Cobaloxime system

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 ... 

Values of the rate constants ( 4) and adduct formation constants (K ) are collected in Table 10. Transmethylation occurs much more readily than transfer of other alkyl groups in the corrinoid and cobaloxime systems. 

The elaborate cobaloxime system (1) has been synthesized (in 7% yield) with a view to modelling the active site of the vitamin Big dependent enzyme methyl-malonyl Co-A mutase. Irradiation of (1) for short periods in methanol causes... 

The synthesis and characterization of cobaloximes that have fluorine-containing alkyl ligands, especially those with fluorine substituents at the carbon atom (a-C) attached to the cobalt ion, are the topic of this report. As noted above, the cobaloximes have been intensely examined spectroscopically and numerous high quality solid-state structural investigations have been previously accomplished (7-5). We have been interested in exploiting these properties of the cobaloxime system in order to assess the effect of fluorine substitution on spectroscopic and structural parameters of this class of compounds. 

Properties and Reactions.—Physicochemical studies of axial ligation of pyridine to cobalt corroles have been carried out/ and the kinetics and mechanism of cobalt-carbon bond cleavage in the alkylation of tin and mercury ions have also been studied. The mechanism of action of the vitamin coenzyme has been reviewed/ and several studies of the mechanism of cobalt-carbon bond cleavage and isomerization reactions of the model cobalt cobaloxime system have been published. ... 

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. 

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. 

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... 

The treatment of the bromoacetal (255) with catalytic amounts ( 10%) of a Co(I) species, generated by the electroreduction of cobaloxime (232) in an Me0H-LiCl04 system at —1.8 V, produces the cis-fused adduct (256) in 60 70% yield (Scheme 96) [392]. Cathodic reduction is used for the synthesis of a [Co(CO)3PBu3] complex in a methanol-methyl formate medium, which catalyzes the alkoxycarbonylation of dichloromethane to dimethyl mal-onate in up to 75% yield [393]. The Co(II) complexes are found to be effective for the homogeneous reduction of gem-dichlorocyclopropanes in the presence of anthracene [394]. The formation of the C—C double bond of (258) may be ascribed to the a-elimination of the Co-H species. Thus, benzalchloride (257) can be converted to a mixture consisting primarily of ds- and trans-stilbenes (258) by the action of electrogenerated Co(I)(salen) (Scheme 97) [395-398]. 

The types of model systems in which 5-coordinate species have been invoked range from the relatively electron-deficient cobaloximes to the relatively electron-rich Schiff base complexes, where the corrin ring in cobal-amins is replaced by two dioximato ligands or by a quadridentate N202 dinegative Schiff base ligand, respectively. These two classes of models, in fact, appear to bracket many characteristics of cobalamins since the cobaloximes are too electron-deficient and the Schiff base complexes are too electron-rich [136], As such, an understanding of the differences in properties between these two types... 

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... 

Bis[2,3-butanedione dioximato(l-)] cobalt derivatives (cobaloximes(I)) appear in most respects to be good model systems for the cobalamins of the vitamin B12 coenzyme.1 The central cobalt atom exhibits three stable oxidation states... 

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. 

Catalytic Processes. Catalytic processes lead to intramolecular and intermolecular C-C bond constructions which are usually directly analogous to the stoichiometric reactions. This topic was reviewed in 1983. Catalytic processes often lead to reduction rather than alkene regeneration this is more likely to happen with B12 as a catalyst than it is with a cohaloxime. Schef-fold pioneered the use of vitamin B12 as a catalyst for C-C bond formation, and Tada pioneered the use of model complexes such as cobaloximes. Several of the reactions described in the section on stoichiometric reactions have also been performed cat-aly tically, as mentioned in that section. Commonly used chemical reductants include Sodium Bomhydride and Zinc metal. Electrochemical reduction has also been used. A novel catalytic system with a Ru trisbipyridine unit covalently tethered to a B12 derivative has been used for photochemically driven catalytic reactions using triethanolamine as the reductant. A catalytic system using DODOH complexes can lead to reduction products or alkene regeneration depending upon the reaction conditions. These catalytic B12 and model complex systems all utilize a... 

Alkylcobalt(III) complexes can also be synthesized in aqueous solution. Two of the best-known systems are methylcobalamin and a group of related cobaloximes, and alkylcobalt(III) complexes having ancillary cyanide ligands. As with the chromium(III) system, alkyl cobalt(III) complexes having dimethylglyoxime (DMG) or cyanide ligands can be synthesized by reaction of the cobalt(II) precursor with alkyl halides (Scheme... 

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... 

In the photodecomposition of polystyrene-bound cobaloximes, the polymer chain decreases the mobility of bis(dimethylglyoximato)pyridinecobalt(L) and increases the probability of recombination of L and a radical fixed on the polymer chain. Retardation of the dissociation resulted in a larger equilibrium constant for the polymeric system than that for the analogous monomeric system.130... 

The active LCo complexes indicated above can be used to test this theory. Porphyrins and phthalocya-nines have an O-shaped system which has a more extended -system than that in cobalamins, but it does not provide a substantial increase in reactivity. It should be noted that the hydrogen bonds of the cobaloxime catalysts are essentially as effective as 7r-bonds in continuing the effects of delocalization around the macrocyclic ring. This effect has been noted elsewhere.142 Catalyst 11 comprises an O-shaped -system. Replacement of one jr-bond with a a-bond in the analogue 13 significantly affects the catalytic properties since both complexes retain their O-shape with -conjugation. Additional replacement of "T-bonds with o-bonds leads to a complete loss of catalytic properties as chelates 13, 20, or 21 indicate. Chelate 22, cannot be a CCT catalyst because of the absence of interaction between the two jr-systems. Chelate 34 is an exception its molecular structure is similar to 21 and 13, but it catalyzes chain transfer with a measurable rate. A possible explanation of this phenomenon will be provided in section 3.7. 

Additional understanding of the role of. T-conjligation is provided by cobaloximes 6—8 and other CCT catalysts with hydrogen bonds which complete the macrocycle (i.e., 32 and 33). The common feature shared by a 7r-bond and these hydrogen-bonded systems is their ability to delocalize their electron density.131132 Resonance isomers with different bonding of the H or BF2 bridges to the oxygen atoms allow the electrons to delocalize around the equatorial plane as shown in Scheme 2. 

The addition of LCoH to olefinic double bonds is well documented in the chemistry of cobaloximes. Because LCoH has the ability to dissociate to H+ and LCo1, both the hydride and Co1 form of the cobalt chelate must be considered in any system where LCoH can be obtained. 

Acidic hydrolysis of CCT catalysts is described as a possible problem. Few actual measurements of the catalyst hydrolysis have been made. Addition of only 1% of acetic acid in MMA leads to a rate of cobaloxime deactivation of about 1.2% min 1 at 60 °C.296 The stability of the BF2-bridged cobaloxime is believed to be higher.119,338,342 The complex (dmgBF2)2-Co11 decays at room temperature at a rate of about 0.6% min 1 at pH = 1 but is practically stable at neutral pH.309,310 It should be noted, however, that these measurements were made in a benign solvent rather than in an active polymerization system involving free radicals. 

---