Applications of NMR spectroscopy of chiral association complexes. 6. rotation about the C(sp2)—C(aryl) bond in 2,6‐disubstituted pivalophenones

For the investigation of the barrier to rotation about the C(sp2)—C(aryl) bond in non‐planar pivalophenones five derivatives were prepared and their 1H and 13C NMR spectra assigned. Methyl and bromine groups in the 3‐position have opposite substituent effects on the chemical shifts of the 1H and 13C signals of Me2 and Me4. The ΔG values were determined from the coalescence temperatures of the signal splittings generated by the addition of optically active shift reagents. The accuracy of this method was estimated by using different signals of 3‐bromo‐2,4,6‐trimethylpivalophenone and by computer simulation of the line shape. A buttressing effect of substituents in the aromatic ring was observed. A change of the twist angle by the substitution of methyl by bromine in the tert‐butyl group was suggested in order to explain the changes in ΔG and the chemical shifts.