Cobalt III cobaloximes

Various Co cobaloximes (90-92) have also been used as catalytic chain transfer agents.To be effective, the complex must be rapidly transformed into the active Co cobaloximes under polymerization conditions. The mechanism of catalytic chain transfer is then identical to that described above (6.2.5.1). 

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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... 

Finally, cobalt(III) cobaloximes and cobalamins have been found to react with vinyl ethers in nucleophilic solvents in the presence of a weak base to form )8-substituted ethylcobalt complexes [71] (e.g. Eqn. 29). 

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... 

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... 

Since bromo(pyridine)cobaloxime(III) was not commerically available and its synthesis was not convenient36, we utilized chloro(pyridine)bis(dimethylglyoximato)-cobalt(III) (Equation 3) (also known as chloro(pyridine)cobaloxime (III)) instead. It has four cathodic waves in polarography when observed in acetonitrile. Its half wave potentials are located at -0.65, -1.45, -2.42, and -2.92 volts vs the Ag/AgNC>3 electrode, corresponding to the reduction of the cobalt from +3 to +2, +1, and 0, and the reduction of the ligand, respectively. 

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... 

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... 

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. 

Alkylcobaloximes can be prepared directly from Co(II) chloride and dimethylglyox-ime. However, more reproducible results and purer products are obtained when the preformed bromobis(dimethylglyoximato)(dimethyl sulfide)cobalt(III) is reduced with NjBH4 and then alkylated . Dimethyl sulfide is easily exchanged, so other base-substituted cobaloximes may be prepared . 

It has long been postulated that these facile reactions occur via formation of a cationic intermediate (upon acid-catalyzed loss of ROH) which can be formulated either as a a-bonded alkylcobalt carbonium ion or a cobalt(III)-olefin m complex. Recently, firm kinetic evidence has been obtained for the occurrence of an intermediate in the acid-catalyzed decomposition of 2-hydroxy- and 2-alkoxyethyl-cobaloximes [94]. Thus, while 2-hydroxyethylcobaloxime decomposes with strictly first-order kinetics in mildly acidic H2SO4/H2O mixtures, the alkoxy derivatives show a substantial lag followed by a first-order decay which is slower than that for the hydroxyethyl complex. In strongly acidic mixtures (//q < —5) all compounds show a rapid burst of absorbance change, followed by a slower first-order decay which is identical for all compounds whether measured spectrophotometrically or manometrically. These observations support the mechanism shown in Eqn. 56. 

The parameters of the inner coordination polyhedron are totally consistent with those reported for a number of organo-cobalt(III) complexes (21) with one exception. The average cobalt-nitrogen (planar) distance, 1.866 A, is shorter, although only slightly, than in other organo-cobalt(III) complexes. The crystal structures of three cobaloxime complexes, which are the most closely related to our organo-cobalt complexes, have mean Co-N (planar) distances which vary between 1.88 and 1.90 A (22,23, 24). 

Bromoacetals containing carbon-carbon triple bonds were cyclized to 5-membered heterocyclic rings (5- xo-exclusively) using [chloropyridine-bis(dimethylglioximato)] cobalt(I) (cobaloxime I) generated from cobaloxime III as an electron transfer reagent (mediator) in alkaline methanol solution at constant current. The most efficient molar substrate/mediator ratio was 2 1 with 70-87% yields of isolated products. The proposed... 

The rate of recombination of methyl radicals and cobaloxime(II) (i.e. the reverse of Eqn. 45) has been measured by flash photolysis and found to be very high k- ca. 5-8 X 10 /M/sec at 25°C) [82,83]. This presumably explains both the slow rate of photolytic decomposition under strictly anaerobic conditions as well as the marked acceleration of the photolytic decomposition rate in the presence of air due to trapping of the radicals by oxygen. In the latter case the final products are cobalt(III) (i.e. aquocobalamin from methylcobalamin) and formaldehyde, with traces of methanol, methane and formic acid. 

R. B. Silverman and D. Dolphin (1973), A direct method for cobalt-carbon bond formation in cobalt(III)-containing cobalamins and cobaloximes. Further support for cobalt (III) r-complexes in coenzyme B12 dependent rearrangements. J. Amer. Chem. Soc. 95, 1686-1688. 

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