C–H Activation by Isolable Cationic Bis(phosphine) Cobalt(III) Metallacycles

Abstract

Five- and six-coordinate cationic bis­(phosphine) cobalt­(III) metallacycle complexes were synthesized with the general structures, [(depe)­Co­(cycloneophyl)­(L)­(L′)]­[BArF 4] (depe = 1,2-bis­(diethylphosphino)­ethane; cycloneophyl = [κ-C:C-(CH2C­(Me)2)­C6H4]; L/L′ = pyridine, pivalonitrile, or the vacant site, BAr4 F = B­[(3,5-(CF3)2)­C6H3]4). Each of these compounds promoted facile directed C­(sp2)–H activation with exclusive selectivity for ortho-alkylated products, consistent with the selectivity of reported cobalt-catalyzed arene-alkene-alkyne coupling reactions. The direct observation of C–H activation by cobalt­(III) metallacycles provided experimental support for the intermediacy of these compounds in this class of catalytic C–H functionalization reaction. Deuterium labeling and kinetic studies provided insight into the nature of C–H bond cleavage and C–C bond reductive elimination from isolable cobalt­(III) precursors.

Mechanism-Driven Development of N-(Quinolin-8-yl)-benzamide Coupling Reactions via C–H or N–H Activation

“…The choice of cobalt among the possible metal catalysts for ligand-directed functionalization was determined by its comparative cheapness and availability as well as by its good catalytic activity in C–H functionalizations, both chemical and electrochemical. − Putatively characterized key intermediates, cobaltacycles, or other cobalt structures are still rare and are given in the Introduction (Scheme ) [e.g., in ref ].…”

“…Ligand-directed transformations are more selective since they proceed through metallacyclic intermediates which have been isolated and characterized in rare cases or through noncovalent coordination. Although a number of metallacyclic structures have been proved for noble metals, fully characterized cobaltacycles are still rare …”

Further Exploration of Aza-Cobalt-Cyclobutenes on CoIII(TIM) Complexes: Reactivity and Spectroelectrochemistry

“…In addition, the facile syntheses of the new compounds 3 and 4 reveal promising opportunities to further explore the coordination of other small-molecules to the aza-cobalt-cyclobutene bearing complexes. Recently, directed C­( sp 2 )–H activation by Co III metallacycle complexes were demonstrated with high selectivity for ortho-alkylation . Inspired by both the activity of Co III metallacycle and recent developments in Co macrocyclic complexes as photocatalysts , and copolymerization catalysts, current efforts are underway to explore the incorporation of photocatalytic activity.…”

“…Recently, directed C(sp 2 )−H activation by Co III metallacycle complexes were demonstrated with high selectivity for ortho-alkylation. 33 Inspired by both the activity of Co III metallacycle and recent developments in Co macrocyclic complexes as photocatalysts 34,35 and copolymerization catalysts, 36 current efforts are underway to explore the incorporation of photocatalytic activity.…”

Iron/Photosensitizer-Catalyzed Directed C–H Activation Triggered by the Formation of an Iron Metallacycle

“…We focused on a metallacycle-mediated C–H activation strategy, where the cleavage of a C–H bond via σ-CAM (σ-complex-assisted metathesis) is triggered by an in situ generated metallacycle formation . Several catalytic directed C–H activation reactions based on this strategy have been reported for Rh and Co catalysts, while Fe catalyst has never been used. Here, we report the iron-catalyzed C­(sp 2 )–H alkenylation of benzamides with 1,6-diynes using an iron/photosensitizer hybrid catalyst (Scheme c).…”