Supramolecular catalysis of esterification by hemicucurbiturils under mild conditions
作者:Hang Cong、Takehiko Yamato、Xing Feng、Zhu Tao
DOI:10.1016/j.molcata.2012.09.002
日期:2012.12
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 k(0.5)= 0.18 h(-1), k(1.0) = 0.36 h(-1) and k(2.0) = 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 Hemi[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. (C) 2012 Elsevier B.V. All rights reserved.