Cobalt hydride complexes catalytic reactions

The reactions are solvent dependent. Non-polar solvents favour the formation of the neutral hydride (68), whereas in polar media, ionic species predominate. Hydroformylation activity for propylene was observed only under conditions where (68) was formed.291 The dinuclear species [Co2(CO)6(PBu"3)2] (69) also appears to be catalytically inactive.290,291 If PBu"3 is added in excess over cobalt, only complexes (68) and (69) were present. The rate of hydroformylation then became first order in hydrogen, owing to the equilibrium shown in equation (60).292... 

However, only alkyl formates are formed in the conventional reactions of alcohols, CO2 and H2 using transition metal complexes, because intermediary hydride complexes generally react with CO2 to give formate complexes. On the other hand, we have found that mthenium cluster anions effectively catalyze the hydrogenation of CO2 to CO, methanol, and methane without forming formate derivatives [2-4]. Ethanol was also directly formed from CO2 and H2 with ruthenium-cobalt bimetallic catalyst [5]. In this paper, we report that this bimetallic catalytic... 

On the basis of these observations the draft mechanism shown in Scheme 3.160 has been proposed for the catalytic reaction, by analogy with the previous reaction. The reaction of CoCl2(dpph) with M( jSiCH2MgOl gives complex 168, which is electron-rich, because of coordination of the Grignard reagent. Complex 168 effects single-electron transfer to an alkyl halide to yield an anion radical of the halide and cobalt complex 169. Immediate loss of halide from the anion radical affords an alkyl radical intermediate, which adds to styrene to yield a benzyl radical. Cobalt species 169 would then recombine with the carbon-centered radical to form cobalt species 170. Finally, /i-hydride elimination provides... 

Catalytic Reactions Involving Cobalt Carbonyl Hydride Complexes... 

Finally, this mechanistic section cannot possibly be concluded without referring to catalytic chain transfer (CCT) this is the most probable side reaction of CMRP,and involves hydrogen abstraction by the cobalt complex, with the release of cobalt hydride ([Co "—Hj) and imsaturated polymer chains (Equation 4.4). Although CCT is mainly used for the preparation of macromonomers, in a process referred to as catalytic chain transfer polymerization (CCTP) [25], the reaction must be minimized in CMRP. 

Discovered more than 70 years ago, hydroformylation is nowadays one of the most important reactions in the chemical industry because aldehydes can be transformed to many other products. In the enantioselective version, rhodium/ diphosphorus ligand complexes are the most important catalytic precursors, although cobalt and platinum complexes have also been widely used. For these systems, the active species are pentacoordinated trigonal-bipyramidal rhodium hydride complexes, [HRh(P-P)(CO)2]. In those complexes, the coordination mode of the bidentate ligand (equatorial-equatorial or equatorial-apical) is an important parameter to explain the outcome of the process. The most common substrates of enantioselective hydroformylation are styrenes followed by vinyl acetate and allyl cyanide. With these substrates, mixtures of the branched (b, chiral) and linear (1, not chiral) aldehydes are usually obtained. In addition, some hydrogenation of the double bond is often observed. Therefore, chemo- and regioselectivity are prerequisites to enan-tioselectivity and all of them must be controlled. An additional eomplieation is that chiral aldehydes are prone to racemise in the presenee of rhodium spe-... 

The catalytic addition of organic and inorganic silicon hydrides to alkenes, ary-lalkenes, and cycloalkenes as well as their derivatives with functional groups leads to their respective alkyl derivatives of silicon and occurs according to the anti-Markovnikov rule. However, under some conditions (e.g., in the presence of Pd catalysts), this product is accompanied by a-adduct (i.e., the one containing an internal silyl group). Moreover, dehydrogenative silylation of alkenes with hydrosilanes, which proceeds particularly in the presence of iron- and cobalt-triad complexes as related to hydrosilylation (and very often its side reaction), is discussed. 

Other metals can catalyze Heck-type reactions, although none thus far match the versatility of palladium. Copper salts have been shown to mediate the arylation of olefins, however this reaction most probably differs from the Heck mechanistically. Likewise, complexes of platinum(II), cobalt(I), rhodium(I) and iridium(I) have all been employed in analogous arylation chemistry, although often with disappointing results. Perhaps the most useful alternative is the application of nickel catalysis. Unfortunately, due to the persistence of the nickel(II) hydride complex in the catalytic cycle, the employment of a stoichiometric reductant, such as zinc dust is necessary, however the nickel-catalyzed Heck reaction does offer one distinct advantage. Unlike its palladium counterpart, it is possible to use aliphatic halides. For example, cyclohexyl bromide (108) was coupled to styrene to yield product 110. 

In some cases where a reaction involving a radical species occurred within cobalt porphyrin complexes, it has been possible to trap transient cobalt porphyrin hydride species. This was indeed observed during the synthesis of organocobalt porphyrin that resulted from the reaction of cobalt(n) porphyrin and dialkylcyanomethylradicals with alkenes, alkynes, alkyl halides, and epoxide. A transient hydride porphyrin complex was also involved in the cobalt porphyrin-catalyzed chain transfer in the free-radical polymerization of methacrylate. The catalytic chain transfer in free-radical polymerizations using cobalt porphyrin systems has been extensively investigated and will not be treated in this section. Gridnev and Ittel have published a comprehensive overview of the catalytic chain transfer in free-radical polymerizations. ... 

Although a cobalt-catalyzed intermolecular reductive aldol reaction (generation of cobalt enolates by hydrometal-lation of acrylic acid derivatives and subsequent reactions with carbonyl compounds) was first described in 1989, low diastereoselectivity has been problematic.3 6 However, the intramolecular version of this process was found to show high diastereoselectivity (Equation (37)).377,377a 378 A Co(i)-Co(m) catalytic cycle is suggested on the basis of deuterium-labeling studies and the chemistry of Co(ll) complexes (Scheme 81). Cobalt(m) hydride 182, which is... 

Variations in reaction conditions, particularly with respect to acid and iodide concentrations, have profound effects on the positions of these equilibria, and these have, for the most part, added significant complexity to the behavior of the systems. It has also been determined that the catalytic activity of the cobalt catalyst system is increased with the introduction of H2 (44) or halides (vide infra), though these increases in rate are quite often at the expense of selectivity. The presence of added H2 serves as an alternate route to the formation of the hydride species ... 

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