Prototropic processes in benzaurins. 19F and 1HNMR spectra of fuoro- and methylsubstituted4-hydroxyphenyl-diphenylcarbinols, relatedfuchsones and benzaurins

摘要

Tautomerism of benzaurins and hydration are studied.~1H and~(19)F chemical shifts have been determined for a number ofsubstituted 4-hydroxyphenyl-diphenyl carbinols containing fuorine in a 3-, 3~*- or 4~*-position, and for similar compoundscontaining additional methyl groups in a position of 3, 3~(**) or 4~(**). The same data have been obtained for the fuchsones prepared bydehydration of the above carbinols. On this basis chemical shifts of fuorine in different positions have been evaluated as a monitor of thetransformation of 4-hydroxyphenyl group to the semiquinone moiety. The~(19)F NMR can be used to monitor the transformation of 4~(**)-fuorobenzaurin and the related 3,3~*-disubstituted and 3,3~*,5,5~*-tetramethylsubstituted compounds to the corresponding carbinols due to theaddition of a water molecule and to study the tautomerism of the two latter benzaurins as well as that of 3,3~*,4~(**)trifuorobenzaurin.Fur thermore, fuorine and methyl group chemical shifts are sensitive to syn-anti-isomerism in substituted fuchsones.The prototropic process of these compounds may be slow or fast on a~1H NMR time scale depending on the solvent and may becatalyzed by water or carbonic acids. On the basis of kinetic and thermodynamic data obtained by dynamic NMR studies, a mechanismfor the process has been proposed.