Catalytic Asymmetric Synthesis of α-Tetrasubstituted α-Trifluoromethyl Homoallylic Amines by Ir-Catalyzed Umpolung Allylation of Imines

Nickel-Catalyzed Asymmetric (3 + 2) Annulations of Propargylic Carbonates and Vinylogous Donors via an Alkenylation Pathway

The transition-metal-catalyzed alkenylation strategy of propargylic alcohol derivatives provides an efficient protocol to access multifunctional products in a double-nucleophilic attack pattern. While limited relevant asymmetric examples have been reported via palladium catalysis, here we first demonstrate that a nonprecious Ni(0)-based chiral complex can efficiently promote the tandem substitution process between propargylic carbonates and N-trifluoroethyl ketimines via consecutive aza-vinylogous activations, finally accomplishing a (3 + 2) annulation reaction to afford products embedding a 4-methylene-3,4-dihydro-2H-pyrrole framework with high regio-, diastereo-, and enantiocontrol. Their assemblies with a few all-carbon-based vinylogous precursors are also successful, and enantioenriched adducts containing a 3-methylenecyclopentene scaffold are furnished effectively. The substitution patterns for both types of substrates are substantial, and an array of synthetic elaborations is conducted to deliver more versatile architectures with high application potential. In addition, density functional theory calculations and control experiments have been conducted to rationalize the catalytic pathways and regio- and enantioselectivity control.

Photocatalytic Decarboxylative Allylation of α-Amino Acids and Peptides under Metal-Free Conditions

We developed an organophotoredox catalytic system to facilitate the decarboxylative allylation coupling process concerning α-amino acids and related C-terminal carboxylate peptides using Morita-Baylis-Hillman adducts as allylic precursors. This metal-free method operates under mild conditions and is compatible with various amino acids. The versatility of this protocol, particularly in chemical biology research, has been preliminarily demonstrated through the ligation of bioactive peptide chains.

Synthesis of Homoallylamines Enabled by Cobalt or Palladium Catalyzed Allylic Substitution of Azaarylmethylamines

Pd(OAc)2/Nixantphos or CoI2/Nixantphos catalyzed allylic substitutions with weakly acidic C(sp)3-H bonds of azaarylmethylamines are described. This method facilitates access to various kinds of heteroaryl rings containing homoallylamines (39 examples, 30-98% yields) with excellent functional group tolerance and diastereoselectivity. Compared with the Pd/Nixantphos complex, the Co/Nixantphos catalysis could obtain the cyclic products with good to excellent diastereoselectivities. Importantly, the CoI2/(R,R)-Me-Duphos catalyzed reactions exhibit moderate enantioselectivity. Additionally, the scalability of this transformation is successfully demonstrated.

The Catalytic Asymmetric Allylic Alkylation of Acyclic Enolates for the Construction of Quaternary and Tetrasubstituted Stereogenic Centres

To facilitate the discovery and development of new pharmaceuticals, the demand for novel stereofunctionalised building blocks has never been greater. Whilst molecules bearing quaternary and tetrasubstituted stereogenic centres are ideally suited to explore untapped areas of chemical space, the asymmetric construction ofsterically congested carbon centres remains a longstanding challenge in organic synthesis. The enantioselective assembly of acyclic stereogenic centres is even more demanding due to the need to restrict a much wider range of geometries and conformations of the intermediates involved. In this context, the catalytic asymmetric allylicalkylation (AAA) of acyclic prochiral nucleophiles, namely enolates, has become an indispensable tool to access a range of linearα-quaternary andα-tetrasubstituted carbonyl compounds. However, unlike the AAA of cyclic enolates with a fixed enolate geometry, to achieve high levels of stereocontrol in the AAA of acyclic enolates, the stereoselectivity of enolisation must be considered. The aim of this review is to offer acomprehensivediscussion of catalytic AAA reactions of acyclic prochiral enolates and their analogues to generate congested quaternary and tetrasubstituted chiral centres using metal, non-metal and dual catalysis, with particular focus given to the control of enolate geometry and its impact on the stereochemical outcome of the reaction.

Regio- and Stereoselective Iodoamination of Ferrocene-Containing Allenylphosphonates: Synthesis of Multifunctional Tetrasubstituted Allylic Amines and Allylic Azides

A general and practical methodology for the regio- and stereoselective synthesis of multifunctional tetrasubstituted allylic amines and azides based on iodoamination of ferrocene-containing allenylphosphonates with anilines and sodium azide is described. A tetrasubstituted olefin moiety, as well as an iodine atom, a phosphonate, and a ferrocene group, are installed to the allylic amine motif simultaneously in moderate to good yields.

Asymmetric α-C(sp3)-H allylic alkylation of primary alkylamines by synergistic Ir/ketone catalysis

Primary alkyl amines are highly reactive in N-nucleophilic reactions with electrophiles. However, their α-C-H bonds are unreactive towards electrophiles due to their extremely low acidity (pKa ~57). Nonetheless, 1,8-diazafluoren-9-one (DFO) can activate primary alkyl amines by increasing the acidity of the α-amino C-H bonds by up to 1044 times. This makes the α-amino C-H bonds acidic enough to be deprotonated under mild conditions. By combining DFO with an iridium catalyst, direct asymmetric α-C-H alkylation of NH2-unprotected primary alkyl amines with allylic carbonates has been achieved. This reaction produces a wide range of chiral homoallylic amines with high enantiopurities. The approach has successfully switched the reactivity between primary alkyl amines and allylic carbonates from intrinsic allylic amination to the α-C-H alkylation, enabling the construction of pharmaceutically significant chiral homoallylic amines from readily available primary alkyl amines in a single step.

Recent advances in the synthesis of chiral α-tertiary amines via transition-metal catalysis

The significance of chiral α-tertiary amines in medicinal chemistry and drug development has been unquestionably established in the last few decades. α-Tertiary amines are attractive structural motifs for natural products, bioactive molecules and pharmaceuticals and are preclinical candidates. Their syntheses have been the focus of intensive research, and the development of new methods has continued to attract more and more attention. In this review, we present the progress in the last decade in the development of synthetic methods for the assembly of chiral ATAs via transition-metal catalysis. To date, the effective approaches in this area could be categorized into three strategies: enantioselective direct and indirect Mannich addition to ketimines; umpolung asymmetric alkylation of imine derivatives; and asymmetric C-N cross-coupling of tertiary alkyl electrophiles. Several related developing strategies for the synthesis of ATAs, such as hydroamination of alkenes, HAT amination approaches and the C-C coupling of α-aminoalkyl fragments, are also described in this article. These strategies have emerged as attractive C-C and C-N bond-forming protocols for enantioselective construction of chiral α-tertiary amines, and to some extent are complementary to each other, showing the prospect of application in medicinal chemistry and chemical biology.

Vicinal stereocenters via asymmetric allylic alkylation and Cope rearrangement: a straightforward route to functionally and stereochemically rich heterocycles

An asymmetric allylic alkylation/Cope rearrangement (AAA/[3,3]) capable of stereoselectively constructing vicinal stereocenters has been developed. Strategically integrated 4-methylation on the 3,3-dicyano-1,5-diene controls stereoselectivity and drives Cope rearrangement equilibrium in the forward direction. The AAA/[3,3] sequence rapidly converts abundant achiral and racemic starting materials into valuable (hetero)cycloalkane building blocks bearing significant functional and stereochemical complexity, highlighting the value of (hetero)cyclohexylidenemalononitriles as launching points for complex heterocycle synthesis. On this line, the resulting alkylidenemalononitrile moiety can be readily converted into amides via Hayashi-Lear amidation to ultimately yield amido-piperidines, tropanes, and related scaffolds with 3-5 stereocenters and drug-like functionality.

Recent advances in metal-catalysed asymmetric sigmatropic rearrangements

Asymmetric sigmatropic rearrangement is a powerful organic transformation via substrate-reorganization to efficiently increase molecular complexity from readily accessible starting materials. In particular, a high level of diastereo- and enantioselectivity can be readily accessed through well-defined and predictable transition states in [3,3], [2,3]-sigmatropic rearrangements, which have been widely applied in the synthesis of various chiral building blocks, natural products, and pharmaceuticals. In recent years, catalytic asymmetric sigmatropic rearrangements involving chiral metal complexes to induce stereocontrol have been intensively studied. This review presents an overview of metal-catalysed enantioselective versions of sigmatropic rearrangements in the past two decades, mainly focusing on [3,3], [2,3], and [1,3]-rearrangements, to show the development of substrate design, new catalyst exploitation, and novel cascade processes. In addition, their application in the asymmetric synthesis of complex natural products is also exemplified.

Preparation of Chiral γ-Secondary Amino Alcohols via Ni-Catalyzed Asymmetric Reductive Coupling of 2-Aza-butadiene with Aldehydes

The first Ni-catalyzed asymmetric reductive coupling of 2-aza-butadiene with aldehydes was achieved to synthesize chiral γ-secondary amino alcohols. This transformation features good enantioselectivity and tolerance to various functional groups, which may serve as a complementary method to previously reported noble-metal-catalyzed protocols. Through competition reaction, 2-aza-butadiene was proved to be a more reactive coupling component than its full-carbon analogue, 1,3-butadiene. Notably, this reaction delivers β-siloxyl imine, an aza-aldol-type product which is difficult to access by conventional methods.

Copper-Catalyzed Umpolung of N-2,2,2-Trifluoroethylisatin Ketimines for the Enantioselective 1,3-Dipolar Cycloaddition with Benzo[b]thiophene Sulfones

A copper-catalyzed umpolung of N-2,2,2-trifluoroethylisatin ketimines for the enantioselective 1,3-dipolar cycloaddition with benzo[b]thiophene sulfones was developed. Using a catalyst system consisting of an (S,S_p)-^tBu-Phosferrox ligand, Cu(OTf)₂, and Cs₂CO₃, a range of pentacyclic spirooxindoles containing pyrrolidine and benzo[b]sulfolane subunits were obtained in high efficiency with excellent regio-, diastereo-, and enantioselectivites under mild conditions. The practicality and versatility of the reaction were also demonstrated.

Recent Advances in Catalytic [3,3]-Sigmatropic Rearrangements

Carbon–carbon bond formation by [3,3]-sigmatropic rearrangement is a fundamental and powerful method that has been used to build organic molecules for a long time. Initially, Claisen and Cope rearrangements proceeded at high temperatures with limited scopes. By introducing catalytic systems, highly functionalized substrates have become accessible for forming complex structures under mild conditions, and asymmetric synthesis can be achieved by using chiral catalytic systems. This review describes recent breakthroughs in catalytic [3,3]-sigmatropic rearrangements since 2016. Detailed reaction mechanisms are discussed to enable an understanding of the reactivity and selectivity of the reactions. Finally, this review is inspires the development of new cascade reaction pathways employing catalytic [3,3]-sigmatropic rearrangement as related methodologies for the synthesis of complex functional molecules.

Recent advances in catalytic asymmetric aza-Cope rearrangement

Aza-Cope rearrangement, as one of the fundamental reactions for C-C and C-N bond formation, has been extensively utilized for the rapid construction of synthetically challenging organic molecules. Despite significant achievements having been made in the past 80 years, catalytic enantioselective versions still remain a challenge, mainly due to the inherent nature of the reversibility of aza-Cope rearrangement. Recently, owing to the intensive development of asymmetric catalysis strategies, various chiral organocatalysts and transition-metal catalysts have been successfully applied to control the stereoselectivity of aza-Cope rearrangement, and remarkable advances have been achieved. This review highlights recent progress relating to catalytic asymmetric aza-Cope rearrangement and covers important features of these studies, including catalytic system design, mechanistic insights, stereochemistry analysis, and synthetic applications.

Transition-metal-free allylation of 2-azaallyls with allyl ethers through polar and radical mechanisms

Allylation of nucleophiles with highly reactive electrophiles like allyl halides can be conducted without metal catalysts. Less reactive electrophiles, such as allyl esters and carbonates, usually require a transition metal catalyst to facilitate the allylation. Herein, we report a unique transition-metal-free allylation strategy with allyl ether electrophiles. Reaction of a host of allyl ethers with 2-azaallyl anions delivers valuable homoallylic amine derivatives (up to 92%), which are significant in the pharmaceutical industry. Interestingly, no deprotonative isomerization or cyclization of the products were observed. The potential synthetic utility and ease of operation is demonstrated by a gram scale telescoped preparation of a homoallylic amine. In addition, mechanistic studies provide insight into these C(sp3)-C(sp3) bond-forming reactions.

Iodide-Mediated [3 + 2]-Cycloaddition Reaction with N-Tosylaziridines and α,β-Unsaturated Ketones

The [3 + 2]-cycloaddition reaction between N-tosylaziridines and α,β-unsaturated ketones was promoted with lithium iodide. The reaction proceeded under mild conditions to provide N-tosylpyrrolidines. Quaternary carbon-possessing 3,3-disubstituted pyrrolidines including spiro compounds were afforded in high yields. A simple procedure with easy to handle reagents makes this reaction concise. The intramolecular version of this reaction was applied to synthesize tropane skeletons.

Asymmetric Catalytic Ketimine Mannich Reactions and Related Transformations

The catalytic enantioselective ketimine Mannich and its related reactions provide direct access to chiral building blocks bearing an α-tertiary amine stereogenic center, a ubiquitous structural motif in nature. Although ketimines are often viewed as challenging electrophiles, various approaches/strategies to circumvent or overcome the adverse properties of ketimines have been developed for these transformations. This review showcases the selected examples that highlight the benefits and utilities of various ketimines and remaining challenges associated with them in the context of Mannich, allylation, and aza-Morita-Baylis-Hillman reactions as well as their variants.

Cobalt-Catalyzed Umpolung Alkylation of Imines To Generate α-Branched Aliphatic Amines

Here we report a general and mild approach to prepare α-branched aliphatic amines from imines. This method capitalizes on a cobalt-catalyzed umpolung alkylation of imines, employs easily available reaction partners, and demonstrates a broad substrate scope. Mechanistic studies suggest this transformation occurs by a radical pathway.

Catalytic asymmetric umpolung reactions of imines via 2-azaallyl anion intermediates

The imine umpolung is a relatively new and interesting strategy, especially in catalytic asymmetric synthesis. A significant development in organo- and transition metal-catalyzed umpolung of imines took place only in the recently concluded decade. A majority of the reports on the asymmetric umpolung of imines involve the initial generation of 2-azaallyl anion intermediates with the chiral catalysts, which serve as a significant driving force for the umpolung addition/substitution reactions. A variety of organocatalysts such as bifunctional cinchona alkaloids including squaramides and thioureas, chiral BINOL derived phosphoric acids, phase transfer catalysts (PTCs), phosphines, and transition metal-complexes of iridium, copper and palladium have been employed to achieve the excellent level of asymmetric induction in such types of umpolung reactions. The asymmetric imine umpolung strategy has been applied successfully to synthesize synthetic amino-acid derivatives and other useful chiral amines, including drugs and potentially bioactive molecules. This review summarizes all the significant recent development in catalytic umpolung reactions of imines involving a 2-azaallyl anion intermediate.

Enantioselective, Catalytic Multicomponent Synthesis of Homoallylic Amines Enabled by Hydrogen-Bonding and Dispersive Interactions

We report a one-step catalytic, enantioselective method for the preparation of homoallylic N-Boc amines directly from acetals. Reactive iminium ion intermediates are generated in situ through the combination of an acetal, a chiral thiourea catalyst, trialkylsilyl triflate, and N-Boc carbamate and are subsequently trapped by a variety of allylsilane nucleophiles. No homoallylic ether byproducts are detected, consistent with allylation of the iminium intermediate being highly favored over allylation of the intermediate oxocarbenium ion. Acetals derived from aromatic aldehydes possessing a variety of functional groups and substitution patterns yield homoallylic amines with excellent levels of enantiomeric enrichment. Experimental and computational data are consistent with an anchoring hydrogen-bond interaction between the protioiminium ion and the amide of the catalyst in the enantiodetermining transition state, and with stereodifferentiation achieved through specific noncovalent interactions (NCIs) with the catalyst pyrenyl moiety. Evidence is provided that the key NCI in the major pathway is a π-stacking interaction, contrasting with the cation-π interactions invoked in previously studied reactions promoted by the same family of aryl-pyrrolidino-H-bond-donor catalysts.

Iridium-Catalyzed Asymmetric Allylic Alkylation of Deconjugated Butyrolactams

Compared with the ever-growing list of nonprochiral nucleophiles in Ir-catalyzed asymmetric allylic substitution reactions, prochiral nucleophiles are less studied. We present a new prochiral nucleophile, namely, deconjugated butyrolactam, for Ir-catalyzed asymmetric allylic alkylation (AAA). This reaction provides access to α-allyl deconjugated butyrolactams with a moderate to good dr and an excellent er. This is the first AAA reaction of deconjugated butyrolactams. Allyl transposition through Cope rearrangement appears to proceed stereospecifically to form γ-allyl conjugated butyrolactams.

Construction of fully substituted carbon centers containing a heteroatom and a CF3 group via in situ generated p-quinone methides

1,6-Conjugate additions of in situ generated δ-CF3-δ-substituted p-quinone methides have been achieved with a variety of heteronucleophiles under mild conditions, which led to facile and practical construction of fully substituted carbon centers including a heteroatom and a CF3 group. In particular, it was revealed that some amines themselves worked for efficient cleavage of the TBS protective group, and addition of a catalytic amount of an appropriate Brønsted acid was found to sometimes improve the progress of the desired process.

Nickel-catalyzed enantioselective vinylation of aryl 2-azaallyl anions

A unique enantioselective nickel-catalyzed vinylation of 2-azaallyl anions is advanced for the first time. This method affords diverse vinyl aryl methyl amines with high enantioselectivities, which are frequently occurring scaffolds in natural products and medications. This C-H functionalization method can also be extended to the synthesis of enantioenriched 1,3-diamine derivatives by employing suitably elaborated vinyl bromides. Key to the success of this process is the identification of a Ni/chiraphos catalyst system and a less reducing 2-azaallyl anion, all of which favor an anionic vinylation route over a background radical reaction. A telescoped gram scale synthesis and a product derivatization study confirmed the scalability and synthetic potential of this method.

Strategies for the Catalytic Enantioselective Synthesis of α-Trifluoromethyl Amines

The exploitation of the α-trifluoromethylamino group as an amide surrogate in peptidomimetics and drug candidates has been on the rise. In a large number of these cases, this moiety bears stereochemistry with the stereochemical identity having important consequences on numerous molecular properties, such as the potency of the compound. Yet, the majority of stereoselective syntheses of α-CF3 amines rely on diastereoselective couplings with chiral reagents. Concurrent with the rapid expansion of fluorine into pharmaceuticals has been the development of catalytic enantioselective means of preparing α-trifluoromethyl amines. In this work, we outline the strategies that have been employed for accessing these enantioenriched amines, including normal polarity approaches and several recent developments in imine umpolung transformations.

Catalytic asymmetric synthesis of quaternary trifluoromethyl α- to ε-amino acid derivatives via umpolung allylation/2-aza-Cope rearrangement

In this study, we developed an efficient Ir-catalyzed cascade umpolung allylation/2-aza-Cope rearrangement of tertiary α-trifluoromethyl α-amino acid derivatives for the preparation of a variety of quaternary α-trifluoromethyl α-amino acids in high yields with excellent enantioselectivities. The umpolung reactivity empowered by the activation of the key isatin-ketoimine moiety obviates the intractable enantioselectivity control in Pd-catalyzed asymmetric linear α-allylation. In combination with quasi parallel kinetic resolution or kinetic resolution, the generality of this method is further demonstrated by the first preparation of enantioenriched quaternary trifluoromethyl β-, γ-, δ- and ε-amino acid derivatives.

Sulfonate N-Heterocyclic Carbene-Copper Complexes: Uniquely Effective Catalysts for Enantioselective Synthesis of C-C, C-B, C-H, and C-Si Bonds

A copper-based complex that contains a sulfonate N-heterocyclic carbene ligand was first reported 15 years ago. Since then, these organometallic entities have proven to be uniquely effective in catalyzing an assortment of enantioselective transformations, including allylic substitutions, conjugate additions, proto-boryl additions to alkenes, boryl and silyl substitutions, hydride-allyl additions to alkenyl boronates, and additions of boron-containing allyl moieties to N-H ketimines. In this review article, we detail the shortcomings in the state-of-the-art that fueled the development of this air stable ligand class, members of which can be prepared on multigram scale. For each reaction type, when relevant, the prior art at the time of the advance involving sulfonate NHC-Cu catalysts and/or subsequent key developments are briefly analyzed, and the relevance of the advance to efficient and enantioselective total or formal synthesis of biologically active molecules is underscored. Mechanistic analysis of the structural attributes of sulfonate NHC-Cu catalysts that are responsible for their ability to facilitate transformations with high efficiency as well as regio- and enantioselectivity are detailed. This review contains several formerly undisclosed methodological advances and mechanistic analyses, the latter of which constitute a revision of previously reported proposals.

Brønsted Acid-Catalyzed General Petasis Allylation and Isoprenylation of Unactivated Ketones

Brønsted acid-catalyzed general Petasis allylation and isoprenylation of unactivated ketones were developed by using o-hydroxyaniline and the corresponding pinacolyl boronic esters. This robust methodology provided access to a broad variety of quaternary homoallylic amines and dienyl amines in high yields, proved to be applicable to a gram-scale synthesis, and allowed the synthesis of a potentially bioactive quaternary homoallylic aminodiol.

Regio- and Enantioselective Palladium-Catalyzed Asymmetric Allylation of N-Fluorenyl Trifluoromethyl Imine

A palladium-catalyzed asymmetric allylation of N-fluorenyl trifluoromethyl imine with allylic acetates is disclosed. This method provides scalable and efficient access to polysubstituted chiral α-trifluoromethyl amines bearing two adjacent stereocenters and one allyl group in high yields with excellent regio-, diastereo-, and enantioselectivity. Importantly, this method also provides a powerful strategy for the synthesis of both regioisomeric products and the regioselectivity is controlled by the chiral catalysts and optically active substrates.

Regio- and Enantioselective Synthesis of Trifluoromethyl-Substituted Homoallylic α-Tertiary NH2 -Amines by Reactions Facilitated by a Threonine-Based Boron-Containing Catalyst

A method for catalytic regio- and enantioselective synthesis of trifluoromethyl-substituted and aryl-, heteroaryl-, alkenyl-, and alkynyl-substituted homoallylic α-tertiary NH₂ -amines is introduced. Easy-to-synthesize and robust N-silyl ketimines are converted to NH-ketimines in situ, which then react with a Z-allyl boronate. Transformations are promoted by a readily accessible l-threonine-derived aminophenol-based boryl catalyst, affording the desired products in up to 91 % yield, >98:2 α:γ selectivity, >98:2 Z:E selectivity, and >99:1 enantiomeric ratio. A commercially available aminophenol may be used, and allyl boronates, which may contain an alkyl-, a chloro-, or a bromo-substituted Z-alkene, can either be purchased or prepared by catalytic stereoretentive cross-metathesis. What is more, Z-trisubstituted allyl boronates may be used. Various chemo-, regio-, and diastereoselective transformations of the α-tertiary homoallylic NH₂ -amine products highlight the utility of the approach; this includes diastereo- and regioselective epoxide formation/trichloroacetic acid cleavage to generate differentiated diol derivatives.

Enantio- and Diastereoselective Synthesis of Homoallylic α-Trifluoromethyl Amines by Catalytic Hydroalkylation of Dienes

We describe a strategy for the enantio- and diastereoselective synthesis of homoallylic α-trifluoromethyl amines by the catalytic hydroalkylation of terminal dienes. Trifluoromethyl-substituted isatin-derived azadienolate nucleophiles undergo γ-selective alkylation with a Pd-DTBM-SEGPHOS catalyst, which additionally promotes regioselective addition to the diene and delivers products in up to 86% yield, 10:1 dr, and 97.5:2.5 er.

Asymmetric synthesis of quaternary α-trifluoromethyl α-amino acids by Ir-catalyzed allylation followed by kinetic resolution

Facile access to quaternary α-trifluoromethyl α-amino acids has been developed. This sequential reaction involves an Ir-catalyzed asymmetric allylation of α-trifluoromethyl aldimine esters followed by an unprecedented kinetic resolution.

Pd-Catalyzed Allylation of Imines to Access α-CF3-Substituted α-Amino Acid Derivatives

We herein report a high yielding protocol for the direct α-allylation of easily accessible trifluoropyruvate-derived imines using Pd-catalysis. The reaction gives access to a variety of different α-allylated-α-CF3-amino acids in a straightforward manner, starting from commercially available trifluoropyruvate. We also provide a proof-of-concept for an enantioselective protocol (up to er = 75:25) by using chiral phosphane ligands.