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

Article abstract of DOI:10.1007/BF01160657

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., 57percent and 43percent in CD2Cl2 at -85deg), the corresponding interconversion barrier (as determined by line shape computer simulation) being 10.7 kcal * mol^-1.