CO complex

Figure C3.1.10. (a) Steady state IR difference spectmm (dark minus light) of cytoclirome c oxidase CO complex measured at low temperature (127 K). This protein contains a copper atom situated immediately adjacent to a haem...

In order to make these oxidative reactions of 1,3-dienes catalytic, several reoxidants are used. In general, a stoichiometric amount of benzoquinone is used. Furthermore, Fe-phthalocyanine complex or Co-salen complex is used to reoxidize hydroquinone to benzoquinone. Also, it was found that the reaction is faster and stereoselectivity is higher when (phenylsulflnyl)benzoquinone (383) is used owing to coordination of the sulfinyl group to Pd, Thus the reaction can be carried out using catalytic amounts of PdfOAcji and (arylsulfinyl)benzoquinone in the presence of the Fe or Co complex under an oxygen atmosphere[320]. Oxidative dicyanation of butadiene takes place to give l,4-dicyano-2-butene(384) (40%) and l,2-dicyano-3-butene (385)[32l]. 

Succinic acid diesters are also obtained by one-step hydrogenation (over Pd on charcoal) and esterification of maleic anhydride dissolved in alcohols (40) carbonylation of acrylates in the presence of alcohols and Co complex catalysts (41—43) carbonylation of ethylene in alcohol in the presence of Pd or Pd—Cu catalysts (44—50) hydroformylation of acetylene with Mo and W complexes in the presence of butanol (51) and a biochemical process from dextrose/com steep Hquor, using Jinaerobiumspirillum succiniciproducens as a bacterium (52). 

The complex CUCN.NH3 provides an unusual example of CN aeting as a bridging ligand at C, a mode which is common in p,-CO complexes (p. 928) indeed, the complex is unique in featuring tridentate CN groups which link the metal atoms into plane nets via the Cu... 

Simple ligand-field arguments, which will be elaborated when M ions of the Ni, Pd, Pt triad are discussed on p. 1157, indicate that the configuration favours a 4-coordinate, square-planar stereochemistry. In the present group, however, the configuration is associated with a lower oxidation state and the requirements of the 18-electron rule, which favour 5-coordination, arc also to be considered. The upshot is that most Co complexes are 5-coordinate, like [Co(CNR)5j, and square-planar Co is apparently unknown. On the other hand, complexes of Rh and Iri are predominantly square planar, although 5-coordination docs also occur. 

Porphyrinic co-complexes as novel multinuclear catalysts for the reduction of dioxygen directly to water 97ACR437. 

When 1 -(4-bromo-3,3-dimethylbutyl)-3-methyl-6-phenylthiouracil (185) was allowed to react with Ph3SnH in the absence and presence of eobaloxime [Co] complex reaction mixtures were obtained, containing a few... 

Initial step is the formation of a dicobalthexacarbonyl-alkyne complex 5 by reaction of alkyne 1 with dicobaltoctacarbonyl 4 with concomitant loss of two molecules of CO. Complex 5 has been shown to be an intermediate by independent synthesis. It is likely that complex 5 coordinates to the alkene 2. Insertion of carbon monoxide then leads to formation of a cyclopentenone complex 6, which decomposes into dicobalthexacarbonyl and cyclopentenone 3 ... 

The principle cost determinant in typical hydrolytic or phenolic resolutions is the cobalt catalyst, despite the relatively low catalyst loadings used in most cases and the demonstrated recyclability with key substrates. From this standpoint, recently developed oligomeric (salen)Co complexes, discussed earlier in this chapter in the context of the hydrolytic desymmetrization of meso-epoxides (Scheme 7.16), offer significant advantages for kinetic resolutions of racemic terminal epoxides (Table 7.3) [29-31]. For the hydrolytic and phenolic kinetic resolutions, the oligo-... 

Cytochrome P450 monooxygenases are characterized through the presence of the heme (protoporphyrin IX) prosthetic group (Scheme 10.1) that is coordinated to the enzyme through a conserved cysteine ligand. They have obtained their name from the signature absorption band with a maximum near 450 nm in the difference spectrum when incubated with CO. The absorption arises from the Soret Jilt transition of the ferrous protoporphyrin IX-CO complex. 

The mechanism proposed for catalytic chain transfer132 is shown in Scheme 6.23 for MMA polymerization. The Co" complex (93) rapidly and reversibly... 

The presence of redox catalysts in the electrode coatings is not essential in the c s cited alx)ve because the entrapped redox species are of sufficient quantity to provide redox conductivity. However, the presence of an additional redox catalyst may be useful to support redox conductivity or when specific chemical redox catalysis is used. An excellent example of the latter is an analytical electrode for the low level detection of alkylating agents using a vitamin 8,2 epoxy polymer on basal plane pyrolytic graphite The preconcentration step involves irreversible oxidative addition of R-X to the Co complex (see Scheme 8, Sect. 4.4). The detection by reductive voltammetry, in a two electron step, releases R that can be protonated in the medium. Simultaneously the original Co complex is restored and the electrode can be re-used. Reproducible relations between preconcentration times as well as R-X concentrations in the test solutions and voltammetric peak currents were established. The detection limit for methyl iodide is in the submicromolar range. 

With respect to CO complexes, the luminescence spectra of a series of Group VI metal carbonyls and substituted carbonyls were obtained in frozen gas matrices at 12K. In addition, the IR spectra of HCo(CO>4 and HCo(CO)3 (proposed as an intermediate in hydroformylation) were observed in an argon matrix. ... 

Next, (1) CO binds to Cluster C to yield a Credi-CO complex (2) CO undergoes attack by the metal-bound hydroxide and is oxidized to CO2 as Cluster C is reduced by two electrons to the Cred2 state (3) CO2 is released and a second CO molecule binds to Cluster C to form a Cred2-CO complex (4) electrons are transferred from Cred2-CO to reduce Cluster B as the second molecule of CO2 is released. This mecha-... 

Fig. 13. Proposed reaction mechanism for ACS. The reaction involves the sequential assembly of acetyl-CoA from a carbonyl, methyl, and CoA. We favor a Ni(l) nucleophile to form a catEdytically competent paramagnetic M-CO complex, but see text for discussion of Em alternative mechanism.

A key step proposed in the radical chain mechanism for the formation of the formyl complex is the coordination of CO to the Rh(OEP)- monomer, to give an intermediate carbonyl complex, Rh(OEP)(CO)- which then abstracts hydride from Rh(OEP)H to give the formyl product.This mechanism was proposed without direct evidence for the CO complex, and since then, again from the research group of Wayland, various Rh(fl) porphyrin CO complexes, Rh(Por)(CO), have been observed spectroscopically along with further reaction products which include bridging carbonyl and diketonate complexes. 

Excess of the olefin is desirable so that the formation of Si—Co complexes [Eq. (112) and Section II,B,7] is suppressed 54, 57). These silyl complexes are not hydrosilylation catalysts. 

Certain quaternary ammonium salts will alkylate [Co (DMG)2py] . The addition of PhCH2NMc3 I to a solution of the complex in methanol gives the PhCH2Co complex in 45% yield. The reaction works more slowly with dimethylpiperidinium iodide to give the CH3—Co complex 15). There is no alkylation with tertiary amines alone 164), but in the presence of equimolar amounts of dimethylacetylenedicarboxylate certain aliphatic tertiary amines can alkylate [Co (DMG)2py] in methanol solution. The reaction also produces the enamine derivative of a maleic ester, and the mechanism appears to involve addition of the amine to the triple bond to form an ammonium salt, which can then attack the Co(I) derivative (75). 

In one case, the insertion of the whole chiral hgand into a Co-exchanged zeohte by subhmation was described [24], Only small ligands, such as li and 2i, can be efficiently introduced into the micropores of the Y zeohte, whereas the bulkier Jacobsen s hgand la only remains on the external surface of the sohd. Unfortunately, these occluded (salen)Co complexes led to very low enantioselectivities (up to 8% ee) in the reduction of acetophenone with NaBH4. 

Chitosan (Fig. 27) was deposited on sihca by precipitation. The palladium complex was shown to promote the enantioselective hydrogenation of ketones [80] with the results being highly dependent on the structure of the substrate. In the case of aromatic ketones, both yield and enantioselectiv-ity depend on the N/Pd molar ratio. Low palladium contents favored enan-tioselectivity but reduced the yield. Very high conversions were obtained with aliphatic ketones, although with modest enantioselectivities. More recently, the immobilized chitosan-Co complex was described as a catalyst for the enantioselective hydration of 1-octene [81]. Under optimal conditions, namely Co content 0.5 mmolg and 1-octene/Co molar ratio of 50, a 98% yield and 98% ee were obtained and the catalyst was reused five times without loss of activity or enantioselectivity. 

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