RESUMEN
A versatile Lewis acid catalyzed tandem cyclization of in situ generated alkynyl o-quinone methides ( o-AQMs) with electron-rich phenols has been developed on the basis of the mode involving an intermolecular 1,4-conjugate addition/6-endo cyclization/1,3-aryl shift/intramolecular 1,4-conjugate addition cascade. This reaction provides a new method for expeditious assembly of synthetically and biologically interesting tetracyclic bridged dioxabicyclo[3.3.1]nonane skeletons featuring a congested bridgehead oxa-quaternary stereocenter.
RESUMEN
A heteropoly acid based organic hybrid heterogeneous catalyst, HMQ-STW, was prepared by combining 8-hydroxy-2-methylquinoline (HMQ) with Keggin-structured H4SiW12O40 (STW). The catalyst was characterized via elemental analysis, X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), thermogravimetric analysis (TG) and potentiometric titration analysis. The catalytic performance of the catalyst was assessed in the ketalization of ketones with glycol or 1,2-propylene glycol. Various reaction parameters, such as the glycol to cyclohexanone molar ratio, catalyst dosage, reaction temperature and time, were systematically examined. HMQ-STW exhibited a relatively high yield of corresponding ketal, with 100% selectivity under the optimized reaction conditions. Moreover, catalytic recycling tests demonstrated that the heterogeneous catalyst exhibited high potential for reusability, and it was revealed that the organic modifier HMQ plays an important role in the formation of a heterogeneous system and the improvement of structural stability. These results indicated that the HMQ-STW catalyst is a promising new type of heterogeneous acid catalyst for the ketalization of ketones.
RESUMEN
Two polyoxometallate-based compounds, tris[1,1'-(butane-1,4-diyl)bis(1H-imidazol-3-ium)] bis[tetracosa-µ2-oxido-dodecaoxido-µ12-phosphato-dodecamolybdenum(VI)], (C10H16N4)3[PMo12O40]2, (I), and 1,1'-(butane-1,4-diyl)bis(1H-imidazol-3-ium) 1-[4-(1H-imidazol-1-yl)butyl]-1H-imidazol-3-ium tetracosa-µ2-oxido-dodecaoxido-µ12-phosphato-dodecamolybdenum(VI) dihydrate, (C10H16N4)(C10H15N4)[PMo12O40]·2H2O, (II), were synthesized by hydrothermal techniques at different pH values. The stoichiometric ratio between the polyoxometallate (POM) anions and organic cations is 2:3 in (I), with one of the cations lying on an inversion centre. The doubly protonated 1,1'-(butane-1,4-diyl)diimidazole (BIM) cations are linked to the [PMo12O40](3-) anions by hydrogen bonds to form a three-dimensional supramolecular network. The stoichiometric ratio of POM anions and organic cations is 1:2 in (II), and the anion is located about a centre of inversion. The partly protonated BIM cations and solvent water molecules form hydrogen bonds with the [PMo12O40](3-) anions, yielding a two-dimensional supramolecular layer. The different lattice architectures of (I) and (II) may be governed by the ratio between the POM anions and organic cations, which, in turn, is determined by the pH value.
