Organocobalt intermediate

Porphyrins and chlorophylls are the most widespread natural pigments. They are associated with the energy-converting processes of respiration and photosynthesis in living organisms, and the synthesis of specific porphyrin derivatives is often motivated by the desire to perform similar processes in the test tube. The structurally and biosynthetically related corrins (e.g. vitamin B,j) catalyze alkylations and rearrangements of carbon skeletons via organocobalt intermediates. The biosyntheses of these chromophores are also of topical interest. 

This is an authentic bimetallic phase transfer reaction in which the second metal species intercepts an organocobalt intermediate. 

According to this mechanism the Co(II) of the B12r formed in Eq. 16-35 has no active role in the isomerization and does not form an organocobalt intermediate as in Eq. 16-36. Its only role is to be available to recombine with the 5 -deoxyadenosyl radical at the end of the reaction sequence. Support for this interpretation has been obtained from study of model reactions and of organic radicals generated in other ways.400... 

Since the butenylcobalt complex was too unstable, the PMR spectrum of the allylcobalt complex was studied 21, 22) to obtain information on the structure of the organocobalt intermediates in these stereoselective reductions (20). This investigation indicated that a-bonded allylic complexes are in equilibrium with 7r-allylic complexes and cyanide ion, thus providing a rationale for the stereoselectivities observed in the reduction of butadiene and butenyl chlorides. 

The intermolecular addition reactions between organocobalt reagents 44 and 38 and a variety of deactivated C=C bonds led to new alkene products 45 and 46 respectively which resulted from radical addition to the C=C bonds followed by dehydro-cobaltation from the presumed [20] organocobalt intermediates (Scheme 16). 

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. 

Two types of mechanism have been suggested for the hydrogenation of conjugated alkenes catalysed by [Co(CN)6] In Scheme 16 organocobalt intermediates... 

The hydrohydrazination represented a general solution for the amination of alkenes, but the protected hydrazines obtained are sometimes difficult to transform to the free amines. At this point, we turned to sulfonyl azides as nitrogen sources, based on then-capacity to react both with enolates and carbon-centered radicals. Mechanistic investigations of the hydrohydrazination reaction had suggested a radical character for the formed organocobalt intermediate. " We were pleased to see that the Cobalt-catalyst 4 was able to promote the hydroazidation of 4-phenylbut-l-ene (3) with ethanesulfonyl azide (7), giving the product derived from the formal Markovnikov addition of hydrazoic acid onto the C-C double bond exclusively, albeit in moderate yields (50%). 

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