Intermediate species, crucial

The crucial role of the bulky substituent at Cl in (, )-l in maintaining the rigidity of the intermediate species is supported by a molecular modeling investigation and it has also been shown experimentally that whereas the introduction of a phenyl group at Cl is sufficient to produce a highly enantioselective catalyst, the introduction of an H atom or a Me group results in deactivation of the catalyst [8 i, 37]. 

The distribution of these agents in their natural reservoirs w ill eventually define the geographic range of the threat the viruses pose. However, these viruses are recent discoveries, and much work remains to be done on their geographic distribution and the reservoir species. The occurrence of the disease in humans has been associated only with infection of an intermediate species such as horses with Hendra and swine with Nipah virus. Early recognition of the disease in the intermediate animal host is probably the most crucial means of limiting future human cases. 

Monolith reactors are composed of a large number of parallel channels, all of which contain catalyst coated on their inner walls (Figure 1.9 [1]). Depending on the porosity of the monolith structure, active metals can be dispersed directly onto the inner channel walls, or the catalyst can be washcoated as a separate layer with a definite thickness. In this respect, monolith reactors can be classified among PER types. However, their characteristic properties are notably dhferent from those of the PBRs presented in Section 1.2.1. Monolith reactors offer structured, well-defined flow paths for the reactive flow, which occurs through random paths in PBRs. In other words, the residence time of the reactive flow is predictable, and the residence time distribution is narrow in monoliths, whereas in a PBR, different elements of the reactive mixture can pass through the bed at different rates, resulting in a wider distribution of residence times. This is a situation that is crucial for reactions where an intermediate species is the desired product and has to be removed from the reactor before it is converted into an undesired species. 

In the oxidative copolymerization of one heterocyclic monomer with another, a crucial parameter is the oxidation potential of the monomer. Because it determines which monomer will oxidize first and the stability of the intermediate species, the oxidation potential of several pertinent monomers as well as the apparent deposition stoichiometry is given in Table... 

The detailed mechanisms (e.g., the intermediate species on the surface) of the above-described systems are still not clear. Nevertheless, the basic features could be modeled successfully ([33, 35, 53-57] for a review, see [58]), since the crucial effects are thermokinetic in nature and can therefore be described by a heat-balance equation in connection with a single chemical variable. 

P-phosphino-NHPs but the reverse reaction of a P-chloro NHP with diphenyl-trimethylsilylphosphine and subsequent reaction with a chloroalkane can be combined to produce high yields of P-alkyl-diphenylphosphines [74], Since the chloro-NHP is recovered in the second step, the overall reaction can be performed by employing this species merely as catalyst (Scheme 13). NMR investigations confirm that the appropriate P-phosphino-NHPs are in fact key intermediates in the resulting catalytic cycle, and it has been pointed out that P-X bond polarization represents a crucial factor for the overall acceleration of the catalyzed P-C... 

The mechanisms of the hydroxycarbonylation and methoxycarbonylation reactions are closely related and both mechanisms can be discussed in parallel (see Section 9.3.6).631 This last reaction has been extensively studied. Two possibilities have been proposed. The first starts the cycle with a hydrido-metal complex.670 In this cycle, an alkene inserts into a Pd—H bond, and then migratory insertion of CO into an alkyl-metal bond produces an acyl-metal complex. Alcoholysis of the acyl-metal species reproduces the palladium hydride and yields the ester. In the second mechanism the crucial intermediate is a carbalkoxymetal complex. Here, the insertion of the alkene into a Pd—C bond of the carbalkoxymetal species is followed by alcoholysis to produce the ester and the alkoxymetal complex. The insertion of CO into the alkoxymetal species reproduces the carbalkoxymetal complex.630 Both proposed cycles have been depicted in Scheme 11. 

This was the first example in which models for presumed Fischer-Tropsch intermediates have been isolated and their sequential reduction demonstrated. Neither methane nor methanol was observed from further reduction of the methyl and the hydroxymethyl complexes. The use of THF/H20 as solvent was crucial in this sytem in THF alone CpRe(C0)(N0)CH3 was the only species observed, probably because the initial formyl complex was further reduced by BH3.— When multihydridic reagents are reacted with metal carbonyl complexes, formyl species are usually not observed. The rapid hydrolysis of BH3 by aqueous THF allowed NaBH to act as a... 

Another effective way of staying clear of the thermodynamic barriers of C-H activation/substitution is the use of the c-bond metathesis reaction as the crucial elementary step. This mechanism avoids intermediacy of reactive metal species that undergo oxidative additions of alkanes, but instead the alkyl intermediate does a o-bond metathesis reaction with a new substrate molecule. Figure 19.13 illustrates the basic sequence [20],... 

The metal-peroxo species are considered to have a side-on structure (bidentate coordination of the peroxide ligand) and to be very unstable in protic medium (8). Under physiological conditions, after the first protonation and formation of a hydroperoxo intermediate (Scheme 2), the second protonation of this intermediate can proceed in two distinctly different pathways. In one case the second protonation results in the release of hydrogen peroxide from the metal center, leaving the metal oxidation state unchanged (Scheme 2). This is a crucial step in the catalytic cycles of SODs and SORs, especially in the catalytic mechanism of manganese SODs, which exist in the hydrophobic mitochondrial matrix. If protonation is not efficient, the... 

Here we presented two general aspects of the interactions between superoxide and metal centers. One is the catalytic decomposition of superoxide by non-heme metal centers (Scheme 9) and the role of the ligand structure in it, and another is the reversible binding of superoxide to the heme metal center and the nature of the product metal(lll)-peroxo species (Scheme 17). In both cases through the same redox reaction steps a metal(III)-peroxo species is formed as the intermediate (Scheme 9), in the catalytic cycle, or the product of stoichiometric reaction (Scheme 17). The crucial difference is in the protonation step. If the protonation of peroxo species is followed by efficient release of hydrogen peroxide (and not 0-0 bond cleavage,... 

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