Organocobalt complexes reactions

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 ... 

This reaction leads to an organocobalt complex, RColnX2, which is reduced to the corresponding RConX. Owing to the low stability of the cobalt(I) species, the reaction proceeds in one step (equation 29). 

The cyclization reactions of organocobalt complexes are very useful, and they offer an excellent alternative to the tin hydride method when reduced products are not desired. Most cobalt cyclizations have been conducted with nucleophilic radicals. Precursors are prepared by alkylation of cobalt(I) anions, and are usually (but not always) isolated. One suspects that alkylcobalt precursors should be useful for slow cyclizations because there are no rapid competing reactions that would consume the initial radical (coupling of the initial radical with cobalt(II) regenerates the starting complex). 

The reaction of both fluorocarbon and hydrocarbon alkynes with the complexes Co3(CO)gCCH3, Co3(CO)gCCeH5, and C03(CO)gCCO2Me has been investigated 31). The products obtained include substituted benzenes and organocobalt complexes of formula Co2(CO)6(alkyne) and Co2(CO)4(alkyne)3. Treatment of some complexes Co3(CO)gCR with OH in R OH followed by the addition of HX produces Co4(CO)ig-(RC2R) in 15% yield 131a). 

B. Giese, A. Ghosez, T. Gobel, J. Hartung, O. Huter, A. Koch, K. Kroder, and R. Springer, Radical reactions with organocobalt complexes. Free Radicals in Synthesis and Biology (F. Minisci, ed.), KJuwer Academic Publishers, 1989, p. 97. 

This paper evaluates the role of the hydrido complex in various carbon-carbon double bond hydrogenations and carbon-halogen bond hydrogenolyses, and the relationship between the two reaction types. Evidence for free radicals, as well as organocobalt complexes ( metal-complexed radicals), as intermediates will be presented. The relationship of organocobalt complex stability to catalytic hydrogenation will also be discussed. 

The addition of hydrido complex-scavenging reagents would be expected either to reduce the reagent (19, 20) (Reaction 14) or to form a stable organocobalt complex (22) (Reaction 15), depending on the re-... 

Neopentyl and Neophyl Iodides. The halides mentioned above formed the corresponding organocobalt complexes on reaction with pentacyanocobaltate(II). While neopentyl radical is stable to rearrangement, the neophyl radical undergoes aryl migration (56) as follows ... 

In addition to the work described here, experiments with styrenes and a,)8-unsaturated acids, esters and ketones, as well as related halides (24), suggest certain generalizations concerning the effect of structure on organocobalt complex stability and catalysis. Table IX lists several organocobalt complexes, whose preparations, either by reaction of the halide with pentacyanocobaltate(II) or by addition of hydrido complex to the corresponding unsaturated substrate, have been attempted. In many cases, both methods were used. 

These reactions are believed to proceed via a reductive process to give an organocobalt complex 37a, which undergoes an intramolecular cyclization of the radical 37b generated by homolytic cleavage of C-Co bond (Scheme 15). 

Because of the toxicity and purification problems associated with tributytin hydride reactions, many ring expansions have been attempted under tin-free conditions. These include the use of reagents such as (Me3Si)3SiH [15a], 138/02 [15b] and SmI [15c], the photoinduced electronic transfer (PET) process [15d], photolysis of thiohydroxamic esters or organocobalt complexes [15e], and electroreductions of fi-keto esters [15f]. 

Besides organocobalt complexes, organostibine has also been found to be able to mediate controlled/ diving polymerization of many vinyl monomers. " For example, Yamago et al reported that at 60 °C the polymerization of VAc mediated by a-dimethylstibanyl ester reached 92% conversion in 5 h and produced narrowly dispersed PVAc (PDl = 1.26), but no detailed reaction kinetics was provided in their study.Kamigaito et al. very recently found that a manganese carbonyl complex [Mn2(CO)io] coupled with an alkyl iodide ( -I)... 

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