Highly enantioselective organocatalytic trifluoromethyl carbinol synthesis--a caveat on reaction times and product isolation

J Am Chem Soc. 2012 Jul 11;134(27):11196-205. doi: 10.1021/ja302511t. Epub 2012 Jun 28.

Abstract

Aldol reactions with trifluoroacetophenones as acceptors yield chiral α-aryl, α-trifluoromethyl tertiary alcohols, valuable intermediates in organic synthesis. Of the various organocatalysts examined, Singh's catalyst [(2S)-N-[(1S)-1-hydroxydiphenylmethyl-3-methylbutyl]-2-pyrrolidinecarboxamide] was found to efficiently promote this organocatalytic transformation in a highly enantioselective manner. Detailed reaction monitoring ((19)F-NMR, HPLC) showed that, up to full conversion, the catalytic transformation proceeds under kinetic control and affords up to 95% ee in a time-independent manner. At longer reaction times, the catalyst effects racemization. For the product aldols, even weak acids (such as ammonium chloride) or protic solvents, can induce racemization, too. Thus, acid-free workup, at carefully chosen reaction time, is crucial for the isolation of the aldols in high (and stable) enantiomeric purity. As evidenced by (19)F-NMR, X-ray structural analysis, and independent synthesis of a stable intramolecular variant, Singh's catalyst reversibly forms a catalytically inactive ("parasitic") intermediate, namely a N,O-hemiacetal with trifluoroacetophenones. X-ray crystallography also allowed the determination of the product aldols' absolute configuration (S).

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acetone / chemical synthesis
  • Acetone / chemistry
  • Acetophenones / chemical synthesis
  • Acetophenones / chemistry*
  • Aldehydes / chemical synthesis
  • Aldehydes / chemistry*
  • Catalysis
  • Crystallography, X-Ray
  • Hydrocarbons, Fluorinated / chemical synthesis*
  • Hydrocarbons, Fluorinated / chemistry
  • Magnetic Resonance Spectroscopy
  • Methanol / chemical synthesis*
  • Methanol / chemistry
  • Models, Molecular
  • Stereoisomerism

Substances

  • Acetophenones
  • Aldehydes
  • Hydrocarbons, Fluorinated
  • Acetone
  • 3-hydroxybutanal
  • Methanol