No abstract

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 H as the sole byproduct.

Electrocatalysis has been identified as a powerful strategy for organometallic catalysis, and yet electrocatalytic C-H activation is restricted to strongly N-coordinating directing groups. The first example of electrocatalytic C-H activation by weak O-coordination is presented, in which a versatile ruthenium(II) carboxylate catalyst enables electrooxidative C-H/O-H functionalization for alkyne annulations in the absence of metal oxidants; thereby exploiting sustainable electricity as the sole oxidant. Mechanistic insights provide strong support for a facile organometallic C-H ruthenation and an effective electrochemical reoxidation of the key ruthenium(0) intermediate.

Sequential twofold meta-C–H/ortho-C–H functionalization was achieved by means of versatile ruthenium­(II) biscarboxylate catalysis. The double C–H activation proved viable in a one-pot fashion with the assistance of synthetically useful imidates. The operationally simple twofold C–H functionalization occurred with high levels of positional selectivity control and was conducted in a nonsequential manner by the judicious choice of the reaction temperature. Detailed experimental mechanistic studies, including unprecedented electron paramagnetic resonance (EPR) experiments, provided strong support for homolytic C–X bond cleavage and facile C–H ruthenation, while a computational density functional theory (DFT) analysis was supportive of a novel mechanistic scenario involving synergistic catalysis via cyclometalated ruthenium­(III) complexes as key intermediates.

The full control of positional selectivity is of prime importance in C–H activation technology. Chelation assistance served as the stimulus for the development of a plethora of ortho-selective arene functionalizations. In sharp contrast, meta-selective C–H functionalizations continue to be scarce, with all ruthenium-catalysed transformations currently requiring difficult to remove or modify nitrogen-containing heterocycles. Herein, we describe a unifying concept to access a wealth of meta-decorated arenes by a unique arene ligand effect in proximity-induced ruthenium(II) C–H activation catalysis. The transformative nature of our strategy is mirrored by providing a step-economical entry to a range of meta-substituted arenes, including ketones, acids, amines and phenols—key structural motifs in crop protection, material sciences, medicinal chemistry and pharmaceutical industries.