On the reactivity of the electrogenerated cobalt(I) species towards aryl halides in the presence of allylethers

The electrochemical behavior of CoBr2 has been investigated in the presence of allylic ethers in an acetonitrile + pyridine (v/v = 9/1) mixture. Cyclic voltametry experiments show a complexation reaction between the electrogenerated cobalt(I) species and allylethers leading thus to a more stable CoI via the formation of a (η2-allyl-OR)CoIBr complex. However, this stabilization is too weak to be observed on the time-scale of cyclic voltametry in the absence of pyridine. The thermodynamic constant for the complexation has been determined. It is also shown that only one molecule of allylether is involved in the stabilisation of CoI. Interestingly, no intra-molecular oxidative addition of allylether to CoI was observed on the time-scale of a preparative electrolysis unless the reaction was carried out in an undivided cell in the presence of a sacrificial iron rod releasing iron(II) ions. Conversely, an oxidative addition has been evidenced between the electrogenerated CoI and aryl halides, ArX. The apparent rate constants for this reaction have been determined for various ArX. Importantly, the rate constants depend on the allylether concentration suggesting that the oxidative addition occurs competitively rather than consecutively with respect to complexation. More importantly, preparative-scale electrolyses of aryl halides and allylphenylether in the presence of CoBr2 as catalyst lead to the coupling product allylaryl, whereas replacement of allylphenylether with allylethylether produces only ArAr along with ArH.