Synthesis and Suzuki Reactions of N-Heterocyclic Carbene Difluoro(aryl)-boranes

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.

Stereoselective Unsymmetrical 1,1-Diborylation of Alkynes with a Neutral sp2 -sp3 Diboron Reagent

The incorporation of two distinct boryl groups at the same carbon center in organic molecules has attracted growing research interest due to its potential for facilitating controlled, precise synthesis through stepwise dual carbon-boron bond transformations. Here we report a method to access unsymmetrical 1,1-diborylalkene (UDBA) stereoselectively via the reaction of readily available alkynes with a neutral sp2 -sp3 diboron reagent (NHC)BH2 -Bpin (NHC=N-heterocyclic carbene). Attributing to the chemically easily distinguishable nature of the sp2 and sp3 boryl moieties, controllable stepwise derivatization of the resultant UDBAs is realized. This process leads to various multifunctionalized olefins and organoborons, such as acylboranes, which are difficult to prepare by other methods.

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.

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.

Boron-Mediated Regioselective Aromatic C-H Functionalization via an Aryl BF2 Complex

An efficient regioselective functionalization of 2-aryl-heteroarenes and aryl aldehydes via an azaaryl BF₂ complex has been developed. Mechanistically the reaction comprises fluoride to bromide ligand exchange on an aryl boron species and consecutive C-B bond cleavage to deliver a broad range of functionalized products. The reaction is high yielding, has a broad substrate scope where several different heteroarenes can be functionalized with chloro, bromo, iodo, hydroxyl, amine and BF₂ in a highly regioselective fashion. The method can be applied for late-stage functionalization or for rapid skeleton remodeling with for instance cross-couplings.

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.

Catalytic asymmetric deuterations rely largely on two-electron reaction manifolds and are mostly limited to benzylic positions. Here, a metal-free platform using peptide- or sugar-derived chiral thiols and deuterium oxide allows for asymmetric open-shell deuteration at non-benzylic positions under visible-light irradiation.

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.

Photocatalytic C-F Bond Borylation of Polyfluoroarenes with NHC-boranes

The first photoredox-catalyzed defluoroborylation of polyfluoroarenes with NHC-BH₃ has been facilely achieved at room temperature via a single-electron-transfer (SET)/radical addition pathway. This new strategy makes full use of the advantage of photoredox catalysis to generate the key boryl radical via direct activation of a B-H bond. Good functional group tolerance and high regioselectivity offer this protocol incomparable advantages in preparing a wide array of valuable polyfluoroarylboron compounds. Moreover, both computational and experimental studies were performed to illustrate the reaction mechanism.

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.

Regioselective radical hydroboration of electron-deficient alkenes: synthesis of α-boryl functionalized molecules

A regioselective radical hydroboration of various electron-deficient alkenes is achieved by the employment of an NHC-boryl radical. A range of α-borylated nitriles, trifluoromethyl molecules, phosphonates, sulfones, and gem-diboron compounds have been prepared from readily available starting materials. Further synthetic applications of these products are also demonstrated.

Highly diastereoselective preparation of chiral NHC-boranes stereogenic at the boron atom

Stereogenic main group elements are clearly generating interest in the enantioselective catalysis field. Surprisingly, while chiral organoboron reagents are very useful in stereoselective transformations, few scaffolds stereogenic at boron and configurationally stable have been reported to date. Herein, we describe an original library of chiral NHC-boranes, stereogenic at the boron atom, that has been prepared in only a few steps and in good yields (up to 93%). Key steps involve a chlorination/arylation sequence in the presence of simple Grignard reagents from bicyclic NHC-boranes. The high and unprecedented diastereoselectivity observed during the second step (up to 99 : 1 dr) has been rationalized through a plausible SRN1 mechanism thanks to EPR observations and DFT calculations.

Regioselective radical α-borylation of α,β-unsaturated carbonyl compounds for direct synthesis of α-borylcarbonyl molecules

Organoboron compounds are highly valuable in synthetic chemistry. In particular, α-borylcarbonyl compounds have shown versatile synthetic applications, owing to fruitful chemistries of both the boryl and carbonyl moieties. However, the synthesis of these molecules still remains tedious and time-consuming. Here we report a straightforward and practical route to synthesize α-borylcarbonyl molecules based on a regioselective radical α-borylation of α,β-unsaturated carbonyl compounds. The reaction features unusual α-regioselectivity and high functional-group compatibility. Further synthetic applications of new α-borylated products were also demonstrated. DFT and kinetic studies implicated that the α-regioselectivity of β-aryl-α,β-unsaturated carbonyl compounds was determined by the thermodynamically more favorable radical α-addition step, whereas the formation of α-addition products from β-alkyl-α,β-unsaturated carbonyl compounds was driven by an energetically favored hydrogen atom transfer step. Given that α,β-unsaturated carbonyl compounds can be easily obtained in abundance and variety, this method enjoys great advantages in diverse and economical synthesis of valuable α-borylcarbonyl molecules.

α-Borylcarbonyl compounds are versatile synthons in organic synthesis, however their preparation is often tedious. Here, the authors report a regioselective radical α-borylation of α,β-unsaturated carbonyl compounds to afford α-borylcarbonyl molecules with high selectivity and functional group compatibility.

Reductive α-borylation of α,β-unsaturated esters using NHC-BH3 activated by I2 as a metal-free route to α-boryl esters

Useful α-boryl esters can be synthesized in one step from α,β-unsaturated esters using just a simple to access NHC-BH₃ (NHC = N-heterocyclic carbene) and catalytic I₂. The scope of this reductive α-borylation methodology is excellent and includes a range of alkyl, aryl substituted and cyclic and acyclic α,β-unsaturated esters. Mechanistic studies involving reductive borylation of a cyclic α,β-unsaturated ester with NHC-BD₃/I₂ indicated that concerted hydroboration of the alkene moiety in the α,β-unsaturated ester proceeds instead of a stepwise process involving initial 1,4-hydroboration; this is in contrast to the recently reported reductive α-silylation. The BH₂(NHC) unit can be transformed into electrophilic BX₂(NHC) moieties (X = halide) and the ester moiety can be reduced to the alcohol with the borane unit remaining intact to form β-boryl alcohols. The use of a chiral auxiliary, 8-phenylmenthyl ester, also enables effective stereo-control of the newly formed C-B bond. Combined two step ester reduction/borane oxidation forms diols, including excellent e.e. (97%) for the formation of S-3-phenylpropane-1,2-diol. This work represents a simple transition metal free route to form bench stable α-boryl esters from inexpensive starting materials.

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 trans-Hydroboration of Alkynes with N-Heterocyclic Carbene Boranes

Hydroboration of internal alkynes with N-heterocyclic carbene boranes (NHC-boranes) occurs to provide stable NHC (E)-alkenylboranes upon thermolysis in the presence of di-tert-butyl peroxide. The E isomer results from an unusual trans-hydroboration, and the E/Z selectivity is typically high (90:10 or greater). Evidence suggests that this hydroboration occurs by a radical-chain reaction involving addition of an NHC-boryl radical to an alkyne to give a β-NHC-borylalkenyl radical. Ensuing hydrogen abstraction from the starting NHC-borane provides the product and returns the starting NHC-boryl radical. Experiments suggest that the observed trans-selectivity results from kinetic control in the hydrogen-transfer reaction.

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.

Highly selective catalytic trans-hydroboration of alkynes mediated by borenium cations and B(C6F5)3

Transition metal free alkyne trans-hydroboration is achieved using strong boron electrophiles in the presence of a B–H moiety.

The trans-hydroboration of terminal alkynes mediated by borenium cations [NHC(9-BBN)]⁺ (NHC = N-heterocyclic carbene, 9-BBN = 9-borabicyclo(3.3.1)nonane) exclusively affords Z-vinylboranes. NHCs and chelating dialkyl substituents on the borenium cation and “non”-basic anions were essential to preclude alternative reactions including dehydroboration. Deuterium labelling studies indicate the mechanism involves addition of the boron electrophile to the alkyne and transfer of hydride to the opposite face of the activated alkyne. trans-Hydroboration proceeds with only catalytic amounts of B(C₆F₅)₃ or [Ph₃C][B(C₆F₅)₄] to activate the (NHC)9-BBN(H) precursor with the borenium regenerated in the hydride transfer step. The NHC can be removed from the trans-hydroborated products by the addition of Et₂O–BF₃ providing access to vinylBBN species effective for Suzuki–Miyaura couplings to generate Z-alkenes. Combinations of catalytic B(C₆F₅)₃ and stoichiometric [HB(C₆F₅)₃]^– also lead to trans-hydroboration of terminal alkynes to form Z-isomers of [arylCH Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHB(C₆F₅)₃]^–.

meso-Tetrakis[4-(methoxycarbonyl)phenyl]porphyrinatopalladium(ii) supported on graphene oxide nanosheets (Pd(ii)-TMCPP-GO): synthesis and catalytic activity

In this study, meso-tetrakis[4-(methoxycarbonyl)phenyl]porphynatopalladium(ii) as a macrocyclic palladium complex was covalently grafted to the surface of graphene oxide (Pd-TMCPP-GO). The prepared catalyst was checked in Suzuki reaction.