Highly chemoselective cobalt-catalyzed biaryl coupling reactions

A practical cobalt-catalyzed hetero-biaryl coupling reaction between aryl chlorides and arylmagnesium halides with unprecedented selectivity has been developed. The protocol utilizes 1 mol% of cheap Co(acac)(3) as pre-catalyst and effects clean reactions of deactivated chlorostyrenes with only 1.1 equiv. of the Grignard reagent under mild conditions (30 degrees C, 5-30 min). Highly chemoselective reactions were realized even in the presence of activated bromoarenes. The olefin substituent facilitates the activation of the C-Cl bond by coordination to the catalyst. Kinetic studies indicate the operation of an arylcobaltate(I) catalyst species. Catalyst formation during the induction period was studied in the presence of cobalt(III), (I), and (-I) pre-catalysts.