Solid support catalysts heterogeneous hydrogenation

Different immobilization methods were employed, and the new heterogenized catalysts were applied in asymmetric hydrogenation and epoxidation of olefins, as well as in ring-opening reactions of epoxides. In some cases, the typical catalytic properties of homogeneous catalysts could be transferred to the heterogeneous systems. The immobilization of complexes on solid supports facilitated recovery and recycling of the catalysts. 

These reactions are unlike any we have encountered so far. They are heterogeneous reactions, which means that the reacting system consists of two or more phases. Usually, the metal catalyst is present as a finely divided solid suspension in the liquid or solution to be reduced. Alternatively, the metal is deposited on an inert solid support such as carbon, barium sulfate, alumina (A1203), or calcium carbonate. Then the mixture of the liquid substrate and solid catalyst is shaken or stirred in a hydrogen atmosphere. However, the actual reaction takes place at the surface of the metal catalyst and is an example of heterogeneous or surface catalysis. 

An example of catalysts which are themselves heterogeneous are the poly-amino acids used for the asymmetric Julia-Colonna-type epoxidation of chalcones using alkaline hydrogen peroxide (Section 10.2) [8]. Because of the highly efficient synthesis of epoxides, this process also has attracted industrial interest (Section 14.3). Since recent work by the Berkessel group revealed that as few as five L-Leu residues are sufficient for epoxidation of chalcone, several solid-phase-bound short-chain peptides have been used, leading to enantioselectivity up to 98% ee [14], For example, (L-Leu)5 immobilized on TentaGel S NH2 , 8, was found to be a suitable solid-supported short-chain peptide catalyst for epoxidations. 

The immobilization of catalysts or catalyst precursors on solid supports is a common technique for simplifying reaction procedures and/or increasing the stability of the catalyst. The homogeneous MTO catalyst can be transformed into a heterogeneous system in a number of different ways. In a recent approach by Saladino and coworkers, poly(4-vinylpyridine) and poly(4-vinylpyridine) //-oxides were used as the catalyst carrier. The MTO-catalyst obtained from 25% cross-linked poly(4-vinylpyridine) with divinylbenzene proved to catalyze efficiently the formation of even hydrolytically sensitive epoxides in the presence of aqueous hydrogen peroxide (Scheme 11). The catalyst could be recycled up to 5 times without any significant loss of activity. 

The topic of this chapter is enantioselective hydrogenation over chiral or chirally modified solid catalysts. Diastereoselective hydrogenation of chiral compounds and asymmetric hydrogenation with heterogenized (supported, embedded) homogeneous transition metal complexes will not be discussed. 

These proceedings from the 19 th Conference represent the work of over 150 scientists from fourteen countries. The proceedings are organized into the following topics immobilized and supported catalysts, practical aspects of catalysis, solid acid and base catalysis, heterogeneous catalysis and commercial processes, Raney-type and base metal catalysis, stereoselective catalysis, and hydrogenation and amination. 

Initial attempts were made by ter Halle et al.[7] to convert the homogeneous ruthenium catalyst into a heterogeneous catalyst by incorporating the metal center in a solid support using polymerizable diamine ligands. Unfortunately, both the conversions and % ee s for the transfer hydrogenations were low compared to the... 

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