Casarini Daniele, Lunazzi Lodovico
Department of Organic Chemistry "A. Mangini", University of Bologna, Risorgimento 4 Bologna 40136, Italy.
J Org Chem. 1996 Sep 6;61(18):6240-6243. doi: 10.1021/jo9604503.
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 (DeltaG) 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 (DeltaG = 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.
通过制备薄层色谱法分离了1,4 - 双（2,2 - 二甲基丙酰基）杜烯（2e）中围绕Ar - C(O)Bu(t)单键的受限旋转产生的正交顺式和反式异构体。在溶液中，它们达到平衡，其中顺式 - 反式比例取决于溶剂的极性。这使我们能够将偶极矩为零的反式结构归属于保留最少的异构体。发现相互转化的活化自由能（ΔG）为22.5 kcal mol⁻¹，该值高到足以将这些物种鉴定为构型异构体。当研究在杜烯部分的1,4位也具有两个RCO（R = Prⁱ、Et、Me）取代基的位阻较小的衍生物时，顺式和反式形式只能在低温下通过核磁共振光谱检测到。实际上，相应的相互转化能垒（分别为ΔG = 13.4、11.7、10.9 kcal mol⁻¹）比R = Bu(t)时低得多，表明在这些情况下我们处理的是构象异构体而非构型异构体。
