5 years ago

Molecular Recognition in Asymmetric Counteranion Catalysis: Understanding Chiral Phosphate-Mediated Desymmetrization

Molecular Recognition in Asymmetric Counteranion Catalysis: Understanding Chiral Phosphate-Mediated Desymmetrization
Robert S. Paton, Fernanda Duarte
We describe the first theoretical study of a landmark example of chiral anion phase-transfer catalysis: the enantioselective ring-opening of meso-aziridinium and episulfonium cations promoted by asymmetric counteranion-directed catalysis (ACDC). The mechanism of ion-pairing, ring-opening, and catalyst deactivation have been studied in the condensed phase with both classical and quantum methods using explicitly and implicitly solvated models. We find that the stability of chiral ion-pairs, a prerequisite for asymmetric catalysis, is dominated by electrostatic interactions at long range and by CH···O interactions at short range. The decisive role of solvent upon ion-pair formation and of nonbonding interactions upon enantioselectivity are quantified by complementary computational approaches. The major enantiomer is favored by a smaller distortion of the substrate, demonstrated by a distortion/interaction analysis. Our computational results rationalize the stereoselectivity for several experimental results and demonstrate a combined classical/quantum approach to perform realistic modeling of chiral counterion catalysis in solution.

Publisher URL: http://dx.doi.org/10.1021/jacs.7b02468

DOI: 10.1021/jacs.7b02468

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