Cobalt(III) Complexes of Pyridine-Functionalized Chelating NHC Ligands: Effective Catalysts for the Olefination of Alcohols Using Sulfones

Several air-stable cobalt(III) complexes 1a-d featuring pyridine-functionalized NHC ligand systems were synthesized and completely characterized. These complexes were then explored as phosphine-free catalysts for the olefination of alcohols with sulfones, and among the complexes studied here, the benzimidazolylidene-supported 1b complex with acetylacetonate (acac) as a labile co-ligand was noted to be the most active one, which was also successful for a broad range of sulfones and alcohols providing diverse olefins. It is important to mention that this is the first example of such a protocol catalyzed by cobalt. Furthermore, the utility of the present protocol was demonstrated by a gram-scale reaction and the postsynthetic modifications of the obtained compounds. Finally, based on various control experiments including the detection of intermediates and kinetic studies, a plausible mechanism is proposed.