[3 + 2] cycloaddition reactions of C, N-diarylnitrone to arylallene and N-aryl-C-carbamoyl nitrones to methyl buta-2, 3-dienoate. A theoretical study of reactivity, site, and regio-selectivity

This study theoretically investigated the selectivity and reactivity in the 32CA reaction of C,N-diarylnitrone (X1) with arylallene (X2) and N-aryl-C-carbamoyl nitrone (B1) with methyl buta-2,3-dienoate (B2), leading to methylideneisoxazolidine. We employed density functional theory (DFT) at the ωB97XD/6-311+G(d,p). The 32CA reaction of X1 to X2 is a one-step mechanism, with path A being the most kinetically favored, leading to the formation of 5-methylideneisoxazolidine diastereoisomeric pairs (P3A + P4A). Substitutions of –CN and -OCH₃ on X2 influence the regioselectivity of the cycloaddition process. CN substitution gives the 5-methylideneisoxazolidine cycloadduct, while OCH₃ substitution gives the 4-methylideneisoxazolidine. The reaction of B1 with B2 occurs at the internal C=C olefinic bond to regioselectively yield the 5-methylideneisoxazolidine adduct. Theoretical computations suggest that the phenyl group on X2 behaves as an electron-withdrawing group. The GEDT values for the 32CA of X1 to X2 exhibit a nonpolar character, classified as REDF, while those involving B1 to B2 display a slightly polar character, classified as FEDF.