Stability and reactivity of electrogenerated cobalt(I) towards aryl halides in the presence of additives such as vinyl acetate or methyl vinyl ketone- Application to the electrosynthesis of arylzinc compounds

The electrochemical reduction of CoBr2 in acetonitrile + pyridine (v/v = 9: 1) has been investigated. In this solvent mixture, the disproportionation r eaction rate of the electrogenerated Co(I) is about twice as slow as in dim ethylformamide + pyridine (9:1) and the reaction between Co(I) and arylhali des, ArX, also is slowed down by the same factor. We show that the use of v inyl acetate (vinyl-OAc) or methyl vinyl ketone (MVK) as additives in aceto nitrile + pyridine increases the stability of Co(I). Thus, in the presence of vinyl-OAc the disproportionation reaction rate of Co(I) is divided by se ven whereas the rate of the oxidative addition of Co(I) with aryl halides i s divided only by two. Preparative-scale electrolyses of arylzine compounds confirm this result since the quantity of catalyst, CoBr2, can be divided by four as compared to the amount used in the absence of additives. In the two cases (presence of vinyl-OAc or MVK) the presence of pyridine is requir ed to stabilize Co(I). In these reactions, the rate constants for both chem ical reactions (disproportionation reaction and oxidative addition) have be en determined and the stabilization of Co(I) is discussed in terms of forma tion of pi -complexes with the additives. (C) 2001 Elsevier Science B.V. Al l rights reserved.