Reaction mechanism proposal

The theoretical approach involved the derivation of a kinetic model based upon the chiral reaction mechanism proposed by Halpem (3), Brown (4) and Landis (3, 5). Major and minor manifolds were included in this reaction model. The minor manifold produces the desired enantiomer while the major manifold produces the undesired enantiomer. Since the EP in our synthesis was over 99%, the major manifold was neglected to reduce the complexity of the kinetic model. In addition, we made three modifications to the original Halpem-Brown-Landis mechanism. First, precatalyst is used instead of active catalyst in om synthesis. The conversion of precatalyst to the active catalyst is assumed to be irreversible, and a complete conversion of precatalyst to active catalyst is assumed in the kinetic model. Second, the coordination step is considered to be irreversible because the ratio of the forward to the reverse reaction rate constant is high (3). Third, the product release step is assumed to be significantly faster than the solvent insertion step hence, the product release step is not considered in our model. With these modifications the product formation rate was predicted by using the Bodenstein approximation. Three possible cases for reaction rate control were derived and experimental data were used for verification of the model. 

In a previous study [33] we confirmed the reaction mechanism proposed by Harris et al. [32] for 40T, which is known to be a general acid-base reaction that takes place in two steps (see Figure 3-1). In the first step, one of the hydrogens from C3... 

Fahey (16) suggests that intermediate 3 dissociates formaldehyde he finds supportive evidence in the rhodium-based system by observation of minor yields of 1,3-dioxolane, the ethylene glycol trapped acetal of formaldehyde. For reasons to be discussed later, we believe the formation of free formaldehyde is not on the principal reaction pathway. (c) We have also rejected two aspects of the reaction mechanism proposed by Keim, Berger, and Schlupp (15a) (i) the production of formates via alcoholysis of a formyl-cobalt bond, and (ii) the production of ethylene glycol via the cooperation of two cobalt centers. Neither of these proposals accords with the observed kinetic orders and the time invariant ratios of primary products. 

Figure 3. Reaction mechanism proposed for purple acid phosphatase.

Figure 11. The reaction mechanism proposed for the interpretation of transient...

Taking into account the reaction mechanism proposed for the same reaction in water, eq (4) and (5), BrN3 should be the reactive intermediate formed in a preequilibrium step, the rate of consumption of the total bromine is therefore expressed by eq (6). At sufficiently high N3 concentration, the BrN3 concentration is simply given by eq (7) and that of total bromine by eq (8). The introduction of eq (7) and (8) in eq (6) gives eq (9) from which the observed third-order rate constant, measurable by monitoring the free Br2 and the observed... 

Figure 17.4 Reaction mechanism proposed for xanthine oxidase. (From Hille, 2005. Copyright 2005, with permission from Elsevier.)...

Dissolution of goethite by oxalate in the presence of different concentrations of ferrous iron. The reaction mechanism proposed is that of Fig. 9.3d. The change in the concentration of Fe(III) is given (preconditioning of the surface introduces some incipient Fe(III)). pH = 3.0, goethite 0.46 gIt, oxalate 0.001 M. 

The reaction mechanism proposed by Noyori and co-workers [12] (Figure 1) is a good example of the generally accepted mechanism for this... 

Figure 15.3 Reaction mechanism proposed for the isomerization 1 6. Cp is pentamethylcydopentadienyl. Adapted from [325],...

The photoreduction of aromatic nitro compounds to the amino compounds can be carried out on the surface of semiconductor particles such as titanium oxide1 with H-atom donors (equation 1). At a shorter duration of the photoinduced reduction of p-nitroacetophenone, the hydroxylamine intermediate can be obtained in about 30% yield. The reaction mechanism proposed is based on the photoexcitation of TiC>2 to generate an electron and a positive hole (equations 2 and 3). Aliphatic nitro compounds such as 12-nitrododecanoic acid can be reduced to 12-amino dodecanoic acid in 90% yield by this method. 

Several aspects of the reaction mechanism still need to be explained. For instance, nonnegligible amounts of C2 compounds (acetic acid, acrolein) are obtained, the formation of which is not satisfactorily explained by reaction mechanisms proposed in the literature. 

ACS isozyme utilizes pyridoxal-5 -phosphate (PLP) as a cofactor and belongs to fold type I PLP-dependent enzymes showing an absorption maximum between 422 and 431 nm, which is due to the internal aldimine. The reaction mechanism proposed for the conversion of SAM to ACC by ACS illustrated in Scheme 2 involves the following steps ... 

The reaction mechanism proposed for the addition of organostannanes [29] is similar to that for organoboronic acids. An example of the reaction of methyl vinyl ketone 42 is outlined in Scheme 3.15. The catalytic cycle involves a cationic rhodium complex G, phenylrhodium H, and oxa-n -allylrhodium I. Stannyl enol ether 44 is formed by the reaction of oxa-n -allylrhodium I with Me3SnBF4, which upon hydrolysis gives the ketone 43. The lower yields in the absence of water were explained by the further reaction of 44 with methyl vinyl ketone 42. The rapid hydrolysis with water may prevent such oligomerization. 

The side chain alkylation of alkyl benzenes is usually performed with alkenes as alkylating agents in the presence of strong-acid catalysts (256,257). The use of highly basic catalysts such as alkali metals, their hydrides, and sodium and potassium complexes for alkylation of alkylaromatic hydrocarbons has also been reported (256,257). The reaction mechanism proposed by Pines et al. (258) involves the addition of a benzylic carboanion to the alkene (Scheme 41). 

The reaction mechanism proposed for the LiBr/NEta induced azomethine ylide cycloadditions to a,p-unsaturated carbonyl acceptors is illustrated in Scheme 11.10. The ( , )-ylides, reversibly generated from the imine esters, interact with acceptors under frontier orbital control, and the lithium atom of ylides coordinates with the carbonyl oxygen of the acceptors. Either through a direct cycloaddition (path a) or a sequence of Michael addition-intramolecular cyclization (path b), the cycloadducts are produced with endo- and regioselectivity. Path b is more likely, since in some cases Michael adducts are isolated. 

The reaction mechanism proposed by Tuazon et al. (1981, 1982) to explain these results is analogous to that for N2H4. In this case, the intermediate CH,N=NH formed in the reaction analogous to (93) was identified by FTIR. Reactions (100a) and (100b) explain the formation of CH2N2 ... 

Fig. 21. A reaction mechanism proposed by Xiong et al. to rationalize the toluene disproportion reaction on zeolites. The benzyl cations and benzenium ions were proposed as reactive intermediates for this zeolite-catalyzed, high-temperature reaction. (Reprinted with permission from Xiong et al. (122). Copyright 1995 American Chemical Society.)...

A rate law derived from a steady-state analysis of a reaction mechanism proposed for the reaction of H2 with NO is given by ... 

In accordance with the reaction mechanism proposed, primary and secondary amines behave as very effective inhibitors, due to their great reactivity towards imides. 

The basic idea in the synthesis approach of isothermal reactor networks with complex reaction mechanisms proposed by Kokossis and Floudas (1990) consists of... 

Figure 3.9 Reaction mechanisms proposed for CO oxidation over supported Au catalysts.

Walling and Kato (1971) modified the reaction mechanism proposed by Merz and Waters as follows ... 

The reaction mechanism proposed by Martin et al. (1996) is presented in Figure 9.5. 4-CP reacts with OH to form the 4-chlorodihydroxycyclodienyl radical (4-CD). After this initial hydroxylation, three parallel reaction pathways exist. In the first pathway, 4-CD is reduced by a conduction band electron to yield 4-CD+, hydroquinone (HQ), and Cl. In the second pathway, 4-CD reacts with oxygen to form the molecule 4-CDO. In the third pathway, ClOy facilitates the abstraction of an electron from 4-CD to yield 4-CD+. 4-CD+ is stabilized by a resonance interaction and by the strong electronreleasing capability of the OH substituent at the 1-position. 

This was ascribed to the short residence times applied (50-100 ms). Under these conditions, assuming the reaction mechanism proposed by Takahashi et al. shown above, carbon monoxide could only be formed by the reverse water-gas shift reaction, which is known to be slower than the reforming reaction. This is the case especially for catalyst systems with low activity towards water-gas shift. Holladay et al. [19] compared the performance of the same proprietary catalyst with that of a Cu/Zn catalyst which produced a higher carbon monoxide concentration of 3.1% in the reformate. 

It should be noted that most of the anion radical simply disappears without giving any new radical species when photobleached with visible light. This observation seems to exclude reaction (26), the key step in the reaction mechanism proposed by Geuskens et al. [31]. The photo-induced disappearance of the anion radical must release an electron, which recombines with some cationic entities. This charge recombination does not actually lead to the formation of the propagating-type radical, in contradiction with the proposed reaction mechanism. 

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