Conformational studies by dynamic NMR .57. Stereodynamics of syn-anti interconversion of disubstituted acyl durenes

The orthogonal syn and anti isomers, originated by the restricted rotation about the Ar-C(O)Bu(t) single bonds in 1,4-bis(2,2-dimethylpropanoyl)durene (2e), have been separated by preparative thin layer chromatography. In solution they reach an equilibrium where the syn-anti ratio depends upon the polarity of the solvent. This allowed us to assign the anti structure, which has a null dipole moment, to the least retained isomer, The free energy of activation (Delta G*) for the interconversion was found to be 22.5 kcal mol(-1), a value high enough for identifying these species as configurational isomers. When less hindered derivatives, also having two RCO (R = Pr-i, Et, Me) substituents in the positions 1,4 of the durene moiety, were examined, the syn and anti forms could be detected only at low temperature by means of NMR, spectroscopy. The corresponding interconversion barriers (Delta G* = 13.4, 11.7, 10.9 kcal mol(-1), respectively) are, in fact, much lower than for R = Bu(t), indicating that in these cases we are dealing with conformational rather than with configurational isomers.