Kinetic chelate effect

Hence, dissociation of tiron occurs in a concerted fashion when the porphyrin is incorporated into the first coordination sphere. Obviously, if proton transfer from H2P has been established, the series of substitutions occurring at MA are rather fast steps dominated by kinetic chelate effects. 

The first dissociation (1) is expected to be slower than a similar dissociation of ammonia, because the ethylenediamine ligand must bend and rotate to move the free amine away from the metal. The reverse reaction associated with the first dissocation is fast. Indeed, the uncoordinated nitrogen is held near the metal by the rest of the ligand, making reattachment more likely. This kinetic chelate effect dramatically reduces aquation reaction rates. 

Octahedral substitution reactions also exhibit a kinetic chelate effect Coordination compounds containing a chelating ligand react more slowly than their counterparts containing two monodentate ligands with similar M-L bond strengths. For example, the rate constant for substitution of Ni[(bpy)] + is 3.3 X 10 s , almost 10 times slower than for [Ni(py)] +, which has a rate constant of 38.5. The pro-... 

An essential feature of template reactions of both the thermodynamic and kinetic type is the formation of a new chelate ring. One of the main difficulties in a discussion of mechanistic aspects of template reactions is the inevitable mixture and overlap of mechanistic effects. Those reactions which clearly exhibit a kinetic template effect could also depend to some extent on a thermodynamic one. Also, in some multistep macrocyclization processes, for example, both effects could be involved. Despite the inherent difficulties, the following discussion in this section will be subdivided into two, to cope predominantly with the thermodynamic and kinetic template effects. A further subdivision of thermodynamic and equilibrium effects will not be made. 

The most important reaction of this type is the formation of imine bonds and Schiff bases. For example, salicylaldehyde and a variety of primary amines undergo reaction to yield the related imines, which can be used as ligands in the formation of metal complexes. However, it is often more desirable to prepare such metal complexes directly by reaction of the amine and the aldehyde in the presence of the metal ion, rather than preform the imine.113 As shown in Scheme 31, imine formation is a reversible process and isolation of the metal complex results from its stability, which in turn controls the equilibrium. It is possible, and quite likely, that prior coordination of the salicylaldehyde to the metal ion results in activation of the carbonyl carbon to amine nucleophilic attack. But it would be impossible for a precoordinated amine to act as a nucleophile and consequently no kinetic template effect could be involved. Numerous macrocyclic chelate systems have been prepared by means of imine bond formation (see Section 61.1.2.1). In mechanistic terms, the whole multistep process could occur without any geometrical influence on the part of the metal ion, which could merely act to stabilize the macrocycle in complex formation. On the other hand,... 

New chelate rings can be formed by the nucleophilic addition of alcohols to imine complexes. For example, the nickel(II) TAAB complex is susceptible to attack by bis-alkoxides (equation 31).127 It is not clear whether or not a kinetic template effect operates by prior coordination of the central oxygen or sulfur atom. However, such an effect is not necessary, as simple alkoxides undergo a similar addition reaction.128... 

The situation is even more clear cut in the formation of thioethers by alkylation of thiolate complexes. Such reactions have been described in Section 7.4.2.1.2, but in those examples new chelate rings were not formed. The reaction which led Busch to propose the kinetic template effect is a thiolate complex alkylation, which does indeed produce a new chelate ring (equation 40).110 162 163 This reaction is an example of the general type shown in equation (35) and it has been established clearly that the sulfur atoms remain coordinated to the metal ion throughout the... 

In the case of inert cobalt(m) complexes it is possible to isolate the chelated products of the reaction. Let us return to the hydrolysis of the complex cations [Co(en)2(H2NCH2C02R)Cl]2+ (3.1), which contain a monodentate iV-bonded amino acid ester, that we encountered in Fig. 3-8. The chelate effect would be expected to favour the conversion of this to the chelated didentate AO-bonded ligand. However, the cobalt(iu) centre is kinetically inert and the chloride ligand is non-labile. When silver(i)... 

Hydration enthalpy Stability or formation constant Overall and stepwise stability constants Chelate effect Macrocyclic effect Preorganization Equilibrium template effect Kinetic template effect Self-assembly... 

Generally, the products derived from the open-chain form are insignificant in the reaction mixture and stereocontrol between the anomeric forms can be achieved. It was observed that the anomeric oxygen in the equatorial alkoxide 276 has enhanced nucleophilicity compared with the axial [284]. Often referred to as the kinetic anomeric effect, this phenomenon can govern the diastereoselective formations of the equatorial glycosides [579]. The anomeric effect, however, favors the formation of the thermodynamically more stable axial glycosides and chelation control can also play a role in determining anomeric stereoselectivity. 

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