Nickel-Catalyzed Electrooxidative C-H Amination: Support for Nickel(IV).

Nickel-catalyzed electrochemical C-H aminations were accomplished by chemo- and position-selective C-H activation with ample scope. Detailed mechanistic studies highlighted a facile C-H cleavage with unique chemo-selectivity, while cyclovoltammetric analysis provided support for a nickel(II/III/IV) manifold.

Electrochemical Cobalt-Catalyzed C-H Activation.

Carbon-heteroatom bonds represent omnipresent structural motifs of the vast majority of functionalized materials and bioactive compounds. C-H activation has emerged as arguably the most efficient strategy to construct C-Het bonds. Despite of major advances, these C-H transformations were largely dominated by precious transition metal catalysts, in combination with stoichiometric, toxic metal oxidants. Herein, we discuss the recent evolution of cobalt-catalyzed C-H activations that enable C-Het formations with electricity as the sole sustainable oxidant until May 2018.

Electroremovable Traceless Hydrazides for Cobalt-Catalyzed Electro-Oxidative C-H/N-H Activation with Internal Alkynes.

Electrochemical oxidative C-H/N-H activations have been accomplished with a versatile cobalt catalyst in terms of [4 + 2] annulations of internal alkynes. The electro-oxidative C-H activation manifold proved viable with an undivided cell setup under exceedingly mild reaction conditions at room temperature using earth-abundant cobalt catalysts. The electrochemical cobalt catalysis prevents the use of transition metal oxidants in C-H activation catalysis, generating H2 as the sole byproduct. Detailed mechanistic studies provided strong support for a facile C-H cobaltation by an initially formed cobalt(III) catalyst. The subsequent alkyne migratory insertion was interrogated by mass spectrometry and DFT calculations, providing strong support for a facile C-H activation and the formation of a key seven-membered cobalta(III) cycle in a regioselective fashion. Key to success for the unprecedented use of internal alkynes in electrochemical C-H/N-H activations was represented by the use of N-2-pyridylhydrazides, for which we developed a traceless electrocleavage strategy by electroreductive samarium catalysis at room temperature.

Electrooxidative Rhodium-Catalyzed C-H/C-H Activation: Electricity as Oxidant for Cross-Dehydrogenative Alkenylation.

Rhodium(III) catalysis has enabled a plethora of oxidative C-H functionalizations, which predominantly employ stoichiometric amounts of toxic and/or expensive metal oxidants. In contrast, we herein describe the first electrochemical rhodium-catalyzed C-H activation that avoids hazardous chemical oxidants. Environmentally benign twofold C-H/C-H functionalizations were accomplished with weakly coordinating benzoic acids and benzamides, employing electricity as the terminal oxidant and generating H2 as the sole byproduct.

Electrochemical C-H Amination by Cobalt Catalysis in a Renewable Solvent.

Syntheses of substituted anilines primarily rely on palladium-catalyzed coupling chemistry with prefunctionalized aryl electrophiles. While oxidative aminations have emerged as powerful alternatives, they largely produce undesired metal-containing by-products in stoichiometric quantities. In contrast, described herein is the unprecedented electrochemical C-H amination by cobalt-catalyzed C-H activation. The environmentally benign electrocatalysis avoids stoichiometric metal oxidants, can be conducted under ambient air, and employs a biomass-derived, renewable solvent for sustainable aminations in an atom- and step-economical manner with H2 as the sole byproduct.

Electrochemical Cobalt-Catalyzed C-H Oxygenation at Room Temperature.

Electrochemical cobalt-catalyzed C-H functionalizations were achieved in terms of C-H oxygenation under mild conditions at 23 °C. The robust electrochemical C-H functionalization was characterized by ample substrate scope, whereas mechanistic studies provided support for a facile C-H cleavage. The electrochemical cobalt-catalyzed C-H oxygenation proved viable on arenes and alkenes with excellent levels of positional and diastereo-selectivity, avoiding the use of stoichiometric silver(I) oxidants under ambient conditions.

Manganese-Catalyzed C-H Alkynylation: Expedient Peptide Synthesis and Modification.

Manganese(I)-catalyzed C-H alkynylations with organic halides occurred with unparalleled substrate scope, and thus enabled step-economical C-H functionalizations with silyl, aryl, alkenyl, and alkyl haloalkynes. The versatility of the manganese(I) catalysis manifold enabled C-H couplings with haloalkynes featuring, among others, fluorescent labels, steroids, and amino acids, thereby setting the stage for peptide ligation as well as the efficient molecular assembly of acyclic and cyclic peptides. A plausible catalytic cycle was proposed.

Cobalt(III)-Catalyzed C-H Alkynylation with Bromoalkynes under Mild Conditions.

Expedient cobalt-catalyzed C-H alkynylation was achieved under exceedingly mild reaction conditions. Thus, chelation-assisted direct alkynylations of heteroarenes occurred with 1-bromoalkynes and ample substrate scope. The optimized catalytic system allowed for step-economical C-H functionalizations with a mild base K2CO3 at reaction temperatures as low as 25 °C.

C-H alkenylations with alkenyl acetates, phosphates, carbonates, and carbamates by cobalt catalysis at 23 °C.

Inexpensive cobalt catalysts with N-heterocyclic carbene ligands enable direct arene alkenylations with easily accessible alkenyl acetates through regioselective C-H/C-O functionalizations in a stereoconvergent fashion. The versatile cobalt catalyst was broadly applicable and thus also allowed for the efficient conversion of alkenyl phosphates, carbonates, and carbamates at ambient temperature.