Isolation of Key Organometallic Aryl-Co(III) Intermediates in Cobalt-Catalyzed C(sp²)–H Functionalizations and New Insights into Alkyne Annulation Reaction Mechanisms

The selective annulation reaction of alkynes with substrates containing inert C–H bonds using cobalt as catalyst is currently a topic attracting significant interest. Unfortunately, the mechanism of this transformation is still relatively poorly understood, with little experimental evidence for intermediates, although an organometallic Co­(III) species is generally implicated. Herein, we describe a rare example of the preparation and characterization of benchtop-stable organometallic aryl-Co­(III) compounds (NMR, HRMS, XAS, and XRD) prepared through a C­(sp2)–H activation, using a model macrocyclic arene substrate. Furthermore, we provide crystallographic evidence of an organometallic aryl-Co­(III) intermediate proposed in 8-aminoquinoline-directed Co-catalyzed C–H activation processes. Subsequent insights obtained from the application of our new organometallic aryl-Co­(III) compounds in alkyne annulation reactions are also disclosed. Evidence obtained from the resulting regioselectivity of the annulation reactions and DFT studies indicates that a mechanism involving an organometallic aryl-Co­(III)-alkynyl intermediate species is preferred for terminal alkynes, in contrast to the generally accepted migratory insertion pathway.