ACIDITIES OF C2 HYDROGEN-ATOMS IN THIAZOLIUM CATIONS AND REACTIVITIES OF THEIR CONJUGATE BASES

The equilibrium acidity for the C2-H bond in the 3,4-dimethylthiazolium cation (TZCH+), a model for thiamin, was estimated to be higher than 16 by direct titrations with indicators in DMSO solution. This result is in agreement with several earlier indirect estimates based on kinetic acidities and Bronsted plots, which place the acidity in the 16-20 pK(HA+) region in aqueous solution, but not with a direct titration in aqueous medium made under stopped-flow conditions, which placed the acidity in the region of pK(HA+) almost-equal-to 13. 3-Methylbenzothiazolium cation (BZCH+), which was found to be considerably more acidic, reacted with Et3N in DMSO to give a dimer BZC=CZB. Evidence is presented to show that this and similar dimerizations occur by addition of the conjugate base of BZCH+ to the H-C=N+ bond to the BZCH+ thiazolium cation, followed by deprotonation. The conjugate base of BZCH+ adds to BZCH+ in preference to reacting with excess of electrophiles such as t-buOH, PhCHO, or PhCH=CH2. Amines, such as piperidine, add rapidly to less acidic thiazolium cations under conditions where little or no deprotonation occurs. These observations exclude a carbene mechanism for dimerization and amine adduct formation.