The Impact of Boron Hybridisation on Photocatalytic Processes

Copper-Catalyzed C-B(sp3) Bond Formation through the Intermediacy of Cu-B(sp3) Complex

The vast majority of transition metal (TM) catalyzed borylative transformations rely on TM-B(sp2) complexes. Contrastingly, the chemistry of TM-B(sp3) species and their potential in catalytic borylation remain surprisingly underdeveloped, due probably to the lack of suitable boron(sp3) reagents. Herein, we employ our recently developed sp2-sp3 diboron reagent to successfully enable a copper-catalyzed hydroboration of allenes for C-B(sp3) bond formation. A comprehensive mechanistic study, including the isolation and structural characterization of a Cu-B(sp3) complex, substantiates the presence of a Cu-B(sp3) intermediate in the catalytic cycle.

Switchable Divergent Photocatalytic C-Glycosylation of Glycosyl Benzoates

Metabolically robust C-glycosides are crucial in various biological and medical applications, underscoring the need for efficient synthesis methods. While radical C-glycosylation reactions are known for their reliability and functional group tolerance, challenges such as glycosyl donor stability and atom economy persist. In this study, we investigate the underexplored potential of condition-controlled divergent synthesis of C-glycosides through a switchable photocatalytic C-glycosylation strategy, involving reductive anomeric C─O bond cleavage. Utilizing simple, readily available, and bench-stable glycosyl benzoates as novel O-based glycosyl radical precursors, we successfully achieve deoxygenative glycosylation of simple alkenes and styryl boronic acids, establishing a versatile platform for C-glycoside synthesis. A critical aspect of the challenging reductive cleavage of these benzoate esters is the introduction of strong single-electron transfer (SET) reductants, combined with Brønsted acids to accelerate fragmentation following substrate reduction. Notably, CO2 •-, generated via the consecutive photon-induced electron transfer process, is utilized for the first time in glycosylation reactions. By meticulously tuning the reaction conditions, including photocatalysts and formate additives, we facilitate the divergent synthesis of alkyl and alkenyl C-glycosides with good to high stereoselectivity and yields. Mechanistic studies provide insight into the reaction pathway and the underlying rationale behind this finely tuned, easily controlled photocatalytic system.

Light-enabled intramolecular [2 + 2] cycloaddition via photoactivation of simple alkenylboronic esters

The photoactivation of organic molecules via energy transfer (EnT) catalysis is often limited to conjugated systems that have low-energy triplet excited states, with simple alkenes remaining an intractable challenge. The ability to address this limitation, using high energy sensitizers, would represent an attractive platform for future reaction design. Here, we disclose the photoactivation of simple alkenylboronic esters established using alkene scrambling as a rapid reaction probe to identify a suitable catalyst and boron motif. Cyclic voltammetry, UV-vis analysis, and control reactions support sensitization, enabling an intramolecular [2 + 2] cycloaddition to be realized accessing 3D bicyclic fragments containing a boron handle.

Precision installation of silyl synthetic handles within arenes by regiocontrolled ruthenium C(sp 2)-H functionalization

The site-selective functionalization of C(sp 2)-H bonds represents a powerful strategy for the synthesis of structurally diverse compounds with broad applicability. Here we report efficient regioselective catalytic methods for the formation of benzyltrimethylsilanes through ruthenium-catalysed C(sp 2)-H silylmethylation. The developed protocols enable selective functionalization at both ortho and meta positions within arenes bearing N-based directing groups. The resulting silylmethyl compounds can undergo diverse transformations, including nucleophilic aromatic substitution, carbonyl addition, olefination and desilylation. Significantly, the regiodivergent installation of silylmethyl synthetic handles allows for the synthesis of the pharmaceutical losmapimod and could further be applied in direct late-stage functionalizations. Mechanistically, an essential role for biscyclometallated ruthenium(II) species has been found, with the formation of intermediate ruthenium(III) species indicated by paramagnetic NMR experiments. These synthetic inventions and mechanistic elucidations signify a transformative step within ruthenium-catalysed C(sp 2)-H functionalization, enabling diverse syntheses and providing a framework for future development.

Energy Transfer (EnT) Catalysis of Non-Symmetrical Borylated Dienes: Origin of Reaction Selectivity in Competing EnT Processes

Energy transfer catalysis (EnT) has had a profound impact on contemporary organic synthesis enabling the construction of higher in energy, complex molecules, via efficient access to the triplet excited state. Despite this, intermolecular reactivity, and the unique possibility to access several reaction pathways via a central triplet diradical has rendered control over reaction outcomes, an intractable challenge. Extended chromophores such as non-symmetrical dienes have the potential to undergo [2+2] cycloaddition, [4+2] cycloaddition or geometric isomerisation, which, in combination with other mechanistic considerations (site- and regioselectivity), results in chemical reactions that are challenging to regulate. Herein, we utilise boron as a tool to probe reactivity of non-symmetrical dienes under EnT catalysis, paying particular attention to the impact of boron hybridisation effects on the target reactivity. Through this, a highly site- and regioselective [2+2] cycloaddition was realised with the employed boron motif effecting reaction efficiency. Subtle modifications to the core scaffold enabled a [4+2] cycloaddition, while a counterintuitive regiodivergence was observed in geometric isomerisation versus [2+2] cycloaddition. The observed reactivity was validated via a mechanistic investigation, determining the origin of regiodivergence and reaction selectivity in competing EnT processes.

Modular Synthesis of Geminal Iododiboron Compounds via Alkylation of Chlorodiborylmethane

α-Halogenated geminal bis(boronates) are emerging as multifunctional building blocks for organic synthesis. Currently, their synthetic utilization is still restricted due to a lack of efficient preparation methods. Herein, we report a direct, modular synthesis of gem-iododiborylalkanes using alkyl halides and a lithiated chlorodiborylmethane reagent. Compared with previously reported methods, this protocol features modular assembly, high efficiency, and good tolerance to various functional groups.

Photochemical 1,3-boronate rearrangement enables three-component N-alkylation for α-tertiary hydroxybenzylamine synthesis

Hydroxybenzylamines are prevalent in drugs and bioactive molecules, including various antimalarial and anticancer drugs. α-tertiary alkylation of amines impacts drug-target interactions significantly through their influence on basicity and lipophilicity. Traditional N-alkylation methods, especially for α-tertiary amines, suffer from limitations due to high energy barriers from steric hindrance. In this work, we leverage visible light irradiation to enable the organoboronic acid addition to sterically hindered ketimines in the excited state. Notably, it overcomes the limitations of the well-explored Petasis reaction, which is restricted to aldimines due to the high energy barrier associated with ketimines (51.3 kcal/mol). This three-component coupling of aliphatic amines, o-phenolic ketones, and organoboronic acids delivers diverse α-tertiary o-hydroxybenzylamines (77 examples, yields up to 82%) with broad functional group tolerance. The light-driven 1,3-boronate rearrangement introduces quaternary carbon centers adjacent to the amine moiety to enable late-stage functionalization of complex bioactive molecules. This versatile tool for complex amine synthesis holds significant potential for accelerating advancements in drug discovery, chemical biology, and materials science research.

Fully conjugated tetraborylethylene: selenium mediated C-C double bond formation from diborylcarbenoid

Heteroatom-substituted ethylenes have long been studied owing to their potential application to electronic devices. In contrast to well-studied π-donor substituted ethylene, the π-acceptor substituted one has only been limitedly reported. While boron can be a candidate of π-acceptors, there has still been no example of fully conjugated tetraborylethylene (TBE). Herein, we synthesized the first fully conjugated TBE 2 by selenium-mediated C-C double bond formation from diborylcarbenoid 1, a synthetic equivalent of diborylcarbene (DBC). An intermediate of bis(diborylmethylene)-λ4-selane 3Se, wherein two DBC fragments were bound to one selenium atom, was confirmed. TBE 2 has a longer C-C bond length of 1.368(2) Å than typical C-C double bonds (1.34 Å) owing to π-electron deficiency. By density functional theory calculations, the LUMO was found to be low-lying at -1.75 eV by the contribution of vacant p-orbitals on the boron atoms adjacent to the C-C double bond.

The current utility and future potential of multiborylated alkanes

Organoboron chemistry has become a cornerstone of modern synthetic methodology. Most of these reactions use an organoboron starting material that contains just one C(sp2)-B or C(sp3)-B bond; however, there has been a recent and accelerating trend to prepare multiborylated alkanes that possess two or more C(sp3)-B bonds. This is despite a lack of general reactivity, meaning many of these compounds currently offer limited downstream synthetic value. This Review summarizes recent advances in the exploration of multiborylated alkanes, including a discussion on how these products may be elaborated in further synthetic manipulations.

The Strategies Towards Electrochemical Generation of Aryl Radicals

The advancement in electrochemical techniques has unlocked a new path for achieving unprecedented oxidations and reductions of aryl radical precursors in a controlled and selective manner. This approach facilitates the construction of aromatic carbon-carbon and carbon-heteroatom bonds. In light of the green merits and the growing importance of this technique in aryl radical chemistry, this review aims to provide an overview of the recent advance in the electrochemical generation of aryl radicals organized by the aryl radical precursor type, with a focus on the substrate scope, limitation, and underlying mechanism, thereby inspiring future work on electrochemical aryl radical generation.

Chemoselective Synthesis of α-Tertiary Hydroxy Oximes via Photochemical 1,3-Boronate Rearrangement

Tertiary alcohols with adjacent nucleophilic labile groups are prevalent in bioactive molecules but are challenging to synthesize. Herein we introduce a direct, protecting group-free method to access α-tertiary hydroxy oximes via photochemical 1,3-boronate rearrangement. This reaction delivers high yields (up to 94%) using readily available boronic acids, is scalable to gram quantities, and allows for further derivatization to other nitrogen-containing molecules.

Electrochemical Generation of Aryl Radicals from Organoboron Reagents Enabled by Pulsed Electrosynthesis

Aryl radicals play a pivotal role as reactive intermediates in chemical synthesis, commonly arising from aryl halides and aryl diazo compounds. Expanding the repertoire of sources for aryl radical generation to include abundant and stable organoboron reagents would significantly advance radical chemistry and broaden their reactivity profile. While traditional approaches utilize stoichiometric oxidants or photocatalysis to generate aryl radicals from these reagents, electrochemical conditions have been largely underexplored. Through rigorous mechanistic investigations, we identified fundamental challenges hindering aryl radical generation. In addition to the high oxidation potentials of aromatic organoboron compounds, electrode passivation through radical grafting, homocoupling of aryl radicals, and decomposition issues were identified. We demonstrate that pulsed electrosynthesis enables selective and efficient aryl radical generation by mitigating the fundamental challenges. Our discoveries facilitated the development of the first electrochemical conversion of aryl potassium trifluoroborate salts into aryl C-P bonds. This sustainable and straightforward oxidative electrochemical approach exhibited a broad substrate scope, accommodating various heterocycles and aryl chlorides, typical substrates in transition-metal catalyzed cross-coupling reactions. Furthermore, we extended this methodology to form aryl C-Se, C-Te, and C-S bonds, showcasing its versatility and potential in bond formation processes.

PolyBorylated Alkenes as Energy-Transfer Reactive Groups: Access to Multi-Borylated Cyclobutanes Combined with Hydrogen Atom Transfer Event

While polyborylated alkenes are being recognized for their elevated status as highly valuable reagents in modern organic synthesis, allowing efficient access to a diverse array of transformations, including the formation of C-C and C-heteroatom bonds, their potential as energy-transfer reactive groups has remained unexplored. Yet, this potential holds the key to generating elusive polyborylated biradical species, which can be captured by olefins, thereby leading to the construction of new highly-borylated scaffolds. Herein, we report a designed energy-transfer strategy for photosensitized [2+2]-cycloadditions of poly-borylated alkenes with various olefins enabling the regioselective synthesis of diverse poly-borylated cyclobutane motifs, including the 1,1-di-, 1,1,2-tri-, and 1,1,2,2-tetra-borylated cyclobutanes. In fact, these compounds belong to a family that presently lacks efficient synthetic pathways. Interestingly, when α-methylstyrene was used, the reaction involves an interesting 1,5-hydrogen atom transfer (HAT). Mechanistic deuterium-labeling studies have provided insight into the outcome of 1,5-hydrogen atom transfer process. In addition, the polyborylated cyclobutanes are then demonstrated to be useful in selective oxidation processes resulting in the formation of cyclobutanones and γ-lactones.

Photogeneration of α-Bimetalloid Radicals via Selective Activation of Multifunctional C1 Units

Light-driven strategies that enable the chemoselective activation of a specific bond in multifunctional systems are comparatively underexplored in comparison to transition-metal-based technologies, yet desirable when considering the controlled exploration of chemical space. With the current drive to discover next-generation therapeutics, reaction design that enables the strategic incorporation of an sp3 carbon center, containing multiple synthetic handles for the subsequent exploration of chemical space would be highly enabling. Here, we describe the photoactivation of ambiphilic C1 units to generate α-bimetalloid radicals using only a Lewis base and light source to directly activate the C-I bond. Interception of these transient radicals with various SOMOphiles enables the rapid synthesis of organic scaffolds containing synthetic handles (B, Si, and Ge) for subsequent orthogonal activation. In-depth theoretical and mechanistic studies reveal the prominent role of 2,6-lutidine in forming a photoactive charge transfer complex and in stabilizing in situ generated iodine radicals, as well as the influential role of the boron p-orbital in the activation/weakening of the C-I bond. This simple and efficient methodology enabled expedient access to functionalized 3D frameworks that can be further derivatized using available technologies for C-B and C-Si bond activation.

Regio- and Stereo-Selective Isomerization of Borylated 1,3-Dienes Enabled by Selective Energy Transfer Catalysis

Configurationally-defined dienes are pervasive across the bioactive natural product spectrum, where they typically manifest themselves as sorbic acid-based fragments. These C5 motifs reflect the biosynthesis algorithms that facilitate their construction. To complement established biosynthetic paradigms, a chemical platform to facilitate the construction of stereochemically defined, functionalizable dienes by light-enabled isomerization has been devised. Enabled by selective energy transfer catalysis, a variety of substituted β-boryl sorbic acid derivatives can be isomerized in a regio- and stereo-selective manner (up to 97 : 3). Directionality is guided by a stabilizing nO→pB interaction in the product: this constitutes a formal anti-hydroboration of the starting alkyne. This operationally simple reaction employs low catalyst loadings (1 mol %) and is complete in 1 h. X-ray analysis supports the hypothesis that the nO→pB interaction leads to chromophore bifurcation: this provides a structural foundation for selective energy transfer.

Modular Synthesis of Complex Benzoxaboraheterocycles through Chelation-Assisted Rh-Catalyzed [2 + 2 + 2] Cycloaddition

Benzoxaboraheterocycles (BOBs) are moieties of increasing interest in the pharmaceutical industry; however, the synthesis of these compounds is often difficult or impractical due to the sensitivity of the boron moiety, the requirement for metalation-borylation protocols, and lengthy syntheses. We report a straightforward, modular approach that enables access to complex examples of the BOB framework through a Rh-catalyzed [2 + 2 + 2] cycloaddition using MIDA-protected alkyne boronic acids. The key to the development of this methodology was overcoming the steric barrier to catalysis by leveraging chelation assistance. We show the utility of the method through synthesis of a broad range of BOB scaffolds, mechanistic information on the chelation effect, intramolecular alcohol-assisted BMIDA hydrolysis, and linear/cyclic BOB limits as well as comparative binding affinities of the product BOB frameworks for ribose-derived biomolecules.

Metal-Free 1,3-Boronate Rearrangement to Ketones Driven by Visible Light

Boronate rearrangements, such as the Matteson and Petasis reactions, are valuable metal-free reactions for the transfer of the carbo group on boron to intramolecular electrophilic sites. However, only highly reactive electrophiles are suitable, and ketones are too inactive for those boronate rearrangements due to the high energy barriers. We disclose here the 1,3-boronate rearrangement to ketones, for which a high energy barrier (44.9 kcal/mol) is prohibitory for thermal reactions in the ground state. The reaction is enabled by the key keto-enol-boronate bidentate complex formation in situ, which absorbs visible light to reach the excited state for the chemoselective 1,3-boronate rearrangement to ketones. Experimental and computational investigations exclude free radical intermediates from organoboronates. The aryl, alkenyl, and alkyl boronic acids react with various 1,3-diketones driven by visible light irradiation to construct structurally diverse β-keto tertiary alcohols under metal-free conditions. The reaction demonstrates substrate diversity with 58 examples, yields up to 98 %, and it is suitable for gram-scale synthesis.

Iron-Catalyzed Enantioselective Multicomponent Cross-Couplings of α-Boryl Radicals

Despite recent interest in the development of iron-catalyzed transformations, methods that use iron-based catalysts capable of controlling the enantioselectivity in carbon-carbon cross-couplings are underdeveloped. Herein, we report a practical and simple protocol that uses commercially available and expensive iron salts in combination with chiral bisphosphine ligands to enable the regio- and enantioselective (up to 91:9) multicomponent cross-coupling of vinyl boronates, (fluoro)alkyl halides, and Grignard reagents. Preliminary mechanistic studies are consistent with rapid formation of an α-boryl radical followed by reversible radical addition to monoaryl bisphosphine-Fe(II) and subsequent enantioselective inner-sphere reductive elimination. From a broader perspective, this work provides a blueprint to develop asymmetric Fe-catalyzed multicomponent cross-couplings via the use of alkenes as linchpins to translocate alkyl radicals, modify their steric and electronic properties, and induce stereocontrol.

A Chemoselective Polarity-Mismatched Photocatalytic C(sp3 )-C(sp2 ) Cross-Coupling Enabled by Synergistic Boron Activation

We report the development of a C(sp3 )-C(sp2 ) coupling reaction using styrene boronic acids and redox-active esters under photoredox catalysis. The reaction proceeds through an unusual polarity-mismatched radical addition mechanism that is orthogonal to established processes. Synergistic activation of the radical precursor and organoboron are critical mechanistic events. Activation of an N-hydroxyphthalimide (NHPI) ester by coordination to boron enables electron transfer, with decomposition leading to a nucleofuge rebound, activating the organoboron to radical addition. The unique mechanism enables chemoselective coupling of styrene boronic acids in the presence of other alkene radical acceptors. The scope and limitations of the reaction, and a detailed mechanistic investigation are presented.

Direct Observation of Triplet States in the Isomerization of Alkenylboronates by Energy Transfer Catalysis

Alkenylboronates are versatile building blocks for stereocontrolled synthesis owing to the traceless nature of the boron group that can be leveraged to achieve highly selective geometric isomerization. Using thioxanthone as an inexpensive photocatalyst, the photoisomerization of these species continues to provide an expansive platform for stereodivergent synthesis, particularly in the construction of bioactive polyenes. Although mechanistic investigations are consistent with light-driven energy transfer, direct experimental evidence remains conspicuously absent. Herein, we report a rigorous mechanistic investigation using two widely used alkenylboronates alongside relevant reference compounds. Through the combination of irradiation experiments, transient absorption spectroscopic studies, kinetic modeling, and DFT calculations with all isomers of the model compounds, it has been possible to unequivocally detect and characterize the perpendicular triplet generated by energy transfer. Our results serve not only as a blueprint for mechanistic studies that are challenging with organic sensitizers, but these guidelines delineated have also enabled the development of more sustainable reaction conditions: for the first time, efficient organocatalytic isomerization under sunlight irradiation has become feasible.

MIDA- and TIDA-Boronates Stabilize α-Radicals Through B-N Hyperconjugation

Multifunctional organoboron compounds increasingly enable the simple generation of complex, Csp3 -rich small molecules. The ability of boron-containing functional groups to modify the reactivity of α-radicals has also enabled a myriad of chemical reactions. Boronic esters with vacant p-orbitals have a significant stabilizing effect on α-radicals due to delocalization of spin density into the empty orbital. The effect of coordinatively saturated derivatives, such as N-methyliminodiacetic acid (MIDA) boronates and counterparts, remains less clear. Herein, we demonstrate that coordinatively saturated MIDA and TIDA boronates stabilize secondary alkyl α-radicals via σB-N hyperconjugation in a manner that allows site-selective C-H bromination. DFT calculated radical stabilization energies and spin density maps as well as LED NMR kinetic analysis of photochemical bromination rates of different boronic esters further these findings. This work clarifies that the α-radical stabilizing effect of boronic esters does not only proceed via delocalization of radical character into vacant boron p-orbitals, but that hyperconjugation of tetrahedral boron-containing functional groups and their ligand electron delocalizing ability also play a critical role. These findings establish boron ligands as a useful dial for tuning reactivity at the α-carbon.

Orange Light-Driven C(sp2)-C(sp3) Cross-Coupling via Spin-Forbidden Ir(III) Metallaphotoredox Catalysis

We report the development and characterization of a library of Ir(III) photocatalysts capable of undergoing spin-forbidden excitation (SFE) under orange light irradiation (595 nm). These catalysts were successfully applied to the construction of synthetically valuable C(sp2)-C(sp3) bonds inaccessible with existing methods of low-energy light-driven dual nickel/photoredox catalysis, demonstrating the synthetic utility of this photocatalyst family. The photocatalysts are capable of accessing both oxidatively and reductively activated coupling partners, illustrated through deaminative arylation and potassium alkyl trifluoroborate cross-coupling reactions with aryl halides. We demonstrate diverse substrate scopes of both cross-coupling paradigms under mild conditions in the first example of low-energy light-driven C(sp2)-C(sp3) metallaphotoredox coupling.

Direct Light-Enabled Access to α-Boryl Radicals: Application in the Stereodivergent Synthesis of Allyl Boronic Esters

Operationally simple strategies to assemble boron containing organic frameworks are highly enabling in organic synthesis. While conventional retrosynthetic logic has engendered many platforms focusing on the direct formation of C-B bonds, α-boryl radicals have recently reemerged as versatile open-shell alternatives to access organoborons via adjacent C-C bond formation. Direct light-enabled α-activation is currently contingent on photo- or transition metal-catalysis activation to efficiently generate radical species. Here, we disclose a facile activation of α-halo boronic esters using only visible light and a simple Lewis base to enable homolytic scission. Intermolecular addition to styrenes facilitates the rapid construction of highly versatile E-allylic boronic esters. The simplicity of activation permits the strategic merger of this construct with selective energy transfer catalysis to enable the complimentary stereodivergent synthesis of Z-allylic boronic esters.

Photoredox Catalytic Installation of an Alkyl/Aryl Side Chain and Deuterium into (S)-Methyleneoxazolidinone: Synthesis of Enantioenriched α-Deuterated α-Amino Acid Derivatives

A photoredox catalytic asymmetric method has been established for the installation of both aliphatic and aromatic side chains and the introduction of deuterium into the chiral methyleneoxazolidinone simultaneously. Efficient coupling of readily available boronic acids with the chiral auxiliary delivers structurally diverse α-deuterated α-amino acid derivatives with a high level of diastereoselectivity and deuteration.

Pd-Catalyzed Organometallic-Free Homologation of Arylboronic Acids Enabled by Chemoselective Transmetalation

A Pd-catalyzed homologation of arylboronic acids is reported. Halomethylboronic acid pinacol esters (Bpin) undergo a remarkably facile, yet rare, oxidative addition enabled by an α-boryl effect. Simultaneous chemoselective transmetalation allows use of these metalloid reagents for formal C₁ insertion to deliver benzyl Bpin products without the requirement for stoichiometric organometallic reagents. The utility of the process is demonstrated by stepwise C(sp³)-C(sp²) cross-coupling of the boronic ester products into diarylmethane pharmacophores and electrophile/nucleophile chemoselective cross-coupling. Control experiments that demonstrate the reactivity enhancement provided by the α-boryl effect are provided, along with a description of the limitations of the formal homologation process.

Photocatalyzed regioselective hydrosilylation for the divergent synthesis of geminal and vicinal borosilanes

Geminal and vicinal borosilanes are useful building blocks in synthetic chemistry and material science. Hydrosilylation/hydroborylation of unsaturated systems offer expedient access to these motifs. In contrast to the well-established transition-metal-catalyzed methods, radical approaches are rarely explored. Herein we report the synthesis of geminal borosilanes from α-selective hydrosilylation of alkenyl boronates via photoinduced hydrogen atom transfer (HAT) catalysis. Mechanistic studies implicate that the α-selectivity originates from a kinetically favored radical addition and an energetically favored HAT process. We further demonstrate selective synthesis of vicinal borosilanes through hydrosilylation of allyl boronates via 1,2-boron radical migration. These strategies exhibit broad scopes across primary, secondary, and tertiary silanes and various boron compounds. The synthetic utility is evidenced by access to multi-borosilanes in a diverse fashion and scaling up by continuous-flow synthesis.