Synthesis of Multisubstituted Allenes via Nickel-Catalyzed Cross-Electrophile Coupling

Pd-catalyzed 5-exo-dig cyclization/etherification cascade of N-propargyl arylamines for the synthesis of polysubstituted furans

A method for the synthesis of furans bearing indoline skeletons was developed via an intramolecular palladium-catalyzed 5-exo-dig cyclization/etherification cascade of N-propargyl arylamines containing a 1,3-dicarbonyl side chain. This method realized the first capture of vinyl carbopalladiums by ketones as O-nucleophiles and showed a wide range of substrate tolerability affording trisubstituted furans in various yields. The enantioselective version for this domino process and diverse derivatizations of the reaction products were also studied.

N-Alkoxyphthalimides as Nitrogen Electrophiles to Construct C-N Bonds via Reductive Cross-Coupling

N-Alkoxyphthalimides, one kind of phthalimide derivative, have great importance in synthesis, mainly used as free radical precursors. While the phthalimide unit, for a long time, was treated as part of the waste stream. Construction of C-N bonds has always been a hot spot, especially in reductive cross-coupling. Herein, a nickel-catalyzed reductive cross-coupling reaction of N-methoxyphthalimides with alkyl halides is described, where N-methoxyphthalimides serve as nitrogen electrophiles. This tactic provides a new approach to construct C-N bonds under mild neutral conditions. Alkyl chlorides, bromides, iodides, and sulfonates are all fit to this transformation. Moreover, the reaction could tolerate a broad substrate scope, especially base-sensitive functional groups (boron or silicon groups), as well as competitive nucleophilic groups (phenols and amides), which are incompatible with traditional Gabriel synthesis under basic conditions, demonstrating a complementary role of this work to Gabriel synthesis.

Ligand-Controlled, Nickel-Catalyzed Stereodivergent Construction of 1,3-Nonadjacent Stereocenters

In contrast to the asymmetric synthesis of molecules with a single stereocenter or 1,2-adjacent stereocenters, the simultaneous construction of acyclic 1,3-nonadjacent stereocenters via a single catalyst in an enantioselective and diastereoselective manner remains a formidable challenge. Here, we demonstrate the enantioselective and diastereodivergent construction of 1,3-nonadjacent stereocenters through Ni-catalyzed reductive cyclization/cross-coupling of alkene-tethered aryl bromides and α-bromoamides, which represents the major remaining stereochemical challenge of cyclization/difunctionalization of alkenes. Using Ming-Phos as ligand, a diverse set of oxindoles containing 1,3-nonadjacent stereocenters were obtained with high levels of enantio- and diastereoselectivity. Mechanistic experiments and density functional theory calculations indicate that magnesium salt plays a key role in controlling the diastereoselectivity. Furthermore, another set of complementary stereoisomeric products were constructed from the same set of starting materials using Ph-Phox as ligand.

Nickel-Catalyzed Asymmetric Propargyl-Aryl Cross-Electrophile Coupling

Performing asymmetric cross-coupling reactions between propargylic electrophiles and aryl nucleophiles is a well-established method to build enantioenriched benzylic alkynes. Here, a catalytic enantioselective propargyl-aryl cross-coupling between two electrophiles was achieved for the first time in a stereoconvergent manner. Propargylic chlorides were treated with aryl iodides as well as heteroaryl iodides in the presence of a chiral nickel complex, and manganese metal was used as a stoichiometric reductant, allowing for the construction of a propargyl C-aryl bond under mild conditions. An alternative dual nickel/photoredox catalytic protocol was also developed for this cross-electrophile coupling in the absence of a metal reductant. The potential utility of this conversion is demonstrated in the facile construction of stereoenriched bioactive molecule derivatives and medicinal compounds based on the diversity of acetylenic chemistry. Detailed experimental studies have revealed the key mechanistic features of this transformation.

Electroreductive Carboxylation of Propargylic Acetates with CO2: Access to Tetrasubstituted 2,3-Allenoates

An electroreductive carboxylation of propargylic alcohols with CO2 and then workup with TMSCHN2 to construct tetrasubstituted 2,3-allenoates is developed. This method allows the incorporation of an external ester group into the resulting allene system through electroreduction, carboxylation, and deacetoxylation cascades. Mechanistically, electricity on/off experiments and cyclic voltammetry analysis support the preferential generation of the CO2 radical anion or the 3-aryl propargylic acetate radical anion based on the electron nature of the aryl rings.

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.

Synthesis of Multisubstituted Allenes by Palladium-Catalyzed Carboetherification of β,γ-Unsaturated Ketoximes with Propargylic Acetates

A highly efficient Pd-catalyzed carboetherification reaction of β,γ-unsaturated ketoximes with propargylic acetates is demonstrated. This method provides a practical protocol for accessing the incorporation of an allene moiety into 3,5-disubstituted and 3,5,5-trisubstituted isoxazolines. The salient features of this transformation include a broad substrate scope, good functional group tolerance, an easy scale-up, versatile transformations, and applications in the late-stage modification of drugs.

Modular and Selective Access to Functionalized Alkynes and Allenes via the Intermediacy of Propargylic Acetates

We report an efficient one-pot, two-step procedure for the modular synthesis of α-difunctionalized alkynes and trisubstituted allenes by sequential cross-coupling of benzal gem-diacetates with organozinc or -copper reagents in the absence of external transition metals. The intermediacy of propargylic acetates enables the divergent and selective synthesis of these valuable products. This method features its readily accessible substrates, relatively mild conditions, wide scope, and scalability in practical synthesis.

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 Three-Component Tandem Radical Cyclization 1,5-Difunctionalization of 1,3-Enynes and Alkyl Bromide

A nickel-catalyzed three-component tandem radical cyclization reaction of aryl bromides with 1,3-enynes and aryl boric acids to construct γ-lactam-substituted allene derivatives has been described. This protocol provides lactam alkyl radicals through the free radical cyclization process, which can be effectively used to participate in the subsequent multicomponent coupling reaction so that 1,3-enynes could directly convert into corresponding poly-substituted allene compounds. In addition, this efficient method enjoys a broad substrate scope and provides a series of 1,5-difunctionalized allenes in a one-pot reaction.

Nickel-Catalyzed Reductive Coupling of γ-Metalated Ketones with Unactivated Alkyl Bromides

A nickel-catalyzed reductive cross-coupling reaction of aryl cyclopropyl ketones with easily accessible unactivated alkyl bromides to access aryl alkyl ketones has been developed. This strategy facilitates access to various of γ-alkyl-substituted ketones via ring opening of cyclopropyl ketones (26 examples, 50-90% yield). Initial mechanistic studies revealed that the reaction proceeds via radical cleavage of the alkyl bromide.

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 Decarboxylative Silylation of Alkynyl Carbonates: Access to Chiral Allenes via Enantiospecific Conversions

A Ni-mediated decarboxylative silylation of alkynyl cyclic carbonates used as versatile propargylic surrogates is reported affording a wide range of highly substituted 2,3- and 3,4-allenol products in good yields. The formal cross-coupling between a tentative intermediate Ni(allenyl) and the silyl reagent was further extended to enantiospecific conversions providing access to chiral allene synthons. This protocol marks the first Ni-catalyzed propargylic silylation proceeding through an S_N2' manifold.