Stability of substituted pyrylium salts and the corresponding 2- H- and 4- H-pyrans

Abstract

The stabilities of a series of 4-substituted pyrylium salts have been compared with those of the correspondingly substituted benzenes using the ab initio STO-3G basis set. It is shown that marked field and resonance effects operate in the pyrylium salts covering a range of more than 50 kcal mol −1 compared to the benzenes. The corresponding 4- H- and 2- H-pyrans that would be produced by hydride ion attack have also been studied, the 2- H-pyrans usually being the more stable. Similar substituent effects occur in these equilibria. The relative stability of the parent 2- H- and 4- H- pyrans is discussed, the 4- H-pyran being the preferred structure at the STO-3G, 3-21G and 6- 31G ∗ bases.