Reduction of Prostaglandin Unsaturated Ketones to Secondary Allylic Alcohols by Hydrogen Transfer over Mesoporous-Supported PtSn Catalysts

Supported PtSn catalysts were prepared by successive impregnation of SiO2, MCM-41, or MCM-48 materials with Pt and Sn, followed by calcination and reduction. The oxidation and aggregation state of the Pt and Sn were studied using combined temperature-programmed reduction, X-ray photoelectron spectroscopy, X-ray diffraction, and transmission electron microscopy experiments. The characteristics of the bimetallic catalysts were related to the catalytic behavior in the hydrogen transfer reduction of the unsaturated ketone moiety in some intermediates of the prostaglandin synthesis; isopropanol was used as the reductant. High chemoselectivities to the allylic alcohol (over 95%) were obtained with catalysts containing 5 wt% Pt and 5–10 wt% Sn on MCM-41 or MCM-48. Characterization of these optimum catalysts evidenced that most of the Pt and part of the Sn form supported alloy particles with PtSn and PtSn2 composition; the rest of the Sn is present as dispersed Sn2+/4+. Activation of the carbonyl group on oxidized Sn, steric shielding of the CC bond by bulky substituents, and use of the hydrogen transfer donor seem crucial for obtaining high allylic alcohol chemoselectivities. In some cases, the mesoporous texture of the support induces significant diastereoselectivity, with one isomer of the secondary allylic alcohol prevailing over its epimer.