Rearrangements of N‐Heterocyclic Carbenes of Pyrazole to 4‐Aminoquinolines and Benzoquinolines

Abstract

Abstract 1‐Phenyl‐substituted pyrazolium salts, formed by quaternization of pyrazoles with benzyl halides or long‐chain alkyl halides, deprotonate to pyrazol‐3‐ylidenes that undergo a sequence of ring‐opening, ring‐closure, and tautomerization to new substituted 4‐aminoquinolines. Similarly, 1‐naphthyl‐substituted pyrazolium‐3‐carboxylates decarboxylate on heating in toluene to give benzoquinolines in excellent yields by an analogous reaction pathway. DFT calculations indicate that the ring transformation proceeds through a sequence of intramolecular elimination, imine inversion and 6π‐electrocyclization steps.