40 Pyridines
Sodium borohydride in a protic solvent reduces pyridinium salts to a tetrahydropyridine oxidation level, via an enam-
ine protonation of a dihydro-intermediate.
The pyridine ring is very resistant to oxidative attack, however N-alkyl pyridinium salts are oxidised in alkaline
solution to N-alkyl 2-pyridones – a small concentration of an adduct, formed by addition of hydroxide, is trapped
by the oxidant.
Pericyclic reactions
There are only a few specialised situations where pyridines take part in electrocyclic processes (this contrasts with
six-membered heterocycles with more than one nitrogen atom, see pages 54–55 and 141) – 2-pyridones reacting as
dienes in Diels–Alder reactions is one of these situations.
An interesting example of another type of cycloaddition – a 1,3-dipolar cycloaddition – is that of the betaines that are
generated by treatment of 3-hydroxypyridinium cations with mild base. These species, for which no neutral mesomeric
structure can be drawn, behave as 1,3-dipoles and add to polarised alkenes – methyl acrylate in the example shown.
Alkyl and carboxylic acid substituents
Alkyl groups at the - and -positions of pyridines are ‘acidic’ in that they can be deprotonated using strong bases to
give anions, which can then be reacted with electrophiles. The analogy to -deprotonation of ketones (formation of
enolates – although requiring somewhat milder bases) is clear, with delocalisation of the negative charge onto nitrogen.
Alkyl at a -position can be deprotonated but under much stronger conditions. The methylpyridines are often called
picolines; dimethylpyridines are called lutidines, for example 2,4-lutidine.