NMR spectroscopic and theoretical evidence of cinchona alkaloid- ketopantolactone complex formation in aprotic solvents: Implications for the mechanism of Pt-catalyzed enantioselective hydrogenation of activated ketones

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Abstract

NMR spectroscopy (standard 2D NMR spectroscopic methods and diffusion-ordered NMR spectroscopy) and theoretical calculations (ab initio modeling at the density functional level and natural bond orbital analysis) were used to verify formation of supramolecular complexes between the pairs O-methylcinchonine-ketopantolactone (KPL) and β-isocinchonine-KPL in deuterobenzene solution. The first direct evidence was found on the interaction of the lone pair of the quinuclidine N atom and the prochiral keto-carbonyl group of the KPL. Strong nN → π* interactions were observed between the nonbonding orbital of the quinuclidine N atom and the π* antibonding orbitals of the C{double bond, long}O bonds. The complex was demonstrated experimentally to be stabilized not only by the H bonds between H 5 of the chiral modifier and the KPL, but also, depending on the structure of the cinchona alkaloid, by those between H8 and H9 and the KPL. In aprotic solvents, this type of experimentally verified adduct may be present on the Pt surface and participate in chiral induction in the Orito reaction.

Original languageEnglish
Pages (from-to)266-276
Number of pages11
JournalJournal of Catalysis
Volume246
Issue number2
DOIs
Publication statusPublished - Mar 10 2007

Keywords

  • Chiral induction
  • Cinchona
  • Enantioselective hydrogenation
  • Intermediate complex
  • Ketopantolactone
  • Molecular modeling
  • NMR
  • Supramolecular complex

ASJC Scopus subject areas

  • Catalysis
  • Physical and Theoretical Chemistry

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