Nickel-Catalyzed Enantioselective α-Alkenylation of N-Sulfonyl Amines: Modular Access to Chiral α-Branched Amines

Regio- and enantioselective nickel-alkyl catalyzed hydroalkylation of alkynes

The migratory insertion of metal-hydride into alkene has allowed regioselective access to organometallics, readily participating in subsequent functionalization as one conventional pathway of hydroalkylation, whereas analogous process with feedstock alkyne is drastically less explored. Among few examples, the regioselectivity of metal-hydride insertion is mostly governed by electronic bias of alkynes. To alter the regioselectivity and drastically expand the intermediate pools that we can access, one aspirational design is through alternative nickel-alkyl insertion, providing opposite regioselectivity induced by steric demand. Leveraging in situ formed nickel-alkyl species, we herein report the regio- and enantioselective hydroalkylation of alkynes with broad functional group tolerance, excellent regio- and enantioselectivity, enabling efficient route to diverse valuable chiral allylic amines motifs. Preliminary mechanistic studies indicate the aminoalkyl radical species can participate in metal-capture and lead to formation of nickel-alkyl, of which the migratory insertion is key to reverse regioselectivity observed in metal-hydride insertion.

P-Stereogenic Phosphorus Ligands in Asymmetric Catalysis

Chiral phosphorus ligands play a crucial role in asymmetric catalysis for the efficient synthesis of useful optically active compounds. They are largely categorized into two classes: backbone chirality ligands and P-stereogenic phosphorus ligands. Most of the reported ligands belong to the former class. Privileged ones such as BINAP and DuPhos are frequently employed in a wide range of catalytic asymmetric transformations. In contrast, the latter class of P-stereogenic phosphorus ligands has remained a small family for many years mainly because of their synthetic difficulty. The late 1990s saw the emergence of novel P-stereogenic phosphorus ligands with their superior enantioinduction ability in Rh-catalyzed asymmetric hydrogenation reactions. Since then, numerous P-stereogenic phosphorus ligands have been synthesized and used in catalytic asymmetric reactions. This Review summarizes P-stereogenic phosphorus ligands reported thus far, including their stereochemical and electronic properties that afford high to excellent enantioselectivities. Examples of reactions that use this class of ligands are described together with their applications in the construction of key intermediates for the synthesis of optically active natural products and therapeutic agents. The literature covered dates back to 1968 up until December 2023, centering on studies published in the late 1990s and later years.

A facile synthesis of α,β-unsaturated imines via palladium-catalyzed dehydrogenation

The dehydrogenation adjacent to an electron-withdrawing group provides an efficient access to α,β-unsaturated compounds that serving as versatile synthons in organic chemistry. However, the α,β-desaturation of aliphatic imines has hitherto proven to be challenging due to easy hydrolysis and preferential dimerization. Herein, by employing a pre-fluorination and palladium-catalyzed dehydrogenation reaction sequence, the abundant simple aliphatic amides are amendable to the rapid construction of complex molecular architectures to produce α,β-unsaturated imines. Mechanistic investigations reveal a Pd(0)/Pd(II) catalytic cycle involving oxidative H-F elimination of N-fluoroamide followed by a smooth α,β-desaturation of the in-situ generated aliphatic imine intermediate. This protocol exhibits excellent functional group tolerance, and even the carbonyl groups are compatible without any competing dehydrogenation, allowing for late-stage functionalization of complex bioactive molecules. The synthetic utility of this transformation has been further demonstrated by a diversity-oriented derivatization and a concise formal synthesis of (±)-alloyohimbane.

Molybdenum-Catalyzed Regio- and Enantioselective Amination of Allylic Carbonates: Total Synthesis of (S)-Clopidogrel

The first molybdenum-catalyzed highly regio- and enantioselective allylic amination of both aryl- and alkyl-substituted branched allylic carbonates has been developed. A wide variety of amines, including drugs and complex bioactive molecules, underwent successful amination with excellent reaction outcomes (up to 96% yield, >99% ee, and >20:1 b/l). The reaction could be scaled up and has been applied to the total synthesis of chiral drug molecule (S)-clopidogrel (Plavix).

A Radical Approach for Asymmetric α-C-H Addition of N-Sulfonyl Benzylamines to Aldehydes

Efficient synthesis of enantioenriched amines is of great importance due to their significant synthetic and biological applications. Photoredox-mediated asymmetric α-amino C(sp3)-H functionalization offers an atom-economical and sustainable approach to access chiral amines. However, the development of analogous reactions is in its early stages, generally affording chiral amines with a single stereocenter. Herein, we present a novel synergistic triple-catalysis approach for the asymmetric α-C-H addition of readily available N-sulfonyl amines to aldehydes under mild conditions. This method allows for the efficient synthesis of a diverse array of valuable β-amino alcohols bearing vicinal stereocenters. Unlike previous reports, our protocol employs a radical approach using earth-abundant Cr catalysis. Quinuclidine plays a dual role by facilitating highly selective hydrogen-atom transfer to generate α-amino radicals and promoting the dissociation of the Cr-O bond, which is crucial for the overall catalytic cycle as evidenced by control, NMR, and DFT experiments. Preliminary mechanistic studies, including radical trapping, nonlinear effect, Stern-Volmer plot, kinetic isotope effect, and Hammett plot, offer valuable insights into the reaction pathway.

Ligand-Controlled Regiodivergent Nickel-Catalyzed Hydroaminoalkylation of Unactivated Alkenes

Ligand modulation of transition-metal catalysts to achieve optimal reactivity and selectivity in alkene hydrofunctionalization is a fundamental challenge in synthetic organic chemistry. Hydroaminoalkylation, an atom-economical approach for alkylating amines using alkenes, is particularly significant for amine synthesis in the pharmaceutical, agrochemical, and fine chemical industries. However, the existing methods usually require specific substrate combinations to achieve precise regio- and stereoselectivity, which limits their practical utility. Protocols allowing for regiodivergent hydroaminoalkylation from the same starting materials, controlling both regiochemical and stereochemical outcomes, are currently absent. Herein, we report a ligand-controlled, regiodivergent nickel-catalyzed hydroaminoalkylation of unactivated alkenes with N-sulfonyl amines. The reaction initiates with amine dehydrogenation and involves aza-nickelacycle intermediates. Tritert-butylphosphine promotes branched regioselectivity and syn diastereoselectivity, whereas ethyldiphenylphosphine enables linear selectivity, yielding regioisomers with inverse orientation. Systematic evaluation of diverse monodentate phosphine ligands reveals distinct regioselectivity cliffs, and % Vbur (min), a ligand steric descriptor, was established as a predictive parameter correlating ligand structure to regioselectivity. Computational investigations supported experimental findings, offering mechanistic insights into the origins of regioselectivity. Our method provides an efficient and predictable route for amine synthesis, demonstrating broad substrate scope, excellent tolerance toward various functional groups, and practical advantages. These include the use of readily available starting materials and cost-effective nickel(II) salts as precatalysts.

p-Cresol-Enabled Nickel-Catalyzed Intermolecular Redox-Economical Coupling of Allyl Alcohols with Alkynes through oxa-Nickelacycle

An intermolecular redox-economical coupling reaction of allyl alcohols with alkynes, catalyzed by Ni-Brønsted acid cocatalysis, has been developed. This method allows for the synthesis of a diverse range of γ,δ-unsaturated ketones with yields ranging from 40% to 94%, while maintaining excellent compatibility with various functional groups. The transformation of the resulting product demonstrates the significant practical value of this method. Further mechanistic investigations have revealed that the reaction proceeds through the formation of an oxa-nickelacycle intermediate.

Catalytic Asymmetric Synthesis of Zinc Metallacycles

Transition-metal-catalyzed reductive coupling reactions of alkynes and imines are attractive methods for the synthesis of chiral allylic amines. Mechanistically, these reactions involve oxidative cyclization of the alkyne and the imine to generate a metallacyclic intermediate, which then reacts with H2 or a H2 surrogate to form the product. As an alternative to this hydrogenolysis pathway, here we show that transmetalation to zinc can occur, forming a zinc metallacycle product. This organozinc product serves as a versatile nucleophile for carbon-carbon and carbon-heteroatom coupling reactions. Mechanistic studies based on isotopic labeling experiments and DFT calculations suggest that the key transmetalation step occurs between a Co(II) species and ZnCl2.

Stereoselective Synthesis of Substituted (Z)-N-Allyl Sulfonamides via a Palladium-Catalyzed Three-Component Tandem Reaction

This paper reports the efficient synthesis of substituted (Z)-N-allyl sulfonamides via a palladium-catalyzed three-component tandem reaction of N-buta-2,3-dienyl sulfonamides with iodides and sulfonyl hydrazide or sulfinic acid sodium salt as nucleophiles. Pd(PPh₃)₄ (2.5 mol %), K₂CO₃, and THF were used as the optimal catalyst, base, and solvent, respectively. The substituted (Z)-N-allyl sulfonamides were obtained in a 30-83% overall yield. Mechanistic investigations revealed that the formation of the single (Z)-isomer was controlled by the formation of a six-membered palladacycle intermediate.

A Ni-catalyzed asymmetric C(sp)-P cross-coupling reaction for the synthesis of P-stereogenic alkynylphosphines

Due to the high reactivity of the triple bond, P-stereogenic alkynylphosphines could be easily derivatized, serving as universal building blocks for structurally diverse phosphine compounds. However, the synthesis of alkynylphosphines via direct P-C bond formation was unprecedented. Here, we report an efficient method for the synthesis of P-stereogenic alkynylphosphines with high enantioselectivity via a Ni-catalyzed asymmetric cross-coupling reaction. The reaction could tolerate a variety of functional groups, affording products that can be converted into useful phosphine derivatives.

Enantioselective Metal-Catalyzed Reductive Coupling of Alkynes with Carbonyl Compounds and Imines: Convergent Construction of Allylic Alcohols and Amines

The use of alkynes as vinylmetal pronucleophiles in intermolecular enantioselective metal-catalyzed carbonyl and imine reductive couplings to form allylic alcohols and amines is surveyed. Related hydrogen auto-transfer processes, wherein alcohols or amines serve dually as reductants and carbonyl or imine proelectrophiles, also are cataloged, as are applications in target-oriented synthesis. These processes represent an emerging alternative to the use of stoichiometric vinylmetal reagents or Nozaki-Hiyama-Kishi (NHK) reactions in carbonyl and imine alkenylation.

External-Ligand-Free, Nickel-Catalyzed Alkenylation of N-Sulfonylamines with Internal Alkynes

Allylic amines were synthesized via a nickel-catalyzed coupling reaction between various N-sulfonylamines and internal alkynes. The catalytic reaction was by-product-free and proceeded without the need for additional oxidant/reductant or activating reagent. As improvements over established methods, the present approach avoids the need for an external ligand, which increases the value of the approach with respect to atom economy, and it uses bench-stable Ni(II)Br2(dme) instead of Ni(0)(COD)2 as the source of the nickel catalyst. Mechanistic studies revealed that a catalytic amount of a strong base (i.e., KO t Bu) was essential for the formation of active Ni(0) catalyst, which, along with an imine intermediate, then initiated the catalytic cycle.

Redox-neutral remote amidation of alkenyl alcohols via long-range isomerization/transformation

A facile and straightforward approach for the construction of amides via redox-neutral Ru-catalyzed cross-coupling reaction of long-range alkenyl alcohols with amines to realize remote site-selective functionalization has been developed.

Redox-Neutral Dehydrogenative Cross-Coupling of Alcohols and Amines Enabled by Nickel Catalysis

Presented herein is a facile and straightforward synthetic method for the construction of amides via Ni/NHC-catalyzed amidation of alcohols with amines. The strategy exhibits various advantages over existing methods, including...

Stereoselective Synthesis of Tertiary Allylic Amines by Titanium-Catalyzed Hydroaminoalkylation of Alkynes with Tertiary Amines

Intermolecular hydroaminoalkylation reactions of symmetrical and unsymmetrical alkynes with tertiary amines take place in the presence of catalytic amounts of TiBn₄, Ph₃C[B(C₆F₅)₄], and a sterically demanding aminopyridinato ligand precursor. The resulting products, synthetically and pharmaceutically useful tertiary β,γ‐disubstituted allylic amines, are formed in convincing yields and with excellent stereoselectivity. Particularly promising for future applications is the fact that even the industrial side product trimethylamine can be used as a substrate.

Ni-catalyzed hydroalkylation of olefins with N-sulfonyl amines

Hydroalkylation, the direct addition of a C(sp³)–H bond across an olefin, is a desirable strategy to produce valuable, complex structural motifs in functional materials, pharmaceuticals, and natural products. Herein, we report a reliable method for accessing α-branched amines via nickel-catalyzed hydroalkylation reactions. Specifically, by using bis(cyclooctadiene)nickel (Ni(cod)₂) together with a phosphine ligand, we achieved a formal C(sp³)–H bond insertion reaction between olefins and N-sulfonyl amines without the need for an external hydride source. The amine not only provides the alkyl motif but also delivers hydride to the olefin by means of a nickel-engaged β–hydride elimination/reductive elimination process. This method provides a platform for constructing chiral α-branched amines by using a P-chiral ligand, demonstrating its potential utility in organic synthesis. Notably, a sulfonamidyl boronate complex formed in situ under basic conditions promotes ring-opening of the azanickellacycle reaction intermediate, leading to a significant improvement of the catalytic efficiency.

Catalytic addition of a carbon chain and a hydrogen across a double bond has often required an added hydride source. Here the authors show a method to add alkanes with an amino functionality to olefins, wherein a nickel catalyst uses the amine itself as the hydride source, obviating an external hydride reagent.

From Hydrogenation to Transfer Hydrogenation to Hydrogen Auto-Transfer in Enantioselective Metal-Catalyzed Carbonyl Reductive Coupling: Past, Present, and Future

Atom-efficient processes that occur via addition, redistribution or removal of hydrogen underlie many large volume industrial processes and pervade all segments of chemical industry. Although carbonyl addition is one of the oldest and most broadly utilized methods for C-C bond formation, the delivery of non-stabilized carbanions to carbonyl compounds has relied on premetalated reagents or metallic/organometallic reductants, which pose issues of safety and challenges vis-à-vis large volume implementation. Catalytic carbonyl reductive couplings promoted via hydrogenation, transfer hydrogenation and hydrogen auto-transfer allow abundant unsaturated hydrocarbons to serve as substitutes to organometallic reagents, enabling C-C bond formation in the absence of stoichiometric metals. This perspective (a) highlights past milestones in catalytic hydrogenation, hydrogen transfer and hydrogen auto-transfer, (b) summarizes current methods for catalytic enantioselective carbonyl reductive couplings, and (c) describes future opportunities based on the patterns of reactivity that animate transformations of this type.