Conformational studies by dynamic NMR. 42. Detection of meso and racemic conformers resulting from a chirality created by the restricted torsion of hindered carbonyls

Abstract

Aromatic derivatives containing two equivalent, sterically hindered, carbonyl groups can exist either as meso or as racemic (d, l) conformers, owing to the noncoplanarity of the Ar and RC=O moieties which creates two centers of chirality. Molecular Mechanics calculations (MM-2) carried out on 2,6-dimethyl-1,5-di(2-methyl)propanoylnaphthalene (1) suggest that the meso form is only 0.5 kcal mol⁻¹ (1 kcal=4.184 J) more stable than the racemic conformer. Solid-state nmr spectra on a bulk sample are not in contrast with the result of the X-ray analysis on a single crystal that the meso conformer is the only species which is present in the solid. On the other hand low-temperature nmr spectra showed that both meso and racemic conformers are present in solution (e.g., 57% and 43% in CD₂Cl₂ at −85°), the corresponding interconversion barrier (as determined by line shape computer simulation) being 10.7 kcal mol⁻¹.