Effect of Acid Modification of Montmorillonite Clays on Synthesis of Octahydro-2H-chromen-4-ol from Vanillin and Isopulegol
Доклады на конференциях
Reaction of homoallylic alcohols with aldehydes, variant of the Prins cyclization reaction, is one of the ways for the synthesis of a wide range of tetrahydropyran derivatives. Recently, acid-activated montmorillonites due to their unique structural and physicochemical properties have received much attention for synthesis of biologically active 2H-chromen-4-ols. The aim of this study was to investigate the effect of the HCl concentration used for the acid-activation on the surface acidity and the porous structure of the montmorillonite clays, and the subsequent influence on the catalytic activity of these solids in the Prins cyclization reaction of (-)-isopulegol (I) with vanillin (II) to octahydro-2H-chromen-4-ol (isomers IIIa and IIIb).
According to experimental data the concentration of HCl used for the acid activation of the clays strongly affects the conversion of (II) and the yield of isomers (IIIa-b) due to influence on the amount of Brønsted acid sites (BAS). It was found that the amount of BAS in acid-activated montmorillonites is a key factor for the adjustment of the reaction rate and the selectivity of the reaction. An increase of the amount of BAS leads to a decrease in the ratio of (III)/(IV) products. The nature of the clay affects the chemo- and stereo-selectivity, i.e. molar ratios of (III)/(IV) and (IIIa)/(IIIb). Reaction rate and isomer selectivity in the presence of clay with large pore diameter and low microporosity (clay 1) were lower in comparison to clay with low pore diameter and large microporosity (clay 2). Noteworthy, in the presence of clay 2 and clay 1 the (IIIa)/(IIIb) molar ratios were 5.3-5.7 and 4.0-4.5, respectively, that can be also related to the differences in the structural parameters