Associative pathway

Dyn is not yet known, it is likely that such changes reflect variations in the activity of the associated pathways. One possible explanation is that increases in neuropeptide tissue levels are due to decreased release of the transmitter, which dunmishes the extracellular peptide metabolism and results in accumulation of these peptide substances. Another possible contributing factor is a drug-related alteration in neuropeptide synthesis. For example, Bannon et al. (1987) reported that METH administration increased the quantity of striatal messenger RNA for the SP precursor preprotachykinin. Thus, increases in peptide synthesis might contribute to increases in peptide content caused by treatment with METH or the other amphetamine analogs. 

The dissociation process should be enhanced by UV irradiation and inhibited by high CO pressure, whereas a reaction through an associative pathway should be unaffected by either. We carried out the corresponding experiments /38/, and found that the chromium carbonyl catalyzed water gas shift reaction is much faster with irradiation than without, and that it is inhibited by high CO pressure. Therefore it seems quite clear that this reaction takes place through a dissociative mechanism. 

In aqueous solutions, a water molecule can bind to the activated complex to complete the coordination sphere. The complex formed, [MLnHjO], has some stability, so it represents a lower energy than the transition state, [ML ], Therefore, the complex [Ml ThO] is known as an intermediate because it is more stable than the transition state either before the H20 enters or after it leaves. Figure 20.2 shows the energy profile for a substitution reaction that follows an associative pathway. 

Ru(II) in (Ru(r 6-arene) complexes may therefore behave more like a Ru(III) center, which usually reacts via associative pathways (79). 

In a recent publication from our laboratory (87), the substitution behavior of the Cu(II) complex of the trimethylated tren (Me3tren, each amino-terminal nitrogen is monomethylated) was studied. The substitution of the coordinated water molecule by pyridine was only slightly slower than in the tren case. The activation volumes of -8.7 4.7 cm3 mol-1 for the forward reaction and -6.2 1.1 cm3 mol-1 for the reverse aquation reaction (see the volume profile in Fig. 7) indicate that substitution occurs via an associative pathway and that the steric influence is not as significant as in the case of Me6tren. 

Upon comparison of the k< m exchange rate of the Tc(V) system with that of the Re(V), the significant increase in reactivity (ca. 3 orders of magnitude) is very prominent and not necessarily indicative of an associative activation. It is, however, possible that the Tc(V) hydroxo complex might be very reactive via an associative pathway, since it is known that the Tc(V) center much more readily accepts electron density than does the corresponding Re(V) complexes (55). The greater ease by which coordination sphere expansion can occur in third-row d-series transition elements such as W(IV) and Re(V) (not very easily... 

Figure 7. The associative pathways calculated by Ziegler and co-workers. ([Rh] = Rh(PH3)2. The two PH3 ligands are perpendicular to the molecular plane shown in this figure.)...

Despite these important studies, most steps of the reaction mechanism are still only postulated. Therefore, we have decided to undertake a theoretical investigation of the Dotz reaction by discussing whether the reaction proceeds via a dissociative or an associative pathway in the initial steps of the process. We have also analyzed the central part of the reaction, the key issue being whether the reaction proceeds through a vinylketene intermediate (route A) or, instead, via a metallacycloheptadienone (route B). As will be seen, we came across a novel third pathway (route C) that turns out to be the best alternative from thermodynamic and kinetic points of view... 

As a whole, the energy barrier from reactants that must be surpassed for the associative pathway (132.6 kJ mol 1) is lower than the dissociation energy of CO from (CO)sCr(C(OH)C2H3) to give 12 (144.7 kJ mol 1) and therefore TDS concluded that the associative route is kinetically favorable. It is important to remark that since we have not taken into account the effect of the solvent, the proposed associative mechanism must be seen as an alternative to explain how the reaction can proceed when it is performed at gas-phase or in noncoordinating solvents. 

Fig. 2.4 A possible pathway for rhodium-catalyzed hydroboration by RhCI(PPh3)3, based on the mechanism of hydrogenation and the contributions of Evans and others [17], This takes into account the theoretical support for an associative pathway [13] and the observed irreversibility of the H-transfer step with styrenes.

The stoichiometric transfer of allenylidene ligands from one metal fragment to another metal center has been scarcely documented, the only examples known involving the allenylidene transfer from chromium compounds [Cr(=C=C= CR R )(C0)5] (R R = aryl, amino or alkoxy groups) to [W(C0)5(THF)] [9d]. DFT calculations indicate that the reaction proceeds by an associative pathway, the initial reaction step involving the coordination of W(CO)5 to the Cc(=Cp bond of the allenylidene ligand in the chromium precursor. 

A quick inspection of Table II soon reveals that octahedral d1 and d2 complexes should be labile (negative CFAE for associative pathways) while d3 species should be inert. High-spin d6 and d7 species should be labile while high-spin d8 complexes should be relatively inert. In the case where there is no stabilization energy contribution, the reactivity is expected to follow that of... 

Figure 6.15. Simplified schematic of the most important reaction pathways of the oxygen reduction reaction. The four-electron pathway results in the formation of water. The two-electron pathway forms hydrogen peroxide. Adsorption of molecular oxygen can form atomic oxygen (dissociative pathway) or form a superoxide species (associative pathway). The formation of Pt—OH and Pt— from water molecules represents the backward reactions of the later portion of the four-electron reduction pathway.

Nonzero values for both A, and k2 in Eqs. 13.10-13.12 indicate that ML2TX is reacting by two different pathways. The A, term, first order with respect to both complex and Y, indicates an associative pathway, A, similar to the SN2 reaction of organic cbemistry. The term arises from the nucleophilic attack of ML TX by Y. As... 

However, because the solvent is present m large excess, its concentration Is essentially constant and therefore A fSj = k,. Asa result, Eq. 13.13 simplifies to Eq. 13.10. The two associative pathways are summarized in the following reaction triangle ... 

One alkyne will displace another from Pt(alkyne)(PR3)2 complexes, the order of alkyne displacement being C2H2 < alkylalkynes < arylalkynes < nitroalkynes. The reaction involves both a dissociative and an associative pathway. The reaction may not lead to a single replacement product in all cases. Thus cr-hydroxyalkynes lead to the formation of dialkynyl platinum(II) complexes.830... 

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