Deoxygenative cross-electrophile coupling of benzyl chloroformates with aryl iodides

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.

Interrogating the Mechanistic Features of Ni(I)-Mediated Aryl Iodide Oxidative Addition Using Electroanalytical and Statistical Modeling Techniques

While the oxidative addition of Ni(I) to aryl iodides has been commonly proposed in catalytic methods, an in-depth mechanistic understanding of this fundamental process is still lacking. Herein, we describe a detailed mechanistic study of the oxidative addition process using electroanalytical and statistical modeling techniques. Electroanalytical techniques allowed rapid measurement of the oxidative addition rates for a diverse set of aryl iodide substrates and four classes of catalytically relevant complexes (Ni(MeBPy), Ni(MePhen), Ni(Terpy), and Ni(BPP)). With >200 experimental rate measurements, we were able to identify essential electronic and steric factors impacting the rate of oxidative addition through multivariate linear regression models. This has led to a classification of oxidative addition mechanisms, either through a three-center concerted or halogen-atom abstraction pathway based on the ligand type. A global heat map of predicted oxidative addition rates was created and shown applicable to a better understanding of the reaction outcome in a case study of a Ni-catalyzed coupling reaction.

Nickel-Catalyzed Decarbonylative Reductive Alkylation of Aroyl Fluorides with Alkyl Bromides

This paper describes the nickel-catalyzed reductive alkylation of aroyl fluorides with alkyl bromides in a decarbonylative manner. In this reaction, various functional groups are well tolerated and the C(sp²)-C(sp³) bond can be constructed directly without the use of organometallic reagents. The present reaction is a cross-electrophile coupling via the radical pathway, affording the corresponding alkylarenes in moderate to good yields.

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

Nickel-Catalyzed Chemo- and Regioselective Benzylarylation of Unactivated Alkenes with o-Bromobenzyl Chlorides

Chemo- and regioselectively nickel-catalyzed reductive benzylarylation of unactivated alkenes with o-bromobenzyl chlorides is disclosed herein, in which electrophiles participate through a single-component double-site approach. Moreover, its utility is underscored by the concise synthesis of bioactive Indane compounds and postreaction functionalizations leading to structurally diverse scaffolds. Preliminary mechanistic investigations suggest a radical chain reaction mechanism.

Metal-catalysed C-Het (F, O, S, N) and C-C bond arylation

The formation of C-aryl bonds has been the focus of intensive research over the last decades for the construction of complex molecules from simple, readily available feedstocks. Traditionally, these strategies involve the coupling of organohalides (I, Br, Cl) with organometallic reagents (Mg, Zn, B, Si, Sn,…) such as Kumada-Corriu, Negishi, Suzuki-Miyaura, Hiyama and Sonogashira cross-couplings. More recently, alternative methods have provided access to these products by reactions with less reactive C-Het (F, O, S, N) and C-C bonds. Compared to traditional methods, the direct cleavage and arylation of these chemical bonds, the essential link in accessible feedstocks, has become increasingly important from the viewpoint of step-economy and functional-group compatibility. This comprehensive review aims to outline the development and advances of this topic, which was organized into (1) C-F bond arylation, (2) C-O bond arylation, (3) C-S bond arylation, (4) C-N bond arylation, and (5) C-C bond arylation. Substantial attention has been paid to the strategies and mechanistic investigations. We hope that this review can trigger chemists to discover more efficient methodologies to access arylation products by cleavage of these C-Het and C-C bonds.

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.

Nickel-catalyzed reductive benzylation of tertiary alkyl halides with benzyl chlorides and chloroformates

We report a Ni-catalyzed cross-electrophile coupling of benzyl chlorides and chloroformates with unactivated tertiary alkyl halides to forge the challenging benzylated all C(sp³)-quaternary carbon centers.

Cross-Electrophile Couplings of Activated and Sterically Hindered Halides and Alcohol Derivatives

Transition metal catalyzed cross-electrophile coupling of alkyl electrophiles has evolved into a privileged strategy that permits the facile construction of valuable C(sp³)-C bonds. Numerous elegant Ni-catalyzed coupling methods, for example, arylation, allylation, acylation, and vinylation of primary and secondary alkyl halides have been developed. This prior work has provided important mechanistic insights into the selectivity and reactivity of the coupling partners, which are largely dictated by both the catalysts and the reactants. In spite of the advances made to date, a number of challenging issues remain, including (1) achieving stereoselective syntheses of C-C bonds that rely primarily on functionalized or activated alkyl precursors, (2) diversifying the electrophiles, and (3) gaining insights into the underlying reaction mechanisms.In this Account, we summarize a number of Ni- and Fe-catalyzed reductive C-C bond forming methods developed in our laboratory, which have allowed us to couple activated, sterically hindered tertiary alkyl and C(sp³)-O bond electrophiles and to access methylated and trifluoromethylated products, esters, C-glycosides, and quaternary carbon centers. We will begin with a brief discussion of Ni-catalyzed chemoselective construction of unactivated alkyl-alkyl bonds, with focus on the effects of ligands and reductants, along with leaving group-directed reactivities of alkyl halides, and the role they play in promoting the reductive coupling of activated electrophiles, including methyl, trifluoromethyl, and glycosyl electrophiles, and chloroformates. Matching the reactivities of these electrophiles with suitable coupling partners is considered essential for success; this is something that can be tuned by means of appropriate Ni catalysts. Second, we will detail how tuning the steric and electronic effects of nickel catalysts with labile pyridine-type ligands and additives (primarily MgCl₂) permits effective creation of arylated all-carbon quaternary centers through the coupling of aryl halides with sterically encumbered tertiary alkyl halides. In contrast, the use of bulkier bipyridine and terpyridine ligands permits the incorporation of relative small-sized acyl and allyl groups into acylated and allylated all-carbon quaternary centers. Finally, we will show how the knowledge gained with halide electrophiles enabled us to develop methods that permit the coupling of tertiary alkyl oxalates with allyl, aryl, and vinyl electrophiles, wherein Barton C-O bond radical fragmentation is mediated by Zn and MgCl₂ and promoted by Ni catalysts. The same protocol is applicable to the arylation of secondary alkyl oxalates derived from α-hydroxyl carbonyl substrates, which involves the formation of relatively stable α-carbonyl carbon centered radicals. Thus, this Account not only summarizes synthetic methods that allow formation of valuable C-C bonds using challenging electrophiles but also provides insight into the relationship between the structure and reactivity of the substrates and catalysts, as well as the effects of additives.