Cobaloximes

The reductions are effected in nature by ferredoxin (p. 1102). This behaviour can be reproduced surprisingly well by simpler, model compounds. Some of the best known of these are obtained by the addition of axial groups to the square-planar complexes of Co with Schiff bases, or substituted glyoximes (giving cobaloximes) as illustrated in Fig. 26.7. The reduced Co species of these, along with vitamin... 

Enikolopyan et al.til found that certain Co11 porphyrin complexes (eg. 87) function as catalytic chain transfer agents. Later work has established that various square planar cobalt complexes (e.g. the cobaloximes 88-92) are effective transfer agents.Ij2 m The scope and utility of the process has been reviewed several times,1 lt>JM ns most recently by Hcuts et al,137 Gridnev,1 3X and Gridnev and Ittel."0 The latter two references1provide a historical perspective of the development of the technique. 

Many catalysts have been screened for activity in catalytic chain transfer. A comprehensive survey is provided in Gridnev and Ittel s review."0 The best known, and to date the most effective, are the cobalt porphyrins (Section 6.2.5.2.1) and cobaloximes (Sections 6.2.5.2.2 and 6.2.5.2.3). There is considerable discrepancy in reported values of transfer constants. This in part reflects the sensitivity of the catalysts to air and reaction conditions (Section 6.2.5.3). 

Much of the recent literature relates to BfVbridged Co" cobaloximes based on dimethyl (89) or diphenyl glyoxime (104).110 The BfVbridged cobaloximes (e.g. 89) show greater stability to hydrolysis than analogous H-bridged species (e.g. 88). The diphenylglyoxime complexes (104) show enhanced air and hydrolytic stability... 

Other complexes also react with propagating radicals by catalytic chain transfer.110 These include certain chromium,151 152 molybdenum152 1" and iron154 complexes. To date the complexes described appear substantially less active than the cobaloximes and are more prone to side reactions. 

It is also known that alkyl cobaloximes are subject to radical-induced decomposition.2 7 This suggests an alternative to the mechanism shown in Scheme 9.28 involving reversible chain transfer (Section 9.5). 

Some related examples with BR2 monocapped ligands are also known [181] and one of these molecules, 129, assembles to an interesting supramolecular structure, in which the cobalt(III) ions in a methylcobaloxime unit are coordinated to the pyridine residues of the bridging B(py)(OMe) group (Fig. 34). The dinuclear complex forms a large rectangular cage that is limited by the two pyridine residues and the cobaloxime moieties. The two pyridine... 

A Ru—Si complex 169), a cobaloxime derivative 216a), a Ni complex 216), and the reported Pd compound (obtained in an impure state) 158) have also been made [Eqs. (20)-(23)]. 

Certain groups of organocobalt(III) complexes have been dealt with in previous reviews. The organo-corrinoids have been mentioned in all reviews on vitamin B, 2 since 1961, when the coenzyme form was identified as an organometallic compound [see, for example, (79, 178) and references therein]. The literature on the corrinoids is too extensive to be treated comprehensively here and for details and references readers are referred to the book on The Inorganic Chemistry of Vitamin B,2 (136)certain other aspects of the organometallic chemistry of cobalt corrinoids are treated elsewhere (137). The pentacyanides were reviewed in 1967 (105), the DMG complexes (cobaloximes) in 1968 (145), and some aspects of salen, BAE, and related complexes in 1970 (17). 

While cobaloximes are the most active CCT catalysts known, they are sensitive to air and moisture, so the more robust porphyrins were considered a better choice for a recoverable catalyst. To better understand how to design an optimal thermomorphic catalyst we initiated an investigation to learn how polyethylene length, number, and the covalent linker influence the catalyst activity and the final color of the methaciylate resin. A series of polyethylene-supported CCT catalysts were thus prepared for study (Scheme 36.2). 

The development of magnetic resonance techniques coupled with computer time averaging has made the study of enzyme structure and function by these techniques more fruitful. H NMR, 13C NMR and 19F NMR have been used successfully to determine the structure of B 12-compounds in solution. We are rapidly approaching the point where the structure and function of the B 12-coenzymes will be completely understood, and the need for the synthesis and study of simple Bi2-model compounds such as the cobaloximes (3) will be no longer necessary. However, even though studies on the chemistry of B 12-coenzymes is a necessary prerequisite to our understanding of their biochemical role, it is a wrong assumption to expect that the chemical properties of free coenzymes in aqueous solution should be duplicated in the enzymes. 

A similar type of oxygen complex has been observed during the oxidation of [Con(CN) s]-3 but it was not possible to show that this species was formed in the initial reaction step since with this system, as with the cobaloxime(II) system, the 1 1 adduct apparently reacts very rapidly with another molecule of pentacyanocobaltate(II) to form a diamagnetic binuclear complex with a bridging peroxide ligand 116). It appears that in the Bi2-system the bulk of the corrin ring does not allow formation of the diamagnetic binuclear complex. 

Table 4. Coalescence temperatures for several six coordinate methyl cobaloximes...

One last point should be mentioned with respect to the 19F NMR results. Included in Table 6 is the chemical shift for CF 3-cobaloxime. The chemical shift for this species is at significantly lower field than that for CF3-cobalamin. It therefore, appears that the bis(dimethylglyoximato) ring system is considerably more electron withdrawing than the corrin system. As a result, the Co—C-bond is considerably more stable in these systems. These data once again lead one to question to what extent these inorganic systems should be considered useful models for B12. 

Branchaud and coworkers have used cobaloximes as alkyl radical precursors for the cross-coupling reaction with nitronates.57 This method is very useful for producing branched-chain monosaccharides, as shown in Eq. 5.39.57b... 

In contrast, the very similar compound methyl (pyridine) cobaloxime captured electrons into the Co-Me o orbital, probably with concomitant loss of pyridine (82). Well-defined doublet splittings were obtained when the 13CH3 derivative was used, and the spectrum had the form expected for a dzz configuration (Figure 6). The contrast in reactivity between these similar compounds is remarkable. 

Cobaloxime(I) generated by the electrochemical reductions of cobaloxime(III), the most simple model of vitamin Bi2, has been shown to catalyze radical cyclization of bromoacetals.307 Cobalt(I) species electrogenerated from [ConTPP] also catalyze the reductive cleavage of alkyl halides. This catalyst is much less stable than vitamin Bi2 derivatives.296 It has, however, been applied in the carboxylation of benzyl chloride and butyl halides with C02.308 Heterogeneous catalysis of organohalides reduction has also been studied at cobalt porphyrin-film modified electrodes,275,3 9-311 which have potential application in the electrochemical sensing of pollutants. 

Microwave-assisted Diels-Alder cycloaddition reactions using water-soluble aquo-cobaloxime complexes have been reported by Welker and coworkers [198]. Many more examples of microwave-assisted cycloaddition processes leading to heterocycles are described in Section 6.24. 

Many model systems which mimic both the redox behaviour [for example, ready reduction to Co(i) species] and the alkyl binding ability of vitamin Bu derivatives have been investigated. The most studied of these has involved bis(dimethylgloximato)cobalt systems of type (307), known as the cobaloximes (Bresciani-Pahor et al., 1985). Other closely related... 

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