Cobalt complexes supported

Several synthetic Co(III) and Fe(III) complexes have been generated as models for the active-site metal center in nitrile hydratases [8, 10]. However, few have been examined in terms of water coordination and acidity. Cobalt complexes supported by Ns-type donor ligands, with two carboxamido nitrogen donors, exhibit a pfCa near 7 (Fig. 8.14a and b) [59, 60]. Introduction of two thiolate sulfur donors into the Co(III) coordination sphere increases the pK of the bound water to 8.3 (Fig. 8.14c) [61]. Interestingly, oxidation of one of the sulfur donors to a S-bound sulfmate (Fig. 8.14d) reduces the pK by around 1 unit [62]. As the active-site metal center in nitrile hydratases contain oxidatively modified cysteine residues coordinated to the metal center [8], it has been suggested that the oxidized sulfur donors play a role in modulating the acidity of the metal-bound water molecule. 

With sp bond angles calculated to be around 162°, macrocycle 131 would be highly strained and was therefore expected to be quite reactive [79]. The octa-cobalt complex 132, on the other hand, should be readily isolable. Indeed, 132 was prepared easily from 133 in five steps, and was isolated as stable, deep maroon crystals (Scheme 30). All spectroscopic data supported formation of the strain-free dimeric structure. Unfortunately, all attempts to liberate 132 from the cobalt units led only to insoluble materials. Diederich et al. observed similar problems when trying to prepare the cyclocarbons [5c]. Whether the failure to prepare these two classes of macrocycles is due to the extreme reactivity of the distorted polyyne moiety or to the lack of a viable synthetic route is not certain. Thus, isolation and characterization of smaller bent hexatriyne- and octatetrayne-containing systems is an important goal that should help answer these questions. 

Since cobalt on kieselguhr in one of the original Fischer-Tropsch catalysts (1-9), it appeared attractive to investigate the catalytic activity of cobalt complexes immobilized on polystyrene. Although there are many supported cobalt-based Fischer-Tropsch catalysts known (see, for example, references 18-21), no polystyrene-bound systems had been reported. During the course of our work 18% (22,60,61) and 20% (23) crosslinked analogs of CpCo(C0)2 were shown to exhibit limited catalytic activity but no CO reduction. A preliminary disclosure of our work has appeared (2)4). 

Carbonylative kinetic resolution of a racemic mixture of trans-2,3-epoxybutane was also investigated by using the enantiomerically pure cobalt complex [(J ,J )-salcy]Al(thf)2 [Co(CO)4] (4) [28]. The carbonylation of the substrate at 30 °C for 4h (49% conversion) gave the corresponding cis-/3-lactone in 44% enantiomeric excess, and the relative ratio (kre ) of the rate constants for the consumption of the two enantiomers was estimated to be 3.8, whereas at 0 °C, kte = 4.1 (Scheme 6). This successful kinetic resolution reaction supports the proposed mechanism where cationic chiral Lewis acid coordinates and activates an epoxide. 

The reaction using 11a as a substrate in the presence of several oxides as additives revealed that addition of tributylphosphine oxide, hexamethylphos-phoric triamide, and dimethyl sulfoxide all accelerate the reaction considerably. Furthermore, when about 10 molar amounts of N-methylmorpholine M-oxide (NMO) is added to the alkyne-cobalt complex 12b in THF,the reaction proceeds even at room temperature and cyclopentenone 13 b is obtained in 37% yield accompanied by another rearranged product, the methylenecyclobutanone 35, obtained in 23% yield as a mixture of ( )-and (Z)-isomers (Scheme 14). These facts indicate that dissociation of the carbonyl ligand of the alkyne-cobalt complex 12 is the rate-determining step in this rearrangement. This is also supported by the fact that under a CO atmosphere in refluxing THF the reaction is completely suppressed. 

In spite of its simplicity and conceptual clearances, the original protocol has suffered from many intrinsic problems in a practical sense. For example, the reaction with the alkyne-cobalt complexes provided low chemical yields and required harsh reaction conditions. In addition, it was also difficult to extract the obtained product from the sticky metallic residue. Those problems can be accounted for based on the widely accepted mechanism given in Scheme 1, which, as proposed by Magnus," is supported by many theoretical studies. ... 

The requirement of the harsh reaction condition is mainly related to the first decarbonylation of the complex 3, as previously mentioned. The first practical and significant rate acceleration had been achieved by the dry support adsorption method on silica gel or alumina. Smit and Gaple used the cobalt-complexed enynes 1 adsorbed on silica gel, and afforded the cyclopentenones in good yields by heating mildly over several hours (Table I). 

This complex is analogous in all ways to the cobalt complex (XXIV), and the proposed structure is supported by its infrared spectrum which shows bands due to the ir-cyclopentadienyl-metal grouping, a methylene group, and conjugated double bonds co-ordinated to the metal atom (at 1451 cm-1). 

Both the rhodium and the cobalt complexes catalyze olefin isomerization as well as olefin hydroformylation. In the case of the rhodium(I) catalysts, the amount of isomerization decreases as the ligands are altered in the order CO > NR3 > S > PR3. When homogeneous and supported amine-rhodium complexes were compared, it was found that they both gave similar amounts of isomerization, whereas with the tertiary phosphine complexes the supported catalysts gave rather less olefin isomerization than their homogeneous counterparts (44, 45). 

Relatively few hydroformylations using supported cobalt complexes have been reported. Moffat (78, 79) showed that poly-2-vinylpyridine reversibly reacted with both Co2(CO) and HCo(CO)4, the cobalt carbonyl being displaced by excess carbon monoxide. This enabled the polymer to pick up the cobalt carbonyl at the end of the reaction and, thus, allowed it to be separated from the products by filtration. The polymer acted as a catalyst reservoir by rapidly releasing the cobalt carbonyl into solution in the presence of further carbon monoxide, so that the actual catalysis was a homogeneous process. More recently, cobalt carbonyl has been irreversibly bound to a polystyrene resin... 

Radical additions of primary, secondary, and tertiary alkyl bromides 249 to diethyl mesaconate 248 catalyzed by 5 mol% vitamin B12a 247 (X=OH2) proceeded in yields of 63-90% [301]. Deuteration experiments and comparison to similar addition processes support that 247 is initially reduced to cobalt complex 253A. This reacts with 249 giving an alkylcobalamin(III) intermediate... 

Arene ruthenium complexes are used frequently in metal-mediated organic synthesis for a wide range of reactions.5 For the purposes of our studies we have focused attention mainly on enol formate synthesis as a representative reaction for comparing the activity of 2 with its non-supported analogue 5. As with the supported cobalt complex, we find that attachment of 5 to a polymer support has little effect in its catalytic activity with a range of enol formates being prepared in high yield. 

The active site of methionine aminopeptidase contains a binuclear cobalt complex that is required for activity, although a number of divalent metal ions support turnover to varying degrees. X-ray crystallographic studies on the enzyme in complexes with transition state analogs suggests that the binuclear metal cluster serves to stabilize the tetrahedral intermediate in peptide hydrolysis. ... 

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