Excited-state proton transfer and ion pair formation in a Cinchona organocatalyst.
The excited-state proton transfer and subsequent intramolecular ion pair formation of a cupreidine-derived Cinchona organocatalyst (BnCPD) were studied in THF-water mixtures using picosecond time-resolved fluorescence together with global analysis. Full spectral and kinetic characterization of all the fluorescent species allowed us to monitor the 3-step process for the ion pair dissociation. In the first step, proton transfer occurs through a water "wire" from the 6-hydroxyquinoline unit (excited-state acid) to the covalently bonded basic quinuclidine moiety, resulting in a hydrogen bonded ion pair. This was confirmed by the observed kinetic isotope effect in the presence of heavy water. In the second step, the formed ions are further solvated by a few solvent molecules, producing the solvent separated ion pair. Finally, a fully solvated ion pair is formed. The 5-exponential global model derived from the reaction scheme describes the experimental data very well.
Van't Hoff Institute for Molecular Sciences, Faculty of Science, University of Amsterdam, PO Box 94157, 1090 GD Amsterdam, The Netherlands.
Tatu
T
Kumpulainen
Albert M
AM
Brouwer
D002384:Catalysis; D002929:Cinchona / Q000737:chemistry*; D017666:Deuterium Oxide / Q000737:chemistry; D006860:Hydrogen Bonding; D007477:Ions / Q000737:chemistry; D007700:Kinetics; D011522:Protons; D011803:Quinine / Q000031:analogs & derivatives / Q000737:chemistry; D013050:Spectrometry, Fluorescence; D014867:Water / Q000737:chemistry
D007477:Ions; D011522:Protons; D014867:Water; C029819:6'-hydroxycinchonidine; D011803:Quinine; D017666:Deuterium Oxide