Mechanisms of acid catalyzed E/Z isomerization of methyl O-methylbenzothiohydroximates.

摘要

The synthesis of (Z) and (E) isomers of methyl O-methylbenzothiohydroximate (7Za, 7Ea), methyl O-methyl-p-chlorobenzothiohydroximate (7Zb, 7Eb), methyl O-methyl-p-methylbenzothiohydroximate (7Zc, 7Ec) and methyl O-methyl-p-methoxybenzothiohydroximate (7Zd, 7Ed) are reported here. The acid-catalyzed isomerization of methyl O-methylbenzothiohydroximate (7Za and 7Ea) was investigated using hydrogen chloride and a variety of other acids with non-nucleophilic or weakly nucleophilic counterions (CF3SO 3H, p-toluenesulfonic acid and CCl3COOH) in acetonitrile at 55.04 +/- 0.01°C. Kinetic measurements demonstrated that the kinetic constant for methyl O-methylbenzothiohydroximate decreases with increasing pKa of the acids with the exception of hydrochloride acid (HCl), which has almost the same acid strength as trichloroacetic acid (CCl 3COOH). With the same reaction conditions, the rate constant of hydrochloric acid with methyl O-methylbenzothiohydroximate is 2600 times faster than that of trichloroacetic acid indicating that the isomerization proceeds by different mechanisms in these two acids. In trichloroacetic acid the isomerization follows the iminium ion rotation mechanism (Mechanism I). If the acid, such as HCl, has a nucleophilic counterion, the isomerization proceeds by the nucleophilic catalysis mechanism (Mechanism II). The Kb of methyl (E)-O-methylbenzothiohydroximate was calculated from the rate constants measured in this work.