Subject cobalt-carbon bond

More recently, van der Donk and coworkers reported radical cyclizations catalyzed by vitamin B12 using titanium(III) citrate as a stoichiometric reducing agent (Fig. 69, entry 17) [330]. Here /V-allylic 2-(isopropenyl) pyrroles 290 or allyl 2-phenylallyl ethers serve as the starting material in tandem hydrocobaltation/ radical 5-exo cyclization sequences giving dihydropyrrolizine derivatives 291 or 292. The mechanistic course is not completely clear. However, it is assumed that the reactions start with an initial hydrocobaltation of the isopropenyl unit in the presence of the allylic alkene (see Sect. 5.7). The benzylic cobalt intermediate is subject to homolysis of the very weak cobalt-carbon bond and initiates the radical 5-exo cyclization. Interestingly, the fate of the cyclized radical is dependent on the... 

These complexes, all of which contain cobalt within a macrocyclic ligand and an axial cobalt-carbon bond, have been found to be photosensitive and have been the subject of numerous photochemical studies. Space does not permit a detailed summary of all the studies which have been conducted on these classes of compounds, and we present here only a general summary of the various observations and discuss pertinent articles from the most recent literature. The reader is referred to an excellent review of the subject by Koerner von Gustorf et al. 108) which presents a detailed discussion of reports that appeared prior to 1975. 

The first common step in AdoCbl-dependent readions is homolytic cleavage of the cobalt-carbon bond to generate a radical pair, cob(ii)alamin and the carbon-centered dAdo radical (Scheme 19.3). This reaction experiences a 10 -fold rate enhancement in B12 enzymes [14, 15] in the presence of substrate, and the mechanism for this rate acceleration has been the subject of extensive scrutiny. Thus, in methylmalonyl-CoA mutase and in glutamate mutase, little if any destabilization of the cobalt-carbon bond is observed in the reactant state, as revealed by resonance Raman spectroscopy [16, 17], and the intrinsic substrate binding is utilized to labilize the bond. In contrast, approximately half of the destabilization of the cobalt-carbon bond in diol dehydratase is expressed in the reactant state. This re-... 

The word cluster in this section will refer to complexes of cobalt containing three or more metals for which there is, moreover, evidence for the existence of metal-metal bonds in the complex. In addition, for such compounds to be reported in this section, the otganometallic nature of the cluster (the direct interaction between one of the cobalt atoms and at least one carbon atom) must be present. In fact, purely inorganic cobalt clusters became more numerous until recently. Some examples will be described at the end of this section however, these compounds will not be coveted in any comprehensive fashion. Polynuclear clusters, which contain other metal atoms associated to cobalt, are the subjects of another chapter of this work. 

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