Pyryliumverbindungen. XXII. 1H-NMR- und UV-spektroskopische Untersuchungen zur Isomerisierung von Pseudobasen unsymmetrisch substituierter 2, 4, 6-Triarylpyryliumsalze

Pyrylium Compounds. XXII. ¹H-N.M.R. and U.V. Spectroscopic Investigations on the Isomerization of Pseudobases of Unsymmetrically Substituted 2, 4, 6-Triarylpyrylium Salts The kinetics and mechanism of the isomerization of the title compounds, the pentene 1,5-diones 2 and 3 , were studied by ¹H-n.m.r. and u.v. spectroscopy. The equilibrium constants measured vary little near 1, indicating the small influence of substituents on the equilibrium position. The mutual rearrangement 2 ⇄ 3 proved to be a base-catalized reaction which is characterized by strictly linear Hammett correlation between 1g k and Σσ_p. The results obtained suggest resonance-stabilized anions of type 4 as intermediates. - Pseudobases of 3-alkyl-2, 4, 6-triarylsubstituted pyrylium salts do not undergo a reversible isomerization.