Pillararenes constitute a class of macrocycles which display planar chirality on account of the methylene bridges that link five disubstituted para-phenylene rings together. Dynamic 1H NMR spectroscopy indicates that A1/A2-dihydroxypillararene undergoes conformational inversion between its enantiomers with an energy barrier of 11.9 kcal mol- 1. This process involving an oxygen-through-the-annulus rotation by all five hydroquinone rings is associated with the breaking of two intramolecular hydrogen bonds between phenolic hydroxyl and methoxyl groups on neighbouring phenylene rings. A combination of molecular mechanics and quantum mechanical calculations reveals that the conformational inversion undergone by A1/A2-dihyroxypillararene involves the breaking of one of these hydrogen bonds in the rate-limiting step of the process. Not only does the calculated energy of activation (13.8 kcal mol- 1) using density functional theory agree well with the experimentally determined value (13.0 kcal mol - 1), it also leads to the identification of the lowest energy pseudorotational pathway involving four intermediates and five transition states. While replacing the two hydroxyl groups in A1/A2-dihydroxypillararene with carbonyl groups leads to much more rapid conformational inversion, placing bromine atoms ortho to the two phenolic hydroxyl groups increases the strength of the intramolecular hydrogen bonds, raising the energy barrier to inversion by 3.9 kcal mol- 1.
|Original language||English (US)|
|Number of pages||13|
|State||Published - Sep 18 2013|
- dynamic NMR spectroscopy
- molecular symmetry
- stereochemical inversion
- topic relationships
ASJC Scopus subject areas
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CCDC 941134: Experimental Crystal Structure Determination
Strutt, N. L. (Creator), Schneebeli, S. T. (Creator) & Stoddart, J. F. (Creator), Cambridge Crystallographic Data Centre, 2013
DOI: 10.5517/cc10lb53, http://www.ccdc.cam.ac.uk/services/structure_request?id=doi:10.5517/cc10lb53&sid=DataCite