A Interação substrato-modificador e sua importância para a hidrogenação enantiosseletiva de alfa-cetoésteres

AUTOR(ES)

Ceciliana da Silva Braga de Oliveira

DATA DE PUBLICAÇÃO

2004

RESUMO

The heterogeneous asymmetric catalysis plays an important and crucial role in many chemical processes and has therefore become a growing and interesting field. The enantioselective hydrogenation of α-ketoesters is one of the best known examples of such process. Methyl pyruvate, which can be converted to (R)- and (S)-methyl lactate with an enantioselectivity of up to 97% on alumina-supported platinum catalyst modified with cinchona alkaloids, has been widely used as a model compound. No enantiodifferentiation is observed in the absence of the alkaloid. In recent years this reaction has been studied in considerable detail; its mechanism, however, is far from being completely resolved. In order to gain additional insight into the interactions between the substrate and the modifier,in the present work, we present high level ab initio and DFT calculations to quantify the interactions between the methyl pyruvate molecule and ammonia, trimethylamine,quinuclidine and cinchonidine. The interactions between these protonated amines and methyl pyruvate as well as between protonated methyl pyruvate and the amines are also studied. We intend to give quantitative indications of the role played by the cinchonidine-methyl pyruvate interactions to the stabilization of the intermediate complex.The NBO population analysis procedure is employed to quantify the donor-acceptor interactions between methyl pyruvate and the amines. The complexation energies between the neutral species have low values and the structures of their complexes do not indicate any specific interaction between the chiral modifier and the substrate. In the case of the protonated species, although the interaction energy is high, the geometries of the complexes do not differentiate clearly a pro-R precursor from a pro-S precursor. These results are not consistent to previous related models.

ASSUNTO(S)

quimica organica enantioselective hydrogenation chiral modifier hidrogenação enantiosseletiva modificador quiral alfa-cetoésteres - hidrogenação mecanismo de reação química orgânica alpha-ketoesteres - hidrogenation

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