Cobaloximes, studies

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. 

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. 

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... 

The hydrogenations become analogous to those involving HMn(CO)5 (see Section II,D), and to some catalyzed by HCo(CN)53 (see below). Use of bis(dimethylglyoximato)cobalt(II)-base complexes or cobaloximes(II) as catalysts (7, p. 193) has been more thoroughly studied (189, 190). Alkyl intermediates have been isolated with some activated olefinic substrates using the pyridine system, and electronic and steric effects on the catalytic hydrogenation rates have been reported (189). Mechanistic studies have appeared on the use of (pyridine)cobaloxime(II) with H2, and of (pyridine)chlorocobaloxime(III) and vitamin B12 with borohydride, for reduction of a,/3-unsaturated esters (190). Protonation of a carbanion... 

Detailed kinetic studies confirmed a two-stage reaction for the cobaloxime(II)-catalyzed autoxidation of this system in methanol (54,55). First, within about 30 s, the reaction reached steady-state conditions via reversible oxygenation of Co(II) to the corresponding... 

Our first choice of alkene was allyltributyltin which had been previously studied by our group (see above). By employing the chloro(pyridine)cobaloxime(III)/magnesium redox couple in which chloro(pyridine)cobaloxime(III) was used in the catalytic amount, we were now able to synthesize the same allylated PCTFE (Eq. 4) with a higher degree of functionalization of approximately 50%. 

A number of model compounds have been used to study the reactions of B,2. The ligands of these compounds are usually oximes and Schiff s bases and pyridine is a common axial base. Cobalt-bisdimethylgiyoxime, Co(dmg)2 (68) is the most popular model and is commonly called cobaloxime (81). 

The term cobaloximes refers to that class of cobalt complexes in which two dimethylglyoximato or other glyoximato monoanions occupy four coordination positions about cobalt and in which the four N donors are in essentially the same plane. A neutral octahedral low-spin d6 Co(III) cobaloxime is shown in Figure 1. Cobaloximes with this oxidation state have been studied most extensively since these are more stable than the oxidation state I, II, and IV compounds. The chemistry of these latter oxidation state compounds will be reviewed... 

The effect of varying P-donor ligands on the C-13 NMR spectra of cobaloximes has not been studied in detail as yet. Limited data are available for C-59 NMR of chloro and methylcobaloximes where the order of shifts (3P < Bu3P < (CH30)3P) is clearly not reflective of the... 

Although the cobalt corrinoids have been studied extensively in. the last two decades (J), the significance of corrin as an equatorial ligand is not well understood. To characterize coenzyme B12 as an organocobalt derivative, a search for model cobalt complexes that can form a Co-C bond axial to a planar equatorial ligand has been stimulated. Studies on model systems (2-13), particularly on the cobaloxime derivatives (2-7), characterized their respective chemistry, but it is still not easy to establish a general correlation between the structure of an equatorial ligand and the properties of cobalt complex... 

Espenson has found that the rhodoxime behaves very much like the cobaloxime in its association with iron(III) ion to form the bimetallic [Rh(DMG)(DMGFe)(H20)Me]2+. Its formation constant (equation 178) is 68.8 + 4.0 (for cobaloxime, Kf— 15.3 + 0.8912). This unusual cation can be considered to be derived from the structure shown in (63), in which X = Me, Y = H20, and one of the hydrogen-bonded hydrogens is replaced by an Fe3+. The presence of the iron adduct is readily signalled by an intense absorption near 440 nm (s ss 47 000), and a comparison of the kinetic and thermodynamic parameters shows a strong similarity to the previously studied Fe3+ adducts of cobaloxime.913 Even in the presence of forcing concentrations of Fe3+, there was no evidence of incorporation of a second iron(III) ion.912... 

Since the recognition of Vitamin B12 and the existence in Nature of Co—C bonds, there has been much study of model systems. These have been mainly complexes of dimethyl and other glyoximes (the so-called cobaloximes), and Schiff base and macrocyclic ligand complexes. The methyl complex [Co(CH3)(NH3)5]2+ may be regarded as the simplest model it has electronic characteristics very similar to those of [Co(NH3)6]3+.31 The main interest has centered on the reduction of Co1 species and on the formation of Co—C bonds and their reactions. 

A significant observation for cobaloximes as B12 models was the reduction of MeCo L4(B) by Cr(aq) +, which gave Co L4(B) and Cr(Me)(aq) + (equation 37). These products are indicative of an inner-sphere (see Inner-sphere Reaction) electron transfer with Me as the bridging ligand. This result has promoted further studies, notably methyl transfer between cobaloximes and nickel tetraazacycle complexes, which provides a possible model for the methylcobalamin alkylation of CO hydrogenase. ... 

In the study of the four conformational polymorphs of a cobaloxime complex, Sawada et al. (1996) were able to establish a quantitative relationship between the size of the reaction cavity for an intramolecular photoisomerization reaction and the rate constant for the process in three of the modifications. Such studies provide a great deal of direct detailed information about the relationship between the environmental influences on the mechanism of a reaction at essentially the atomic level, information which is much more difficult to obtain from studies on a single crystalline form. 

The complexity of vitamin B12 led to the preparation and investigation of model compounds, of which derivatives of bis(dimethylglyoximato)-cobalt(III), termed cobaloximes, have been the most extensively studied (29, 235-237). These derivatives, especially methylaquocobaloxime, have helped cast considerable light on the mechanisms involved in the biological action of vitamin B g. 

The cobaloxime system has been well studied, and stereochemical tests show complete racemization of the alkyl group during insertion. Rearrangements expected for a radical mechanism [reaction (g)] are also observed . A reaction between O2 an organic free radical has been suggested as a key step for most of the above reactions ... 

Theoretical investigations have been used to model coenzyme Bi2. While important conclusions may be drawn, the work has not been extended with CCT in mind. Due to computational limitations, the initial work was limited to the triaminomethylcobalt111— amide system.67 Force fields specifically designed to do molecular mechanical calculations on cobalamins68 and cobaloximes69 are relevant for conformational studies but cannot elucidate the mechanisms of cobaloxime reactions because they do not account for electronic effects. Computational investigation of... 

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