Regioselective Radical Borylation of α,β-Unsaturated Esters and Related Compounds by Visible Light Irradiation with an Organic Photocatalyst

Cu(II)-Catalyzed Hydroboration Reactions of 1,1-Disubstituted α,β-Unsaturated Ketones, Esters, and Amides in Pure Water

Here, a Cu2(OH)2CO3-catalyzed hydroboration reaction of 1,1-disubstituted α,β-unsaturated compounds has been developed. The reaction was carried out using water as a solvent at room temperature except for N-monosubstituted α,β-unsaturated amides. This method is applicable to diverse 1,1-disubstituted α,β-unsaturated ketones, esters, and amides, showing excellent reactivity (up to 98% yield). Gram-scale experiments and functional group transformations further demonstrated the practicality of this method.

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.

Exhaustive Hydrodefluorination or Deuterodefluorination of Trifluoromethylarenes via Metal-Free Photoredox Catalysis

Perfluoroalkyl compounds are persistent environmental pollutants due to their chemical and thermal stability. Hydrodefluorination is one of the most promising strategies for the disposal of fluorine-containing compounds, which has attracted much attention from a broad spectrum of scientific communities. Herein, we disclose a metal-free, visible-light-promoted protocol for the exhaustive hydrodefluorination of a wide variety of trifluoromethylarenes with up to 95% yields. Moreover, methyl-d3 groups can be obtained via deuterium water with a D ratio of up to 94%.

Metal-organic framework boosts heterogeneous electron donor-acceptor catalysis

Metal-organic framework (MOF) is a class of porous materials providing an excellent platform for engineering heterogeneous catalysis. We herein report the design of MOF Zr-PZDB consisting of Zr6-clusters and PZDB (PZDB = 4,4'-(phenazine-5,10-diyl)dibenzoate) linkers, which served as the heterogeneous donor catalyst for enhanced electron donor-acceptor (EDA) photoactivation. The high local concentration of dihydrophenazine active centers in Zr-PZDB can promote the EDA interaction, therefore resulting in superior catalytic performance over homogeneous counterparts. The crowded environment of Zr-PZDB can protect the dihydrophenazine active center from being attacked by radical species. Zr-PZDB efficiently catalyzes the Minisci-type reaction of N-heterocycles with a series of C-H coupling partners, including ethers, alcohols, non-activated alkanes, amides, and aldehydes. Zr-PZDB also enables the coupling reaction of aryl sulfonium salts with heterocycles. The catalytic activity of Zr-PZDB extends to late-stage functionalization of bioactive and drug molecules, including Nikethamide, Admiral, and Myristyl Nicotinate. Systematical spectroscopy study and analysis support the EDA interaction between Zr-PZDB and pyridinium salt or aryl sulfonium salt, respectively. Photoactivation of the MOF-based EDA adduct triggers an intra-complex single electron transfer from donor to acceptor, giving open-shell radical species for cross-coupling reactions. This research represents the first example of MOF-enabled heterogeneous EDA photoactivation.

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.

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.

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.

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.

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.

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.

t BuOLi-Promoted Hydroboration of Esters and Epoxides

Commercially available and inexpensive lithium tert-butoxide ( ^t BuOLi) acts as a good precatalyst for the hydroboration of esters, lactones, and epoxides using pinacolborane as a borylation agent. Functional groups such as cyano-, nitro-, amino-, vinyl, and alkynyl are unaffected under the presented hydroboration process, representing high chemoselectivity. This transformation has also been effectively applied to the synthesis of key intermediates of Erlotinib and Cinacalcet. Preliminary investigations of the mechanism show that the hydroboration proceeds through the in situ formed BH₃ species.

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.

Visible-light-induced 1,2-diphenyldisulfane-catalyzed regioselective hydroboration of electron-deficient alkenes

A photoinduced PhSSPh-catalyzed regioselective borylation of electron-deficient alkenes has been developed for the synthesis of borylated carbonyl, nitrile, sulfone, phosphonate, trifluoromethyl, and gem-diboron compounds.

Copper-catalyzed B−H bond insertion reaction of azide-ynamide with borane adducts via α-imino copper carbenes

A new copper-catalyzed B−H bond insertion reaction between azide-ynamides and borane adducts has been developed, which represents the first B−H bond insertion into α-imino metal carbenes. This protocol enables the...

NHC induced radical formation via homolytic cleavage of B–B bonds and its role in organic reactions

New borylation methodologies have been reported recently, wherein diboron(4) compounds apparently participate in free radical couplings via the homolytic cleavage of the B–B bond. We report herein that bis-NHC adducts of the type (NHC)₂·B₂(OR)₄, which are thermally unstable and undergo intramolecular ring expansion reactions (RER), are sources of boryl radicals of the type NHC–BR₂˙, exemplified by Me₂Im^Me·Bneop˙ 1a (Me₂Im^Me = 1,3,4,5-tetramethyl-imidazolin-2-ylidene, neop = neopentylglycolato), which are formed by homolytic B–B bond cleavage. Attempts to apply the boryl moiety 1a in a metal-free borylation reaction by suppressing the RER failed. However, based on these findings, a protocol was developed using Me₂Im^Me·B₂pin₂3 for the transition metal- and additive-free boryl transfer to substituted aryl iodides and bromides giving aryl boronate esters in good yields. Analysis of the side products and further studies concerning the reaction mechanism revealed that radicals are likely involved. An aryl radical was trapped by TEMPO, an EPR resonance, which was suggestive of a boron-based radical, was detected in situ, and running the reaction in styrene led to the formation of polystyrene. The isolation of a boronium cation side product, [(Me₂Im^Me)₂·Bpin]⁺I⁻7, demonstrated the fate of the second boryl moiety of B₂pin₂. Interestingly, Me₂Im^Me NHC reacts with aryl iodides and bromides generating radicals. A mechanism for the boryl radical transfer from Me₂Im^Me·B₂pin₂3 to aryl iodides and bromides is proposed based on these experimental observations.

Bis-NHC adducts of the type (NHC)₂·B₂(OR)₄ are sources of boryl radicals of the type NHC–BR₂˙, which are formed by homolytic B–B bond cleavage.

Copper-Catalyzed Cyclization of N-Propargyl Ynamides with Borane Adducts through B-H Bond Insertion

An efficient copper-catalyzed cyclization of N-propargyl ynamides with borane adducts through B-H bond insertion has been developed. A series of valuable organoboron compounds are constructed in generally good yields with a wide substrate scope and good functional group tolerance under mild reaction conditions. Importantly, this protocol via vinyl cation intermediates constitutes a novel way of B-H bond insertion.