Enhanced Activity in the Tosylation of Tolanophanes via Supramolecular HgCl2 Recognition

Abstract

The ‘self-activation’ of host molecules via the incorporation of a guest has received considerable attention in supramolecular catalysis. Here, we demonstrate how HgCl2 effects the tosylation rate of tolanophanes (1a–c: n=2−4) with different alkyl chain lengths. Among these substrates, 1a has the highest strain in sp carbons and, therefore, is active even without assistance of HgCl2. In contrast, 1c is inert and needs to be activated in the presence of HgCl2. Therefore, the averaged reaction rate is in the following order: 1c>1b>1a, confirming the role of the supramolecular cavity of 1 over the strain of alkyne bonds. Ab initio calculations are consistent with the experimentally derived reactivity, supporting our size-fitting hypothesis. In contrast, acyclic analogues showed lower activity in the presence of HgCl2 to confirm the effect of the ring cavity. To gain more information, the HgCl2 complexation of 1b was examined by using 1H NMR and UV-vis spectroscopies. All products 5 are new and well characterized. The hydration of isomeric mixtures of 5b,c gave the corresponding single products 4b,c.