Conformational studies by dynamic NMR .62. Stereomutations of rotamers and of conformational enantiomers in 1,2-diacylbenzenes

A number of 1,2-diacylbenzenes have been investigated by dynamic NMR spectroscopy. The 1,2-formyl derivative 1 was found to exist (at -162 degrees C) in the nearly coplanar ZE and EE conformations (70% and 30%, respectively) that interconvert with a free energy of activation of 4.9 kcal mol(-1). On the contrary, the more hindered 1,2-diisobutanoylbenzene (4) adopts a twisted conformation (as indicated by the C-13 spectrum at -157 degrees C) which, in principle, might correspond either to a meso or to a racemic stereolabile structure. Only the racemic conformer is, however, believed to be populated, and the interconversion barrier between the RR and SS conformational enantiomers has been determined (5.6 kcal mol(-1)). Although the 1,2-diacetyl- and 1,2-dipropanoylbenzenes did not exhibit dynamic NMR effects in solution, they were assigned a twisted (racemic) conformation on the basis of the corresponding solid state C-13 CP-MAS spectra, in that they display a 1:1 doublet signal for each pair of enantiotopic carbons. All these conclusions agree with the predictions of molecular mechanics calculations.