1974 年 94 巻 5 号 p. 533-539
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 hO12, hS13, and hx14 may be caused by the protonation. The increase of hS13 and/or the decrease of hx14 makes the partial bond orders PHO8, 13 negative and PHO8, 14 positive resulting in the decrease of P8, 13 and the increase of P8, 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.