Cobalamin catalyst

Only a few other cobalt complexes of the type covered in this review (and therefore excluding, for example, the cobalt carbonyls) have been reported to act as catalysts for homogeneous hydrogenation. The complex Co(DMG)2 will catalyze the hydrogenation of benzil (PhCOCOPh) to benzoin (PhCHOHCOPh). When this reaction is carried out in the presence of quinine, the product shows optical activity. The degree of optical purity varies with the nature of the solvent and reaches a maximum of 61.5% in benzene. It was concluded that asymmetric synthesis occurred via the formation of an organocobalt complex in which quinine was coordinated in the trans position (133). Both Co(DMG)2 and cobalamin-cobalt(II) in methanol will catalyze the following reductive methylations ... 

Vitamin Bjj (8.50, cobalamin) is an extremely complex molecule consisting of a corrin ring system similar to heme. The central metal atom is cobalt, coordinated with a ribofuranosyl-dimethylbenzimidazole. Vitamin Bjj occurs in liver, but is also produced by many bacteria and is therefore obtained commercially by fermentation. The vitamin is a catalyst for the rearrangement of methylmalonyl-CoA to the succinyl derivative in the degradation of some amino acids and the oxidation of fatty acids with an odd number of carbon atoms. It is also necessary for the methylation of homocysteine to methionine. 

Giese and coworkers determined on the basis of experimental results obtained by Scheffold et al. [268, 299] that addition reactions of alkyl bromides 249 to a,p-unsaturated nitriles or esters 248 catalyzed by cobalamine 247 are free radical reactions (Fig. 61) [300], This conclusion was based on the similar cis/trans-selectivities in addition reactions of the 4-tert-butylcyclohexyl radical to different electron-poor alkenes 248 using 247 as a catalyst on one hand and classical tributyltin hydride conditions on the other. The kinetics of the radical addition was determined. 

Organometallic reactions can create species that are otherwise not attainable. Cobalamin enzymes are particular examples of these catalysts. 

Other uses of cobalt(I) catalysts include the reductive intramolecular cyclization of bromocyclohexenones to form bicyclic ketones [391] and the radical cyclization of bro-moacetals [392,393]. Krautler and coworkers [394] found that 1,4-dibromobutane interacts with electrogenerated cob(I)alamin to afford a tetramethylene-l,4-di = Co -cobalamin species. In a recent study of the reactions of cobalt(I) tetraphenyl porphyrin with benzyl chloride or 1-chlorobutane, Zheng and coworkers [395] reported that alkyl radicals are transferred from the cobalt center to a nitrogen of a pyrrole ring, leading to formation of an A-alkyl cobalt porphyrin complex. 

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. 

All known ACS enzymes are bifunctional in that they possess a C cluster with COdFI activity in addition to an A cluster (the ACS active site. Scheme 9). In the enzymes, a CO tunnel is described through which GO can pass directly from the C cluster, where it is generated from CO2, to the A cluster, where acetyl GoA synthesis takes place. Again, two mechanisms were proposed that differ in the order of binding events and redox states involved. In essence, however, GO binds to an Ni-GHs species, followed by insertion and generation of an Ni-acetyl species, which upon reaction with GoA liberates the acetyl GoA product. It is interesting to note that methylation of Ni occurs by reaction with methyl cobalamin (Scheme 7). In M. thermoacetica, the cobalamin is the cofactor for a rather unique protein called the corrinoid iron sulfur protein (GFeSP). The above process, even if mechanistic details still remain in question, resembles the industrial Monsanto acetic acid synthesis process (Scheme 9, bottom). In this case, however, the reaction is catalyzed by a low-valent Rh catalyst. 

The relative catalytic activities of a series of cobaloximes and of aquo-cobalamine for homogeneous oxidation of formaldehyde have been qualitatively assessed. Vitamin Biaa is by far the most effective the rate law for the process is complicated. The effect of these catalysts on decomposition of the formic acid produced was also studied. Somewhat simpler cobalt complexes catalyse the oxidation of alkylbenzenes. Thus cobalt(ii) acetate in hydrochloric acid is a good catalyst for the oxidation of p-xylene or p-toluic acid to terephthalic acid. Co(acac>3 catalyses oxidation of toluene, and a cobalt stearate complex catalyses the oxidation of isopropylbenzene. ... 

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