Cobaloxime alkylated

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). 

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... 

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... 

Steric factors play an important role in reactions of bromomethylaqua-cobaloxime with substituted imidazoles - 1-methylimidazole reacts at approximately the same rate as imidazole itself, but the presence of an alkyl substituent in the 2-position of the incoming imidazole reduces the rate dramatically (72). 

In abroad sense, the model developed for the cobaloxime(II)-catalyzed reactions seems to be valid also for the autoxidation of the alkyl mercaptan to disulfides in the presence of cobalt(II) phthalocyanine tetra-sodium sulfonate in reverse micelles (142). It was assumed that the rate-determining electron transfer within the catalyst-substrate-dioxygen complex leads to the formation of the final products via the RS and O - radicals. The yield of the disulfide product was higher in water-oil microemulsions prepared from a cationic surfactant than in the presence of an anionic surfactant. This difference is probably due to the stabilization of the monomeric form of the catalyst in the former environment. 

Vitamin B12 derivatives and their model compounds have recently been used as recyclable electrocatalysts for the reduction of alkyl halides since low-valent Co species are good nucleophiles toward organic substrates [367-369]. Examples of such elec-trocatalysts are the vitamin B12 derivatives aquocobalamin (230), dibromo[l-hydr-oxy-2,2,3,3,7,7,8,8,12,12,13,13,17,17,18,18-hexadecamethyl-10,20-diazaoctahydropor-phinato]cobalt(III) (231), and cobaloxim (232). The above Co(I) complexes can be... 

The dehydration and deamination reactions appear to operate in a parallel fashion and will be considered together. Schrauzer and Silbert propose a base-catalyzed cleavage of the carbon-cobalt bond in the B12-catalyzed diol dehydration reaction as shown in Fig. 17 (81), based on demonstrated lability of the beta hydrogens in alkyl cobaloximes with electronegative groups in this position. 

These cobaloximes (1) are generated by reaction of alkyl halides with in situ generated Co (dmgH)2py. They are orange crystals that are relatively stable to air and silica gel, but on photolysis they generate R-, which can be trapped in generally high yield or which cyclize in the case of alkenyl radicals.1... 

Reaction of electron-rich olefins such as vinyl ethers have not enabled us to observe spectroscopically a complex with cobalt(III) cobaloximes. Nevertheless, the reaction between a cobal(III)oxime, ethyl-vinyl ether, and ethanol gives complete alkylation of the cobalt (33). When the ethanol is not vigorously anhydrous, some formyl-methylcobaloxime (25) (Scheme 7) is formed also and these observations are consistent (34) with the reactions outlined in Scheme 7. When the water concentration was increased, the amount of formylmethyl-cobaloxime also increased at the expense of the acetal. Thus, we anticipated that the analogous reaction with vitamin Bi2, which is impossible to obtain in an anhydrous condition and is best manipulated in... 

Alkylated cobaloximes yield the corresponding dimeric species of alkyl radicals by photolysis under acidic conditions. But the BDHC complex with a hexyl or benzyl group at its axial site does not yield the corresponding dimeric species by photolysis (dodecane and bibenzyl, respectively). Consequently, the hydrogenation product must be obtained through the formation of a carbanion by heterolytic cleavage of the Co-C bond, followed by its protonation. 

Recrystallization from water/acetone (even at a basic pH) causes some hydrolysis of the acetal. (2,2-Diethoxyethyl)cobalamin, like all alkyl cobalamins, is light sensitive in solution in addition, and unlike most other alkylcobalamins, it is add sensitive, decomposing to both (formylmethyl)cobalamin and aqua-cobalamin.2 The formation of other alkylcobalamins and cobaloximes by reaction with enol ethers has been described.2... 

Recently we found that the chiral alkyl group bonded to the cobalt atom in a cobaloxime complex crystal is almost (82%) inverted to the opposite configuration only by photoirradiation [12]. In this chapter we describe the processes of the racemic-to-chiral transformation and also the chirality inversion in the... 

Such crystalline-state racemizations by photoirradiation were observed in the cobaloxime complexes not only with the chiral 1-cyanoethyl group but also with the chiral l-(methoxycarbonyl)ethyl [18-20], 1-(ethoxycarbonyl)ethyl [12,21], 1,2-bis(methoxycarbonyl)ethyl [22], l,2-bis(ethoxycarbonyl)ethyl [23], and l,2-bis(allyloxycarbonyl)ethyl [24] groups as axial alkyl groups. The concept of a reaction cavity was also applicable to the racemization of the cobaloxime complexes with such bulky chiral alkyl groups. 

Reduction of Co(dmg)2(R)(L) gives the supemucleophilic cobaloxime(I) which undergoes oxidative addition with alkyl halides to give Co111 alkyls. The [Co(dmgBF2)2(py)] anion is square-pyramidal, with very short Co—N bonds.32... 

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. ... 

Stoichiometric Processes. In 1964, Schrauzer published the first of many papers on the synthesis and properties of alkyl cobaloximes. This work led to the development of cobaloximes and related compounds as vitamin B12 model compounds, e.g. (l)-(4), summarized in a review in 1976. By the mid 1970 s, many of the fundamental reactions of vitamin B12 and model complexes were well established. This work is summarized in several reviews. ... 

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... 

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