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The meso Helix: Symmetry and Symmetry‐Breaking in Dynamic Oligourea Foldamers with Reversible Hydrogen‐Bond Polarity
Author(s) -
Wechsel Romina,
Raftery James,
Cavagnat Dominique,
Guichard Gilles,
Clayden Jonathan
Publication year - 2016
Publication title -
angewandte chemie international edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 1433-7851
DOI - 10.1002/anie.201604496
Subject(s) - hydrogen bond , chemistry , conformational isomerism , cyclohexane , monomer , crystallography , enantiomer , stereochemistry , molecule , polymer , organic chemistry
Oligoureas (up to n= 6) of meso cyclohexane‐1,2‐diamine were synthesized by chain extension with an enzymatically desymmetrized monomer 2 . Despite being achiral, the meso oligomers adopt chiral canonical 2.5‐helical conformations, the equally populated enantiomeric screw‐sense conformers of which are in slow exchange on the NMR timescale, with a barrier to screw‐sense inversion of about 70 kJ mol −1 . Screw‐sense inversion in these helical foldamers is coupled with cyclohexane ring‐flipping, and results in a reversal of the directionality of the hydrogen bonding in the helix. The termini of the meso oligomers are enantiotopic, and desymmetrized analogues of the oligoureas with differentially and enantioselectively protected termini display moderate screw‐sense preferences. A screw‐sense preference may furthermore be induced in the achiral, meso oligoureas by formation of a 1:1 hydrogen‐bonded complex with the carboxylate anion of Boc‐ d ‐proline. The meso oligoureas are the first examples of hydrogen‐bonded foldamers with reversible hydrogen‐bond directionality.