Ab initio and PM3 analysis of 1,3-dipolar cycloaddition reaction between pyridine N-oxides and isocyanates. Theoretical evidence of concerted and nonsynchronous mechanism with zwitterionic character

Abstract

The 1,3-dipolar cycloaddition of nitrone (1a) and pyridine N-oxide (1b) with isocyanates (2) was evaluated in terms of ab initio and semiempirical PM3 calculations. In the ab initio transition structure of 1a with isocyanic acid (2a) by MP2/6-31G ∗ level, the length of the newly forming bonds of O-C and C-N are 1.740 and 2.197 Å, respectively, indicative of the former bond formation being significantly advanced than that of the latter. The PM3 transition structure (TSba) of the cycloaddition between 1b with 2a showed similar result. The O-C bond (1.445 Å) is ca. 76% formed at the transition state, significantly advanced than the C-N bond (2.165 Å, ca. 20%). Moreover, the atoms forming the C-N bond have opposite charge, indicating a zwitterionic character of TSba.