Ternary catalytic systems consisting of cobalt salts, phosphine ligands, and Grignard reagents promote addition of arylpyridines and imines to unactivated internal alkynes with high regio- and stereoselectivities. Deuterium-labeling experiments suggest that the reaction involves chelation-assisted oxidative addition of the aryl C-H bond to the cobalt center and insertion of the C-C triple bond into the Co-H bond, followed by reductive elimination of the resulting diorganocobalt species.
A quaternary catalytic system consisting of a cobalt salt, a triarylphosphine ligand, a Grignard reagent, and pyridine has been developed for chelation-assisted C-H bond activation of an aromatic imine, followed by insertion of an unactivated internal alkyne that occurs at ambient temperature. The reaction not only tolerates potentially senstitive functional groups (e.g., Cl, Br, CN, and tertiary amide), but also displays a unique regioselectivity. Thus, the presence of substituents such as methoxy, halogen, and cyano groups at the meta-position of the imino group led to selective C-C bond formation at the more sterically hindered ortho positions. Under acidic conditions, the hydroarylation products of dialkyl- and alkylarylacetylenes underwent cyclization to afford benzofulvene derivatives, while those of diarylacetylenes afforded the corresponding ketones in moderate to good yields. A mechanistic investigation into the reaction with the aid of deuterium-labeling experiments and kinetic analysis has indicated that oxidative addition of the ortho C-H bond is the rate-limiting step of the reaction. The kinetic analysis has also shed light on the complexity of the quaternary catalytic system.
A cobalt-chiral phosphoramidite catalyst promotes enantioselective imine-directed C2-alkylation of Boc-protected indoles with styrenes. The reaction affords 1,1-diarylethane products in moderate to good yields with good enantioselectivities under mild conditions. A deuterium-labeling experiment suggests that the enantioselectivity is controlled by both the styrene insertion and the C-C reductive elimination steps.
An ortho-arylation reaction of aromatic imines with aryl chlorides has been achieved using a cobalt-N-heterocyclic carbene catalyst in combination with a neopentyl Grignard reagent. The reaction takes place at room temperature to afford biaryl products in moderate to good yields.
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