Nickel-Catalyzed Enantioconvergent Reductive Hydroalkylation of Olefins with α-Heteroatom Phosphorus or Sulfur Alkyl Electrophiles

Enantioselective Synthesis of Allylic Sulfones via Rhodium-Catalyzed Direct Hydrosulfonylation of Allenes and Alkynes

A highly regio- and enantioselective hydrosulfonylation using commercially available sodium sulfinates is reported, providing the first direct asymmetric rhodium-catalyzed hydrosulfonylation of allenes/alkynes to synthesize chiral allylic sulfones. Ligand screening studies demonstrated the indispensable role of the C1-symmetric P,N-ligand (Rax,S,S)-StackPhim for achieving both high regioselecitivity (>20:1) and enantioselectivity (up to 97% ee). Notably, the operationally simple method and mild conditions allow for the rapid preparation of chiral allylic sulfones with a wide scope of functional groups. Moreover, the use of sodium tert-butyldimethylsilyloxymethanesulfinate enables the collective synthesis of various chiral sulfone derivatives after simple transformations of the protected hydroxymethyl product.

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.

Enantioselective synthesis of chiral α,α-dialkyl indoles and related azoles by cobalt-catalyzed hydroalkylation and regioselectivity switch

General, catalytic and enantioselective construction of chiral α,α-dialkyl indoles represents an important yet challenging objective to be developed. Herein we describe a cobalt catalyzed enantioselective anti-Markovnikov alkene hydroalkylation via the remote stereocontrol for the synthesis of α,α-dialkyl indoles and other N-heterocycles. This asymmetric C(sp3)-C(sp3) coupling features high flexibility in introducing a diverse set of alkyl groups at the α-position of chiral N-heterocycles. The utility of this methodology has been demonstrated by late-stage functionalization of drug molecules, asymmetric synthesis of bioactive molecules, natural products and functional materials, and identification of a class of molecules exhibiting anti-apoptosis activities in UVB-irradiated HaCaT cells. Ligands play a vital role in controlling the reaction regioselectivity. Changing the ligand from bi-dentate L6 to tridentate L12 enables CoH-catalyzed Markovnikov hydroalkylation. Mechanistic studies disclose that the anti-Markovnikov hydroalkylation involves a migratory insertion process while the Markovnikov hydroalkylation involves a MHAT process.

Regiodivergent and Enantioselective Synthesis of Cyclic Sulfones via Ligand-Controlled Nickel-Catalyzed Hydroalkylation

Cyclic sulfones have demonstrated important applications in drug discovery. However, the catalytic and enantioselective synthesis of chiral cyclic sulfones remains challenging. Herein, we develop nickel-catalyzed regiodivergent and enantioselective hydroalkylation of sulfolenes to streamline the synthesis of chiral alkyl cyclic sulfones. The method has broad scope and high functional group tolerance. The regioselectivity can be controlled by ligands only. A neutral PYROX ligand favors C3-alkylation whereas an anionic BOX ligand favors C2-alkylation. This control is kinetic in origin as the C2-bound Ni intermediates are always thermodynamically more stable. Reactivity study of a wide range of relevant Ni intermediates reveal a NiI/NiIII catalytic cycle with a NiII-H species as the resting state. The regio- and enantio-determining step is the insertion of this NiII-H species into 2-sulfolene. This work provides an efficient catalytic method for the synthesis of an important class of organic compounds and enhances the mechanistic understanding of Ni-catalyzed stereoselective hydroalkylation.

Computational Exploration of 1,2-Carboamine Carbonylation Catalyzed by Nickel

Nickel-catalyzed carbonylation of alkenes is a stereoselective and regioselective method for the synthesis of amide compounds. Theoretical predictions with density functional theory calculations revealed the mechanism and origin of stereoselectivity and regioselectivity for the nickel-catalyzed carbonylation of norbornene. The carbonylation reaction proceeds through oxidative addition, migration insertion of alkenes, and subsequent reduction elimination to afford cis-carbonylation product. The C-N bond activation of amides is unfavorable because the oxidative addition ability of the C-C bond is stronger than that of the C-N bond. The determining step of stereoselectivity is the migratory insertion of the strained olefin. The structural analysis shows that steroselectivity is controlled by the steric hindrance of methyl groups to olefins and substituents to IMes in ligands.

Ligand Relay Catalysis Enables Asymmetric Migratory Hydroarylation for the Concise Synthesis of Chiral α-(Hetero)Aryl-Substituted Amines

Complementary to the design of a single structurally complex chiral ligand to promote each step in transition-metal catalysis, multiligand relay catalysis through dynamic ligand exchange with each step in the catalytic cycle promoted by its best ligand provides an attractive approach to enhance the whole reaction reactivity and selectivity. Herein, a regio- and enantioselective NiH-catalyzed migratory hydroarylation process with a simple combination of a chain-walking ligand and an asymmetric arylation ligand, producing high-value chiral α-(hetero)aryl-substituted amines and their derivatives under mild conditions, is reported. The potential synthetic applications of this transformation are demonstrated by the concise synthesis of (S)-nicotine and a CDK8 inhibitor.

A general copper-catalysed enantioconvergent C(sp3)-S cross-coupling via biomimetic radical homolytic substitution

Although α-chiral C(sp3)-S bonds are of enormous importance in organic synthesis and related areas, the transition-metal-catalysed enantioselective C(sp3)-S bond construction still represents an underdeveloped domain probably due to the difficult heterolytic metal-sulfur bond cleavage and notorious catalyst-poisoning capability of sulfur nucleophiles. Here we demonstrate the use of chiral tridentate anionic ligands in combination with Cu(I) catalysts to enable a biomimetic enantioconvergent radical C(sp3)-S cross-coupling reaction of both racemic secondary and tertiary alkyl halides with highly transformable sulfur nucleophiles. This protocol not only exhibits a broad substrate scope with high enantioselectivity but also provides universal access to a range of useful α-chiral alkyl organosulfur compounds with different sulfur oxidation states, thus providing a complementary approach to known asymmetric C(sp3)-S bond formation methods. Mechanistic results support a biomimetic radical homolytic substitution pathway for the critical C(sp3)-S bond formation step.

Ligand-Controlled On-Off Switch of a Silicon-Tethered Directing Group Enabling the Regiodivergent Hydroalkylation of Vinylsilanes under Ni-H Catalysis

A regiodivergent Ni-H-catalyzed hydroalkylation of vinylsilanes is described. Depending on the ancillary ligand at the nickel catalyst, the regioselectivity can be steered by a directing group attached to the silicon atom. The mild protocols allow for the selective synthesis of either branched or linear alkylsilanes. An example of a vinylgermane is also reported. The method features a broad scope with high functional-group tolerance and follows a radical mechanism.

Ligand-Controlled Cobalt-Catalyzed Regio-, Enantio-, and Diastereoselective Oxyheterocyclic Alkene Hydroalkylation

Metal-hydride-catalyzed alkene hydroalkylation has been developed as an efficient method for C(sp3)-C(sp3) coupling with broad substrate availability and high functional group compatibility. However, auxiliary groups, a conjugated group or a chelation-directing group, are commonly required to attain high regio- and enantioselectivities. Herein, we reported a ligand-controlled cobalt-hydride-catalyzed regio-, enantio-, and diastereoselective oxyheterocyclic alkene hydroalkylation without chelation-directing groups. This reaction enables the hydroalkylation of conjugated and unconjugated oxyheterocyclic alkenes to deliver C2- or C3-alkylated tetrahydrofuran or tetrahydropyran in uniformly good yields and with high regio- and enantioselectivities. In addition, hydroalkylation of C2-substituted 2,5-dihydrofuran resulted in the simultaneous construction of 1,3-distereocenters, providing convenient access to polysubstituted tetrahydrofuran with multiple enantioenriched C(sp3) centers.

CAMDOL-enabled diastereoselective synthesis of α-substituted phosphonates

Enantiopure α-substituted phosphonic acids are widely utilized as drugs, pesticides, and ligands. Despite numerous synthetic approaches having been investigated, precise construction of P-adjacent chiral tertiary carbon centres by the employment of recoverable chiral auxiliaries is traditional and still one of the most reliable and practical synthetic methodologies so far. Herein, we present a highly diastereoselective synthesis of α-substituted phosphonates via the unique CAMDOL-derived P-substrates by an efficient sequential deprotonation with LiHMDS and alkylation/arylation with RI. A wide range of 30 structurally diverse α-substituted phosphonate products, including the well-known P-analogues of naproxen and ibuprofen, were thus afforded conveniently in up to 92% yields and 99 : 1 diastereomeric ratios. The related chiral phosphonic acid could be easily obtained by simple acidic hydrolysis with fully recovered auxiliary. This CAMDOL-enabled asymmetric synthetic protocol exhibits comparative advantages over known chiral-induction methods with easy accessibility and compatibility of furnishing a variety of C-stereogenic centres in the proximity of the phosphorus atom, including some rare examples.

Nickel-Catalyzed Migratory Cross-Coupling Reactions: New Opportunities for Selective C-H Functionalization

ConspectusMigratory cross-coupling via metal migration is a process of significant academic and industrial interest. It provides an attractive alternative for the selective installation of a functional group at remote C-H positions from simple precursors, thus enabling the direct synthesis of challenging structures not accessible with traditional cross-coupling. In particular, with the merger of 1,n-Ni/H shift and cross-coupling of nickel, the Ni-catalyzed migratory functionalization of simple precursors has undergone particularly intense development and emerged as a valuable field of research in the past few years. This Account will outline the recent progress made in this arena in terms of migration-functionalization modes, diverse functionalizations, and strategies for regio- and stereocontrol. Mechanistic studies and synthetic applications are also discussed.In detail, we systematically categorize our work into two parts based on the migration modes. In the first part, a platform is created for Ni-catalyzed migratory sp3 C-H functionalization of alkenes or alkyl halides via iterative 1,2-Ni/H shift-selective cross-coupling. The key reactive Ni(II)H species for chain-walking could be generated in situ either in a polarity-reversed fashion relying on stoichiometric reductants (X-Ni(II)-H) or in a redox-neutral fashion with the participation of nucleophilic coupling partners (FG-Ni(II)-H). One significant advantage associated with the polarity-reversed NiH system is the use of relatively stable, abundant, and safe olefin surrogates or alkyl halides instead of the sensitive organometallics required in traditional cross-coupling reactions. Another advantage is that diverse functionalizations, including carbonation and more challenging amination and thiolation could be smoothly achieved with suitable electrophiles or their precursors. Finally, to address the challenging multifaceted selectivity and reactivity issues in asymmetric migratory cross-coupling reactions, we have developed a feasible ligand relay catalytic strategy. In this dynamic ligand exchange process, one ligand promotes rapid migration while the other promotes highly regio- and stereoselective coupling. This innovative strategy overcomes the formidable challenge stemming from the difficulty of designing a single ligand to efficiently promote both steps of chain-walking and asymmetric coupling. In the second part, a new platform for Ni-catalyzed migratory sp2 C-H functionalization via 1,4-Ni/H shift-selective cross-coupling has been reported. Starting from readily available aryl or vinyl coupling partners, the in situ-generated aryl- or vinylnickel(II) species could undergo a rapid and reversible 1,4-Ni/H shift along an sp2 backbone, and subsequent selective coupling with various coupling partners would allow regio- and stereoselective access to diverse 1,4-migratory functionalization products. The key to success was the discovery of an appropriate ligand to efficiently promote both migration and subsequent selective cross-coupling. A vinyl-to-aryl 1,4-Ni/H shift successfully enables the modular ipso/ortho difunctionalization of aryl coupling partners, while an aryl-to-vinyl 1,4-Ni/H shift enables regio- and stereoselective access to functionalized trisubstituted alkenes.We hope that this Account will inspire broad interest and future development of migratory cross-coupling reactions. We strongly believe that continued efforts in this fascinating field will overcome many of the remaining challenges, including cutting-edge ligand/catalyst design to enhance reactivity and selectivity, conceptually new migration modes for additional transformations, and in-depth mechanistic studies for rational reaction design.

Understanding the Reaction Mechanism of Ni-Catalyzed Regio- and Enantioselective Hydroalkylation of Enamines: Chemoselectivity of (Bi-oxazoline)NiH

This density functional theory study explores the detailed mechanism of nickel-catalyzed hydroalkylation of the C═C bond of N-Cbz-protected enamines (Cbz = benzyloxycarbonyl) with alkyl iodides to give chiral α-alkyl amines. The active catalyst (biOx)NiH, a chiral bioxazoline (biOx)-chelated Ni(I) hydride, exhibits chemoselectivity that favors single electron transfer to the alkyl iodide over C═C hydrometalation with the enamine. This generates an alkyl radical and a Ni(II) intermediate, which takes up the enamine substrate CbzNHCH═CH2CH3 via a regio- and enantioselective C═C insertion into the NiII-H bond. The resulting Ni(II) alkyl complex combines with the alkyl radical, forming a Ni(III) intermediate, from which the alkyl-alkyl reductive elimination delivers the chiral amine product. The regioselectivity arises from a combination of orbital and noncovalent interactions, both of which are induced by the Cbz group. Thus, Cbz plays an additional role in controlling regioselectivity. The enantioselectivity stems from the differing distortion energies of CbzNHCH═CH2CH3. The reductive elimination is the rate-determining step (ΔG⧧ = 18.7 kcal/mol). In addition, the calculations show a noninnocent behavior of the biOx ligand induced by the insertion of CbzNHCH═CH2CH3 into the Ni-H bond of (biOx)NiH. These computationally gained insights can have implications for developing new Ni(I)-catalyzed reactions.

Enantio- and Diastereoselective NiH-Catalyzed Hydroalkylation of Enamides or Enecarbamates with Racemic α-Bromoamides

Catalytic methods which control multiple stereogenic centers simultaneously are highly desirable in modern organic synthesis and chemical manufacturing. Herein, we report a regio-, enantio-, and diastereoselective NiH-catalyzed hydroalkylation process which proceeds with simultaneous control of vicinal stereocenters originating from two readily accessible partners, prochiral internal alkenes (enamides or enecarbamates) and racemic alkyl electrophiles (α-bromoamides or Katritzky salts). This reaction produces high-value β-aminoamides and their derivatives under mild conditions and with precise selectivity. Preliminary studies of the mechanism indicate that the reaction involves an enantioselective syn-hydronickelation to generate an enantiomerically enriched alkylnickel(II) species. Subsequent enantioconvergent alkylation with a racemic alkyl electrophile generates the desired product as a single stereoisomer.

Metal-Catalyzed Enantioconvergent Transformations

Enantioconvergent catalysis has expanded asymmetric synthesis to new methodologies able to convert racemic compounds into a single enantiomer. This review covers recent advances in transition-metal-catalyzed transformations, such as radical-based cross-coupling of racemic alkyl electrophiles with nucleophiles or racemic alkylmetals with electrophiles and reductive cross-coupling of two electrophiles mainly under Ni/bis(oxazoline) catalysis. C-H functionalization of racemic electrophiles or nucleophiles can be performed in an enantioconvergent manner. Hydroalkylation of alkenes, allenes, and acetylenes is an alternative to cross-coupling reactions. Hydrogen autotransfer has been applied to amination of racemic alcohols and C-C bond forming reactions (Guerbet reaction). Other metal-catalyzed reactions involve addition of racemic allylic systems to carbonyl compounds, propargylation of alcohols and phenols, amination of racemic 3-bromooxindoles, allenylation of carbonyl compounds with racemic allenolates or propargyl bromides, and hydroxylation of racemic 1,3-dicarbonyl compounds.

NiH-Catalyzed Regio- and Enantioselective Hydroalkylation for the Synthesis of β- or γ-Branched Chiral Aromatic N-Heterocycles

A nickel hydride-catalyzed regio- and enantioselective hydroalkylation reaction was developed to give access to a library of chiral β- or γ-branched aromatic N-heterocycles. This intriguing asymmetric transformation features excellent selectivities, step- and atom-economies, and generating two kinds of chiral products through one synthetic strategy. Furthermore, the possible reaction mechanism was extensively investigated using numerous control experiments and density functional theory calculations.

Synthesis of Enantioenriched 1,2-cis Disubstituted Cycloalkanes by Convergent NiH Catalysis

Enantioenriched multi-substituted cycloalkanes constitute an essential class of compounds in pharmaceuticals, natural products and agrochemicals. Here we report an NiH-catalyzed asymmetric migratory hydroalkylation process for the efficient and selective construction of such compounds. Through a dynamic kinetic asymmetric transformation (DYKAT), easily accessible racemic and isomeric mixtures of cycloalkenes could be directly utilized as starting materials, convergently producing thermo-dynamically disfavored chiral 1,2-cis disubstituted cycloalkanes bearing vicinal stereocenters with high levels of regio-, diastereo- and enantioselectivity. In addition, prochiral cyclic alkenes can be also employed, and deliver chiral 1,2-cis disubstituted cycloalkanes through desymmetrization process.

Nickel-Catalyzed Hydroamination and Hydroalkoxylation of Enelactams with Unactivated Amines and Alcohols

Nickel-catalyzed hydroamination and hydroalkoxylation of enelactams with unactivated amines and alcohols are reported. This method showed good functional group tolerance and delivered the corresponding hydrofunctionalized products in good to excellent yields (≤98%). Furthermore, an intramolecular hydroalkoxylation of an enelactam was also realized, giving a cyclization product in a good yield. Mechanistic studies indicated that tBuI acts as a hydride donor and radical precursor, which is crucial for the success of the reaction.

Recent Progress in NiH-Catalyzed Linear or Branch Hydrofunctionalization of Terminal or Internal Alkenes

The construction of C-C and C-X (X = N, O, Si, etc.) bonds is an important field in organic synthesis and methodology. In recent decades, studies on transition metal-catalyzed functionalization of alkenes have been on the rise. The individual properties of different transition metals determine the type of reaction that can be applied. Generally, post-transition metals with a large number of electrons in the d-orbit such as Mn, Fe, Co, Ni, Cu and Zn, etc., can be applied to more reaction types than pre-transition metals with a small number of electrons (e.g., Ti, Zr, etc.). Alkyl nickel intermediates formed by oxidative addition could couple with various of nucleophiles or electrophiles. Moreover, nickel has several oxidation valence states, which can flexibly realize a variety of catalytic cycles. These characteristics make nickel favored by researchers in the field of functionalization of alkenes, especially for the hydrofunctionalization of alkenes. Both terminal and internal alkenes could be converted, and the strategies of synthesizing linear and branched compounds have been expanded. Moreover, the guiding groups in alkenes played an almost decisive role in the regional selectivity, and the ligand or temperature also had regulating effects. Herein, we will give a comprehensive and timely overview of the works about the Ni-catalyzed hydrofunctionalization of alkenes and some insights on regional selectivity.

Regioselective Hydroalkylation of Vinyl- and Allylsilanes as Well as Vinylgermanes under Ni-H Catalysis

A Ni-H-catalyzed hydroalkylation of vinylsilanes and -germanes as well as allylsilanes with unactivated alkyl iodides is reported. Unlike related reactions of styrene or vinyl boronate esters, the addition across the C-C double bond proceeds with anti-Markovnikov selectivity to deliver the linear regioisomer. Mechanistic control experiments support a radical mechanism, and a competition experiment reveals that the chemoselectivity is in favor of the vinyl over the allyl group.

Enantioselective Csp3-Csp3 formation by nickel-catalyzed enantioconvergent cross-electrophile alkyl-alkyl coupling of unactivated alkyl halides

The pervasive occurrence of saturated stereogenic carbon centers in pharmaceuticals, agrochemicals, functional organic materials, and natural products has stimulated great efforts toward the construction of such saturated carbon centers. We report a reaction mode for the enantioselective construction of alkyl-alkyl bond to access saturated stereogenic carbon centers by asymmetric reductive cross-coupling between different alkyl electrophiles in good yields with great levels of enantioselectivity. This reaction mode uses only alkyl electrophiles for enantioselective C_sp3-C_sp3 bond-formation, rendering reductive alkyl-alkyl cross-coupling as an alternative to traditional alkyl-alkyl cross-coupling reactions between alkyl nucleophiles and alkyl electrophiles to access saturated stereogenic carbon centers without the use of organometallic reagents. The reaction displays a broad scope for two alkyl electrophiles with good functional group tolerance. Mechanistic studies reveal that the reaction undergoes a single electron transfer that enabled the reductive coupling pathway to form the alkyl-alkyl bond.

Ligand-enabled Ni-catalysed enantioconvergent intermolecular Alkyl-Alkyl cross-coupling between distinct Alkyl halides

α-Tertiary aliphatic amides are key elements in organic molecules, which are abundantly present in natural products, pharmaceuticals, agrochemicals, and functional organic materials. Enantioconvergent alkyl-alkyl bond-forming process is one of the most straightforward and efficient, yet highly challenging ways to build such stereogenic carbon centers. Herein, we report an enantioselective alkyl-alkyl cross-coupling between two different alkyl electrophiles to access α-tertiary aliphatic amides. With a newly-developed chiral tridentate ligand, two distinct alkyl halides were successfully cross-coupled together to forge an alkyl-alkyl bond enantioselectively under reductive conditions. Mechanistic investigations reveal that one alkyl halides exclusively undergo oxidative addition with nickel versus in-situ formation of alkyl zinc reagents from the other alkyl halides, rendering formal reductive alkyl-alkyl cross-coupling from easily available alkyl electrophiles without preformation of organometallic reagents.

Nickel-Catalyzed Remote Asymmetric Hydroalkylation of Alkenyl Ethers to Access Ethers of Chiral Dialkyl Carbinols

Site- and enantio-selective alkyl-alkyl bond formation is privileged in the retrosynthetic analysis due to the universality of sp³-hybridized carbon atoms in organic molecules. Herein, we report a nickel-catalyzed remote asymmetric hydroalkylation of alkenyl ethers via synchronous implementation of alkene isomerization and enantioselective C(sp³)-C(sp³) bond formation. Regression analysis of catalyst structure-activity relationships accelerates the rational ligand modification through modular regulation. This reaction has several advantages for synthesizing chiral dialkyl carbinols and their ether derivatives, including the broad substrate scope, good functional group tolerance, excellent regioselectivity (>20:1 regioisomeric ratio), and high enantioselectivity (up to 95% enantiomeric excess).

Ligand-Controlled Nickel-Catalyzed Regiodivergent Cross-Electrophile Alkyl-Alkyl Couplings of Alkyl Halides

Functionalizing specific positions on a saturated alkyl molecule is a key challenge in synthetic chemistry. Herein, a ligand-controlled regiodivergent alkylations of alkyl bromides at different positions by Ni-catalyzed alkyl-alkyl cross-electrophile coupling with the second alkyl bromides has been developed. The reaction undergoes site-selective isomerization on one alkyl bromides in a controlled manner, providing switchable access to diverse alkylated structures at different sites of alkyl bromides. The reaction occurs at three similar positions with excellent chemo- and regioselectivity, representing a remarkable ligand tuned reactivity between alkyl-alkyl cross-coupling and nickel migration along the hydrocarbon side chain. This reaction offers a catalytic platform to diverse saturated architectures by alkyl-alkyl bond-formation from identical starting materials.

Enantioselective C(sp3)-C(sp3) Reductive Cross-Electrophile Coupling of Unactivated Alkyl Halides with α-Chloroboronates via Dual Nickel/Photoredox Catalysis

Substantial advances in enantioconvergent C(sp³)-C(sp³) bond formations have been made with nickel-catalyzed cross-coupling of racemic alkyl electrophiles with organometallic reagents or nickel-hydride-catalyzed hydrocarbonation of alkenes. Herein, we report an unprecedented enantioselective C(sp³)-C(sp³) reductive cross-coupling by the direct utilization of two different alkyl halides with dual nickel/photoredox catalysis system. This highly selective coupling of racemic α-chloroboronates and unactivated alkyl iodides furnishes chiral secondary alkyl boronic esters, which serve as useful and important intermediates in the realm of organic synthesis and enable a desirable protocol to fast construction of enantioenriched complex molecules.

Copper-Catalyzed Asymmetric Boroprotonation of Phosphinylallenes

Synthesis of chiral phosphorus compounds from readily available substrates by a facile method is an attractive strategy. In this study, an efficient route for copper-catalyzed asymmetric boroprotonation of phosphinylallenes with bis(pinacolato)diboron with high regioselectivity was developed, affording chiral allylphosphine oxides in high yields with high enantioselectivities of up to 98% ee. The synthetic utility was further demonstrated by the facile transformation of the chiral allylphosphine oxides to several stereospecific products.

NiH-Catalyzed Functionalization of Remote and Proximal Olefins: New Reactions and Innovative Strategies

Transition metal hydride catalyzed functionalization of remote and proximal olefins has many advantages over conventional cross-coupling reactions. It avoids the separate, prior generation of stoichiometric amounts of organometallic reagents and the use of preformed organometallic reagents, which are sometimes hard to access and may compromise functional group compatibility. The migratory insertion of metal hydride complexes generated in situ into readily available alkene starting materials, the hydrometalation process, provides an attractive and straightforward route to alkyl metal intermediates, which can undergo a variety of sequential cross-coupling reactions. In particular, with the synergistic combination of chain-walking and cross-coupling chemistry of nickel, NiH-catalyzed functionalization of remote and proximal olefins has undergone particularly intense development in the past few years. This Account aims to chronicle the progress made in this arena in terms of activation modes, diverse functionalizations, and chemo-, regio-, and enantioselectivity.We first provide a brief introduction to the general reaction mechanisms. Taking remote hydroarylation as an example, the four oxidation states of Ni have allowed us to develop two different reaction strategies to form the final product: a Ni(I)-H/X-Ni(II)-H platform that relies on stoichiometric reductants and a Ni(I/II/III) cycle and a redox-neutral functional group or FG-Ni(II)-H platform that reacts with an alkene substrate and forms the migratory products via a Ni(0/II) pathway. We also demonstrate that diverse functionalization, including general C-C bond-forming reactions and the more challenging C-N/C-S bond-forming reactions could be realized. Moreover, the employment of appropriate chiral ligands has allowed us to successfully realize the corresponding asymmetric hydrofunctionalization reactions of olefins, including hydroalkylation, hydroarylation, hydroalkenylation, hydroalkynylation, and hydroamination. Interestingly, the enantio-determining step could be enantioselective hydronickelation, selective oxidative addition, or selective reductive elimination. To realize more challenging asymmetric migratory hydrofunctionalization, we have developed a general ligand relay catalytic strategy with a combination of two simple ligands, the first for chain-walking and the second for asymmetric coupling. This novel strategy avoids the design of a single, possibly structurally complex chiral ligand to promote both steps of chain-walking and asymmetric coupling. In addition, the success of multicomponent hydrofunctionalization provides a convenient approach to gain simple access to complex molecules. Finally, alkyl halides could be used as olefin precursors to undergo a variety of reductive migratory cross-electrophile coupling reactions. Applications of these remote hydrofunctionalization reactions are also discussed. We hope this Account will inspire future development in the field to overcome key challenges, including conceptually new catalytic strategies, development of high-performance systems with enhanced reactivity and selectivity, cutting-edge catalyst design, and further mechanistic studies.

Enantioselective synthesis of N-alkylindoles enabled by nickel-catalyzed C-C coupling

Enantioenriched N-alkylindole compounds, in which nitrogen is bound to a stereogenic sp³ carbon, are an important entity of target molecules in the fields of biological, medicinal, and organic chemistry. Despite considerable efforts aimed at inventing methods for stereoselective indole functionalization, straightforward access to a diverse range of chiral N-alkylindoles in an intermolecular catalytic fashion from readily available indole substrates remains an ongoing challenge. In sharp contrast to existing C-N bond-forming strategies, here, we describe a modular nickel-catalyzed C-C coupling protocol that couples a broad array of N-indolyl-substituted alkenes with aryl/alkenyl/alkynyl bromides to produce chiral N-alkylindole adducts in single regioisomeric form, in up to 91% yield and 97% ee. The process is amenable to proceed under mild conditions and exhibit broad scope and high functional group compatibility. Utility is highlighted through late-stage functionalization of natural products and drug molecules, preparation of chiral building blocks.

Access to dialkylated allylic stereogenic centers by Ni-catalysed enantioselective hydrovinylation of unactivated alkenes

Tertiary dialkylated allylic stereogenic centers are widespread substructures in bioactive molecules and natural products. However, enantioselective access to dialkyl substituted allylic motifs remains a long-term challenge. Herein, a straightforward protocol to build allylic dialkylated stereogenic centers enabled by nickel-catalysed regio- and enantioselective hydrovinylation of isolated unactivated alkenes facilitated by a weakly coordinating group with vinyl bromides was developed, affording dialkylated allylic species in good yields with excellent enantioselectivities. The reaction distinguishes distinct alkenes and works for both terminal and internal aliphatic alkenes. The reaction proceeds under mild conditions and tolerates a wide range of functional groups.

Enantioselective synthesis of α-aminoboronates by NiH-catalysed asymmetric hydroamidation of alkenyl boronates

Chiral α-aminoboronic acids and their derivatives are generally useful as bioactive compounds and some have been approved as therapeutic agents. Here we report a NiH-catalysed asymmetric hydroamidation process that with a simple amino alcohol ligand can easily produce a wide range of highly enantioenriched α-aminoboronates from alkenyl boronates and dioxazolones under mild conditions. The reaction is proposed to proceed by an enantioselective hydrometallation followed by an inner-sphere nitrenoid transfer and C–N bond forming sequence. The synthetic utility of this transformation was demonstrated by the efficient synthesis of a current pharmaceutical agent, Vaborbactam.

Enantioenriched α-aminoboronic acid, a structural unit in many bioactive molecules, is also a valuable synthon in organic synthesis. Here, the authors disclose a NiH-catalysed asymmetric hydroamidation process for their direct synthesis.

Direct Electrooxidative Selenylation/Cyclization of Alkynes: Access to Functionalized Benzo[b]furans

A mild, practical, metal and oxidant-free methodology for the synthesis of various C-3 selenylated benzo[b]furan derivatives was developed through the intramolecular cyclization of alkynes promoted with diselenides via electrooxidation. A wide range of selenium-substituted benzo[b]furan derivatives were obtained in good to excellent yields with high regioselectivity under constant current in an undivided cell equipped with carbon and platinum plates as the anode and cathode, respectively. Moreover, the convergent approach exhibited good functional group tolerance and could be easily scaled up with good efficiency, providing rapid access to a diverse range of selenylated benzo[b]furans derivatives from simple, readily available starting materials.

Ligand-Controlled NiH-Catalyzed Regiodivergent Chain-Walking Hydroalkylation of Alkenes

A NiH-catalyzed migratory hydroalkylation of alkenyl amines with predictable and switchable regioselectivity is reported. By utilizing a ligand-controlled, directing group-assisted strategy, various alkyl units are site-selectively installed at inert sp³ C-H sites far away from the original C=C bonds. A range of structurally diverse α- and β-branched protected amines are conveniently synthesized via stabilization of 5- and 6-membered nickelacycles respectively. This method exhibits broad scope and high functional group tolerance, and can be applied to late-stage modification of medicinally relevant molecules.

Ligand-Controlled Cobalt-Catalyzed Regiodivergent Alkyne Hydroalkylation

Regiodivergent alkyne hydroalkylation to generate different isomers of an alkene from the same alkyne starting material would be beneficial; however, it remains a challenge. Herein, we report a ligand-controlled cobalt-catalyzed regiodivergent alkyne hydroalkylation. The sensible selection of bisoxazoline (L1) and pyridine-oxazoline (L8) ligands led to reliable and predictable protocols that provided (E)-1,2-disubstituted and 1,1-disubstituted alkenes with high E/Z stereoselectivity and regioisomeric ratio starting from identical terminal alkyne and alkyl halide substrates and produced trisubstituted alkenes in the case of internal alkynes. This method exhibits a broad scope for terminal and internal alkynes with a wide range of activated and unactivated alkyl halides and shows excellent functional group compatibility.

Ni-Catalyzed Enantioselective Dialkyl Carbinol Synthesis via Decarboxylative Cross-Coupling: Development, Scope, and Applications

The first enantioselective decarboxylative Negishi-type alkylations of α-oxy carboxylic acids are reported via the intermediacy of redox-active esters (RAEs). This transformation enables a radical-based retrosynthesis of seemingly trivial enantiopure dialkyl carbinols. This article includes a discussion of the history of such couplings, the retrosynthetic ramifications of such a coupling, the development of general conditions, and an extensive series of applications that vividly demonstrate how it can simplify synthesis.

Regio- and enantioselective remote hydroarylation using a ligand-relay strategy

The design of a single complicated chiral ligand to well-promote each step of an asymmetric cascade reaction is sometimes a formidable challenge in transition metal catalysis. In this work, a highly regio- and enantioselective Ni-catalysed migratory hydroarylation relay process has been achieved with the combination of two simple ligands, one which accomplishes chain-walking and the other causing asymmetric arylation. This formal asymmetric C(sp³)−H arylation provides direct access to a wide range of structurally diverse chiral 1,1-diarylalkanes, a structural unit found in a number of bioactive molecules. The value of this strategy was further demonstrated by the Ni-catalysed migratory asymmetric 1,3-arylboration.

Migratory alkene isomerizations and cross-coupling reactions are both possible under nickel catalysis, but usually require different conditions. Here the authors show a combined protocol to isomerize a double bond and then, via an in-situ exchange of ligands, perform an enantioselective C(sp2)–C(sp3) cross-coupling.

Streamlined Alkylation via Nickel-Hydride-Catalyzed Hydrocarbonation of Alkenes

Compounds rich in sp³-hybridized carbons are desirable in drug discovery. Nickel-catalyzed hydrocarbonation of alkenes is a potentially efficient method to synthesize these compounds. By using abundant, readily available, and stable alkenes as pro-nucleophiles, these reactions can have broad scope and high functional group tolerance. However, this methodology is still in an early stage of development, as the first efficient examples were reported only in 2016. Herein, we summarize the progress of this emerging field, with an emphasis on enantioselective reactions. We highlight major developments, critically discuss a wide range of possible mechanisms, and offer our perspective of the state and challenges of the field. We hope this Perspective will stimulate future works in this area, making the methodology widely applicable in organic synthesis.

NiH-catalysed proximal-selective hydroalkylation of unactivated alkenes and the ligand effects on regioselectivity

Alkene hydrocarbonation reactions have been developed to supplement traditional electrophile-nucleophile cross-coupling reactions. The branch-selective hydroalkylation method applied to a broad range of unactivated alkenes remains challenging. Herein, we report a NiH-catalysed proximal-selective hydroalkylation of unactivated alkenes to access β- or γ-branched alkyl carboxylic acids and β-, γ- or δ-branched alkyl amines. A broad range of alkyl iodides and bromides with different functional groups can be installed with excellent regiocontrol and availability for site-selective late-stage functionalization of biorelevant molecules. Under modified reaction conditions with NiCl₂(PPh₃)₂ as the catalyst, migratory hydroalkylation takes place to provide β- (rather than γ-) branched products. The keys to success are the use of aminoquinoline and picolinamide as suitable directing groups and combined experimental and computational studies of ligand effects on the regioselectivity and detailed reaction mechanisms.

Difunctionalization of olefins is an ongoing and important focus of synthetic organic chemistry. Here the authors report a nickel-catalysed hydroalkylation of unactivated alkenes to obtain branched alkyl carboxylic acids or alkyl amines, using aminoquinoline and picolinamide as directing groups.

DFT insight into asymmetric alkyl-alkyl bond formation via nickel-catalysed enantioconvergent reductive coupling of racemic electrophiles with olefins

A DFT study has been conducted to understand the asymmetric alkyl–alkyl bond formation through nickel-catalysed reductive coupling of racemic alkyl bromide with olefin in the presence of hydrosilane and K₃PO₄. The key findings of the study include: (i) under the reductive experimental conditions, the Ni(ii) precursor is easily activated/reduced to Ni(0) species which can serve as an active species to start a Ni(0)/Ni(ii) catalytic cycle. (ii) Alternatively, the reaction may proceed via a Ni(i)/Ni(ii)/Ni(iii) catalytic cycle starting with a Ni(i) species such as Ni(i)–Br. The generation of a Ni(i) active species via comproportionation of Ni(ii) and Ni(0) species is highly unlikely, because the necessary Ni(0) species is strongly stabilized by olefin. Alternatively, a cage effect enabled generation of a Ni(i) active catalyst from the Ni(ii) species involved in the Ni(0)/Ni(ii) cycle was proposed to be a viable mechanism. (iii) In both catalytic cycles, K₃PO₄ greatly facilitates the hydrosilane hydride transfer for reducing olefin to an alkyl coupling partner. The reduction proceeds by converting a Ni–Br bond to a Ni–H bond via hydrosilane hydride transfer to a Ni–alkyl bond via olefin insertion. On the basis of two catalytic cycles, the origins for enantioconvergence and enantioselectivity control were discussed.

The enantioconvergent alkyl–alkyl coupling involves two competitive catalytic cycles with nickel(0) and nickel(i) active catalysts, respectively. K₃PO₄ plays a crucial role to enable the hydride transfer from hydrosilane to nickel–bromine species.

Diverse Synthesis of Chiral Trifluoromethylated Alkanes via Nickel-Catalyzed Enantioconvergent Reductive Hydroalkylation of Unactivated Olefins

Here, we report a nickel-catalyzed enantioconvergent hydroalkylation of olefins with trifluoromethyl-containing α-alkyl halides for the synthesis of enantioenriched trifluoromethylated alkanes. This reaction employs readily available and bench-stable alkenes as alkyl coupling partners, featuring mild conditions, a broad substrate scope, and high functional group tolerance. The synthetic utility of this method is further demonstrated in the late-stage functionalization of a range of drug molecules and natural products.