Nickel-Catalyzed Reductive Alkylation of Heteroaryl Imines

A General Enantioselective α-Alkyl Amino Acid Derivatives Synthesis Enabled by Cobalt-Catalyzed Reductive Addition

Enantioenriched unnatural amino acids represent a prevalent motif in organic chemistry, with profound applications in biochemistry, medicinal chemistry, and materials science. Herein, we report a cobalt-catalyzed aza-Barbier reaction of dehydroglycines with unactivated alkyl halides to afford unnatural α-amino esters with high enantioselectivity. This catalytic reductive alkylative addition protocol circumvents the use of moisture-, air-sensitive organometallic reagents, and stoichiometric chiral auxiliaries, enabling the conversion of a variety of primary, secondary, and even tertiary unactivated alkyl halides to α-alkyl-amino esters under mild conditions, thus leading to broad functional group tolerance. The expedient access to biologically active motifs demonstrates the practicality of this protocol by reducing the number of synthetic steps and enhancing the reaction efficiency.

Cobalt-Catalyzed Enantioselective Reductive Arylation, Heteroarylation, and Alkenylation of Michael Acceptors via an Elementary Mechanism of 1,4-Addition

Cobalt complexes with chiral quinox ligands effectively promote the enantioselective conjugate addition of enones using aryl, heteroaryl, and alkenyl halides and sulfonates. Additionally, a cobalt complex with a strongly donating diphosphine, BenzP*, successfully catalyzes the asymmetric reductive arylation and alkenylation of α,β-unsaturated amides. Both catalytic systems show broad scopes and tolerance of sensitive functional groups. Both reactions can be scaled up with low loadings of cobalt catalysts. Experimental results and density functional theory (DFT) calculations suggest a new mechanism of elementary 1,4-addition of aryl cobalt(I) complexes.

Cobalt-Catalyzed Enantioselective Reductive α-Chloro-Carbonyl Addition of Ketimine to Construct the β-Tertiary Amino Acid Analogues

β-Tertiary amino acid derivatives constitute one of the most frequently occurring units in natural products and bioactive molecules. However, the efficient asymmetric synthesis of this motif still remains a significant challenge. Herein, we disclose a cobalt-catalyzed enantioselective reductive addition reaction of ketimine using α-chloro carbonyl compound as a radical precursor, providing expedient access to a diverse array of enantioenriched β-quaternary amino acid analogues. This protocol exhibits outstanding enantioselectivity and broad substrate scope with excellent functional group tolerance. Preliminary mechanism studies rule out the possibility of Reformatsky-type addition and confirm the involvement of radical species in stereoselective addition process. The synthetic utility has been demonstrated through the rapid assembly of iterative amino acid units and oligopeptide, showcasing its versatile platform for late-stage modification of drug candidates.

Multicomponent Synthesis of α-Branched Amines via a Zinc-Mediated Carbonyl Alkylative Amination Reaction

Methods for the synthesis of α-branched alkylamines are important due to their ubiquity in biologically active molecules. Despite the development of many methods for amine preparation, C(sp3)-rich nitrogen-containing compounds continue to pose challenges for synthesis. While carbonyl reductive amination (CRA) between ketones and alkylamines is the cornerstone method for α-branched alkylamine synthesis, it is sometimes limited by the sterically demanding condensation step between dialkyl ketones and amines and the more restricted availability of ketones compared to aldehydes. We recently reported a "higher-order" variant of this transformation, carbonyl alkylative amination (CAA), which utilized a halogen atom transfer (XAT)-mediated radical mechanism, enabling the streamlined synthesis of complex α-branched alkylamines. Despite the efficacy of this visible-light-driven approach, it displayed scalability issues, and competitive reductive amination was a problem for certain substrate classes, limiting applicability. Here, we report a change in the reaction regime that expands the CAA platform through the realization of an extremely broad zinc-mediated CAA reaction. This new strategy enabled elimination of competitive CRA, simplified purification, and improved reaction scope. Furthermore, this new reaction harnessed carboxylic acid derivatives as alkyl donors and facilitated the synthesis of α-trialkyl tertiary amines, which cannot be accessed via CRA. This Zn-mediated CAA reaction can be carried out at a variety of scales, from a 10 μmol setup in microtiter plates enabling high-throughput experimentation, to the gram-scale synthesis of medicinally-relevant compounds. We believe that this transformation enables robust, efficient, and economical access to α-branched alkylamines and provides a viable alternative to the current benchmark methods.

Ni-catalyzed enantioconvergent deoxygenative reductive cross-coupling of unactivated alkyl alcohols and aryl bromides

Transition metal-catalyzed enantioconvergent cross-coupling of an alkyl precursor presents a promising method for producing enantioenriched C(sp3) molecules. Because alkyl alcohol is a ubiquitous and abundant family of feedstock in nature, the direct reductive coupling of alkyl alcohol and aryl halide enables efficient access to valuable compounds. Although several strategies have been developed to overcome the high bond dissociation energy of the C - O bond, the asymmetric pattern remains unknown. In this report, we describe the realization of an enantioconvergent deoxygenative reductive cross-coupling of unactivated alkyl alcohol (β-hydroxy ketone) and aryl bromide in the presence of an NHC activating agent. The approach can accommodate substituents of various sizes and functional groups, and its synthetic potency is demonstrated through a gram scale reaction and derivatizations into other compound families. Finally, we apply our convergent method to the efficient asymmetric synthesis of four β-aryl ketones that are natural products or bioactive compounds.

Modular α-tertiary amino ester synthesis through cobalt-catalysed asymmetric aza-Barbier reaction

Unnatural chiral α-tertiary amino acids containing two different carbon-based substituents at the α-carbon centre are widespread in biologically active molecules. This sterically rigid scaffold is becoming a growing research interest in drug discovery. However, a robust protocol for chiral α-tertiary amino acid synthesis remains scarce due to the challenge of stereoselectively constructing sterically encumbered tetrasubstituted stereogenic carbon centres. Herein we report a cobalt-catalysed enantioselective aza-Barbier reaction of ketimines with various unactivated alkyl halides, including alkyl iodides, alkyl bromides and alkyl chlorides, enabling the formation of chiral α-tertiary amino esters with a high level of enantioselectivity and excellent functional group tolerance. Primary, secondary and tertiary organoelectrophiles are all tolerated in this asymmetric reductive addition protocol, which provides a complementary method for the well-exploited enantioselective nucleophilic addition with moisture- and air-sensitive organometallic reagents. Moreover, the three-component transformation of α-ketoester, amine and alkyl halide represents a formal asymmetric deoxygenative alkylamination of the carbonyl group.

Cobalt-Catalyzed Asymmetric Aza-Nozaki-Hiyama-Kishi (NHK) Reaction of α-Imino Esters with Alkenyl Halides

Chromium-catalyzed enantioselective Nozaki-Hiyama-Kishi (NHK) reaction represents one of the most powerful approaches for the formation of chiral carbon-heteroatom bond. However, the construction of sterically encumbered tetrasubstituted stereocenter through NHK reaction still posts a significant challenge. Herein, we disclose a cobalt-catalyzed aza-NHK reaction of ketimine with alkenyl halide to provide a convenient synthetic approach for the manufacture of enantioenriched tetrasubstituted α-vinylic amino acid. This protocol exhibits excellent functional group tolerance with excellent 99 % ee in most cases. Additionally, this asymmetric reductive method is also applicable to the aldimine to access the trisubstituted stereogenic centers.

Selective and Functional-Group-Tolerant Photoalkylation of Imines by Energy-Transfer Photocatalysis

Basic amines show broad bioactivity and remain a promising source of new medicines. The direct photoalkylation of imines offers a promising strategy for complex amines. However, the lack of efficient imine photoreactivity hinders this reaction and remains a fundamental limitation in organic photochemistry. We report an efficient photoalkylation of imines that provides primary amines directly without protecting or leaving groups. The transformation effects C-H addition across N-H imines under energy-transfer photocatalysis by a ketone. Our method is distinguished from organometallic, metal-catalyzed, and photoredox approaches to imine alkylation by its lack of protecting groups and its broad scope, which includes unactivated alkanes, protic substrates, basic amines, heterocycles, and ketone imines. We highlight this scope through the condensation and alkylation of two pharmaceutical ketones, providing complex amines succinctly. Our mechanistic analysis supports a three-step process, involving hydrogen-atom transfer to an imine triplet excited state, intersystem crossing, and radical recombination, with photocatalytic enhancement through energy transfer. We further show that N-H imines are more photoreactive than N-substituted imines, a distinction partially explained by sterics and side reactions. To fully explain this distinction, we introduce the thermodynamic parameter excited-state hydrogen-atom affinity, which is highly effective at predicting the photoreactivity of imines.

Deep Electroreductive Chemistry: Harnessing Carbon- and Silicon-based Reactive Intermediates in Organic Synthesis

This Viewpoint outlines our recent contribution in electroreductive synthesis. Specifically, we leveraged deeply reducing potentials provided by electrochemistry to generate radical and anionic intermediates from readily available alkyl halides and chlorosilanes. Harnessing the distinct reactivities of radicals and anions, we have achieved several challenging transformations to construct C-C, C-Si, and Si-Si bonds. We highlight the mechanistic design principle that underpinned the development of each transformation and provide a view forward on future opportunities in growing area of reductive electrosynthesis.

Nickel-Catalyzed Enantioselective Reductive Arylation and Heteroarylation of Aldimines via an Elementary 1,4-Addition

Nickel catalysts of chiral pyrox ligands promoted enantioselective reductive arylation and heteroarylation of aldimines, using directly (hetero)aryl halides and sulfonates. The catalytic arylation can also be conducted with crude aldimines generated from condensation of aldehydes and azaaryl amines. Mechanistically, density functional theory (DFT) calculations and experiments pointed to an elementary step of 1,4-addition of aryl nickel(I) complexes to N-azaaryl aldimines.

Dual-Catalytic Enantioselective Allylation of N-(Heteroaromatic-methyl)imine Derivatives

We report the dual-catalytic enantioselective allylic alkylation of 2-(pyridylmethyl)amine-derived ketimines with allylic carbonates. The reaction proceeds under mild reaction conditions to generate α-amino heteroaryl benzylamine stereocenters in good yield and enantioselectivity. Enantioselectivity is achieved through the use of a copper catalyst modified with chiral bisphosphine ligand (2S,4S)-bis(diphenylphosphino)pentane.