Kinetics and Mechanism of the Methyltrioxorhenium-Catalyzed Sulfoxidation of Thioketones and Sulfines
作者:Ruili Huang、James H. Espenson
DOI:10.1021/jo990621e
日期:1999.8.1
Kinetic studies have been carried out on the oxidation of thiobenzophenones and thiocamphor by hydrogen peroxide and on the second step in the sequence, oxidation of the resulting sulfines (Ar2C=S=O). The first reaction follows the pattern now common to methyltrioxorhenium/H2O2 reactions, in that the rate constant for the reaction between the peroxorhenium intermediate and the thioketone follows a Hammett; correlation such that electron-releasing substrates react more rapidly. The reaction constant is rho = -1.12. However, the plot of log(k(X)/k(H)) for the sulfines against 2 sigma, determined over the range -1.6 < 2 sigma < +1.5, is markedly U-shaped. This suggests a mechanism for the sulfines in which the direction of the electron flow in the transition state changes with the electron demand of the substituents on the sulfines. The product of oxidation of the sulfine is a transient sultine (from epoxidation of the C=S bond). It cannot be detected, however, because it so rapidly yields sulfur monoxide. The SO was oxidized to SO2 under these conditions rather than undergoing disproportionation to SO2 and S. Also, SO was trapped with a 1,3-diene as a thiophene-1-oxide.