Chelate effect thermodynamic effects

The thallium intermediates can be useful in directing substitution to specific positions when the site of thallation can be controlled in an advantageous way. The two principal means of control are chelation and the ability to effect thermal equilibration of arylthallium intermediates. Oxygen-containing groups normally direct thallation to the ortho position by a chelation effect. The thermodynamically favored position is... 

Romeo et al. (1978) clearly indicate that complexes of divalent metal ions with 1,2-diaminoethane are more stable than those with 1,3-diaminopro-pane. Moreover, in a thorough discussion of the relations between the chelate effect and the ring size, Anderegg (1980) has listed thermodynamic data of complex formation between divalent metal ions and ligand [45], showing that almost invariably the stability of chelate rings decreases with increasing n in the order 5 > 6 > 7. 

Larger chelate rings should be easier to open and of course they are known to have a smaller thermodynamic chelate effect. 

U. Wahlgren, and 1. Grenthe, Chelate Effect and Thermodynamics of Metal Complex Formation in Solution A Quantum Chemical Study, J. Am. Chem. 

Tetranuclear clusters, 815-816 Thermal ellipsoids, 234, 724 Thermochemical calculations, predictive power of, 127-129 Thermochemical radii. 117 Thermodynamics, and chelate effect. 523... 

Thermodynamic contributions to the chelate effect in complexes of nickel(ll) and copperfll)"... 

As a very simple case, consider the reactions, and the pertinent thermodynamic data for them, given in Table 1-1. In this case the enthalpy difference is well within experimental error the chelate effect can thus be traced entirely to the entropy difference. 

It should be pointed out, however, that the thermodynamic explanation of the chelate effect, in particular the contribution of entropy as presented above, is actually not as straightforward as it might appear. The entropy change for a reaction depends on the standard state chosen for reference and for very concentrated solutions one might chose unit mole fraction instead of one molal and the chelate effect would disappear. However, this is not realistic and for solutions one molal (or less) there is a real chelate effect. In very dilute solutions (0.1 M or less) where complexation of metal ions is generally most important, the chelate effect is of major importance and is properly understood as entropically driven. 

The thermodynamic chelate effect, which causes polydentate complexes to be thermodynamically more stable than their monodentate counterparts, was described in Section 10-1-1. The difference in the attachment and dissociation of the second (and third or higher numbered) point of attachment for the ligand is also observed kineticaUy. 

Threo diastereoselectivity is consistent with a chelation-controlled (Cram cyclic model) organolithium addition (Figure 8a). Since five-membered chelation of lithium is tenuous, an alternative six-membered chelate involving the dimethylamino nitrogen atom of the thermodynamically less stable (Z)-hydrazone (in equilibrium with the ( )-isomer) cannot be discounted. The trityl ether (entry 4, Table 9) eliminates the chelation effect of the oxygen atom such that the erythro diastereomer predominates (via normal Felkin-Ahn addition) (Figure 8b). 

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