The Manganese-Catalyzed Cross-Coupling Reaction

Herein it is presented the MnCl₂-catalyzed cross-coupling reaction between aryl halides and Grignard reagents. Aryl chlorides containing a cyano or an ester group in the para or ortho position react smoothly and in good yield. A variety of alkyl- and arylmagnesium chlorides can be used in this cross-coupling reaction. The mechanism of the cross-coupling is believed to proceed by a SRN₁ mechanism and radical clock experiments were performed in order to elucidate this pathway. A triorganomanganate complex is believed to be formed by the reaction between the organomagnesium halide and manganese chloride, and it serves both as the nucleophile and the single electron donor. Other mechanistic hypotheses were excluded on the basis of the results of the performed experiments.
An improved protocol has been developed for the manganese catalyzed cross-coupling of two arylmagnesium bromides under an atmosphere of dioxygen. The reaction is performed with a 2:1 ratio between the Grignard reagents and 20% of MnCl₂. When the limiting Grignard regent undergoes little homo-coupling under the reaction conditions, very good yields of the hetero-coupling product can be achieved. Arylmagnesium bromides with 4-methoxy, 4-dimethylamino, 4-fluoro and 4-chloro substituents give high yields in the cross-coupling while heterocyclic Grignard reagents turned out to be poor substrates for the reaction.