Long-range substituent and temperature effect on prototropic tautomerism in 2-(acylmethyl)quinolines

Tautomeric equilibria between 2‐(cinnamoylmethyl)quinoline, (Z)‐1,2‐dihydro‐2‐(cinnamoylmethylene)quinoline and (Z)‐4‐phenyl‐1‐(2‐quinolyl)‐1,3‐butadien‐2‐ol were studied by 1H, 13C and 15N NMR methods. The —CHCH— fragment conjugated with phenyl and a strong electron donor p‐(1‐pyrrolidine) substituent were found to favour the enolimine tautomer. This undergoes fast exchange (on the NMR time‐scale) with the enaminone form. The amount of the latter tautomer was found to increase at low temperatures. Copyright © 2001 John Wiley & Sons, Ltd.