N-Heterocyclic Carbene Boranes Research Articles

Borylation with an N-Heterocyclic Carbene-Borane and a CdS Photocatalyst

Borylation with an N-Heterocyclic Carbene-Borane and a CdS Photocatalyst

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.

Visible-Light-Enabled Radical Alkynylborylation of Activated Alkenes

AbstractA photoredox-catalyzed protocol for performing radical difunctionalization of alkenes using N-heterocyclic carbene (NHC) boranes and alkynyl bromines is described. The alkynylborylation difunctionalization reaction involves photoredox generation of boryl radical, with subsequent radical addition to the double bond followed by the capture of alkynyl bromide to form a C–C bond. This method features mild reaction conditions, remarkable chemoselectivity, broad substrate scope and good to excellent yields (up to 89%). The modification of coumarin derivatives indicates that this approach can provide a useful route for the synthesis of complex alkynylborylated products.

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 nano-materials chemistry as highly stabilizing ligands enabling formation of strong and covalent C-metal bonds. Applied to gold nanoparticles synthesis, the most common strategy consists in the reduction of a preformed NHC-Au(I) complex with a large excess of a reducing agent rendering the particle size difficult to control. In this paper, we report on the straightforward synthesis of NHC-coated gold nanoparticles (NHC-AuNPs) by reacting a commercially-available gold(I) precursor with an easy-to-synthesize NHC-BH3reagent. 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-BH2Cl as a byproduct. This strategy garners efficient control over the nucleation and growth of gold particles by varying the NHC-borane/gold(I) ratio, allowing unparalleled particle size variation in the range of 4.9 ± 0.9 nm to 10.0 ± 2.7 nm. Our strategy also allows the unprecedented precise and controlled seeded growth of gold nanoparticles. In addition, the as-prepared NHC-AuNPs exhibit narrow size distributions without engaging 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.

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

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

Light-Promoted Dearylation of Perfluorinated Aryl Sulfides with N-Heterocyclic Carbene-Borane.

The removal of the tetrafluoropyridinyl group tethered to a sulfur atom using a complex of N-heterocyclic carbene (NHC) with borane is described. The reaction is performed under blue light irradiation with a disulfide as radical initiator. The selective cleavage of S-Ar in preference to the weaker S-Alk bond is achieved as a result of aromatic radical substitution of the thiyl group by a NHC-stabilized boron-centered radical. Alkyl thiols, which are the primary products of the dearylation, are in situ oxidized or alkylated, affording disulfides or sulfides, respectively.

Copper-catalyzed radical trans-selective hydroboration of ynamides with N-heterocyclic carbene boranes

SummaryVinylboron compounds are important compounds in organic chemistry and biology. In this communication, we developed a copper(I)-catalyzed, highly regio- and stereoselective radical trans-hydroboration of ynamides with N-heterocyclic carbene (NHC)-ligated borane is reported, which leads to a series of trans-boryl enmides that can be conveniently transformed into various multi-substituted enamides. Further investigation showcased that our method is robust and scalable. The mechanism of this unique reaction is studied and discussed.

Radical Reduction of Polymer Chain-End Functionality by Stoichiometric N-Heterocyclic Carbene Boranes

Radical Reduction of Polymer Chain-End Functionality by Stoichiometric N-Heterocyclic Carbene Boranes

Development of a Borane-(Meth)acrylate Photo-Click Reaction.

In the development of 3D printing fuels, there is a need for new photoinitiating systems working under mild conditions and/or leading to polymers with new and/or enhanced properties. In this context, we introduce herein N-heterocyclic carbene-borane complexes as reagents for a new type of photo-click reaction, the borane-(meth)acrylate click reaction. Remarkably, the higher bond number of boranes relative to thiols induced an increase of the network density associated with faster polymerization kinetics. Solid-state NMR evidenced the strong participation of the boron centers on the network properties, while DMA and AFM showed that the materials exhibit improved mechanical properties, as well as reduced solvent swelling.

Radical Borylative Cyclization of Isocyanoarenes with N-Heterocyclic Carbene Borane: Synthesis of Borylated Aza-arenes.

Borylated aza-arenes are of great importance in the area of organic synthesis. A radical borylative cyclization of isocyanoarenes with N-heterocyclic carbene borane (NHC-BH3) under metal-free conditions was developed. The reaction allows the efficient assembly of several types of borylated aza-arenes (phenanthridines, benzothiazoles, etc.), which are difficult to access using alternative methods. Mild reaction conditions, a good functional-group tolerance, and generally good efficiencies were observed. The utility of these products is demonstrated, and the mechanism is discussed.

Radical trans-Hydroboration of Substituted 1,3-Diynes with an N-Heterocyclic Carbene Borane.

Monohydroboration of substituted 1,3-diynes with an N-heterocyclic carbene borane (NHC-borane) occurs under radical conditions using an azo initiator, such as ACCN and AIBN, and a thiol as a polarity-reversal catalyst. The reaction is highly regio- and stereoselective and provides stable NHC-(E)-alkynylalkenylboranes.

1,4-Hydroboration Reactions of Electron-Poor Aromatic Rings by N-Heterocyclic Carbene Boranes.

Reactions of N-heterocyclic carbene boranes (NHC-boranes) with electron-poor aromatic rings under photoredox conditions provide dearomatized 3-NHC-boryl-1,5-cycohexadienes, which are formally products of 1,4-hydroboration reactions. When regioisomers are possible, the more crowded (doubly ortho-substituted) product is formed preferably or exclusively. The mechanism may involve oxidation of the NHC-borane to an NHC-boryl radical, reduction of the electron-poor aromatic ring to a radical anion, coupling of the radical and the radical anion to give a cyclohexadienyl anion, and finally regioselective protonation.

Revisiting Polyfluoroarenes as Radical Acceptors: Radical C-F Bond Borylation of Polyfluoroarenes with N-Heterocyclic Carbene Boranes and Synthesis of Borane-Containing Liquid Crystals.

Polyfluoroarenes work as good radical acceptors in the thermal radical reaction with N-heterocyclic carbene boranes (NHC-boranes) in the presence of di-tert-butyl peroxide. In this reaction, a C-F bond of polyfluoroarenes is substituted with an NHC-boryl group to provide B-aryl NHC-borane derivatives. The developed synthetic method was applied to the synthesis of novel borane-containing liquid crystalline molecules having highly thermal stability.

Visible-Light-Induced Selective Defluoroborylation of Polyfluoroarenes, gem-Difluoroalkenes, and Trifluoromethylalkenes.

Fluorinated organoboranes serve as versatile synthetic precursors for the preparation of value-added fluorinated organic compounds. Recent progress has been mainly focused on the transition-metal catalyzed defluoroborylation. Herein, we report a photocatalytic defluoroborylation platform through direct B-H activation of N-heterocyclic carbene boranes, through the synergistic merger of a photoredox catalyst and a hydrogen atom transfer catalyst. This atom-economic and operationally simple protocol has enabled defluoroborylation of an extremely broad scope of multifluorinated substrates including polyfluoroarenes, gem-difluoroalkenes, and trifluoromethylalkenes in a highly selective fashion. Intriguingly, the defluoroborylation protocol can be transition-metal free, and the regioselectivity obtained is complementary to the reported transition-metal-catalysis in many cases.

The Thermal Rearrangement of an NHC-Ligated 3-Benzoborepin to an NHC-Boranorcaradiene.

An N-heterocyclic-carbene-ligated 3-benzoborepin with a bridged structure has been synthesized by double radical trans-hydroboration of benzo[3,4]cycloundec-3-ene-1,5-diyne with an N-heterocyclic carbene borane. The thermal reaction of the NHC-ligated borepin at 150 °C gives an isolable NHC-boranorcaradiene. Experiments and density functional theory calculations support a mechanism whereby the borepin initially rearranges to a boranorcaradiene by a thermal 6π-electrocyclic reaction. This is followed by 1,5-boron shift to give a rearranged boranorcaradiene. This shift occurs with stereoinversion at boron through a transition state with open-shell diradical character. This is the first example of the isolation of a boranorcaradiene from a thermal reaction of a borepin.

Direct Synthesis of N‐Heterocyclic Carbene‐Stabilized Copper Nanoparticles from an N‐Heterocyclic Carbene–Borane

N-Heterocyclic carbene (NHC)-stabilized copper nanoparticles (NPs) were synthesized from an NHC-borane adduct and mesitylcopper(I) under thermal conditions (refluxing toluene for 2.5 h). NPs with a size distribution of 11.6±1.8 nm were obtained. The interaction between Cu NPs and NHC ligands was probed by X-ray photoelectron spectroscopy, which showed covalent binding of the NHC to the surface of the NPs. Mechanistic studies suggested that NHC-borane plays two roles: contributing to the reduction of [CuMes]2 to release Cu0 species and providing NHC ligands to stabilize the copper NPs.

Facile Synthesis of α-N-Heterocyclic Carbene-Boryl Ketones from N-Heterocyclic Carbene-Boranes and Alkenyl Triflates.

Reactions of readily available alkenyl triflates with N-heterocyclic carbene (NHC)-boranes in the presence of diisopropyl ethyl amine provided about three dozen stable α-NHC-boryl ketones. Isolated yields were typically 40-56% for B-unsubstituted NHC-boranes (NHC-BH3), and somewhat lower for NHC-boranes with B-substituents (NHC-BH2R). The requisite alkenyl triflates can be made separately or prepared in situ from either ketones or alkynes. The experimental evidence supports a radical chain mechanism that involves the following: (1) addition of an NHC-boryl radical to the alkenyl triflate, (2) fragmentation to give the α-NHC-boryl ketone, SO2, and trifluoromethyl radical, and (3) hydrogen abstraction by trifluoromethyl radical from the starting NHC-borane to return the NHC-boryl radical along with trifluoromethane. Reactions 1 and 3 are both new and evidently rather fast.

Esters as Radical Acceptors: β-NHC-Borylalkenyl Radicals Induce Lactonization by C-C Bond Formation/Cleavage on Esters.

Substituted propargyl acetates are converted into 4-boryl-2(5H)-furanones upon thermolysis in the presence of an N-heterocyclic carbene borane (NHC-borane) and di-tert-butyl peroxide. The acetyl methyl group is lost during the reaction as methane. Evidence suggests that the reaction proceeds by a sequence of radical events including: 1) addition of an NHC-boryl radical to the triple bond; 2) cyclization of the resultant β-borylalkenyl radical to the ester carbonyl group; 3) β-scission of the so-formed alkoxy radical to provide the 4-boryl-2(5H)-furanone and a methyl radical; and 4) hydrogen abstraction from the NHC-borane to return the initial NHC-boryl radical and methane.