Intramolecular carbonyl-ene reaction of citronellal to isopulegol over ZnBr 2-loading mesoporous silica catalysts

Abstract

We report the preparation of ZnBr 2-loading mesoporous catalysts and their application to the intramolecular carbonyl-ene reaction of (+)-citronellal to (−)-isopulegol. The mesoporous catalysts were characterized by N 2 adsorption–desorption isotherms, X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and inductively coupled plasma atomic emission spectrometry (ICP-AES). Among a series of mesoporous supports, C8-HMS, which was synthesized from Si(OEt) 4 via a sol–gel process in the presence of octylamine as a templating agent, was found to be an optimal support to load ZnBr 2 without damage of inherent mesoporosity of the support. The ZnBr 2 loaded on C8-HMS (ZnBr 2/C8-HMS) showed higher catalytic activity and diastereoselectivity than ZnBr 2 on other HMS with different particle and pore sizes, MCM-41, mesoporous alumina, and Al-HMS. The amount of zinc ions eluted into the solution phase from ZnBr 2/C8-HMS was only at a less than 1 ppm level. The results suggested that clumped ZnBr 2 was no longer formed on the mesoporous silica surface. The ZnBr 2/C8-HMS catalyst which had been washed well with excess EtOH still contained zinc species, but had neither catalytic activity nor bromide ions. It was concluded that at least two different kinds of Zn sites existed on the surface of ZnBr 2/C8-HMS; one site was the crystallite ZnBr 2 which was finely dispersed on the silica surface, and the other was oxygenated zinc species such as Zn(OSi ) 2 and/or Zn(OH)(OSi ).