Cobalt, complexes alkyl

Alkyl radicals have also been prepared by reaction of alkylbromides with photolytically generated Re(CO)5 (from Re2(CO)io) [17], photolysis of cobalt-alkyl complexes [20], photolysis of AIBN [17, 21, 22] or thermolysis of TEMPO adducts [23]. 

In reductive acylation and dimerization, the cathode is often superior to dissolving metal or radical anions reductants. So a, j6-unsaturated ketones or esters can be acylated in high yield to 1,4-dicarbonyl compounds at the mercury cathode [39], but the corresponding reaction with sodium in tetrahydrofuran (THE) fails [40]. On the other hand, reductive acylation of double bonds becomes possible in high yield, when vitamin Bj2 is used as mediator [41]. Here cobalt-alkyl complexes play a decisive role as intermediates. 

Cyclooctene also reacts with CoH(CO)3(PBu3) to give a cobalt-alkyl complex however, the latter complex decomposes upon heating to give back the reactants rather than a hydrogenated product. In many studies, Co2(CO)g is reacted in situ with phosphine ligands to generate catalytic species . 

It is only during the past ten years that reliable and widely applicable methods for determining homolytic metal-alkyl bond dissociation energies of stable or-ganometallic compounds in solution have been developed and that information about such bond dissociation energies has become available. Today about one hundred transition metal-alkyl bond dissociation energies have been determined, the majority for cobalt-alkyl complexes. Most of these have been from kinetic measurements. The scope, limitations and results of such determinations are discussed. 

The majority of BDE determinations of the type described above have been made on cobalt-alkyl complexes. Among the reasons for this emphasis are (a) the relevance of such complexes as models for coenzyme B, whose action is triggered by enzyme-induced cobalt-carbon bond nomolysis (Equation 4) (4), (b) the typically low range... 

Table V. Simultaneous Radical Formation and Olefin Elimination During Thermolysis of Cobalt Alkyl Complexes...

The catalyst precursor is the 18-electron hydrido cobalt tetracarbonyl complex A, which dissociates a CO ligand to give the 16-electron active catalyst B. The next step is the coordination of alkene to give the 18-electron it complex C. This is followed by rapid insertion of the alkene into the metaTTiydrogen bond by hydride migration to form the cobalt alkyl complex D. The next step is addition of CO from the gas phase to afford the 18-electron tetracarbonyl complex E, which undergoes CO insertion to give the 16-electron acyl complex F. This is followed by oxidative addition of H2 to the Co acyl complex to form the 18-electron Co dihydrido complex G. 

V. Gibson, K. Tellmann, M. Humphries, D. Gould, Bis(imino)pyridine cobalt alkyl complexes and their reactivity towards ethylene a model system for /9-hydrogen chain transfer. (Them. Commun. 20, 2316-2317 (2002)... 

A comparison of the rates of insertion of ethylene into metal-hydride and metal-alkyl bonds. The cobalt-hydride comply inserts ethylene, but the cobalt-alkyl complex does not. 

Reversible formation of a cobalt-alkyl complex from HCo(CO) and olefin has been established. The reversible formation of a cobalt alkyl is required to explain observations of alkene isomerization during hydroformylation by HCo(CO) and scrambling of deuterium in substrates, such as CD3(CHj)3CH=CH2, mider relatively low pressures of the mixture of CO and (Equation 17.4). Little deuterium is lost during transformation of the alkene to product, but the deuterium atoms are distributed nearly statistically along the chain. ... 

In other cases, it has been shown that isomeric cobalt-alkyl complexes interconvert without dissociation of alkene from the hydridocobalt-alkene complex. For example, isomerization through a tertiary alkyl intermediate without alkene dissociation is required by retention of optical activity and deuterium labeling in the products of hydroformylation of the non-racemic alkene in Equation 17.5. - These results are consistent with extensive, rapid, intramolecular isomerization of the intermediate cobalt alkyl to place the metal at terminal and internal positions along the alkyl chain. 

Thus, when the diiodide derivative was used in combination with MMAO (300 < Al/Co < 1000), turnover frequencies of ethylene polymerization of up to 630 TON h were obtained, the catalyst being moreover stable for hours at RT. The analysis of the catalytic properties of this compound led to the conclusion that the catalyst resting state is a cobalt alkyl complex and not a cobalt alkyl ethylene complex. Thus, the cationic methyl acetonitrile or arylnitrile species were active for the polymerization of ethylene and, not requiring the presence of MMAO as under related reaction conditions, they led to polyethylene products with similar molecular weights and branching levels as compared to the polyethylene produced with the diiodide species. 

In addition, Coates et al. have developed a cobalt-catalyzed carbonylation of epoxides for the synthesis of substituted 3-hydroxy-8-lactones [113] (Scheme 66). After screening for several catalysts, HCo(CO)4 was identified as the best catalyst to effect this transformation. The proposed mechanism of the carbonylation involves protonation and ring opening of the epoxide 302 by the catalyst to form cobalt alkyl complex 303, followed by insertion of CO and subsequent cyclization to generate the 3-hydroxy-8-lactone framework 305. 

A related, tetradentate ligand bis(acetylacetone)ethylenediamine, (MeCOCH2CMe=NCH2)2 (abbreviated to BAE), also stabilizes cobalt-alkyl complexes, including the pentaco-ordinate (BAE)CoR [132a]. 

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