Highly nucleophilic vitamin B12-assisted nickel-catalysed reductive coupling of aryl halides and non-activated alkyl tosylates

Electrocatalytic Formal C(sp2)-H Alkylations via Nickel-Catalyzed Cross-Electrophile Coupling with Versatile Arylsulfonium Salts

Producing sp3-hybridized carbon-enriched molecules is of particular interest due to their high success rate in clinical trials. The installation of aliphatic chains onto aromatic scaffolds was accomplished by nickel-catalyzed C(sp2)-C(sp3) cross-electrophile coupling with arylsulfonium salts. Thus, simple non-prefunctionalized arenes could be alkylated through the formation of aryldibenzothiophenium salts. The reaction employs an electrochemical approach to avoid potentially hazardous chemical redox agents, and importantly, the one-pot alkylation proved also viable, highlighting the robustness of our approach.

Cross-selective Deoxygenative Coupling of Aliphatic Alcohols: Installation of Methyl Groups including Isotopic Labels by Nickel Catalysis

Nickel-catalyzed cross-electrophile coupling reactions of two aliphatic alcohol derivatives remain a challenge. Herein, we report a nickel-catalyzed reductive methylation reaction of aliphatic mesylates with methyl tosylate. This reaction provides straightforward access to compounds bearing aliphatic methyl groups from alkyl alcohol derivatives. Isotopically labelled substrates and reagents can be employed in the reaction to provide perdeuterated and 13C-labelled products. This transformation can be achieved by employing stoichiometric Mn reductant or electrochemically. Additionally, mechanistic experiments show that alkyl iodides are key intermediates in the transformation which undergo a stereoablative reaction via radical intermediates.

Deoxygenative Transformation of Alcohols via Phosphoranyl Radical from Exogenous Radical Addition

A general approach to the direct deoxygenative transformation of primary, secondary, and tertiary alcohols has been developed. It undergoes through phosphoranyl radical intermediates generated by the addition of exogenous iodine radical to trivalent alkoxylphosphanes. Since these alkoxylphosphanes are readily in situ obtained from alcohols and commercially available, inexpensive chlorodiphenylphosphine, a diverse range of alcohols with various functional groups can be utilized to proceed deoxygenative cross-couplings with alkenes or aryl iodides. The selective transformation of polyhydroxy substrates and the rapid synthesis of complex organic molecules are also demonstrated with this method.

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.

Multimetallic-Catalyzed C-C Bond-Forming Reactions: From Serendipity to Strategy

The use of two or more metal catalysts in a reaction is a powerful synthetic strategy to access complex targets efficiently and selectively from simple starting materials. While capable of uniting distinct reactivities, the principles governing multimetallic catalysis are not always intuitive, making the discovery and optimization of new reactions challenging. Here, we outline our perspective on the design elements of multimetallic catalysis using precedent from well-documented C-C bond-forming reactions. These strategies provide insight into the synergy of metal catalysts and compatibility of the individual components of a reaction. Advantages and limitations are discussed to promote further development of the field.

Zinc-Mediated Transformation of 1,3-Diols to Cyclopropanes for Late-Stage Modification of Natural Products and Medicinal Agents

A method for incorporating cyclopropane motifs into complex molecules has been developed. Herein we report a zinc dust-mediated cross-electrophile coupling reaction of 1,3-dimesylates to synthesize cyclopropanes. 1,3-Dimesylates can be readily accessed from 1,3-diols, a functionality prevalent in many natural products and medicinal agents. The reaction conditions are mild, such that functional groups, including amides, esters, heterocycles, and alkenes, are tolerated. Notably, we have demonstrated late-stage cyclopropanation of statin medicinal agents.

Recent Progress on Transition-Metal-Mediated Reductive C(sp3)–O Bond Radical Addition and Coupling Reactions

In this short review, we summarize the recent developments on thermo-driven C(sp3)–O bond radical scission methods and their applications in the construction of C(sp3)–C bonds via conjugate addition with activated double bonds and reductive coupling mediated by economic 3d metals, in particular nickel. We have arranged the review based on three approaches for C(sp3)–O bond radical scission (vide infra). After generating the radical intermediates, their subsequent transformation into C(sp3)–C bonds enabled by C(sp3)–O cross-electrophile coupling with carbon electrophiles is discussed in detail.

1 Introduction

2 Direct Single-Electron Transfer to a C(sp3)–O Bond

3 Radical Scission of Activated C(sp3)–O Bonds via Single-Electron Transfer to Protecting Groups

4 In Situ Activation of Alcohols

5 Summary and Outlook

Homogeneous Organic Electron Donors in Nickel-Catalyzed Reductive Transformations

Many contemporary organic transformations, such as Ni-catalyzed cross-electrophile coupling (XEC), require a reductant. Typically, heterogeneous reductants, such as Zn⁰ or Mn⁰, are used as the electron source in these reactions. Although heterogeneous reductants are highly practical for preparative-scale batch reactions, they can lead to complications in performing reactions on process scale and are not easily compatible with modern applications, such as flow chemistry. In principle, homogeneous organic reductants can address some of the challenges associated with heterogeneous reductants and also provide greater control of the reductant strength, which can lead to new reactivity. Nevertheless, homogeneous organic reductants have rarely been used in XEC. In this Perspective, we summarize recent progress in the use of homogeneous organic electron donors in Ni-catalyzed XEC and related reactions, discuss potential synthetic and mechanistic benefits, describe the limitations that inhibit their implementation, and outline challenges that need to be solved in order for homogeneous organic reductants to be widely utilized in synthetic chemistry. Although our focus is on XEC, our discussion of the strengths and weaknesses of different methods for introducing electrons is general to other reductive transformations.

Cu(ii)-vitamin C-complex catalyzed photo-induced homocoupling reaction of aryl boronic acid in base-free and visible light conditions

In this work, the photocatalytic efficiency of Cu(ii)-vitamin C complex immobilized on titanium dioxide nanoparticles was exploited in the photo-assisted homocoupling reaction of aryl boronic acids under heterogeneous conditions. The homocoupling reaction affords the corresponding symmetrical biaryls in 50-97% yields at ambient temperature in the air under visible light irradiation without any need for any additive such as base or oxidant. This method tolerates various substituents on the aryl boronic acids such as halogen, carbonyl, and a nitro group. The light-dependent photocatalytic performance of the title catalyst evaluated by action spectra revealed a maximum apparent quantum efficiency (AQYs) at 410 nm demonstrating the visible-light-driven photocatalytic reaction. The as-prepared nano biophotocatalyst proved to be reusable at least six times without losing its activity. Thus this work exhibits a favorable method from the environmental and economic point of view which enables the industrially important reactions such as coupling reactions, to be carried out efficiently under photocatalytic and practically attainable conditions.

Nickel-Catalyzed Decarboxylative Coupling of Redox-Active Esters with Aliphatic Aldehydes

The addition of alkyl fragments to aliphatic aldehydes is a highly desirable transformation for fragment couplings, yet existing methods come with operational challenges related to the basicity and instability of the nucleophilic reagents commonly employed. We report herein that nickel catalysis using a readily available bioxazoline (BiOx) ligand can catalyze the reductive coupling of redox-active esters with aliphatic aldehydes using zinc metal as the reducing agent to deliver silyl-protected secondary alcohols. This protocol is operationally simple, proceeds under mild conditions, and tolerates a variety of functional groups. Initial mechanistic studies suggest a radical chain pathway. Additionally, alkyl tosylates and epoxides are suitable alkyl precursors to this transformation providing a versatile suite of catalytic reactions for the functionalization of aliphatic aldehydes.

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

A nickel-catalyzed reductive cross coupling with phosphonium salts and allylic C(sp³)-O bond electrophiles, which granted direct construction of the C(sp²)-C(sp³) bond, is successfully developed. The protocol features broad substrate scope, high-functional-group tolerance, and heterocycle compatibility. Notably, the much more challenging reductive cross coupling with heterocyclic thiazolylphosphonium salts has also been accomplished for the first time.

Nickel-Catalyzed Negishi-Type Arylation of Trialkylsulfonium Salts

Negishi-type arylation of trialkylsulfonium salts with arylzinc reagents has been accomplished under nickel catalysis. The use of cyclohexanethiol as an additional ligand was found to be particularly important to promote C–S cleavage. The present reaction accommodates one-pot arylation of dialkyl sulfides by combining with S-methylation with MeOTf. Mechanistic experiments suggest that C–S cleavage would proceed via single-electron transfer (SET) to generate the most stable carbon-centered radical and that the thiolate ligand would promote the C–S cleavage and radical recombination step.

Cobalt Catalyst Determines Regioselectivity in Ring Opening of Epoxides with Aryl Halides

Ring-opening of epoxides furnishing either linear or branched products belongs to the group of classic transformations in organic synthesis. However, the regioselective cross-electrophile coupling of aryl epoxides with aryl halides still represents a key challenge. Herein, we report that the vitamin B₁₂/Ni dual-catalytic system allows for the selective synthesis of linear products under blue-light irradiation, thus complementing methodologies that give access to branched alcohols. Experimental and theoretical studies corroborate the proposed mechanism involving alkylcobalamin as an intermediate in this reaction.

Acylation of electrophilic bicyclo[1.1.0]butanes via Co/Ni-catalyzed reductive cross-coupling

Cyclobutane scaffold is appreciated in medicinal chemistry for its strained, well-defined three-dimensional structure. Consequently, methods for the synthesis of cyclobutyl derivatives have become highly desired, particularly those offering access to compounds with new patterns of substituents. Herein, an acylation of electrophilic strained molecules at the bridgehead carbon with [Formula: see text]-acyl-glutarimides is reported. For this, the polarity-reversal strategy based on cobalt catalysis that enables the generation of cyclobutyl radicals in a strain release event was harnessed. These nucleophilic species, in the presence of a Ni-complex, couple with [Formula: see text]-acyl-glutarimides to give cyclobutyl ketones in decent yields.

Formate-Mediated Cross-Electrophile Reductive Coupling of Aryl Iodides and Bromopyridines

Two catalytic systems for the formate-mediated cross-electrophile reductive coupling of 4-iodoansiole with 6-bromopyridines are described. Using homogenous rhodium or heterogeneous palladium catalysts, the product of reductive biaryl cross-coupling could be formed in moderate yield with excellent levels of chemoselectivity.

Electrochemically Enabled, Nickel-Catalyzed Dehydroxylative Cross-Coupling of Alcohols with Aryl Halides

As alcohols are ubiquitous throughout chemical science, this functional group represents a highly attractive starting material for forging new C-C bonds. Here, we demonstrate that the combination of anodic preparation of the alkoxy triphenylphosphonium ion and nickel-catalyzed cathodic reductive cross-coupling provides an efficient method to construct C(sp²)-C(sp³) bonds, in which free alcohols and aryl bromides-both readily available chemicals-can be directly used as coupling partners. This nickel-catalyzed paired electrolysis reaction features a broad substrate scope bearing a wide gamut of functionalities, which was illustrated by the late-stage arylation of several structurally complex natural products and pharmaceuticals.

Visible light-driven Giese reaction with alkyl tosylates catalysed by nucleophilic cobalt

The scope of the Giese reaction is expanded using readily available alkyl tosylates as substrates and nucleophilic cobalt(i) catalysts under visible-light irradiation. The reaction proceeds preferentially with less bulky primary alkyl tosylates. This unique reactivity enables the regio-selective Giese reaction of polyol derivatives.

We report an efficient approach for the Giese reaction with abundant alkyl tosylates as alkyl radical sources using a nucleophilic cobalt catalyst under visible-light irradiation.

Vitamin B12-Catalyzed Dicarbofunctionalization of Bromoalkenes Under Visible Light Irradiation

Vitamin B12 plays a crucial role in enzymatic transformations. This natural compound proved also useful as a catalyst in numerous organic reactions. Commercial availability and lower cost than precious metal complexes, make cobalamin an attractive candidate for a broader use as a benign Co-catalyst. Herein, the vitamin B12-catalyzed dicarbofuntionalization of bromoalkenes with electrophilic olefins is reported leading to substituted pyrrolidines and piperidines in decent yields after only 15 minutes under light irradiation.

A Widely Applicable Dual Catalytic System for Cross-Electrophile Coupling Enabled by Mechanistic Studies

A dual catalytic system for cross-electrophile coupling reactions between aryl halides and alkyl halides that features a Ni catalyst, a Co cocatalyst, and a mild homogeneous reductant is described. Mechanistic studies indicate that the Ni catalyst activates the aryl halide, while the Co cocatalyst activates the alkyl halide. This allows the system to be rationally optimized for a variety of substrate classes by simply modifying the loadings of the Ni and Co catalysts based on the reaction product profile. For example, the coupling of aryl bromides and aryl iodides with alkyl bromides, alkyl iodides, and benzyl chlorides is demonstrated using the same Ni and Co catalysts under similar reaction conditions but with different optimal catalyst loadings in each case. Our system is tolerant of numerous functional groups and is capable of coupling heteroaryl halides, di-ortho-substituted aryl halides, pharmaceutically relevant druglike aryl halides, and a diverse range of alkyl halides. Additionally, the dual catalytic platform facilitates a series of selective one-pot three-component cross-electrophile coupling reactions of bromo(iodo)arenes with two distinct alkyl halides. This demonstrates the unique level of control that the platform provides and enables the rapid generation of molecular complexity. The system can be readily utilized for a wide range of applications as all reaction components are commercially available, the reaction is scalable, and toxic amide-based solvents are not required. It is anticipated that this strategy, as well as the underlying mechanistic framework, will be generalizable to other cross-electrophile coupling reactions.

Ni-Catalyzed Reductive and Merged Photocatalytic Cross-Coupling Reactions toward sp3/sp2-Functionalized Isoquinolones: Creating Diversity at C-6 and C-7 to Address Bioactive Analogues

Naturally occurring isoquinolones have gained considerable attention over the years for their bioactive properties. While the late-stage introduction of various functionalities at certain positions, namely, C-3, C-4, and C-8, has been widely documented, the straightforward introduction of challenging sp3 carbon-linked acyclic aminoalkyl or aza- and oxacyclic appendages at C-6 and C-7 remains largely underexplored. Interest in 6-substituted azacyclic analogues has recently garnered attention in connection with derivatives exhibiting anticancer activity. Reported here is the first application of the versatile and recently emerging field of Ni-catalyzed reductive cross-coupling reactions to the synthesis of 6- and 7- hetero(cyclo)alkyl-substituted isoquinolones. In a second and complementary approach, a new set of C-6- and C-7-substituted positional isomers of hetero(cyclo)alkyl appendages were obtained from the merging of photocatalytic and Ni-catalyzed coupling reactions. In both cases, 6- and 7-bromo isoquinolones served as dual-purpose reacting partners with readily available tosylates and carboxylic acids, respectively.

Polarity-Reversal Strategy for the Functionalization of Electrophilic Strained Molecules via Light-Driven Cobalt Catalysis

Strain-release-driven methodology is a powerful tool for accessing structural motifs, highly desirable by the pharmaceutical industry. The reactivity of spring-loaded cyclic reagents is dominated by transformations relying on their inherent electrophilic reactivity. Herein, we present a polarity-reversal strategy based on light-driven cobalt catalysis, which enables the generation of nucleophilic radicals through strain release. The applicability of this methodology is demonstrated by the design of two distinct types of reactions: Giese-type addition and Co/Ni-catalyzed cross-coupling. Moreover, a series of electrochemical, spectroscopic, and kinetic experiments as well as X-ray structural analysis of the intermediate alkylcobalt(III) complex give deeper insight into the mechanism of the reaction.

Nickel-Catalyzed Alkyl-Alkyl Cross-Electrophile Coupling Reaction of 1,3-Dimesylates for the Synthesis of Alkylcyclopropanes

Cross-electrophile coupling reactions of two Csp³-X bonds remain challenging. Herein we report an intramolecular nickel-catalyzed cross-electrophile coupling reaction of 1,3-diol derivatives. Notably, this transformation is utilized to synthesize a range of mono- and 1,2-disubstituted alkylcyclopropanes, including those derived from terpenes, steroids, and aldol products. Additionally, enantioenriched cyclopropanes are synthesized from the products of proline-catalyzed and Evans aldol reactions. A procedure for direct transformation of 1,3-diols to cyclopropanes is also described. Calculations and experimental data are consistent with a nickel-catalyzed mechanism that begins with stereoablative oxidative addition at the secondary center.

Reductive amidation of alkyl tosylates with isocyanates by a Ni/Co-dual catalytic system

Reductive amidation of alkyl tosylates with isocyanates using the Ni/Co-dual catalytic system is disclosed. The method proceeds efficiently under mild conditions, giving rise to the corresponding alkyl amides. Notably, the protocol can discriminate the steric environment of two alkyl tosylate moieties, enabling regioselective mono-amidation at the less-bulky site.

Ni-catalyzed cross-electrophile coupling between vinyl/aryl and alkyl sulfonates: synthesis of cycloalkenes and modification of peptides

We report here the coupling reactions between vinyl/aryl and alkyl C-O electrophiles that can be derived from chemical feedstocks and naturally occurring functional groups. This method provides an efficient approach to the synthesis of a wide range of functionalized, and/or secondary alkyl substituted cycloalkenes. These compounds are difficult to produce by conventional methods. The reaction proceeds with broad substrate scope, and tolerates various functional groups such as alcohol, aldehyde, ketone, ester, amide, alkene, alkyne, heterocycles, organotin and organosilicon compounds. The synthetic utility of this method has been demonstrated by providing facile access to important building blocks. We also demonstrated the possibility to apply this method for late-stage modification of peptides. A broad range of functionalized alkyl groups could be selectively introduced into tyrosine in peptides via C-C bond formation, which has been a challenge to the existing procedures.

Ni/Co-Catalyzed Homo-Coupling of Alkyl Tosylates

A direct reductive homo-coupling of alkyl tosylates has been developed by employing a combination of nickel and nucleophilic cobalt catalysts. A single-electron-transfer-type oxidative addition is a pivotal process in the well-established nickel-catalyzed coupling of alkyl halides. However, the method cannot be applied to the homo-coupling of ubiquitous alkyl tosylates due to the high-lying σ*(C-O) orbital of the tosylates. This paper describes a Ni/Co-catalyzed protocol for the activation of alkyl tosylates on the construction of alkyl dimers under mild conditions.

Reductive Cyclization of Unactivated Alkyl Chlorides with Tethered Alkenes under Visible-Light Photoredox Catalysis

The chemical inertness of abundant and commercially available alkyl chlorides precludes their widespread use as reactants in chemical transformations. Presented in this work is a metallaphotoredox methodology to achieve the catalytic intramolecular reductive cyclization of unactivated alkyl chlorides with tethered alkenes. The cleavage of strong C(sp3 )-Cl bonds is mediated by a highly nucleophilic low-valent cobalt or nickel intermediate generated by visible-light photoredox reduction employing a copper photosensitizer. The high basicity and multidentate nature of the ligands are key to obtaining efficient metal catalysts for the functionalization of unactivated alkyl chlorides.

Mechanistic Interrogation of Co/Ni-Dual Catalyzed Hydroarylation

Cobalt/nickel-dual catalyzed hydroarylation of terminal olefins with iodoarenes builds complexity from readily available starting materials, with a high preference for the Markovnikov (branched) product. Here, we advance a mechanistic model of this reaction through the use of reaction progress kinetic analysis (RPKA), radical clock experiments, and stoichiometric studies. Through exclusion of competing hypotheses, we conclude that the reaction proceeds through an unprecedented alkylcobalt to nickel direct transmetalation. Demonstration of catalytic alkene prefunctionalization, via spectroscopic observation of an organocobalt species, distinguishes this Csp2-Csp3 cross-coupling method from a conventional transmetalation process, which employs a stoichiometric organometallic nucleophile, and from a bimetallic oxidative addition of an organohalide across nickel, described by radical scission and subsequent alkyl radical capture at a second nickel center. A refined understanding of the reaction leads to an optimized hydroarylation procedure that excludes exogenous oxidant, demonstrating that the transmetalation is net redox neutral. Catalytic alkene prefunctionalization by cobalt and engagement with nickel catalytic cycles through direct transmetalation provides a new platform to merge these two rich areas of chemistry in preparatively useful ways.