Conformational Studies by Dynamic NMR. 94. Cogwheel Pathway for the Stereomutations of Durene Derivatives Containing the Mesityl Ring

The low-temperature NMR spectra of 1,4-bis(mesitoyl)durene, 1, and of 1,4-bis(mesitylethenyl)durene, 2, reveal the presence of syn and anti rotamers at the equilibrium, their relative proportions
depending on the dielectric constant of the solvent. In solution the more stable rotamer of 1 is the
anti whereas, in the case of 2, the more stable is the syn. Depending on the crystallization solvent
employed the more (anti) and the less stable (syn) rotamers were both observed (X-ray diffraction)
in the solid state of 1. On the other hand, only the less stable rotamer (anti) was found to be present
in the solid state of 2. As shown by MM calculations, the syn-to-anti interconversion occurs via a
correlated process (cogwheel pathway) involving the mesityl-C and durene-C bond rotations:  the
dynamic NMR technique yields an experimental barrier of 8.2 kcal mol-1 for 1 and 13.1 kcal mol-1
for 2. In the case of derivative 2 a second barrier, due to a second type of correlated rotation process
(torsion), was also determined (8.6 kcal mol-1). As a consequence of the restriction of this second
torsional motion the anti rotamer of 2 displays two distinguishable NMR spectra at −133 °C,
corresponding to a pair of conformers with different symmetry (anti Ci and anti C2).