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Effect of π‐Electron Delocalization on Tautomeric Equilibria – Benzoannulated 2‐Phenacylpyridines
Author(s) -
Gawinecki Ryszard,
Kolehmainen Erkki,
LoghmaniKhouzani Hossein,
Ośmiałowski Borys,
Lovász Tamás,
Rosa Pavel
Publication year - 2006
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200500840
Subject(s) - tautomer , chemistry , pyridine , intramolecular force , ring (chemistry) , annulation , delocalized electron , computational chemistry , stereochemistry , medicinal chemistry , organic chemistry , catalysis
Most 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)