Recent Developments in Asymmetric Allylic Amination Reactions

Iridium-Catalyzed Regio- and Enantioselective N-Allylation of Pyrazoles with Dienyl/Monoallylic Alcohols

Here we report the first example of iridium-catalyzed asymmetric N-allylation of pyrazoles with dienyl allylic alcohols under mild conditions with broad functional group tolerance, exhibiting excellent N1/C3-site selectivities and enantioselectivities (up to >99% ee). In addition to pyrazoles, other nucleophiles including benzotriazole, triazole, and pyrazole precursors (aryl vinyldiazos) are also suitable in this method. Notably, with the use of Sc(OTf)3 as additive and reactions performed at 30 °C for 24 h, the N1-C5 or N1-C1 selective alkylated pyrazoles are also obtained.

Recent advances in catalytic asymmetric synthesis

Asymmetric catalysis stands at the forefront of modern chemistry, serving as a cornerstone for the efficient creation of enantiopure chiral molecules characterized by their high selectivity. In this review, we delve into the realm of asymmetric catalytic reactions, which spans various methodologies, each contributing to the broader landscape of the enantioselective synthesis of chiral molecules. Transition metals play a central role as catalysts for a wide range of transformations with chiral ligands such as phosphines, N-heterocyclic carbenes (NHCs), etc., facilitating the formation of chiral C-C and C-X bonds, enabling precise control over stereochemistry. Enantioselective photocatalytic reactions leverage the power of light as a driving force for the synthesis of chiral molecules. Asymmetric electrocatalysis has emerged as a sustainable approach, being both atom-efficient and environmentally friendly, while offering a versatile toolkit for enantioselective reductions and oxidations. Biocatalysis relies on nature's most efficient catalysts, i.e., enzymes, to provide exquisite selectivity, as well as a high tolerance for diverse functional groups under mild conditions. Thus, enzymatic optical resolution, kinetic resolution and dynamic kinetic resolution have revolutionized the production of enantiopure compounds. Enantioselective organocatalysis uses metal-free organocatalysts, consisting of modular chiral phosphorus, sulfur and nitrogen components, facilitating remarkably efficient and diverse enantioselective transformations. Additionally, unlocking traditionally unreactive C-H bonds through selective functionalization has expanded the arsenal of catalytic asymmetric synthesis, enabling the efficient and atom-economical construction of enantiopure chiral molecules. Incorporating flow chemistry into asymmetric catalysis has been transformative, as continuous flow systems provide precise control over reaction conditions, enhancing the efficiency and facilitating optimization. Researchers are increasingly adopting hybrid approaches that combine multiple strategies synergistically to tackle complex synthetic challenges. This convergence holds great promise, propelling the field of asymmetric catalysis forward and facilitating the efficient construction of complex molecules in enantiopure form. As these methodologies evolve and complement one another, they push the boundaries of what can be accomplished in catalytic asymmetric synthesis, leading to the discovery of novel, highly selective transformations which may lead to groundbreaking applications across various industries.

Copper-Catalyzed Enantioconvergent Radical N-Alkylation of Diverse (Hetero)aromatic Amines

The 3d transition metal-catalyzed enantioconvergent radical cross-coupling provides a powerful tool for chiral molecule synthesis. In the classic mechanism, the bond formation relies on the interaction between nucleophile-sequestered metal complexes and radicals, limiting the nucleophile scope to sterically uncongested ones. The coupling of sterically congested nucleophiles poses a significant challenge due to difficulties in transmetalation, restricting the reaction generality. Here, we describe a probable outer-sphere nucleophilic attack mechanism that circumvents the challenging transmetalation associated with sterically congested nucleophiles. This strategy enables a general copper-catalyzed enantioconvergent radical N-alkylation of aromatic amines with secondary/tertiary alkyl halides and exhibits catalyst-controlled stereoselectivity. It accommodates diverse aromatic amines, especially bulky secondary and primary ones to deliver value-added chiral amines (>110 examples). It is expected to inspire the coupling of more nucleophiles, particularly challenging sterically congested ones, and accelerate reaction generality.

Naturally occurring phytic acid: an advanced Brønsted acid catalyst for direct amination reactions of allylic alcohols

Phytic acid is abundant in various plant-based foods and is considered agricultural waste. Here, we demonstrate the effectiveness of this organophosphorus acid as a sustainable catalyst for the direct amination reactions of allylic alcohols. This approach is successfully performed in air using technical grade solvents, affording allylanilines in moderate to excellent yields. Challenging electron-rich anilines react effectively, and their corresponding Friedel-Crafts side products can be minimised under the optimised reaction conditions. A variety of asymmetrically substituted allylic alcohols are tolerated, while the scope is extended to amide, and C-, O- and S-nucleophiles.

Iridium-catalysed reductive allylic amination of α,β-unsaturated aldehydes

Allylic amination is a powerful tool for constructing N-allylic amines widely found in bioactive molecules. Generally, allylic alcohols and unsaturated hydrocarbons have been considered for allylic amination reactions to minimize waste production. Herein, we present an iridium-catalysed method for reductive allylic amination of α,β-unsaturated aldehydes with amines to afford N-allylic amines under air conditions. This protocol is demonstrated to provide products from many substrates (41 examples) in moderate-to-excellent yields. This synthetic methodology is also highlighted by the synthesis of drug molecules, optically pure products, as well as scale-up experiments.

Recent developments for intermolecular enantioselective amination of non-acidic C(sp3)-H bonds

Enantioenriched chiral amines are of exceptional importance in the pharmaceutical industry. Recently, several new methods for the installation of these functional groups directly from non-acidic C(sp3)-H bonds by catalytic intermolecular enantioselective amination have been reported. These methods represent significant advances of the field and most of them display high levels of enantioselectivity, utilize the C(sp3)-H substrate as the limiting reagent, feature good functional group tolerance, and show compatibility with late-stage C(sp3)-H amination of advanced substrates. This perspective provides an overview of the recent developments in this rapidly advancing field and outlines possibilities and limitations, which will help identify unsolved challenges and guide future research efforts.

Desymmetrization of Geminal Difluoromethylenes using a Palladium/Copper/Lithium Ternary System for the Stereodivergent Synthesis of Fluorinated Amino Acids

Fluorinated amino acids and related peptides/proteins have been found widespread applications in pharmaceutical and agricultural compounds. However, strategies for introducing a C-F bond into amino acids in an enantioselective manner are still limited and no such asymmetric catalysis strategy has been reported. Herein, we have successfully developed a Pd/Cu/Li ternary system for stereodivergent synthesis of chiral fluorinated amino acids. This method involves a sequential desymmetrization of geminal difluoromethylenes and allylic substitution with amino acid Schiff bases via Pd/Li and Pd/Cu dual activation, respectively. A series of non-natural amino acids bearing a chiral allylic/benzylic fluorine motif are easily synthesized in high yields with excellent regio-, diastereo-, and enantioselectivities (up to >20 : 1 dr and >99 % ee). A density functional theory (DFT) study revealed the F-Cu interaction of the allylic substrate and the Cu catalyst significantly influence the stereoselectivity.

Copper-Catalyzed Enantioconvergent Radical C(sp3)-N Cross-Coupling of Activated Racemic Alkyl Halides with (Hetero)aromatic Amines under Ambient Conditions

The enantioconvergent C(sp³)-N cross-coupling of racemic alkyl halides with (hetero)aromatic amines represents an ideal means to afford enantioenriched N-alkyl (hetero)aromatic amines yet has remained unexplored due to the catalyst poisoning specifically for strong-coordinating heteroaromatic amines. Here, we demonstrate a copper-catalyzed enantioconvergent radical C(sp³)-N cross-coupling of activated racemic alkyl halides with (hetero)aromatic amines under ambient conditions. The key to success is the judicious selection of appropriate multidentate anionic ligands through readily fine-tuning both electronic and steric properties for the formation of a stable and rigid chelating Cu complex. Thus, this kind of ligand could not only enhance the reducing capability of a copper catalyst to provide an enantioconvergent radical pathway but also avoid the coordination with other coordinating heteroatoms, thereby overcoming catalyst poisoning and/or chiral ligand displacement. This protocol covers a wide range of coupling partners (89 examples for activated racemic secondary/tertiary alkyl bromides/chlorides and (hetero)aromatic amines) with high functional group compatibility. When allied with follow-up transformations, it provides a highly flexible platform to access synthetically useful enantioenriched amine building blocks.

Copper-catalysed enantioconvergent alkylation of oxygen nucleophiles

Carbon-oxygen bonds are commonplace in organic molecules, including chiral bioactive compounds; therefore, the development of methods for their construction with simultaneous control of stereoselectivity is an important objective in synthesis. The Williamson ether synthesis, first reported in 18501, is the most widely used approach to the alkylation of an oxygen nucleophile, but it has significant limitations (scope and stereochemistry) owing to its reaction mechanism (SN2 pathway). Transition-metal catalysis of the coupling of an oxygen nucleophile with an alkyl electrophile has the potential to address these limitations, but progress so far has been limited2-7, especially with regard to controlling enantioselectivity. Here we establish that a readily available copper catalyst can achieve an array of enantioconvergent substitution reactions of α-haloamides, a useful family of electrophiles, by oxygen nucleophiles; the reaction proceeds under mild conditions in the presence of a wide variety of functional groups. The catalyst is uniquely effective in being able to achieve enantioconvergent alkylations of not only oxygen nucleophiles but also nitrogen nucleophiles, giving support for the potential of transition-metal catalysts to provide a solution to the pivotal challenge of achieving enantioselective alkylations of heteroatom nucleophiles.

Palladium-Catalyzed Branch- and Z-Selective Allylic C-H Amination with Aromatic Amines

Allylamines are important building blocks in the synthesis of bioactive compounds. The direct coupling of allylic C-H bonds and commonly available amines is a major synthetic challenge. An allylic C-H amination of 1,4-dienes has been accomplished by palladium catalysis. With aromatic amines, branch-selective allylic aminations are favored to generate thermodynamically unstable Z-allylamines. In addition, more basic aliphatic cyclic amines can also engage in the reaction, but linear dienyl allylic amines are the major products.

P,S-Bidentate Phosphoramidites with (Ra)-BINOL Core in Palladium-Catalyzed Asymmetric Allylic Substitution

Abstract

New P,S-bidentate phosphoramidite ligands, including these having a stereogenic phosphorus atom in the 1,3,2-dioxaphosphepine ring, have been synthesized on the basis of (Ra)-BINOL and its adamantanyl derivatives. With their participation in the Pd-catalyzed asymmetric allylic alkylation of (E)-1,3-diphenylallyl acetate with dimethyl malonate and the amination with pyrrolidine involving these ligands have afforded selectivity up to 84 and 75% ee, respectively.

Modular Synthesis of Enantioenriched α-Chiral Homoallylic Amidines Enabled by Relay Ir/Cu Catalysis

The cascade of Ir-catalyzed enantioselective allylic amination and Cu-catalyzed alkyne-azide cycloaddition was designed for the asymmetric synthesis of homoallylic amidines. The nucleophilic addition of an in situ-generated enantioenriched tertiary allylamine to a ketenimine intermediate triggers a rapid and stereospecific zwitterionic aza-Claisen rearrangement in a 1,3-chiral transfer manner. The approach allows modular access to enantioenriched α-chiral homoallylic amidines in high yields with a high level of enantiomeric purity.

Biomacromolecule-Assisted Screening for Reaction Discovery and Catalyst Optimization

Reaction discovery and catalyst screening lie at the heart of synthetic organic chemistry. While there are efforts at de novo catalyst design using computation/artificial intelligence, at its core, synthetic chemistry is an experimental science. This review overviews biomacromolecule-assisted screening methods and the follow-on elaboration of chemistry so discovered. All three types of biomacromolecules discussed─enzymes, antibodies, and nucleic acids─have been used as "sensors" to provide a readout on product chirality exploiting their native chirality. Enzymatic sensing methods yield both UV-spectrophotometric and visible, colorimetric readouts. Antibody sensors provide direct fluorescent readout upon analyte binding in some cases or provide for cat-ELISA (Enzyme-Linked ImmunoSorbent Assay)-type readouts. DNA biomacromolecule-assisted screening allows for templation to facilitate reaction discovery, driving bimolecular reactions into a pseudo-unimolecular format. In addition, the ability to use DNA-encoded libraries permits the barcoding of reactants. All three types of biomacromolecule-based screens afford high sensitivity and selectivity. Among the chemical transformations discovered by enzymatic screening methods are the first Ni(0)-mediated asymmetric allylic amination and a new thiocyanopalladation/carbocyclization transformation in which both C-SCN and C-C bonds are fashioned sequentially. Cat-ELISA screening has identified new classes of sydnone-alkyne cycloadditions, and DNA-encoded screening has been exploited to uncover interesting oxidative Pd-mediated amido-alkyne/alkene coupling reactions.

Photoinduced, Copper-Catalyzed Enantioconvergent Alkylations of Anilines by Racemic Tertiary Electrophiles: Synthesis and Mechanism

Transition-metal catalysis of substitution reactions of alkyl electrophiles by nitrogen nucleophiles is beginning to emerge as a powerful strategy for synthesizing higher-order amines, as well as controlling their stereochemistry. Herein, we report that a readily accessible chiral copper catalyst (commercially available components) can achieve the photoinduced, enantioconvergent coupling of a variety of racemic tertiary alkyl electrophiles with aniline nucleophiles to generate a new C-N bond with good ee at the fully substituted stereocenter of the product; whereas this photoinduced, copper-catalyzed coupling proceeds at -78 °C, in the absence of light and catalyst, virtually no C-N bond formation is observed even upon heating to 80 °C. The mechanism of this new catalytic enantioconvergent substitution process has been interrogated with the aid of a wide array of tools, including the independent synthesis of proposed intermediates and reactivity studies, spectroscopic investigations featuring photophysical and EPR data, and DFT calculations. These studies led to the identification of three copper-based intermediates in the proposed catalytic cycle, including a chiral three-coordinate formally copper(II)-anilido (DFT analysis points to its formulation as a copper(I)-anilidyl radical) complex that serves as a persistent radical that couples with a tertiary organic radical to generate the desired C-N bond with good enantioselectivity.

Recent Advances in the Enantioselective Synthesis of Chiral Amines via Transition Metal-Catalyzed Asymmetric Hydrogenation

Chiral amines are key structural motifs present in a wide variety of natural products, drugs, and other biologically active compounds. During the past decade, significant advances have been made with respect to the enantioselective synthesis of chiral amines, many of them based on catalytic asymmetric hydrogenation (AH). The present review covers the use of AH in the synthesis of chiral amines bearing a stereogenic center either in the α, β, or γ position with respect to the nitrogen atom, reported from 2010 to 2020. Therefore, we provide an overview of the recent advances in the AH of imines, enamides, enamines, allyl amines, and N-heteroaromatic compounds.

Medium and Large N-Heterocycle Formation via Allene Hydroamination with a Bimetallic Rh(II) Catalyst

We report the synthesis of a 2-phosphinoimidazole-derived bimetallic Rh(II) complex that enables intramolecular allene hydroamination to form 7- to 10-member rings in high yield. Monometallic Rh complexes, in contrast, fail to achieve any product formation. We demonstrate a broad substrate scope for formation of various N-heterocycles. Macrocyclizations that form 11- to 15-member N-heterocycles are also demonstrated. Mechanistic studies suggest that the reaction proceeds via reversible allene insertion with a Rh-hydride followed by C-N bond-forming reductive elimination. We hypothesize that the reactivity observed with our catalyst vs monometallic Rh complexes is derived from the bimetallic nature of our complex.

Electrochemical Synthesis of Allylic Amines from Terminal Alkenes and Secondary Amines

Allylic amines are valuable synthetic targets en route to diverse biologically active amine products. Current allylic C-H amination strategies remain limited with respect to the viable N-substituents. Herein, we disclose a new electrochemical process to prepare aliphatic allylic amines by coupling two abundant starting materials: secondary amines and unactivated alkenes. This oxidative transformation proceeds via electrochemical generation of an electrophilic adduct between thianthrene and the alkene substrates. Treatment of these adducts with aliphatic amine nucleophiles and base provides allylic amine products in high yield. This synthetic strategy is also amenable to functionalization of feedstock gaseous alkenes at 1 atm. In the case of 1-butene, high Z-selective crotylation is observed. This strategy, however, is not limited to the synthesis of simple building blocks; complex biologically active molecules are suitable as both alkene and amine coupling partners. Preliminary mechanistic studies implicate vinylthianthrenium salts as key reactive intermediates.

Photoinduced copper-catalysed asymmetric amidation via ligand cooperativity

The substitution of an alkyl electrophile by a nucleophile is a foundational reaction in organic chemistry that enables the efficient and convergent synthesis of organic molecules. Although there has been substantial recent progress in exploiting transition-metal catalysis to expand the scope of nucleophilic substitution reactions to include carbon nucleophiles1-4, there has been limited progress in corresponding reactions with nitrogen nucleophiles5-8. For many substitution reactions, the bond construction itself is not the only challenge, as there is a need to control stereochemistry at the same time. Here we describe a method for the enantioconvergent substitution of unactivated racemic alkyl electrophiles by a ubiquitous nitrogen-containing functional group, an amide. Our method uses a photoinduced catalyst system based on copper, an Earth-abundant metal. This process for asymmetric N-alkylation relies on three distinct ligands-a bisphosphine, a phenoxide and a chiral diamine. The ligands assemble in situ to form two distinct catalysts that act cooperatively: a copper/bisphosphine/phenoxide complex that serves as a photocatalyst, and a chiral copper/diamine complex that catalyses enantioselective C-N bond formation. Our study thus expands enantioselective N-substitution by alkyl electrophiles beyond activated electrophiles (those bearing at least one sp- or sp2-hybridized substituent on the carbon undergoing substitution)8-13 to include unactivated electrophiles.

Rhodium-Catalyzed Regio- and Enantioselective Allylic Amination of Racemic 1,2-Disubstituted Allylic Phosphates

Alkynylphosphines are rarely used as ligands in asymmetric metal catalysis. We synthesized a series of chiral bis(oxazoline)alkynylphosphine ligands and used them in Rh-catalyzed highly regio- and enantioselective allylic amination reactions of 1,2-disubstituted allylic phosphates. Chiral 1,2-disubstituted allylic amines were synthesized in up to 95% yield with >20:1 branched/linear (b/l) ratio and 99% ee from racemic 1,2-disubstituted allylic precursors. The sterically smaller linear alkynyl group on the P atom in the bis(oxazoline)alkynylphosphine ligands was the key to fit the new requirements of the introduction of bulky 2-R' groups.

Recent Advances in Enantioselective Pd-Catalyzed Allylic Substitution: From Design to Applications

This Review compiles the evolution, mechanistic understanding, and more recent advances in enantioselective Pd-catalyzed allylic substitution and decarboxylative and oxidative allylic substitutions. For each reaction, the catalytic data, as well as examples of their application to the synthesis of more complex molecules, are collected. Sections in which we discuss key mechanistic aspects for high selectivity and a comparison with other metals (with advantages and disadvantages) are also included. For Pd-catalyzed asymmetric allylic substitution, the catalytic data are grouped according to the type of nucleophile employed. Because of the prominent position of the use of stabilized carbon nucleophiles and heteronucleophiles, many chiral ligands have been developed. To better compare the results, they are presented grouped by ligand types. Pd-catalyzed asymmetric decarboxylative reactions are mainly promoted by PHOX or Trost ligands, which justifies organizing this section in chronological order. For asymmetric oxidative allylic substitution the results are grouped according to the type of nucleophile used.

Regioselective molybdenum-catalyzed allylic substitution of tertiary allylic electrophiles: methodology development and applications

The first molybdenum-catalyzed allylic sulfonylation of tertiary allylic electrophiles is described. The method employs a readily accessible catalyst (Mo(CO)₆/2,2′-bipyridine, both are commercially available) and represents the first example of the use of a group 6 transition metal-catalyst for allylic sulfonylation of substituted tertiary allylic electrophiles to form carbon–sulfur bonds. This atom economic and operationally simple methodology is characterized by its relatively mild conditions, wide substrate scope, and excellent regioselectivity profile, thus unlocking a new platform to forge sulfone moieties, even in the context of late-stage functionalization and providing ample opportunities for further derivatization through traditional Suzuki cross-coupling reactions.

The first general example of Mo-catalyzed allylic sulfonylation of tertiary allylic electrophile provides an efficient way to forge sulfone moieties, and providing ample opportunities for further transformation through traditional Suzuki cross-coupling.

Allylic C(sp3)-H alkylation via synergistic organo- and photoredox catalyzed radical addition to imines

A new catalytic method for the direct alkylation of allylic C(sp³)–H bonds from unactivated alkenes via synergistic organo- and photoredox catalysis is described. The transformation achieves an efficient, redox-neutral synthesis of homoallylamines with broad functional group tolerance, under very mild reaction conditions. Mechanistic investigations indicate that the reaction proceeds through the N-centered radical intermediate which is generated by the allylic radical addition to the imine.

A new catalytic method for the direct alkylation of allylic C(sp³)–H bonds from unactivated alkenes via synergistic organo- and photoredox catalysis is described.

Stereocontrolled Microwave-Assisted Domino [3,3]-Sigmatropic Reactions: A Winstein-Overman Rearrangement for the Formation of Differentiated Contiguous C-N Bonds

A domino [3,3]-sigmatropic rearrangement sequence employing a sequential reversible allylic azide rearrangement followed by an irreversible Overman reaction provides a new route to the formation of two contiguous C-N bonds. The reaction occurs in a stereocontrolled fashion in two steps from readily available alkenyl epoxides via initial azide anion ring opening of the epoxides.

Pd-Catalyzed Asymmetric N-Allylation of Amino Acid Esters with Exceptional Levels of Catalyst Control: Stereo-Divergent Synthesis of ProM-15 and Related Bicyclic Dipeptide Mimetics

A general and powerful method for the stereo‐controlled Pd‐catalyzed N‐allylation of amino acid esters is reported, as a previously largely unsolved synthetic challenge. Employing a new class of tartaric acid‐derived C₂‐symmetric chiral diphosphane ligands the developed asymmetric amination protocol allows the conversion of various amino acid esters to the N‐allylated products with highest levels of enantio‐ or diastereoselectivity in a fully catalyst‐controlled fashion and predictable configuration. Remarkably, the in situ generated catalysts also exhibit outstanding levels of activity (ligand acceleration). The usefulness of the method was demonstrated in the stereo‐divergent synthesis of a set of new conformationally defined dipeptide mimetics, which represent new modular building blocks for the development of peptide‐inspired bioactive compounds.

K2CO3 Promoted Cascade Reaction for the Preparation of 1H-Imidazol-4- yl-1-amine Derivatives

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A K2CO3 promoted efficient one pot two-step method for the preparation of 1H-imidazol-4- yl-1-amine derivatives has been developed. A series of second amines with an imidazole group were obtained with 56%-91% yields by the K2CO3 promoted amination of acetates and nitrogen deprotection of the imidazole process.

Enantioconvergent Alkylations of Amines by Alkyl Electrophiles: Copper-Catalyzed Nucleophilic Substitutions of Racemic α-Halolactams by Indoles

Transition-metal catalysis has the potential to address shortcomings in the classic S_N2 reaction of an amine with an alkyl electrophile, both with respect to reactivity and to enantioselectivity. In this study, we describe the development of a user-friendly method (reaction at room temperature, with commercially available catalyst components) for the enantioconvergent nucleophilic substitution of racemic secondary alkyl halides (α-iodolactams) by indoles. Mechanistic studies are consistent with the formation of a copper(I)-indolyl complex that reacts at different rates with the two enantiomers of the electrophile, which interconvert under the reaction conditions (dynamic kinetic resolution). This investigation complements earlier work on photoinduced enantioconvergent N-alkylation, supporting the premise that this important challenge can be addressed by a range of strategies.