Intramolecular carbonyl-ene reaction of citronellal to isopulegol over ZnBr2-loading mesoporous silica catalysts

We report the preparation of ZnBr2-loading mesoporous catalysts and their application to the intramolecular carbonyl-ene reaction of (+)-citronellal to (−)-isopulegol. The mesoporous catalysts were characterized by N2 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 ZnBr2 without damage of inherent mesoporosity of the support. The ZnBr2 loaded on C8-HMS (ZnBr2/C8-HMS) showed higher catalytic activity and diastereoselectivity than ZnBr2 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 ZnBr2/C8-HMS was only at a less than 1 ppm level. The results suggested that clumped ZnBr2 was no longer formed on the mesoporous silica surface. The ZnBr2/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 ZnBr2/C8-HMS; one site was the crystallite ZnBr2 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).

The preparation of ZnBr2-loading mesoporous catalysts and their application to the intramolecular carbonyl-ene reaction of (+)-citronellal to (−)-isopulegol were studied. 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 ZnBr2 without damage of inherent mesoporosity of the support. The ZnBr2 loaded on C8-HMS showed higher catalytic activity and diastereoselectivity than ZnBr2 on other supports, namely HMS, MCM-41, mesoporous alumina, and Al-HMS.Figure optionsDownload as PowerPoint slide