Dynamics and Entropy of Cyclohexane Rings Control pH-Responsive Reactivity.
Dynamics and Entropy of Cyclohexane Rings Control pH-Responsive Reactivity.
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作者：
Kang S;Noh C;Kang H;Shin JY;Kim SY;Kim S;Son MG;Park E;Song HK;Shin S;Lee S;Kim NK;Jung Y;Lee Y
活化熵（ΔS）通常不被认为是决定单分子反应活性的主要因素。本文报道了六元环化合物的分子内降解主要由ΔS ε决定，而Δ S ε受环翻转运动和取代基几何构型的强烈影响.从1，2-环己烷羧酸酰胺（1，2-CHCAA）几何异构体的pH依赖性降解动力学之间的独特差异（其中只有顺式异构体在弱酸性条件（pH < 5.5）下容易降解）出发，我们发现这种差异源于ΔS ε的巨大差异（16.02 cal·mol-1·K-1）。虽然顺式-1，2-CHCAA保持了经典的椅式环己烷构象的偏好，反式-1，2-CHCAA的椅子和扭曲的船构象之间的动态相互转换，这是支持的MD模拟和VT-NMR分析。反式异构体的大体积1，2-取代基之间的空间排斥是环构象之间能垒降低的主要原因之一，从而促进动态环翻转运动。因此，由于反应物的预置，更具动态的反式异构体在活化过程中表现出比顺式异构体更大的熵损失，并且反式异构体的pH依赖性降解被有效抑制。当环反转运动被抑制的环己烷环上的一个额外的甲基取代基，pH降解性可以显着提高，即使是反式异构体。本研究提供了一个独特的例子，其中空间排列和动态性质可以强烈影响单分子反应中的分子反应性，这将有助于未来设计的反应性结构取决于动态构象变化。
Activation entropy (ΔS‡) is not normally considered the main factor in determining the reactivity of unimolecular reactions. Here, we report that the intramolecular degradation of six-membered ring compounds is mainly determined by the ΔS‡, which is strongly influenced by the ring-flipping motion and substituent geometry. Starting from the unique difference between the pH-dependent degradation kinetics of geometric isomers of 1,2-cyclohexanecarboxylic acid amide (1,2-CHCAA), where only the cis isomer can readily degrade under weakly acidic conditions (pH < 5.5), we found that the difference originated from the large difference in ΔS‡ of 16.02 cal·mol–1·K–1. While cis-1,2-CHCAA maintains a preference for the classical chair cyclohexane conformation, trans-1,2-CHCAA shows dynamic interconversion between the chair and twisted boat conformations, which was supported by both MD simulations and VT-NMR analysis. Steric repulsion between the bulky 1,2-substituents of the trans isomer is one of the main reasons for the reduced energy barrier between ring conformations that facilitates dynamic ring inversion motions. Consequently, the more dynamic trans isomer exhibits much a larger loss in entropy during the activation process due to the prepositioning of the reactant than the cis isomer, and the pH-dependent degradation of the trans isomer is effectively suppressed. When the ring inversion motion is inhibited by an additional methyl substituent on the cyclohexane ring, the pH degradability can be dramatically enhanced for even the trans isomer. This study shows a unique example in which spatial arrangement and dynamic properties can strongly influence molecular reactivity in unimolecular reactions, and it will be helpful for the future design of a reactive structure depending on dynamic conformational changes.
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影响因子：
18.3
作者：
Kohen A
通讯作者：
Kohen A
影响因子：
3.7
作者：
Budi, A.;Stipp, S. L. S.;Andersson, M. P.
通讯作者：
Andersson, M. P.
影响因子：
2.4
作者：
Dragojlovic, Veljko
通讯作者：
Dragojlovic, Veljko
DOI：
10.1039/p29720001206
发表时间：
1972-01-01
期刊：
JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2
影响因子：
--
作者：
KIRBY, AJ;LANCASTER, PW
通讯作者：
LANCASTER, PW
影响因子：
3.6
作者：
Gill, G;Pawar, DM;Noe, EA
通讯作者：
Noe, EA