Morris, David TJ, Wales, Steven M, Echavarren, Javier, Zabka, Matej, Marsico, Giulia, Ward, John W 
ORCID: 0000-0001-7186-6416, Pridmore, Natalie E and Clayden, Jonathan
(2023)
Dynamic and Persistent Cyclochirality in Hydrogen-Bonded Derivatives of Medium-Ring Triamines.
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 145 (34).
pp. 19030-19041.
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Abstract
Cyclic triureas derived from 1,4,7-triazacyclononane (TACN) were synthesized; X-ray crystallography showed a chiral bowl-like conformation with each urea hydrogen-bonded to its neighbor with uniform directionality, forming a "cyclochiral" closed loop of hydrogen bonds. Variable-temperature <sup>1</sup>H NMR, <sup>1</sup>H-<sup>1</sup>H exchange spectroscopy, Eyring analysis, computational modeling, and studies in various solvents revealed that cyclochirality is dynamic (Δ<i>G</i><sup>‡</sup><sub>25°C</sub> = 63-71 kJ mol<sup>-1</sup> in noncoordinating solvents), exchanging between enantiomers by two mechanisms: bowl inversion and directionality reversal, with the former subject to a slightly smaller enantiomerization barrier. The enantiomerization rate substantially increased in the presence of hydrogen-bonding solvents. Population of only one of the two cyclochiral hydrogen-bond directionalities could be induced by annulating one ethylene bridge with a <i>trans</i>-cyclohexane. Alternatively, enantiomerization could be inhibited by annulating one ethylene bridge with a <i>cis</i>-cyclohexane (preventing bowl inversion) and replacing one urea function with a formamide (preventing directionality reversal). Combining these structural modifications resulted in an enantiomerization barrier of Δ<i>G</i><sup>‡</sup><sub>25°C</sub> = 93 kJ mol<sup>-1</sup>, furnishing a planar-chiral, atropisomeric bowl-shaped structure whose stereochemical stability arises solely from its hydrogen-bonding network.
| Item Type: | Article |
|---|---|
| Divisions: | Faculty of Science and Engineering > School of Physical Sciences |
| Depositing User: | Symplectic Admin |
| Date Deposited: | 29 Sep 2023 15:30 |
| Last Modified: | 29 Sep 2023 15:30 |
| DOI: | 10.1021/jacs.3c06570 |
| Open Access URL: | https://pubs.acs.org/doi/10.1021/jacs.3c06570 |
| Related URLs: | |
| URI: | https://livrepository.liverpool.ac.uk/id/eprint/3173234 |
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