Advances in chemistry of N-heterocyclic carbene boryl radicals

N-Heterocyclic Carbene-Borane Adducts with Chiral (R)-Chloroethyl and Vinyl Substituents

This research provides how chiral imidazolium salts {1,3-bis[1'-chloro-1'-phenylpropan-2'-yl]-imidazolium} (1a) and enantiopure {1,3-bis[(R)-1-chlorobutan-2-yl]-imidazolium} [1b] can be utilized in diverse synthetic pathways to obtain new carbene-borane adducts (2a-2d): {1,3-bis[1'-chloro-1'-phenylpropan-2'-yl]-imidazolyl-2-ylidene-borane} (2a), {1,3-bis[(Z)-1'-phenylpropen-2'-yl]-imidazolyl-2-ylidene-borane} (2b), {1,3-bis[(R)-1-chlorobutan-2-yl]-imidazolyl-2-ylidene-borane} (2c), and {1,3-bis[but-1-en-2-yl]-imidazolyl-2-ylidene-borane} (2d). The carbene-borane adducts were synthesized and characterized by 13C, 1H, and 11B nuclear magnetic resonance spectroscopy and time-of-flight mass spectrometry. The X-ray crystal analyses of compounds 2a and 2b were performed, and to understand the structure and interactions of 2a, a computational study was carried out. The effect of N-substituents in the NHC-borane adducts was clearly observed in the C-B bond lengths obtained by single-crystal X-ray diffraction, where the C-B bond is longer for adducts with N-(R)-chloroethyl substituents than for vinyl substituents. The analysis of the reduced density gradient and the bond critical point calculations of 2a showed intramolecular proton-hydride and Cl···N interactions. These chiral imidazolium salts could have applicability in the development of new materials and possibly in pharmaceutical research.

Harnessing transient CAAC-stabilized mesitylborylenes for chalcogen activation

Newly synthesized adducts of CAAC-bound mesitylborylene with carbon monoxide (CO) and trimethylphosphine (PMe3) are established as efficient precursors for the in situ generation of the dicoordinate borylene [(CAAC)BMes] (CAAC = cyclic(alkyl)(amino)carbene), as demonstrated by their ability to activate elemental chalcogens. Upon thermal or photolytic activation, these precursors readily react with sulfur and selenium, yielding boron chalcogenides characterized by terminal boron-chalcogen double bonds. In contrast, the reaction with tellurium leads to the formation of an unusual diradical ditelluride species with a Te-Te bond. Quantum chemical calculations of its electronic structure indicate an open-shell singlet ground state characterized by significant diradical character. Further investigations into the redox behavior of these boron chalcogenides reveal intriguing transformations, including the redox-induced formation and cleavage of E-E bonds.

Photoinduced Reductive [4 + 2] Cycloaddition of Nitroarenes

Direct C-N bond formation from nitroarenes is a valuable synthetic tool for quick access to aniline derivatives. Transformation via nitroso intermediates could be useful due to their unique properties, but the generation of nitrosoarenes in situ from nitroarenes is challenging due to the tendency for various side reactions. Herein we report the photoinduced reductive [4 + 2] cycloaddition of nitroarenes. The method begins with the photoinduced reduction of nitroarenes to give nitrosoarenes by hydrogen atom abstraction (HAA). The generated nitrosoarenes undergo a nitroso Diels-Alder (NDA) reaction. The key to success is the use of N-heterocyclic carbene (NHC) borane, which promotes efficient HAA, enabling the NDA reaction to proceed without the need for transition metals, strong bases, or elevated temperatures. The developed conditions allow high functional group tolerance, enabling late-stage functionalization and further derivatization of biologically active compounds.

A Radical Precursor Based on the Aromatization of p-Quinol Esters Enabled by Pyridine-Boryl Radical

A class of prearomatic carboxylic acid p-quinol ester radical precursors has been developed successfully, which could undergo homolytic cleavage of the para C-O bond of p-quinol esters via pyridine-boryl radical-induced aromatization in the presence of pyridines and diboron reagents, affording the corresponding alkyl radical via decarboxylation from the carboxyl radical in situ. In addition, the prearomatic radical precursors were further applied in radical substitution with phenylsulfonyl compounds and radical self-coulpings. This method not only provides a new approach to the generation of a radical intermediate but also expands the application of boron radicals.

Modular assembly of amines and diborons with photocatalysis enabled halogen atom transfer of organohalides for C(sp3)-C(sp3) bond formation

In the past few years, the direct activation of organohalides by ligated boryl radicals has emerged as a potential synthetic tool for cross-coupling reactions. In most existing methods, ligated boryl radicals are accessed from NHC-boranes or amine-boranes. In this work, we report a new photocatalytic platform by modular assembly of readily available amines and diboron esters to access a library of ligated boryl radicals for reaction screening, thus enabling the cross-coupling of organohalides and alkenes including both activated and unactivated ones for C(sp3)-C(sp3) bond formation by using the assembly of DABCO A1 and B2Nep2B1. The strategy features operational simplicity, mild conditions and good functional group tolerance. A range of organohalides including activated alkyl chlorides, alkyl bromides (1°, 2° and 3° C-Br) as well as aromatic bromides are applicable in the strategy. Experimental and computational studies rationalize the proposed mechanism.

Quenching Rate Constants of Lewis Base-Boryl Radical by Substrates: a Laser Flash Photolysis Study

The advanced strategy using Lewis base-boryl radicals (LBRs) has recently been proposed for the addition of alkyl substituents to the full-carbon quaternary center of an organic molecule. However, as the rate-determining step in the whole route, reaction rate constants of LBRs with substrates are extremely lacking. In this paper, 4-dimethylaminopyridine (DMAP)-BH2⋅ was selected as a representative of LBRs, and its reactions with six monochloro-substituted substrates, including three methyl chlorobenzoates and three chlorinated acetanilides were studied in experiments and theoretical calculations. The bimolecular reaction rate constants, kq, were determined using laser flash photolysis approach. By comparing activation energies along the two addition pathways, we have clarified the rate-determining step as the attacking to carbonyl oxygen instead of chlorine atom. Furthermore, noncovalent interaction (NCI) analyses on these substrates indicate that weak interactions, such as hydrogen-bonding and van der Waals interactions, have significant influence on the reactivity of these substrates. Our study provides concrete clues to extend this synthetic strategy.

Photocatalytic [3 + 2]-annulation via sodium tetraarylborate: a fundamental approach for synthesizing 1,4,2-diazaborole analogs

Substantial advancements have been achieved in the field of photocatalytic borylation utilizing 4c-7e Lewis base-boryl radicals. However, the utilization of 3c-5e neutral boryl radicals for C-B bond formation remains relatively underexplored due to their inherent instability. In this study, we successfully demonstrated the direct construction of C-B bonds using sodium tetraarylborate as a key reagent. This was accomplished by effectively stabilizing diaryl boryl radicals with nitrile compounds, thereby facilitating the synthesis of valuable boron-containing compounds. Overall, our research elucidates the significant role played by sodium tetraarylborate in enabling an efficient and versatile approach for synthesizing of 1,4,2-diazaborole analogs through a photocatalyzed [3 + 2]-annulation reaction. This mild and adaptable methodology expands synthetic strategies for obtaining diverse derivatives of 1,4,2-diazaboroles, with the RCN-BAr2 complex serving as an effective boron-nitrogen synthon that opens up pathways to multiple boron-nitrogen heterocycles. Furthermore, this breakthrough significantly enhances the applicability of sodium tetraarylborate in photoredox catalysis.

Photocatalytic Direct Borylation of Benzothiazole Heterocycles via a Triplet Activation Strategy

Boron compounds are widely employed in organic chemistry, pharmaceuticals, and materials science. Among them, borylated heterocycles serve as versatile synthons for the construction of new C-C or C-heteroatom bonds via coupling or radical processes. Such methods for direct C-H borylation reactions are of high synthetic value to reduce the number of synthetic steps and the amount of waste and to improve efficiency. Despite significant advances, the borylation of heterocycles remains an ongoing challenge with great potential for applications in chemical synthesis. Herein, we describe a photocatalytic C-H borylation reaction of five-membered ring heterocycles by employing a stable N-heterocyclic carbene borane as the borylating reagent and a photoredox catalyst. Under green and mild conditions, C-H borylation was achieved on a series of benzo-fused five-membered heterocyclic compounds. Further studies demonstrate the utility of this approach for applications in pharmaceutical and agrochemical research.

C(sp3)-F Bond Activation by Lewis Base-Boryl Radicals via Concerted Electron-Fluoride Transfer

Selective C-F bond activation through a radical pathway in the presence of multiple C-H bonds remains a formidable challenge, owing to the extraordinarily strong bond strength of the C-F bond. By the aid of density functional theory calculations, we disclose an innovative concerted electron-fluoride transfer mechanism, harnessing the unique reactivity of Lewis base-boryl radicals to selectively activate the resilient C-F bonds in fluoroalkanes. This enables the direct abstraction of a fluorine atom and subsequent generation of an alkyl radical, thus expanding the boundaries of halogen atom transfer reactions.

Naphthalene Hydrodearomatization via Controllable Photocatalytic Hydroboration

The photocatalytic dearomative 1,4-hydroboration of naphthalenes with an N-heterocyclic carbene borane (NHC-BH3) complex was reported herein with controllable regioselectivity and chemoselectivity. This protocol yielded a wide range of naphthalene derivatives bearing various functional groups, notably bioactive compounds. Hydroboration occurred through the cooperation of photoredox and hydrogen atom transfer via boryl radical addition to naphthalene and further selective protonation. Notably, tuning the conditions could lead to further hydrogenation of the tetrahydroboration products. Mechanistic studies, including experimental and computational studies, elucidated the mechanism for the regioselectivity.

Visible-Light-Mediated Radical trans-Hydroboration of Alkynes with NHC Borane

Although the radical hydroboration of alkenes with N-heterocyclic carbene (NHC) borane is well documented, the radical hydroboration of alkynes, especially terminal alkynes, remains challenging. Herein, a photoredox-catalyzed radical trans-hydroboration of alkynes with NHC borane has been developed, which provided various alkenyl boron compounds in moderate to good yields. This protocol exhibits a broad substrate scope, as both internal and terminal alkynes were compatible. The synthetic value of this method was further demonstrated by its applicability in the late-stage modification of bioactive molecules. A preliminary mechanism for this reaction was proposed.

Boryl Radical Mediated Hydro(gem-diboryl)alkylation of Alkenes with Sterically Hindered NHC Boranes

NHC boryl radical mediated halogen atom transfer (XAT) is useful in organic synthesis. Yet, most of the reaction ends only with reducing the halogen to hydrogen, that is, the C-X to C-H. This is especially dominant for electron-deficient alkyl halides, where the formed electrophilic radical reacts rapidly with NHC boranes. Herein, by employing a sterically hindered NHC borane as the boryl radical precursor (IPr·BH3), we were able to use the electrophilic-deficient alkyl halide (α-Iodide gem-di(B(pin))methane) in the C-C bond formation reaction. Mono-, disubstituted styrene, aliphatic alkenes, and heteroatom-substituted alkenes were used as reaction partners. Forty hydro(gem-diboryl)methylation products were obtained at room temperature in moderate to good yields. Detailed mechanistic studies revealed that the reaction mainly involved the radical process.

Photoinduced B-H Arylation of N-Heterocyclic Carbene Boranes with Sulfonyl(hetero)arenes

Arylborane complexes ligated by N-heterocyclic carbenes (NHCs) can be synthesized by photoirradiation of a mixture of NHC-boranes and sulfonyl(hetero)arenes. The reaction occurs under mild and convenient conditions without any photocatalyst, which are realized by a radical chain mechanism involving NHC-boryl radicals and sulfonyl radicals. This reaction offered the opportunity to reveal the photophysical property of a 2-borylnaphtho[1,2-d]thiazole derivative.

Photocatalytic Borylation of Imines and Alkenes via Decarboxylation of Trimethylamine Carboxyborane: A New Approach for Generating Boryl Radicals

In this study, we investigated the photocatalytic decarboxylation of trimethylamine carboxyborane under mild conditions, successfully generating boryl radicals for subsequent borylation reactions with imines and various alkenes. This method exhibited broad substrate compatibility, including the functionalization of biologically relevant molecules. Our findings expand the utility of trimethylamine carboxyborane beyond its role as a carbon-monoxide- and drug-releasing agent, highlighting its potential in radical chemistry through decarboxylation. This work establishes a robust foundation for further exploration of the synthetic application of trimethylamine carboxyborane.

Radical Replacement Process for Ligated Boryl Radical-Mediated Activation of Unactivated Alkyl Chlorides for C(sp3)-C(sp3) Bond Formation

The ligated boryl radical (LBR) has emerged as a potent tool for activating alkyl halides in radical transformations through halogen-atom transfer (XAT). However, unactivated alkyl chlorides still present an open challenge for this strategy. We herein describe a new activation mode of the LBR for the activation of unactivated alkyl chlorides to construct a C(sp3)-C(sp3) bond. Mechanistic studies reveal that the success of the protocol relies on a radical replacement process between the LBR and unactivated alkyl chloride, forming an alkyl borane intermediate as the alkyl radical precursor. Aided with the additive K3PO4, the alkyl borane then undergoes one-electron oxidation, generating an alkyl radical. The incorporation of the radical replacement activation model to activate unactivated alkyl chlorides significantly enriches LBR chemistry, which has been applied to activate alkyl iodides, alkyl bromides, and activated alkyl chlorides via XAT.

Synthesis and Suzuki-Miyaura Cross-Coupling of Alkyl Amine-Boranes. A Boryl Radical-Enabled Strategy

Alkyl organoborons are powerful materials for the construction of C(sp3)-C(sp2) bonds, predominantly via Suzuki-Miyaura cross-coupling. These species are generally assembled using 2-electron processes that harness the ability of boron reagents to act as both electrophiles and nucleophiles. Herein, we demonstrate an alternative borylation strategy based on the reactivity of amine-ligated boryl radicals. This process features the use of a carboxylic acid containing amine-ligated borane that acts as boryl radical precursor for photoredox oxidation and decarboxylation. The resulting amine-ligated boryl radical undergoes facile addition to styrenes and imines through radical-polar crossover manifolds. This delivers a new class of sp3-organoborons that are stable solids and do not undergo protodeboronation. These novel materials include unprotected α-amino derivatives that are generally unstable. Crucially, these aliphatic organoboron species can be directly engaged in Suzuki-Miyaura cross-couplings with structurally complex aryl halides. Preliminary studies suggest that they enable slow-release of the corresponding and often difficult to handle alkyl boronic acids.

Halogen Atom Transfer-Induced Homolysis of C-F Bonds by the Excited-State Boryl Radical

A novel reactivity toward C-F bond functionalization has been developed, which could be designated as fluorine atom transfer (FAT). A photoexcited state of an N-heterocyclic carbene-ligated boryl radical exhibits a transcendent reactivity, capable of activating chemically inert carbon-fluorine bonds through homolysis. Combined experimental and computational studies suggest that the ligated boryl radical species directly abstracts a fluorine atom from the organofluoride substrates to provide valuable carbon-centered radicals.

Visible light-induced halogen-atom transfer by N-heterocyclic carbene-ligated boryl radicals for diastereoselective C(sp3)-C(sp2) bond formation

Photoinduced halogen-atom transfer (XAT) has rapidly emerged as a programmable approach to generate carbon-centered radical intermediates, mainly relying on silyl and α-aminoalkyl radicals as halogen abstractors. More recently, ligated boryl radicals have also been proposed as effective halogen abstractors under visible-light irradiation. In this study, we describe the use of this approach to enable C(sp3)-C(sp2) bond formation via radical addition of carbon-centered radicals generated via XAT onto chloroalkynes. Our mechanistic investigation reveals a complex interplay of highly reactive radical intermediates which, under optimized conditions, delivered the targeted vinyl chlorides in excellent yields and Z : E ratios. Finally, we demonstrated the synthetic value of these products in transition metal-based cross-coupling reactions.

Regioselective Homolytic C2-H Borylation of Unprotected Adenosine and Adenine Derivatives via Minisci Reaction

A Minisci-type borylation of unprotected adenosine, adenine nucleotide, and adenosine analogues was successfully achieved through photocatalysis or thermal activation. Despite the challenges posed by the presence of two potential reactive sites (C2 and C8) in the purine motif, the unique nucleophilic amine-ligated boryl radicals effortlessly achieved excellent C2 site selectivity and simultaneously avoided the formation of multifunctionalized products. This protocol proved effective for the late-stage borylation of some important biomolecules as well as a few antiviral and antitumor drug molecules, such as AMP, cAMP, Vidarabine, Cordycepin, Tenofovir, Adefovir, GS-441524, etc. Theoretical calculations shed light on the site selectivity, revealing that the free energy barriers for the C2-Minisci addition are further lowered through the chelation of additive Mg2+ to N3 and furyl oxygen. This phenomenon has been confirmed by an IGMH analysis. Preliminary antitumor evaluation, derivation of the C2-borylated adenosine to other analogues with high-value functionalities, along with the CuAAC click reactions, suggest the potential application of this methodology in drug molecular optimization studies and chemical biology.

Photoinduced Intermolecular Radical Hydroalkylation of Olefins via Ligated Boryl Radicals-Mediated Halogen Atom Transfer

Light-mediated Halogen-Atom Transfer (XAT) has become a significant methodology in contemporary synthesis. Unlike α-aminoalkyl and silyl radicals, ligated boryl radicals (LBRs) have not been extensively explored as halogen atom abstractors. In this study, we introduce NHC-ligated boranes as optimal radical chain carriers for the intermolecular reductive radical hydroalkylation and hydroarylation of electron-deficient olefins by using direct UV-A light irradiation. DFT analysis allowed us to rationalize the critical role of the NHC ligand in facilitating efficient chain propagation.

Unified Approach to Deamination and Deoxygenation Through Isonitrile Hydrodecyanation: A Combined Experimental and Computational Investigation

Herein, we describe a general hydrodefunctionalization protocol of alcohols and amines through a common isonitrile intermediate. To cleave the relatively inert C-NC bond, we leveraged dual hydrogen atom transfer (HAT) and photoredox catalysis to generate a nucleophilic boryl radical, which readily forms an imidoyl radical intermediate from the isonitrile. Rapid β-scission then accomplishes defunctionalization. This method has been applied to the hydrodefunctionalization of both amine and alcohol-containing pharmaceuticals, natural products, and biomolecules. We extended this approach to the reduction of carbonyls and olefins to their saturated counterparts, as well as the hydrodecyanation of alkyl nitriles. Both experimental and computational studies demonstrate a facile β-scission of the imidoyl radical, and reconcile differences in reactivity between nitriles and isonitriles within our protocol.

Boryl Radical as a Catalyst in Enabling Intra- and Intermolecular Cascade Radical Cyclization Reactions: Construction of Polycyclic Molecules

Cascade radical cyclization constitutes an atom- and step-economic route for rapid assembly of polycyclic molecular skeletons. Although an array of redox-active metal catalysts has recently shown robust applications in enabling various catalytic cascade radical processes, the use of free organic radical as the catalyst, which is capable of triggering strategically distinct cascades, has rarely been developed. Here, we disclosed that the benzimidazolium-based N-heterocyclic carbene (NHC)-boryl radical is capable of catalyzing cascade cyclization reactions in both intra- and intermolecular pathways, assembling [5,5] fused bicyclic and [6,6,6] fused tricyclic molecules, respectively. The catalytic reactions start with the chemo- and regioselective addition of the boryl radical catalyst to a tethered alkene or alkyne moiety, followed by either an intramolecular formal [3+2] or an intermolecular [2+2+2] cycloaddition process to construct bicyclo[3.3.0]octane or tetrahydrophenanthridine skeletons, respectively. Eventually, a β-elimination occurs to release the boryl radical catalyst, completing a catalytic cycle. High to excellent diastereoselectivity is achieved in both catalytic reactions under substrate control.

Diastereoselective dearomatization of indoles via photocatalytic hydroboration on hydramine-functionalized carbon nitride

A protocol for trans-hydroboration of indole derivatives using heterogeneous photocatalysis with NHC-borane has been developed, addressing a persistent challenge in organic synthesis. The protocol, leveraging high crystalline vacancy-engineered polymeric carbon nitride as a catalyst, enables diastereoselective synthesis, expanding substrate scope and complementing existing methods. The approach emphasizes eco-friendliness, cost-effectiveness, and scalability, making it suitable for industrial applications, particularly in renewable energy contexts. The catalyst's superior performance, attributed to its rich carbon-vacancies and well-ordered structure, surpasses more expensive homogeneous alternatives, enhancing viability for large-scale use. This innovation holds promise for synthesizing bioactive compounds and materials relevant to medicinal chemistry and beyond.

Radical hydroboration for the synthesis of organoboron compounds

Organoboron compounds demonstrate diverse applications in the fields of organic synthesis, materials science, and medicinal chemistry. Compared to the conventional hydroboration reaction, radical hydroboration serves as an alternative approach for the synthesis of organoborons via different mechanisms. In radical hydroboration, a boryl radical is initially generated from homolytic cleavage of a B-H or a B-B bond, which is then added to an unsaturated double bond to deliver a carbon radical. Subsequent hydrogen atom transfer or reduction of the carbon radical to form a carbanion followed by protonation gave the final product. Over the past few years, numerous efforts have been made for efficient synthesis of boryl radicals and the expansion of substrate scope of the radical hydroboration reaction. Here, we discuss the recent advancement of radical hydroboration and its associated mechanisms. Numerous radical hydroboration strategies employing N-heterocyclic carbene borane, bis(pinacolato)diboron and pinacolborane as the boron source were illustrated. Thermochemical, photochemical and electrochemical strategies for the generation of boryl radicals were also discussed in detail.

Screening seven-electron boron-centered radicals for dinitrogen activation

The activation of dinitrogen is significant as nitrogen-containing compounds play an important role in industries. However, the inert NN triple bond caused by its large HOMO-LUMO gap (10.8 eV) and high bond dissociation energy (945 kJ mol-1 ) renders its activation under mild conditions particularly challenging. Recent progress shows that a few main group species can mimic transition metal complexes to activate dinitrogen. Here, we demonstrate that a series of seven-electron (7e) boron-centered radical can be used to activate N2 via density functional theory calculations. It is found that boron-centered radicals containing amine ligand perform best on the thermodynamics of dinitrogen activation. In addition, when electron-donating groups are introduced at the boron atom, these radicals can be used to activate N2 with low reaction barriers. Further analysis suggests that the electron transfer from the boron atom to the π* orbitals of dinitrogen is essential for its activation. Our findings suggest great potential of 7e boron radicals in the field of dinitrogen activation.

Optically induced charge-transfer in donor-acceptor-substituted p- and m- C2B10H12 carboranes

Icosahedral carboranes, C2B10H12, have long been considered to be aromatic but the extent of conjugation between these clusters and their substituents is still being debated. m- and p-Carboranes are compared with m- and p-phenylenes as conjugated bridges in optical functional chromophores with a donor and an acceptor as substituents here. The absorption and fluorescence data for both carboranes from experimental techniques (including femtosecond transient absorption, time-resolved fluorescence and broadband fluorescence upconversion) show that the absorption and emission processes involve strong intramolecular charge transfer between the donor and acceptor substituents via the carborane cluster. From quantum chemical calculations on these carborane systems, the charge transfer process depends on the relative torsional angles of the donor and acceptor groups where an overlap between the two frontier orbitals exists in the bridging carborane cluster.

Dianionic and Neutral Diboron-Centered Classical Diradicaloids

Herein, we report the syntheses and electronic structures of crystalline dianionic as well as neutral diboron-centered classical diradicaloids as boron analogues of classical Thiele, Chichibabin, and Müller (this only for dianionic diradicaloids!) hydrocarbons. These are based on borane radical anion and NHC-stabilized boryl radical spin carriers, respectively. All these dianionic diboron-centered diradicaloids exhibit triplet population at room temperature regardless of the π-conjugated spacer: p-phenylene, p,p'-biphenylene, or p,p″-terphenylene. In the case of neutral diboron-centered diradicaloids, the employed π-conjugated spacer plays a crucial role for the triplet population at room temperature: EPR inactive for p-phenylene vs EPR active for p,p'-biphenylene. The findings emphasize the importance of the spin carriers for the resulting ground-state: borane radical anion vs NHC-stabilized boryl radical along with the pivotal role of the π-conjugated spacer as spin-coupler between two spins. Notably, 100 years (a century) after the first report by Krause of the triphenyl borane radical-anion, being isoelectronic to the triphenylmethyl radical, we convey borane radical anion-based diradicaloids. Furthermore, while donor-stabilized boryl radicals were introduced in the 1980s by Giles and Roberts, said concept is herewith being extended to NHC-stabilized boryl radical-based diradicaloids.

Boryls, their compounds and reactivity: a structure and bonding perspective

Boryls and their compounds are important due to their diverse range of applications in the fields of materials science and catalysis. They are an integral part of boron chemistry, which has attracted tremendous research interest over the past few decades. In this perspective, we provide an in-depth analysis of the reaction chemistry of boryl compounds from a structure and bonding perspective. We discuss the reactivity of boryls in various transition metal complexes and diborane(4) compounds towards different substrate molecules, with a focus on their nucleophilic and electrophilic properties in various reaction processes. Additionally, we briefly discuss the reactivity of boryl radicals. Our analysis sheds new light on the unique properties of boryls and their potential for catalytic applications.

Photocatalytic Boryl Radicals Triggered Sequential B─N/C─N Bond Formation to Assemble Boron-Handled Pyrazoles

Vinyldiazo compounds are one of the most important synthons in the construction of a cyclic ring. Most photochemical transformations of vinyldiazo compounds are mainly focusing on utilization of their C═C bond site, while reactions taking place at terminal nitrogen atom are largely unexplored. Herein, a photocatalytic cascade radical cyclization of LBRs with vinyldiazo reagents through sequential B─N/C─N bond formation is described. The reaction starts with the addition of LBRs (Lewis base-boryl radicals) at diazo site, followed by intramolecular radical cyclization to access a wide range of important boron-handled pyrazoles in good to excellent yields. Control experiments, together with detailed mechanism studies well explain the observed reactivity. Further studies demonstrate the utility of this approach for applications in pharmaceutical and agrochemical research.

Formation of C-B, C-C, and C-X Bonds from Nonstabilized Aryl Radicals Generated from Diaryl Boryl Radicals

With the development of organoboron chemistry, boron-centered radicals have become increasingly attractive. However, their synthetic applications remain limited in that they have been used only as substrates for addition reactions or as initiators for catalytic reactions. We have achieved a new reaction pathway in which tetraarylborate salts are used as precursors for aryl radicals via boron radicals, by introducing a simple activation reagent. In addition, we carried out a diverse array of transformations involving these aryl radical precursors, which allowed the construction of new C-B, C-C, and C-X bonds in the presence of visible light.

From Carbene-Dithiolene Zwitterion Mediated B-H Bond Activation to BH3·SMe2-Assisted Boron-Boron Bond Formation

The 1:1 reaction of the carbene-stabilized dithiolene zwitterion 1 with BH3·SMe2 gave the dithiolene-based hydroborane 2 and the doubly hydrogen-capped CAAC species 3 via hydride-coupled reverse electron transfer processes. The mechanism of this transformation was probed computationally using density functional theory. The subsequent 2:1 reaction of 2 with 1 resulted in 4 and 3, suggesting that 1 can mediate the B-H bond activation not only for BH3 but also for monohydroboranes. In the presence of BH3·SMe2, 2 was unexpectedly converted to the corresponding diborane(4) complex 5 through a dehydrocoupling reaction at an elevated temperature.

Boryl radical catalysis enables asymmetric radical cycloisomerization reactions

The development of functionally distinct catalysts for enantioselective synthesis is a prominent yet challenging goal of synthetic chemistry. In this work, we report a family of chiral N-heterocyclic carbene (NHC)-ligated boryl radicals as catalysts that enable catalytic asymmetric radical cycloisomerization reactions. The radical catalysts can be generated from easily prepared NHC-borane complexes, and the broad availability of the chiral NHC component provides substantial benefits for stereochemical control. Mechanistic studies support a catalytic cycle comprising a sequence of boryl radical addition, hydrogen atom transfer, cyclization, and elimination of the boryl radical catalyst, wherein the chiral NHC subunit determines the enantioselectivity of the radical cyclization. This catalysis allows asymmetric construction of valuable chiral heterocyclic products from simple starting materials.

Stereoselective Unsymmetrical 1,1-Diborylation of Alkynes with a Neutral sp2 -sp3 Diboron Reagent

The incorporation of two distinct boryl groups at the same carbon center in organic molecules has attracted growing research interest due to its potential for facilitating controlled, precise synthesis through stepwise dual carbon-boron bond transformations. Here we report a method to access unsymmetrical 1,1-diborylalkene (UDBA) stereoselectively via the reaction of readily available alkynes with a neutral sp2 -sp3 diboron reagent (NHC)BH2 -Bpin (NHC=N-heterocyclic carbene). Attributing to the chemically easily distinguishable nature of the sp2 and sp3 boryl moieties, controllable stepwise derivatization of the resultant UDBAs is realized. This process leads to various multifunctionalized olefins and organoborons, such as acylboranes, which are difficult to prepare by other methods.

Visible-Light-Promoted Regioselective Hydroborylation of Ketene Dithioacetals with NHC-Boranes

NHC-boranes have been treated as a reliable source of boryl radicals. In this study, regioselective hydroborylation of ketene dithioacetals with NHC-borane was achieved under mild conditions via a visible-light-promoted radical chain process using thiophenol as a proton donor and hydrogen atom transfer. This protocol features a low-cost catalyst, good functional group tolerance, a relatively broad range of substrate scope, and good to excellent yields. Moreover, mechanism of this hydroborylation reaction was preliminarily studied.

Photocatalytic and Electrochemical Borylation and Silylation Reactions

Due to their high versatility borylated and silylated compounds are inevitable synthons for organic chemists. To escape the classical hydroboration/hydrosilylation paradigm, chemists turned their attention to more modern and green methods such as photoredox chemistry and electrosynthesis. This account focuses on novel methods for the generation of boryl and silyl radicals to forge C-B and C-Si bonds from our group.

Aluminum(III) Cations [(NHC) ⋅ AlMes2 ]+ : Synthesis, Characterization, and Application in FLP-Chemistry

The three-coordinate aluminum cations ligated by N-heterocyclic carbenes (NHCs) [(NHC) ⋅ AlMes2 ]+ [B(C6 F5 )4 ]- (NHC=IMeMe 4, IiPrMe 5, IiPr 6, Mes=2,4,6-trimethylphenyl) were prepared via hydride abstraction of the alanes (NHC) ⋅ AlHMes2 (NHC=IMeMe 1, IiPrMe 2, IiPr 3) using [Ph3 C]+ [B(C6 F5 )4 ]- in toluene as hydride acceptor. If this reaction was performed in diethyl ether, the corresponding four-coordinate aluminum etherate cations [(NHC) ⋅ AlMes2 (OEt2 )]+ [B(C6 F5 )4 ]- 7-9 (NHC=IMeMe 7, IiPrMe 8, IiPr 9) were isolated. According to a theoretical and experimental assessment of the Lewis-acidity of the [(IMeMe ) ⋅ AlMes2 ]+ cation is the acidity larger than that of B(C6 F5 )3 and of similar magnitude as reported for Al(C6 F5 )3 . The reaction of [(IMeMe ) ⋅ AlMes2 ]+ [B(C6 F5 )4 ]- 4 with the sterically less demanding, basic phosphine PMe3 afforded a mixed NHC/phosphine stabilized cation [(IMeMe ) ⋅ AlMes2 (PMe3 )]+ [B(C6 F5 )4 ]- 10. Equimolar mixtures of 4 and the sterically more demanding PCy3 gave a frustrated Lewis-pair (FLP), i.e., [(IMeMe ) ⋅ AlMes2 ]+ [B(C6 F5 )4 ]- /PCy3 FLP-11, which reacts with small molecules such as CO2 , ethene, and 2-butyne.

Programmable synthesis of difluorinated hydrocarbons from alkenes through a photocatalytic linchpin strategy

The introduction of difluoromethylene moieties into organic molecules has garnered significant attention due to their profound influence on the physicochemical and biological properties of compounds. Nonetheless, the existing approaches for accessing difluoroalkanes from readily available feedstock chemicals remain limited. In this study, we present an efficient and modular protocol for the synthesis of difluorinated compounds from alkenes, employing the readily accessible reagent, ClCF2SO2Na, as a versatile "difluoromethylene" linchpin. By means of an organophotoredox-catalysed hydrochlorodifluoromethylation of alkenes, followed by a ligated boryl radical-facilitated halogen atom transfer (XAT) process, we have successfully obtained various difluorinated compounds, including gem-difluoroalkanes, gem-difluoroalkenes, difluoromethyl alkanes, and difluoromethyl alkenes, with satisfactory yields. The practical utility of this linchpin strategy has been demonstrated through the successful preparation of CF2-linked derivatives of complex drugs and natural products. This method opens up new avenues for the synthesis of structurally diverse difluorinated hydrocarbons and highlights the utility of ligated boryl radicals in organofluorine chemistry.

Experimental and Computational Study of a Confirmed Borylene-to-Diborene Dimerization

While the dimerization of heavier group 13 carbene analogues to the corresponding alkene analogues is known and relatively well understood, the dimerization of dicoordinate borylenes (LRB:, L = neutral donor; R = anionic substituent) to the corresponding diborenes (LRB═BRL) has never been directly observed. In this study we present the first example of a formal borylene-to-diborene dimerization through abstraction of a labile phosphine ligand from the tricoordinate hydroborylene precursor (CAAC)(Me3P)BH (CAAC = cyclic alkyl(amino)carbene) by bulky Lewis-acidic dihaloboranes (BX2Y, X = Cl, Br, Y = aryl, boryl), generating the corresponding dihydrodiborene (CAAC)HB═BH(CAAC) and (Me3P)BX2Y as the byproduct. An in-depth experimental and computational mechanistic analysis shows that this seemingly simple process (2 LL'BH + 2 BX2Y → LHB═BHL + 2 L'BX2Y) is in fact based on a complex sequence of finely tuned processes, involving the one-electron oxidation of and PMe3 abstraction from the borylene precursor by BX2Y, multiple halide transfers between (di)boron intermediates and BX2Y/[BX3Y]-, and multiple one-electron redox processes between diboron intermediates and the borylene precursor, which make the reaction ultimately autocatalytic in [(CAAC)(Me3P)BH]•+. The findings suggest that [LBXR]• boryl radicals are more likely coupling partners than dicoordinate LRB: borylenes in the reductive coupling of base-stabilized LBX2R boranes to LRB═BRL diborenes.

Facile Borylation of Alkenes, Alkynes, Imines, Arenes and Heteroarenes with N-Heterocyclic Carbene-Boranes and a Heterogeneous Semiconductor Photocatalyst

Although the development of radical chain and photocatalytic borylation reactions using N-heterocyclic carbene (NHC)-borane as boron source is remarkable, the persistent problems, including the use of hazardous and high-energy radical initiators or the recyclability and photostability issues of soluble homogeneous photocatalysts, still leave great room for further development in a sustainable manner. Herein, we report a conceptually different approach toward highly functionalized organoborane synthesis by using recoverable ultrathin cadmium sulfide (CdS) nanosheets as a heterogeneous photocatalyst, and a general and mild borylation platform that enables regioselective borylation of a wide variety of alkenes (arylethenes, trifluoromethylalkenes, α,β-unsaturated carbonyl compounds and nitriles), alkynes, imines and electron-poor aromatic rings with NHC-borane as boryl radical precursor. Mechanistic studies and density functional theory (DFT) calculations reveal that both photogenerated electrons and holes on the CdS fully perform their own roles, thereby resulting in enhancement of photocatalytic activity and stability of CdS.

1,3-Imidazole-Based Mesoionic Carbenes and Anionic Dicarbenes: Pushing the Limit of Classical N-Heterocyclic Carbenes

Classical N-heterocyclic carbenes (NHCs) featuring the carbene center at the C2-position of 1,3-imidazole framework (i.e. C2-carbenes) are well acknowledged as very versatile neutral ligands in molecular as well as in materials sciences. The efficiency and success of NHCs in diverse areas is essentially attributed to their persuasive stereoelectronics, in particular the potent σ-donor property. The NHCs with the carbene center at the unusual C4 (or C5) position, the so-called abnormal NHCs (aNHCs) or mesoionic carbenes (iMICs), are however superior σ-donors than C2-carbenes. Hence, iMICs have substantial potential in sustainable synthesis and catalysis. The main obstacle in this direction is rather demanding synthetic accessibility of iMICs. The aim of this review article is to highlight recent advances, particularly by the author's research group, in accessing stable iMICs, quantifying their properties, and exploring their applications in synthesis and catalysis. In addition, the synthetic viability and use of vicinal C4,C5-anionic dicarbenes (ADCs), also based on an 1,3-imidazole framework, are presented. As will be apparent on following pages, iMICs and ADCs hold potentials in pushing the limit of classical NHCs by enabling access to conceptually new main-group heterocycles, radicals, molecular catalysts, ligands sets, and more.

Selective multifunctionalization of N-heterocyclic carbene boranes via the intermediacy of boron-centered radicals

The selective difunctionalization of N-heterocyclic carbene (NHC) boranes with alkenes has been achieved via decatungstate and thiol synergistic catalysis. The catalytic system also allows stepwise trifunctionalization, leading to complex NHC boranes with three different functional groups which are challenging to prepare by other methods. The strong hydrogen-abstracting ability of the excited decatungstate enables the generation of boryl radicals from mono- and di-substituted boranes for realizing borane multifunctionalization. This proof-of-principle research provides a new chance for fabricating unsymmetrical boranes and developing boron-atom-economic synthesis.

Chalcogen-substituted carbenes: a density functional study of structure, stability, and donor ability

Chalcogen-substituted carbenes are examined computationally using density functional theory. Several approaches are used to assess the stability and reactivity of chalcogenazol-2-ylidene carbenes (NEHCs; E = O, S, Se, Te). The known unsaturated species 1,3-dimethylimidazol-2-ylidene is studied at the same level of theory as the NEHC molecules, as a reference. Electronic structures, stability towards dimerization, and ligand properties are discussed. The results highlight the NEHCs as potentially valuable ancillary ligands for stabilizing low-valent metals or paramagnetic main group molecules. A simple, effective computational method for evaluating σ donor ability and π acidity of carbenes is presented.

Formation and reactivity of NHC-boryl radicals: insight into substituent effect from theoretical calculations

Substituent modification effects of N-heterocyclic carbene (NHC) boranes on their hydrogen atom abstraction (HAA) reactions and the chemical reactivities of corresponding NHC-boryl radicals have been investigated by density functional theory calculations. The substituent modification of NHC-boranes may notably affect the HAA reaction, both kinetically and thermodynamically, and shows remarkable substitution position dependence. The multi-site-modification of NHC-boranes is proved to be more effective for reduction of the B-H bond dissociation energy (BDE), promotion of the HAA reaction, and the reactivity regulation of their corresponding NHC-boryl radicals. Computational screening reveals that the spin density and the charge population of the radical boron center have good correlation with the B-H BDEs of NHC-boranes and the chemical reactivities of NHC-boryl radicals, and they can be considered as property and reactivity descriptors of these boron-based systems. The present results and established scaling relationships are beneficial to promote the advancement of design of NHC-boryl radical catalysis.

Photoinduced Dehydrogenative Borylation via Dihydrogen Bond Bridged Electron Donor and Acceptor Complexes

Air-stable amine- and phosphine-boranes are discovered as donors to integrate with pyridinium acceptor for generating photoactive electron-donor-acceptor (EDA) complexes. Experimental results and DFT calculations suggest a dihydrogen bond bridging the donor and acceptor. Irradiating the EDA complex enables an intra-complex single electron transfer to give a boron-centered radical for dehydrogenative borylation with no need of external photosensitizer and radical initiator. The deprotonation of Wheland-like radical intermediate rather than its generation is believed to determine the good ortho-selectivity based on DFT calculations. A variety of α-borylated pyridine derivatives have been readily synthesized with good functional group tolerance.

Lewis Base-Boryl Radicals Enabled Borylation Reactions and Selective Activation of Carbon-Heteroatom Bonds

ConspectusThe past decades have witnessed tremendous progress on radical reactions. However, in comparison with carbon, nitrogen, oxygen, and other main group element centered radicals, the synthetic chemistry of boron centered radicals was less studied, mainly due to the high electron-deficiency and instability of such 3-center-5-electron species. In the 1980s, Roberts and co-workers found that the coordination of a Lewis base (amines or phosphines) with the boron center could form 4-center-7-electron boryl radicals (Lewis base-boryl radicals, LBRs) that are found to be more stable. However, only limited synthetic applications were developed. In 2008, Curran and co-workers achieved a breakthrough with the discovery of N-heterocyclic carbene (NHC) boryl radicals, which could enable a range of radical reduction and polymerization reactions. Despite these exciting findings, more powerful and valuable synthetic applications of LBRs would be expected, given that the structures and reactivities of LBRs could be easily modulated, which would provide ample opportunities to discover new reactions. In this Account, a summary of our key contributions in LBR-enabled radical borylation reactions and selective activation of inert carbon-heteroatom bonds will be presented.Organoboron compounds have shown versatile applications in chemical society, and their syntheses rely principally on ionic borylation reactions. The development of mechanistically different radical borylation reactions allows synthesizing products that are inaccessible by traditional methods. For this purpose, we progressively developed a series of NHC-boryl radical mediated chemo-, regio-, and stereoselective radical borylation reactions of alkenes and alkynes, by which a wide variety of structurally diverse organoboron molecules were successfully prepared. The synthetic utility of these borylated products was also demonstrated. Furthermore, we disclosed a photoredox protocol for oxidative generation of NHC-boryl radicals, which enabled useful defluoroborylation and arylboration reactions.Selective bond activation is an ideal way to convert simple starting materials to value-added products, while the cleavage of inert chemical bonds, in particular the chemoselectivity control when multiple identical bonds are present in similar chemical environments, remains a long-standing challenge. We envisaged that finely tuning the properties of LBRs might provide a new solution to address this challenge. Recently, we disclosed a 4-dimethylaminopyridine (DMAP)-boryl radical promoted sequential C-F bond functionalization of trifluoroacetic acid derivatives, in which the α-C-F bonds were selectively snipped via a spin-center shift mechanism. This strategy enables facile conversion of abundantly available trifluoroacetic acid to highly valuable mono- and difluorinated molecules. Encouraged by this finding, we further developed a boryl radical enabled three-step sequence to construct all-carbon quaternary centers from a range of trichloromethyl groups, where the three C-Cl bonds were selectively cleaved by the rational choice of suitable boryl radical precursors in each step. Furthermore, a boryl radical promoted dehydroxylative alkylation of α-hydroxy carboxylic acid derivatives was achieved, allowing for the efficient conversion of some biomass platform molecules to high value products.

Neutral Boryl Radicals in Mixed-Valent B(III) Br-B(II) Adducts

The general strategies to stabilize a boryl radical involve single electron delocalization by π-system and the steric hinderance from bulky groups. Herein, a new class of boryl radicals is reported, with intramolecular mixed-valent B^(III) Br-B^(II) adducts ligated by a cyclic (alkyl)(amino)carbene (CAAC). The radicals feature a large spin density on the boron center, which is ascertained by EPR spectroscopy and DFT calculations. Structural and computational analyses revealed that the stability of radical species was assisted by the CAAC ligand and a weak but significant B(III)Br-B(II) interaction, suggesting a cooperative avenue for stabilization of boryl radicals. Two-electron reduction of these new boryl radicals provides C-H insertion products via a borylene intermediate.

Transition metal-free radical trans-hydroboration of alkynes with NHC-boranes via visible-light photoredox catalysis

Visible-light-induced 4-CzIPN-catalyzed trans-hydroboration of alkynes with NHC-boranes to generate diverse (E)-alkenylboranes with high selectivity.

Photoinduced Halogen-Atom Transfer by N-Heterocyclic Carbene-Ligated Boryl Radicals for C(sp3)-C(sp3) Bond Formation

Herein, we present a comprehensive study on the use of N-heterocyclic carbene (NHC)-ligated boryl radicals to enable C(sp³)-C(sp³) bond formation under visible-light irradiation via Halogen-Atom Transfer (XAT). The methodology relies on the use of an acridinium dye to generate the boron-centered radicals from the corresponding NHC-ligated boranes via single-electron transfer (SET) and deprotonation. These boryl radicals subsequently engage with alkyl halides in an XAT step, delivering the desired nucleophilic alkyl radicals. The present XAT strategy is very mild and accommodates a broad scope of alkyl halides, including medicinally relevant compounds and biologically active molecules. The key role of NHC-ligated boryl radicals in the operative reaction mechanism has been elucidated through a combination of experimental, spectroscopic, and computational studies. This methodology stands as a significant advancement in the chemistry of NHC-ligated boryl radicals, which had long been restricted to radical reductions, enabling C-C bond formation under visible-light photoredox conditions.