Kinetics and Mechanism of Tautomerism of a Hydroxy Schiff Base, N-[2-{2-Hydroxyethylimino(methyl)methyl}phenyl]-2-chlorpropamide, in Solution

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The tautomerism between a hydroxy Schiff base (I), the titled compound, and the corresponding ring-closed oxazolidine (II) was kinetically studied in solution. In chloroform, the rates of tautomerism (I⇄^^k_1__k_2) were determined using NMR and UV spectroscopies, and both methods gave the almost identical pseudo first-order rate constants (k_<obs>=k_1+k_2) of 5.4×10^<-4>s^<-1>. The ratio of I to II at equilibrium, estimated by an NMR method in chloroform-d_1,was 1 : 1 and 7 : 1 in methanol-d_4.The molecular species of compound I in various pH buffers were deduced by NMR, UV and other spectroscopies. In an acid solution (e.g., pH 3.0) compound I existed as the protonated Schiff base (IH^+) at the imine nitrogen atom, and in the alkaline region (e.g., pH 9.0) as the oxazolidine form II. The tautomerism rates in the aqueous solutions were measured by a pH-jump method using a stopped-flow apparatus. The rate (I⇄II) in the alkaline region was faster than that (IH^+ ⇄ IIH^+) in the acid region, where IIH^+ represents an N-protonated oxazolidine form of II.
社団法人日本薬学会の論文
1994-02-15