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

Abstract

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(sp)-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.

Cobalt‐Catalyzed Annulation Reactions via C−H Bond Activation

“…Recently, extensive mechanistic studies and isolation of possible cobaltacycles in bidentate‐directing group assisted C−H bond activations were reported by Ribas and Maiti . The details of these reports are not discussed here.…”

Model Macrocyclic Ligands for Proof‐of‐Concept Mechanistic Studies in Transition‐Metal Catalysis

“…We realized that the square planar geometrical constraints imposed by the triazamacrocyclic substrates were not suitable to accommodate the octahedral geometry imposed by d 6 electronic configuration of the Co III center. Therefore, we designed another smaller arene‐containing macrocycle bearing two secondary amines and a pyridine moiety, thus enforcing a O h coordination at Co with two cis ‐labile sites (Scheme ) . The use of this model arene substrate allowed us i) to determine that the C−H activation step occurs at Co III , ii) to obtain the spectroscopic signature of a well‐defined octahedral aryl–Co III and iii) to study its reactivity with phenylacetylenes.…”

“… Intramolecular C−H bond activation of a small macrocyclic substrate by Co II under aerobic condition to form the O h aryl–Co III and subsequent differing reactivity with internal and terminal alkynes …”

“…Therefore, we designeda nother smaller arene-containing macrocycle bearing two secondary amines and ap yridine moiety, thus enforcing aO h coordination at Co with two cislabile sites (Scheme 15). [30] The use of this model arene substrate allowed us i) to determine that the CÀHa ctivation step occurs at Co III ,i i) to obtain the spectroscopic signature of a well-defined octahedral aryl-Co III and iii)tos tudy its reactivity Scheme12. a) Formation of ad icationic square-planar Ag III complex by C sp2 ÀBr oxidative addition followedb yA g ÀBr bond precipitation,a nd b) catalyticC ÀX (X = O, C) bond formation taking advantageoft he Ag I /Ag III 2e À redox chemistry.…”

“…Intramolecular CÀHb ond activationofasmall macrocyclic substrate by Co II under aerobic condition to form the O h aryl-Co III and subsequent differingr eactivity with internal and terminal alkynes. [30] with phenylacetylenes. It is worth mentioning that at ertiary amine N3C ligand analoguew as used in 2015 by Mirica to stabilize organometallic high valentn ickel complexes.…”