有機イオウ化合物に関する研究(第15報)脱硫閉環反応におけるプロトンならびに重金属イオンの役割

抄録

A role of proton and heavy metallic ions in the desulfurization ring-closure reaction of 4-(o-aminophenyl)-3-thioallophanic acid ester (Ib) is discussed from the concept of charge and frontier controlled reactions based on a quantum chemical perturbation theory. It is assumed that the protonation to Ib may occur at the position of O-12, S-13, and N-14 atoms but heavy metallic ions, which are known as soft acids, make the selective coordination at the position of S-13, which has the largest highest-occupied density. The energy change in the π-conjugated intramolecular ring-closure reaction is estimated from the perturbation term by the HMO approximation on the assumption that the increase of Coulomb integrals h_O12, h_S13, and h_x14 may be caused by the protonation. The increase of h_S13 and/or the decrease of h_x14 makes the partial bond orders P^HO_{8, 13} negative and P^HO_{8, 14} positive resulting in the decrease of P_{8, 13} and the increase of P_{8, 14} Namely, the protonation to S-13 (the increasing electronegativity of S-13) suggests the tendency of the formation of a new bond between C-8 and N-14, and the weakening of the bond between C-8 and S-13. However, the protonation to O-12 or N-14 (the increasing electronegativity of O-12 or N-14) suggests the opposite tendency. It is understandable that the presence of heavy metallic ions which may cause the selective addition to S-13 accelerates the ring-closure reaction of Ib.