Bambusurils as a mechanistic tool for probing anion effects

Bambusuril macrocycles have high affinity towards anions (Xˉ) such as PF6ˉ, SbF6ˉ or BF4ˉ, ClO4ˉ. Therefore, addition of bambusurils to the reaction mixtures containing these anions effectively removes the anions from the reaction process. Hence, comparing reactions with and without addition of bambusurils can demonstrate, whether the anions actively participate in reaction mechanism or not. We show this approach for gold(I) mediated addition of methanol to an alkyne. The reaction mechanism can proceed via monoaurated intermediates (e.g., in catalysis with [(IPr)AuX]) or via diaurated intermediates (e.g., in catalysis with [(PPh3)AuX]). We show that anions Xˉ slightly affect the reaction rates, however the effect stays almost the same even after their encapsulation to the cavity of bambusurils. We also demonstrate that Xˉ affects the overall reaction rate in the very same way as the reaction rate of the protodeauration step. All results are consistent with the indirect effect of Xˉ by the acidity of the conjugated acid HX on the rate-determining step. There is no evidence that a direct involvement of Xˉ would affect the reaction rate.