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Dianionic Phase-Transfer Catalyst for Asymmetric Fluoro-cyclization.

Hiromichi Egami1, Tomoki Niwa1, Hitomi Sato1

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|February 10, 2018
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Researchers developed new dicarboxylic acid precatalysts for asymmetric fluorination. These catalysts enable efficient enantioselective synthesis of fluorinated dihydrooxazines from allylic amides via a novel 6-endo-fluoro-cyclization.

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Area of Science:

  • Organic Chemistry
  • Asymmetric Catalysis
  • Fluorination Chemistry

Background:

  • Electrophilic fluorinating reagents like Selectfluor are crucial in organic synthesis.
  • Developing efficient and selective catalytic methods for introducing fluorine is a significant challenge.
  • Phase-transfer catalysis offers a versatile platform for various organic transformations.

Purpose of the Study:

  • To design and synthesize novel dicarboxylic acid precatalysts for asymmetric fluorination.
  • To investigate the catalytic activity of these precatalysts in the enantioselective fluorination of alkenes.
  • To explore the scope and limitations of the developed catalytic system.

Main Methods:

  • Rational design of dicarboxylic acid precatalysts inspired by the structure of Selectfluor.
  • Deprotonation of precatalysts to generate anionic phase-transfer catalysts.
  • Asymmetric fluorination reactions using allylic amides and acyclic alkenes as substrates.
  • Analysis of enantioselectivity and diastereoselectivity using chiral chromatography and NMR spectroscopy.

Main Results:

  • A series of dicarboxylic acid precatalysts were synthesized and evaluated.
  • The precatalyst 2a demonstrated high efficiency in catalyzing the 6-endo-fluoro-cyclization of allylic amides.
  • Fluorinated dihydrooxazine products were obtained with excellent enantioselectivity (up to 99% ee).
  • Acyclic trisubstituted alkenes reacted with good diastereoselectivity, while linear disubstituted alkenes showed lower selectivity.

Conclusions:

  • The developed dicarboxylic acid precatalysts function effectively as anionic phase-transfer catalysts for asymmetric fluorination.
  • The 6-endo-fluoro-cyclization represents an unprecedented transformation for synthesizing fluorinated dihydrooxazines.
  • The reaction mechanism likely involves a fluoro-carbocation intermediate, providing insights for further catalyst development.