Der ‘Push‐Pull’‐Effekt von ‘Push‐Pull’‐Oligoacetylenen. Eine 13C‐NMR‐Untersuchung

Abstract

The ‘Push‐Poll’ Effect of ‘Push‐Pull’ Oligoacetylenes. A 13C‐NMR InvestigationAccording to 13C‐chemicaI shifts of ‘push‐pull’ oligoacetylenes 1–4, the ‘push‐pull’ effect (i.e. π delocalization induced by ‘push‐pull’ substituents) rapidly decays in this series. To correct for other than π ‐charge‐density effects, Δδ values of symmetrically placed C‐atoms of the oligoacetylene chain are discussed. Stereoelectronic resteffects (SER) of the substituents on terminal C‐atoms of PP‐ketones 1a–3a and PP‐esters 1b–4b are estimated from the residual Δδ of the asymptotes of Fig. 3. Fig. 4 convincingly shows that Δδ values are dramatically decreasing with increasing number n of acetylene units between the push and pull substituents. Assignment problems of ‘push‐pull’ triacetylenes 3 have been solved by 13C labelling of the CO group of 3a.