Effect of π‐Electron Delocalization on Tautomeric Equilibria – Benzoannulated 2‐Phenacylpyridines

Abstract

AbstractMost benzoannulated 2‐methylpyridines react with phenyllithium and substituted alkyl benzoates to give the corresponding 2‐phenacylpyridines. 3‐Methylisoquinoline is transformed into 2‐benzoyl‐3‐methyl‐1‐phenyl‐1,2‐dihydroisoquinoline under these conditions, but replacement of phenyllithium with lithium isopropylcyclohexylamide is effective for production of 3‐phenacylisoquinolines. Except in the cases of some substituted 6‐phenacylphenanthridines, tautomeric mixtures of benzoannulated 2‐phenacylpyridines in chloroform solution always contain the ketimine forms.(Z)‐2‐(2‐Hydroxy‐2‐phenylvinyl)pyridine (enolimine) forms also contribute if the pyridine ring is not benzoannulated or if such annulation is at positions 4,5. On the other hand,(Z)‐2‐benzoylmethylene‐1,2‐dihydropyridine (enaminone) forms exist in equilibrium with the ketimine tautomers if the pyridine ring is benzoannulated at positions 3,4 or 5,6, or at both of these locations. As well as the effectiveness of π‐electron delocalization, other effects, such as the strength of the intramolecular hydrogen bonding, should also be considered in order to infer the tautomeric preferences. Strongly electron‐donating substituents were found to stabilize theketimine forms in each series. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)