Nickel-Catalyzed Reductive Csp2-Csp3 Cross Coupling Using Phosphonium Salts

One-Pot Chlorination and Cross-Electrophile Coupling of Alcohols with Aryl Chlorides

Although alkyl alcohols and aryl chlorides are the two most abundant substrate pools for cross-electrophile coupling, methods to couple them remain limited. Herein we demonstrate a simple procedure for the in situ deoxychlorination of alcohols followed by XEC with aryl chlorides. A broad substrate scope can be achieved by tuning the rate of the reaction via halide exchange. Key to success is the identification of 1-chloro-N,N,2-trimethyl-1-propenylamine as a mild, noninterfering halogenation reagent.

Ni-catalyzed regioselective and site-divergent reductive arylalkylations of allylic amines

Catalytic methods by switching the least parameters for regioselective and site-divergent transformations to construct different architectures from identical and readily available starting materials are among the most ideal catalytic protocols. However, the associated challenge to precisely control both regioselectivity and site diversity renders this strategy appealing yet challenging. Herein, Ni-catalyzed cross-electrophile regioselective and site-divergent 1,2- and 1,3-arylalkylations of N-acyl allylic amines have been developed. This Ni-catalyzed reductive three-component protocol enables 1,2-arylalkylation and 1,3-arylalkylation of allylic amines with aryl halides and alkyl halides with excellent chemo-, regio- and site-selectivity, representing the first example of controlled migratory difunctionalization of alkenes under reductive conditions. A wide range of terminal and internal unactivated allylic amines, aryl halides and alkyl precursors were tolerated, providing straightforward and efficient access to diverse C(sp3)-rich branched aliphatic amines from identical starting materials.

Nickel-Catalyzed Cross-Electrophile Coupling of Aryl Triflates with Alkyl Halides: Mechanism-Informed Design of More General Conditions

Aryl triflates make up a class of aryl electrophiles that are available in a single step from the corresponding phenol. Despite the known reactivity of nickel complexes for aryl C-O bond activation of phenol derivatives, nickel-catalyzed cross-electrophile coupling using aryl triflates has proven challenging. Herein, we report a method to form C(sp2)-C(sp3) bonds by coupling aryl triflates with alkyl bromides and chlorides using phenanthroline (phen) or pyridine-2,6-bis(N-cyanocarboxamidine) (PyBCamCN)-ligated nickel catalysts. The scope of the reaction is demonstrated with 38 examples (61 ± 14% average yield). Mechanistic studies provide a rationale for the conditions used and a roadmap for further applications of cross-electrophile coupling. First, the rate of alkyl radical generation is controlled by maintaining the majority of alkyl halide as the alkyl chloride, which is unreactive, and utilizing a dynamic halide exchange process to adjust the concentration of reactive alkyl bromide or iodide. Second, the challenge of using electron-rich aryl triflates appears to be due to off-cycle transmetalation to form unproductive aryl zinc reagents. The optimal PyBCamCN ligand together with LiCl avoids this deleterious transmetalation step.

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.

An SN1-Approach to Cross-Coupling: Deoxygenative Arylation Facilitated by the β-Silicon Effect

We report a dual metal-catalyzed method for the cross-coupling of unprotected alcohols by exploiting the β-Si effect. This deoxygenative Suzuki-Miyaura reaction tolerates a range of primary, secondary, and tertiary alcohol substrates along with diverse functional groups and heterocycles. Mechanistic experiments including KIE, VTNA, and Eyring analyses suggest the existence of a carbocation intermediate on the reaction pathway, consistent with a rare SN1 pathway for the activation of an electrophile in cross-coupling reactions. A novel bis-imidazolium N-heterocyclic carbene (NHC) ligand was found to be optimal for reactivity, and nickel(0)-, nickel(I)- and nickel(II)- complexes of this ligand were isolated and characterized. In contrast to more well-established shorter chain ligands, these long-chain NHCs are found to have characteristically large bite angles, which may be critical for enabling the deoxygenative arylation of aliphatic alcohols.

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

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.

Accessing Ni(0) to Ni(IV) via nickel-carbon-phosphorus bond reorganization

Two-electron oxidation of a NiIIPh(PCP) pincer complex initiates phosphine ligand insertion, generating an η6-arylphosphonium moiety coordinated to NiII. The reaction is fully reversible under reducing conditions. X-ray crystallography, NMR/EPR spectroscopy, electrochemistry, and DFT calculations support the proposed Ni-C-P bond reorganization mechanisms, which access oxidation states from Ni0 to NiIV.

1-Hydroxyalkylphosphonium Salts-Synthesis and Properties

An efficient and convenient method for the synthesis of 1-hydroxyalkylphosphonium salts is described. Reactions were carried out at room temperature, in a short time, and without chromatography for product isolation. The properties of the obtained phosphonium salts were examined and discussed. In this paper, primary attention was paid to the stability of phosphonium salts, depending on the structure of the aldehydes used as substrates in their preparation. Other conditions such as the type of solvent, temperature, and molar ratio of the substrates were also investigated. Finally, the high reactivity of 1-hydroxyalkylphosphonium salts was demonstrated in reactions with amide-type substrates and (hetero)aromatic compounds. The developed step-by-step procedure (with the isolation of 1-hydroxyphosphonium salts) was compared to the one-pot protocol (in situ formation of such phosphonium salts).

Brønsted acid-mediated tandem cyclization of triarylphosphines and in situ generated ortho-alkynyl quinone methides: access to heterocyclic quaternary phosphonium salts

Heterocyclic Quaternary Phosphonium Salts (HQPS) have emerged as promising chemicals for organic synthesis and medicinal chemistry. However, the present synthetic methodology of this type of compound is still limited. Here, we report a deconstructive reorganization strategy based on Brønsted acid-mediated tandem 1,4 addition/intramolecular cyclization of triphenylphosphine derivatives and in situ generated o-AQMs for the first time. This protocol provides a novel approach to heterocyclic quaternary phosphonium salts. The method also features a non-metal catalyst, mild reaction conditions, high efficiency and wide substrate scope. Moreover, a series of obtained heterocyclic phosphonium salts can be converted to isotopically labelled 2-benzofuran compounds directly by simple deuteration reactions.

Dehydroxylative Arylation of Alcohols via Paired Electrolysis

Nonactivated alcohols along with arene compounds are used in electrochemical dehydroxylative arylation for constructing C(sp³)-C(sp²) bonds. The P^III reagent undergoes single-electron anodic oxidation to form its radical cation, which reacts with the alcohol to produce an alkoxytriphenylphosphine radical. Through spontaneous β-scission of the phosphoranyl radical, the C-O bond is cleaved to form an alkyl radical species, which couples with the radical anion generated by cathodic reduction of the electron-poor arene to afford the dehydroxylative arylated product.

C(sp2)-C(sp2) Reductive Cross-Coupling of Triarylphosphines with Aryl Halides by Palladium/Nickel Co-catalysis

Herein, we report the first general C(sp²)-C(sp²) reductive cross-coupling reaction of diverse triarylphosphines with a wide range of aryl halides by palladium/nickel co-catalysis. This protocol offers a unique route for the synthesis of biaryl compounds via the activation of inert C(Ar)-P bonds. The mechanistic studies demonstrate that the formation of the phosphonium salts in situ plays a key role in the catalytic cycle.

Nickel-catalysed diversification of phosphine ligands by formal substitution at phosphorus

We report a diversification strategy that enables the direct substituent exchange of tertiary phosphines. Alkylated phosphonium salts, prepared by standard alkylation of phosphines, are selectively dearylated in a nickel-catalysed process to access alkylphosphine products via a formal substitution at the phosphorus center. The reaction can be used to introduce a wide range of alkyl substituents into both mono- and bisphosphines. We also show that the alkylation and dearylation steps can be conducted in a one-pot sequence, enabling accelerated access to derivatives of the parent ligand. The phosphine products of the reaction are converted in situ to air-stable borane adducts for isolation, and versatile derivatisation reactions of these adducts are demonstrated.

Phosphine substituents can be exchanged by standard alkylation of a phosphine and a subsequent dearylation of the resulting phosphonium salt. A wide variety of alkyl groups can be introduced into both mono- and bidentate ligands using this method.