Conformational Studies by Dynamic NMR. 65. Interconversion of Stereolabile Meso and Racemic Diastereoisomers of Hindered 1,4-Diacylnaphthalenes

1,4-Diacylnaphthalenes, bearing methyl groups in positions 2 and 3, display, at appropriate temperatures, NMR spectra due to meso and racemic stereolabile diastereoisomers, brought about by the restricted rotation of the two RCO substituents. As indicated by the diastereotopicity of appropriate prochiral substituents, these acyl groups are not coplanar to the naphthalene ring, and their CO moieties can adopt either a syn or an anti arrangement. The meso (syn) and racemic (anti) structures were assigned, in solution, by taking into account the changes of their relative proportions in solvents of different polarity and, in the solid state, by X-ray diffraction. The free energies of activation for the interconversion were determined by computer line-shape simulation of the variable-temperature NMR spectra, and their values were found to increase substantially with the increasing bulkiness of the carbonyl-bonded alkyl groups (from 10.2 to 22.1 kcal mol^-1 for R = Me and R = t-Bu, respectively).