Conformational Studies by Dynamic NMR. 57.(1) Stereodynamics of Syn-Anti Interconversion of Disubstituted Acyl Durenes.
Conformational Studies by Dynamic NMR. 57.(1) Stereodynamics of Syn-Anti Interconversion of Disubstituted Acyl Durenes.
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通过动态核磁共振进行构象研究。
DOI：
10.1021/jo9604503
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发表时间：
1996
期刊：
影响因子：
--
通讯作者：
L. Lunazzi
中科院分区：
文献类型：
--
作者：
D. Casarini;L. Lunazzi
用制备薄层色谱法分离了1，4-二（2，2-二甲基丙酰基）均四甲苯（2 e）中由Ar-C（O）Bu（t）单键限制性旋转产生的顺反异构体。在溶液中，它们达到平衡，其中顺-反比例取决于溶剂的极性。这使我们能够分配的反结构，它有一个零的偶极矩，保留最少的异构体。发现相互转化的活化自由能（DeltaG）为22.5 kcal mol（-）（1），该值足以将这些物质鉴定为构型异构体。当较少受阻的衍生物，也有两个RCO（R = Pr（i），Et，Me）取代基的1，4位的杜烯部分，进行了检查，顺式和反式只能在低温下通过NMR光谱检测。相应的互变势垒（Δ G分别为13.4、11.7、10.9千卡mol（-）（1））实际上比R = Bu（t）低得多，表明在这些情况下我们处理的是构象异构体而不是构型异构体。
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.