Hemicucurbit[6]uril-induced aerobic oxidation of heterocyclic compounds

The aerobic oxidation of furan in aqueous solution in the presence of HemiQ[16] has been investigated, and the product furan-2,5-diol is stabilized by encapsulation of HemiQ[16], which could be transformed to the dione confirmation in acidic solution and escape from the macrocyclic compound. The H-1 NMR titration experiments of the host-guest interaction at different pH values suggest protonation should improve the encapsulation, and therefore an unique property that HemiQI6] can be protonated has been revealed. The oxidizing kinetics suggests that the procedure is a consecutive reaction with a series of constants k(1) =2.9 x 10(-2) min(-1) , k(2) = 2.7 x 10(-2) min(-1), and k(3) = 5.7 x 10(-3) min(-1), respectively. The kinetic investigation at pD =2.0 indicates the HemiQ[6]-catalytic oxidation of furan could be accelerated by acidification. As a consequence, a plausible mechanism has been established on the above evidences. 2-Methylfuran is employed to give the product 2-methylfuran-5-ol exhibiting a satisfied activity in this aerobic oxidation with the supramolecular catalysis of HemiQ[6], but the oxidation of thiophene is very slow in either neutral or acidic condition. (C) 2013 Elsevier B.V. All rights reserved.