MCM-41-supported ruthenium carbonyl cluster-derived catalysts for asymmetric hydrogenation reactions

Abstract

The anionic ruthenium carbonyl cluster [Ru₄(μ-H)₃(CO)₁₂]^- has been ion-paired with (3-chloropropyl)trimethoxysilyl-cinchonidium or sparteinium groups chemically bound to the surfaces of MCM-41 [(MCM-41-)(-O)₃SiCH₂CH₂CH₂NR₃⁺Cl^-, NR₃ = cinchonidine or sparteine]. The resultant materials have been characterized before and after use as hydrogenation catalysts by IR, XPS, and TEM. The molecular identity of the cluster is retained in the fresh catalysts, but under hydrogen pressure (≥ 30 bar) the carbonyl groups are lost. Ruthenium (0) and a small amount of Ru²⁺ can be observed on the fresh catalyst by XPS, while the used catalyst shows only Ru(0). TEM micrographs show retention of the MCM structure and no observable aggregation of metal. Under optimal conditions, good (≤75%) and observable (≤30%) enantioselectivities are obtained for the hydrogenations of methyl pyruvate and acetophenone, respectively. The enantioselectivity of the catalyst towards methylpyruvate hydrogenation is retained even with relatively high turnovers. This behavior is in sharp contrast to that of the Chini cluster [Pt₁₂(CO)₂₄]^2-, derived analogous catalyst reported earlier by us.