REACTION OF AROMATIC N-OXIDES WITH DIPOLAROPHILES .19. PM3 ANALYSIS OF SEQUENTIAL PERICYCLIC-REACTIONS BETWEEN PYRIDINE N-OXIDES AND ALLENES

Sequential pericyclic reactions between pyridine N-oxides and allenes were evaluated in terms of PM3 calculation. Structures corresponding t o the perturbating system were calculated by varying the distance of t he addends. The cycloaddition occurs via pi-complex. In the transition structure TS1 of primary cycloaddition, the lengths of the newly crea ted bonds of O-C and C-C are 1.864 and 2.204 Angstrom, respectively. T he bond between the O atom of pyridine N-oxide and the central carbon of allene is ca. 35 % formed at TS1, slightly advanced than the other (ca. 30 %). The PM3 calculation at the UHF level predicts that [1,5]-s igmatropic rearrangement of the primary cycloadduct proceeds via a tra nsition state with a biradical character.