Cobalt complexes hydrogenation

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 ... 

The hexamine cobalt (II) complex is used as a coordinative catalyst, which can coordinate NO to form a nitrosyl ammine cobalt complex, and O2 to form a u -peroxo binuclear bridge complex with an oxidability equal to hydrogen peroxide, thus catalyze oxidation of NO by O2 in ammoniac aqueous solution. Experimental results under typical coal combusted flue gas treatment conditions on a laboratory packed absorber- regenerator setup show a NO removal of more than 85% can be maitained constant. 

Reduction of low-spin Fe(m) imide 165 with hydrogen (1 atm, 20 °C) proceeds stepwise leading first to anilido complex 168 and then to 77-cyclohexadienyl complex 169 via hydrogenolysis of the Fe=NR linkage (Scheme 64). It should be mentioned that the similar low-spin cobalt complex [PhB(CH2PPh2)3]Co N-/>-Tol is stable to hydrogen pressure (1-3 atm) up to 70 °C <2004JA4538>. 

The cobalt complex 37 was used in combination with quinine as a chiral coordinating base to hydrogenate l,2-diphenyl-2-propene-l-one in 49% ee (Fig. 29.22) [50]. However, no further studies of this type of catalyst were reported. 

For a decade or so [CoH(CN)5] was another acclaimed catalyst for the selective hydrogenation of dienes to monoenes [2] and due to the exclusive solubility of this cobalt complex in water the studies were made either in biphasic systems or in homogeneous aqueous solutions using water soluble substrates, such as salts of sorbic add (2,4-hexadienoic acid). In the late nineteen-sixties olefin-metal and alkyl-metal complexes were observed in hydrogenation and hydration reactions of olefins and acetylenes with simple Rii(III)- and Ru(II)-chloride salts in aqueous hydrochloric acid [3,4]. No significance, however, was attributed to the water-solubility of these catalysts, and a new impetus had to come to trigger research specifically into water soluble organometallic catalysts. 

In Reaction (7.20) is reported the cyclization of a thermally unstable propar-gyl bromide cobalt complex mediated by Ph2SiH2 at room temperature and Et3B/02 as the radical initiator. However, a mixture of reduced and bromine atom-transfer products (1 1.8 ratio) are isolated due to the low hydrogen donation of the employed silane [31]. 

Cobalt(ll)-EDTA complex, hydrogen peroxide determination, 628, 639 Cobalt(ll)-hexacyanoferrate, hydrogen peroxide determination, 651 Cobalt(lll)-phthalocyaninetetrasulfonate, hydroperoxide determination, 677 CocrystaUization, alkyl hydroperoxides-ether, 111, 113... 

Hydrogenation of acetic anhydride to acetaldehyde (equation 23) has been demonstrated utilizing cobalt carbonyl under one atmosphere of hydrogen. However, the cobalt complex is short lived. A more efficient cobalt catalyzed reaction with substantial catalyst longevity was realized at a temperature of 190 and 3000 psi pressure CO and hydrogen. The main products were equal amounts of EDA and acetic acid. Upon investigation, this reaction was found exceptionally efficient at a more reasonable 1500 psi pressure provided that the temperature was maintained... 

Rathke and Feder have employed Co2(CO)8 as the catalyst precursor in their studies. Samples withdrawn from reactions under pressure were analyzed for both total cobalt and for HCo(CO)4 (35) conversion to HCo(CO)4 was observed to the extent of 50-90%, varying according to (14) with temperature and hydrogen pressure. Experiments with different levels of catalyst showed that the overall rate of CO reduction was first-order in the HCo(CO)4 concentration, as determined by titration of reaction samples. Thus, there is substantial evidence that the catalyst in this system (or more precisely, the species present in the transition state of the rate-determining catalytic step) is a mononuclear cobalt complex. The observed kinetic dependences [Eq. 

It is suggested that this occurs by interaction of the isopropyl side chain with the cobalt complexes in solution, leading to C—C bond cleavage and subsequent hydrogenation (97). 

Electrochemical (24) and chemical (25, 26) techniques have been utilized to investigate the kinetics and the mechanisms of the addition of dioxygen to a metal center, and to follow its subsequent reduction to hydrogen peroxide when catalyzed by cobalt(III) complexes of macro-cyclic amine ligands. Such complexes have also been involved in the general investigation of dioxygen addition to cobalt complexes (27,28). 

The [Con(bipy)2 ]2+ species has also been reported to activate hydrogen peroxide and ter -butyl hydroperoxide for the selective ketonization of methylenic carbons, the oxidation of alcohols and aldehydes, and the dioxygenation of aryl olefins and acetylenes (36). Later reports (37), however, while confirming that the cobalt complexes did indeed cata-... 

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