI eliminate the possibility of a stepwise mechanism for C–H oxidation in the fluorenyl benzoate series. Implications for asynchronous (imbalanced) MS-CPET mechanisms are discussed with respect to classical Marcus theory and the quantum-mechanical treatment of concerted proton–electron transfer.

2-Fluorenyl benzoates were recently shown to undergo C–H bond oxidation through intramolecular proton transfer coupled with electron transfer to an external oxidant.

Bimodal Evans-Polanyi Relationships in Hydrogen Atom Transfer from C(sp3)-H Bonds to the Cumyloxyl Radical. A Combined Time-Resolved Kinetic and Computational Study

The applicability of the Evans-Polanyi (EP) relationship to HAT reactions from C(sp3)-H bonds to the cumyloxyl radical (CumO•) has been investigated. A consistent set of rate constants, kH, for HAT from the C-H bonds of 56 substrates to CumO•, spanning a range of more than 4 orders of magnitude, has been measured under identical experimental conditions. A corresponding set of consistent gas-phase C-H bond dissociation enthalpies (BDEs) spanning 27 kcal mol-1 has been calculated using the (RO)CBS-QB3 method. The log kH' vs C-H BDE plot shows two distinct EP relationships, one for substrates bearing benzylic and allylic C-H bonds (unsaturated group) and the other one, with a steeper slope, for saturated hydrocarbons, alcohols, ethers, diols, amines, and carbamates (saturated group), in line with the bimodal behavior observed previously in theoretical studies of reactions promoted by other HAT reagents. The parallel use of BDFEs instead of BDEs allows the transformation of this correlation into a linear free energy relationship, analyzed within the framework of the Marcus theory. The ΔG⧧HAT vs ΔG°HAT plot shows again distinct behaviors for the two groups. A good fit to the Marcus equation is observed only for the saturated group, with λ = 58 kcal mol-1, indicating that with the unsaturated group λ must increase with increasing driving force. Taken together these results provide a qualitative connection between Bernasconi's principle of nonperfect synchronization and Marcus theory and suggest that the observed bimodal behavior is a general feature in the reactions of oxygen-based HAT reagents with C(sp3)-H donors.

Visible Light-Promoted Magnesium, Iron, and Nickel Catalysis Enabling C(sp3)-H Lactonization of 2-Alkylbenzoic Acids

A mild and practical C(sp³)-H lactonization protocol has been achieved by merging photocatalysis and magnesium (iron, nickel) catalysis. A diverse range of 2-alkylbenzoic acids with a variety of substitution patterns could be transformed into the corresponding phthalide products. Based on the mechanistic experimentation and reported prior studies, a possible mechanism for the benzylic oxidative lactonization reaction was proposed with the hypothetic photoactive ternary complex formed between the 2-alkylbenzoic acid substrate, magnesium ion, and bromate anion.

Electrocatalytic Reduction of C-C π-Bonds via a Cobaltocene-Derived Concerted Proton-Electron Transfer Mediator: Fumarate Hydrogenation as a Model Study

Reductive concerted proton-electron transfer (CPET) is poorly developed for the reduction of C-C π-bonds, including for activated alkenes that can succumb to deleterious pathways (e.g., a competing hydrogen evolution reaction or oligomerization) in a standard electrochemical reduction. We demonstrate herein that selective hydrogenation of the C-C π-bond of fumarate esters can be achieved via electrocatalytic CPET (eCPET) using a CPET mediator comprising cobaltocene with a tethered Brønsted base. High selectivity for electrocatalytic hydrogenation is observed only when the mediator is present. Mechanistic analysis sheds light on two distinct kinetic regimes based on the substrate concentration: low fumarate concentrations operate via rate-limiting CPET followed by an electron-transfer/proton-transfer (ET/PT) step, whereas high concentrations operate via CPET followed by a rate-limiting ET/PT step.

Hydrogen Atom Transfer from Benzyl Alcohols to N-Oxyl Radicals. Reactivity Parameters

A model for predicting the rate constants of hydrogen atom transfer (HAT) from the α-C-H bond of p-substituted benzyl alcohols to N-oxyl radicals was proposed. To quantify the factors governing the reactivity of both N-oxyl radicals and benzyl alcohols, multivariate regression analysis was performed using various combinations of reactivity parameters. The analysis was based on a 2D array of 35 HAT reactions, the rate constants of which span 4 orders of magnitude. The proposed polyparameter equation approximates the experimental rate constants of reactions with high accuracy using three independent parameters: Brown and Okamoto's substituent constants σ⁺ in alcohol molecules and the spin population on O and N atoms in the N-O^• fragment of N-oxyl radicals [calculated by DFT/B3LYP/6-31G(d,p)]. The rate constants of HAT reactions from p-substituted benzyl alcohols to a series of aryl-substituted phthalimide-N-oxyl radicals containing either electron-withdrawing or electron-donating substituents (4-Cl, 4-HOOC, 4-CH₃O), quinolinimide-N-oxyl, benzotriazole-N-oxyl, and violuric acid radicals were experimentally determined at 30 °C in acetonitrile.

Two-Electron-Two-Proton Transfer from Colloidal ZnO and TiO2 Nanoparticles to Molecular Substrates

Transfers of multiple electrons and protons are challenging yet central to many energy-conversion processes and other chemical and biochemical reactions. Semiconducting oxides can hold multiple redox equivalents. This study describes the 2e^-/2H⁺ transfer reactivity of photoreduced ZnO and TiO₂ nanoparticle (NP) colloids with molecular 2e^-/2H⁺ acceptors, to form new O-H, N-H, and C-H bonds. The reaction stoichiometries were monitored by NMR and optical spectroscopies. Faster 2e^-/2H⁺ transfer rates were observed for substrates forming O-H or N-H bonds, presumably due to initial hydrogen bonding at the oxide surface. Chemically reduced ZnO NPs stabilized by Na⁺ or Ca²⁺ also engage in 2e^-/2H⁺ transfer reactivity, showing that protons transferred in these processes are inherent to the oxide nanoparticles and do not exclusively stem from photoreduction. These results highlight the potential of ZnO and TiO₂ for multiple proton-coupled electron transfer (PCET) reactions.

Carbonyl-Directed Aliphatic Fluorination: A Special Type of Hydrogen Atom Transfer Beats Out Norrish II

Recently, our group reported that enone and ketone functional groups, upon photoexcitation, can direct site-selective sp³ C-H fluorination in terpenoid derivatives. How this transformation actually occurred remained mysterious, as a significant number of mechanistic possibilities came to mind. Herein, we report a comprehensive study describing the reaction mechanism through kinetic studies, isotope-labeling experiments, ¹⁹F NMR, electrochemical studies, synthetic probes, and computational experiments. To our surprise, the mechanism suggests intermolecular hydrogen atom transfer (HAT) chemistry is at play, rather than classical Norrish hydrogen atom abstraction as initially conceived. What is more, we discovered a unique role for photopromoters such as benzil and related compounds that necessitates their chemical transformation through fluorination in order to be effective. Our findings provide documentation of an unusual form of directed HAT and are of crucial importance for defining the necessary parameters for the development of future methods.

Concerted proton-electron transfer reactions of manganese-hydroxo and manganese-oxo complexes

The enzymes manganese superoxide dismutase and manganese lipoxygenase use MnIII-hydroxo centres to mediate proton-coupled electron transfer (PCET) reactions with substrate. As manganese is earth-abundant and inexpensive, manganese catalysts are of interest for synthetic applications. Recent years have seen exciting reports of enantioselective C-H bond oxidation by Mn catalysts supported by aminopyridyl ligands. Such catalysts offer economic and environmentally-friendly alternatives to conventional reagents and catalysts. Mechanistic studies of synthetic catalysts highlight the role of Mn-oxo motifs in attacking substrate C-H bonds, presumably by a concerted proton-electron transfer (CPET) step. (CPET is a sub-class of PCET, where the proton and electron are transferred in the same step.) Knowledge of geometric and electronic influences for CPET reactions of Mn-hydroxo and Mn-oxo adducts enhances our understanding of biological and synthetic manganese centers and informs the design of new catalysts. In this Feature article, we describe kinetic, spectroscopic, and computational studies of MnIII-hydroxo and MnIV-oxo complexes that provide insight into the basis for the CPET reactivity of these species. Systematic perturbations of the ligand environment around MnIII-hydroxo and MnIV-oxo motifs permit elucidation of structure-activity relationships. For MnIII-hydroxo centers, electron-deficient ligands enhance oxidative reactivity. However, ligand perturbations have competing consequences, as changes in the MnIII/II potential, which represents the electron-transfer component for CPET, is offset by compensating changes in the pKa of the MnII-aqua product, which represents the proton-transfer component for CPET. For MnIV-oxo systems, a multi-state reactivity model inspired the development of significantly more reactive complexes. Weakened equatorial donation to the MnIV-oxo unit results in large rate enhancements for C-H bond oxidation and oxygen-atom transfer reactions. These results demonstrate that the local coordination environment can be rationally changed to enhance reactivity of MnIII-hydroxo and MnIV-oxo adducts.

Proton Transfer Can Govern Regioselectivity Assisted by Iron Catalysis

Ortho-selective aromatic C-H functionalization is frequently used in organic synthesis and chemical/pharmaceutical industries. However, this reaction relies heavily on the use of directing groups suffering from limited substrate scope and extra steps to put on and remove the directing/protecting groups. Herein we present the previously neglected concept that enables good to nearly complete selective ortho position. Proton transfer was utilized to tune the electron density on the aryl ring and determine the positional selectivity of electrophilic substitution. Consistently with deuteration experiments and DFT studies, this work demonstrates that acid-promoted proton transfer directs accelerated ortho-selective halogenation of NH/OH contained aromatic amines/phenols with excellent selectivity (>40 examples; up to 98:2 ortho/para selectivity). The application potential of this Fe-catalyzed method is demonstrated by the convenient synthesis of three alkaloids and tizanidine. This report raises the possibility that proton transfer could serve as the basis of developing new selective C-H functionalization reactions.

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Highly ortho-selective halogenations of anilines and carbazoles

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Lewis acids being able to accelerate EAS reactions

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Proton shift found to be crucial for the regioselectivity

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Practical iron sulfonate catalysis being scaled up to 100 g

Base-Directed Photoredox Activation of C-H Bonds by PCET

Photoredox catalysis using proton-coupled electron transfer (PCET) has emerged as a powerful method for bond transformations. We previously employed traditional chemical oxidants to achieve multiple-site concerted proton-electron transfer (MS-CPET) activation of a C-H bond in a proof-of-concept fluorenyl-benzoate substrate. As described here, photoredox oxidation of the fluorenyl-benzoate follows the same rate constant vs driving force trend determined for thermal MS-CPET. Analogous photoredox catalysis enables C-H activation and H/D exchange in a number of additional substrates with favorably positioned bases. Mechanistic studies support our hypothesis that MS-CPET is a viable pathway for bond activation for substrates in which the C-H bond is weak, while stepwise carboxylate oxidation and hydrogen atom transfer likely predominate for stronger C-H bonds.

Kinetic Isotope Effects and Hydrogen Tunnelling in PCET Oxidations of Ascorbate: New Insights into Aqueous Chemistry?

Recent experimental studies of kinetic isotope effects (KIE-s) and hydrogen tunnelling comprising three proton-coupled electron transfer (PCET) oxidations of ascorbate monoanion, (a) in aqueous reaction solutions, (b) in the mixed water-organic cosolvent systems, (c) in aqueous solutions of various salts and (d) in fairly diluted aqueous solutions of the various partial hydrophobes are reviewed. A number of new insights into the wealth of the kinetic isotope phenomena in the PCET reactions have been obtained. The modulation of KIE-s and hydrogen tunnelling observed when partially hydrophobic solutes are added into water reaction solution, in the case of fairly diluted solutions is revealed as the strong linear correlation of the isotopic ratios of the Arrhenius prefactors Ah/Ad and the isotopic differences in activation energies ΔE_a (D,H). The observation has been proposed to be a signature of the involvement of the collective intermolecular excitonic vibrational dynamics of water in activation processes and aqueous chemistry.

Excited-state proton-coupled electron transfer within ion pairs

The use of light to drive proton-coupled electron transfer (PCET) reactions has received growing interest, with recent focus on the direct use of excited states in PCET reactions (ES-PCET). Electrostatic ion pairs provide a scaffold to reduce reaction orders and have facilitated many discoveries in electron-transfer chemistry. Their use, however, has not translated to PCET. Herein, we show that ion pairs, formed solely through electrostatic interactions, provide a general, facile means to study an ES-PCET mechanism. These ion pairs formed readily between salicylate anions and tetracationic ruthenium complexes in acetonitrile solution. Upon light excitation, quenching of the ruthenium excited state occurred through ES-PCET oxidation of salicylate within the ion pair. Transient absorption spectroscopy identified the reduced ruthenium complex and oxidized salicylate radical as the primary photoproducts of this reaction. The reduced reaction order due to ion pairing allowed the first-order PCET rate constants to be directly measured through nanosecond photoluminescence spectroscopy. These PCET rate constants saturated at larger driving forces consistent with approaching the Marcus barrierless region. Surprisingly, a proton-transfer tautomer of salicylate, with the proton localized on the carboxylate functional group, was present in acetonitrile. A pre-equilibrium model based on this tautomerization provided non-adiabatic electron-transfer rate constants that were well described by Marcus theory. Electrostatic ion pairs were critical to our ability to investigate this PCET mechanism without the need to covalently link the donor and acceptor or introduce specific hydrogen bonding sites that could compete in alternate PCET pathways.

Hydrogen and proton exchange at carbon. Imbalanced transition state and mechanism crossover

A recent remarkable study of the C-H oxidation of substituted fluorenyl-benzoates together with the transfer of a proton to an internal receiving group by means of electron transfer outer-sphere oxidants, in the noteworthy absence of hydrogen-bonding interactions, is taken as an example to uncover the existence of a mechanism crossover, making the reaction pass from a CPET pathway to a PTET pathway as the driving force of the global reaction decreases. This was also the occasion to stress that considerations based on "imbalanced" or "asynchronous" transition states cannot replace activation/driving force models based on the quantum mechanical treatment of both electrons and transferring protons.

Elucidating Proton-Coupled Electron Transfer Mechanisms of Metal Hydrides with Free Energy- and Pressure-Dependent Kinetics

Proton-coupled electron transfer (PCET) was studied in a series of tungsten hydride complexes with pendant pyridyl arms ([(PyCH₂Cp)WH(CO)₃], PyCH₂Cp = pyridylmethylcyclopentadienyl), triggered by laser flash-generated Ru^III-tris-bipyridine oxidants, in acetonitrile solution. The free energy dependence of the rate constant and the kinetic isotope effects (KIEs) showed that the PCET mechanism could be switched between concerted and the two stepwise PCET mechanisms (electron-first or proton-first) in a predictable fashion. Straightforward and general guidelines for how the relative rates of the different mechanisms depend on oxidant and base are presented. The rate of the concerted reaction should depend symmetrically on changes in oxidant and base strength, that is on the overall ΔG⁰_PCET, and we argue that an "asynchronous" behavior would not be consistent with a model where the electron and proton tunnel from a common transition state. The observed rate constants and KIEs were examined as a function of hydrostatic pressure (1-2000 bar) and were found to exhibit qualitatively different dependence on pressure for different PCET mechanisms. This is discussed in terms of different volume profiles of the PCET mechanisms as well as enhanced proton tunneling for the concerted mechanism. The results allowed for assignment of the main mechanism operating in the different cases, which is one of the critical questions in PCET research. They also show how the rate of a PCET reaction will be affected very differently by changes of oxidant and base strength, depending on which mechanism dominates. This is of fundamental interest as well as of practical importance for rational design of, for example, catalysts for fuel cells and solar fuel formation, which operate in steps of PCET reactions. The mechanistic richness shown by this system illustrates that the specific mechanism is not intrinsic to a specific synthetic catalyst or enzyme active site but depends on the reaction conditions.

Understanding Chemoselectivity in Proton-Coupled Electron Transfer: A Kinetic Study of Amide and Thiol Activation

While the mechanistic understanding of proton-coupled electron transfer (PCET) has advanced significantly, few reports have sought to elucidate the factors that control chemoselectivity in these reactions. Here we present a kinetic study that provides a quantitative basis for understanding the chemoselectivity in competitive PCET activations of amides and thiols relevant to catalytic olefin hydroamidation reactions. These results demonstrate how the interplay between PCET rate constants, hydrogen-bonding equilibria, and rate-driving force relationships jointly determine PCET chemoselectivity under a given set of conditions. In turn, these findings predict reactivity trends in a model hydroamidation reaction, rationalize the selective activation of amide N-H bonds in the presence of much weaker thiol S-H bonds, and deliver strategies to improve the efficiencies of PCET reactions employing thiol co-catalysts.

Strategies for Enhancing the Rate Constant of C-H Bond Cleavage by Concerted Proton-Coupled Electron Transfer

Recently selective C-H bond cleavage under mild conditions with weak oxidants was reported for fluorenyl-benzoates. This mechanism is based on multi-site concerted proton-coupled electron transfer (PCET) involving intermolecular electron transfer to an outer-sphere oxidant coupled to intramolecular proton transfer to a well-positioned proton acceptor. The electron transfer driving force depends predominantly on the oxidant, and the proton transfer driving force depends mainly on the basicity of the carboxylate, which is influenced by the substituent on the benzoate fragment. Experiments showed that the rate constants are much more sensitive to the carboxylate basicity than to the redox potential of the oxidant. Herein a vibronically nonadiabatic PCET theory is used to explain how changing the driving force for the electron and proton transfer components of the reaction through varying the oxidant and the substituent, respectively, impacts the PCET rate constant. In addition to increasing the driving force for proton transfer, enhancing the basicity of the carboxylate also decreases the equilibrium proton donor-acceptor distance, thereby facilitating the sampling of shorter proton donor-acceptor distances. This additional effect arising from the strong dependence of proton transfer on the proton donor-acceptor distance provides an explanation for the greater sensitivity of the rate constant to the carboxylate basicity than to the redox potential of the oxidant. These fundamental insights have broad implications for developing new strategies to activate C-H bonds, specifically by designing systems with shorter equilibrium proton donor-acceptor distances.

C-H Alkylation via Multisite-Proton-Coupled Electron Transfer of an Aliphatic C-H Bond

The direct, site-selective alkylation of unactivated C(sp³)-H bonds in organic substrates is a long-standing goal in synthetic chemistry. General approaches to the activation of strong C-H bonds include radical-mediated processes involving highly reactive intermediates, such as heteroatom-centered radicals. Herein, we describe a catalytic, intermolecular C-H alkylation that circumvents such reactive species via a new elementary step for C-H cleavage involving multisite-proton-coupled electron transfer (multisite-PCET). Mechanistic studies indicate that the reaction is catalyzed by a noncovalent complex formed between an iridium(III) photocatalyst and a monobasic phosphate base. The C-H alkylation proceeds efficiently using diverse hydrocarbons and complex molecules as the limiting reagent and represents a new approach to the catalytic functionalization of unactivated C(sp³)-H bonds.

Transition State Asymmetry in C-H Bond Cleavage by Proton-Coupled Electron Transfer

The selective transformation of C-H bonds is a longstanding challenge in modern chemistry. A recent report details C-H oxidation via multiple-site concerted proton-electron transfer (MS-CPET), where the proton and electron in the C-H bond are transferred to separate sites. Reactivity at a specific C-H bond was achieved by appropriate positioning of an internal benzoate base. Here, we extend that report to reactions of a series of molecules with differently substituted fluorenyl-benzoates and varying outer-sphere oxidants. These results probe the fundamental rate versus driving force relationships in this MS-CPET reaction at carbon by separately modulating the driving force for the proton and electron transfer components. The rate constants depend strongly on the pK_a of the internal base, but depend much less on the nature of the outer-sphere oxidant. These observations suggest that the transition states for these reactions are imbalanced. Density functional theory (DFT) was used to generate an internal reaction coordinate, which qualitatively reproduced the experimental observation of a transition state imbalance. Thus, in this system, homolytic C-H bond cleavage involves concerted but asynchronous transfer of the H⁺ and e^-. The nature of this transfer has implications for synthetic methodology and biological systems.

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

The photochemical reactivity of the triplet state of pyrano- and furano-1,4-naphthoquinone derivatives (1 and 2) has been examined employing nanosecond laser flash photolysis. The quinone triplets were efficiently quenched by l-tryptophan methyl ester hydrochloride, l-tyrosine methyl ester hydrochloride, N-acetyl-l-tryptophan methyl ester and N-acetyl-l-tyrosine methyl ester, substituted phenols and indole (k q ∼109 L mol-1 s-1). For all these quenchers new transients were formed in the quenching process. These were assigned to the corresponding radical pairs that resulted from a coupled electron/proton transfer from the phenols, indole, amino acids, or their esters, to the excited state of the quinone. The proton coupled electron transfer (PCET) mechanism is supported by experimental rate constants, isotopic effects and theoretical calculations. The calculations revealed differences between the hydrogen abstraction reactions of phenol and indole substrates. For the latter, the calculations indicate that electron transfer and proton transfer occur as discrete steps.

Rate-Driving Force Relationships in the Multisite Proton-Coupled Electron Transfer Activation of Ketones

Here we present a detailed kinetic study of the multisite proton-coupled electron transfer (MS-PCET) activations of aryl ketones using a variety of Brønsted acids and excited-state Ir(III)-based electron donors. A simple method is described for simultaneously extracting both the hydrogen-bonding equilibrium constants and the rate constants for the PCET event from deconvolution of the luminescence quenching data. These experiments confirm that these activations occur in a concerted fashion, wherein the proton and electron are transferred to the ketone substrate in a single elementary step. The rates constants for the PCET events were linearly correlated with their driving forces over a range of nearly 19 kcal/mol. However, the slope of the rate-driving force relationship deviated significantly from expectations based on Marcus theory. A rationalization for this observation is proposed based on the principle of non-perfect synchronization, wherein factors that serve to stabilize the product are only partially realized at the transition state. A discussion of the relevance of these findings to the applications of MS-PCET in organic synthesis is also presented.

Ultrafast photoactivation of C─H bonds inside water-soluble nanocages

Light energy absorbed by molecules can be harnessed to activate chemical bonds with extraordinary speed. However, excitation energy redistribution within various molecular degrees of freedom prohibits bond-selective chemistry. Inspired by enzymes, we devised a new photocatalytic scheme that preorganizes and polarizes target chemical bonds inside water-soluble cationic nanocavities to engineer selective functionalization. Specifically, we present a route to photoactivate weakly polarized sp³ C─H bonds in water via host-guest charge transfer and control its reactivity with aerial O₂. Electron-rich aromatic hydrocarbons self-organize inside redox complementary supramolecular cavities to form photoactivatable host-guest charge transfer complexes in water. An ultrafast C─H bond cleavage within ~10 to 400 ps is triggered by visible-light excitation, through a cage-assisted and solvent water-assisted proton-coupled electron transfer reaction. The confinement prolongs the lifetime of the carbon-centered radical to enable a facile yet selective reaction with molecular O₂ leading to photocatalytic turnover of oxidized products in water.

Proton-coupled electron transfer oxidation of O–H bond by the N-radical cation of Wurster's blue salt (TMPDA˙+)

Beyond a single electron oxidant, the N-radical cation is also a good PCET reagent.