Catalytic Asymmetric Synthesis of Zinc Metallacycles

Cobalt-Catalyzed Enantioselective Reductive Coupling of Imines and Internal Alkynes

Chiral allylamines are important structural components in natural products, pharmaceuticals, and chiral catalysts. Herein, we report a cobalt-catalyzed enantioselective reductive coupling of imines with internal alkynes to synthesize chiral allylamines. The reaction is catalyzed by a cobalt complex derived from commercially available bisphosphine ligand utilizing zinc as the electron donor. The substrate scope is extensive. Symmetric and unsymmetric alkyl and aryl alkynes have been successfully coupled with various imines derived from aryl and alkyl aldehydes. Tri- and tetra-substituted allyl amines were isolated in high yields, with enantiomeric excess surpassing >99.9 % and regioselectivities exceeding >20 : 1. These chiral allyl amines can serve as versatile platforms for subsequent transformations while preserving their stereochemical integrity. Extensive experimental and computational mechanistic studies were performed to elucidate the mechanism. These investigations have indicated that an in situ cobalt(I) catalyst enables the oxidative cyclization of alkynes and imines, and a spin crossover occurs during the enantio-determining step. Zinc plays a pivotal role in facilitating the transmetallation of the resulting azacobaltacycle. The observed enantioselectivity was interpreted by the stabilization of the transition state through higher stabilizing interaction energy from high negative polarization, dispersion, and C-H⋅⋅⋅π interactions.

Bis(diiminate)-Supported Bimetallic Complexes: Tri-Coordinated Zinc for Nitrile and Carbodiimide Hydroboration

We report the synthesis and characterization of bis(diiminate)-supported tricoordinated zinc complexes (1-4) and demonstrate the catalytic activity of one representative compound in the hydroboration of nitriles and carbodiimides using pinacolborane (HBpin). Experimental and theoretical studies were performed to elucidate the reaction mechanism. Our findings indicate that the hydroboration reaction initiates with the formation of a tricoordinated zinc hydride intermediate, followed by the subsequent attack of nitriles and carbodiimides. This leads to the formation of a four-membered metallacycle before the release of the diborylated amine. This work provides access to new types of zinc complexes and highlights its effectiveness in the hydroboration of nitriles and carbodiimides, offering a milder alternative to existing reduction methods.

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.

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.

Enhanced Co-Adsorption of Alcohols and Amines for Visible Light Driven Oxidative Condensation Using Iron-Based MOF

Imines are essential intermediates in organic transformations, and is generally produced by dehydrogenative condensation of alcohols and amines with the assist of specialized catalysts and additives. Heterogeneous photocatalysis provides a sustainable platform for such process without the using of toxic oxidants, yet a functionalized photocatalyst with optimized co-adsorption of reactants needs to be developed to promote the stoichiometric oxidative condensation under ambient conditions. Here, we show that benzyl alcohol and aniline adsorb non-interferingly on the Fe node and the linker sites of the MIL-53(Fe) metal organic frameworks (MOFs), respectively. The co-adsorption of both reactants barely influences the reduction of molecular oxygen to generate oxygen radicals, resulting in efficient formation of benzaldehyde under visible light. Additionally, the weak adsorption of water together with surface acidity of the MIL-53(Fe) promote a rapid condensation of benzaldehyde with aniline and the depletion of generated water, achieving an efficient C-N bond creation for a wide range of substrates.

Nickel-Catalyzed Chemoselective Carbomagnesiation for Atroposelective Ring-Opening Difunctionalization

There is a pressing need for methods that can connect enantioenriched organic compounds with readily accessible building blocks via asymmetric functionalization of unreactive chemical bonds in organic synthesis and medicinal chemistry. Herein, the asymmetric chemoselective cleavage of two unactivated C(Ar)-O bonds in the same molecule is disclosed for the first time through an unusual nickel-catalyzed carbomagnesiation. This reaction facilitates the evolution of a novel atroposelective ring-opening difunctionalization. Utilizing readily available dibenzo bicyclic substrates, diverse valuable axially chiral biaryls are furnished with high efficiencies. Synthetic elaborations showcase the application potential of this method. The features of this method include good atom-economy, multiple roles of the nucleophile, and a simple catalytic system that enables the precise magnesiation of an α-C(Ar)-O bond and arylation of a β-C(Ar)-O bond.

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.

Reductive anti-Dizincation of Arylacetylenes

Arylacetylenes undergo anti-1,2-dizincation to afford trans-1,2-dizincioalkenes. The process employs sodium dispersion as a reducing agent and zinc chloride TMEDA complex as a reduction-resistant zinc electrophile. This reductive anti-dizincation contrasts with the conventional additive syn-dimetalation like silylzincation. The resulting dizincated alkenes undergo the cross-coupling to yield multi-substituted alkenes stereoselectively.

Pathway-divergent coupling of 1,3-enynes with acrylates through cascade cobalt catalysis

Catalytic cascade transformations of simple starting materials into highly functionalized molecules bearing a stereochemically defined multisubstituted alkene, which are important in medicinal chemistry, natural product synthesis, and material science, are in high demand for organic synthesis. The development of multiple reaction pathways accurately controlled by catalysts derived from different ligands is a critical goal in the field of catalysis. Here we report a cobalt-catalyzed strategy for the direct coupling of inexpensive 1,3-enynes with two molecules of acrylates to construct a high diversity of functionalized 1,3-dienes containing a trisubstituted or tetrasubstituted olefin. Such cascade reactions can proceed through three different pathways initiated by oxidative cyclization to achieve multiple bond formation in high chemo-, regio- and stereoselectivity precisely controlled by ligands, providing a platform for the development of tandem carbon-carbon bond-forming reactions.

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.