Heterocyclic Chemistry at a Glance, Second Edition. John A. Joule and Keith Mills.
© 2013 John Wiley & Sons, Ltd. Published 2013 by John Wiley & Sons, Ltd.
Pyrylium salts
Pyrylium salts, especially perchlorates (ClO 4 ), tetrafl uoroborates (BF 4 ) or hexachloroantimonates(V) (SbCl 6 ), are
stable but reactive aromatic compounds. Historically, perchlorate salts have been used, but tetrafl uoroborates or hexa-
chloroantimonates are preferred. (CAUTION: all perchlorates, particularly when dry, are potentially explosive and
should be handled with great care.) Alternatively, trifl uoromethanesulfonates have a particular advantage in that they
are more soluble in organic solvents than the other salts mentioned above.
Electrophiles
Pyrylium cations do not react with electrophiles. There are no examples of electrophilic substitutions of pyryliums or,
indeed, on the benzene ring of benzopyryliums.
Nucleophilic addition
Addition of nucleophiles, especially negatively charged nucleophiles, to pyrylium cations is extremely easy, as one
would anticipate for the interaction of a positively charged substrate with a negatively charged reagent. The normal
position of attack is an -position – the analogy with nucleophilic addition to an O-protonated carbonyl group is clear.
Such additions produce neutral 1,2-dihydro-products (2H-pyrans) and it is the further chemistry of these adducts
which comprises the bulk of pyrylium chemistry.
Substituents at the -positions stabilise pyryliums towards nucleophilic addition to an extent – for example, while the
pyrylium cation itself is attacked even by neutral water, at 0 °C, 2,4,6-trimethylpyrylium is stable in water at 100 °C.
Hydroxide, as a negatively charged nucleophile, is more reactive, adding to all pyrylium cations at low temperature.
Ring-opening reactions of 2H-pyrans
Hydroxide adds reversibly, for example to 2,4,6-trimethylpyrylium cation, giving a cyclic hemiacetal, which is in equi-
librium with various ring-opened isomers. On heating, an intramolecular aldol reaction involving an enolate anion
leads, after loss of water and tautomerism, to 3,5-dimethylphenol.