This thesis describes a series of attempts to synthesise 2,5- and 1,4-polymethylene bridged pyridines.
Nuclear magnetic resonance theory predicts that protons,
which are held directly over an aromatic ring, will be
abnormally shielded compared with protons in aliphatic
straight-chain hydrocarbons. This prediction has been
verified for the central methylene protons of paracyclophanes. The degree of shielding, expressed in terms of
the distance from the aromatic ring, is a measure of the
induced ring current and hence the aromaticity of the
benzene ring. Similar measurements upon 2,5- or 1,4—
polymethylene bridged pyridines would make it possible
to determine the degree of aromaticity of the pyridine
ring relative to benzene.
A review of the subject of aromaticity is presented
in which special reference has been made to its interpretation by nuclear magnetic resonance.
The synthetic work has not been brougL.t to a truly
satisfactory conclusion. However, the synthetic routes to
2,5-dialkylpyridines have been thoroughly investigated
and a wide variety of such compounds prepared. The functional groups at the ends of the alkyl chains have
been varied in an effort to produce a derivative which
would cyclise to give a 2,5-bridged pyridine. The
attempted intramolecular oxidative coupling of 2,5-dihex51 -ynylpyridine received much attention. In the attempts
to obtain a 1,4-bridged pyridine, two tricyclic compounds,
each containing two quaternised pyridine rings linked by
polymethylene chains, were obtained.
As a result of these investigations, several alternative routes to bridged pyridines are suggested.
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