Prochiral molecules interaction with chiral

Prochiral Molecules Interacting with Chiral Surfaces... 

In this work a new approach is desribed, which can help to understand ED over heterogeneous catalysts We also hope that this approach can be used to find new modifiers for enantioselective heterogeneous catalytic reactions. The basis for this approach is the steric shielding known in organic chemistry [7,8]. A chiral template molecule can induce shielding effect (SE) in such a way that it preferentially interacts with one of the prochiral sites of the substrate. If a substrate is preferentially shielded its further reaction can take place only fi"om its unshielded site resulting in ED. 

Because proteins are made up of L-amino acids, they exhibit chirality in their structures, lacking planes or points of symmetry. Proteins also can exhibit chirality in their interactions with other chiral molecules as well as prochiral centers in other molecules. This latter point is beautifully illustrated by fumarase s catalysis of the dehydration of L-malate, a molecule containing two seemingly equivalent hydrogen atoms ... 

The interaction of a prochiral molecule with a chiral homogeneous catalyst results in the formation of diastereomeric intermediates and transition states. High enantioselectivity is obtained if for the rate-determining step out of the many possible diastereomeric transition states, one is energetically favored. In such a situation the reaction follows mainly this path. Other possible pathways that cause dilution of optical purity are avoided. A fundamental point to note is that diastereomers, unlike enantiomers, need not have identical energies. The presence of a C2 axis of symmetry in these ligands makes some of the possible diastereomeric transition states structurally and energetically equivalent. As the... 

It is interesting to note that the interaction of a prochiral reactant with a chiral step-kink site and with an adsorbed chiral modifier each fiilfills the three-point contact model required for chiral recognition [102-104]. At a step-kink site, the reactant adsorbed in the pro-(l ) geometry contacts the surface plane, the step, and the kink in a way that is not equivalent to the pro-(S) adsorbate. Similarly, in the modifier-reactant interaction between methyiacetoacetate and pyroglutamate, it is proposed that the reactant has three key points of contact comprising the metal-molecule interaction and two intermolecular H-bonds with the modifier [101]. 

Analysis of natural abundance deuterium distribution in organic molecules, an important step in the study of kinetic isotope effects associated with enzyme-catalyzed reactions, by the use of chiral anisotropic media has been explored. An aspect of this analysis is the discrimination of the enantiotopic deuterons in prochiral molecules and the quantification of isotopic fractionation on methylene prostereogenic sites. Towards this an approach has been presented which is based on the use of natural abundance 2-dimensional NMR experiments on solutes oriented by chiral liquid crystalline solvents and the separation of the deuterium signals based on the quadrupolar interaction. The case of 1,1 -bis(phenylthio)hexane derived by cleavage from methyl linoleate of safflower has been used to illustrate the method with (D/H)pro-R and (D/H)pro-S measured at the same methylene position of a fatty acid chain. Enantiomers of water soluble materials can be observed using deuterium NMR spectroscopy in the lyotropic mesophase formed by glucopon/hexanol/buffered water. ... 

It is generally accepted that one of the remaining challenges in organic chemistry is to induce efficient asymmetric synthesis on a prochiral precursor, much as the enzyme does. One way to get around this problem is to use a chiral reagent that would produce diastereotopic interactions with a reactant molecule and lead to an asymmetric product. 

In all the examples of exodendrally functionalized enantioselective den-drimer catalysts, the active sites in the periphery of the support were well-defined immobilized molecular catalysts. An alternative is provided by the possibility of attaching chiral multi-functional molecules to the end groups of dendrimers which, due to their high local concentrations, may interact more or less strongly with an achiral reagent and thus induce enantioselectivity in a transformation of a prochiral substrate. Asymmetric induction thus occurs by way of a chiral functionalized microenvironment for a given reaction. 

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