Two thiazolium compounds were synthesized specifically labeled at their C2 positions: 3,4,5-trimethyl[2-13C]thiazolium nitrate and 3-benzyl-5-(β-ethoxyethyl)-4-methyl [2-13C] thiazolium bromide, with a view to examine their pathways leading to dimerization in strongly basic medium using 13C NMR. On addition of less than 1 equiv of base the N-methyl ion first formed an unsymmetrical dimer in which the C2 atoms of two molecules were bonded to each other and only one of them still carried a hydrogen; that unsymmetrical dimer upon addition of excess base lost the remaining hydrogen at C2 and was converted to a mixture of syn and anti symmetrical dimers in nearly equal amounts. The sequence of observations on addition of base to the N-methyl derivative is consistent with nucleophilic addition of the conjugate base to a second thiazolium ion at its C2 position. Since the unsymmetrical dimer is formed first, rather than the symmetrical dimer, the latter cannot result from direct dimerization of two conjugate bases (ylides) by a carbene mechanism. Instead, a carbanion-addition mechanism was further supported by two experiments. A “crossover” experiment was designed in which unsymmetrical dimers were detected in Me2SO on addition of limiting potassium tert-butoxide to thiazolium ions containing [2-13C]-H and [2-12C]-D, under conditions such that there was little H/D exchange observed at the C2 position. Also, N-3-alkenyithiazolium ions were synthesized, that, if carbenic reactivity had existed, would have resulted in formation of cyclopropanes. In preference to the intramolecular reaction, intermolecular unsymmetrical dimers resulted in each case, consistent with nucleophilic addition. On addition of base to the N-benzylthiazolium ion, the first product to be detected by 13C NMR was the syn/anti symmetrical dimer mixture (again bonded via the C2 atoms), that underwent a [1,3]-sigmatropic rearrangement of one of the benzyl groups from N3 to C2. According to 1H NMR recorded within minutes of mixing, the unsymmetrical dimer precedes the symmetrical one for this salt as well. The reactivity of the C2 ylide derived from the N-methyl and N-benzylthiazolium ions can be rationalized according to an ionic addition reaction, implying that the related thiamin (vitamin B1) conjugate base (ylide) behaves similarly.
All Science Journal Classification (ASJC) codes
- Organic Chemistry