Structural features and crystallographic examination of 5-acetyl- and 5-trifluoroacetyl-2-(N,N-disubstituted amino)thiazoles

Abstract

5-Acetyl-2-(N,N-disubstituted amino)thiazoles, unsubstituted or with Me, Bu^t, or Ph groups at position 4, have been prepared by unambiguous methods and examined spectrometrically; crystallographic studies have been carried out on four of the compounds and a related 4-aryl-5-trifluoroacetylthiazole. In solution the 4-H, 4-Bu^t, and 4-Ar compounds exist predominantly in one conformation but with the 4-Me compounds, two forms (carbonyl O,S-syn or anti, related by rotation of the 5-acetyl group) of approximately equal stability are present. The X-ray work establishes that for 5-acetylthiazoles, as solids, the 4-H and 4-Ph derivatives have the carbonyl O, S-syn stereochemistry whereas the 4-Me and 4-Bu^t derivatives have the anti arrangement; in contrast with the 5-acetyl-4-phenyl compound the 4-aryl-5-trifluoroacetylthiazole adopts the anti arrangement. These findings validate a correlatioh, for solutions, between the stereochemistry of the rotational isomers and the positions of their i.r. CO bands.

The results confirm the operation of a strong mesomeric interaction between the 2-NR₂ and 5-CH₃(CF₃)CO groups.