STUDY OF THE PI-ELECTRON DISTRIBUTION IN PUSH-PULL ALKENES BY H-1 AND C-13 NMR-SPECTROSCOPY .3. THE CONFORMATION OF 3,4-DIHYDRO-4-OXO-2H-THIINS

The H-1 and C-13 NMR spectra of a variety of differently 2,5,6-trisubstituted 3,4-dihydro-4-oxo-2H-thiins (integrated push-pull alkenes) were unequivocally assigned by means of a wide variety of 1D and 2D NMR spectroscopic methods. The NMR parameters thus obtained, together with the results of accompanying force-field calculations, proved useful in determining the preferred ring conformation of 3,4-dihydro-4-oxo-2H-thiin (half-chair). Additionally, the C-13 chemical shirt difference, Delta delta(C-5, C-6), could be employed to indicate the degree of bond polarization of the partial C-5-C-6 double bond (push-pun character); the barrier to rotation, otherwise used to estimate this parameter characterizing the push-pull alkenes, cannot be applied owing to the ring fusion. Interestingly, NH2 proved less effective in C-5-C-6 bond polarization than NHPh.