Solvent effects in nuclear magnetic resonance spectroscopy. Part XV. Solvent shifts of methoxy-group resonances induced by trifluoroacetic acid as an aid to structure elucidation

Abstract

The benzene-induced solvent shifts [Δ(chloroform/benzene)] of certain methoxy-groups in flavones are appreciably enhanced by the addition of a small quantity (3% v/v) of trifluoroacetic acid (TFA) to the solution of the flavone in benzene. The TFA-addition shifts [Δ(benzene/benzene–TFA)] for methoxy-groups at C-5, C-7, C-2′ and C-4′ are very small in the absence of methoxy- or hydroxy-substituents ortho to these groups. In contrast, methoxy-groups at C-3, or those flanked by two ortho-methoxy-functions (or by one ortho-methoxy- and one ortho-hydroxy-function) show appreciable positive addition shifts (+0·18 to +0·45 p.p.m.). A methoxy-group at C-5 suffers a marked increase in addition shift when a 6-methoxy-group is introduced. Use of TFA-induced solvent shifts [Δ(chloroform/TFA)] distinguishes a 5-methoxy-group. Protonation by TFA at both methoxy- and carbonyl groups is suggested as the principal cause of these effects.