Heterocyclic analogues of cyclopentadienylidenecycloheptatriene derived from pyridine

Abstract

Quaternary pyridinium salts and their methyl homologues with a vacant 4-position react with cyclopentadiene and with indene to give 4-cyclopentadienylidenedihydropyridines and their 4-inden-1-ylidene analogues, respectively. If a 4-methyl group is present traces of 2-substituted dihydropyridines are formed. The n.m.r. spectra of these bases, which are iso-π-electronic with cyclopentadienylidenecycloheptatriene, have been determined. Deuteriation occurs at the cyclopentadiene 2- and 5-positions in the cyclopentadiene compounds but solely at the indene 3-position in the indene analogues. Similarly, protonation, azo-coupling, and condensation with p-dimethylaminobenzaldehyde takes place at the indene 3-position in the indene bases; the cyclopentadiene compounds give rise to two protonated species, 80% at the cyclopentadiene 2-position and 20% at the cyclopentadiene 3-position. Evidence for rotation round the central bond has been obtained from a variable temperature n.m.r. study of the indenylidenedihydropyridines; this and the site of deuterium exchange are discussed in terms of molecular orbital theory.