Asymmetric Synthesis of Secondary and Tertiary Boronic Esters

Stereospecific Phosphination and Thioetherification of Organoboronic Esters

Alkyllithium-activated organoboronic esters are found to undergo stereospecific phosphination with copper chloride and chlorophosphines. They also react with thiolsulfonate electrophiles under copper catalysis. These reactions enable stereospecific phosphination and thiolation of organoboronic esters, which are further applied in preparation of chiral ligands and biologically active molecules.

Enantioconvergent Hydroboration of E/Z-Mixed Trisubstituted Alkenes

The lack of mode for chirality recognition makes it particularly challenging to carry out asymmetric transformations on E/Z-mixed minimally functionalized trisubstituted alkenes. Here, we report a catalytic enantioconvergent hydroboration of minimally functionalized trisubstituted E/Z-mixed alkenes to construct chiral organoboronic esters with excellent enantioselectivity using chiral radical cobalt catalyst. This C(sp3)-H borylation protocol showed good functional group tolerance and products could be converted to valuable compounds via C-B derivatizations. The mechanistic studies, which included control experiments, nonlinear effect experiments, deuterated labeling experiments, and X-ray diffraction, demonstrated that the favorable compatibility between the thermodynamically unfavorable isomerization and hydroboration was the key factor in achieving convergent transformation.

Rhodium-Catalyzed Highly Enantioselective Hydroboration of Acyclic Tetrasubstituted Alkenes Directed by an Amide

Although progress has been made in enantioselective hydroboration of di- and trisubstituted alkenes over the past decades, enantioselective hydroboration of tetrasubstituted alkenes with high diastereo- and enantioselectivities continues as an unmet challenge since the 1950s due to its extremely low reactivity and the difficulties to simultaneously control the regio- and stereoselectivity of a tetrasubstituted alkene. Here, we report highly regio-, diastereo-, and enantioselective catalytic hydroboration of diverse acyclic tetrasubstituted alkenes. The delicate interplay of an electron-rich rhodium complex and coordination-assistance forms a highly adaptive catalyst that effectively overcomes the low reactivity and controls the stereoselectivity. The generality of the catalyst system is exemplified by its efficacy across various tetrasubstituted alkenes with diverse steric and electronic properties.

Enantio- and Regioselective Cascade Hydroboration of Methylenecyclopropanes for Facile Access to Chiral 1,3- and 1,4-Bis(boronates)

Chiral 1, n-bis(boronate) plays a crucial role in organic synthesis and medicinal chemistry. However, their catalytic and asymmetric synthesis has long posed a challenge in terms of operability and accessibility from readily available substrates. The recent discovery of the C═C bond formation through β-C elimination of methylenecyclopropanes (MCP) has provided an exciting opportunity to enhance molecular complexity. In this study, the catalyzed asymmetric cascade hydroboration of MCP is developed. By employing different ligands, various homoallylic boronate intermediate are obtained through the hydroboration ring opening process. Subsequently, the cascade hydroboration with HBpin or B2pin2 resulted in the synthesis of enantioenriched chiral 1,3- and 1,4-bis(boronates) in high yields, accompanied by excellent chemo- and enantioselectivities. The selective transformation of these two distinct C─B bonds also demonstrated their application potential in organic synthesis.

Simultaneous Stereoinvertive and Stereoselective C(sp3)-C(sp3) Cross-Coupling of Boronic Esters and Allylic Carbonates

With increasing interest in constructing more three-dimensional entities, there has been growing interest in cross-coupling reactions that forge C(sp3)-C(sp3) bonds, which leads to additional challenges as it is not just a more difficult bond to construct but issues of stereocontrol also arise. Herein, we report the stereocontrolled cross-coupling of enantioenriched boronic esters with racemic allylic carbonates enabled by iridium catalysis, leading to the formation of C(sp3)-C(sp3) bonds with single or vicinal stereogenic centers. The method shows broad substrate scope, enabling primary, secondary, and even tertiary boronic esters to be employed, and can be used to prepare any of the four possible stereoisomers of a coupled product with vicinal chiral centers. The new method, which combines the simultaneous enantiospecific reaction of a chiral nucleophile with the enantioselective reaction of a chiral electrophile in a single process, offers a solution for stereodivergent cross-coupling of two C(sp3) fragments.

Dinuclear gold-catalyzed divergent dechlorinative radical borylation of gem-dichloroalkanes

The enormous and widespread use of organoboronic acids has prompted the development of innovative synthetic methodologies to meet the demands on structural diversity and functional group tolerance. The existing photoinduced defunctionalization radical borylation, typically focused on the conversion of one C-X bond (X= Br, I, or other leaving group) into only one C-B bond. Herein, we disclose a divergent radical dechloroborylation reaction enabled by dinuclear gold catalysis with visible light irradiation. A wide range of structurally diverse alkyl boronic, α-chloroboronic, and gem-diboronic esters can be synthesized in moderate to good yields (up to 92%). Its synthetic robustness is further demonstrated on a preparative scale and applied to late-stage diversification of complex molecules. The process hinges on a C-Cl bond relay activation in readily available gem-dichloroalkanes through inner-sphere electron transfer, overcoming the redox potential limits of unreactive alkyl chlorides.

Cobalt-catalyzed double hydroboration of pyridines

Cobalt(ii) complexes were prepared from a modular phosphinopyridonate platform and applied to the hydroboration of pyridines. The synthetically useful, yet challenging, double hydroboration toward tetrahydropyridine derivatives was successfully performed with high activity and regiocontrol. This new method enabled the direct synthesis of N-heterocyclic allylic boronates from commercial pyridines and pinacolborane (HBpin). One-pot acetylation afforded the bench-stable borylated N-acetyl tetrahydropyridines in good yields. The synthetic utility of this procedure was demonstrated by a gram-scale double hydroboration-acetylation sequence followed by chemical diversification. Mechanistic experiments indicated metal-ligand cooperativity involving ligand-centered C-H activation and the intermediacy of a cobalt(iii) hydride species.

Atroposelective Chan-Evans-Lam Amination

The synthetic control of atropoisomerism along C-N bonds is a major challenge, and methods that allow C-N atroposelective bond formation are rare. This is a problem because each atropoisomer can feature starkly differentiated biological properties. Yet, among the three most practical and applicable classical amination methods available: 1) the Cu-catalyzed Ullmann-Goldberg reaction, 2) the Pd-catalyzed Buchwald-Hartwig reaction, and 3) the Cu-catalyzed Chan-Evans-Lam reaction, none has truly been rendered atroposelective at the newly formed C-N bond. The first ever Chan-Evans-Lam atroposelective amination is herein described with a simple copper catalyst and newly designed PyrOx chiral ligand. This method should find important applications in asymmetric synthesis, in particular for medicinal chemistry.

Enantioselective Copper-Catalyzed Ring-Opening Diboration of Arylidenecyclopropanes to Access Chiral Skipped 1,4- and 1,3-Diboronates

Two enantioselective approaches to synthesize chiral skipped diboronate compounds have been developed, relying on copper-catalyzed one-pot asymmetric ring-opening diboration of arylidenecyclopropanes. A wide range of arylidenecyclopropanes react smoothly with HBpin in the presence of CuOAc and (R)-DTBM-Segphos, affording chiral 1,4-diboronates with high enantioselectivity (up to 99% ee). Meanwhile, a variety of arylidenecyclopropanes react selectively with HBpin and B2pin2 in the presence of CuOAc and (S,S)-Ph-BPE with the sequential addition of MeOH, providing chiral 1,3-diboronates with high enantioselectivity (up to 98% ee). These enantioenriched 1,3- and 1,4-diboronates can undergo various enantiospecific transformations with minimal loss of their enantiopurity. Mechanistic studies reveal that these two diboration processes start with CuH-catalyzed ring-opening hydroboration of arylidenecyclopropanes to form a mixture of Z/E-homoallyl boronate intermediates, which subsequently undergo enantioselective CuH-catalyzed second hydroboration or Cu-Bpin-catalyzed protoboration to produce chiral 1,4-diboronates or 1,3-diboronates, respectively.

Dual Function of N-Iodosuccinimide for C(sp3)-B Bond Activation

A practical method for C(sp3)-B bond activation was developed. Using a combination of alkyl trifluoroborates and N-iodosuccinimide (NIS), various C(sp3)-heteroatom bonds were readily generated in an efficient manner. Mechanistic studies revealed the bifunctional ability of NIS: mediating the formation of reactive halogenated intermediates and activating them via halogen bonding. This electrophilic activation of the reaction center enables the utilization of general heteroatom nucleophiles, which are used in a limited capacity in traditional 1,2-metalate rearrangements.

Stereospecific Conversion of Boronic Esters into Enones using Methoxyallene: Application in the Total Synthesis of 10-Deoxymethynolide

Enones are widely utilized linchpin functional groups in chemical synthesis and molecular biology. We herein report the direct conversion of boronic esters into enones using commercially available methoxyallene as a three-carbon building block. Following boronate complex formation by reaction of the boronic ester with lithiated-methoxyallene, protonation triggers a stereospecific 1,2-migration before oxidation generates the enone. The protocol shows broad substrate scope and complete enantiospecificity is observed with chiral migrating groups. In addition, various electrophiles could be used to induce 1,2-migration and give a much broader range of α-functionalized enones. Finally, the methodology was applied to a 14-step synthesis of the enone-containing polyketide 10-deoxymethynolide.

Iron-Catalyzed Enantioselective Multicomponent Cross-Couplings of α-Boryl Radicals

Despite recent interest in the development of iron-catalyzed transformations, methods that use iron-based catalysts capable of controlling the enantioselectivity in carbon-carbon cross-couplings are underdeveloped. Herein, we report a practical and simple protocol that uses commercially available and expensive iron salts in combination with chiral bisphosphine ligands to enable the regio- and enantioselective (up to 91:9) multicomponent cross-coupling of vinyl boronates, (fluoro)alkyl halides, and Grignard reagents. Preliminary mechanistic studies are consistent with rapid formation of an α-boryl radical followed by reversible radical addition to monoaryl bisphosphine-Fe(II) and subsequent enantioselective inner-sphere reductive elimination. From a broader perspective, this work provides a blueprint to develop asymmetric Fe-catalyzed multicomponent cross-couplings via the use of alkenes as linchpins to translocate alkyl radicals, modify their steric and electronic properties, and induce stereocontrol.

Metal-Free Stereoconvergent C-H Borylation of Enamides

Enamides, functional derivatives of enamines, play a significant role as synthetic targets. However, the stereoselective synthesis of these molecules has posed a longstanding challenge in organic chemistry, particularly for acyclic enamides that are less thermodynamically stable. In this study, we present a general strategy for constructing β-borylenamides by C-H borylation, which provides a versatile platform for generating the stereodefined enamides. Our approach involves the utilization of metalloid borenium cation, generated through the reaction of BBr3 and enamides in the presence of two different additives, avoiding any exogenous catalyst. Importantly, the stereoconvergent nature of this methodology allows for the use of starting materials with mixed E/Z configurations, thus highlighting the unique advantage of this chemistry. Mechanistic investigations have shed light on the pivotal roles played by the two additives, the reactive boron species, and the phenomenon of stereoconvergence.

MIDA- and TIDA-Boronates Stabilize α-Radicals Through B-N Hyperconjugation

Multifunctional organoboron compounds increasingly enable the simple generation of complex, Csp3 -rich small molecules. The ability of boron-containing functional groups to modify the reactivity of α-radicals has also enabled a myriad of chemical reactions. Boronic esters with vacant p-orbitals have a significant stabilizing effect on α-radicals due to delocalization of spin density into the empty orbital. The effect of coordinatively saturated derivatives, such as N-methyliminodiacetic acid (MIDA) boronates and counterparts, remains less clear. Herein, we demonstrate that coordinatively saturated MIDA and TIDA boronates stabilize secondary alkyl α-radicals via σB-N hyperconjugation in a manner that allows site-selective C-H bromination. DFT calculated radical stabilization energies and spin density maps as well as LED NMR kinetic analysis of photochemical bromination rates of different boronic esters further these findings. This work clarifies that the α-radical stabilizing effect of boronic esters does not only proceed via delocalization of radical character into vacant boron p-orbitals, but that hyperconjugation of tetrahedral boron-containing functional groups and their ligand electron delocalizing ability also play a critical role. These findings establish boron ligands as a useful dial for tuning reactivity at the α-carbon.

Substrate Plasticity Enables Group-Selective Transmetalation: Catalytic Stereospecific Cross-Couplings of Tertiary Boronic Esters

Activation of enantiomerically enriched tertiary alkylboronic esters with adamantyllithium generated in situ enables stereoretentive boron-to-copper transmetalation. The resulting alkylcopper species can undergo cross-coupling reactions with an array of electrophiles to furnish synthetically useful compounds bearing quaternary stereocenters. DFT calculations of the transmetalation process provide insights for reactivity and selectivity.

Asymmetric anti-Selective Borylalkylation of Terminal Alkynes by Nickel Catalysis

Selective transformation of alkyne triple bonds to double bonds serves as an efficient platform to construct substituted alkenes. While significant advances have been made in its spatiotemporal regulation, achieving a multicomponent enantioselective reaction that requires multifaceted selectivity issues to be overcome is still uncommon. Here, we report an unprecedented asymmetric anti-stereoselective borylcarbofunctionalization of terminal alkynes by nickel catalysis. The utilization of an inexpensive chiral diamine ligand enables the three-component cross-coupling of terminal alkynes, a diboron reagent, and prochiral alkyl electrophiles with high levels of regio-, stereo-, and enantioselectivities. This reaction provides an efficient protocol to access enantioenriched alkenyl esters bearing an α-stereogenic center, is remarkably practical, and has a broad scope and an outstanding functional group compatibility. In addition, the value of this method has been highlighted in a diversity of follow-up stereoretentive derivatizations and the stereoselective concise synthesis of complex drug molecules.

External Flash Generation of Carbenoids Enables Monodeuteration of Dihalomethanes

In this study, incorporation of one deuterium atom was achieved by H-D exchange of one of the two identical methylene protons in various dihalomethanes (halogen=Cl, Br, and I) through a rapid-mixing microflow reaction of lithium diisopropylamide as a strong base and deuterated methanol as a deuteration reagent. Generation of highly unstable carbenoid intermediate and suppression of its decomposition were successfully controlled under high flow-rate conditions. Monofunctionalization of diiodomethane afforded various building blocks composed of boryl, stannyl, and silyl groups. The monodeuterated diiodomethane, which served as a deuterated C1 source, was subsequently subjected to diverted functionalization methods to afford various products including biologically important molecules bearing isotope labelling at specific positions and homologation products with monodeuteration.

Iridium-Catalyzed Asymmetric Difunctionalization of C-C σ-Bonds Enabled by Ring-Strained Boronate Complexes

Enantioenriched organoboron intermediates are important building blocks in organic synthesis and drug discovery. Recently, transition metal-catalyzed enantioselective 1,2-metalate rearrangements of alkenylboronates have emerged as an attractive protocol to access these valuable reagents by installing two different carbon fragments across C═C π-bonds. Herein, we report the development of an iridium-catalyzed asymmetric allylation-induced 1,2-metalate rearrangement of bicyclo[1.1.0]butyl (BCB) boronate complexes enabled by strain release, which allows asymmetric difunctionalization of C-C σ-bonds, including dicarbonation and carboboration. This protocol provides a variety of enantioenriched three-dimensional 1,1,3-trisubstituted cyclobutane products bearing a boronic ester that can be readily derivatized. Notably, the reaction gives trans diastereoisomers that result from an anti-addition across the C-C σ-bond, which is in contrast to the syn-additions observed for reactions promoted by Pd^II-aryl complexes and other electrophiles in our previous works. The diastereoselectivity has been rationalized based on a combination of experimental data and density functional theory calculations, which suggest that the BCB boronate complexes are highly nucleophilic and react via early transition states with low activation barriers.

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.

Stereospecific Transformations of Alkylboronic Esters Enabled by Direct Boron-to-Zinc Transmetalation

Chiral secondary organoboronic esters, when activated with t-butyllithium, are shown to undergo efficient stereoretentive transmetalation with either zinc acetate or zinc chloride. This reaction provides chiral secondary alkylzinc reagents that are configurationally stable under practical experimental conditions. The organozinc compounds were found to engage in stereospecific reactions with difluorocarbene, catalytic cross-couplings with palladium-based catalysts, and trifluoromethylation with a copper(III) complex. Mechanistic and computational studies shed light on the inner workings of the transmetalation event.

Ligand Control in Co-Catalyzed Regio- and Enantioselective Hydroboration: Homoallyl Secondary Boronates via Uncommon 4,3-Hydroboration of 1,3-Dienes

Enantiopure homoallylic boronate esters are versatile intermediates because the C-B bond in these compounds can be stereospecifically transformed into C-C, C-O, and C-N bonds. Regio- and enantioselective synthesis of these precursors from 1,3-dienes has few precedents in the literature. We have identified reaction conditions and ligands for the synthesis of nearly enantiopure (er >97:3 to >99:1) homoallylic boronate esters via a rarely seen cobalt-catalyzed [4,3]-hydroboration of 1,3-dienes. Monosubstituted or 2,4-disubstituted linear dienes undergo highly efficient regio- and enantioselective hydroboration with HBPin catalyzed by [(L*)Co]+[BARF]-, where L* is typically a chiral bis-phosphine ligand with a narrow bite angle. Several such ligands (e.g., i-PrDuPhos, QuinoxP*, Duanphos, and BenzP*) that give high enantioselectivities for the [4,3]-hydroboration product have been identified. In addition, the equally challenging problem of regioselectivity is uniquely solved with a dibenzooxaphosphole ligand, (R,R)-MeO-BIBOP. A cationic cobalt(I) complex of this ligand is a very efficient (TON >960) catalyst while also providing excellent regioselectivities (rr >98:2) and enantioselectivities (er >98:2) for a broad range of substrates. A detailed computational investigation of the reactions using Co complexes from two widely different ligands (BenzP* and MeO-BIBOP) employing the B3LYP-D3 density functional theory provides key insights into the mechanism and the origins of selectivities. The computational results are in full agreement with the experiments. For the complexes we have examined thus far, the relative stabilities of the diastereomeric diene-bound complexes [(L*)Co(η4-diene)]+ lead to the initial diastereofacial selectivity, which in turn is retained in the subsequent steps, providing exceptional enantioselectivity for the reactions.

Stereospecific Acylative Suzuki–Miyaura Cross-Coupling: General Access to Optically Active α-Aryl Carbonyl Compounds

A novel strategy for the stereospecific Pd-catalyzed acylative cross-coupling of enantiomerically enriched alkylboron compounds has been developed. The protocol features an extremely high level of enantiospecificity to allow facile access to synthetically challenging and valuable chiral ketones and carboxylic acid derivatives. The use of a sterically encumbered and electron-rich phosphine ligand proved to be crucial for the success of the reaction. Furthermore, on the basis of experimental and computational studies, a unique mechanism for the transmetalation, assisted by the noncovalent interactions of the C(sp³)-based organoboron reagent, has been identified.

Comparing B-H Bond Activation in NiIIX(NNN)-Catalyzed Nitrile Dihydroboration (X = Anionic N-, C-, O-, S-, or P-donor)

One of the key steps in many metal complex-catalyzed hydroboration reactions is B-H bond activation, which results in metal hydride formation. Anionic ligands that include multiple lone pairs of electrons, in cooperation with a metal center, have notable potential in redox-neutral B-H bond activation through metal-ligand cooperation. Herein, using an easily prepared N_pyridineN_imineN_pyrrolide ligand (L²)^-, a series of divalent Ni^IIX(NNN) complexes were synthesized, with X = bromide (2), phenoxide (3), thiophenoxide (4), 2,5-dimethylpyrrolide (5), diphenylphosphide (6), and phenyl (7). The complexes were characterized using ¹H and ¹³C NMR spectroscopy, mass spectrometry, and X-ray crystallography and employed as precatalysts for nitrile dihydroboration. Superior activity of the phenoxy derivative (3) [vs thiophenoxy (4) or phenyl (7)] suggests that B-H bond activation occurs at the Ni-X (vs ligand Ni-N_pyrrolide) bond. Furthermore, stoichiometric treatment of 2-7 with a nitrile showed no reaction, whereas stoichiometric reactions of 2-7 with pinacolborane (HBpin) gave the same Ni-H complex for 2, 3, and 5. Considering that only 2, 3, and 5 successfully catalyzed nitrile dihydroboration, we suggest that the catalytic cycle involves a conventional inner sphere pathway initiated by substrate insertion into Ni-H.

Recent progress in nickel-catalyzed carboboration of alkenes

Alkenes represent one of the most useful building blocks for organic synthesis, owing to their abundance and versatile reactivity. Transition metal (Pd, Cu, Co, Ni, Fe, etc.) catalyzed difunctionalization of alkenes provides efficient access to substituted molecules from readily available alkenes by installing functional groups across their carbon-carbon double bonds. Particularly, Nickel-based catalytic complexes have attracted a great deal of attention. This is because they are prone to undergoing oxidative addition and slow β-hydride elimination, and can access both two-electron and radical pathways. Numerous elegant Ni-catalyzed cross-coupling methods, e.g., (hetero)arylboration, alkenylboration, alkylboration and alkynylboration of alkenes, have been developed with broad scopes and a high tolerance to a variety of functional groups. Therefore, the Ni-catalyzed carboboration of alkenes has become an efficient synthetic protocol to deliver substituted compounds by the cross-coupling of alkenes, electrophiles, and B₂Pin₂. Despite this progress, a number of challenging issues remaining in the field include broadening the types of carboboration reactions, especially the asymmetric ones, diversifying electrophile types (which is limited to halogens for now) and gaining profound insight into the reaction mechanisms. This review summarizes the recent progress in this emerging field from the literature published since 2018. It will provide the scientific community with convenience to access collective information and to accelerate their further research in order to broaden the scope of methodology and application in drug discovery programs.

General and Selective Metal-Free Radical α-C-H Borylation of Aliphatic Amines

Despite recent developments, selective C(sp3)-H borylation of feedstock amines remains a formidable challenge. Herein, we have developed a general, mild, and photoinduced transition metal- and strong base-free method for α-C(sp3)-H borylation of amines. This protocol features a regioselective 1,5-hydrogen atom transfer process to access key α-aminoalkyl radical intermediate using commercially available easy-to-install/remove iodobenzoyl radical translocating group. Remarkably, this general, efficient, and operationally simple method allows activation of primary and secondary α-C-H sites of a broad range of acyclic and cyclic amines toward highly regio- and diastereoselective synthesis of valuable α-aminoboronates. Utility of this protocol has been demonstrated by its employment in late-stage borylation of structurally complex amines and formal C-H arylation reaction of amines. Thus, it is expected that this operationally simple, general, and practical method will find broad application in organic synthesis and drug discovery.

Enantio- and Diastereoselective Cu(II)-Catalyzed Conjugate Borylation/Michael Addition Cascade: Synthesis of Spiroindane Boronates

We report a Cu(II)-(S,S)-ⁱPr-FOXAP-catalyzed borylative Michael/Michael addition cascade cyclization of unsymmetrical dienone for the synthesis of highly substituted and functionalized all-carbon spiroindane boronates under mild conditions. A series of optically active spiroindanes bearing boronic ester were obtained with excellent yields and good to excellent enantioselectivities (≤97% ee) and diastereoselectivities (up to >20:1 dr). In addition, scale-up synthesis of this method and synthetic transformations of spiroindane boronates are also illustrated.

Enantioselective Cobalt-Catalyzed Hydroboration of Fluoroalkyl-Substituted Alkenes to Access Chiral Fluoroalkylboronates

Selective defluoroborylation and asymmetric hydroboration reactions of fluoroalkyl-substituted terminal alkenes with pinacolborane (HBpin) have been developed with cobalt catalysts generated from Co(acac)₂ and bisphosphine ligands. A variety of fluoroalkyl-substituted terminal alkenes undergo this enantioselective hydroboration, affording the corresponding chiral alkylboronates containing fluoroalkyl-substituted stereogenic carbon centers with high enantioselectivity (up to 98% ee). This asymmetric hydroboration provides a versatile foundation for the synthesis of a variety of chiral organofluorine compounds containing fluoroalkyl-substituted stereogenic carbon centers.

Enantioselective Cobalt-Catalyzed Hydroboration of Ketone-Derived Silyl Enol Ethers

Catalytic asymmetric hydroboration of alkenes is a powerful tool for the synthesis of natural products, agrochemicals, and pharmaceuticals via the versatile transformations of chiral alkyl boronic esters. However, the scope of available alkenes is limited to styrenes, activated alkenes, and compounds with directing groups. The catalytic enantioselective hydroboration of heteroatom-substituted alkenes is rarely explored and those catalyzed by earth-abundant metals are yet to be reported. Herein, we report a cobalt-catalyzed asymmetric hydroboration of ketone-derived silyl enol ethers and provide a convenient approach to access valuable enantiopure β-hydroxy boronic esters. This protocol features mild reaction conditions, a broad substrate scope, and excellent enantioselectivities (up to 99 % ee). This approach was applied in the successful synthesis of salmeterol and albuterol, demonstrating its potential to streamline complex molecule synthesis.

Remote Stereocenter through Amide-Directed, Rhodium-Catalyzed Enantioselective Hydroboration of Unactivated Internal Alkenes

Despite the frequent occurrence of γ-branched amines in bioactive molecules, the direct catalytic asymmetric synthesis of this structural motif containing a remote stereocenter remains an important synthetic challenge. Here, we report an amide-directed, rhodium-catalyzed highly diastereo- and enantioselective hydroboration of unactivated internal alkenes. This method provided facile access to enantioenriched amines containing β,γ-vicinal stereocenters. The application of this strategy to the synthesis of bioactive molecules was demonstrated. Computational studies indicated that migratory insertion of the alkene into rhodium hydride controls the enantioselectivity.

Copper-Catalyzed Stereospecific Transformations of Alkylboronic Esters

Copper-catalyzed stereospecific cross-couplings of boronic esters are reported. Boron "ate" complexes derived from pinacol boronic esters and tert-butyl lithium undergo stereospecific transmetalation to copper cyanide, followed by coupling with alkynyl bromides, allyl halides, propargylic halides, β-haloenones, hydroxylamine esters, and acyl chlorides. Through this simple transformation, commercially available inexpensive compounds can be employed to convert primary and secondary alkylboronic esters to a wide array of synthetically useful compounds.

Asymmetric Synthesis of Tertiary and Secondary Cyclopropyl Boronates via Cyclopropanation of Enantioenriched Alkenyl Boronic Esters

The cyclopropanation of alkenyl boronates and subsequent derivatization of the boronate handle are a convenient strategy to quickly build molecular complexity and access diverse compounds with a high sp³ fraction. Herein, we describe the asymmetric cyclopropanation of enantioenriched hydrobenzoin-derived alkenyl boronic esters toward the synthesis of tertiary and secondary cyclopropyl boronates.

Selective Coupling of 1,2-Bis-Boronic Esters at the more Substituted Site through Visible-Light Activation of Electron Donor-Acceptor Complexes

1,2‐Bis‐boronic esters are useful synthetic intermediates particularly as the two boronic esters can be selectively functionalized. Usually, the less hindered primary boronic ester reacts, but herein, we report a coupling reaction that enables the reversal of this selectivity. This is achieved through the formation of a boronate complex with an electron‐rich aryllithium which, in the presence of an electron‐deficient aryl nitrile, leads to the formation of an electron donor–acceptor complex. Following visible‐light photoinduced electron transfer, a primary radical is generated which isomerizes to the more stable secondary radical before radical‐radical coupling with the arene radical‐anion, giving β‐aryl primary boronic ester products. The reactions proceed under catalyst‐free conditions. This method also allows stereodivergent coupling of cyclic cis‐1,2‐bis‐boronic esters to provide trans‐substituted products, complementing the selectivity observed in the Suzuki–Miyaura reaction.

Transition Metal Catalyst Free Synthesis of Olefins from Organoboron Derivatives**

Stereoselective preparation of highly substituted olefins is still a severe challenge that requires well defined elimination precursors. Organoboron chemistry is particularly suited for the preparation of molecules with adjacent stereocenters. As organo boron substrates with leaving groups in β‐position can undergo stereospecific syn‐ or anti‐elimination, this chemistry harbors great potential for the synthesis of complex olefins. In recent years three main strategies emerged, which differ in their approach to the β‐functionalized organoboron elimination precursor. (i) Stereoselective preparation of such elimination precursor can be achieved by addition of a boron‐stabilized anion (d¹) to an aldehyde or ketone (a¹) or diastereoselective 1,3‐rearrangement of suitable boron‐ate‐complexes. Stereospecific methods rely either on (ii) diastereospecific 1,2‐metalate rearrangement of boron‐ate‐complexes that involve opening of appropriate heterocycles or (iii) addition of chiral carbenoids (d¹*) to chiral boronates (a¹*) with a leaving group in α‐position.

Recent Advances in BODIPY Compounds: Synthetic Methods, Optical and Nonlinear Optical Properties, and Their Medical Applications

Organoboron compounds are attracting immense research interest due to their wide range of applications. Particularly, low-coordinate organoboron complexes are receiving more attention due to their improbable optical and nonlinear optical properties, which makes them better candidates for medical applications. In this review, we summarize the various synthetic methods including multicomponent reactions, microwave-assisted and traditional pathways of organoboron complexes, and their optical and nonlinear properties. This review also includes the usage of organoboron complexes in various fields including biomedical applications.

Aluminium complexes: next-generation catalysts for selective hydroboration

Organoboranes obtained from hydroboration reactions are one of the important classes of compounds that could be used to provide valuable synthons for follow-up transformations such as various functional group incorporation or C-C bond forming reactions. For decades, various transition metals were utilised as catalysts in such transformations. Recently Earth-abundant and less toxic main group metals have revived their importance in hydroboration chemistry, among which the suitable candidates are aluminium complexes as catalysts. In this regard, the development of aluminium complexes to achieve more robust catalytic systems with greater efficiency is appreciable.

Enantioselective difunctionalization of alkenes by a palladium-catalyzed Heck/borylation sequence

A palladium catalyzed enantioselective Heck/borylation reaction of alkene-tethered aryl iodides was realized, delivering a variety of 2,3-dihydrobenzofuranyl boronic esters in high yield with excellent enantioselectivity. Asymmetric synthesis of chromane boronic ester, indane boronic ester and indoline boronic ester was also accomplished. The protocol offers an efficient access to the corresponding chiral benzocyclic boronic esters, which are notably important chemical motifs in synthetic transformations.

A palladium catalyzed enantioselective Heck/borylation reaction of alkene-tethered aryl iodides was realized, delivering a variety of 2,3-dihydrobenzofuranyl boronic esters in high yield with excellent enantioselectivity.

Development of Methods to the Synthesis of β-Boryl Acyls, Imines and Nitriles

Organoboron compounds are highly important and versatile synthetic intermediates for the preparation of a wide range of organic molecules. Organoboron compounds have drawn significant attention among organic chemists due to their Lewis acidic property, non-toxicity, and commercial availability. Over the last several decades, there has been a substantial development of new organoboron compounds, useful in organic synthesis. Among all other organoboron compounds, β-boryl carbonyl compounds are the important ones. The β-boryl compounds have appeared as promising intermediates for various synthetic transformations. The 1,4-conjugate addition of diboron reagents to carbon-carbon double bond in the presence of different transition-metal catalysts has been extensively reported by various research groups across the globe. This mini-review outlines the numerous racemic as well as asymmetric β-borylation methods developed to date.