Computational studies on the reaction pathways of trifluoromethyloxazolones with both α,β and α,β,γ,δ-unsaturated Fischer carbenes

Abstract

The reaction pathways between α,β and α,β,γ,δ-unsaturated Fischer carbenes with 2-trifluoromethyl oxazolone were analyzed from the selectivity and reactivity perspective. The potential energy surface (PES) of the reaction pathways was explored, chemical descriptors were calculated along the PES by means of QTAIM and conceptual-DFT. We have found that the oxazolone ionic form is the reactive specie instead of dipolar oxazolone (münchnone), due to the HOMO energetic character of the dipolar form compared with the ionic specie. For α,β-unsaturated systems, the pyrrole is originated via a stepwise mechanism instead a cycloaddition reaction, while the regioselectivity can be explained by the energy differences between the corresponding intermediates of the two principal nucleophilic resonance structures of the ionic oxazolone. The preferred mechanism for the α,β,γ,δ-unsaturated Fischer carbene is the one in which the CF3-carbon base of 2-trifluoromethyl oxazolone goes through the 1,6-addition to the unsaturated system.