Ni-Catalyzed Reductive Arylcyanation of Alkenes

Nickel-Catalyzed Cross-Electrophile Coupling to Access Polysubstituted Cyclobutenes

The synthesis of cyclobutenes remains inefficient owing to inherent ring strain and poor regioselectivity. We describe herein a nickel-catalyzed cross-electrophile coupling (XEC) between readily accessible homopropargyl halides and commercially available aryl/vinyl electrophiles for direct access to polysubstituted cyclobutenes. This approach provides the first reductive 4-endo-dig cyclization with high chemo- and regioselectivity, which paves the way for the synthesis of diverse cyclobutene containing compounds (including synthetically challenging macrocycles). The synthesis of an antitumor-active combretastatin A-4 analog has been significantly optimized; the original six-step procedure yielding 14% has been streamlined into a three-step process with a markedly improved yield of 51%. Experimental data and computational studies support a mechanism involving an alkenyl nickel(I) intermediate, which undergoes facile back-side SH2 attack to produce the strained cyclobutene ring with overall stereoinversion at the homopropargylic position.

Cross-Electrophile Coupling: Principles, Methods, and Applications in Synthesis

Cross-electrophile coupling (XEC), defined by us as the cross-coupling of two different σ-electrophiles that is driven by catalyst reduction, has seen rapid progression in recent years. As such, this review aims to summarize the field from its beginnings up until mid-2023 and to provide comprehensive coverage on synthetic methods and current state of mechanistic understanding. Chapters are split by type of bond formed, which include C(sp3)-C(sp3), C(sp2)-C(sp2), C(sp2)-C(sp3), and C(sp2)-C(sp) bond formation. Additional chapters include alkene difunctionalization, alkyne difunctionalization, and formation of carbon-heteroatom bonds. Each chapter is generally organized with an initial summary of mechanisms followed by detailed figures and notes on methodological developments and ending with application notes in synthesis. While XEC is becoming an increasingly utilized approach in synthesis, its early stage of development means that optimal catalysts, ligands, additives, and reductants are still in flux. This review has collected data on these and various other aspects of the reactions to capture the state of the field. Finally, the data collected on the papers in this review is offered as Supporting Information for readers.

Modular synthesis of 3,3-disubstituted oxindoles from nitrones and acrylic acids

We developed a modular synthesis for 3,3-disubstituted oxindoles, utilising readily accessible nitrones and acrylic acids. This approach facilitates the preparation of a diverse array of oxindoles through the variation of the starting materials. We demonstrated the applicability of this method through a gram-scale reaction and a synthesis of esermethole.

Green and efficient synthesis of dibenzyl cyanamides and ureas with cyanamide as a block

A method for the two-step synthesis of dibenzyl cyanamide and dibenzyl urea via cyanamide is presented. This approach is both efficient and environmentally friendly. Various N,N-dibenzyl ureas could be obtained by reactions of N,N-dibenzyl cyanamides and N,N-dibenzyl cyanamides as intermediates formed from cyanamide. In the absence of metal, ligand and hydrogen peroxide as the oxidant, products with moderate yields have been obtained under mild conditions. Key features include the use of widely available and easily handled cyanamide sources as starting materials.

EMM-Promoted Pd-Catalyzed Solid State Intramolecular Heck-Type Cyclization/Boronation and Suzuki Couplings: Access to Functionalized Indolines

EMM (electromagnetic mill)-promoted Pd-catalyzed solid state intramolecular Heck-type cyclization/boronation and Suzuki couplings are reported. Compared to previous mechanochemistry that constructed one chemical bond through a cross-coupling reaction, this strategy realizes cascade transformation along with multiple chemical bond formation. This conversion does not require organic solvents or additional heating, and it shows a good substrate scope and high functional group tolerance.

Recent Progress in Asymmetric Domino Intramolecular Cyclization/Cascade Reactions of Substituted Olefins

The domino cyclization/coupling strategy is one of the most effective methods to produce cyclized and multi-functionalized compounds from olefins, which has attracted huge attention from chemists and biochemists especially for its considerable potential of enantiocontrol. Nowadays, more and more studies are developed to achieve difunctionalization of substituted olefins through an asymmetric domino intramolecular cyclization/cascade reaction, which is still an elegant choice to accomplish several synthetic ideas such as complex natural products and drugs. This review surveys the recent advances in this field through reaction type classification. It might serve as useful knowledge desktop for the community and accelerate their research.

Ni-Catalyzed Enantioselective Intramolecular Mizoroki-Heck Reaction for the Synthesis of Phenanthridinone Derivatives

A Ni-catalyzed enantioselective intramolecular Mizoroki-Heck reaction has been developed to transform symmetrical 1,4-cyclohexadienes with attached aryl halides into phenanthridinone analogues containing quaternary stereocenters. Herein, we report important advances in reaction optimization enabling control of unwanted proto-dehalogenation and alkene reduction side products. Moreover, this approach provides direct access to six-membered ring heterocyclic systems bearing all-carbon quaternary stereocenters, which have been much more challenging to form enantioselectively with nickel-catalyzed Heck reactions. A wide range of substrates were demonstrated to work in good to excellent yields. Good enantioselectivity was demonstrated using a new synthesized chiral iQuinox-type bidentate ligand (L27). The sustainability, low price of nickel catalysts, and significantly faster reaction rate (1 h) versus that of a recently reported palladium-catalyzed reaction (20 h) make this process an attractive alternative.

Nickel-Catalyzed Ligand-Controlled Selective Reductive Cyclization/Cross-Couplings

ConspectusThe use of quaternary stereocenters during lead candidate optimization continues to grow because of improved physiochemical and pharmacokinetic profiles of compounds with higher sp³ fraction. Pd-catalyzed redox-neutral alkene difunctionalization involving carbopalladation of alkenes followed by nucleophilic-trapping σ-alkyl-palladium intermediates has been developed as an efficient method to construct quaternary stereocenters. However, the low chemoselectivity and air sensitivity of organometallic nucleophiles, as well as their low availability and accessibility, limit the scope of application of this elegant strategy. Recently, Ni-catalyzed reductive cross-coupling has evolved into a privileged strategy to easily construct valuable C(sp³)-C bonds. Despite great progress, the enantioselective coupling of C(sp³) electrophiles still relies on activated or functionalized alkyl precursors, which are often unstable and require multiple steps to prepare. Therefore, Ni-catalyzed reductive difunctionalization of alkenes via selective cyclization/cross-coupling was developed. This strategy not only offers a robust and practical alternative for traditional redox-neutral alkene difunctionalization but also provides strategic complementarity for reductive cross-coupling of activated alkyl electrophiles. In this Account, we summarize the latest results from our laboratory on this topic. These findings mainly include our explorations in modulating the enantioselectivity and cyclization mode of reductive cyclization/cross-couplings.We will first discuss Ni-catalyzed enantioselective reductive cyclization/cross-coupling to construct valuable chiral heterocycles with quaternary stereocenters and focus on the effects of ligands, reductants, and additives and their roles in reductive cross-coupling. A wide range of electrophiles have been explored, including aryl halides, vinyl halides, alkynyl halides, gem-difluoroalkenes, CO₂, trifluoromethyl alkenes, and cyano electrophiles. The synthetic potential of this approach has also been demonstrated in the synthesis of biologically active natural products and drug molecules. Second, we will detail how to tune the steric effects of nickel catalysts by modifying bipyridine ligands for regiodivergent cyclization/cross-couplings. Specifically, the use of bidentate ligands favors exo-selective cyclization/cross-coupling, while the use of a carboxylic acid-modified bipyridine ligand permits endo-selective cyclization/cross-coupling. We will also show how to activate the amide substrate by altering the electronic and steric properties of substituents on the nitrogen, thereby enabling the nucleophilic addition of aryl halides to amide carbonyls. Further investigation of ligand properties has led to tunable cyclization/cross-couplings (addition to the amide carbonyl vs 7-endo-cyclization) for the divergent synthesis of pharmacologically important 2-benzazepine frameworks. Finally, we serendipitously discover that modifying the ligands of nickel catalysts and changing the oxidation state of nickel can control the migratory aptitude of different groups, thus providing a switchable skeletal rearrangement strategy. This transformation is of high synthetic value because it represents a conceptually unprecedented new approach to C-C bond activation. Thus, this Account not only summarizes synthetic methods that allow the formation of valuable chiral heterocycles with quaternary stereocenters using a wide variety of electrophiles but also provides insight into the relationship between ligand structure, substrate, and cyclization selectivity.

Nickel-Catalyzed Reductive Dicarbofunctionalizations of Alkenes for the Synthesis of Difluorocarbonyl Oxindoles and Isoquinoline-1,3-diones

A novel and efficient strategy for the construction of difluorocarbonyl-oxindole and difluorocarbonyl-isoquinoline-1,3-dione derivatives involving nickel-catalyzed intramolecular Heck-type cyclizations followed by intermolecular cross-couplings has been developed. This approach features high functional group tolerance, broad substrate scope, and operational simplicity under mild conditions, thus providing a new method for the rapid difluorocarbonyl-functionalization of alkenes to construct the structurally diversified five- and six-membered heterocycles.

Nickel-catalyzed asymmetric reductive arylcyanation of alkenes with acetonitrile as the cyano source

Chiral 3-cyanomethyl oxindoles were synthesized in high enantioselectivities and yields. The employment of acetonitrile as a cyano source via Zn(OTf)2-assisted β-carbon elimination is distinct from the common cyanation reaction modes.

Synthesis of Carbonyl-Containing Oxindoles via Ni-Catalyzed Reductive Aryl-Acylation and Aryl-Esterification of Alkenes

Carbonyl-containing oxindoles are ubiquitous core structures present in many biologically active natural products and pharmaceutical molecules. Nickel-catalyzed reductive aryl-acylation of alkenes using aryl anhydrides or alkanoyl chlorides as acyl sources is developed, providing 3,3-disubstituted oxindoles bearing ketone functionality at the 3-position. Moreover, nickel-catalyzed reductive aryl-esterification of alkenes using chloroformate as ester sources is further developed, affording 3,3-disubstituted oxindoles bearing ester functionality at the 3-position. This strategy has the advantages of good yields and high functional group compatibility.

Ni-Catalyzed Divergent Synthesis of 2-Benzazepine Derivatives via Tunable Cyclization and 1,4-Acyl Transfer Triggered by Amide N-C Bond Cleavage

Ligand-directed divergent synthesis can transform common starting materials into distinct molecular scaffolds by simple tuning different ligands. This strategy enables the rapid construction of structurally rich collection of small molecules for biological evaluation and reveals novel modes of catalytic transformation, representing one of the most sought-after challenges in synthetic chemistry. We herein report a Ni-catalyzed ligand-controlled tunable cyclization/cross-couplings for the divergent synthesis of pharmacologically important 2-benzazepine frameworks. The bidentate ligand facilitates the nucleophilic addition of the aryl halides to the amide carbonyl, followed by 1,4-acyl transfer and cross-coupling to obtain 2-benzazepin-5-ones and benzo[c]pyrano[2,3-e]azepines. The tridentate ligand promotes the selective 7-endo cyclization/cross-coupling to access to 2-benzazepin-3-ones. The protocol operates under mild reaction conditions with divergent cyclization patterns that can be easily modulated through the ligand backbone.

Nickel-Catalyzed Reductive Cascade Arylalkylation of Alkenes with Alkylpyridinium Salts

Herein, we describe a nickel-catalyzed reductive deaminative arylalkylation of tethered alkenes with pyridinium salts as C(sp³) electrophiles. This two-component dicarbofunctionalization reaction enables the efficient synthesis of various benzene-fused cyclic compounds bearing all-carbon quaternary centers. The approach presented in this paper proceeds under mild conditions, tolerating a wide variety of functional groups and heterocycles. It has been used to functionalize complicated molecules at a late stage.

Ni-Catalyzed enantioselective reductive arylcyanation/cyclization of N-(2-iodo-aryl) acrylamide

A Ni/(S,S)-BDPP-catalyzed intramolecular Heck cyclization of N-(2-iodo-aryl) acrylamide with 2-methyl-2-phenylmalononitrile was developed to give oxindoles with good enantioselectivities. We found that utilizing such an electrophilic cyanation reagent could tackle the deleterious effect of the coordinative cyanide ion in the asymmetric alkene arylcyanation.

Visible-light-induced radical cascade reaction to prepare oxindoles via alkyl radical addition to N-arylacryl amides

A photochemical approach towards oxindoles with C3 quaternary centers by the radical cascade reaction of α,β-unsaturated N-arylacryl amides with alkyl bromides or iodides upon visible light irradiation under mild reaction conditions was developed.