Chelate effect, mechanism

In the last 5 years there has been a growing interest in the Pt—S interactions from a chemical point of view. The main reason for this is to understand the exact role vide supra) of such interactions in the mechanism of the antitumor activity of cis-Pt. In this section the currently known products between Pt amine compounds and sulfur-containing biomolecules will be evaluated, with special attention to cis-Pt and the antitumor-inactive complexes trans-Pt and [Pt (dien)Cl]Cl. [Pt(dien)Cl]Cl is monofunctional in nature and is often used to model the first binding step of cj s)-Pt (66). In addition, stable complexes can be expected (originating from the chelate effect of dien), whereas cis-Pt often gives complicated mixtures of products with all kinds of degradation products (vide infra). 

The design of polydentate ligands containing imines has exercised many minds over many years, and imine formation is probably one of the commonest reactions in the synthetic co-ordination chemist s arsenal. Once again, the chelate effect plays an important role in stabilising the co-ordinated products and the majority of imine ligands contain other donor atoms that are also co-ordinated to the metal centre. The above brief discussion of imine formation will have shown that the formation of the imine from amine and carbonyl may be an intra- or intermolecular process. In many cases, the detailed mechanism of the imine formation reaction is not fully understood. In particular, it is not always clear whether the nucleophile is metal-co-ordinated amine or amide. Some intramolecular imine formation reactions at cobalt(m) are known to proceed through amido intermediates. A particularly useful intermediate (5.24) in metal-directed amino acid chemistry is... 

The Pd-catalyzed hydrocarbonations of methyleneaziridines 14 do not proceed through the formation of a Jt-allylpalladium intermediates, instead via a chelation effect. The hydropalladation of methyleneaziridines with the Pd(II) hydride species 16, followed by reductive elimination gives the non-ring-opened products 15. It is noteworthy to mention that the palladium-catalyzed intermolecular or intramolecular addition of nitriles to carbon-carbon multiple bonds can be explained by the hydropalladation mechanism, except for the intramolecular addition to the C=C triple bond of alkynes (vide infra). 

In this bicyclic case the palladium and methoxyl groups are trans to each other 1X>. A cis stereochemistry would have been expected on the basis of the ethylene oxidation mechanism. Trans-addition, however, is unusually favorable in the bicyclic examples. Although addition to the exo positions is generally strongly preferred, it cannot occur here if the favorable chelating effect of the second double bond is to be obtained. As a result, only the solvent methanol can attack from the exo side. The endo cis adduct has not been prepared and it conceivably could rearrange to the trans isomer even if it were formed initially. Clearly, more work needs to be done on the stereochemistry of the addition reactions. 

The effect of ionic liquid solvents on the stereochemical selectivity of allylation of 2-methoxycyclohexanone has been investigated, and found a higher selectivity (axial alcohol 34/equatorial alcohol 35) toward the chelation-controlled mechanism in ionic liquid than in conventional solvents such as water and THF. The use of 0.1 equiv. of indium, combined with Mn and TMSC1 (2 equiv. of each), results in the isolation of the desired products in good purity, with an overall conversion of 81% (Scheme 39).168... 

The simultaneous interaction of multiple saccharide epitopes of polyvalent ligands with oligomeric receptors can lead to interactions of high functional affinity when the orientation of the saccharide recognition elements corresponds to that of the receptor (O Fig. 8). The mechanism of affinity increase is often described as the chelate effect [52,53]. This idea is... 

In light of the work carried out on boron systems as described above, a silacrown (28) was synthesized in fair yield via nine steps (87). This transported bromide ions more effectively than chloride, but no evidence for the binding mechanism or evaluation of binding affinity was provided. In recent studies, Compound 29 showed, as would be expected, a chelate effect with fluoride ions, exhibiting a high-binding constant (log K > 9 in acetone-d6), which had to be determined via a stepwise procedure (88). 

Fig. 2.4 Quantification of the chelate effect in the presence of two interaction mechanisms plot of experimental free complexation energies on aromatic ion pairs against the number m of pairwise interactions, after deducting the contribution of a primary interac-...

To explore structure-function relationships and optimize their activities, NeuAc-substituted acrylamide polymers were prepared with a variety of appended functional groups (Fig. 22) [99]. These studies generated polymers with very high inhibitory potencies toward HA. For example, an acrylamide copolymer substituted with 10% benzylamine and 20% NeuAc residues had a Ki of 0.6 nM [100]. Both the chelate effect and steric stabilization were mechanisms suggested to contribute to the potent activity of this and related materials (Fig. 5). The NeuAc-substituted acrylamide... 

The lithium aluminum hydride reduction of /i-chirar / -alkyl dialkylamino ketones has also been investigated776. Although some of the reactions were effectively unselective, others showed a modest syn selectivity, for example, the reduction of 3-dimethylamino-l-phenylbu-tanone. The sense of the asymmetric induction is consistent with a chelation-controlled mechanism analogous to that of the. svn-selective reductions of /1-hydroxv ketones (see Section 2.3.3.1.1.2.3.). 

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