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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. Dynamics and Entropy of Cyclohexane Rings Control pH-Responsive Reactivity. JACS AU 2021; 1:2070-2079. [PMID: 34841418 PMCID: PMC8611792 DOI: 10.1021/jacsau.1c00354] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/13/2021] [Indexed: 05/31/2023]
Abstract
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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Affiliation(s)
- Sunyoung Kang
- Department
of Chemistry, Seoul National University, Seoul 08826, Republic of Korea
| | - Chanwoo Noh
- Department
of Chemistry, Seoul National University, Seoul 08826, Republic of Korea
| | - Hyosik Kang
- Department
of Chemistry, Gachon University, Seongnam, Gyunggido 13120, Republic of Korea
| | - Ji-Yeon Shin
- Advanced
Analysis Center, Korea Institute of Science
and Technology, Seoul 02792, Republic of Korea
- Department
of Life Sciences, Korea University, Seoul 02841, Republic of Korea
| | - So-Young Kim
- Advanced
Analysis Center, Korea Institute of Science
and Technology, Seoul 02792, Republic of Korea
| | - Seulah Kim
- Department
of Chemistry, Seoul National University, Seoul 08826, Republic of Korea
| | - Moon-Gi Son
- Department
of Chemistry, Seoul National University, Seoul 08826, Republic of Korea
| | - Eunseok Park
- Bruker
Biospin Korea, Seongnam, Gyunggido 13493, Republic of Korea
| | - Hyun Kyu Song
- Department
of Life Sciences, Korea University, Seoul 02841, Republic of Korea
| | - Seokmin Shin
- Department
of Chemistry, Seoul National University, Seoul 08826, Republic of Korea
| | - Sanghun Lee
- Department
of Chemistry, Gachon University, Seongnam, Gyunggido 13120, Republic of Korea
| | - Nak-Kyoon Kim
- Advanced
Analysis Center, Korea Institute of Science
and Technology, Seoul 02792, Republic of Korea
| | - YounJoon Jung
- Department
of Chemistry, Seoul National University, Seoul 08826, Republic of Korea
| | - Yan Lee
- Department
of Chemistry, Seoul National University, Seoul 08826, Republic of Korea
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Francisco CB, Fernandes CS, de Melo UZ, Rittner R, Gauze GF, Basso EA. Strong hyperconjugative interactions limit solvent and substituent influence on conformational equilibrium: the case of cis-2-halocyclohexylamines. Beilstein J Org Chem 2019; 15:818-829. [PMID: 31019574 PMCID: PMC6466766 DOI: 10.3762/bjoc.15.79] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/21/2019] [Accepted: 03/20/2019] [Indexed: 01/17/2023] Open
Abstract
The presence of strong stereoelectronic interactions involving the substituents in cis-2-substituted cyclohexanes may lead to results different from those expected. In this work, we studied the conformational behavior of cis-2-fluoro- (F), cis-2-chloro- (Cl), cis-2-bromo- (Br) and cis-2-iodocyclohexylamine (I) by dynamic NMR and theoretical calculations. The experimental data pointed to an equilibrium strongly shifted toward the ea conformer (equatorial amine group and axial halogen), with populations greater than 90% for F, Cl and Br in both dichloromethane-d2 and methanol-d4. Theoretical calculations (M06-2X/6-311++G(2df,2p)) were in agreement with the experimental, with no influence of the solvent or the halogen on the equilibrium. A principal component analysis of natural bond orbital energies pointed to the σ*C–X and σC–H orbitals and the halogen lone pairs (LPX) as the most significant for the hyperconjugative interactions that influenced the equilibrium. The σC–H → σ*C–X hyperconjugation and the interactions involving the LPX counterbalance each other, explaining the non-influence of the halogen on the conformational equilibrium. These interactions are responsible for the strong preference for the ea conformer in cis-2-halocyclohexylamines, being strong enough to restrain the shift in the equilibrium due to other factors such as steric repulsion or solvent effects.
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Affiliation(s)
- Camila B Francisco
- Chemistry Department, State University of Maringá, 5790, Maringá 87020-900, Brazil
| | | | - Ulisses Z de Melo
- Chemistry Department, State University of Maringá, 5790, Maringá 87020-900, Brazil
| | - Roberto Rittner
- Chemistry Institute, University of Campinas, 6154, Campinas 13083-970, Brazil
| | - Gisele F Gauze
- Chemistry Department, State University of Maringá, 5790, Maringá 87020-900, Brazil
| | - Ernani A Basso
- Chemistry Department, State University of Maringá, 5790, Maringá 87020-900, Brazil
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