Cobalt isomerization reactions

Cobalt, tetraammineaquahydroxy-reactions, 1, 27 Cobalt, tetraamminedichloro-isomerization, 1,182, 201,467 Cobalt, tetraamminedihydroxy-cobalt(III) salt structure, 1,184... 

Isomerization has been observed with many a,j3-unsaturated carboxylic acids such as w-cinnamic 10), angelic, maleic, and itaconic acids (94). The possibility of catalyzing the interconversion of, for example, 2-ethyl-butadiene and 3-methylpenta-l,3-diene has not apparently been explored. The cobalt cyanide hydride will also catalyze the isomerization of epoxides to ketones (even terminal epoxides give ketones, not aldehydes) as well as their reduction to alcohols. Since the yield of ketone increases with pH, it was suggested that reduction involved reaction with the hydride [Co" (CN)jH] and isomerization reaction with [Co (CN)j] 103). A related reaction is the decomposition of 2-bromoethanol to acetaldehyde... 

The next five transition metals iron, cobalt, nickel, copper and zinc are of undisputed importance in the living world, as we know it. The multiple roles that iron can play will be presented in more detail later in Chapter 13, but we can already point out that, with very few exceptions, iron is essential for almost all living organisms, most probably because of its role in forming the amino acid radicals required for the conversion of ribonucleotides to deoxyribonucleotides in the Fe-dependent ribonucleotide reductases. In those organisms, such as Lactobacilli6, which do not have access to iron, their ribonucleotide reductases use a cobalt-based cofactor, related to vitamin B12. Cobalt is also used in a number of other enzymes, some of which catalyse complex isomerization reactions. Like cobalt, nickel appears to be much more extensively utilized by anaerobic bacteria, in reactions involving chemicals such as CH4, CO and H2, the metabolism of which was important... 

The cis/trans isomerization reaction, Eq. (24), has been applied in the preparation of salts of the cis isomers of the chromium(III) complexes with L3 = (NH3)3 or tacn (319). For these species Eq. (24) equilibrium is shifted to the right, while the corresponding equilibria with the diaqua or dihydroxo species, respectively, are shifted to the left (Table X). The increased stability of the cis aqua hydroxo species can be explained in terms of intramolecular hydrogen bond formations (Section VI,C). As mentioned above, the corresponding cobalt(III) and rhodium(III) complexes have been isolated as salts only in the case of the trans-(H20)L3M(0H)2ML3(H20)4+ cations, but it seems very probable that their cis isomers could be prepared by reaction Eq. (24). 

The data in Table XXXV show that common features for these ammonia and amine complexes are very fast isomerization between the cis and trans isomers of the diaqua species and the fact that the trans diaqua isomers are generally more stable than the cis isomers. In the ammine system the activation parameters for k2 and k 2 are consistent with an isomerization process at cobalt(III), but it is at present not clear how this occurs. It need not be a simple cis-trans isomerization occurring at one of the Co(III) centers, but might involve the participation of both metal centers. The isomerization reaction may proceed via intramolecular proton transfer between a water ligand and one of the two hydroxo bridges with simultaneous bridge cleavage and formation... 

It must be concluded, therefore, that the ligands do not become completely detached from the metal ion in isomerization reactions. Comparable results have been observed in the isomerization95 of potassium diaquodioxalatochromium(III) and the racemization96 of optically active potassium tris(oxalato)chromium(III) when no exchange with free ligand in solution occurs. Thus, although it is not practicable to take advantage of the desirable properties of individual isomers of 2 1 chromium and cobalt complexes of tridentate azo compounds because of the facility with which such compounds isomerize in solution, the technically important unsymmetrical 2 1 complexes are capable of practical application because they show little or no tendency to disproportionate in solution. 

Vitamin B12 is a biologically active corrinoid, a group of cobalt-containing compounds with macrocyclic pyrrol rings. Vitamin B12 functions as a cofactor for two enzymes, methionine synthase and L-methylmalonyl coenzyme A (CoA) mutase. Methionine synthase requires methylcobalamin for the methyl transfer from methyltetrahydrofolate to homocysteine to form methionine tetrahy-drofolate. L-methylmalonyl-CoA mutase requires adenosylcobalamin to convert L-methylmalonyl-CoA to succinyl-CoA in an isomerization reaction. An inadequate supply of vitamin B12 results in neuropathy, megaloblastic anemia, and gastrointestinal symptoms (Baik and Russell, 1999). 

The cobalt—hydride intermediate has not been isolated and is virtually undetected. Cobalt hydride reaction with an olefin by first migrating the hydrogen atom from cobalt to a porphyrin nitrogen atom (78) or carbon atom (79) is precedented by the isomerization of a benzylcobalt chelate. In that case, the benzyl migrates reversibly from the cobalt to the carbon atom of the equatorial ligand.253... 

Vitamin B12 is known for its ability to catalyze molecular rearrangements. A variety of cobalt chelates are logical models for vitamin B12, and their stoichiometric and catalytic activities in a variety of reactions,403 particularly olefin isomerizations, were studied intensively.404-411 Noncatalytic isomerization reactions based upon the synthesis of alkylcobalt chelates as model intermediates were favored. A variety of catalytic oxidations of substrates such as hydroquinone, azo compounds, phosphines, and olefins were also investigated.412-415 Copolymerization of a-methylstyrene and other monomers with oxygen in the presence of CoTPP led to alternating polyperoxides.416 418 Cobaloximes were found to catalyze... 

Other reactions are alkane formation by hydrogenation, ketone formation (especially with ethylene ), ester formation through hydrogen transfer and formate ester synthesis. An improved catalyst system in which one CO ligand of CoH(CO)4 is substituted with a trialkylphosphine ligand , was disclosed by Shell workers in the early 1960s. With this catalyst, which is more thermally stable than the unsubstituted cobalt carbonyl, reaction proceeds at 140-190 C with 3-7 MPa of CO and Hj. Additionally, mostly linear aldehydes are obtained from linear terminal and internal olefins. This remarkable result arises from the high preference for the terminal addition to an a-olefin, and the isomerization of the olefinic position which occurs simultaneously with hydroformyiation. 

For some complexes, the isomerization is very slow (as for the inert cobalt(III) complex [Co(en)3]3+), whereas for others the process is sufficiently fast (as for thenickel(II) analogue CNi(en)3]3+) that the complex cannot be easily resolved into its optical forms through conventional crystallization methods. The isomerization reaction can be readily followed by observing the loss of optical rotation at a selected wavelength over time this is usually a simple first order exponential decay process. 

It is remarkable that with Pt-Co catalysts under hydrogen, in isomerization reactions (28-29), cobalt behaves as a poison. With Pt-Ni catalysts, in the same conditions nickel plays the role of an additive except for the 50 atom% in Ni and Pt (30). In the oxidation reaction it is exactly in opposite behavior Pt-Co is the best system and Ni is a poison for our studied systems. 

The cobalt which is a poison when it is added to platinum for the isomerization reactions is the best additive in our case for the CO oxidation reaction. 

The critical functionality of radical SAM enzymes arises through the generation of the 5 -deoxyadenosyl radical intermediate. Nature has exploited this chemical reactivity in one other system, that of the radical Bi2-dependent enzymes, which utilize 5 -deoxyadenosylcobalamin (AdoCbl), also known as coenzyme B12, to catalyze isomerization reactions (Figure 3)." Homolytic cleavage of the cobalt(III)-deoxyadenosine bond results in a cobalt(II) center and the 5 -deoxyadenosyl radical, which generates a substrate-derived radical via the abstraction of a hydrogen atom from the substrate molecule, just as it does in the [4Fe S] reductive... 

Cobaltjpcntaamminenitrilotriacetato-eleclron transfer with hexaaquairon, 369 Cobalt, pentaammincnitrito-isomerization, 465 linkage isomerization reactions, 465 Cobalt, pentaamminenitro-dichloride... 

Linkage isomerization reactions, 465 Linked redox centres, 493 Liquid-liquid extraction, 538-549 Lowry, Thomas Martin, 16 Luteocobalt—see Cobalt, hexaammine-Luteocobaltic chloride —see Cobalt, hexaammine-, chloride... 

Propybtaphthalene Cobalt modified and also nonmo Cied NaY catalysts can induce the polymerization of l-propyl n hthalene. However the mechanism of this polymerization is obscure and probably invdves isomerization reaction. 

---