Cobalt, complexes cobaloxim

Enikolopyan et al.til found that certain Co11 porphyrin complexes (eg. 87) function as catalytic chain transfer agents. Later work has established that various square planar cobalt complexes (e.g. the cobaloximes 88-92) are effective transfer agents.Ij2 m The scope and utility of the process has been reviewed several times,1 lt>JM ns most recently by Hcuts et al,137 Gridnev,1 3X and Gridnev and Ittel."0 The latter two references1provide a historical perspective of the development of the technique. 

Certain groups of organocobalt(III) complexes have been dealt with in previous reviews. The organo-corrinoids have been mentioned in all reviews on vitamin B, 2 since 1961, when the coenzyme form was identified as an organometallic compound [see, for example, (79, 178) and references therein]. The literature on the corrinoids is too extensive to be treated comprehensively here and for details and references readers are referred to the book on The Inorganic Chemistry of Vitamin B,2 (136)certain other aspects of the organometallic chemistry of cobalt corrinoids are treated elsewhere (137). The pentacyanides were reviewed in 1967 (105), the DMG complexes (cobaloximes) in 1968 (145), and some aspects of salen, BAE, and related complexes in 1970 (17). 

A special mention13 may be made of cobalt complexes the [Co(HDMG)2] (w = 0, 1) has been named cobaloxime by analogy with cobalamin , which is another name of vitamin B,2. 

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... 

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... 

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... 

Interestingly the radical cyclization of acetal 40 in the presence of cobaloxime leads to the cw-ring-fused alkyl-cobalt complex 41 which can be converted in a preparative manner to 42a,b following 1,2-elimination and hydrolysis/oxidation. Similarly the lactone 43 can be obtained from 40 following insertion of molecular oxygen and hydrolysis/oxidation protocol (Scheme 14). 

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). 

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). 

Square planar cobalt complexes including porphyrin and cobaloxime derivatives rapidly and reversibly add carbon-centred radicals. An alternative pathway to the reverse reaction is an elimination to form the olefin and a cobalt hydride species, which can initiate polymerization by donating hydrogen to monomer.24... 

Despite being studied several decades ago, cobaloxime -n-cations have seen modest use in organic synthesis. The cations can be generated in situ by treatment of p-hydroxy or p-acetoxy cobaloxime complexes 6349 with acid (Scheme 6.129). Although these reactions seem to resemble those of the Fp complexes, structurally, these cobalt complexes are not straightforward coordinated alkenes. Low-temperature C NMR studies have... 

It is also known that a malonic ester substituted in the j5-position with cobalt can rearrange into the corresponding succinic ester. To test the possible role of the metal and to better simulate the situation of the enzyme, the group of J. Retey (266) synthesized the following cobalt complex in which the substrate is anchored covalently by two methylene bridges to planar cobaloxime. 

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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