A MCM-41-supported platinum carbonyl cluster-derived asymmetric hydrogenation catalyst
Introduction
Anchoring of homogeneous catalysts to insoluble polymeric supports has been much investigated in the search for a simple and easy method for the separation of catalyst [1], [2], [3]. Grafting of proven asymmetric homogeneous catalysts to a solid support is a viable strategy but in most cases requires multistep syntheses of expensive chiral ligands and/or functionalization of a given support with such ligands [3], [4], [5]. Another approach that has been the focus of much research avoids the costly synthesis of expensive ligands and/or organometallic complexes. In this approach a conventional heterogeneous catalyst such as platinum or raney nickel is modified by treatment with easily available chiral substances [6], [7], [8], [9], [10]. Although the overall success of this approach has been limited, platinum on alumina modified by cinchona alkaloids has been found to be a particularly effective catalyst for the enantioselective hydrogenation of α-ketoesters in general and pyruvate esters in particular (Orito reaction) [6], [7], [8], [9], [10].
The potential of Chini clusters as homogeneous catalysts has been investigated by us, and as precursors to supported catalysts by us and by others [11], [12], [13], [14], [15], [16]. Very recently we reported hydrogenation of ketones where the Chini cluster [Pt12(CO)24]2−, ion paired on functionalized fumed silica, was used as the precatalyst [12]. The superior performance of this catalyst prompted us to explore the potential of analogous but chiral materials in the asymmetrical hydrogenation of ketones. We find that an enantioselective catalyst can indeed be obtained with this general method of catalyst synthesis.
Moreover, in view of the recent observations of enhancement of enantioselectivity via constraining chiral asymmetric catalysts within the nanopores of a support, we also investigated the effect of porosity, if any, on the enantioselectivity of our catalysts [17], [18]. Our findings establish that the pore size distribution of the support has a critical and remarkable effect. Thus, with fumed silica as the support (see 1 in Scheme 1), no enantioselectivity could be achieved in the hydrogenation of methyl pyruvate or acetophenone. However, with 2, a precatalyst with MCM-41 as the support, excellent () enantiomeric excess is obtained in the Orito reaction, and moderate () enantioselectivity is achieved for acetophenone.
Section snippets
Experimental
All preparations and manipulations were performed with standard Schlenk techniques under an atmosphere of nitrogen. Solvents were dried by standard procedures (toluene over Na/benzophenone; methanol over Mg-turnings/iodine), distilled under nitrogen, and used immediately. Chloroplatinic acid was purchased from Johnson Mathey (London). Methyl pyruvate, methyl lactate, cinchonidine, and colloidal silica were purchased from Fluka (Switzerland). Fumed silica, (3-chloropropyl)trimethoxysilane,
Results and discussion
The functionalization of MCM-41 with trialkoxy chloropropylsilane followed by further reactions with amines including ephedrine has been reported [22]. The latter material was used as a potential chiral catalyst in the alkylation of benzaldehyde by diethyl zinc but was found to give only moderate enantioselectivity. The proposed structural formulations of the resultant precatalysts (1 and 2) are shown in Scheme 1.
Chemical and EDAX analyses for both 1 and 2 did not show the presence of sodium,
Conclusion
In conclusion, we have shown that by ion pairing [Pt12(CO)24]2− with cinchonidium groups chemically linked to the surface of MCM-41, we obtained an asymmetric hydrogenation catalyst. An analogous material with fumed silica as the support does not give any enantioselectivity, indicating that the pore size distribution of MCM-41 is of critical importance. Research aimed at the enhancement of enantioselectivity under high conversion, through the optimization of degree of functionalization and pore
Acknowledgment
Financial assistance from Reliance Industries Limited, Mumbai, India, for this work is gratefully acknowledged.
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Effective and durable Pt nanocatalyst supported on three-dimensionally ordered macroporous carbon for asymmetric hydrogenation
2017, Catalysis TodayCitation Excerpt :Nevertheless, Pt/Al2O3 catalysts always exhibited limited reusability owing to the inferior stability of alumina in acetic acid, one of the best solvents for this asymmetric hydrogenation catalytic system [12,13]. Besides alumina [14–20], a variety of other solid materials such as silica [21–25], carbon materials [12,26–30], and polymers [31], have been developed as supports for Pt nanoparticles. Improved stability was often achieved compared with Pt/Al2O3, despite the relatively low enantioselectivities except in the case of carbon nanotube as support [30].
High enantioselectivity in the asymmetric hydrogenation of ketones by a supported Pt nanocatalyst on a mesoporous modified MCM-41 support
2015, Cuihua Xuebao/Chinese Journal of CatalysisElectrocatalytic oxidation of methanol on Pt catalyst supported on nitrogen-doped graphene induced by hydrazine reduction
2013, Journal of Physics and Chemistry of SolidsSelective hydrogenation of chloronitrobenzenes with an MCM-41 supported platinum allyl complex derived catalyst
2011, Applied Catalysis A: GeneralCitation Excerpt :Our interest in supported and tethered metal complexes as precatalysts arises from the fact that they have the potential to generate small metal particles of unusual selectivity under the catalytic conditions. We have used tethered and supported metal complexes as precursors of nanoparticles and showed that organometallic derived MCM-41 supported Ru- and Pt-nanocatalysts exhibit high chemo- and enantioselectivity in hydrogenation reactions [48–50]. Of special relevance in the present context, is our earlier observation that a palladium allyl complex tethered onto diamine functionalized MCM-41 is a selective precatalyst for the hydrogenations of o- and m-CNB [51].