Cobalt hydride intermediate

Indeed the cobalt hydride disproportionates to Co" and hydrogen and at high conversions the addition is suppressed because cobalt(II) couples with starting radicals (to regenerate the starting complex). This is one reason why excess alkene is required. Hydrocobaltation of the acceptor by the cobalt hydride intermediate can also cause problems, see W. M. Bandaranayake and G. Pattenden, J. Chem. Soc., Chem. Commun., 1988, 1179. 

The cobalt—hydride intermediate has not been isolated and is virtually undetected. Cobalt hydride reaction with an olefin by first migrating the hydrogen atom from cobalt to a porphyrin nitrogen atom (78) or carbon atom (79) is precedented by the isomerization of a benzylcobalt chelate. In that case, the benzyl migrates reversibly from the cobalt to the carbon atom of the equatorial ligand.253... 

For the cobalt catalyst the story appears somewhat clearer. Both experiment and theory have been shown to be in agreement and support a stepwise pathway for the reaction of the cobalt(I) alkyl complexes with 1-alkenes, reacting by (3-hydride transfer via a cobalt hydride intermediate [135]. Such cobalt alkyls have also been shown to contain low-spin cobalt(II) antiferromagnetically coupled to a hgand radical anion. The lowest triplet state is thermaUy accessible and accounts for the observed H NMR chemical shifts at room temperature [66]. 

The dominant reactions that occur during the early portion of the induction period when significant amounts of (TMP)Co are present are shown in Scheme 5.2, eqn (5.2.1)-(5.2.6). Hydrogen abstraction from a methyl group on the tertiary carbon radical produced by V-70 forms a transient cobalt hydride intermediate (TMP)Co-H which adds with the methyl acrylate monomer to produce an organometallic complex, (TMP)Co-CH(CH3)C02CH3 (eqn (5.2.2),... 

Cobalt(II) complexes of three water-soluble porphyrins are catalysts for the controlled potential electrolytic reduction of H O to Hi in aqueous acid solution. The porphyrin complexes were either directly adsorbed on glassy carbon, or were deposited as films using a variety of methods. Reduction to [Co(Por) was followed by a nucleophilic reaction with water to give the hydride intermediate. Hydrogen production then occurs either by attack of H on Co(Por)H, or by a disproportionation reaction requiring two Co(Por)H units. Although the overall I easibility of this process was demonstrated, practical problems including the rate of electron transfer still need to be overcome. " " ... 

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). 

Pettit and coworkers—metal hydride intermediates by weak base attack over Fe carbonyl catalysts. Pettit et al.ls approached the use of metal carbonyl catalysts for the homogeneous water-gas shift reaction from the standpoint of hydroformyla-tion by the Reppe modification.7 In the typical hydroformylation reaction, an alkene is converted to the next higher aldehyde or alcohol through reaction of CO and H2 with the use of a cobalt or rhodium carbonyl catalyst. However, in the Reppe modification, the reduction is carried out with CO and H20 in lieu of H2 (Scheme 6) ... 

Sodium borohydride (160) was found to serve as a hydrogen donor in the asymmetric reduction of the presence of an a,pi-unsaturated ester or amide 162 catalyzed by a cobalt-Semicorrin 161 complex, which gave the corresponding saturated carbonyl compound 163 with 94-97% ee [93]. The [i-hydrogen in the products was confirmed to come from sodium borohydride, indicating the formation of a metal enolate intermediate via conjugate addition of cobalt-hydride species (Scheme 2.17). 

Thus when the >73-l,l-dimethylallyltris(trifluorophosphine) complexes of cobalt or rhodium are gently warmed a rearrangement to the 73-l,2-isomer occurs (56, 295). The postulated mechanism involves a diene-metal hydride intermediate (Scheme 9). The small PF3 ligand can also be added directly to coordinatively unsaturated 3-allylic systems or to chloro-bridged structures (method H). 

A third reaction scheme (eq 14 and 15) encompasses what is the currently accepted mechanism. This scheme was suggested simultaneously with eqs 10 and 12 and calls for the formation of the cobalt hydride, LCoH, as an intermediate species.161 162... 

Species such as 74 and 75 could be intermediates in the trans addition of cobalt hydride to acetylene. 

Bridged species such as 76 are well documented in rhodium porphyrin chemistry.240-241 An acetylene bonded to one metal-centered radical is presumed to be trapped by addition of a second metal-centered radical. Lower bond dissociation energies of cobalt relative to rhodium would disfavor species such as 76 and facilitate the reaction with metal—hydride intermediates to form a trans product. 

The use of organometallic compounds as chain-transfer catalysts in free-radical polymerization has been widely studied. One objective is the production of polymers with terminal vinyl groups and lower molecular weight components compared with polymerization in the absence of chain-transfer catalysts. Gomplexes of cobalt(ii) have been used as effective catalysts, but the instability of the intermediate cobalt hydride does not permit firm establishment of the reaction mechanism. To address this issue, several chromium compounds have been applied as catalysts for the polymerization of methylmethacrylate (MMA) and styrene. The temperature dependence of the rate constant for free-radical polymerization of MMA for catalyzed chain transfer by (GsPh5)Gr(GO)3 has been determined using the Mayo equation. ... 

An even more active version of the catalysts containing Lewis acids and metal anions allows the reactions to be conducted at ambient temperatures with 1 atm of CO. This catalyst appears to suppress the competitive formation of ketones from isomerization of the epoxide ° that possibly occurs through p-hydride elimination by the cobalt-alkyl intermediate generated prior to CO insertion. Reactions of a range of epoxides catalyzed by [(salph)Cr(THF)2] [Co(CO)J occur with balloon pressures of carbon monoxide, even on the multigram scale. ... 

A novel method of generating carbocation intermediates has been developed, which utilizes a cobalt complex, A-fluoropyridinium salt, and silane (Scheme 13). The chemistry has been used to prepare hydroxalkoxylation products (51) from olefins by Markovnikov addition. A mechanism has been proposed in which a cobalt hydride species adds to the olefin to provide intermediate (52). Initial formation of the carbon-centered radical (53) is followed by oxidation to the carbocation (54). Subsequent trapping of the carbocation leads to the hydroxalkoxylation product (51). 

This system was disclosed in 1942 by Igushi, and it involves H2 addition onto the 17-electron cobalt center of [Co(CN)5] " that serves as a reservoir of hydrogen atoms. The latter are transferred, one by one, by the 18-electron cobalt hydride complex [HCo(CN)5] onto the olefin. The intermediate organic radical must be sufficiently stable, which is the case for the benzyl radical. Only activated olefins can react in this way thus, this mechanism is rather rare. ... 

Carbonylation of alcohols such as ethanol and -propanol with cobalt, rhodium, and iridium catalysts have also been studied. With propanol and higher alcohols, the product is found to have both the linear and the branched isomer. Thus carbonylation of n-propanol gives both butyric and isobutyric acid. In these reactions, a metal hydride intermediate such as 4.9 plays a crucial role. 

The chemistry of organorhodium and -iridium porphyrin derivatives will be addressed in a separate section. Much of the exciting chemistry of rhodium (and iridium) porphyrins centers around the reactivity of the M(ll) dimers. M(Por) 2-and the M(III) hydrides, M(Por)H. Neither of these species has a counterpart in cobalt porphyrin chemistry, where the Co(ll) porphyrin complex Co(Por) exists as a monomer, and the hydride Co(Por)H has been implicated but never directly observed. This is still the case, although recent developments are providing firmer evidence for the existence of Co(Por)H as a likely intermediate in a variety of reactions. 

In contrast to the rhodium porphyrin hydride complexes, Rh(Por)H, which play a central role in many of the important developments in rhodium porphyrin chemistry, the corresponding cobalt porphyrin hydride complexes have been implicated as reaction intermediates in a variety of processes, but a stable, i.solable example has yet to be achieved. 

The corresponding reactions of transient Co(OEP)H with alkyl halides and epoxides in DMF has been proposed to proceed by an ionic rather than a radical mechanism, with loss of from Co(OEP)H to give [Co(TAP), and products arising from nucleophilic attack on the substrates. " " Overall, a general kinetic model for the reaction of cobalt porphyrins with alkenes under free radical conditions has been developed." Cobalt porphyrin hydride complexes are also important as intermediates in the cobalt porphyrin-catalyzed chain transfer polymerization of alkenes (see below). 

Tetraazamacrocyclic complexes131 of cobalt and nickel were found110 to be effective in facilitating the reduction of C02 at -1.3 to -1.6 V versus SCE (Table 8). An acetonitrile-water mixture and water were used as solvents, while in dry dimethylsulfoxide no catalytic reduction of C02 took place. Using an Hg electrode, both CO and H2 were produced, where total current efficiencies were greater than 90%. The turnover numbers of the catalysts were 2-9 h 1. The catalytic activity lasted for more than 24 h and the turnover numbers of the catalysts exceeded 100. A protic source was required to produce both CO and H2, and the authors suggested that both products may arise from a common intermediate, which is most likely a metal hydride. The applied potential for C02 reduction was further reduced by using illuminated p- Si in the presence of the above catalysts.111... 

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