The relative electron donating ability of substituents, determined in 2,6-disubstituted pyrylium cations
摘要:
The carbon-13 NMR spectra of a series of 2,6-disubstituted pyrylium compounds have been investigated. The chemical shift of C(4) is a measure of the charge density in that position of the ring. The variation of the chemical shift with the change of substituent is determined by the ability of substituents to delocalize the positive charge from C(4), an effect which for substituents at C(2) and C(6) is free from any other shielding or deshielding effects. The electron-donating ability of substituents vary in the order: H < i-Pr < Me < p-ClC6H4 < p-FC6H4 less-than-or-equal-to Ph << 2-thienyl less-than-or-equal-to c-Pr less-than-or-equal-to p-MeOC6H4. The ordering of alkyl groups indicates that C-H hyperconjugation is stronger than C-C hyperconjugation. For the aryl substituents the chemical shifts are correlated linearly with the partial rate factor (log (P(f)) for the protiodesilylation of the parent arenes. The ability of cyclopropyl to delocalize the positive charge is similar to that of 2-thienyl and p-anisyl.