Carboxylate-Assisted Formation of Aryl-Co(III) Masked-Carbenes in Cobalt-Catalyzed C–H Functionalization with Diazo Esters

Herein we describe the synthesis of a family of aryl-Co­(III)-carboxylate complexes and their reactivity with ethyl diazoacetate. Crystallographic, full spectroscopic characterization, and theoretical evidence of unique C-metalated aryl-Co­(III) enolate intermediates is provided, unraveling a carboxylate-assisted formation of aryl-Co­(III) masked-carbenes. Moreover, additional evidence for an unprecedented Co­(III)-mediated intramolecular SN2-type C–C bond formation in which the carboxylate moiety acts as a relay is disclosed. This novel strategy is key to tame the hot reactivity of a metastable Co­(III)-carbene and elicit C–C coupling products in a productive manner.