Ionization Shifts in 1H‐NMR Spectra of α,β‐Unsaturated Carboxylic Acids

Abstract

AbstractChanges in chemical shifts of olefinic protons in a number of α,β‐ and α,β,γ,δ‐unsaturated carboxylic acids caused by ionization of the COOH group were investigated. The ionization shifts of α‐H‐atoms are −0.09 to 0.07 ppm, those of β‐H‐atoms are 0.32−0.47 ppm. The ionization shifts of δ‐H‐atoms are substantially larger than those of γ‐H‐atoms. The ionization shifts can be used for immediate determination of the esterification site in monoesters of (2E,4Z)‐2,4‐hexadienedioic (muconic) acid, which are of interest in connection with synthetic studies on verrucarins. Thus, isomerization by heating in aqueous solution of monoesters of (2Z,4Z)‐2,4‐hexadienedioic acid yields 1‐monoesters rather than 6‐monoesters of (2E,4Z)‐2,4‐hexadienedioic acid, in accordance with the isomerization mechanism involving anchimeric assistance of the free COOH group. Solutions of the ABXY spectra of olefinic protons of monomethyl (2E,4E)‐ and (2Z,4Z)‐2,4‐hexadienedioate are reported.