A correlative IR, MS, 1H, 13C and 15N NMR and theoretical study of 4-arylthiazol-2(3H)-ones

Abstract

Sixteen 4-arylthiazol-2(3H)-ones (3) were synthesised by cyclisation of α-thiocyanatoacetophenones (1) in acid solution. They appear to prefer greatly the oxo tautomeric forms. In CCl₄ solution an equilibrium between the free CO bond and a “dimeric” hydrogen-bonded form exists in which the latter predominates. Several IR and NMR (¹H, ¹³C and ¹⁵N) spectral properties are shown to correlate with Hammett σ-values and/or atomic Mulliken charges and bond orders, the latter being estimated by PM3 or AM1 semiempirical methods. The electron-impact mass spectra were also recorded and the fragmentation mechanisms interpreted in terms of the energetics of the ionic species. In addition, the geometric and electronic properties of 4-phenylthiazol-2(3H)-one (3a) and the related benzothiazol-2(3H)-one (4) based on ab initio HF/6-31 G* calculations are compared with each other.