Intramolecular fluorine migration via four-member cyclic transition states

Area of Science:

• Physical Chemistry
• Organic Chemistry
• Computational Chemistry

Background:

• Gaseous trifluoromethyl cation (CF(3)(+)) is known to react with carbonyl compounds.
• The mechanism of fluorine-oxygen exchange in these reactions requires further elucidation.

Purpose of the Study:

• To investigate the reaction pathways of CF(3)(+) with propionaldehyde.
• To understand the mechanism of fluorine for oxygen (F(+)/O) metathesis in carbonyl compounds.
• To explore the influence of substituents on the F(+)/O metathesis reaction.

Main Methods:

• Gas-phase reactions of CF(3)(+) with isotopically labeled propionaldehyde.
• (19)F NMR spectroscopy for product analysis.
• Electron bombardment flow (EBFlow) reactor for deprotonation.
• Density functional theory (DFT) calculations.

Main Results:

• CF(3)(+) reacts with propionaldehyde via two pathways, with the major pathway (80%) yielding a product with the label in a methyl group.
• A minor pathway (20%) results in the label at the central carbon, indicating F(+)/O transposition.
• DFT calculations confirm a four-member cyclic transition state for F(+)/O metathesis.
• Transition state energetics do not systematically vary with thermochemistry, but correlate with bond lengths in an inverse Hammond postulate manner.

Conclusions:

• The F(+)/O metathesis mechanism involves adduct formation, 1,3-atom transfer, and isomerization.
• Computational and experimental data provide insights into the reaction mechanism and transition state behavior.
• The observed correlation between transition state bond lengths and thermochemistry challenges naive interpretations of the Hammond postulate.