Cyclization and rearrangement of monofluoroallylic cations from halonium metathesis in the gas phase

Abstract

Products of the F+-for-O metathesis between a variety of fluorinated cations and α,β-unsaturated ketones have been examined using FT-ICR and sector mass spectrometry. The reactant ions CF3+, C3F7+, and CFO+ all undergo ion–molecule reactions with carbonyl compounds to replace oxygen with F+. In principle, the simple transposition of atoms corresponds to the transformation CO→CF+ and ought to produce monofluorinated allylic cations, which DFT calculations predict to be highly stable. The metathesis, however, is so exothermic that cationic rearrangements take place, as attested by several experimental data, including: (1) unimolecular loss of HF from the ion created by methacrolein; (2) the Brønsted acidity of the ion created by methacrolein in its subsequent ion–molecule reactions; and (3) unimolecular loss of ethylene from the ion created by senecialdehyde. DFT calculations suggest that cyclization of unsaturated cations takes place, even though that mechanism removes the positive charge from conjugation with double bonds. Evidence for cyclization is to be found in CF3+-adduct ions as well as the metathesis ions (for instance, the reaction of sorbital with CF3+, which forms CH2OCF3+, as well as the metathesis ion). Electrocyclic ring closure of fluoroallylic ions creates cyclopropyl cations, which represent transition states rather than stable structures on the DFT potential energy surface. Calculated energy barriers to forming monofluorinated cyclopropyl cations range from ΔH=160 to 266kJmol−1. The exothermicities of metatheses with CF3+ are calculated to be >50kJmol−1 higher than the respective barrier heights.