A theoretical study on the regio- and stereoselectivity of [3+2] cycloddition of β-trifluoroacetyl vinyl ethyl ether with 3-oxo-1,2-pyrazolidinium ylides

[3+2] Cycloaddition reactions of 2-(trifluoroacetyl)vinyl ethyl ether ( 1 ) to substituted and unsubstitued 2-arylidene-5-oxopyrazolidin-2-ium-1-ides ( 2a – e ) were studied using density functional theory (DFT) methods at the cc-pVDZ level. The mechanistic details of these reactions, especially with respect to regio- and stereoselectivity, were analyzed. Analysis of the relative energies that are associated with the different reaction pathways indicated that the pre­sence of the trifluoroacetyl group in the dipolarophile and substituents on the aryl ring in the dipolar have a remarkable effect on selectivity. In addition, it was found that the ortho–endo pathway with the lowest activation energy is preferred, which is in good agreement with the experimental data. Moreover, the elimination of ethanol from the [3+2] cycloadducts and the formation of bicyclic pyrazolidinones are explained in order to give a total description of the complete domino processes. The inclusion of solvent effects increased the acti­vation energies and the exothermic character of the cyclo­adducts, but did not change the gas phase selectivity. The DFT-based reactivity indices clearly pre­dicted the experimental regiochemistry.