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

Mechanism and Origin of Nickel-Catalyzed Decarbonylative Construction of C(sp2)-C(sp3) Bonds from Carboxylic Acids and Their Derivatives

Nickel-catalyzed arylation of carboxylic acids provides a ligand-controlled chemoselectivity-switchable method for the construction of C(sp2)-C(sp3) bonds. Here, we employed density functional theory to provide a detailed understanding of the mechanism and origin of nickel-catalyzed ligand-controlled carbonyl transformation. This reaction generates decarbonylation products through oxidative addition, activation of C-C bonds, decarbonylation, binding of alkyl radicals with Ni(III) complexes, and final reduction elimination step. The activation of C-C bonds in aromatic carboxylate esters is more favorable than C-O bond activation because of the interaction between the nickel catalyst and the π orbitals of the substrate's aromatic moiety during C-C bond activation. The induction effect of the ligand and the carbonyl group together determines the transfer tendency of the carbonyl group.

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.

Magnesium-Mediated Cross-Electrophile Couplings of Aryl 2-Pyridyl Esters with Aryl Bromides for Ketone Synthesis through In Situ-Formed Arylmagnesium Intermediates

Aryl 2-pyridyl esters could efficiently undergo cross-electrophile couplings with aryl bromides with the aid of magnesium as a reducing metal in the absence of a transition-metal catalyst, leading to the unsymmetrical diaryl ketones in modest to good yields with wide functionality compatibility. In addition, the reaction could be easily scaled up and applied in the late-stage modification of biologically active molecules. Preliminary mechanistic study showed that the coupling reaction presumably proceeds through the in situ formation of arylmagnesium reagents as key intermediates.

Nickel-catalyzed γ-alkylation of cyclopropyl ketones with unactivated primary alkyl chlorides: balancing reactivity and selectivity via halide exchange

A novel method was developed for synthesizing γ-alkyl ketones via nickel-catalyzed cross-electrophile coupling of cyclopropyl ketones and non-activated primary alkyl chlorides. High reactivity and selectivity can be achieved with sodium iodide as a crucial cocatalyst that generates a low concentration of alkyl iodide via halide exchange, thus avoiding the formation of alkyl dimers. This reaction possessed excellent regioselectivity and high step economy circumventing in situ or pregenerated organometallics.

Ligand Relay for Nickel-Catalyzed Decarbonylative Alkylation of Aroyl Chlorides

Transition metal-catalyzed direct decarboxylative transformations of aromatic carboxylic acids usually require high temperatures, which limit the substrate's scope, especially for late-stage applications. The development of the selective decarbonylative of carboxylic acid derivatives, especially the most fundamental aroyl chlorides, with stable and cheap electrophiles under mild conditions is highly desirable and meaningful, but remains challenging. Herein, a strategy of nickel-catalyzed decarbonylative alkylation of aroyl chlorides via phosphine/nitrogen ligand relay is reported. The simple phosphine ligand is found essential for the decarbonylation step, while the nitrogen ligand promotes the cross-electrophile coupling. Such a ligand relay system can effectively and orderly carry out the catalytic process at room temperature, utilizing easily available aroyl chlorides as an aryl electrophile for reductive alkylation. This discovery provides a new strategy for direct decarbonylative coupling, features operationally simple, mild conditions, and excellent functional group tolerance. The mild approach is applied to the late-stage methylation of various pharmaceuticals. Extensive experiments are carried out to provide insights into the reaction pathway and support the ligand relay process.

Dehalogenative Arylation of Unactivated Alkyl Halides via Electroreduction

Herein, a facile and efficient dehalogenative arylation of unactivated alkyl halides enabled by electrochemical reductive coupling is developed, affording a series of C(sp2)-C(sp3) products in moderate to good yields. This protocol proceeds in the absence of transition metal catalysts and redox mediators. The reaction features mild conditions, broad substrate scope, and high tolerance of functional groups and is demonstrated to be applicable for gram-scale synthesis and late-stage functionalization of natural products.

Formal Cross-Coupling of Amines and Carboxylic Acids to Form sp3-sp2 Carbon-Carbon Bonds

Amines and carboxylic acids are abundant synthetic building blocks that are classically united to form an amide bond. To access new pockets of chemical space, we are interested in the development of amine-acid coupling reactions that complement the amide coupling. In particular, the formation of carbon-carbon bonds by formal deamination and decarboxylation would be an impactful addition to the synthesis toolbox. Here, we report a formal cross-coupling of alkyl amines and aryl carboxylic acids to form C(sp3)-C(sp2) bonds following preactivation of the amine-acid building blocks as a pyridinium salt and N-acyl-glutarimide, respectively. Under nickel-catalyzed reductive cross-coupling conditions, a diversity of simple and complex substrates are united in good to excellent yield, and numerous pharmaceuticals are successfully diversified. High-throughput experimentation was leveraged in the development of the reaction and the discovery of performance-enhancing additives such as phthalimide, RuCl3, and GaCl3. Mechanistic investigations suggest phthalimide may play a role in stabilizing productive Ni complexes rather than being involved in oxidative addition of the N-acyl-imide and that RuCl3 supports the decarbonylation event, thereby improving reaction selectivity.

Formation of C(sp2)-C(sp3) Bonds Instead of Amide C-N Bonds from Carboxylic Acid and Amine Substrate Pools by Decarbonylative Cross-Electrophile Coupling

Carbon-heteroatom bonds, most often amide and ester bonds, are the standard method to link together two complex fragments because carboxylic acids, amines, and alcohols are ubiquitous and the reactions are reliable. However, C-N and C-O linkages are often a metabolic liability because they are prone to hydrolysis. While C(sp2)-C(sp3) linkages are preferable in many cases, methods to make them require different starting materials or are less functional-group-compatible. We show here a new, decarbonylative reaction that forms C(sp2)-C(sp3) bonds from the reaction of activated carboxylic acids (via 2-pyridyl esters) with activated alkyl groups derived from amines (via N-alkyl pyridinium salts) and alcohols (via alkyl halides). Key to this process is a remarkably fast, reversible oxidative addition/decarbonylation sequence enabled by pyridone and bipyridine ligands that, under reaction conditions that purge CO(g), lead to a selective reaction. The conditions are mild enough to allow coupling of more complex fragments, such as those used in drug development, and this is demonstrated in the coupling of a typical Proteolysis Targeting Chimera (PROTAC) anchor with common linkers via C-C linkages.