C(sp3)–H Bond Acylation with N-Acyl Imides under Photoredox/ Nickel Dual Catalysis

A General Direct Aldehyde C-H Alkylation via TBADT-Nickel Synergistic Catalysis

Herein, we present a nickel/tetrabutylammonium decatungstate (TBADT)-catalyzed protocol for general C-H alkylation of aldehydes, enabling the efficient synthesis of aliphatic ketones through ligand-controlled cross-coupling. This mild and cost-effective methodology demonstrates broad substrate compatibility with various commercially available aldehydes and both activated and unactivated alkyl bromides, delivering target products in high yields. Notably, the practical utility of this catalytic system has been highlighted through a concise two-step synthesis of the commercially valuable musk odorant Aurelione from readily available starting materials.

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.

Stereoretentive cross-coupling of chiral amino acid chlorides and hydrocarbons through mechanistically controlled Ni/Ir photoredox catalysis

The direct modification of naturally occurring chiral amino acids to their amino ketone analogs is a significant synthetic challenge. Here, an efficient and robust cross-coupling reaction between chiral amino acid chlorides and unactivated C(sp³)–H hydrocarbons is achieved by a mechanistically designed Ni/Ir photoredox catalysis. This reaction, which proceeds under mild conditions, enables modular access to a wide variety of chiral amino ketones that retain the stereochemistry of the starting amino acids. In-depth mechanistic analysis reveals that the strategic generation of an N-acyllutidinium intermediate is critical for the success of this reaction. The barrierless reduction of the N-acyllutidinium intermediate facilitates the delivery of chiral amino ketones with retention of stereochemistry. This pathway avoids the formation of a detrimental nickel intermediate, which could be responsible for undesirable decarbonylation and transmetalation reactions that limit the utility of previously reported methods.

Chiral α-amino ketones are privileged motifs in bioorganic and medicinal chemistry. Here, the authors develop an efficient method to synthesize these structures via stereoretentive direct cross-coupling of amino acid chlorides with simple aliphatic substrates.

Nickel-Mediated Photoreductive Cross Coupling of Carboxylic Acid Derivatives for Ketone Synthesis

A simple visible light photochemical, nickel-catalyzed synthesis of ketones from carboxylic acid-derived precursors is presented. Hantzsch ester (HE) functions as a cheap, green and strong photoreductant to facilitate radical generation and also engages in the Ni-catalytic cycle to restore the reactive species. With this dual role, HE allows for the coupling of a large variety of radicals (1°,2°, benzylic, α-oxy & α-amino) with aroyl and alkanoyl moieties, a new feature in reactions of this type. With both precursors deriving from abundant carboxylic acids, this protocol is a welcome addition to the organic chemistry toolbox. The reaction proceeds under mild conditions without the need for toxic metal reagents or bases and shows a wide scope, including pharmaceuticals and complex molecular architectures.