Supramolecular catalysis of esterification by hemicucurbiturils under mild conditions

Hemicucurbit[6]uril (HemiQ[6])-induced esterification of acids with CH3OH was investigated. Esterification of the model substrate MA in the presence of different amounts of HemiQ[6] had reaction rate constants of k0.5 = 0.18 h−1, k1.0 = 0.36 h−1 and k2.0 = 0.52 h−1. These results confirm that the reaction rate increases with the ratio of catalyst to substrate. Ineffective catalysis of MA esterification with a stoichiometric amount of MeOH suggests that the mechanism for HemiQ[6]-catalyzed esterification is solvolysis. Comparing the HemiQ[6]-catalytic kinetics of MA (4-methoxy-4-oxobut-2-enoic acid) with AA (acrylic acid) and BA (benzoic acid) shows that the catalytic activities should bear relation to the size of substrates. The different conversion of sorts of substrates in the presence of HemiQ[6] reveals that the supramolecular catalysis is favor in the conjugated structures. The inefficacy of HemiQ[12] demonstrates that the catalytic capability depends on the structure of the macrocyclic compound used.

Graphical abstractThe supramolecular catalytic capability of hemicucurbit[n]uril (n = 6 or 12) for esterification of a series of acids with CH3OH was investigated.Figure optionsDownload full-size imageDownload high-quality image (129 K)Download as PowerPoint slideHighlights► The macrocyclic compound, hemicucurbit[6]uril, has been investigated as a kind of catalyst in esterification. ► The selective esterification under the supramolecular catalysis has been developed. ► The esterification has been preformed directly without any catalysis of acid.