Catalytic sequence

Ammonia is a cyclic reagent that is recovered by the end of the Solvay process for sodium carbonate from hme and salt. Although there is nothing obscure about the intermediate reactions, ammonia definitely participates in a catalytic sequence. 

In fact, the sun is not a first-generation main-sequence star since spectroscopic evidence shows the presence of many heavier elements thought to be formed in other types of stars and subsequently distributed throughout the galaxy for eventual accretion into later generations of main-sequence stars. In the presence of heavier elements, particularly carbon and nitrogen, a catalytic sequence of nuclear reactions aids the fusion of protons to helium (H. A. Bethe... 

Hie metliod involves a tegioselective, /vons-diasteteoselective, and eaantioselective tliree-component coupling, as shown in Sdieme 7.26. In tliis case, tlie zinc eaolate tesulling from tlie 1,4-addilion is trapped in a palladiuni-calalyzed allyla-tion [64] to afford /voiis-2,3-disubstituted cyclobexanone 96. Subsequent palladiuni-calalyzed Wacket oxidation [82] yields tlie metliylketone 97, wbidi in tlie presence of/-BuOK undergoes an aldol cyclization. This catalytic sequence provides tlie 5,6-i98) and 5,7- i99) annulated structures witli ees of 9696. 

Ribozymes that are small RNA molecules with endoribonuclease activity and exhibit catalytic sequence-specific cleavage of the target. 

The release of N2 occurs within function 3. It involves the dissociation of NO (via a dinitrosyl-adsorbed intermediate), followed by subsequent formation of N2 and scavenging of the adsorbed oxygen species left from NO dissociation. The removal of adsorbed oxygen is due to the total oxidation of an activated reductant (CxHyOz). This reaction corresponds to a supported homogeneous catalytic process involving a surface transition metal complex. The corresponding catalytic sequence of elementary steps occurs in the coordinative sphere of the metal cation. 

Figure 1.1 Schematic representation of a well known catalytic reaction, the oxidation of carbon monoxide on noble metal catalysts CO + Vi 02 —> C02. The catalytic cycle begins with the associative adsorption of CO and the dissociative adsorption of 02 on the surface. As adsorption is always exothermic, the potential energy decreases. Next CO and O combine to form an adsorbed C02 molecule, which represents the rate-determining step in the catalytic sequence. The adsorbed C02 molecule desorbs almost instantaneously, thereby liberating adsorption sites that are available for the following reaction cycle. This regeneration of sites distinguishes catalytic from stoichiometric reactions.

If one would be able to derive from the experimental data an accurate rate equation like (12) the number of terms in the denominator gives us the number of reactions involved in forward and backward direction that should be included in the scheme of reactions, including the reagents involved. The use of analytical expressions is limited to schemes of only two reaction steps. In a catalytic sequence usually more than two reactions occur. We can represent the kinetics by an analytical expression only, if a series of fast pre-equilibria occurs (as in the hydroformylation reaction, Chapter 9, or as in the Wacker reaction, Chapter 15) or else if the rate determining step occurs after the resting state of the catalyst, either immediately, or as the second one as shown in Figure 3.1. In the examples above we have seen that often the rate equation takes a simpler form and does not even show all substrates participating in the reaction. 

ACC external aldimine undergoes transaldimination to release ACC and complete the catalytic sequence (Scheme 2(e)), according to McCarthy et... 

The method involves a regioselective, trans-diastereoselective, and enantioselective three-component coupling, as shown in Scheme 7.26. In this case, the zinc enolate resulting from the 1,4-addition is trapped in a palladium-catalyzed allyla-tion [64] to afford trans-2,3-disubstituted cyclohexanone 96. Subsequent palladium-catalyzed Wacker oxidation [82] yields the methylketone 97, which in the presence of t-BuOK undergoes an aldol cyclization. This catalytic sequence provides the 5,6-(98) and 5,7- (99) annulated structures with ees of 96%. 

Any catalytic sequence needs to fulfill certain thermochemical boimdary conditions as far as the elementary steps are concerned. For illustration, consider the oxygenation of an organic substrate S by a transition-metal oxo species [M]0 according to reaction 1, where [M] stands for a bare or Hgated, neutral or charged metal fragment ... 

In treating catalytic sequences of elementary steps, Christiansen adopted the simplification that each elementary step is first order in both directions with respect to concentration of a single active species. The resulting rates... 

In the previous sections, we have discussed the pathways by which PASCs are desulfurized and what is presently known about the structure of the active species in supported Co(Ni)-Mo-S catalysts. In this section, we discuss the chemical reactions and intermediates involved in the catalytic sequence that results in desulfurization. This sequence is often called the catalytic mechanism. 

Fig. 22. Proposed catalytic sequence of reactions catalyzed by glutamine synthetase.

The effect of combining the deactivation model with the simple catalytic sequence of the Michaelis-Menten relation is shown below ... 

The committed step in a metabolic pathway is usually under metabolic control. Inhibition of the committed step in a metabolic sequence or pathway prevents the accumulation of unneeded intermediates and effectively precludes activity of the enzymes using those intermediates as substrates. The decarboxylation of pyruvate and the oxidative transfer of the hydroxyethyl group by pyruvate dehydrogenase constitutes the committed step in the pyruvate dehydrogenase catalytic sequence and is a logical control point. 

C Kinetic isotope effects (KIEs) of a xylose reductase-catalysed cinnamalde-hyde reduction have been determined by 13C NMR using competition reactions with reactants at natural 13C abundance. The primary KIEs indicated that the chemical reaction steps are only partly rate limiting during reduction of aromatic aldehydes and slow steps occur outside the catalytic sequence. The aldo-keto reductase-catalysed... 

At very negative potentials neither the TAA+ cation nor the electrode are inert, instead they combine to form reduced TAA-metals. Recent work has led us to formulate the hypothesis that TAA-metals are involved in many organic reductions done in the presence of TAA+ salts at very negative potentials. These species are formed at the electrode surface and may act as mediators for electron-transfer to the organic substrates. A simplified mechanism for this catalytic sequence is shown below. 

Carbamyl phosphate synthetase catalyzes the synthesis of carbamyl-P from HCO3-, glutamine, and 2 moles of ATP. The enzyme also catalyzes the HC03 -dependent hydrolysis of ATP. Raushel and Villafranca (5) followed the exchange of from the bridge to the nonbridge position of [y- 0]ATP after Incubation with enzyme and bicarbonate. The exchange rate was O.A times the rate of ADP formation. These results support the formation of carboxy phosphate as the first Intermediate In the catalytic sequence. 

Scheme 1. Possible catalytic sequences forthe Pd-catalyzed intermolecular arylation of five-membered heteroaromatic compounds involving one heteroatom.

Scheme 2. Possible catalytic sequences for the Pd-catalyzed intermolecular arylation of azole compounds at the 2-position.

Figure 4. Possible flavin peroxide and 1-deazaflavin peroxide species in p-hydroxybenzoate-hydroxylase-mediatea catalytic sequence...

The first step of peroxidase catalysis involves binding of the peroxide, usually H2C>2, to the heme iron atom to produce a ferric hydroperoxide intermediate [Fe(IE)-OOH]. Kinetic data identify an intermediate prior to Compound I whose formation can be saturated at higher peroxide concentrations. This elusive intermediate, labeled Compound 0, was first observed by Back and Van Wart in the reaction of HRP with H2O2 [14]. They reported that it had absorption maxima at 330 and 410 nm and assigned these spectral properties to the ferric hydroperoxide species [Fe(III)-OOH]. They subsequently detected transient intermediates with similar spectra in the reactions of HRP with alkyl and acyl peroxides [15]. However, other studies questioned whether the species with a split Soret absorption detected by Back and Van Wart was actually the ferric hydroperoxide [16-18], Computational prediction of the spectrum expected for Compound 0 supported the structure proposed by Baek and Van Wart for their intermediate, whereas intermediates observed by others with a conventional, unsplit Soret band may be complexes of ferric HRP with undeprotonated H2O2, that is [Fe(III)-HOOH] [19]. Furthermore, computational analysis of the peroxidase catalytic sequence suggests that the formation of Compound 0 is preceded by formation of an intermediate in which the undeprotonated peroxide is coordinated to the heme iron [20], Indeed, formation of the [Fe(III)-HOOH] complex may be required to make the peroxide sufficiently acidic to be deprotonated by the distal histidine residue in the peroxidase active site [21],... 

Cofactor Requirements and Multiple Enzyme Catalytic Sequences Many enzyme-catalyzed reactions of potential interest for synthetic chemistry require cofactor substrates which serve as electron donors or acceptors or phosphate donors. These chemicals are much too costly to be used as process feedstocks, so economical practice of these cofactor-requiring enzyme-catalyzed reactions necessitates recovery and chemical regeneration of the required cofactor substrate. In spite of significant research aimed at solving this problem, no general solution exists at present. 

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