New Radical Borylation Pathways for Organoboron Synthesis Enabled by Photoredox Catalysis

Photocatalytic Regioselective Redox-Neutral 1,3-Oxypyridylation of Aryl Cyclopropanes

Pyridines and cyclopropanes are important structural units in chemistry. Herein, we introduce a photoredox-catalyzed approach for the ring opening and 1,3-oxypyridylation of aryl cyclopropanes using 4-cyanopyridines and carboxylic acids. This sequential process involves single-electron oxidation of the aryl cyclopropane, leading to nucleophilic ring opening and radical pyridylation at the benzylic position. The redox-neutral reaction exhibits high regioselectivity under mild reaction conditions, offering a broad substrate scope and wide applicability.

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.

Dual Nickel- and Photoredox-Catalyzed Asymmetric Reductive Cross-Couplings: Just a Change of the Reduction System?

ConspectusIn recent years, nickel-catalyzed asymmetric coupling reactions have emerged as efficient methods for constructing chiral C(sp3) carbon centers. Numerous novel approaches have been reported to rapidly construct chiral carbon-carbon bonds through nickel-catalyzed asymmetric couplings between electrophiles and nucleophiles or asymmetric reductive cross-couplings of two different electrophiles. Building upon these advances, our group has been devoted to interrogating dual nickel- and photoredox-catalyzed asymmetric reductive cross-coupling reactions.In our endeavors over the past few years, we have successfully developed several dual Ni-/photoredox-catalyzed asymmetric reductive cross-coupling reactions involving organohalides. While some probably think that this system is just a change of the reduction system from traditional metal reductants to a photocatalysis system, a question that we also pondered at the beginning of our studies, both the achievable reaction types and mechanisms suggest a different conclusion: that this dual catalysis system has its own advantages in the chiral carbon-carbon bond formation. Even in certain asymmetric reactions where the photocatalysis regime functions only as a reducing system, the robust reducing capability of photocatalysts can effectively accelerate the regeneration of low-valent nickel species, thus expanding the selectable scope of chiral ligands. More importantly, in many transformations, besides reducing nickel catalysts, the photocatalysis system can also undertake the responsibility of alkyl radical formation, thereby establishing two coordinated, yet independent catalytic cycles. This catalytic mode has been proven to play a crucial role in achieving diverse asymmetric coupling reactions with great challenges.In this Account, we elucidate our understanding of this system based on our experience and findings. In the Introduction, we provide an overview of the main distinctions between this system and traditional Ni-catalyzed asymmetric reductive cross-couplings with metal reductants and the potential opportunities arising from these differences. Subsequently, we outline various chiral carbon-carbon bond-forming types obtained by this dual Ni/photoredox catalysis system and their mechanisms. In terms of chiral C(sp3)-C(sp2) bond formation, extensive discussion focuses on the asymmetric arylations of α-chloroboronates, α-trifluoromethyl alkyl bromides, α-bromophosphonates, and so on. In the realm of chiral C(sp3)-C(sp) bond formation, asymmetric alkynylations of α-bromophosphonates and α-trifluoromethyl alkyl bromides have been presented herein. Regarding C(sp3)-C(sp3) bond formation, we take the asymmetric alkylation of α-chloroboronates as a compelling example to illustrate the great efficiency of this dual catalysis system. This summary would enable a better grasp of the advantages of this dual catalysis system and clarify how the photocatalysis regime facilitates enantioselective transformations. We anticipate that this Account will offer valuable insights and contribute to the development of new methodologies in this field.

Halogen-Atom Transfer Enabled Catalytic Enantioselective Coupling to Chiral Trifluoromethylated Alkynes via Dual Nickel and Photocatalysis

With halogen-atom transfer as an effective tool, a novel catalytic enantioselective protocol to generate chiral trifluoromethylated alkynes has been established by a cooperative photoredox and nickel catalysis system, providing a straightforward and modular route to access this type of product in good yields and enantioselectivities. The halogen-atom transfer process is essential for the reaction and this novel strategy offers another promising way to utilize alkyl halides with highly negative reduction potentials. It firstly expands nickel-catalyzed asymmetric reductive cross-couplings of organohalides from the traditional single-electron transfer to halogen-atom transfer.

Catalyst-Free α-trans-Selective Hydroboration and (E)-Selective Deuterated Semihydrogenation of Alkynyl Sulfones

Here, we present a straightforward α-trans-selective hydroboration of alkynyl sulfones with NHC-boranes without the need for a catalyst. This reaction is compatible with a wide range of substrates for efficiently producing structurally diverse α-borylated vinyl sulfones in satisfactory yields. The hydride transfer from NHC-borane 2a to alkynyl triflone 1b is studied by density functional theory (DFT) calculations for trans-hydroboration. Moreover, a regiodivergent deuterated semihydrogenation of alkynyl triflones has also been developed using D2O as the deuterium source. A variety of diversity-oriented D-containing vinyl triflones were prepared in good to excellent yields with excellent deuterium incorporation ratios. Synthetic manipulations of the deuterated products are achieved for the conversion into valuable deuterated molecules, indicating the utility of this protocol.

Nickel-catalyzed hydrodefluorination/deuterodefluorination of CF3-alkenes with formic acid

The synthesis of deuterated gem-difluoroalkenes via selective deuterodefluorination of β-CF3-cinnamates using a nickel catalyst has been reported for the first time. Commercially available deuterated formic acid is a cheap and convenient deuterium source. The nickel-catalyst showed high selectivity for monodefluorination and avoided competitive reactions such as multiple defluorination or hydrogenation.

Photoredox-Catalyzed Acylchlorination of α-CF3 Alkenes with Acyl Chloride and Application as Masked Access to β-CF3-enones

We disclose a photocatalytic strategy that simultaneously addresses the construction of trifluoromethylated quaternary carbon centers and the preparation of β-CF3-enones through radical difunctionalization of α-CF3 alkenes with acyl chlorides. This method is characterized by its broad functional group compatibility, high efficiency, and atom economy. The versatility of this transformation is poised to broaden the applications of α-CF3 alkenes, providing new pathways for the rapid assembly of structurally diverse fluorinated compounds.

Visible Light-Induced 1,2-Diphenyldisulfane-Mediated Defluoroborylation of Polyfluoroarenes

A green and practical protocol of defluoroborylation of polyfluoroarenes with stable and readily accessible NHC-borane was developed, using 1,2-diphenyldisulfane as a hydrogen atom transfer (HAT) and single electron transfer (SET) reagent precursor under visible-light irradiation, leading to the concise formation of value-added fluorinated organoboron scaffolds. Mechanism studies revealed the method underwent a boryl radical addition reaction with polyfluoroarene, followed by successive single electron transfer pathways and defluorination of the C-F bond to offer the targeted product. This unprecedented platform relies on 1,2-diphenyldisulfane and base without using expensive photocatalysts, highlighting the methodology has promising application value to prepare borylated polyfluoroarene compounds.

Radical Cross Coupling and Enantioselective Protonation through Asymmetric Photoredox Catalysis

An unprecedented enantioselective protonation reaction enabled by photoredox catalytic radical coupling is developed. Under cooperative dicynopyrazine-derived chromophore (DPZ) as a photosensitizer and a chiral phosphoric acid catalyst, and Hantzsch ester as a sacrificial reductant, the transformations between α-substituted enones and cyanoazaarenes or 2-(chloromethyl)azaaren-1-iums can proceed a tandem reduction, radical coupling, and enantioselective protonation process efficiently. Two classes of pharmaceutically important enantioenriched azaarene variants, which contain a synthetically versatile ketone-substituted tertiary carbon stereocenter at the β- or γ-position of the azaarenes, are synthesized with high yields and ees.

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.

Photoredox Enabled Defluorinative Benzylation of Trifluoromethyl Alkenes with Alkylarenes

Herein, we report a photoredox enabled defluorinative benzylation of trifluoromethyl alkenes with readily available alkylarenes, which provides convenient access to a series of structurally valuable benzylated gem-difluoroalkenes under mild reaction conditions. The synthetic value of this protocol has been demonstrated by the transformations of several substrates bearing drug moieties, gram-scale reactions, and various further derivatizations of the gem-difluoroalkene products. The preliminary mechanistic investigations suggest a reaction pathway with rate-determining benzyl C-H bond cleavage of toluene followed by benzylic radical formation.

Solvent-Controllable C-F Bond Activation for Masked Formylation of α-Trifluoromethyl Alkenes via Organo-Photoredox Catalysis

A solvent-controllable organo-photoredox-catalyzed C-F bond activation for masked formylation of α-trifluoromethyl alkenes with low-priced 1,3-dioxolane as masked formyl radical equivalent has been described. Consequently, a diversity of masked formylated gem-difluoroalkenes and monofluoroalkenes are constructed in moderate to high yields. This approach merits readily available starting materials, mild reaction conditions, and broad substrate scope. The feasibility of this approach has been highlighted by the one-pot masked formylation/hydrolysis sequence to form γ,γ-difluoroallylic aldehydes and late-stage modification of pharmaceutical and natural product derivatives.

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.

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.

Scandium-Catalyzed Electrochemical Synthesis of α-Pyridinyl Tertiary Amino Acids and Esters

α-Pyridyl tertiary amino acids have potential pharmaceutical applications because of their structural features. However, their synthesis is still highly limited. Herein, we report a straightforward approach for the electrochemical synthesis of tertiary α-substituted amino acid derivatives via three-component reductive coupling. Using gaseous ammonia as both the N and H source, the α-keto ester reacts directly with 4-CN-pyridine. The application of scandium catalysis is the key for achieving chemoselectivity among various side reaction pathways.

Regioselective Electrophilic Addition to Propargylic B(MIDA)s Enabled by β-Boron Effect

Electrophilic addition reaction to alkynes is of fundamental importance in organic chemistry, yet the regiocontrol when reacting with unsymmetrical 1,2-dialkyl substituted alkynes is often problematic. Herein, it is demonstrated that the rarely recognized β-boron effect can confer a high level of site-selectivity in several alkyne electrophilic addition reactions. A broad range of highly functionalized and complex organoborons are thus formed under simple reaction conditions starting from propargylic MIDA (N-methyliminodiacetic acid) boronates. These products are demonstrated to be valuable building blocks in organic synthesis. In addition to the regiocontrol, this study also observes a drastic rate enhancement upon B(MIDA) substitution. Theoretical calculation reveals that the highest occupied molecular obital (HOMO) energy level of propargylic B(MIDA) is significantly raised by 0.3 eV, and the preferential electrophilic addition to the γ position is due to its higher HOMO orbital coefficient and more negative natural bond orbital (NBO) charge compared to the β position. This study demonstrates the potential of utilizing the β-boron effect in stereoelectronic control of chemical transformations, which can inspire further research in this area.

Base-Mediated α-gem-Difluoroalkenylations of Aldehydes and Ketones

Herein, we present a base-mediated nucleophilic substitution reaction of α-trifluoromethylstyrenes with simple silyl enol ethers, enabling the efficient synthesis of carbonyl-substituted gem-difluoroalkenes. The merit of this protocol is exhibited by its mild reaction conditions, broad substrate scope, and scalable preparation. Notably, this method demonstrates its applicability for late-stage functionalization of structurally complex molecules. Moreover, we illustrate that the resulting products can serve as valuable precursors for the synthesis of diverse medicinally relevant compounds.

N-Heterocyclic Carbene Boranes: Dual Reagents for the Synthesis of Gold Nanoparticles

N-Heterocyclic carbenes (NHCs) have drawn considerable interest in the field of nanomaterials chemistry as highly stabilizing ligands enabling the formation of strong and covalent carbon-metal bonds. Applied to gold nanoparticles synthesis, the most common strategy consists of the reduction of a preformed NHC-AuI complex with a large excess of a reducing agent that makes the particle size difficult to control. In this paper, we report the straightforward synthesis of NHC-coated gold nanoparticles (NHC-AuNPs) by treating a commercially available gold(I) precursor with an easy-to-synthesize NHC-BH3 reagent. The latter acts as both the reducing agent and the source of surface ligands operating under mild conditions. Mechanistic studies including NMR spectroscopy and mass spectrometry demonstrate that the reduction of gold(I) generates NHC-BH2 Cl as a by-product. This strategy gives efficient control over the nucleation and growth of gold particles by varying the NHC-borane/gold(I) ratio, allowing unparalleled particle size variation over the range of 4.9±0.9 to 10.0±2.7 nm. Our strategy also allows an unprecedented precise and controlled seeded growth of gold nanoparticles. In addition, the as-prepared NHC-AuNPs exhibit narrow size distributions without the need for extensive purification or size-selectivity techniques, and are stable over months.

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.

Fe-Catalyzed Alkylazidation of α-Trifluoromethylalkenes: An Access to Quaternary Stereocenters Containing CF3 and N3 Groups

A concise Fe-catalyzed alkylazidation of α-trifluoromethylalkenes via a C-C bond cleavage/radical addition/azidation cascade is described. This protocol features a broad substrate scope, excellent functional group compatibility, and the ability to be performed on a gram scale, thus offering a practical and step-economic approach to the synthetically useful tertiary α-trifluoromethyl azides.

Synthesis of α-Boryl Ketones via Hydration or Oxidation of B(MIDA)-Decorated Alkynes

α-Boryl ketones are traditionally challenging targets in organic synthesis. Reported herein is a mild and metal-free synthesis of α-boryl ketones via the hydration or oxidation of N-methyliminodiacetyl boronate (B(MIDA))-decorated alkynes. A new hydration system comprised of AcCl and H₂O in HFIP allows the hydration of arylethynyl B(MIDA)s at room temperature with decent functional group tolerance. An oxidative carbon deletion process of propargylic B(MIDA)s is also developed for the synthesis of aliphatic α-boryl ketones. An intriguing β-boron effect was observed to account for the unique site- and chemoselectivities. The application of the products in the synthesis of borylated heterocycles was demonstrated.

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.

Tuning the Photocatalytic Performance of Ruthenium(II) Polypyridine Complexes Via Ligand Modification for Visible-Light-Induced Phosphorylation of Tertiary Aliphatic Amines

Tuning the redox potential of commonly available photocatalyst to improve the catalytic performance or expand its scope for challenging synthetic conversions is an ongoing demand in synthetic chemistry. Herein, the excited state properties and redox potential of commercially available [Ru(bpy)₃ ]²⁺ photocatalyst were tuned by modifying the structure of the bipyridine ligands with electron-donating/withdrawing units. The visible-light-mediated photoredox phosphorylation of tertiary aliphatic amines was demonstrated under mild conditions. A series of cross-dehydrogenative coupling reactions were performed employing the Ru^II complexes as photocatalyst giving the corresponding α-aminophosphinoxides and α-aminophosphonates via carbon-phosphorus (C-P) bond formation.

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.

C4-Selective C-H Borylation of Pyridinium Derivatives Driven by Electron Donor-Acceptor Complexes

The photoinduced C4-selective C-H borylation of pyridines was achieved using electron donor-acceptor complexes derived from a Lewis base and N-amidopyridinium salts under external oxidant- and photocatalyst-free conditions. Notably, the nucleophilic character of phosphite-ligated boryl radicals enables addition of a radical to position C4 of pyridinium salts to afford C4-borylated heteroarenes that are otherwise difficult to obtain. This approach provides a versatile platform for the installation of both phosphite- and amine-coordinated boron groups on a series of pyridines under mild conditions, demonstrating excellent C4-positional selectivity for the pyridine scaffolds.

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.

Synthesis of remote fluoroalkenyl ketones by photo-induced ring-opening addition of cyclic alkoxy radicals to fluorinated alkenes

Fluoroalkenyl moieties are often used as carbonyl mimics in medicine preparation, and thus the development of facile routes for the synthesis of such compounds is of great importance. In this work, we report a photocatalytic ring-opening addition of cyclic alcohols to α-(trifluoromethyl)styrenes, which underwent a proton-coupled electron transfer and β-scission process, delivering a great variety of remote gem-difluoroalkenyl ketone derivatives. This methodology can also be applied in the reaction of gem-difluorostyrenes and 1,1,2-trifluorostyrenes to access monofluoro- and 1,2-difluoroalkenyl ketones.

A linear Di-coordinate boron radical cation

The pursuit of di-coordinate boron radical has been continued for more than a half century, and their stabilization and structural characterization remains a challenge. Here we report the isolation and structural characterization of a linear di-coordinate boron radical cation, achieved by stabilizing the two reactive atomic orbitals of the central boron atom by two orthogonal π-donating and π-accepting functionalities. The electron deficient radical cation undergoes facile one-electron reduction to borylene and binds Lewis base to give heteroleptic tri-coordinate boron radical cation. The co-existence of half-filled and empty p orbitals at boron also allows the CO-regulated electron transfer to be explored. As the introduction of CO promotes the electron transfer from a tri-coordinate neutral boron radical to a boron radical cation, the removal of CO under vacuum furnishes the reverse electron transfer from borylene to yield a solution consisting of two boron radicals.

Palladium-catalyzed asymmetric allylic 4-pyridinylation via electroreductive substitution reaction

The enantioselective pyridinylation is important for providing chiral compounds bearing heterocycles of pharmaceutical interests. 4-CN-pyrinde is extensively applied in the radical pyridinylation reaction, however, its' enantioselective application is highly challenging. To achieve this goal, we propose an electrochemical catalytic activation of 4-CN-pyridine with a chiral transition metal complex instead of direct cathodic reduction. The chiral catalyst acts as the electron mediator and the transition metal catalysis in turn. The radical species from 4-CN-pyridine is captured via radical rebound by chiral catalyst, and undergoes enantioselective pyridinylation reaction. Here, we show the first method for catalytic asymmetric allylic 4-pyridinylation reactions using 4-CN-pyridine under electrochemical conditions.

Site-Selective Functionalization of Carboranes at the Electron-Rich Boron Vertex: Photocatalytic B-C Coupling via a Carboranyl Cage Radical

Functionalization of carboranes in a vertex-specific manner is a perennial challenge. Here, we report a photocatalytic B-C coupling for the selective functionalization of carboranes at the boron site which is most distal to carbon. This reaction was achieved by the photo-induced decarboxylation of carborane carboxylic acids to generate boron vertex-centered carboranyl radicals. Theoretical calculations also demonstrate that the reaction more easily occurs at the boron site bearing higher electron density owing to the lower energy barrier for a single-electron transfer to generate a carboranyl radical. By using this strategy, a number of functionalized carboranes could be accessed through alkylation, alkenylation, and heteroarylation under mild conditions. Moreover, both a highly efficient blue emitter with a solid-state luminous efficiency of 42 % and a drug candidate for boron neutron capture therapy (BNCT) containing targeting and fluorine units were obtained.

Visible-light mediated catalytic asymmetric radical deuteration at non-benzylic positions

Site- and enantioselective incorporation of deuterium into organic compounds is of broad interest in organic synthesis, especially within the pharmaceutical industry. While catalytic approaches relying on two-electron reaction manifolds have allowed for stereoselective delivery of a formal deuteride (D-) or deuteron (D+) at benzylic positions, complementary strategies that make use of one-electron deuterium atom transfer and target non-benzylic positions remain elusive. Here we report a photochemical approach for asymmetric radical deuteration by utilizing readily available peptide- or sugar-derived thiols as the catalyst and inexpensive deuterium oxide as the deuterium source. This metal-free platform enables four types of deuterofunctionalization reactions of exocyclic olefins and allows deuteration at non-benzylic positions with high levels of enantioselectivity and deuterium incorporation. Computational studies reveal that attractive non-covalent interactions are responsible for stereocontrol. We anticipate that our findings will open up new avenues for asymmetric deuteration.

Visible-light-driven PhSSPh-catalysed regioselective hydroborylation of α,β-unsaturated carbonyl compounds with NHC-boranes

A photo-induced transition-metal-free regioselective hydroborylation of α,β-unsaturated carbonyl compounds is developed. The PhSSPh reagent was employed as the photocatalyst, and NHC-BH₃ was used as the boron source. This transformation shows a broad substrate scope and provides a wide range of α-borylcarbonyl molecules in good to excellent yields.

Modular synthesis of α-arylated carboxylic acids, esters and amides via photocatalyzed triple C-F bond cleavage of methyltrifluorides

α-Arylated carboxylic acids, esters and amides are widespread motifs in bioactive molecules and important building blocks in chemical synthesis. Thus, straightforward and rapid access to such structures is highly desirable. Here we report an organophotocatalytic multicomponent synthesis of α-arylated carboxylic acids, esters and amides from exhaustive defluorination of α-trifluoromethyl alkenes in the presence of alkyltrifluoroborates, water and nitrogen/oxygen nucleophiles. This operationally simple strategy features a unified access to functionally diverse α-arylated carboxylic acids, esters, and primary, secondary, and tertiary amides through backbone assembly from simple starting materials enabled by consecutive C–F bond functionalization at room temperature. Preliminary mechanistic investigations reveal that the reaction operates through a radical-triggered three-step cascade process, which involves distinct mechanisms for each defluorinative functionalization of the C–F bond.

Here we report an organophotocatalytic synthesis of α-arylated carboxylic acids, esters and amides from exhaustive defluorination of α-trifluoromethyl alkenes in the presence of alkyltrifluoroborates, water and nitrogen/oxygen nucleophiles.

Light-Mediated Defluorosilylation of α-Trifluoromethyl Arylalkenes through Hydrogen Atom Transfer

Herein, we report a direct, light-mediated defluorosilylation protocol for converting α-trifluoromethyl arylalkenes and alkyl silanes into γ,γ-difluoroallylic compounds via a combination of photoredox catalysis and hydrogen atom transfer. The clean, convenient protocol can be scaled to the gram level, and its mild conditions make it very suitable for late-stage functionalization of complex natural products and drugs.

A general electron donor-acceptor complex for photoactivation of arenes via thianthrenation

General photoactivation of electron donor-acceptor (EDA) complexes between arylsulfonium salts and 1,4-diazabicyclo[2.2.2]octane with visible light or natural sunlight was discovered. This practical and efficient mode enables the production of aryl radicals under mild conditions, providing an unrealized opportunity for two-step para-selective C-H functionalization of complex arenes. The novel mode for generating aryl radicals via an EDA complex was well supported by UV-vis absorbance measurements, nuclear magnetic resonance titration experiments, and density functional theory (DFT) calculations. The method was applied to the regio- and stereo-selective arylation of various N-heterocycles under mild conditions, yielding an assembly of challengingly linked heteroaryl-(hetero)aryl products. Remarkably, the meaningful couplings of bioactive molecules with structurally complex drugs or agricultural pharmaceuticals were achieved to display favorable in vitro antitumor activities, which will be of great value in academia or industry.

Radical borylation of vinyl azides with NHC-boranes: divergent synthesis of α-boryl ketones and borylated triazoles

A divergent radical borylation of vinyl azides with N-heterocyclic carbene (NHC) boranes in the presence of ^tBuSH is described. The protocol enables the divergent synthesis of α-boryl ketones and borylated triazoles with excellent functional group tolerance and a broad substrate scope. Remarkably, this work shows that vinyl azides can serve as unprecedented five-atom synthons for the construction of 1,2,3-triazoles without N₂ extrusion.

Catalytic Asymmetric Reductive Azaarylation of Olefins via Enantioselective Radical Coupling

Visible-light-driven photocatalytic reductive azaarylation has been widely used to construct the important imine-containing azaarene derivatives. In addition to the direct use of various commercially available cyanoazaarenes as feedstocks, the synthetic advantages include precise regioselectivity, high efficiency, mild reaction conditions, and good functional group tolerance. However, although many efficient reductive azaarylation methods have been established, the example of an enantioselective manner is still unmet, which most likely can be ascribed to the highly reactive radical coupling as the key step of forming stereocenters. Exploring the feasibility of enantiocontrol thus constitutes an attractive but highly challenging task. Here, we demonstrate that chiral hydrogen-bonding/photosensitizer catalysis is a viable platform as it enables the realization of the first enantioselective manifold. A variety of acyclic and cyclic enones as the reaction partners are compatible with the dual catalyst system, leading to a wide array of valuable enantioenriched azaarene variants with high yields and ees. Regulating the types of chiral catalysts represents one of the important manners to success, in which several readily accessible Cinchona alkaloid-derived bifunctional catalysts are introduced in asymmetric photochemical reactions.

Hydropyridylation of α,β-Unsaturated Esters through Electroreduction of 4-Cyanopyridine

A mild and highly efficient method for the hydropyridylation of α,β-unsaturated esters has been developed. This protocol provides the products smoothly with a wide substrate scope in an undivided cell under ambient conditions. Moreover, studies showed that the scope could be extended to other unsaturated compounds, including enones and aldehydes.

Photo-induced trifunctionalization of bromostyrenes via remote radical migration reactions of tetracoordinate boron species

Tetracoordinate boron species have emerged as radical precursors via deboronation by photo-induced single electron transfer (SET) pathway. These reactions usually produce an alkyl radical and boron-bound species, and the valuable boron species are always discarded as a by-product. Given the importance of boron species, it will be very attractive if the two parts could be incorporated into the eventual products. Herein we report a photo-catalyzed strategy in which in situ generated tetracoordinated boron species decomposed into both alkyl radicals and boron species under visible light irradiation, due to the pre-installation of a vinyl group on the aromatic ring, the newly generated alkyl radical attacks the vinyl group while leaving the boron species on ipso-position, then both radical part and boron moiety are safely incorporated into the final product. Tertiary borons, secondary borons, gem-diborons as well as 1,2-diborons, and versatile electrophiles are all well tolerated under this transformation, of note, ortho-, meta- and para-bromostyrenes all demonstrated good capabilities. The reaction portraits high atom economy, broad substrate scope, and diversified valuable products with tertiary or quaternary carbon center generated, with diborons as substrates, Csp²-B and Csp³-B are established simultaneously, which are precious synthetic building blocks in chemical synthesis.

Tetracoordinate boron species are common radical precursors in organic synthesis, but the boron species are discarded as by-products. Herein the authors report a strategy to incorporate both the alkyl moiety and boron species into the eventual products, yielding organoboron compounds.

Recent Advances in the Construction of Fluorinated Organoboron Compounds

Fluorinated organoboron compounds are important synthetic building blocks that combine the unique characteristics of a fluorinated motif with the versatile synthetic applications of organoboron moiety. This review article guides the research on fluorinated organoboron compounds mainly from four aspects in recent years: selective monodefluoroborylation of polyfluoroarenes and polyfluoroalkenes, selective borylation of fluorinated substrates, selective fluorination of organoboron compounds, and borofluorination of alkynes/olefins. In addition, this review will provide a necessary guidance and inspiration for the research on the valuable synthetic building block fluorinated organoboron compounds.

Recent advances in radical enabled selective Csp3-F bond activation of multifluorinated compounds

Fluorine-containing molecules have found broad applications in pharmaceutical and agrochemical industries as introducing fluorine into a molecule could significantly tune the biological activities of parent molecules. Thus, the synthesis of fluorine-containing molecules has received substantial attention over the past few decades. As a complementary strategy for the synthesis of fluorinated compounds through new C_sp³-F bonds formation, selective cleavage of inert C_sp³-F bonds from easily-available and cost-effective multifluorinated molecules, such as fluoroalkylaromatics, α-trifluoromethyl alkenes and α-multifluorinated carbonyl compounds, has been emerging as an attractive alternative to access fluorine-containing molecules. Moreover, the inherent nature of radical reactions offers the opportunity for the selective C_sp³-F functionalizations to occur under mild conditions. In this regard, the development of photoredox catalysis, transition-metal catalysis, or electrochemistry to enable radical species generation via selective C_sp³-F cleavage has gained broad attention and substantial progress has been made over recent years. This highlight summerizes the recent advances in the single-electron-transfer enabled selective functionalizations of C_sp³-F bonds in multifluorinated compounds via radical pathways.