Radical deoxygenation of xanthates and related functional groups with new minimalist N-heterocyclic carbene boranes

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.

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.

Six-Membered Saturated NHC-Stabilized Borenium Cations: Isolation of a Cationic Analogue of Borinic Acid

The reaction of six-membered saturated NHC [1,3-di(2,6-diisopropylphenyl) tetrahydropyrimidine-2-ylidene; henceforth abbreviated as 6-SIDipp] with PhBCl₂ yields a Lewis base adduct, 6-SIDipp·PhBCl₂ (1), which readily undergoes nucleophilic substitution reaction with AgNO₃, leading to the single (2) and double (3) substitution of both chlorides with ONO₂ moieties at the boron atom. The reaction of 1 with 1 equiv of AlCl₃ resulted in a borenium cation of composition [6-SIDipp·B(Ph)Cl]⁺ (4) with AlCl₄^- as the counteranion. Although borenium cations with different substituents on boron have been reported, a structurally characterized phenylchloroborenium cation remains unknown. Similarly, the reaction of 1 with triflic acid provides the first representative of a new class of borenium cations bearing one hydroxyl and one phenyl group on boron (5), a cationic analogue of borinic acid.

Nucleophilic Substitution at a Coordinatively Saturated Five-Membered NHC∙Haloborane Centre

In this paper, we have used a saturated five-membered N-Heterocyclic carbene (5SIDipp = 1,3-bis-(2,6-diisopropylphenyl)imidazolin-2-ylidine) for the synthesis of SNHC-haloboranes adducts and their further nucleophilic substitutions to put unusual functional groups at the central boron atom. The reaction of 5-SIDipp with RBCl2 yields Lewis-base adducts, 5-SIDipp·RBCl2 [R = H (1), Ph (2)]. The hydrolysis of 1 gives the NHC stabilized boric acid, 5-SIDipp·B(OH)3 (3), selectively. Replacement of chlorine atoms from 1 and 2 with one equivalent of AgOTf led to the formation of 5-SIDipp·HBCl(OTf) (4) and 5-SIDipp·PhBCl(OTf) (5a), where all the substituents on the boron atoms are different. The addition of two equivalents of AgNO3 to 2 leads to the formation of rare di-nitro substituted 5-SIDipp·BPh(NO3)2 (6). Further, the reaction of 5-SIDipp with B(C6F5)3 in tetrahydrofuran and diethyl ether shows a frustrated Lewis pair type small molecule activated products, 7 and 8.

Dehydroxylative Alkylation of α-Hydroxy Carboxylic Acid Derivatives via a Spin-Center Shift

A strategically distinct dehydroxylative alkylation reaction of α-hydroxy carboxylic acid derivatives with alkenes is developed. The reaction starts with the attack of a 4-dimethylaminopyridine (DMAP)-boryl radical to the carbonyl oxygen atom, followed by a spin-center shift (SCS) to trigger the C-O bond scission. The resulting α-carbonyl radicals couple with a wide range of alkenes to furnish various alkylated products. This strategy allows for the efficient conversion of a wide array of α-hydroxy amides and esters derived from several biomass molecules and natural products to value-added compounds. Experimental and computational studies verified the reaction mechanism.

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.

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.

Advances in chemistry of N-heterocyclic carbene boryl radicals

Boron-centred radicals (boryl radicals) are potential and attractive species in main group chemistry and synthetic chemistry. Recently, the development of boron compounds ligated by N-heterocyclic carbenes (NHCs) has sparked off advavnces in boryl radical chemistry because NHCs can highly stabilise boryl radicals by electronic and steric factors. This review highlights recent synthesis and reactions of such NHC-boryl radicals. From the standpoint of main group chemistry, examples of isolation or detection of unique NHC-boryl radicals are presented. From the standpoint of synthetic chemistry, on the other hand, the development of reactions of user-friendly NHC-boryl radicals, which has contributed to radical chemistry, organoboron chemistry and polymer science, is comprehensively described.

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.

DFT Investigation of Hydrogen Atom Abstraction from NHC-Boranes by Methyl, Ethyl and Cyanomethyl Radicals-Composition and Correlation Analysis of Kinetic Barriers

Understanding the hydrogen atom abstraction (HAA) reactions of N-heterocyclic carbene (NHC)-boranes is essential for extending the practical applications of boron chemistry. In this study, density functional theory (DFT) computations were performed for the HAA reactions of a series of NHC-boranes attacked by ^•CH₂CN, Me^• and Et^• radicals. Using the computed data, we investigated the correlations of the activation and free energy barriers with their components, including the intrinsic barrier, the thermal contribution of the thermodynamic reaction energy to the kinetic barriers, the activation Gibbs free energy correction and the activation zero-point vibrational energy correction. Furthermore, to describe the dependence of the activation and free energy barriers on the thermodynamic reaction energy or reaction Gibbs free energy, we used a three-variable linear model, which was demonstrated to be more precise than the two-variable Evans–Polanyi linear free energy model and more succinct than the three-variable Marcus-theory-based nonlinear HAA model. The present work provides not only a more thorough understanding of the compositions of the barriers to the HAA reactions of NHC-boranes and the HAA reactivities of the substrates but also fresh insights into the suitability of various models for describing the relationships between the kinetic and thermodynamic physical quantities.

Site-Selective α-Alkoxyl Alkynation of Alkyl Esters Mediated by Boryl Radicals

A novel method for site-selective C-H functionalization of ethyl acetate mediated by pyridine-boryl radicals is presented, delivering a variety of 4-phenylbut-3-yn-2-yl acetate derivatives under mild conditions. A distinguishing feature of this reaction is that the pyridine-ligated boryl radicals can abstract the inactive α-hydrogen of the alkoxyl group instead of the α-hydrogen of carbonyl groups described in a previous report using amine-ligated boryl radicals. Significantly, substrates with halogen atoms are compatible under the reaction conditions.

EPR and Preparative Studies of 5- endo Cyclizations of Radicals Derived from Alkenyl NHC-Boranes Bearing tert-Butyl Ester Substituents

Radical H atom abstraction from a set of N-heterocyclic carbene (NHC) complexes of alkenylboranes bearing two tert-butyl ester substituents was studied by EPR spectroscopy. The initial boraallyl radical intermediates rapidly ring closed onto the O atoms of their distal ester groups in 5- endo mode to yield 1,2-oxaborole radicals. Unexpectedly, two structural varieties of these radicals were identified from their EPR spectra. These proved to be two stable rotamers, in which the carbonyl group of the tert-butyl ester was oriented toward and away from the NHC ring. These rotamers were akin to the s- trans and s- cis rotamers of α,β-unsaturated carbonyl compounds. Their stability was attributed to the quasi-allylic interaction of their unpaired electrons with the carbonyl units of their adjacent ester groups. EPR spectroscopic evidence for two rotamers of the analogous methyl ester containing NHC-oxaborole radicals was also obtained. An improved synthetic procedure for preparing rare NHC-boralactones was developed involving treatment of the alkenyl NHC-boranes with AIBN and tert-dodecanethiol.

Difluorination at Boron Leads to the First Electrophilic Ligated Boryl Radical (NHC-BF2. )

1,3-Dimethylimidazol-2-ylidene difluoroborane (NHC-BF₂ H) was prepared in a one-pot, two-step reaction from the parent ligated borane (NHC-BH₃ ). The derived difluoroboryl radical (NHC-BF₂^. ) was generated by laser flash photolysis experiments and characterized by UV spectroscopy and rate-constant measurements. It is transient and reacts quickly with O₂ . Unusually, it also reacts more rapidly with ethyl vinyl ether than with methyl acrylate. By this measure, it is the first electrophilic ligated boryl radical. Both NHC-BH₃ and NHC-BF₂ H serve as co-initiators in bulk photopolymerizations, converting both electron-poor and electron-rich monomers at roughly similar rates. However, the difluorinated coinitiator provides polymers with dramatically increased chain lengths from both monomers.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

Radical Borylative Cyclization of 1,6-Dienes: Synthesis of Boron-Substituted Six-Membered Heterocycles and Carbocycles

A radical borylative cyclization reaction of 1,6-dienes was developed to assemble boron-handled six-membered heterocycles and carbocycles. This reaction was initiated by the chemo- and regio-controlled addition of an N-heterocyclic carbene-boryl radical to one of the alkene tethers, followed by an intramolecular 6- exo cyclization to afford a six-membered ring framework. The utility of this method was demonstrated in the synthesis of diverse paroxetine analogues through late-stage derivatization of the boryl functional unit.

Boryl Radicals-Triggered Selective C-H Functionalization for the Synthesis of Diverse Phenanthridine Derivatives

A boryl radical-triggered C-H functionalization of aliphatic ethers/amines or DMF with isocyanides is developed to deliver diverse phenanthridine derivatives in good to excellent yields. The substrate scope is broad, and a wide range of functional groups are tolerated under the standard conditions. The rapid removal of HBPin species by 4-cyanopyridine 1-oxide provides the driving force for this reaction. This new method should make boryl radicals widely applicable in organic synthesis.

N-Heterocyclic Carbene Boranes are Hydrogen Donors in Masamune-Bergman Reactions of Benzo[3,4]cyclodec-3-ene-1,5-diynes

Thermal reactions of benzo[3,4]cyclodec-3-ene-1,5-diyne with N-heterocyclic carbene boranes (NHC-boranes) provided mixtures of 9-borylated 1,2,3,4-tetrahydroanthracenes along with 1,2,3,4-tetrahydroanthracene. These products indicate that NHC-boranes serve as hydrogen donors to a p-benzyne intermediate formed by the Masamune-Bergman reaction. Experimental results support a radical mechanism in nonpolar solvents, but suggest that ionic mechanisms compete in the production of 1,2,3,4-tetrahydroanthracene when the reaction is performed in a polar solvent.

1-Butyl-3-methylimidazol-2-ylidene Borane: A Readily Available, Liquid N-Heterocyclic Carbene Borane Reagent

1-Butyl-3-methylimidazol-2-ylidene borane has been synthesized directly from two inexpensive commercial reagents: 1-butyl-3-methylimidazolium bromide and sodium borohydride. This NHC-borane reagent is a stable, free-flowing liquid that shows promise for use in radical, ionic, and metal-catalyzed reactions.

Borylative Radical Cyclizations of Benzo[3,4]cyclodec-3-ene-1,5-diynes and N-Heterocyclic Carbene-Boranes

Borylative radical cyclization of benzo[3,4]cyclodec-3-ene-1,5-diynes to provide 5-borylated 6,7,8,9-tetrahydrobenzo[a]azulenes has been developed. The experimental results suggest that the reaction proceeds by a radical chain mechanism, in which di-tert-butyl hyponitrite (TBHN) works as a good radical initiator to form boryl radicals from N-heterocyclic carbene-boranes (NHC-boranes). The present reaction is a rare model that illustrates addition of boryl radicals to alkynes.

Influence of Electronic Effects on the Reactivity of Triazolylidene-Boryl Radicals: Consequences for the use of N-Heterocyclic Carbene Boranes in Organic and Polymer Synthesis

A small library of triazolylidene-boranes that differ only in the nature of the aryl group on the external nitrogen atom was prepared. Their reactivity as hydrogen-atom donors, as well as that of the corresponding N-heterocyclic carbene (NHC)-boryl radicals toward methyl acrylate and oxygen, was investigated by laser flash photolysis, molecular orbital calculations, and ESR spin-trapping experiments, and benchmarked relative to the already known dimethyltriazolylidene-borane. The new NHC-boranes were also used as co-initiators for the Type I photopolymerization of acrylates. This allowed a structure-reactivity relationship with regard to the substitution pattern of the NHC to be established and the role of electronic effects in the reactivity of NHC-boryl radicals to be probed. Although their rate of addition to methyl acrylate depends on their electronegativity, the radicals are all nucleophilic and good initiators for photopolymerization reactions.