Cyclopropyl‐substituted pyrylium salts

Abstract

AbstractThe previously unknown 2,4,6‐trisubstituted pyrylium salts 1‐4, carrying cyclopropyl and methyl or phenyl substituents in the same molecule, and 5‐8, substituted with isopropyl groups instead of cyclopropyl, were synthesized as perchlorates. The electronic spectra and the C‐13 nmr spectra of the two groups of compounds were compared. A cyclopropyl group has a batochromic effect on the electronic spectrum roughly half that exerted by a phenyl; replacement of an alkyl by a cyclopropyl shifts the highest wavelength absorption band by about 20 nm. Presence of phenyl substituents reduces the batochromic effects of cyclopropyl substituents. The effects of both cyclopropyl and phenyl substituents on the electronic spectra are smaller for pyrylium than for tropylium. The nmr signals for all the ring carbons are shifted upfield upon replacement of isopropyl by cyclopropyl; in particular, the effect of substituents in position 2 upon the chemical shift of C(4) indicates that the electron‐releasing effect varies in the series alkyl < phenyl < cyclopropyl, in agreement with the findings for 2,6‐disubstituted pyrylium salts. The difference between the chemical shifts for the methine and methylene carbons of the three‐membered ring is a function of the electron demand of the cationic heterocycle.