Dual Mechanisms of Nucleophilic Substitution of Thiamin Analogs: Estimating the Special Kinetic Effect of Sulfite Ion

✍ Scribed by G. Uray; I. Kriessmann; J.A. Zoltewicz

• Book ID: 102966029
• Publisher: Elsevier Science
• Year: 1993
• Tongue: English
• Weight: 739 KB
• Volume: 21
• Category: Article
• ISSN: 0045-2068
• DOI: 10.1006/bioo.1993.1025

✦ Synopsis

The thiamin derivative 4-amino-5-bromomethyl-1,2-dimethylpyrimidinium bromide (2a) reacts with (p)-nitrobenzenethiolate, thiosulfate, and azide ions and the corresponding 5 -chloromethyl derivative. (\mathbf{2 b}), reacts with thiosulfate and azide ions, each substrate by an (\mathrm{S}{\mathrm{N}} 2) mechanism. Large kinetic leaving group effects (\left(k{\mathrm{Br}} / k_{\mathrm{Cl}}>30\right)) are found with thiosulfate and azide ions but not with sulfite ion. With sulfite ion (2 \mathbf{a}) reacts by two competing mechanisms, mainly nucleophilic substitution by the unique multistep (\mathrm{S}{\mathrm{N}}(\mathrm{AE})) route found for thiamin and many of its analogs and also by a competing (\mathrm{S}{\mathrm{N}} 2) mechanism. A linear free energy relationship comparing the reactivities of (2 \mathbf{a}) and benzyl bromide toward a series of nucleophiles shows that the rate acceleration for the (S_{N}(A E)) route over that for the (S_{N^{2}}) pathway is negligible for 2a and sulfite ion. 1993 Academic Press. Inc.