A molecular electron density theory study for [3+2] cycloaddition reactions of1-pyrroline-1-oxide with disubstituted acetylenes leading to bicyclic 4-isoxazolines

The [3 + 2] cycloaddition (32CA) reactions of 1-pyrroline-1-oxide with acetylene and with a series of four symmetrically disubstituted acetylenes (XC equivalent to CX, X = CH3, NH2, OH, F), leading to 4,5-disubstituted bicyclic 4-isoxazolines, have been studied within molecular electron density theory at the B3LYP-D3/6-311++G(d,p) computational level. These 32CA reactions take place through aone-stepmechanism, with activation enthalpies in toluene between 24.4 (X = NH2) and 12.9 (X = F) kcal mol(-1). Due to the strong exergonic character of these 32CA reactions, between -12.0 and -47.6 kcal mol(-1), they are irreversible. The 32CA reaction of difluoroacetylene, involving fluorine as the most electronegative atom of this series attached to the acetylene carbons, presents the lowest activation enthalpy due to the increase of the polar character of this 32CA reaction, evidenced by the analysis of the global electron density transfer at the corresponding transition state structure. The topological analysis of the electron localization function along the reaction paths of these 32CA reactions allows us to characterize them as the zwitterionic-type ones. The formation of the new C-C and C-O single bonds takes place at the end of the reactions through the coupling ofpseudoradicalcenters.