Carbene Boranes Research Articles

Alkoxylation of Ketone Mediated by N‐Heterocyclic Carbene Borane and Sulfuric Acid in Alcohol

AbstractIn this study, a novel method for the alkoxylation of ketones has been developed, employing sulfuric acid and N‐heterocyclic carbene boranes (NHC−boranes). The ketones are reduced using NHC−borane (diMe‐Imd‐BH3) and subsequently alkoxylate with sulfuric acid in alcohol solvents. Due to the stability of NHC−borane in the presence of both moisture and air, this reaction emerges as an effective and accessible approach for organic synthesis.

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.

MeOH‐Mediated Efficient Reduction with N‐Heterocyclic Carbene Boranes Towards the Synthesis of Cyclic Sulfamates and Sulfonamides

AbstractAn efficient approach for the reduction of cyclic N‐sulfonylimines using N‐heterocyclic carbene boranes as the hydride donor was developed, leading to various cyclic sulfamates and sulfonamides in excellent yields. The approach had broad scope including both the N‐sulfonyl aldimines and ketimines under mild reaction conditions, which proves that the NHC‐boranes are good alternatives as the reductive reagents.

Preparation of Mono and Bis‐Hydrazino‐Substituted N‐Heterocyclic Carbene Boranes

AbstractWe report the formation of NHC complexes of mono‐ and bis‐hydrazino boranes with N−N−B and N−N−B−N−N chains via the reaction of NHC‐boranes with electrophilic azo reagents. The influence of steric hindrance on the carbene and on the azo reagent has been shown to be crucial for the reactivity. The hydroboration of dissymmetric azo reagents is regioselective only when the latter is sufficiently electronically twisted. Bis‐hydrazino borane complexes with two different hydrazine arms could be obtained via sequential addition.

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.

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.

The Cation‐π Interactions in a Potassium Alkylideneborane Complex

AbstractThe complexation of dibromo‐fluorenylborane (FluH‐BBr2) with bulky bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPr) afforded N‐heterocyclic carbene (NHC) borane adduct 1, which could be subsequently dehydrobrominated by smaller 1,3‐diisopropyl‐4,5‐dimethylimidazol‐2‐ylidene (IPr2Me2) to give the alkylideneborane‐NHC complex 2. Species 2 readily binds with potassium ion to form a potassium alkylideneborane complex 3 through exclusive cation‐π interactions.

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.

A boron-transfer mechanism mediating the thermally induced revival of frustrated carbene\u2013borane pairs from their shelf-stable adducts

Chemists have designed strategies that trigger the conformational isomerization of molecules in response to external stimuli, which can be further applied to regulate the complexation between Lewis acids and bases. We have recently developed a system in which frustrated carbene–borane pairs are revived from shelf-stable but external-stimuli-responsive carbene–borane adducts comprised of N-phosphine-oxide-substituted imidazolylidenes (PoxIms) and triarylboranes. Herein, we report the detailed mechanism on this revival process. A thermally induced borane-transfer process from the carbene carbon atom to the N-phosphinoyl oxygen atom initiates the transformation of the carbene–borane adduct. Subsequent conformational isomerization via the rotation of the N-phosphinoyl group in PoxIm moieties eventually leads to the revival of frustrated carbene–borane pairs that can cleave H2. We believe that this work illustrates an essential role of dynamic conformational isomerization in the regulation of the reactivity of external-stimuli-responsive Lewis acid-base adducts that contain multifunctional substituents.

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.

Development of a Borane–(Meth)acrylate Photo‐Click Reaction

AbstractIn 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.

B(C6F5)3‐Catalyzed Hydroboration of Alkenes with N‐Heterocyclic Carbene Boranes via B—H Bond Activation

Main observation and conclusionIn this work, a novel mode for the activation of N‐heterocyclic carbene boranes (NHC‐boranes) was developed by generating the highly reactive zwitterion species through hydride abstraction with Lewis acid B(C6F5)3 in an frustrated Lewis pairs manner. A broad range of alkenes including stilbenes, β‐methylstyrenes, styrenes, and alkyl‐alkenes were suitable substrates for the B(C6F5)3‐catalyzed hydroboration to furnish the desired products in good to high yields. Significantly, excellent regioselectivities were obtained in some cases. Mechanistic studies indicate that the B—H bond cleavage is likely involved in the rate‐determining step. In addition, an electrophilic addition of NHC‐borenium cation to alkenes and the subsequent formation of carbocation are also postulated. The current work provides a promising method for the activation of stable borane adducts, which might lead to some interesting transformations in the future.

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.

The [(NHC)B(H)C6 F5 ]+ Cations and Their [B](H)-CO Borane Carbonyls.

Hydride abstraction from the heterocyclic carbene borane adducts (NHC)BH2 C6 F5 (NHC: IMes or IMe4 ) gave the B-H containing [(NHC)B(H)C6 F5 ]+ borenium cations. They added carbon monoxide to give the respective [(NHC)B(H)(C6 F5 )CO]+ boron carbonyl cations. Carbon nucleophiles add to the boron carbonyl to give [B](H) acyls. Hydride reduced the [B]CO cation to hydroxymethylborane derivatives.

The [(NHC)B(H)C6F5]+ Cations and Their [B](H)−CO Borane Carbonyls

AbstractHydride abstraction from the heterocyclic carbene borane adducts (NHC)BH2C6F5 (NHC: IMes or IMe4) gave the B−H containing [(NHC)B(H)C6F5]+ borenium cations. They added carbon monoxide to give the respective [(NHC)B(H)(C6F5)CO]+ boron carbonyl cations. Carbon nucleophiles add to the boron carbonyl to give [B](H) acyls. Hydride reduced the [B]CO cation to hydroxymethylborane derivatives.

Visible‐Light‐Induced Radical Defluoroborylation of Trifluoromethyl Alkenes: An Access to gem‐Difluoroallylboranes

AbstractA photoredox‐catalyzed defluoroborylation of trifluoromethyl alkenes with N‐heterocyclic carbene boranes is described for the synthesis of gem‐difluoroallylboranes. This protocol exhibits a broad substrate scope and good functional group compatibility, which enables the late‐stage functionalization of structurally complex compounds. Further transformations of the defluoroborylation products to valuable CF2‐containing molecules are also demonstrated.magnified image