Conformational analysis of trigonal and planar rotors attached to Δ4-azoline-2-thiones. The effect of ring geometry

Abstract

The barriers to rotation of isopropyl and aryl groups (unsubstituted in the ortho positions) in position 3 in 4,5-dimethyl-oxazoline-2-thiones, -imidazoline-2-thiones, and -thiazoline-2-thiones have been studied by temperature-dependent ¹H n.m.r. spectra. The barriers are 44.2,50.4, and 59.0 kJ mol^–1 for the Prⁱ groups, and 38.9, 51.5, and 74.0 kJ mol^–1 for the Ar groups. The barriers are analysed in terms of the apparent overlap of van der Waals radii in idealized transition states, and the barrier differences are shown to depend on the effects of the ring elements in position 1 on the geometry of the ring.