Cobalt-Catalyzed Asymmetric Markovnikov Hydroboration of Styrenes

Copper-catalysed asymmetric hydroboration of alkenes with 1,2-benzazaborines to access chiral naphthalene isosteres

Bioisosteric replacement has emerged as a clear strategy for drug-structure optimization. Naphthalene is the core element of many chiral pharmaceuticals and drug candidates. However, as a promising isostere of naphthalene, the chiral version of 1,2-benzazaborine has rarely been explored due to the lack of efficient synthetic methods. Here we describe a copper-catalysed enantioselective hydroboration of alkenes with 1,2-benzazaborines. The method provides a general platform for the atom-economic and efficient construction of diverse chiral 1,2-benzazaborine compounds (more than 60 examples) that bear a 2-carbon-stereogenic centre or allene skeleton in high yields and excellent enantioselectivities. Three 1,2-benzazaborine analogues of bioactive chiral naphthalene-containing molecules have been prepared, and a series of transformations around chiral 1,2-benzazaborines have also been developed. Notably, the hydroboration process of this study reveals that the identity of 1,2-benzazaborine plays an essential role in the rate-determining step and catalyst resting state.

Pd-Catalyzed Hydroboration of Vinylarenes with B2pin2

A palladium(II)-catalyzed Markovnikov hydroboration of aryl alkenes with readily available bis(pinacolato)diboron (B2pin2) is reported. The reaction proceeded with low catalyst loading (0.5 mol %) in the absence of N- or P-containing ligands, affording the products in up to 90% yield. Trifluoracetic acid serves as the hydrogen source, enabling the synthesis of benzylic boronic esters under mild ambient conditions.

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.

Cobalt-Catalyzed Enantioselective Hydroboration of α-Substituted Acrylates

Even though metal-catalyzed enantioselective hydroborations of alkenes have attracted enormous attention, few preparatively useful reactions of α-alkyl acrylic acid derivatives are known, and most use rhodium catalysts. No examples of asymmetric hydroboration of the corresponding α-arylacrylic acid esters are known. In our continuing efforts to search for new applications of earth-abundant cobalt catalysts for broadly applicable organic transformations, we have identified 2-(2-diarylphosphinophenyl)oxazoline ligands and mild reaction conditions for efficient and highly regio- and enantioselective hydroboration of α-alkyl- and α-aryl- acrylates, giving β-borylated propionates. Since the C-B bonds in these compounds can be readily replaced by C-O, C-N, and C-C bonds, these intermediates could serve as valuable chiral synthons, some from feedstock carbon sources, for the synthesis of propionate-bearing motifs including polyketides and related molecules. Two-step syntheses of "Roche" ester from methyl methacrylate (79%; er 99:1), arguably the most widely used chiral fragment in polyketide synthesis, and tropic acid esters (∼80% yield; er ∼93:7), which are potential intermediates for several medicinally important classes of compounds, illustrate the power of the new methods. Mechanistic studies confirm the requirement of a cationic Co(I) species [(L)Co]+as the viable catalyst in these reactions and rule out the possibility of a [L]Co-H-initiated route, which has been well-established in related hydroborations of other classes of alkenes. A mechanism involving an oxidative migration of a boryl group to the β-carbon of an η4-coordinated acrylate-cobalt complex is proposed as a plausible route.

Thiyl Radical Trapped by Cobalt Catalysis: An Approach to Markovnikov Thiol-Ene Reaction

In the presence of a thiyl radical species, the catalytic Markovnikov thiol-ene reaction is challenging because it prefers to proceed via a radical pathway, thereby leading to anti-Markovnikov selectivity. In this work, a rare example of thiyl radical engaged in Markovnikov thiol-ene reaction enabled by cobalt catalysis is reported. This protocol features the avoidance of unique oxidants, exclusive regioselectivity, and broad substrate scope. Scalable synthesis and late-stage modification of complex molecules demonstrate the practicability of the protocol.

Homoleptic octahedral CoII complexes as precatalysts for regioselective hydroboration of alkenes with high turnover frequencies

Homoleptic complexes adopting octahedral coordination modes are usually less active in catalysis due to the saturated coordination around metal centers that prevents substrate activation in a catalytic event. In this work, we demonstrated that a homoleptic octahedral cobalt complex (1) of 4'-pyridyl-2,2';6',2''-terpyridine that experienced monoprotonation at the non-coordinating pyridyl moiety upon crystallization could serve as a highly efficient precatalyst for the hydroboration of styrene derivatives with Markovnikov selectivity. The solid-state structure of this precatalyst along with relevant homoleptic CoII and FeII complexes has been characterized by X-ray crystallography. In the solid state, 1 features one-dimensional hydrogen-bonded chains that are further stacked by interchain π⋯π interactions. The newly synthesized complexes (1-3) along with several known analogues (4-6) were examined as precatalysts for the hydroboration of alkenes. The best-performing system, 1/KOtBu was found to promote Markovnikov hydroboration of substituted styrenes with high turnover frequencies (TOFs) up to ∼47 000 h-1, comparable to the most efficient polymeric catalyst [Co(pytpy)Cl2]n reported to date. Although some limitations in substrate scope as well as functional group tolerance exist, the catalyst shows good promise for several relevant hydrofunctionaliation reactions.

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.

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.

Undirected, Asymmetric Alkyl Group Functionalizations through Alkane Dehydrogenation

Direct asymmetric alkyl group functionalizations can potentially convert abundant and inexpensive hydrocarbon feedstocks into value-added chiral fine chemicals. Here, we report a one-pot, dehydrogenation-based strategy for enantioselective formal benzylic C(sp³)-H bond borylation. Dehydrogenation of alkylarenes by a pincer-Ir complex produces aryl alkenes via a tandem dehydrogenation/alkene-isomerization catalysis. The subsequent Cu-catalyzed asymmetric alkene hydroboration affords benzylic boronate esters with excellent site- and enantioselectivity. The generality of this strategy has been further demonstrated by asymmetric alkyl group amination.

Hexamethyldisilazane Lithium (LiHMDS)-Promoted Hydroboration of Alkynes and Alkenes with Pinacolborane

Lithium-promoted hydroboration of alkynes and alkenes using commercially available hexamethyldisilazane lithium as a precatalyst and HBpin as a hydride source has been developed. This method will be appealing for organic synthesis because of its remarkable substrate tolerance and good yields. Mechanistic studies revealed that the hydroboration proceeds through the in situ-formed BH₃ species, which acts to drive the turnover of the hydroboration of alkynes and alkenes.

Desymmetrizing Isomerization of Alkene via Thiazolinyl Iminoquinoline Cobalt Catalysis

We report a cobalt-catalyzed desymmetrizing isomerization of exo-cyclic alkenes to generate chiral 1-methylcyclohexene derivatives with good yields and enantioselectivities. A novel chiral thiazolinyl iminoquinoline ligand and its cobalt complex were designed and synthesized to control the establishment of tertiary or quaternary carbon centers at a remote position. This protocol is operationally simple, and a model for the stereochemical outcome has been proposed.

The transition metal-catalysed hydroboration reaction

This review covers the development of the transition metal-catalysed hydroboration reaction, from its beginnings in the 1980s to more recent developments including earth-abundant catalysts and an ever-expanding array of substrates.

Solvent-free hydroboration of alkynes catalyzed by an NHC–cobalt complex

A new cobalt complex bearing a bulky N-heterocyclic carbene (NHC) ligand is described as a pre-catalyst for alkyne hydroboration. The proposed catalytic system, synthesized using easily accessible reagents, allowed obtaining a series of mono- and dialkenylboranes in solvent-free conditions with excellent efficiency and selectivity. The results have been compared to those obtained in the presence of the same cobalt complex containing smaller NHC ligands and those achieved for the catalytic system based on a CoCl₂ – NHC precursor.

Cobalt complex bearing bulky NHC ligand enable selective hydroboration of alkynes in solvent-free conditions.

The Co(ii)/spiroBox-catalyzed enantioselective Mukaiyama-Mannich reaction for the synthesis of quaternary α-amino acid derivatives

Co(ii)/spiroBox-catalyzed Mukaiyama-Mannich reactions of enol silyl ethers with cyclic N-sulfonyl ketimino esters were examined and showed excellent yields and enantioselectivity values.

Nickel-Catalyzed Enantioselective Hydroboration of Vinylarenes

The enantioselective hydroboration of vinylarenes catalyzed by a chiral, nonracemic nickel catalyst is presented as a facile method for generating chiral benzylic boronate esters. Various vinylarenes react with bis(pinacolato)diboron (B₂pin₂) in the presence of MeOH as a hydride source to form chiral boronate esters in up to 92% yield with up to 94% ee. The use of anhydrous Me₄NF to activate B₂pin₂ is crucial for ensuring fast transmetalation to achieve high enantioselectivities.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

Cobalt-Catalyzed Asymmetric 1,4-Hydroboration of Enones with HBpin

Herein, a series of new 8-OIQ cobalt complexes were synthesized and used for cobalt-catalyzed chemo- and enantioselective 1,4-hydroboration of enones with HBpin to access chiral β,β-disubstituted ketones with good to excellent chemo- and enantioselectivties. This protocol is operationally simple and shows a broad substrate scope.

Recent advances in asymmetric borylation by transition metal catalysis

Chiral organoboronates have played a critical role in organic chemistry and in the development of materials science and pharmaceuticals. Much effort has been devoted to exploring synthetic methodologies for the preparation of these compounds during the past few decades. Among the known methods, asymmetric catalysis has emerged as a practical and highly efficient strategy for their straightforward preparation, and recent years have witnessed remarkable advances in this respect. Approaches such as asymmetric borylative addition, asymmetric allylic borylation and stereospecific cross-coupling borylation, have been extensively explored and well established employing transition-metal catalysis with a chiral ligand. This review provides a comprehensive overview of transition metal-catalysed asymmetric borylation processes to construct carbon-boron, carbon-carbon, and other carbon-heteroatom bonds. It summarises a range of recent achievements in this area of research, with considerable attention devoted to the reaction modes and the mechanisms involved.

Enantioselective Rh-Catalyzed Hydroboration of Silyl Enol Ethers

The asymmetric hydroboration of alkenes has proven to be among the most powerful methods for the synthesis of chiral boron compounds. This protocol is well suitable for activated alkenes such as vinylarenes and alkenes bearing directing groups. However, the catalytic enantioselective hydroboration of O-substituted alkenes has remained unprecedented. Here we report a Rh-catalyzed enantioselective hydroboration of silyl enol ethers (SEEs) that utilizes two new chiral phosphine ligands we developed. This approach features mild reaction conditions and a broad substrate scope as well as excellent functional group tolerance, and enables highly efficient preparation of synthetically valuable chiral borylethers.

Iridium-Catalyzed Enantioconvergent Allylation of a Boron-Stabilized Organozinc Reagent

An iridium-catalyzed enantioconvergent coupling of the versatile boron-stabilized organozinc reagent BpinCH₂ZnI with a racemic branched allylic carbonate has been developed here, which differs from our previous work by using 1,1-bisborylmethane through the kinetic resolution process. The reaction has a broad substrate scope, and various chiral homoallylic organoboronic esters could be obtained in good yields with excellent enantioselectivities. The synthetic practicability of the products was demonstrated by their conversion to other useful families of compounds.

Iron- and Cobalt-Catalyzed Asymmetric Hydrofunctionalization of Alkenes and Alkynes

Transition metal catalyzed asymmetric hydrofunctionalization of readily available unsaturated hydrocarbons presents one of the most straightforward and atom-economic protocols to access valuable optically active products. For decades, noble transition metal catalysts have laid the cornerstone in this field, on account of their superior reactivity and selectivity. In recent years, from an economical and sustainable standpoint, first-row, earth-abundant transition metals have received considerable attention, due to their high natural reserves, affordable costs, and low toxicity. Meanwhile, the earth-abundant metal catalyzed hydrofunctionalization reactions have also gained much interest and been investigated gradually. However, since chiral ligand libraries for earth-abundant transition-metal catalysis are limited to date, the development of highly enantioselective versions remains a significant challenge.This Account summarizes our recent efforts in developing suitable chiral ligands for iron and cobalt catalysts and their applications in the highly enantioselective hydrofunctionalization reactions (hydroboration and hydrosilylation) of alkenes and alkynes. In ligand design, we envisioned that chiral unsymmetric NNN-tridentate (UNT) ligand scaffolds could promote these enantioselective transformations with earth-abundant metals. Therefore, several types of chiral UNT ligands were designed and prepared in our laboratory, utilizing readily available natural amino acids as chiral sources. In the very beginning, chiral oxazoline iminopyridine (OIP) ligands were proposed and investigated through the rational combination of nitrogen-containing ligand scaffolds. After a systematic survey of the ligand effects, the imine moiety in the rigid OIP ligands was replaced by a conformationally more flexible amine unit, leading to the construction of reactive oxazoline aminoisopropylpyridine (OAP) ligands. Subsequently, imidazoline iminopyridine (IIP) and thiazoline iminopyridine (TIP) ligands were prepared by altering the oxygen atom of oxazoline with nitrogen and sulfur linkers, respectively. To further expand the chiral ligand library, other tridentate ligands containing a twisted pincer, anionic, and nonrigid backbone were also designed and synthesized, including iminophenyl oxazolinyl phenylamine (IPOPA) and imidazoline phenyl picolinamide (ImPPA). The efficacy of these chiral UNT ligands for asymmetric induction in iron and cobalt catalysis has been demonstrated through asymmetric hydrofunctionalization of alkenes and asymmetric sequential hydrofunctionalization of alkynes, which exhibit excellent reactivity as well as high chemo-, regio-, and stereoselectivity with broad functional group tolerance. Notably, highly regio- and enantioselective hydrofunctionalization of challenging substrates, such as 1,1-disubstituted aryl alkenes and terminal aliphatic alkenes, was also achieved. Furthermore, the development of asymmetric sequential isomerization/hydroboration of internal alkenes and sequential hydrofunctionalization of alkynes further demonstrates the synthetic power of these catalytic systems. The chiral enantioenriched products obtained by these methodologies could be potentially utilized in organic synthesis, medicinal chemistry, and materials science. We believe that our continuous efforts in this field would be beneficial to the development of asymmetric earth-abundant metal catalysis.

Hydroboration of Nitriles, Esters, and Carbonates Catalyzed by Simple Earth-Abundant Metal Triflate Salts

During the past decade earth-abundant metals have become increasingly important in homogeneous catalysis. One of the reactions in which earth-abundant metals have found important applications is the hydroboration of unsaturated C-C and C-X bonds (X=O or N). Within these set of transformations, the hydroboration of challenging substrates such as nitriles, carbonates and esters still remain difficult and often relies on elaborate ligand designs and highly reactive catalysts (e. g., metal alkyls/hydrides). Here we report an effective methodology for the hydroboration of challenging C≡N and C=O bonds that is simple and applicable to a wide set of substrates. The methodology is based on using a manganese(II) triflate salt that, in combination with commercially available potassium tert-butoxide and pinacolborane, catalyzes the hydroboration of nitriles, carbonates, and esters at room temperature and with near quantitative yields in less than three hours. Additional studies demonstrated that other earth-abundant metal triflate salts can facilitate this reaction as well, which is further discussed in this report.

Dual Ni/photoredox-catalyzed asymmetric cross-coupling to access chiral benzylic boronic esters

The flourishing Ni/photoredox-catalyzed asymmetric couplings typically rely on redox-neutral reactions. In this work, we report a reductive cross-coupling of aryl iodides and α-chloroboranes under a dual catalytic regime to further enrich the metallaphotoredox chemistry. This approach proceeds under mild conditions (visible light, ambient temperature, no strong base) to access the versatile benzylic boronic esters with good functional group tolerance and excellent enantioselectivities.

Metallaphototoredox catalysis has been rarely applied to reductive cross-couplings, in contrast to typical redox-neutral methods. Here, the authors report a mild Ni/photoredox-catalyzed reductive cross-coupling of aryl iodides and α-chloroboranes, further enriching the metallaphotoredox chemistry.

Manganese-Catalyzed Hydroboration of Terminal Olefins and Metal-Dependent Selectivity in Internal Olefin Isomerization-Hydroboration

In the past decade, the use of earth-abundant metals in homogeneous catalysis has flourished. In particular, metals such as cobalt and iron have been used extensively in reductive transformations including hydrogenation, hydroboration, and hydrosilylation. Manganese, on the other hand, has been considerably less explored in these reductive transformations. Here, we report a well-defined manganese complex, [Mn(^iPrBDI)(OTf)₂] (2a; BDI = bipyridinediimine), that is an active precatalyst in the hydroboration of a variety of electronically differentiated alkenes (>20 examples). The hydroboration is specifically selective for terminal alkenes and occurs with exclusive anti-Markovnikov selectivity. In contrast, when using the analogous cobalt complex [Co(^iPrBDI)(OTf)₂] (3a), internal alkenes are hydroborated efficiently, where a sequence of isomerization steps ultimately leads to their hydroboration. The contrasting terminal versus internal alkene selectivity for manganese and cobalt was investigated computationally and is further discussed in the herein-reported study.

Bottleable NiCl2(dppe) as a catalyst for the Markovnikov-selective hydroboration of styrenes with bis(pinacolato)diboron

Although transition-metal-catalysed hydroboration reactions of alkenes have been extensively studied, only three examples using Ni complexes have been reported so far. In this study, we have examined hydroboration reactions of alkenes using Ni/phosphine complexes. The commercially available and bottleable complex NiCl2(dppe) (dppe = 1,2-bis(diphenylphosphino)ethane) serves as a catalyst for the highly Markovnikov-selective hydroboration of styrene derivatives that affords the desired Markovnikov products in high yield.

Zirconium-Catalyzed Atom-Economical Synthesis of 1,1-Diborylalkanes from Terminal and Internal Alkenes

A general and atom‐economical synthesis of 1,1‐diborylalkanes from alkenes and a borane without the need for an additional H₂ acceptor is reported for the first time. The key to our success is the use of an earth‐abundant zirconium‐based catalyst, which allows a balance of self‐contradictory reactivities (dehydrogenative boration and hydroboration) to be achieved. Our method avoids using an excess amount of another alkene as an H₂ acceptor, which was required in other reported systems. Furthermore, substrates such as simple long‐chain aliphatic alkenes that did not react before also underwent 1,1‐diboration in our system. Significantly, the unprecedented 1,1‐diboration of internal alkenes enabled the preparation of 1,1‐diborylalkanes.

Rh-Catalyzed Asymmetric Hydrogenation of α,β- and β,β-Disubstituted Unsaturated Boronate Esters

A highly enantioselective hydrogenation of α,β-unsaturated boronate esters catalyzed by Rh-(S)-DTBM-Segphos complex has been developed. Both (Z)-α,β- and β,β-disubstituted substrates can be successfully hydrogenated to afford chiral boronates with excellent enantioselectivities, up to 98 % ee. Furthermore, the obtained chiral boronate esters, as important versatile synthetic intermediates are successfully transformed to the corresponding chiral alcohols, amines and other important derivatives with maintained enantioselectivities.

Selective Synthesis of Secondary Alkylboronates: Markovnikov-Selective Hydroboration of Vinylarenes with Bis(pinacolato)diboron Catalyzed by a Nickel Pincer Complex

A nickel pincer complex bearing a β-aminoketonato-based O,N,P-tridentate ligand (1a) has been employed for the highly Markovnikov-selective hydroboration of vinylarenes using bis(pinacolato)diboron. This reaction proceeds smoothly under mild reaction conditions and affords the corresponding Markovnikov products in good to high yield.

Enantioselective Synthesis of 4-Allyl Tetrahydroquinolines via Copper(I) Hydride-Catalyzed Hydroallylation of 1,2-Dihydroquinolines

CuCl/(R,R)-Ph-BPE-catalyzed asymmetric hydroallylation of 1,2-dihydroquinolines, prepared from readily available quinolines, was developed. The optically active tetrahydroquinolines (THQs) bearing an allylic functionality at position 4 were obtained in good yields and excellent enantioselectivity. The introduced allylic groups are amenable to diverse transformations, thus offering chances to rapidly expand the THQ libraries.

Ligand-controlled cobalt-catalyzed regiodivergent hydroboration of aryl,alkyl-disubstituted internal allenes

We report a stereoselective regiodivergent hydroboration of aryl,alkyl-disubstituted internal allenes with pinacolborane (HBpin) in the presence of cobalt catalysts generated from bench-stable Co(acac)2 and bisphosphine ligands. An interesting correlation between the regioselectivity of this hydroboration and the bite angles of bisphosphine ligands was identified. When hydroboration was conducted with cobalt catalysts containing bisphosphines with medium bite angles (e.g. 98° for dppb and 96° for dppf), HBpin was selectively added to the alkyl-substituted double bond. However, HBpin was selectively added to the aryl-substituted double bond when the reactions were conducted with cobalt catalysts containing bisphosphines with large bite angles (e.g. 111° for xantphos and 114° for Nixantphos). A range of internal allenes underwent these Co-catalyzed hydroboration reactions in a regiodivergent manner to yield the corresponding (Z)-alkenylboronates in high isolated yields and with high regioselectivity. These reactions show good functional group compatibility and can be readily scaled up to gram scales without using a dry box. In addition, the comparison of regioselectivity between the Co-catalyzed hydrosilylation and hydroboration reactions of the same allene substrate suggests that this Co-catalyzed regiodivergent hydroboration of allenes proceeds through a Co-Bpin intermediate. Deuterium-labeling experiments suggest that the Co-Bpin intermediates react with allenes to form allylcobalt species which then react with HBpin to release (Z)-alkenylboronate products.

Copper-Catalyzed Asymmetric Hydroboration of 2 H-Chromenes Using a Chiral Diphosphine Ligand

A highly regioselective asymmetric hydroboration of 2 H-chromenes catalyzed by the complex of CuCl and diphosphine ligand ( S, R)-DuanPhos has been realized under mild conditions to produce 3-boryl chromans, achieving good yields and excellent enantioselectivities up to 96% ee. This work provides an efficient approach to the synthesis of chiral 3-boryl chromans and derivatives.