The effect of solvent on chemical-shift non-equivalence of diastereotopic geminal nuclei in (pro)-chiral N,N-disubstituted 5-oxo-4-phenyl-2,5-dihydroisoxazol-2-ium-3-olates

Abstract

The ¹H and ¹³C resonance shifts of the geminal nuclei in N,N-diethyl, -dibenzyl, -di-isopropyl, and -tetramethylene derivatives of 5-oxo-4-phenyl-2,5-dihydroisoxazol-2-ium-3-olates have been measured in various solvents and (CD₃)₂SO–CDCl₃ mixtures. It is concluded that the accidental isochrony of the geminal hydrogens of the N,N-diethyl derivative in (CD₃)₂SO is not due to a change in conformation. The aim of this work is to discover whether the changes of the relevant chemical shifts are due to the change in the reaction field at the dipole of the molecule, which differs according to the change in dielectric constant of the medium. The use of solvents less polar than (CD₃)₂SO as well as of some of higher polarity results in well separated anisochrony of the two geminal hydrogens. The different directions and magnitudes of the solvent effects on the chemical shifts of each of the nuclei in the compounds studied are attributed to their different angles, relative to the direction of the reaction field.