Cobalt complexes as catalysts

In the presence of cobalt complexes as catalysts, the following stoichiometry was reported for the autoxidation of H2DTBC in chloroform (52) ... 

Two types of intermediates, i.e., radicals or carbanions or their organometallic equivalents, can be used to perform addition reactions to Michael acceptors. The free-radical route has already been investigated with nickel or cobalt complexes as catalysts [62-64]. These studies have been reinvestigated recently with the aim of improving the turn-over of the catalyst and/or using easily prepared cheap complexes. 

Three commercial homogeneous catalytic processes for the hydroformyla-tion reaction deserve a comparative study. Two of these involve the use of cobalt complexes as catalysts. In the old process a cobalt salt was used. In the modihed current version, a cobalt salt plus a tertiary phosphine are used as the catalyst precursors. The third process uses a rhodium salt with a tertiary phosphine as the catalyst precursor. Ruhrchemie/Rhone-Poulenc, Mitsubishi-Kasei, Union Carbide, and Celanese use the rhodium-based hydroformylation process. The phosphine-modihed cobalt-based system was developed by Shell specih-cally for linear alcohol synthesis (see Section 7.4.1). The old unmodihed cobalt process is of interest mainly for comparison. Some of the process parameters are compared in Table 5.1. 

The cobalt mediated homo Diels-Alder reaction of norbomadiene (560) with phenyl acetylene (568a), affording a phenyl substituted deltacyclene, demonstrated the potential of low-valent cobalt complexes as catalysts . Lautens and coworkers extended the scope of this reaction and were able to synthesize a wide range of substituted deltacyclenes from alkynes 568 (equation 164, Table 33). The low-valent cobalt(I) or cobalt(O) species to be used was prepared in situ by reduction of Co(acac)3 with Et2AlCl. Monosubstituted... 

Novel platinum(ii) Schiff base complexes were found to be efficient photosensitizers for H2 evolution from mixed solvents (e.g. DMF/H2O) under visible light irradiation, with a cobalt complex as catalyst and TEA as sacrificial electron donor. Catal5Aic activity is influenced by the presence of substituents on the ligand electron-withdrawing groups, in particular, have a positive effect. ... 

This chapter describes our initial investigations of cobalt complexes bound to colloidal polymers for the autoxidations (in which dioxygen is the stoichiometric oxidant) of tetralin, 2,6-di-r rr-butylphenol, and 1-decanethiol. The reactions of these water-insoluble organic compounds proceed faster in the colloids that they do with the same cobalt complexes as catalysts in aqueous solution. Autoxidations have been chosen because of their potential for low cost use for the decontamination of water and for chemical manufacturing processes. 

A more efficient system was obtained if copper acetate was replaced by salen-type cobalt complexes as catalysts, operating together with hydroquinone or quinone. Finally, incorporation of a hydroqninone as part of the salen ligand gave an even more efficient catalyst that did not reqnire cocatalysis by quinone (Scheme 11). 

An early attempt to hydroformylate butenediol using a cobalt carbonyl catalyst gave tetrahydro-2-furanmethanol (95), presumably by aHybc rearrangement to 3-butene-l,2-diol before hydroformylation. Later, hydroformylation of butenediol diacetate with a rhodium complex as catalyst gave the acetate of 3-formyl-3-buten-l-ol (96). Hydrogenation in such a system gave 2-methyl-1,4-butanediol (97). 

There are currentiy no commercial producers of C-19 dicarboxyhc acids. During the 1970s BASF and Union Camp Corporation offered developmental products, but they were never commercialized (78). The Northern Regional Research Laboratory (NRRL) carried out extensive studies on preparing C-19 dicarboxyhc acids via hydroformylation using both cobalt catalyst and rhodium complexes as catalysts (78). In addition, the NRRL developed a simplified method to prepare 9-(10)-carboxystearic acid in high yields using a palladium catalyst (79). 

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... 

Polymerization employing Co complexes as catalysts or else polymers incorporating functionality that includes Co ions represent aspects of polymerization reactions of interest here. Cobalt-mediated free-radical polymerization of acrylic monomers has been reviewed.55 Co11 porphyrins act as traps for dialkylcyanomethyl radicals.1098 Alkyl complexes of Co(TMesP)... 

Electrocatalysis employing Co complexes as catalysts may have the complex in solution, adsorbed onto the electrode surface, or covalently bound to the electrode surface. This is exemplified with some selected examples. Cobalt(I) coordinatively unsaturated complexes of 2,2 -dipyridine promote the electrochemical oxidation of organic halides, the apparent rate constant showing a first order dependence on substrate concentration.1398,1399 Catalytic reduction of dioxygen has been observed on a glassy carbon electrode to which a cobalt(III) macrocycle tetraamine complex has been adsorbed.1400,1401... 

Tinnemans et al.132 have examined the photo(electro)chemical and electrochemical reduction of C02 using some tetraazamacrocyclic Co(II) and Ni(II) complexes as catalysts. CO and H2 were the products. Pearce and Pletcher133 have investigated the mechanism of the reduction of C02 in acetonitrile-water mixtures by using square planar complexes of nickel and cobalt with macrocyclic ligands in solution as catalysts. CO was the reduction product with no significant amounts of either formic or oxalic acids... 

A relatively high molecular weight copolymer can be synthesized by using an acyl-cobalt complex as an initiator such a catalyst system is not accompanied by dehydration theoretically because the terminal group of a growing species is not a free alcohol but protected by an acyl group (Scheme 13). 

Organic electroreductive coupling reactions using transition-metal complexes as catalysts have been widely investigated. Reviews on the subject have been published [89, 90]. The method involving the most common transition-metal complexes (nickel, cobalt, palladium) appears to be a useful tool to synthetize heterocycles from organic halides via radical intermediates. Nickel catalyst precursors are nickel(II) salts that are cathodically reduced either to nickel(I) or to nickel(O) and cobalt catalyst... 

Yamada and coworkers [25-27] have employed chiral p-ketoiminato cobalt (III) complexes as catalysts for the same type of reaction. The cycloadducts were obtained with complete endo selectivity and in moderate to high ee (Scheme 7). 

On the other hand, in two other papers, the formation of hydrogen gas was not mentioned, whereas carbon monoxide and formic acid were both observed as products. In studies carried out by Ogura and coworkers [123], electrogenerated [Co(PPh3)2L] (where L is a substituted quinoline, bipyridine, or phenan-throline moiety) was employed as a catalyst for the reduction of CO2 in anhydrous organic solvents, conditions for which the current efficiency for production of CO (the main product) was 83%. Similarly, in an investigation done by Behar et al. [124], who used cobalt porphyrins as catalysts in an acetonitrile medium, the formation of both carbon monoxide and formic acid was noted however, the catalytic species did not appear to contain cobalt(I), but rather a cobalt(O) species complexed with carbon dioxide. 

Radical reactions involving cobalt(I) catalysis find their model in Nature, which uses vitamin B12 247 extensively for catalyzing difficult transformations [267-273]. This served as an early inspiration to use cobalt complexes as mediators and catalysts for radical reactions [268, 298]. 

The low-pressure oxo process based on rhodium complex catalysts has largely replaced the older, high-pressure process, which used cobalt carbonyls as catalyst. The low-pressure process is operated at about 100°C and 200psig. A new generation oxo process with bisphosphite modified rhodium catalyst is shown schematically in Fig. 10.5. 

The aerobic oxidation of phenols in the presence of cobalt-Schiffs base complexes as catalysts is facilitated by (electron-donating) alkyl substituents in the ring and affords the corresponding p-quinones, e.g. the Vitamin E intermediate drawn in Fig. 4.87. When the para-position is occupied the reaction may be directed to the ortho-position [252, 253]. Copper compounds also mediate this type of oxidation, e.g. the Mitsubishi Gas process for the Vitamin E intermediate... 

Homogeneous catalysis concerns processes in which a catalyst is in solution with at least one of the reactants. An example of homogeneous catalysis is the industrial Oxo process for manufacturing normal isobutylaldehyde. ft has propylene, carbon monoxide, and hydrogen as the reactants and a liquid-phase cobalt complex as the catalyst. 

In contrast to the behavior of the foregoing nickel(I) complexes as catalysts, the catalytic reactions of alkyl halides with cobalt(I) species such as vitamin Bi2s, cobaloximes(I), and cobalt(I) salen exhibit a significant difference. Cobalt(I) species, acting as potent nucleophiles in Sn2 reactions with alkyl halides, give stable alkylcobalt(III) intermediates. Lexa and coworkers [318] have discussed this mechanistic scheme for the catalytic reduction of l-bromobutane by the electrogenerated cobalt(I) tetraphenylpor-phin complex, where TPP denotes the ligand. Reversible one-electron reduction of the parent cobalt(II) species... 

The hydroformylation of alkenes is commonly run using soluble metal carbonyl complexes as catalysts but there are some reports of heterogeneously catalyzed reactions of olefins with hydrogen and carbon monoxide. Almost all of these are vapor phase reactions of ethylene or propylene with hydrogen and carbon monoxide catalyzed by rhodium, " 20 ruthenium,nickel, 22,123 cobalt, 23,124 and cobalt-molybdenum 23 catalysts as well as various sulfided metal catalysts. 23,125,126... 

In 1989 Oehme et al. reported a photoassisted synthesis of a-substituted pyridines under mild conditions using -Cp-cobalt complexes as the catalyst [21]. 

Bdnnemann and co-workers [22] and others [23] have tried acetylacetonato-and -Cp-rhodium as well as resin-attached / -Cp-rhodium complexes as catalysts in the pyridine synthesis [23]. However, rhodium catalysts are generally less effective than the analogous cobalt systems. 

Cp—Cp coupling occurs, probably via the first-formed palladium phenolate (315) to give the bisquinone methide (316), and the latter spontaneously undergoes intramolecular Diels-Alder reaction to the natural lignan carpanone (317) in 46% yield, with stereocontrol at five chiral centers. High yields, up to 94%, have been recorded using oxygen as oxidant with a metal(II)-salen complex as catalyst, e.g. cobalt(II) salen. A low yield of carpanone was also obtained in electrooxidation. 8... 

The use of zero-valent transition metal (mainly nickel and cobalt) complexes as promoters of the homo-Diels-Alder reaction has been an important development. These catalysts allow 1,4-dienes to react with nonactivated alkenes and alkynes, broadening the scope of the homo-... 

The use of transition metal complexes as catalysts allows 1,4-cycloadditions to be involved as the major pathway in several cases when conjugated dienes are reacted with norbornadiene. No normal homo-Diels-Alder reaction was observed by reaction of the latter with buta-1,3-diene in the presence of an iron complex catalyst, the main product obtained was such a 1,4-adduct 2f the same adduct 2 was obtained in good yield and selectivity when a catalyst formed from cobalt(II) chloride, diethylaluminum chloride and bis(l,2-diphenylphos-phinojethane was used. ... 

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