Cobalt complexes mechanism

In its complex compounds, of which there are many thousands, Co almost invariably has a +3 oxidation number. Apparently, Co+s ion accompanied by six coordinating groups is particularly stable. Cobalt complexes are important in biochemistry. Some enzyme reactions go through a cobalt-complexing mechanism. Although only small traces are needed, cobalt is essential to the diet. 

The abrasion resistance of cobalt-base alloys generally depends on the hardness of the carbide phases and/or the metal matrix. For the complex mechanisms of soHd-particle and slurry erosion, however, generalizations cannot be made, although for the soHd-particle erosion, ductihty may be a factor. For hquid-droplet or cavitation erosion the performance of a material is largely dependent on abiUty to absorb the shock (stress) waves without microscopic fracture occurring. In cobalt-base wear alloys, it has been found that carbide volume fraction, hence, bulk hardness, has Httie effect on resistance to Hquid-droplet and cavitation erosion (32). Much more important are the properties of the matrix. 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

One possibility is a limiting SN1 mechanism with a five-coordinate cobalt complex as an intermediate,... 

Although the actual reaction mechanism of hydrosilation is not very clear, it is very well established that the important variables include the catalyst type and concentration, structure of the olefinic compound, reaction temperature and the solvent. used 1,4, J). Chloroplatinic acid (H2PtCl6 6 H20) is the most frequently used catalyst, usually in the form of a solution in isopropyl alcohol mixed with a polar solvent, such as diglyme or tetrahydrofuran S2). Other catalysts include rhodium, palladium, ruthenium, nickel and cobalt complexes as well as various organic peroxides, UV and y radiation. The efficiency of the catalyst used usually depends on many factors, including ligands on the platinum, the type and nature of the silane (or siloxane) and the olefinic compound used. For example in the chloroplatinic acid catalyzed hydrosilation of olefinic compounds, the reactivity is often observed to be proportional to the electron density on the alkene. Steric hindrance usually decreases the rate of... 

The d5Tiamic stereochemistries of M(dtc)3 and [M(dtc)3] (M = Fe, Co, or Rh) complexes have been studied (315). The cobalt complex is non-rigid, but the mechanism of optical inversion could not be determined. The Rh complex is stereochemically rigid up to 200°. The optical inversion of (-l-)546 [Colpyr-dtcla] in chloroform has been studied, by loss of optical activity, by polarimetry (316). 

This may be of significance in connetion with the biosynthesis of acetate from carbon dioxide, because the next step, the fixation of carbon monoxide, was demonstrated by B. Krautler. He irradiated methyl cobalamin under Co at 30 atm and obtained the acyl cobalamin as the product. Interestingly, a radical mechanism was iproposed, involving the reaction of methyl radicals with CO to give acyl radicals, which then recombine with the cobalt complex /55/. 

Cobalt complexes are used for the living radical polymerization of acrylates to give a high molecular weight polymer with a narrow molecular weight distribution (Mw/Mn 1.2) (Eq. 71), whereas the complex is applied to the introduction of an unsaturated group into the methacrylate polymers with a high efficiency via a reaction mechanism illustrated in Eq. (72) [27,28,267,268]. 

A mechanism was proposed in which entry into the catalytic cycle is achieved via Et2AlCl-mediated cobalt hydride generation. Diene hydrometallation affords the cobalt-complexed -jr-allyl A-5, which inserts the tethered alkene to furnish intermediate B-4. Elimination of LnCoOBn provides the cyclization product. Reduction of LnCoOBn by Et2AlCl regenerates cobalt hydride to complete the catalytic cycle (Scheme 17). 

The reaction of binuclear cobalt complexes is somewhat different. For Co2(CO)8 the overall mechanism proposed by Heck24 is... 

Carbonylative kinetic resolution of a racemic mixture of trans-2,3-epoxybutane was also investigated by using the enantiomerically pure cobalt complex [(J ,J )-salcy]Al(thf)2 [Co(CO)4] (4) [28]. The carbonylation of the substrate at 30 °C for 4h (49% conversion) gave the corresponding cis-/3-lactone in 44% enantiomeric excess, and the relative ratio (kre ) of the rate constants for the consumption of the two enantiomers was estimated to be 3.8, whereas at 0 °C, kte = 4.1 (Scheme 6). This successful kinetic resolution reaction supports the proposed mechanism where cationic chiral Lewis acid coordinates and activates an epoxide. 

DR. DAVID STANBURY (Rice University) With regard to the problem of the electrocatalytic reduction of oxygen, I have attempted to formalize some ideas regarding the constraints of thermodynamics in order to elucidate the probable character of cobalt complexes which may catalyze the oxygen electrode via binuclear peroxo-bridged intermediates. The following gross mechanism is presupposed ... 

The fact that from a chloro-cobalt complex a chloro-chromium complex is formed, suggests that the reaction must proceed through an intermediate state that enables the transfer of a chlorine atom from cobalt to chromium. The proposed mechanism for this reaction is ... 

The overall response to the reaction variables is very similar in the carbonylation and reductive carbonylation reactions. This may indicate similar catalysts and reaction mechanisms. In the carbonylation reaction Co(CO) " was identified by its characteristic CO stretching frequency ( v(CO) r 1890 cm" as the dominant species present in high pressure infrared experiments carried out at 170 °C and 5000 psig. Similar results were obtained in the reductive carbonylation of methanol. It is known that Co(CO) " rapidly reacts with CH I to yield CH C(0)Co(C0) (J9) however, in the carbonylation and reductive carbonylation reactions acyl-cobalt complexes are not observed by infrared under catalytic conditions. This indicates that once formed, the acyl complex rapidly reacts as outlined by Equations 7 and 8. 

In spite of its simplicity and conceptual clearances, the original protocol has suffered from many intrinsic problems in a practical sense. For example, the reaction with the alkyne-cobalt complexes provided low chemical yields and required harsh reaction conditions. In addition, it was also difficult to extract the obtained product from the sticky metallic residue. Those problems can be accounted for based on the widely accepted mechanism given in Scheme 1, which, as proposed by Magnus," is supported by many theoretical studies. ... 

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 catalysis of the selective oxidation of alkanes is a commercially important process that utilizes cobalt carboxylate catalysts at elevated (165°C, 10 atm air) temperatures and pressures (98). Recently, it has been demonstrated that [Co(NCCH3)4][(PF6)2], prepared in situ from CoCl2 and AgPF6 in acetonitrile, was active in the selective oxidation of alkanes (adamantane and cyclohexane) under somewhat milder conditions (75°C, 3 atm air) (99). Further, under these milder conditions, the commercial catalyst system exhibited no measurable activity. Experiments were reported that indicated that the mechanism of the reaction involves a free radical chain mechanism in which the cobalt complex acts both as a chain initiator and as a hydroperoxide decomposition catalyst. 

It appears difficult to propose a unified mechanism to explain all experimental observations of the cyclotrimerization of acetylene. The most common pathway, studied mainly with cobalt complexes,72 73 involves a metallacyclopentadiene intermediate ... 

Because such alkylation proceeds by S l mechanism, even cobalt complexes derived from unreactive (in an SN2 sense) halides can be formed. The cobalt complexes are air-stable compounds, but are affected by direct daylight. The incorporated Co—C bond is weak and, therefore, photolysis of 33 sets free the anomeric radical 11. In the presence of olefins 12 this radical adds to the double bond, followed by subsequent combination to give the insertion product 35 (Scheme 9). 

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