Anwendung der NMR-Spektroskopie chiraler Assoziate 8: rotation about the C(sp²)—C(aryl) bond in 2,6-disubstituted Benzamides

Abstract

The barrier to rotation about the C(sp2)C(aryl) single bond in non-planar benzoyl compounds was investigated using N,N-dimethyl, N,N-tetramethylene and N,N-diisopropyl derivatives of 2,4,6-trimethylbenzamide and 2,6-dimethoxybenzamide and N,N-dimethyl derivatives of the corresponding thiobenzamides. Their 1H and 13C NMR spectra were determined and assigned and the splittings in the 1H spectra due to the addition of the optically active shift reagent (+)-Eu(hfbc)3, are discussed. The free enthalpy of activation was calculated from the coalescence temperature of the ortho-methyl or -methoxyl signals and from a line-shape analysis. An exact equation for the determination of the rate constant at coalescence and a valid approximation which includes the line-width are presented. A distinct effect of the size of the N-substituent on the barrier to rotation in 2,6-dimethoxybenzamides was observed, while the π values for the 2,4,6-trimethylbenzamides studied are practically identical.