Electrochemistry of Organocobalt Compounds

Abstract This chapter deals with the electrochemical preparation and reactivity of electrogenerated alkyl‐ and aryl‐cobalt complexes. Alkylcobalts have originally been obtained from the electrochemical reduction of square planar cobalt(II) complexes, mimicking the ability of vitamin B12 to reversibly form cobalt–carbon bonds. In this respect, the electrosynthetic processes involving alkyl halides and Schiff‐base or porphyrin cobalt complexes have been extensively developed on various substrates, allowing reductive dehalogenation of the alkyl halides, their cyclization, carboxylation, etc. The reduction waves of both the cobalt(II) precursors and the alkylcobalt(III) complexes that result from the reaction of electrogenerated cobalt(I) on alkyl halides are observed in the [−1 V vs. SCE/−1.4 V vs. SCE] region, making accurate mechanistic predictions difficult from simple electrolyses. However, thorough electroanalytical investigations have provided robust reaction frameworks, namely, on the stability and reactivity of the alkylcobalt(III) intermediates. The activation of C(sp 2 )–X bonds (aromatic halides) has been observed from mono‐ or bi‐dentate electrogenerated cobalt(I) complexes like those obtained in DMF or acetonitrile mixed with pyridine as cosolvent. In such media, the oxidative addition of cobalt(I) on the C(sp 2 )–X bond competes with the disproportionation of very instable electrogenerated cobalt(I). Aromatic iodides, bromides, and chlorides undergo cobalt‐catalyzed electroreductive coupling with, namely, allyl and vinyl acetates, allyl ethers, olefins. The preparation of stable arylzinc compounds is also achieved in mild conditions. The mechanistic aspects of the reactions between aromatic halides and electrogenerated cobalt(I) have been extensively explored by electroanalytical techniques. The oxidative addition between ArX and cobalt(I) yields arylcobalt(III) intermediates which are immediately reduced into arylcobalt(II) complexes, which are the species responsible for the transmetalation with Zn(II) salts in the formation or arylzincs, or undergo an additional reduction step to yield arylcobalt(I), prone to engage in a new oxidative addition.