Dual Mechanisms of Bimolecular Nucleophilic Subtitution of a Heterobenzylic Bromide Related to Thiamin

Article abstract of DOI:10.1021/jo00267a026

4-Amino-5-(bromomethyl)-1,2-dimethylpyrimidinium ion at 25 deg C in 0.9 M KCl made to an ionic strength of 1 M with buffers react with hydroxide ion with a rate constant 725 M^-1s^-1.Evidence for an intermediate is found in the large common ion rate retarding effect of added bromide ion.An addition-elimination mechanism (S_N(AE)) involving the addition of hydroxide ion to give a pseudobase intermediate followed by loss of bromide ion is advanced.Chloride ion reacts by an S_N2 mechanism (5.9E-5 M^-1s^-1).When 2-thiopyridone acts as a nucleophile by the S_N2 mechanism, its sulfur atom is heterobenzylated with a rate constant of 0.28 M^-1s^-1 (31 deg C).Two different bimolecular mechanisms of nucleophilic substitution are thereby observed.