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

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