Theoretical Analysis of Regioselectivity in 1,3-Dipolar Cycloadditions of Unstabilized Azomethine Ylide --Methaniminioanilino (methylthio)methylide--

Otohiko TSUGE*, Taizo HATTA, Tomoko OHTO and Kenichi SOMEKAWA†*

Department of Applied Chemistry, Faculty of Engineering, Sojo University; Ikeda, Kumamoto-shi 860-0082 Japan
Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima University; Korimoto, Kagoshima-shi 890-0065 Japan

1,3-Dipolar cycloadditions of unstabilized 1,1-disubstituted azomethine ylide--methaniminioanilino(methylthio)methylide--(1a), to some electron-deficient double bonds gave regioselective single adducts, respectively. The regioselectivities could not be explained by Frontier Molecular Orbital (FMO) method, but could be understood only through the activation energies calculated by transition state (TS) analysis. The TS surfaces to the regioisomer assumed by FMO method have similar reaction distances. Due to two bulky substituents in 1a, on the contrary, the two reaction distances at TS leading to the real reaction products are different: the one at no-substituent site is shorter, 1.9−2.1 , and the other is longer, 2.4−3.4 . The shorter TS distance may cause smaller activation energies. These phenomena seem to come from steric influence by two bulky substituents not to be coplanar to one another in 1a. Namely, steric hindrance of two substituents in the 1,3-dipolar seems to cause asymmetric TS and to derive two-step reactions of lower activation energy.

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