Stabilization of Co~i by Zn~(II) in pure acetonitrile and its reaction with aryl halides

摘要

The study of the electrochemical behavior of cobalt(II) bromide (CoBr)_2 in pure acetonitrile allowed us to demonstrate that Co~(2+) is the catalyst precursor involved in the electrochemical and chemical conversions of arylhalides, ArX, to arylzinc compounds in that solvent. The reduction of Co~(2+) leads to the Co~+ species, which disproportionates too rapidly to react further with aryl halides. However, the presence of zinc(II) bromide allows us to stabilize the electrogenerated cobalt(I) and to observe it on the timescale of slow cyclic voltammetry. Under such conditions, the Co~I species has time to react with aryl halides and produce [Co~(III)ArX]~+ complexes that are reduced into [Co~(II)ArX] by a single electron uptake at the same potential at which Co~(2+) is reduced. Rate constants for the oxidative addition of ArX to Co~I have been determined for various aryl halides and compared to the values obtained in an acetonitrile (ACN)/pyridine (9:1, v/v) mixture. It is shown that Co~I is stabilized more by ZnBr_2 than by pyridine. A transmetallation reaction between [Co~(II)ArX] and ZnBr_2 has also been observed. We finally propose a mechanism for the cobalt-catalyzed electrochemical conversion of aryl bromides into organozinc species in pure acetonitrile.