5 years ago

Substituent Effects in Parallel-Displaced π–π Stacking Interactions: Distance Matters

Substituent Effects in Parallel-Displaced π–π Stacking Interactions: Distance Matters
François Diederich, Leslie-Joana Riwar, Nils Trapp, Bernd Kuhn
Host–guest systems with Rebek imide type receptors and a 2,6-di(isobutyramido)pyridine ligand were employed to investigate substituent effects in parallel-displaced π–π stacking interactions. Changing the intermolecular distance between the para substituent on the aromatic platform of the receptor and the pyridine ring of the guest results in a strongly different substituent effect. With a short ethyne-1,2-diyl spacer between the Rebek imide and the aromatic platform, partial overlap of substituent and guest stabilizes the π–π stacking interactions independent of the electronic nature of the substituent (Wheeler–Houk model). When the substituent is shifted further away by using a buta-1,3-diyne-1,4-diyl spacer, direct, through-space interactions between substituent and guest are prevented. A linear correlation between logKa (Ka=association constant) and the Hammett substituent constant σpara is observed, confirming predictions by the Hunter–Sanders model experimentally. Distance matters: Substituents have a different effect on π–π stacking interactions depending on their position. Stacking interactions in Rebek imide model systems with partial substituent overlap follow the Wheeler–Houk model whereas a larger displacement supports the Hunter–Sanders model. Two controversial theoretical predictions were confirmed experimentally.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/anie.201703744

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