Postulated mechanism

Mechanisms. Mechanism is a technical term, referring to a detailed, microscopic description of a chemical transformation. Although it falls far short of a complete dynamical description of a reaction at the atomic level, a mechanism has been the most information available. In particular, a mechanism for a reaction is sufficient to predict the macroscopic rate law of the reaction. This deductive process is vaUd only in one direction, ie, an unlimited number of mechanisms are consistent with any measured rate law. A successful kinetic study, therefore, postulates a mechanism, derives the rate law, and demonstrates that the rate law is sufficient to explain experimental data over some range of conditions. New data may be discovered later that prove inconsistent with the assumed rate law and require that a new mechanism be postulated. Mechanisms state, in particular, what molecules actually react in an elementary step and what products these produce. An overall chemical equation may involve a variety of intermediates, and the mechanism specifies those intermediates. For the overall equation... 

Solid Catalyzed Reaction The pioneers were Langmuir (J. Am. Chem. Soc., 40, 1361 [1918]) and Hinshelwood Kinetics of Chemical Change, Oxford, 1940). For a gas phase reaction A + B Products, catalyzed by a solid, the postulated mechanism consists of the following ... 

Kinetics of the reaction of p-nitrochlorobenzene with the sodium enolate of ethyl cyanoacetate are consistent with this mechanism. Also, radical scavengers have no effect on the reaction, contrary to what would be expected for a chain mechanism in which aryl radicals would need to encounter the enolate in a propagation step. The reactant, /i-nitrophenyl chloride, however, is one which might also react by the addition-elimination mechanism, and the postulated mechanism is essentially the stepwise electron-transfer version of this mechanism. The issue then becomes the question of whether the postulated radical pair is a distinct intermediate. 

Eberhard, A., and Hastings, J. W. (1972). A postulated mechanism for the bioluminescent oxidation of reduced flavin mononucleotide. Biochem. Biophys. Res. Commun. 47 348-353. 

Watanabe, H., et al. (1992). Chemiluminescence in the crude extracts of soybean seedlings. Postulated mechanism on the formation of hydroperoxide intermediates. Biochim. Biophys. Acta 1117 107-113. 

Other postulated mechanisms for spontaneous initiation include electron transfer followed by proton transfer to give two monoradicals, hydrogen atom transfer between a charge-transfer complex and solvent,110 and formation of a di radical from a charge-transfer complex, JJ[Pg.111]

For the purposes of this chapter, an arbitrary distinction is made between protonic and thermal activation, wherein protonic activation is caused by the action of acid at room temperature or lower, and thermal activation refers to the use of elevated temperatures with or without the addition of acid. In fact, in both cases, the initial steps in the postulated mechanisms are protonation of the C-2 oxygen atom followed by elimination of the aglycone to yield a ketohexofuranosyl or pyranosyl cation, which is the reactive intermediate in certain circumstances, this might be in equilibrium with the derived glycosyl fluoride. 

The postulated mechanism shown by Scheme 2 was also supported by iH NMR measurements. In the presence of a weak base (such as pyridine-ds) in DMSO-dg-D2O solution 65 % of la remained unchanged after 3 days but a complete elimination leading to flavone (2) was observed in the case of Ih and no considerable deuteration was found at C-3 atom of 2. 

If we can develop accurate quantitative models that simulate how cells respond to various enviromnental changes, we can better utilize the chemical synthesis capabilities of cells. Steps toward this goal are being taken. Models of the common gut bacterium Escherichia coli have been developed from mechanisms of subcellular processes discovered or postulated by molecular biologists. These models have progressed to the point where they can be used with experiments to discriminate among postulated mechanisms for control of subcellular processes. 

Scheme 2.15 Postulated mechanism for the copper-catalysed hydroamination of electron-deficient alkenes...

Anfj-Markovnikov products are only observed. The postulated mechanism for these reactions is analogous to the previously discussed for the copper-catalysed hydroamination (Scheme 2.15) with the coordinated thiolate (rather than the amide) acting as nucleophile [82, 85]. 

Scheme 2.18 Postulated mechanism for the Au-NHC catalysed hydroalkoxylation of aUenes...

The postulated mechanism for the reaction involves activation of the alkyne by jt-coordination to the cationic (IPr)Au% followed by direct nucleophilic attack by the electron-rich aromatic ring to form product 111. Alternatively, two 1,2-acetate migrations give the activated aUene complex, which can be cyclised to product 110 by nucleophilic attack of the aromatic ring on the activated aUene (Scheme 2.21) [92]. 

Scheme 2.21 Postulated mechanism for the synthesis of substituted indenes by intramolecular hydroarylation of propargylic acetates...

Bromomethyl-3,4-dibromo-3,4-dihydrocoumarin 1 (Fig. 11.4) and its chloro-methylated analogue 2b rapidly and progressively inactivate a-chymotrypsin and also the activities of a series of trypsin-like proteases. A benzyl substituent characteristic of good substrates of a-chymotrypsin was introduced at the 3-position to make inhibition more selective. This substituted dihydrocoumarin 3 irreversibly inhibited a-chymotrypsin and other proteases. These functionalized six-membered aromatic lactones, and their five- and seven-membered counterparts, 3//-benzofuran-2-ones 2a26 and 4,5-dihydro-3//-benzo[b]oxepin-2-ones 2c,27 were the first efficient suicide inhibitors of serine proteases. Their postulated mechanism of action is shown in Scheme 11.2. 

SCHEME 11.2 Postulated mechanism of inactivation of a serine protease by a functionalized dihydrocoumarin such as molecule 3 (i i = benzyl).28... 

SCHEME 11.3 Postulated mechanisms for the inhibition of serine proteases by coumarin derivatives. NuH nucleophile. Pathway a suicide-type inactivation (suicide substrate). Pathway b transient inactivation by formation of a stable acyl-enzyme (alternate substrate-inhibitor). 

SCHEME 11.4 Postulated mechanism for inactivation of DPP IV by a functionalized cyclopeptide 11.32... 

In acetic acid, or aqueous acetone with subsequent treatment with acetic anhydride, the esters (196 R=4-Me0 or 4-MeS) give rise to the expected esters(197) or (198). For the 4-chlorophenyl derivative, a mixture of the unexpected products (199, 200 Ar=4-chlorophenyl) was obtained. The unsubstituted compound (196 R=H) gave only (199) in aqueous acetone, but (200) in acetic acid. The postulated mechanism tor such a rearrangement centres around pseudophosphonium and-or phosphorane... 

Proposed intermediates in the above reaction include atomic hydrogen [27, 28], hydride ions [29, 30], metal hydroxides [31], metaphosphites [32, 33], and excitons [34]. In general, the postulated mechanisms are not supported by direct independent evidence for these intermediates. Some authors [35] maintain that the mechanism is entirely electrochemical (i.e. it is controlled by electron transfer across the metal-electrolyte interface), but others [26] advocate a process involving a surface-catalyzed redox reaction without interfacial electron transfer. 

Guideline 7. A postulated mechanism for a reaction in the forward direction must also hold for the reverse reaction. This guideline is a consequence of the principle of microscopic reversibility. (See Section 4.1.5.4.) Three corollaries of this guideline should also be kept in mind when postulating a reaction mechanism. First, the rate limiting step for the reverse reaction must... 

Note that the overall stoichiometry requires [d(02)ldt = [d(CH3CHO)/dt]. What assumptions are necessary to make the postulated mechanism agree with the observed kinetics ... 

Scheme 6/1.30. Postulated mechanism for the reaction of 6/1-113 to give annulated oxacycles 6/1-113.

Fig. 13. Postulated mechanism by which cutin monomers produced by constitutive expression of c f2/3 induce cut ...

Fig. 9. Postulated mechanism of lactone polymerization catalyzed by lipase...

Insulin binding to the extracellular side of cell membranes initiates the insulin cascade , a series of phosphorylation/dephosphorylation steps. A postulated mechanism for vanadium is substitution of vanadate for phosphate in the transition state structure of protein tyrosine phosphatases (PTP).267,268 In normal physiological conditions, the attainable oxidation states of vanadium are V111, Viv and Vv. Relevant species in solution are vanadate, (a mixture of HV042-/ H2VOO and vanadyl V02+. Vanadyl is not a strong inhibitor of PTPs, suggesting other potential mechanisms for insulin mimesis for this cation. 

Figure 5.12. Postulated mechanism for ethylene oligomerization according to Shell s SHOP process with a P,0-...

All of these make the formation of the first protocell rather critical. There is also the general problem of membrane transport and how molecules might form within the protocell. The simplest postulated mechanism for the initial inclusion of molecules in protcells is that of encapsulation. 

Most hydroformylation investigations reported since 1960 have involved trialkyl or triarylphosphine complexes of cobalt and, more recently, of rhodium. Infrared studies of phosphine complex catalysts under reaction conditions as well as simple metal carbonyl systems have provided substantial information about the postulated mechanisms. Spectra of a cobalt 1-octene system at 250 atm pressure and 150°C (21) contained absorptions characteristic for the acyl intermediate C8H17COCo(CO)4 (2103 and 2002 cm-1) and Co2(CO)8. The amount of acyl species present under these steady-state conditions increased with a change in the CO/ H2 ratio in the order 3/1 > 1/1 > 1/3. This suggests that for this system under these conditions, hydrogenolysis of the acyl cobalt species is a rate-determining step. 

Scheme 1. Postulated mechanism of methanol and ethylene glycol ester formation...

FIGURE 8. Postulated mechanism for the formation of Os(SnMe3)2H2(CO)(PPh3)2. Reproduced from Reference 53 by permission of Elsevier Sequoia S.A. 

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