Cobalt-Catalyzed Asymmetric Hydroboration/Cyclization of 1,6-Enynes with Pinacolborane

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 Hydroboration of Terminal and Internal Alkynes

A novel methodology to access synthetically versatile vinylboronic esters through a ligand-controlled cobalt-catalyzed hydroboration of terminal and internal alkynes is reported. The approach relies on the in situ reduction of Co(II) by H-BPin in the presence of bisphosphine ligands generating catalytically active Co(I) hydride complexes. This procedure avoids the use of stoichiometric amounts of base, and no boron-containing byproducts are generated which is translated into high functional group tolerance and atom economy.

Diastereo- and enantioselective rhodium(III)-catalyzed reductive cyclization of cyclohexadienone-containing 1,6-dienes

A diastereo- and enantioselective rhodium(III)-catalyzed reductive cyclization of cyclohexadienone-tethered terminal alkenes and (E)-1,2-disubstituted alkenes (1,6-dienes) is reported, providing cis-bicyclic products bearing three contiguous stereocenters with good yields and high diastereo- and enantioselectivities. The kinetic resolution of the racemic precursor is also achieved with good efficiency. Moreover, a subgram-scale experiment, several transformations of the cyclization product, and one-pot preparation of bridged polycyclic frameworks are presented.

Reductive hydrobenzylation of terminal alkynes via photoredox and nickel dual catalysis

A photoredox/nickel dual catalyzed reductive hydrobenzylation of alkynes and benzyl chlorides by employing alkyl amines as a stoichiometric reductant is described. This synergistic protocol proceeds via Markovnikov-selective migratory insertion of an alkyne into nickel hydride, followed by cross-coupling with benzyl chloride, providing facile access to important 1,1-disubstituted olefins. This reaction enables the generation of nickel hydride by utilizing readily available alkyl amines as the hydrogen source. The mild conditions are compatible with a wide range of aryl and alkyl alkynes as well as chlorides.

Cationic Co(I) Catalysts for Regiodivergent Hydroalkenylation of 1,6-Enynes. An Uncommon cis-β-C-H Activation Leads to Z-Selective Coupling of Acrylates

Two intermolecular hydroalkenylation reactions of 1,6-enynes are presented which yield substituted 5-membered carbo- and -heterocycles. This reactivity is enabled by a cationic bis-diphenylphosphinopropane (DPPP)Co^I species which forms a cobaltacyclopentene intermediate by oxidative cyclization of the enyne. This key species interacts with alkenes in distinct fashion, depending on the identity of the coupling partner to give regiodivergent products. Simple alkenes undergo insertion reactions to furnish 1,3-dienes whereby one of the alkenes is tetrasubstituted. When acrylates are employed as coupling partners, the site of intermolecular C-C formation shifts from the alkyne to the alkene motif of the enyne, yielding Z-substituted-acrylate derivatives. Computational studies provide support for our experimental observations and show that the turnover-limiting steps in both reactions are the interactions of the alkenes with the cobaltacyclopentene intermediate via either a 1,2-insertion in the case of ethylene, or an unexpected β-C-H activation in the case of most acrylates. Thus, the H syn to the ester is activated through the coordination of the acrylate carbonyl to the cobaltacycle intermediate, which explains the uncommon Z-selectivity and regiodivergence. Variable time normalization analysis (VTNA) of the kinetic data reveals a dependance upon the concentration of cobalt, acrylate, and activator. A KIE of 2.1 was observed with methyl methacrylate in separate flask experiments, indicating that C-H cleavage is the turnover-limiting step in the catalytic cycle. Lastly, a Hammett study of aryl-substituted enynes yields a ρ value of -0.4, indicating that more electron-rich substituents accelerate the rate of the reaction.

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.

Enantioselective Hydroesterificative Cyclization of 1,6-Enynes to Chiral γ-Lactams Bearing a Quaternary Carbon Stereocenter

A palladium-catalyzed asymmetric hydroesterification-cyclization of 1,6-enynes with CO and alcohol was developed to efficiently prepare a variety of enantioenriched γ-lactams bearing a chiral quaternary carbon center and a carboxylic ester group. The approach featured good to high chemo-, region-, and enantioselectivities, high atom economy, and mild reaction conditions as well as broad substrate scope. The correlation between the multiple selectivities of such process and the N-substitutes of the amide linker in the 1,6-enyne substrate has been depicted by the crystallographic evidence and control experiments.

Backbone-Modified C2-Symmetrical Chiral Bisphosphine TMS-QuinoxP*: Asymmetric Borylation of Racemic Allyl Electrophiles

A new C₂-symmetrical P-chirogenic bisphosphine ligand with silyl substituents on the ligand backbone, (R,R)-5,8-TMS-QuinoxP*, has been developed. This ligand showed higher reactivity and enantioselectivity for the direct enantioconvergent borylation of cyclic allyl electrophiles than its parent ligand, (R,R)-QuinoxP* (e.g., for a piperidine-type substrate: 95% ee vs 76% ee). The borylative kinetic resolution of linear allyl electrophiles was also achieved using (R,R)-5,8-TMS-QuinoxP* (up to 90% ee, s = 46.4). An investigation into the role of the silyl groups on the ligand backbone using X-ray crystallography and computational studies displayed interlocking structures between the phosphine and silyl moieties of (R,R)-5,8-TMS-QuinoxP*. The results of DFT calculations revealed that the entropy effect thermodynamically destabilizes the dormant dimer species in the catalytic cycle to improve the reactivity. Furthermore, in the direct enantioconvergent case, detailed calculations indicated a pronounced enantioselective recognition of carbon-carbon double bonds, which is virtually unaffected by the chirality at the allylic position, as a key for the borylation from both enantiomers of racemic allyl electrophiles.

Cobalt-Catalyzed Dearomatization of Indoles via Transfer Hydrogenation To Afford Polycyclic Indolines

A cobalt-catalyzed dearomatization of indoles via transfer hydrogenation with HBpin and H₂O has been developed. This reaction offered a straightforward platform to access hexahydropyrido[1,2-a]indoles in high regio- and chemoselectivity. A preliminary reaction mechanism was proposed on the basis of deuterium-labeling experiments, and a cobalt hydride species was involved in the reaction.

Copper(ii)-catalyzed protoboration of allenes in aqueous media and open air

A copper(ii)-catalyzed internal protoboration of monosubstituted allenes efficiently occurs in water at room temperature and open air to generate 1,1-disubstituted vinyl boronic acid derivatives.

The Use of Domino Reactions for the Synthesis of Chiral Rings

This short review highlights the recent developments reported in the last four years on the asymmetric construction of chiral rings based on enantioselective domino reactions promoted by chiral metal catalysts.

1 Introduction

2 Formation of One Ring Containing One Nitrogen Atom

3 Formation of One Ring Containing One Oxygen/Sulfur Atom

4 Formation of One Ring Containing Several Heterocyclic Atoms

5 Formation of One Carbon Ring

6 Formation of Two Rings

7 Conclusion

Enantioselective Cobalt-Catalyzed Intermolecular Hydroacylation of 1,6-Enynes

We report a cobalt-catalyzed hydroacylation of 1,6-enynes with exogenous aldehydes in a domino sequence to construct enantioenriched ketones. The products were obtained in good yields with excellent regio-, diastereo-, and enantioselectivity. Furthermore, the chiral products served as valuable precursors to access complex spirocyclic scaffolds with three contiguous stereocenters. The asymmetric hydroacylation process exhibited no C-H crossover and no KIE, thus indicating that the C-H bond cleavage was not involved in the turnover-limiting step.

Cobalt-Catalyzed Enantioselective Hydroarylation of 1,6-Enynes

An asymmetric hydroarylative cyclization of enynes involving a C-H bond cleavage is reported. The cobalt-catalyzed cascade generates three new bonds in an atom-economical fashion. The products were obtained in excellent yields and excellent enantioselectivities as single diastereo- and regioisomers. Preliminary mechanistic studies indicate that the reaction shows no intermolecular C-H crossover. This work highlights the potential of cobalt catalysis in C-H bond functionalization and enantioselective domino reactivity.

Efficient Co-Catalyzed Double Hydroboration of Nitriles: Application to One-Pot Conversion of Nitriles to Aldimines

The commercially available and bench-stable Co(acac)₂ /dpephos system is employed as a precatalyst for selective and efficient room temperature hydroboration of organic nitriles with HBPin to produce a series of N,N-diborylamines [RN(BPin)₂ ], which react in situ with aldehydes to give aldimines. Formation of aldimines from N,N-diborylamines does not require a dehydrating agent, is applicable to a wide range of N,N-diborylamine and aldehyde substrates and is highly chemoselective, being unaffected by various common functional groups, such as alkenes, alkynes, secondary amines, ketones, esters, amides, carboxylic acids, pyridines, nitriles, and nitro compounds. The overall transformation represents a synthetically valuable approach to aldimines from nitriles and can be performed in a sequential one-pot manner, tolerating ester, lactone, carboxamide and unactivated alkene functionalities.

Versatile cobalt-catalyzed regioselective chain-walking double hydroboration of 1,n-dienes to access gem-bis(boryl)alkanes

Double hydroboration of dienes is the addition of a hydrogen and a boryl group to the two double bonds of a diene molecule and represents a straightforward and effective protocol to prepare synthetically versatile bis(boryl)alkanes, provided that this reaction occurs selectively. However, this reaction can potentially yield several isomeric organoboron products, and it still remains a challenge to control the regioselectivity of this reaction, which allows the selective production of a single organoboron product, in particular, for a broad scope of dienes. By employing a readily available cobalt catalyst, here we show that this double hydroboration yields synthetically useful gem-bis(boryl)alkanes with excellent regioselectivity. In addition, the scope of dienes for this reaction is broad and encompasses a wide range of conjugated and non-conjugated dienes. Furthermore, mechanistic studies indicate that this cobalt-catalyzed double hydroboration occurs through boryl-directed chain-walking hydroboration of alkenylboronates generated from anti-Markovnikov 1,2-hydroboration of 1,n-diene.

Control of regioselectivity in the double hydroboration of dienes to obtain a single organoboron compound is a considerable synthetic challenge. Here, the authors show a cobalt-catalyzed chain-walking double hydroboration of 1,n-dienes to access gem-bis(boryl)alkanes with regioselective control.

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.

Nickel-catalyzed regio- and diastereoselective hydroarylative and hydroalkenylative cyclization of 1,6-dienes

By using methanol as the hydrogen source and commercially available nickel complex as the catalyst, the hydroarylative and hydroalkenylative cyclization of unsymmetrically substituted 1,6-dienes with organoboronic acid was developed to afford products with high regio- and diastereoselectivities.

Mechanistic Insights into the Ru(II)-Catalyzed Intramolecular Formal [3 + 2] Cycloaddition of (E)-1,6-Enynes

Design of a unique reaction pathway in transition-metal-catalyzed 1,6-enynes cyclization to construct valuable synthetic motifs is a significant challenge in organic chemistry. Herein, we report a Ru(II)-catalyzed formal [3 + 2] cycloaddition as an efficient method to prepare unprecedented bicyclo[3.3.0]octenes from readily available (E)-1,6-enynes. Mechanistic studies based on the deuterium labeling experiments and the DFT calculation disclose a reasonable mechanistic pathway, where a ruthenacyclopentene generated by an ene-yne oxidative cyclization undergoes a sequential ß-hydride elimination and intramolecular hydroruthenation to form a ruthenacyclohexene, producing the desirable bicyclo[3.3.0]octenes.

Atom-Economical Ni-Catalyzed Diborylative Cyclization of Enynes: Preparation of Unsymmetrical Diboronates

We report a Ni-catalyzed diborylative cyclization of enynes that affords carbo- and heterocycles containing both alkyl- and alkenylboronates. The reaction is fully atom-economical, shows a broad scope, and employs a powerful and inexpensive catalytic Ni-based system. The reaction mechanism seems to involve activation of the enyne by Ni(0) through oxidative cyclometalation of the enyne prior to diboron reagent activation. An unprecedented dinuclear bis(organometallic) Ni(I) intermediate complex was isolated.

Base-catalysed reductive relay hydroboration of allylic alcohols with pinacolborane to form alkylboronic esters

An unprecedented base-catalysed reductive relay hydroboration of allylic alcohols is described. Commercially available nBuLi was found to be a robust transition metal-free initiator for this protocol, affording various boronic esters in high yield and selectivity. Mechanistically, this methodology involves a one-pot three-step successive process (dehydrocoupling/allylic hydride substitution/anti-Markovnikov hydroboration).

Cationic Co(I)-Intermediates for Hydrofunctionalization Reactions: Regio- and Enantioselective Cobalt-Catalyzed 1,2-Hydroboration of 1,3-Dienes

Much of the recent work on catalytic hydroboration of alkenes has focused on simple alkenes and styrene derivatives with few examples of reactions of 1,3-dienes, which have been reported to undergo mostly 1,4-additions to give allylic boronates. We find that reduced cobalt catalysts generated from 1,n- bis-diphenylphosphinoalkane complexes [Ph₂P-(CH₂) _n-PPh₂]CoX₂; n = 1-5) or from (2-oxazolinyl)phenyldiarylphosphine complexes [(G-PHOX)CoX₂] (G = 4-substituent on oxazoline ring) effect selective 1,2-, 1,4-, or 4,3-additions of pinacolborane (HBPin) to a variety of 1,3-dienes depending on the ligands chosen. Conditions have been found to optimize the 1,2-additions. The reactive catalysts can be generated from the cobalt(II)-complexes using trimethylaluminum, methyl aluminoxane, or activated zinc in the presence of sodium tetrakis[(3,5-trifluoromethyl)phenyl]borate (NaBARF). The complex, (dppp)CoCl₂, gives the best results (ratio of 1,2- to 1,4-addition >95:5) for a variety of linear terminal 1,3-dienes and 2-substituted 1,3-dienes. The [(PHOX)CoX₂] (X = Cl, Br) complexes give mostly 1,4-addition with linear unsubstituted 1,3-dienes, but, surprisingly, selective 1,2-additions with 2-substituted or 2,3-disubstituted 1,3-dienes. Isolated and fully characterized (X-ray crystallography) Co(I)-complexes, (dppp)₃Co₂Cl₂ and [( S,S)-BDPP]₃Co₂Cl₂, do not catalyze the reaction unless activated by a Lewis acid or NaBARF, suggesting a key role for a cationic Co(I) species in the catalytic cycle. Regio- and enantioselective 1,2-hydroborations of 2-substituted 1,3-dienes are best accomplished using a catalyst prepared via activation of a chiral phosphinooxazoline-cobalt(II) complex with zinc and NaBARF. A number of common functional groups, among them, -OBn, -OTBS, -OTs, N-phthalimido- groups, are tolerated, and er's > 95:5 are obtained for several dienes including 1-alkenylcycloalk-1-enes. This operationally simple reaction expands the realm of asymmetric hydroboration to provide direct access to a number of nearly enantiopure homoallylic boronates, which are not readily accessible by current methods. The resulting boronates have been converted into the corresponding alcohols, potassium trifluororoborate salts, N-BOC amines, and aryl derivatives by C-BPin to C-aryl transformation.

Earth-Abundant Metal Catalysis Enabled by Counterion Activation

A precatalyst activation strategy has been developed for earth-abundant metal catalysis enabled by counterion dissociation and demonstrated through alkene hydroboration. Commercially available iron and cobalt tetrafluoroborate salts were found to catalyze the hydroboration of aryl and alkyl alkenes with good functional group tolerance (Fe, 12 substrates; Co, 13 substrates) with three structurally distinct ligands. Key to this endogenous activation was counterion dissociation to generate fluoride which indirectly activates the precatalyst by reaction with pinacol borane.

Atom-economical regioselective Ni-catalyzed hydroborylative cyclization of enynes: development and mechanism

We report a full study on the novel regioselective Ni-catalyzed hydroborylative cyclization of enynes using HBpin as the borylation agent.