Isomerism mechanism

The result of such a series of steps is depicted in Figure 9. This method has proved effective in isolating reaction pathways for conformational transitions involving localized torsional transitions including those involving a subtle isomerization mechanism [56]. 

The isomerization mechanism is clearly established by labeling experiments. The rearrangement of a to c via a 7i-allyl hydride complex b in the coordination sphere of the metal is a key step in this cayalytic cycle (Scheme 54) [174, 175]. hi case of polyunsaturated derivatives, formation of a stable q" complexes (Scheme 55) is preferred over the rearrangement (a c). 

Scheme 54 Isomerization mechanism of allylic alcohols to ketones...

Let us first recall some basic characteristics of the cyanine isomerization mechanism, as emerging from simple MO correlation diagrams like those of Fig. 1. In... 

To distinguish between simple, reversible slow binding (scheme B) and an enzyme isomerization mechanism (scheme C), one can examine the dependence of kobs on inhibitor concentration. If the slow onset of inhibition merely reflects inherently slow binding and/or dissociation, then the term kobs in Equations (6.1) and (6.2) will depend only on the association and dissociation rate constants k3 and k4 as follows ... 

For the enzyme isomerization mechanism illustrated in scheme C of Figure 6.3, there are two steps involved in formation of the final enzyme-inhibitor complex an initial encounter complex that forms under rapid equilibrium conditions and the slower subsequent isomerization of the enzyme leading to the high-affinity complex. The value of kohs for this mechanism is a saturable function of [/], conforming to the following equation ... 

In a two-step enzyme isomerization mechanism, as in scheme C, the affinity of the inhibitor encounter complex and the affinity of the final E I complex are reflected in the diminutions of v, and of vs, respectively, that result from increasing concen-... 

Both Vioxx and Celebrex derive their COX2 selectivity from the same type of isozyme-specific slow enzyme isomerization mechanism that was detailed here for DuP697. 

The energy minima between the energy barriers for the monomer coordination and insertion correspond to alkene-bound intermediates of the kind simulated by our molecular mechanics calculations (Figures 1.7 and 1.9). The possible dissociation of the monomer coordinated with the wrong enantioface can lead back to the alkene-free intermediate or, directly, to the alkene-bound intermediate with the right enantioface (through some isomerization mechanism, for which the monomer does not leave the coordination sphere of the metal). 

Figure 3. Acid catalyzed xylene isomerization mechanism.

G. K. Anderson and R. J. Cross, Chem. Soc. Revs. 9, 185 (1980) for a comprehensive and incisive review of isomerization mechanisms of square-planar complexes. 

A general two-step ligand-induced isomerization mechanism, as described by Eigen , can be written as ... 

A graphical procedure for characterizing isomerization mechanisms . The protocol uses data from product inhibition, and l/[v[p]=o vpjo] is plotted versus 1/[P] at various constant concentrations of the substrate (where Vp=o is the initial velocity in the absence of product and V[p]o is the initial velocity in the presence of product). Secondary and ternary replots allows one to characterize the mechanism . This procedure requires very accurate estimation of initial rates. 

Chance Bi Bi mechanism. (This expression also applies to isomerization mechanisms for the ordered and Theorell-Chance schemes in which a stable enzyme form undergoes isomerization.) The points of intersection for each different mechanism can be used to exclude certain possibilities, depending on the quality of the rate data. 

ENZYME COMMISSION NOMENCLATURE ISOMERIZATION Isomerization mechanisms,... 

ISOMERIZATION MECHANISMS MONO-ISO PING PONG Bl Bl REACTION SCHEME... 

Scheme 13 Topochemical photoisomerization of dialkyl (Z,Z)-muconates 13 and 15 in the crystalline state in a one-way isomerization mechanism...

In the context of the accepted olefin isomerization mechanism, our results illustrate that fra/rsition-state theory can accurately model the competition between C-H and C-D activation for olefin exchange (isomerization) for the case of... 

R.S. Disselkamp, K.M. Denslow, T.R. Hart, C.H.F. Peden Non-equilibrium Effects in the Hydrogenation-mediated Isomerization Mechanism of Olefins during Cavitating Ultrasound Processing, Catal Commun., in press (2006). R.S. Disselkamp, K.M. Judd, T.R. Hart, C.H.F. Peden, G.J. Posakony, L.J. 

This reaction is particularly interesting because the starting compound has the trans, trans structure and the product must arise from a cis intermediate in order to form the ring. A possible isomerization mechanism involves a ketene intermediate. 

It has been already mentioned in passing that indications exist in the literature showing that the 3C isomerization can take place by formation of at least two different 3C complexes, having different activation energies of isomerization, different particle size effects, different responses to alloying, etc. (157, 195-198). The suggestions presented above offer a choice of different complexes for further speculations. However, a definitive description of isomerization mechanisms under different conditions (H2 pressure, temperature, etc.) and with different catalysts (pure metals, alloys, etc.) is not yet possible. 

ABSTRACT The acid-catalysed epimerization reaction of bioactive indole alkaloids and their derivatives is reviewed. The three mechanisms, which have been proposed for the (J-carboline-type indole alkaloids, are discussed. Through recent developments, evidence for all three mechanisms has been obtained, which shows the complexity of the epimerization reaction. The epimerization seems to depend on structural features and reaction conditions making it difficult to define one universal mechanism. On the other hand, the isomerization mechanism of oxindole alkaloids has been widely accepted. The acid-catalysed epimerization reaction provides a powerful tool in selectively manipulating the stereochemistry at the epimeric centre and it can also have a marked effect on the pharmacology of any epimerizable compound. Therefore, examples of this reaction in die total synthesis of indole alkaloids are given and pharmacological activities of some C-3 epimeric diastereomers are compared. Finally, literature examples of acid-catalysed epimerization reactions are presented. 

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