Coluccini Carmine, Grilli Stefano, Lunazzi Lodovico, Mazzanti Andrea
Department of Organic Chemistry "A. Mangini", University of Bologna, Viale Risorgimento, 4 Bologna 40136, Italy.
J Org Chem. 2003 Sep 19;68(19):7266-73. doi: 10.1021/jo034737t.
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 degrees C, corresponding to a pair of conformers with different symmetry (anti C(i)() and anti C(2)).
1,4 - 双（均三甲酰基）杜烯（1）和1,4 - 双（均三甲基乙烯基）杜烯（2）的低温核磁共振谱表明，在平衡状态下存在顺式和反式旋转异构体，它们的相对比例取决于溶剂的介电常数。在溶液中，1的更稳定旋转异构体是反式，而对于2，更稳定的是顺式。根据所用的结晶溶剂，在1的固态中通过X射线衍射观察到了更稳定的（反式）和较不稳定的（顺式）旋转异构体。另一方面，在2的固态中仅发现了较不稳定的旋转异构体（反式）。如分子力学计算所示，顺式到反式的相互转化通过涉及均三甲苯基 - C和杜烯 - C键旋转的相关过程（齿轮途径）发生：动态核磁共振技术得出1的实验能垒为8.2 kcal mol⁻¹，2的为13.1 kcal mol⁻¹。对于衍生物2，由于第二种相关旋转过程（扭转）还确定了第二个能垒（8.6 kcal mol⁻¹）。由于这种第二种扭转运动的限制，2的反式旋转异构体在 - 133℃时显示出两个可区分的核磁共振谱，对应于一对具有不同对称性的构象体（反式Cᵢ和反式C₂）。
