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.

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.

Asymmetric alkene-alkene reductive cross-coupling reaction via visible-light photoredox/cobalt dual catalysis

The first example of asymmetric alkene-alkene reductive coupling is demonstrated via visible-light-fueled photoredox/cobalt dual catalysis. The desymmetrization reaction provided products (>20 examples) with up to five chiral centers in single-step operation in up to 95% yields with very high relative (>99 : 1 dr) and absolute stereochemistry (up to 98 : 2 er) control. The preliminary mechanistic investigations suggested that the critical mechanistic steps involved light-mediated controlled low-valent cobalt complex generation, oxidative ene-ene cyclization, and protonation.

Highly Regio- and Enantioselective Reductive Coupling of Alkynes and Aldehydes via Photoredox Cobalt Dual Catalysis

A Co-catalyzed highly regio- and enantioselective reductive coupling of alkynes and aldehydes has been developed under visible light photoredox dual catalysis. A variety of enantioenriched allylic alcohols have been obtained by using unsymmetrical internal alkynes and commercially available catalyst, chiral ligand, and reagents. It is noteworthy that this approach has considerable advantages, such as excellent regio- (>95:5 for >40 examples), stereo- (up to >95:5 E/Z), and enantioselectivity (92-99% ee, >35 examples) control, mild reaction conditions, broad substrate scope, and good functional group compatibility, making it a great improvement to enantioselective alkyne-aldehyde reductive coupling reactions.

Highly Enantioselective Cobalt-Catalyzed (3+2) Cycloadditions of Alkynylidenecyclopropanes

Low-valent cobalt complexes equipped with chiral ligands can efficiently promote highly enantioselective (3+2) cycloadditions of alkyne-tethered alkylidenecyclopropanes. The annulation allows to assemble bicyclic systems containing five-membered rings in good yields and with excellent enantiomeric ratios. We also present a mechanistic discussion based on experimental and computational data, which support the involvement of Co^I /Co^III catalytic cycles.

Theoretical study of rhodium- and cobalt-catalyzed decarboxylative transformations of isoxazolones: origin of product selectivity

DFT calculations were performed to elucidate the origins of catalyst-controlled product selectivity in the decarboxylative transformations of isoxazolones.

Nickel-Catalyzed Three-Component Cycloadditions of Enoates, Alkynes, and Aldehydes

A method for the three-component cycloaddition of enoates, alkynes, and aldehydes has been developed. Building upon previous work by this group in which stoichiometrically generated metallacycles undergo alkylation, we report a catalytic, alkylative [3 + 2] cycloaddition. From simple starting materials, structurally complex cyclopentenones may be rapidly assembled. Computational investigation of the mechanism (ωB97X-D3/cc-pVTZ//ωB97X/6-31G(d)) identified three energetically feasible pathways. Based on the relative rates of ketene formation compared to isomerization to a seven-membered metallacycle, the most likely mechanism has been determined to occur "ketene-first", with carbocyclization prior to aldol addition. Deuterium labeling studies suggest that formation of the seven-membered metallacycle becomes possible when an α-substituted enoate is used. This observed change in selectivity is due to the increased difficulty of phenoxide elimination with the inclusion of additional steric bulk of the α-substituent. The net transformation results in a [3 + 2] cycloaddition accompanied by an alkylation of the enoate substituent.

Divergent ring-opening coupling between cyclopropanols and alkynes under cobalt catalysis

Cobalt catalysts promote ring-opening coupling between cyclopropanols and unactivated alkynes to give β-alkenyl ketones or cyclopentenols with exquisite chemoselectivity control.

Cobalt–diphosphine catalysts promote ring-opening coupling reactions between cyclopropanols and unactivated internal alkynes, affording either β-alkenyl ketones or multisubstituted cyclopentenol derivatives in good yields with good to excellent regioselectivities. The chemoselectivity between these β-alkenylation and [3 + 2] annulation reactions, which likely share a cobalt homoenolate as a key catalytic intermediate, is exquisitely controlled by the reaction conditions, with the solvent being a major controlling factor. The reactions are proposed to involve ring opening of cobalt cyclopropoxide into homoenolate, migratory insertion of the alkyne into the Co–C bond, and protodemetalation or intramolecular carbonyl addition of the resulting alkenylcobalt species. The feasibility of these reaction steps was supported by DFT calculations.

Diols as Dienophiles: Bridged Carbocycles via Ruthenium(0)-Catalyzed Transfer Hydrogenative Cycloadditions of Cyclohexadiene or Norbornadiene

Highly exo-selective ruthenium(0) catalyzed transfer hydrogenative cycloadditions of 1,2-diols with cyclohexadiene or norbornadiene are described. Novel bridged bicyclic ring systems are accessible from the diol, α-ketol or 1,2-dione oxidation levels with complete control of diastereoselectivity.

Unpredictable cycloisomerization of 1,11-dien-6-ynes by a common cobalt catalyst

1,11-Dien-6-ynes undergo cycloisomerization in the presence of the cobalt catalytic system CoBr₂/phosphine ligand/Zn/ZnI₂ giving cyclohexene, diene or cyclopropane structures depending on the type of the phosphine ligand. This unpredictable behaviour suggests that, although the availability of the cobalt catalytic system is appealing, the development of well-defined catalysts is desirable for further progress.

Enantioselective synthesis of bicyclo[3.n.1]alkanes by chiral phosphoric acid-catalyzed desymmetrizing Michael cyclizations

2,2-Disubstituted cyclic 1,3-diketones containing a tethered electron-deficient alkene undergo chiral phosphoric acid-catalyzed desymmetrizing Michael cyclizations to give bridged bicyclic products in high enantioselectivities. Both bicyclo[3.2.1]octanes and bicyclo[3.3.1]nonanes are accessible using this methodology.

Cobalt catalysis involving π components in organic synthesis

Over the last three decades, transition-metal-catalyzed organic transformations have been shown to be extremely important in organic synthesis. However, most of the successful reactions are associated with noble metals, which are generally toxic, expensive, and less abundant. Therefore, we have focused on catalysis using the abundant first-row transition metals, specifically cobalt. In this Account, we demonstrate the potential of cobalt catalysis in organic synthesis as revealed by our research. We have developed many useful catalytic systems using cobalt complexes. Overall, they can be classified into several broad types of reactions, specifically [2 + 2 + 2] and [2 + 2] cycloadditions; enyne reductive coupling; reductive [3 + 2] cycloaddition of alkynes/allenes with enones; reductive coupling of alkyl iodides with alkenes; addition of organoboronic acids to alkynes, alkenes, or aldehydes; carbocyclization of o-iodoaryl ketones/aldehydes with alkynes/electron-deficient alkenes; coupling of thiols with aryl and alkyl halides; enyne coupling; and C-H bond activation. Reactions relying on π components, specifically cycloaddition, reductive coupling, and enyne coupling, mostly afford products with excellent stereo- and regioselectivity and superior atom economy. We believe that these cobalt-catalyzed π-component coupling reactions proceed through five-membered cobaltacyclic intermediates formed by the oxidative cyclometalation of two coordinated π bonds of the substrates to the low-valent cobalt species. The high regio- and stereoselectivity of these reactions are achieved as a result of the electronic and steric effects of the π components. Mostly, electron-withdrawing groups and bulkier groups attached to the π bonds prefer to be placed near the cobalt center of the cobaltacycle. Most of these transformations proceed through low-valent cobalt complexes, which are conveniently generated in situ from air-stable Co(II) salts by Zn- or Mn-mediated reduction. Overall, we have shown these reactions to be excellent substitutes for less desirable noble-metal systems. Recent successes in cobalt-catalyzed C-H activation have especially advanced the applicability of cobalt in this field. In addition to the more common low-valent-cobalt-catalyzed C-H activation reactions, an in situ-formed cobalt(III) five-membered complex with a 1,6-enyne effectively couples with aromatic ketones and esters through ortho C-H activation, opening a new window in this research area. Interestingly, this reaction proceeds under milder reaction conditions with broad substrate scope. Furthermore, many of the reactions we have developed are highly enantioselective, including enantioselective reductive coupling of enones and alkynes, addition of organoboronic acids to aldehydes, and the cyclization of 2-iodobenzoates with aldehydes. Overall, this Account demonstrates the versatility and utility of cobalt catalysis in organic synthesis.

Synthesis of highly strained bicyclic[3.n.1]alkenes by a metal-catalyzed Conia-ene reaction

A high yielding metal-catalysed Conia-ene reaction of 2-acetylenic ketones for the synthesis of bicyclo[3.n.1]alkenes has been developed.

A general approach to mechanism in multiproduct reactions: product-specific intermolecular kinetic isotope effects

Here we report a general method for the measurement of (13)C kinetic isotope effects at natural abundance for reactions that yield two or more products concurrently. We use, as an example, a recently reported Co-catalyzed reaction between cyclopentene and 1-phenyl-1-propyne. High-precision intermolecular (13)C isotope effects are reported for both the formal [2+2] cycloaddition (major) and Alder-ene (minor) reaction products. Mechanistic possibilities that are in accord with observed isotope effect measurements are discussed.

Regioselective hydroacylation of 1,3-dienes by cobalt catalysis

We describe a cobalt-catalyzed hydroacylation of 1,3-dienes with non-chelating aldehydes. Aromatic aldehydes provide 1,4-addition products as the major isomer, while aliphatic aldehydes favor 1,2-hydroacylation products. The kinetic profile supports an oxidative cyclization mechanism involving a cobaltacycle intermediate that undergoes transformation with high regio- and stereoselectivity.

Iron-catalyzed aerobic oxidation of allylic alcohols: the issue of C═C bond isomerization

An aerobic oxidation of allylic alcohols using Fe(NO3)3·9H2O/TEMPO/NaCl as catalysts under atmospheric pressure of oxygen at room temperature was developed. This eco-friendly and mild protocol provides a convenient pathway to the synthesis of stereodefined α,β-unsaturated enals or enones with the retention of the C-C double-bond configuration.

Successive C-C coupling of dienes to vicinally dioxygenated hydrocarbons: ruthenium catalyzed [4 + 2] cycloaddition across the diol, hydroxycarbonyl, or dione oxidation levels

The ruthenium(0) catalyst generated from Ru3(CO)12 and tricyclohexylphosphine or BIPHEP promotes successive C-C coupling of dienes to vicinally dioxygenated hydrocarbons across the diol, hydroxyketone, and dione oxidation levels to form products of [4 + 2] cycloaddition. A mechanism involving diene-carbonyl oxidative coupling followed by intramolecular carbonyl addition from the resulting allylruthenium intermediate is postulated.