Cobalt cleavage reaction

Shortly after Footes initial report on cobalt catalyzed reactions (see Sect. 5.8), Noyori and coworkers found that Ru(PPh)3Cl2 catalyzes the reductive cleavage of endoperoxides (Fig. 47) [152, 259]. While 1,2-dioxanes provided a mixture of... 

It is generally believed that the cobalt-carbon a-bond is rather unstable and reactive. However, cobaloximes, model complexes of vitamin B12, form simple alkylcobaloximes by several ways, such as reductive alkylation, addition of olefins, and substitution reaction. The cobalt-carbon bonds thus formed are surprisingly stable, and undergo a variety of cleavage reactions, such as/3-elimination, coupling, substitution with nucleophiles, and other reactions. The effect of this specific ligand is remarkable. 

The area of carbon-cobalt bond cleavage reactions is of perhaps the greatest direct relevance to the mechanism of action of the B,2 coenzymes. Thus, while it is not yet clear if new organocobalt intermediates are formed (and decomposed) from substrates and the cobalt-containing moiety of adenosylcobalamin during the enzyme-catalyzed intramolecular rearrangements (see Section 4), it is clear that the activation of adenosylcobalamin, as well as the coenzymic action of methylcobalamin require that the carbon-cobalt bond of the coenzymes be cleaved. 

The hydrolysis of ethylene phosphate in hydroxide ion solution proceeds with a rate constant of 5 x 10" L-mol" s" (100). The O-P-0 angle in the ring of ethylene phosphate of 99° is expected to be rather similar to that for the four-membered ring incorporating the cobalt and phosphorus centers. Therefore it is likely that reaction at the strained P center of the complex is eclipsed by a more rapid metal-ligand cleavage reaction. This problem can be circumvented by the use of metal ion complexes of Ir(III) where the metal-ligand bonds are more inert as the locus for the reaction. 

Khand lU, Knox GR, Pauson PL, Watts WE (1971) A cobalt induced cleavage reaction and a new series of arenecobalt carbonyl complexes. Chem Commun 36a... 

Cleavage, as depicted, would give a PPhj cobalt complex. Such PPh3 would only build up in solution to a small degree since it would compete with the resin for bonding to the Co and would itself be susceptible to the phosphorus-carbon bond cleavage reaction. 

Similar attempted cleavage reactions of the lead-iron derivative with cadmium(II) iodide, copper(II) chloride dihydrate, cobalt(II) chloride hexahydrate, bismuth(III) bromide, and K[Pbl3] failed to give any iron carbonyl derivatives of these metals 198). The black air-sensitive derivative of divalent lead [PbFe(CO)4] has been obtained on treatment of [HFe(CO)4] with basic lead(II) acetate 199). 

---