Understanding the mechanism and regio- and stereo selectivity of [3 + 2] cycloaddition reactions between substituted azomethine ylide and 3,3,3-trifluoro-1-nitroprop-1-ene, within the molecular electron density theory.

The selectivity and the nature of the molecular mechanism of the [3 + 2] cycloaddition (32CA) reaction between 2-(dimethylamino)-1H-indene-1,3(2H)-dione (AY11) and trans(E)-3,3,3-trifluoro-1-nitroprop-1-ene(FNP10) has been studied, in which the molecular electron density theory using density functional theory methods at the MPWB1K/6-31G(d) computational level was used. Analysis of the global reactivity indices permits us to characterize FNP10 as a strong electrophile and AY11 as a strong nucleophile. Four reactive pathways associated with the ortho/meta regioselective channels and endo/exo stereoselective approaches modes have been explored and characterized in the gas phase and in the benzene solvent. The analysis of the relative energies associated with the different reaction pathways indicates that the 32CA reactions of the azomethine ylide (AY) with the nitroalkene (FNP) is meta regioselective with high endo stereoselectivity. This result is in good agreement with the experimental observations. electron localization function topological analysis of the most favored reactive pathways allows for characterizing the mechanism of this 32CA reactions as a non-concerted two-stage one-step mechanism. Finally, non-covalent interactions and quantum theory of atoms in molecule analyses at the meta/endo transition state structure indicate that the presence of different several weak interactions, namely, CF and NH contributed in favoring the formation of a meta-endo cycloadduct.

Effect of hydrogen bonds and CF3 group on the regioselectivity and mechanism of [3 + 2] cycloaddition reactions between nitrile oxide and 2,4-disubstituted cyclopentenes. A MEDT study.

The mechanism and the regioselectivity of the non-polar [3 + 2] cycloaddition reactions between nitrile oxide and two cyclopentenes were theoretically investigated within the molecular electron density theory (MEDT) using DFT methods, namely the B3LYP, MPWB1K and ωB97XD functionals together with the standard 6-31G(d) basis set. The activation energies of these 32CA reactions are relatively high, due to the low nucleophilic power of both cycloalkenes and the relatively moderate electrophilic nature of the nitrile oxide. The B3LYP and MPWB1K functionals reproduced high relative energies values, while the ωB97XD one yields better values in the kinetic and thermodynamic energies. The completely 4-regioselectivity that observed experimentally has been explained by the analysis of the relative energies, in which the formation of 4-regioismeric cycloadducts is always the more kinetically favored one. The electron localization function (ELF) topological analysis showed that the studied 32CA reactions proceed through two-stage one-step nonconcerted mechanism. The effects of the hydrogen bonds on the regioselectivity determination have been investigated by mean both noncovalent interactions (NCI) and quantum theory of atoms in molecule (QTAIM) analyses, which confirm the presence of high strength N - H···O hydrogen bond and a supplementary weak stabilized H…F hydrogen bonds.