Nickel‐Catalyzed N ‐Arylation of NH ‐Sulfoximines with Aryl Halides via Paired Electrolysis

Photoelectrochemical Ni-catalyzed cross-coupling of aryl bromides with amine at ultra-low potential

Photoelectrochemical (PEC) cell is an ideal platform for organic transformation because of its green benefits and minimal energy consumption. As an emerging methodology, the reaction types of photoelectrocatalytic organic synthesis (PECOS) are limited to simple oxidation and C-H activation at current stage. Metal catalysis for the construction of C(sp2)-N bonds has not been touched yet in PECOS. We introduce here a PEC method that successfully engages Ni catalysis for the mild production of aniline derivatives. Experimental and computational investigations elucidate that the addition of photoanode-generated amine radical to Ni catalyst avoids the sluggish nucleophilic attack, enabling the reaction to proceed at an ultra-low potential (-0.4 V vs. Ag/AgNO3) and preventing the overoxidation of products in conventional electrochemical synthesis. This synergistic catalysis strategy exhibits good functional group tolerance and wide substrate scope on both aryl halides and amines, by which some important natural products and pharmaceutical chemicals have been successfully modified.

Counter Electrode Reactions-Important Stumbling Blocks on the Way to a Working Electro-organic Synthesis

Over the past two decades, electro-organic synthesis has gained significant interest, both in technical and academic research as well as in terms of applications. The omission of stoichiometric oxidizers or reducing agents enables a more sustainable route for redox reactions in organic chemistry. Even if it is well-known that every electrochemical oxidation is only viable with an associated reduction reaction and vice versa, the relevance of the counter reaction is often less addressed. In this Review, the importance of the corresponding counter reaction in electro-organic synthesis is highlighted and how it can affect the performance and selectivity of the electrolytic conversion. A selection of common strategies and unique concepts to tackle this issue are surveyed to provide a guide to select appropriate counter reactions for electro-organic synthesis.

Electrochemical Amidation: Benzoyl Hydrazine/Carbazate and Amine as Coupling Partners

An electrochemical amidation of benzoyl hydrazine/carbazate and primary/secondary amine as coupling partners via concomitant cleavage and formation of C(sp²)-N bonds has been achieved. This methodology proceeds under metal-free and exogenous oxidant-free conditions producing N₂ and H₂ as byproducts. Mechanistic studies reveal the in situ generations of both acyl and N-centered radicals from benzoyl hydrazines and amines. The utility of this protocol is demonstrated through a large-scale, and synthesis of bezafibrate, a hyperlipidemic drug.

Mapping Dual-Base-Enabled Nickel-Catalyzed Aryl Amidations: Application in the Synthesis of 4-Quinolones

The C-N cross-coupling of (hetero)aryl (pseudo)halides with NH substrates employing nickel catalysts and organic amine bases represents an emergent strategy for the sustainable synthesis of (hetero)anilines. However, unlike protocols that rely on photoredox/electrochemical/reductant methods within Ni^I/III cycles, the reaction steps that comprise a putative Ni^0/II C-N cross-coupling cycle for a thermally promoted catalyst system using organic amine base have not been elucidated. Here we disclose an efficient new nickel-catalyzed protocol for the C-N cross-coupling of amides and 2'-(pseudo)halide-substituted acetophenones, for the first time where the (pseudo)halide is chloride or sulfonate, which makes use of the commercial bisphosphine ligand PAd2-DalPhos (L4) in combination with an organic amine base/halide scavenger, leading to 4-quinolones. Room-temperature stoichiometric experiments involving isolated Ni^{0, I, and II} species support a Ni^0/II pathway, where the combined action of DBU/NaTFA allows for room-temperature amide cross-couplings.

Chemoselective, Scalable Nickel-Electrocatalytic O-Arylation of Alcohols

The formation of aryl-alkyl ether bonds through cross coupling of alcohols with aryl halides represents a useful strategic departure from classical SN 2 methods. Numerous tactics relying on Pd-, Cu-, and Ni-based catalytic systems have emerged over the past several years. Herein we disclose a Ni-catalyzed electrochemically driven protocol to achieve this useful transformation with a broad substrate scope in an operationally simple way. This electrochemical method does not require strong base, exogenous expensive transition metal catalysts (e.g., Ir, Ru), and can easily be scaled up in either a batch or flow setting. Interestingly, e-etherification exhibits an enhanced substrate scope over the mechanistically related photochemical variant as it tolerates tertiary amine functional groups in the alcohol nucleophile.

Low-Temperature Nickel-Catalyzed C-N Cross-Coupling via Kinetic Resolution Enabled by a Bulky and Flexible Chiral N-Heterocyclic Carbene Ligand

The transition-metal-catalyzed C-N cross-coupling has revolutionized the construction of amines. Despite the innovations of multiple generations of ligands to modulate the reactivity of the metal center, ligands for the low-temperature enantioselective amination of aryl halides remain a coveted target of catalyst engineering. Designs that promote one elementary reaction often create bottlenecks at other steps. We here report an unprecedented low-temperature (as low as -50 °C), enantioselective Ni-catalyzed C-N cross-coupling of aryl chlorides with sterically hindered secondary amines via a kinetic resolution process (s factor up to >300). A bulky yet flexible chiral N-heterocyclic carbene (NHC) ligand is leveraged to drive both oxidative addition and reductive elimination with low barriers and control the enantioselectivity. Computational studies indicate that the rotations of multiple σ-bonds on the C₂ -symmetric chiral ligand adapt to the changing needs of catalytic processes. We expect this design would be widely applicable to diverse transition states to achieve other challenging metal-catalyzed asymmetric cross-coupling reactions.