Conformational studies by nuclear magnetic resonance—IV: Restricted rotation in vinylogous thioamides

Enamino ‐thial and ‐thiones R 1 C(S)CHCHNR   2 2(R 1 = H or alkyl; R 2 = Me or Et) have been shown by NMR spectra to exist in two rotational forms, s ‐ cis and s ‐ trans , the populations of the latter being approximately the same as in the case of the parent oxa analogues. An increase of the order of 2 to 4 Kcal/mole in the heights of CC and CN rotation barriers (Δ G *) was found on comparing the title compounds with their oxa analogues. IR spectra failed as a tool to establish the rotational equilibrium. IR absorption bands of the ν CC , ν CH (in the NMe 2 group) and γ HCCH vibrations have been found, but the ν CS band could not be assigned unambiguously. Anomalies concerning the frequency and intensity of the ν CC band are discussed.