Cobalt ring cleavage reactions

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. 

The proposed mechanism, which is based on the double carbonylation of styrene oxide, is shown in Scheme 6.2. The generation of an acylcobalt carbonyl complex from the reaction of cobalt tetracarbonyl anion with an alkyl halide is followed by reaction with a thiirane. This species can undergo carbonylation, the thioester function can undergo hydrolysis to reveal a sulfido nucleophile, and intramolecular cydization then produces thietan-2-one. The thietan-2-one can undergo ring cleavage and the mercapto acid results by protonahon. 

This result suggests that diene itself is not involved in the cleavage reaction. A TT-allyl-metal species w ith agostic interaction of H with Co is the key intermediate. The same type of reaction also proceeds in 5-membered ring compounds on Co (Eq. 14) [23]. A simple cobalt rr-allyl complex reacted w ith 2-bu-tyne to give a 7-membered ring cobalt complex where one carbon and two carbons of the allyl moiety were separated in the final product [11]. The mechanism is not yet clear. 

In the presence of nickel(0), tethered diene-VCPs react to produce eight- and five-membered ring products (Scheme 2). Palladium(O) and cobalt(m) were also tried but produced only decomposition products. However, in the presence of Wilkinson s catalyst (RhCl(PPh3)3), tethered diene-VCP 1 was cleanly converted to triene 4 in 91% yield. Although the desired cycloaddition reaction was not obtained, the cleavage of the cyclopropane ring was encouraging.22... 

Examination of the reaction products indicated that the primary products of reaction were probably butadiene and H2S. The rates of hydrogenation of butadiene and butene were found to be consistent with the amounts appearing in the reaction products (provided, in the case of cobalt molybdate catalyst, that H2S was present to simulate reaction conditions). The results support the view that C-S bond cleavage is the first step in thiophene desulfurization, rather than hydrogenation of the ring. 

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