Bond-weakening catalysis: conjugate aminations enabled by the soft homolysis of strong N-H bonds

Coordination-induced O-H/N-H bond weakening by a redox non-innocent, aluminum-containing radical

Several renewable energy schemes aim to use the chemical bonds in abundant molecules like water and ammonia as energy reservoirs. Because the O-H and N-H bonds are quite strong (>100 kcal/mol), it is necessary to identify substances that dramatically weaken these bonds to facilitate proton-coupled electron transfer processes required for energy conversion. Usually this is accomplished through coordination-induced bond weakening by redox-active metals. However, coordination-induced bond weakening is difficult with earth's most abundant metal, aluminum, because of its redox inertness under mild conditions. Here, we report a system that uses aluminum with a redox non-innocent ligand to achieve significant levels of coordination-induced bond weakening of O-H and N-H bonds. The multisite proton-coupled electron transfer manifold described here points to redox non-innocent ligands as a design element to open coordination-induced bond weakening chemistry to more elements in the periodic table.

Coordination-induced bond weakening and small molecule activation by low-valent titanium complexes

Bond activation of small molecules through coordination to low valent metal complexes in M⋯X-H type interactions (where X = O, N, B, Si, etc.) leads to the formation of unusually weak X-H bonds and provides a powerful approach for the synthesis of target compounds under very mild conditions. Coordination of small molecules like water, amides, silanes, boranes, and dinitrogen to Ti(III) or Ti(II) complexes results in the synergetic redistribution of electrons between the metal orbitals and the ligand orbitals which weakens and enables the facile cleavage of the X-H or N-N bonds of the ligands. This review presents an overview of coordination-induced bond activation of small molecules by low valent titanium complexes. In particular, the applications of low valent titanium-induced bond weakening in nitrogen fixation are presented. The review concludes with potential future directions for work in this area including low-valent Ti-based PCET systems, photocatalytic nitrogen reduction, and approaches to tailoring complexes for optimal bond activation.

Visible-Light-Promoted α-C(sp3)-H Amination of Ethers with Azoles and Amides

A visible-light-induced highly efficient C(sp3)-H amination of ethers with amides and azoles has been presented under mild conditions via a nitrogen- and carbon-centered radical coupling process. This protocol successfully utilizes 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and tert-butyl nitrite (TBN) as cocatalysts to deliver the aminated products of ethers under aerobic conditions. Notably, the developed reaction features the corresponding products in good yields (up to 93%) with a wide substrate scope. The mechanistic study indicates that C-N bond formation proceeds via a direct radical cross-coupling process. Preliminary biological activity analysis indicates that the resulting products have good and selective inhibitory activity on osteosarcoma (OS) cell lines and are promising for use as hits for drug discovery.

Frustrated Radical Pairs in Organic Synthesis

Frustrated radical pairs (FRPs) describe the phenomenon that two distinct radicals─which would otherwise annihilate each other to form a closed-shell covalent adduct─can coexist in solution, owing to steric repulsion or weak bonding association. FRPs are typically formed via spontaneous single-electron transfer between two sterically encumbered precursors─an oxidant and a reductant─under ambient conditions. The two components of a FRP exhibit orthogonal chemical properties and can often act in cooperativity to achieve interesting radical reactivities. Initially observed in the study of traditional frustrated Lewis pairs, FRPs have recently been shown to be capable of homolytically activating various chemical bonds. In this Perspective, we will discuss the discovery of FRPs, their fundamental reactivity in chemical bond activation, and recent developments of their use in synthetic organic chemistry, including in C-H bond functionalization. We anticipate that FRPs will provide new reaction strategies for solving challenging problems in modern organic synthesis.

Accessing Unusual Reactivity through Chelation-Promoted Bond Weakening

Highly reducing Sm(II) reductants and protic ligands were used as a platform to ascertain the relationship between low-valent metal-protic ligand affinity and degree of ligand X-H bond weakening with the goal of forming potent proton-coupled electron transfer (PCET) reductants. Among the Sm(II)-protic ligand reductant systems investigated, the samarium dibromide N-methylethanolamine (SmBr₂-NMEA) reagent system displayed the best combination of metal-ligand affinity and stability against H₂ evolution. The use of SmBr₂-NMEA afforded the reduction of a range of substrates that are typically recalcitrant to single-electron reduction including alkynes, lactones, and arenes as stable as biphenyl. Moreover, the unique role of NMEA as a chelating ligand for Sm(II) was demonstrated by the reductive cyclization of unactivated esters bearing pendant olefins in contrast to the SmBr₂-water-amine system. Finally, the SmBr₂-NMEA reagent system was found to reduce substrates analogous to key intermediates in the nitrogen fixation process. These results reveal SmBr₂-NMEA to be a powerful reductant for a wide range of challenging substrates and demonstrate the potential for the rational design of PCET reagents with exceptionally weak X-H bonds.

Radical Activation of N-H and O-H Bonds at Bismuth(II)

The development of unconventional strategies for the activation of ammonia (NH₃) and water (H₂O) is of capital importance for the advancement of sustainable chemical strategies. Herein we provide the synthesis and characterization of a radical equilibrium complex based on bismuth featuring an extremely weak Bi-O bond, which permits the in situ generation of reactive Bi(II) species. The ensuing organobismuth(II) engages with various amines and alcohols and exerts an unprecedented effect onto the X-H bond, leading to low BDFE_X-H. As a result, radical activation of various N-H and O-H bonds─including ammonia and water─occurs in seconds at room temperature, delivering well-defined Bi(III)-amido and -alkoxy complexes. Moreover, we demonstrate that the resulting Bi(III)-N complexes engage in a unique reactivity pattern with the triad of H⁺, H^-, and H^• sources, thus providing alternative pathways for main group chemistry.

Reversible Homolysis of a Carbon-Carbon σ-Bond Enabled by Complexation-Induced Bond-Weakening

A case study of catalytic carbon-carbon σ-bond homolysis is presented. The coordination of a redox-active Lewis acid catalyst reduces the bond-dissociation free energies of adjacent carbon-carbon σ-bonds, and this complexation-induced bond-weakening is used to effect reversible carbon-carbon bond homolysis. Stereochemical isomerization of 1,2-disubstituted cyclopropanes was investigated as a model reaction with a ruthenium (III/II) redox couple adopted for bond weakening. Results from our mechanistic investigation into the stereospecificity of the isomerization reaction are consistent with selective complexation-induced carbon-carbon bond homolysis. The ΔG‡ of catalyzed and uncatalyzed reactions were estimated to be 14.4 and 40.0 kcal/mol, respectively with the computational method, (U)PBE0-D3/def2-TZVPP-SMD(toluene)//(U)B3LYP-D3/def2-SVP. We report this work as the first catalytic example where the complexation-induced bond-weakening effect is quantified through transition state analysis.

Spontaneous Ammonia Activation Through Coordination-Induced Bond Weakening in Molybdenum Complexes of a Dianionic Pentadentate Ligand Platform

Ammonia oxidation catalyzed by molecular compounds is of current interest as a carbon-free source of dihydrogen. Activation of N-H bonds through coordination to transition metal centers is a key reaction in this process. We report the substantial activation of ammonia via reaction with low-valent molybdenum complexes of a diborate pentadentate ligand system. Spontaneous loss of dihydrogen from (B₂ Pz₄ Py)Mo^II -NH₃ at room temperature to produce the dinuclear μ-nitrido compound (B₂ Pz₄ Py)Mo-N-Mo(B₂ Pz₄ Py) is observed due to substantial N-H bond weakening upon coordination to Mo. Mechanistic details are supported through the experimental observation/characterization of terminal amido, imido and nitrido complexes and density functional theory computations. The generally under-appreciated role of bridging nitrido intermediates is revealed and discussed, providing guidance for further catalyst development for this process.

Redox-Neutral Generation of Iminyl Radicals by N-Heterocyclic Carbene Catalysis: Rapid Access to Phenanthridines from Vinyl Azides

An N-heterocyclic carbene-catalyzed oxidant-, metal- and light-free iminyl radical generation pathway stemming from the reaction of vinyl azide and α-bromo ester is uncovered. This newly developed methodology is successfully applied to the redox-neutral construction of a number of diversified phenanthridine derivatives with nice functional group compatibility. Insights from the mechanism study reveal that this NHC-catalyzed transformation potentially proceeds through an alkyl radical addition-initiated HAS process, with the iminyl radical as an active intermediate.

Coordination-Induced Bond Weakening

Coordination-induced bond weakening is a phenomenon wherein ligand X-H bond homolysis occurs in concert with the energetically favorable oxidation of a coordinating metal complex. The coupling of these two processes enables thermodynamically favorable proton-coupled electron transfer reductions to form weak bonds upon formal hydrogen atom transfer to substrates. Moreover, systems utilizing coordination-induced bond weakening have been shown to facilitate the dehydrogenation of feedstock molecules including water, ammonia, and primary alcohols under mild conditions. The formation of exceptionally weak substrate X-H bonds via small molecule homolysis is a powerful strategy in synthesis and has been shown to enable nitrogen fixation under mild conditions. Coordination-induced bond weakening has also been identified as an integral process in biophotosynthesis and has promising applications in renewable chemical fuel storage systems. This review presents a discussion of the advances made in the study of coordination-induced bond weakening to date. Because of the broad range of metal and ligand species implicated in coordination-induced bond weakening, each literature report is discussed individually and ordered by the identity of the low-valent metal. We then offer mechanistic insights into the basis of coordination-induced bond weakening and conclude with a discussion of opportunities for further research into the development and applications of coordination-induced bond weakening systems.

A Germanium Catalyst Accelerates the Photoredox α-C(sp3)-H Alkylation of Primary Amines

Site-selective C(sp³)-H functionalizations using photoredox catalysis (PC) and hydrogen atom transfer (HAT) catalysis have received increasing attention. Here, we report a Ph₂GeCl₂ cocatalyst that greatly improves the yield of α-C(sp³)-H alkylation of primary amines catalyzed by a PC-HAT hybrid system. The α-position of the amino group selectively reacted even when weaker C-H bonds existed in the substrates. This finding may help the design of a novel site-selective hybrid catalysis.

Enantioselective Addition of Remote Alkyl Radicals to Double Bonds by Photocatalytic Proton-Coupled Electron Transfer (PCET) Deconstruction of Unstrained Cycloalkanols

Herein, we report the enantioselective addition of remote alkyl radicals, generated from the ring opening of unstrained cycloalkanols by a proton-coupled electron transfer (PCET) process, to 2-acyl imidazoles previously coordinated to a rhodium-based chiral Lewis acid. High yields and enantioselectivites up to 99% are achieved in 1 h. Mechanistic investigations support the formation of the remote alkyl radical by a PCET process, and theoretical studies explain the observed stereochemistry in the addition step.

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.

Exploring the potential of the ammine complexes M(NH3)n+ (M = Zr, Re) to activate NH3

NH3 is activated by the complex Zr(NH3)7+ through a mechanism involving radical species.

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.

Facile proton-coupled electron transfer enabled by coordination-induced E-H bond weakening to boron

We report the facile activation of aryl E-H (ArEH; E = N, O, S; Ar = Ph or C6F5) or ammonia N-H bonds via coordination-induced bond weakening to a redox-active boron center in the complex, (1-). Substantial decreases in E-H bond dissociation free energies (BDFEs) are observed upon substrate coordination, enabling subsequent facile proton-coupled electron transfer (PCET). A drop of >50 kcal mol-1 in H2N-H BDFE upon coordination was experimentally determined.

Multiple N-H and C-H Hydrogen Atom Abstractions Through Coordination-Induced Bond Weakening at Fe-Amine Complexes

We report the use of the reported Fe-phthalocyanine complex, PcFe (1; Pc = 1,4,8,11,15,18,22,25-octaethoxy-phthalocyanine), to generate PcFe-amine complexes 1-(NH₃)₂, 1-(MeNH₂)₂, and 1-(Me₂NH)₂. Treatment of 1 or 1-(NH₃)₂ to an excess of the stable aryloxide radical, 2,4,6-tritert-butylphenoxyl radical (^tBuArO^•), under NH₃ resulted in catalytic H atom abstraction (HAA) and C-N coupling to generate the product 4-amino-2,4,6-tritert-butylcyclohexa-2,5-dien-1-one (2) and ^tBuArOH. Exposing 1-(NH₃)₂ to an excess of the trityl (CPh₃) variant, 2,6-di-tert-butyl-4-tritylphenoxyl radical (^TrArO^•), under NH₃ did not lead to catalytic ammonia oxidation as previously reported in a related Ru-porphyrin complex. However, pronounced coordination-induced bond weakening of both α N-H and β C-H in the alkylamine congeners, 1-(MeNH₂)₂ and 1-(Me₂NH)₂, led to multiple HAA events yielding the unsaturated cyanide complex, 1-(MeNH₂)(CN), and imine complex, 1-(MeN═CH₂)₂, respectively. Subsequent C-N bond formation was also observed in the latter upon addition of a coordinating ligand. Detailed computational studies support an alternating mechanism involving sequential N-H and C-H HAA to generate these unsaturated products.

Testing the limits of radical-anionic CH-amination: a 10-million-fold decrease in basicity opens a new path to hydroxyisoindolines via a mixed C-N/C-O-forming cascade

An intramolecular C(sp3)-H amidation proceeds in the presence of t-BuOK, molecular oxygen, and DMF. This transformation is initiated by the deprotonation of an acidic N-H bond and selective radical activation of a benzylic C-H bond towards hydrogen atom transfer (HAT). Cyclization of this radical-anion intermediate en route to a two-centered/three-electron (2c,3e) C-N bond removes electron density from nitrogen. As this electronegative element resists such an "oxidation", making nitrogen more electron rich is key to overcoming this problem. This work dramatically expands the range of N-anions that can participate in this process by using amides instead of anilines. The resulting 107-fold decrease in the N-component basicity (and nucleophilicity) doubles the activation barrier for C-N bond formation and makes this process nearly thermoneutral. Remarkably, this reaction also converts a weak reductant into a much stronger reductant. Such "reductant upconversion" allows mild oxidants like molecular oxygen to complete the first part of the cascade. In contrast, the second stage of NH/CH activation forms a highly stabilized radical-anion intermediate incapable of undergoing electron transfer to oxygen. Because the oxidation is unfavored, an alternative reaction path opens via coupling between the radical anion intermediate and either superoxide or hydroperoxide radical. The hydroperoxide intermediate transforms into the final hydroxyisoindoline products under basic conditions. The use of TEMPO as an additive was found to activate less reactive amides. The combination of experimental and computational data outlines a conceptually new mechanism for conversion of unprotected amides into hydroxyisoindolines proceeding as a sequence of C-H amidation and C-H oxidation.

Recent Advances in Titanium Radical Redox Catalysis

New catalytic strategies that leverage single-electron redox events have provided chemists with useful tools for solving synthetic problems. In this context, Ti offers opportunities that are complementary to late transition metals for reaction discovery. Following foundational work on epoxide reductive functionalization, recent methodological advances have significantly expanded the repertoire of Ti radical chemistry. This Synopsis summarizes recent developments in the burgeoning area of Ti radical catalysis with a focus on innovative catalytic strategies such as radical redox-relay and dual catalysis.

Bimetallic Radical Redox-Relay Catalysis for the Isomerization of Epoxides to Allylic Alcohols

Organic radicals are generally short-lived intermediates with exceptionally high reactivity. Strategically, achieving synthetically useful transformations mediated by organic radicals requires both efficient initiation and selective termination events. Here, we report a new catalytic strategy, namely, bimetallic radical redox-relay, in the regio- and stereoselective rearrangement of epoxides to allylic alcohols. This approach exploits the rich redox chemistry of Ti and Co complexes and merges reductive epoxide ring opening (initiation) with hydrogen atom transfer (termination). Critically, upon effecting key bond-forming and -breaking events, Ti and Co catalysts undergo proton transfer/electron transfer with one another to achieve turnover, thus constituting a truly synergistic dual catalytic system.

Roles of Iron Complexes in Catalytic Radical Alkene Cross-Coupling: A Computational and Mechanistic Study

A growing and useful class of alkene coupling reactions involve hydrogen atom transfer (HAT) from a metal-hydride species to an alkene to form a free radical, which is responsible for subsequent bond formation. Here, we use a combination of experimental and computational investigations to map out the mechanistic details of iron-catalyzed reductive alkene cross-coupling, an important representative of the HAT alkene reactions. We are able to explain several observations that were previously mysterious. First, the rate-limiting step in the catalytic cycle is the formation of the reactive Fe-H intermediate, elucidating the importance of the choice of reductant. Second, the success of the catalytic system is attributable to the exceptionally weak (17 kcal/mol) Fe-H bond, which performs irreversible HAT to alkenes in contrast to previous studies on isolable hydride complexes where this addition was reversible. Third, the organic radical intermediates can reversibly form organometallic species, which helps to protect the free radicals from side reactions. Fourth, the previously accepted quenching of the postcoupling radical through stepwise electron transfer/proton transfer is not as favorable as alternative mechanisms. We find that there are two feasible pathways. One uses concerted proton-coupled electron transfer (PCET) from an iron(II) ethanol complex, which is facilitated because the O-H bond dissociation free energy is lowered by 30 kcal/mol upon metal binding. In an alternative pathway, an O-bound enolate-iron(III) complex undergoes proton shuttling from an iron-bound alcohol. These kinetic, spectroscopic, and computational studies identify key organometallic species and PCET steps that control selectivity and reactivity in metal-catalyzed HAT alkene coupling, and create a firm basis for elucidation of mechanisms in the growing class of HAT alkene cross-coupling reactions.

Reversible homolytic activation of water via metal-ligand cooperativity in a T-shaped Ni(ii) complex

A T-shaped Ni(ii) complex [Tol,PhDHPy]Ni has been prepared and characterized. EPR spectra and DFT calculations of this complex suggest that the electronic structure is best described as a high-spin Ni(ii) center antiferromagnetically coupled with a ligand-based radical. This complex reacts with water at room temperature to generate the dimeric complex [Tol,PhDHPy]Ni(μ-OH)Ni[Tol,PhDHPyH] which has been thoroughly characterized by SXRD, NMR, IR and deuterium-labeling experiments. Addition of simple ligands such as phosphines or pyridine displaces water and demonstrates the reversibility of water activation in this system. The water activation step has been examined by kinetic studies and DFT calculations which suggest an unusual homolytic reaction via a bimetallic mechanism. The ΔH ‡, ΔS ‡ and KIE (k H/k D) of the reaction are 5.5 kcal mol-1, -23.8 cal mol-1 K-1, and 2.4(1), respectively. In addition to the reversibility of water addition, this system is capable of activating water towards net O-atom transfer to substrates such as aromatic C-H bonds and phosphines. This reactivity is facilitated by the ability of the dihydrazonopyrrole ligand to accept H-atoms and illustrates the utility of metal ligand cooperation in activating O-H bonds with high bond dissociation energies.

Aminoxyl-Catalyzed Electrochemical Diazidation of Alkenes Mediated by a Metastable Charge-Transfer Complex

We report the development of a new aminoxyl radical catalyst, CHAMPO, for the electrochemical diazidation of alkenes. Mediated by an anodically generated charge-transfer complex in the form of CHAMPO-N₃, radical diazidation was achieved across a broad scope of alkenes without the need for a transition metal catalyst or a chemical oxidant. Mechanistic data support a dual catalytic role for the aminoxyl serving as both a single-electron oxidant and a radical group transfer agent.

Evidence of proton-coupled mixed-valency by electrochemical behavior on transition metal complex dimers bridged by two Ag+ ions

H-Bonded metal complex dimers with reversible redox behaviour, which are connected by a low-barrier hydrogen bond (LBHB) with a very low energy barrier for proton transfer, can provide a unique mixed-valency state stabilized by the proton-coupled electron transfer (PCET) phenomenon. Using cyclic voltammetry measurements, newly prepared [ReIIICl2(PnPr3)2(Hbim)]2 (2) and [OsIIICl2(PnPr3)2(Hbim)]2 (3) existing as H-bonded dimers in a CH2Cl2 solution showed a four-step and four-electron transfer containing two mixed-valency states of ReIIReIII and ReIIIReIV, and OsIIOsIII and OsIIIOsVI, respectively. Furthermore, [ReIIICl2(PnPr3)2(Agbim)]2 (4) and [OsIIICl2(PnPr3)2(Agbim)]2 (5), bridged by two Ag+ ions instead of two H-bonding protons, were prepared, and their electrochemical behaviours changed to a two-step and four-electron transfer. It is clear that the H-bonded complex dimers 2 and 3, connected by an LBHB, can be electrochemically stabilized into unique pairs of mixed-valency states by PCET, and the H-bonding proton transfer also controls the electrochemical redox behaviour.

O–H and (CO)N–H bond weakening by coordination to Fe(ii)

Coordination of hydroxyl/amide groups to Fe(ii) diminishes BDFEs of O–H and (CO)N–H bonds down to 76.0 and 80.5 kcal mol⁻¹ respectively.

Coordination-induced O–H bond weakening in Sm(ii)-water complexes

Coordination of water to low-valent Sm leads to O–H bond-weakening that enables PCET to substrates.

Demystifying Cp2Ti(H)Cl and its Enigmatic Role in the Reactions of Epoxides with Cp2TiCl

The role of Cp₂Ti(H)Cl in the reactions of Cp₂TiCl with trisubstituted epoxides has been investigated in a combined experimental and computational study. Although Cp₂Ti(H)Cl has generally been regarded as a robust species, its decomposition to Cp₂TiCl and molecular hydrogen was found to be exothermic (ΔG = -11 kcal/mol when the effects of THF solvation are considered). In laboratory studies, Cp₂Ti(H)Cl was generated using the reaction of 1,2-epoxy-1-methylcyclohexane with Cp₂TiCl as a model. Rapid evolution of hydrogen gas was demonstrated, indicating that Cp₂Ti(H)Cl is indeed a thermally unstable molecule, which undergoes intermolecular reductive elimination of hydrogen under the reaction conditions. The stoichiometry of the reaction (Cp₂TiCl:epoxide = 1:1) and the quantity of hydrogen produced (1 mole per 2 moles of epoxide) is consistent with this assertion. The diminished yield of allylic alcohol from these reactions under the conditions of protic versus aprotic catalysis can be understood in terms of the predominant titanium(III) present in solution. Under the conditions of protic catalysis, Cp₂TiCl complexes with collidine hydrochloride and the titanium(III) center is less available for "cross-disproportionation" with carbon-centered radicals; this leads to by-products from radical capture by hydrogen atom transfer, resulting in a saturated alcohol.

Intermolecular Reductive Couplings of Arylidene Malonates via Lewis Acid/Photoredox Cooperative Catalysis

A cooperative Lewis acid/photocatalytic reduction of arylidene malonates yields a versatile radical anion species. This intermediate preferentially undergoes intermolecular radical-radical coupling reactions, and not the conjugate addition-dimerization reactivity typically observed in the single-electron reduction of conjugate acceptors. Reported here is the development of this open-shell species in intermolecular radical-radical cross couplings, radical dimerizations, and transfer hydrogenations. This reactivity underscores the enabling modularity of cooperative catalysis and demonstrates the utility of stabilized enoate-derived radical anions in intermolecular bond forming reactions.