Nickel-Catalyzed Negishi Cross-Coupling of Bromodifluoroacetamides

Low-cost transition metal catalysed Negishi coupling: an update

The Negishi coupling is a significant C-C bond-forming reaction to access synthetically valuable organic compounds. In recent years, researchers have developed sustainable first-row transition metal (Fe, Co, Ni and Cu) based complexes in place of the conventional Pd catalyst for this reaction. Several such low-cost metal-based catalysts showed high efficiency and potential application in natural product synthesis. This review focuses on the recent achievements in low-cost transition metal-based Negishi coupling reactions, covering reports from 2016.

Synthesis of α-Aryl Secondary Amides via Nickel-Catalyzed Reductive Coupling of Redox-Active Esters

The transition-metal-catalyzed α-arylation of secondary amides remains a synthetic challenge due to the presence of a free N-H bond. We report a strategy to synthesize secondary α-aryl amides via a Ni-catalyzed reductive arylation of redox-active N-hydroxyphthalimide (NHP) esters of malonic acid half amides. This transformation proceeds under mild conditions and displays excellent chemoselectivity for amide α-arylation in the presence of other enolizable carbonyls. The NHP ester substrates are readily prepared from Meldrum's acid.

Theoretical study of NiI-NiIII cycle mediated by heterogeneous zinc in C-N cross-coupling reaction

Photoredox/transition-metal dual catalysis could efficiently construct C-N bonds by a cross-coupling reaction. The limitations of low recovery, low utilization rate and high cost have hindered the application and development of low-cost and efficient transition metal catalytic cycles. The integration of heterogeneous metal and transition metal catalysis is an appealing alternative to realize the oxidation state modulation of active species. With the support of density functional theory (DFT) calculation, we have explored the mechanistic details of Ni-catalyzed C-N cross-coupling of aryl bromide and cyclic amine assisted by zinc powder. Zinc successfully regulates the oxidation state of Ni^II → Ni^I, thus achieving the Ni^I-Ni^III-Ni^I catalytic cycle in the absence of light. In comparison, when the Ni(0) complex is employed as the initial catalyst, organic zinc reagents can still be involved in the transmetalation process to accelerate the cross-coupling reaction. We hope that such computational studies can provide theoretical reference for the design and development of low-cost and efficient catalytic systems for C-N cross-couplings.

Synthesis of 2,2-difluoro-2-arylethylamines as fluorinated analogs of octopamine and noradrenaline

A series of 2,2-difluoro-2-arylethylamines was synthesized as fluorinated analogs of octopamine and noradrenaline with the expectation of bioisosteric OH/F exchanges. The syntheses of these compounds were performed by a Suzuki–Miyaura cross-coupling reaction of 4-(bromodifluoroacetyl)morpholine with aryl boronic acids to produce the intermediate 2,2-difluoro-2-arylacetamides, followed by transformation of difluoroacetamide to difluoroethylamine.

Applications of Nickel(II) Compounds in Organic Synthesis

This review article summarizes the applications of nickel(II) compounds in organic synthesis since 2016. In recent years, the field of nickel(II) catalysis is gaining considerable interest due to readily available, low-cost nickel(II)-compounds and several key properties of nickel. This review article is organized by the reaction type, although some reactions can be placed in multiple sections.

Using JPP to Identify Ni Bidentate Phosphine Complexes In Situ

Identifying intermediates of Ni-containing reactions can be challenging due to the high reactivity of Ni complexes and their sensitivity toward air and moisture. Many Ni bidentate phosphine complexes are diamagnetic and can be analyzed in situ via ³¹P NMR spectroscopy, but the oxidation state of Ni is difficult to determine using ³¹P chemical shift analysis alone. The J-coupling between P atoms, J_PP, has been proposed to correlate with oxidation state, but few investigations have looked at how J_PP is affected by parameters such as length of the linker or identity of the phosphine or other ligands. The present investigation into the J_PP values of Ni bidentate phosphine complexes with two-carbon and three-carbon linkers shows that the J_PP values observed in ³¹P NMR spectra, |J_PP|, are competent indicators of the oxidation state at Ni. For complexes with two-carbon linkers, |J_PP| > 40 Hz is typical of Ni⁰ while |J_PP| < 30 Hz is typical of Ni^II; this trend is reversed for complexes with three-carbon linkers. Additionally, the Lewis acidity of the Ni and Lewis basicity of the phosphine ligand affect J_PP predictably. For example, increased P-to-Ni donation arising from more-donating phosphines or more-withdrawing ligands trans to the P atoms causes a more negative J_PP. These results should enable the oxidation state of Ni and properties of ligands in Ni bidentate phosphine complexes to be determined in situ during reactions containing these species.

Cobalt-Catalyzed 1,4-Aryl Migration/Desulfonylation Cascade: Synthesis of α-Aryl Amides

A cobalt-catalyzed 1,4-aryl migration/disulfonylation cascade applied to α-bromo N-sulfonyl amides was developed. The reaction was highly chemoselective, allowing the preparation of α-aryl amides possessing a variety of functional groups. The method was used as the key step to synthesize an alkaloid, (±)-deoxyeseroline. Mechanistic investigations suggest a radical process.

The Fascinating Chemistry of α-Haloamides

The aim of this review is to highlight the rich chemistry of α-haloamides originally mainly used to discover new C-N, C-O and C-S bond forming reactions, and later widely employed in C-C cross-coupling reactions with C(sp3), C(sp2) and C(sp) coupling partners. Radical-mediated transformations of α-haloamides bearing a suitable located unsaturated bond has proven to be a straightforward alternative to access diverse cyclic compounds by means of either radical initiators, transition metal redox catalysis or visible light photoredox catalysis. On the other hand, cycloadditions with α-halohydroxamate-based azaoxyallyl cations have garnered significant attention. Moreover, in view of the important role in life and materials science of difluoroalkylated compounds, a wide range of catalysts has been developed for the efficient incorporation of difluoroacetamido moieties into activated as well as unactivated substrates.

Nickel-catalysed radical tandem cyclisation/arylation: practical synthesis of 4-benzyl-3,3-difluoro-γ-lactams

Enabled by nickel catalysis, a practical access to the synthesis of 4-benzyl-3,3-difluoro-γ-lactams has been developed via radical tandem cyclisation/arylation. This method features a nickel catalyst, high reaction efficiency, and good substrate tolerance and scope. This protocol proceeds through an intramolecular radical addition to form a primary alkyl radical followed by intermolecular Suzuki-type coupling.

Transition-Metal (Cu, Pd, Ni)-Catalyzed Difluoroalkylation via Cross-Coupling with Difluoroalkyl Halides

Difluoroalkylated compounds play a remarkably important role in life and materials sciences because of the unique characteristics of the difluoromethylene (CF₂) group. In particular, precise introduction of a CF₂ group at the benzylic position can dramatically improve the biological properties of the corresponding molecules. As a consequence, difluoroalkylation of aromatic compounds has become a powerful strategy in modulating the bioactivities of organic molecules. However, efficient strategies to selectively synthesize difluoroalkylated arenes had been very limited before 2012. Traditional synthetic methods in this regard suffer from either harsh reaction conditions or narrow substrate scope, significantly restricting their widespread applications, particularly for late-stage difluoroalkylation of bioactive molecules. To overcome these limitations, a straightforward route to access these valuable difluoroalkylated skeletons is the direct introduction of the difluoroalkylated group (CF₂R) onto aromatic rings through transition-metal-catalyzed cross-coupling. However, because of the instability of some difluoroalkylated metal species, which are prone to protonation, dimerization, and/or generation of other unknown byproducts, it is difficult to selectively control the catalytic cycle to suppress these side reactions. In this context, we proposed the use of low-cost and widely available difluoroalkyl halides as fluoroalkyl sources for transition-metal-catalyzed difluoroalkylation reactions via cross-coupling. In this Account, we summarize our major efforts on copper-, palladium-, and nickel-catalyzed difluoroalkylations of aromatics with low-cost and widely available difluoroalkyl halides as fluoroalkyl sources. Four modes of catalytic difluoroalkylation reactions, including nucleophilic difluoroalkylation, electrophilic difluoroalkylation, radical difluoroalkylation, and metal-difluorocarbene coupling (MeDiC), have been demonstrated through careful modulation of the catalytic systems. Among these reactions, the MeDiC reaction represents a new mode of fluoroalkylation. These processes enable difluoroalkylation of a variety of aryl halides and arylboron reagents under mild reaction conditions. A wide range of difluoroalkyl halides, including activated difluoroalkyl halides (Cl/BrCF₂R, R = π system), unactivated difluoroalkyl halides (BrCF₂R, R = alkyl, H), and especially the inert and inexpensive industrial chemical chlorodifluoromethane (ClCF₂H), are applicable to these reactions, providing straightforward and facile routes to a diverse range of difluoroalkylated (hetero)arenes. These difluoroalkyl halide-based strategies can also be applied to prepare difluoroalkylated alkenes, alkynes, and alkanes and feature impressive advantages over conventional methods for the synthesis of difluoroalkylated compounds in terms of synthetic efficiency, functional group tolerance, and structural diversity. In particular, the late-stage difluoroalkylation of bioactive molecules through these processes offers good opportunities for the synthesis and development of new medicinal agents without the need for multistep de novo syntheses.

Nickel-catalyzed direct difluoromethylation of aryl boronic acids with BrCF2H

Nickel-catalyzed direct difluoromethylation of aryl boronic acids has been developed with the industrial raw material BrCF₂H.

A convenient approach to difluoromethylated all-carbon quaternary centers via Ni(ii)-catalyzed enantioselective Michael addition

A Ni(ii)-catalyzed enantioselective Michael addition of 2-acetyl azarenes with β-difluoromethyl substituted nitroalkenes was successfully realized, which afforded chiral CF₂H-containing compounds in good enantioselectivities (up to 93% ee). This protocol provides a new convenient approach to all-carbon quaternary stereogenic centers featuring a CF₂H group.

Ni(ii)-catalyzed enantioselective Michael addition afforded compounds with all-carbon quaternary stereogenic centers featuring a CF₂H group in good enantioselectivities.

Cobalt-Catalyzed Cross-Coupling of α-Bromo Amides with Grignard Reagents

A cobalt-catalyzed cross-coupling between α-bromo amides and Grignard reagents is disclosed. The reaction is general and allows access to a large variety of α-aryl and β,γ-unsaturated amides. Some mechanistic investigations have been undertaken to determine the nature of the intermediate species.

Highly selective nickel-catalyzed gem-difluoropropargylation of unactivated alkylzinc reagents

In spite of the important applications of difluoroalkylated molecules in medicinal chemistry, to date, the reaction of difluoroalkylating reagents with unactivated, aliphatic substrates through a controllable manner remains challenging and has not been reported. Here we describe an efficient nickel-catalyzed cross-coupling of unactivated alkylzinc reagen\ts with gem-difluoropropargyl bromides. The reaction proceeds under mild reaction conditions with high efficiency and excellent regiochemical selectivity. Transformations of the resulting difluoroalkylated alkanes lead to a variety of biologically active molecules, providing a facile route for applications in drug discovery and development. Preliminary mechanistic studies reveal that an alkyl nickel intermediate [Ni(tpy)alkyl] (tpy, terpyridine) is involved in the catalytic cycle.

Difluoroalkylated compounds find wide application in medicinal chemistry. Here, the authors describe the mild nickel-catalyzed difluoroalkylation of aliphatic alkylzinc reagents with gem-difluoropropargylbromides and show its utility in a number of post-synthetic transformations.

Visible-Light-Driven, Radical-Triggered Tandem Cyclization of o-Hydroxyaryl Enaminones: Facile Access to 3-CF2 /CF3-Containing Chromones

A practical and straightforward synthetic route to construct a variety of 3-CF₂/CF₃-containing chromones via photoredox catalysis was developed. This novel protocol features a visible-light-induced radical-triggered tandem cyclization.