Phosphines enantioselective hydrogenation catalysts containing

Tocopherol can be produced as the pure 2R,4 R,8 R stereoisomer from natural vegetable oils. This is the most biologically active of the stereoisomers. The correct side-chain stereochemistry can be obtained using a process that involves two successive enantioselective hydrogenations.28 The optimum catalyst contains a 6, 6 -dimethoxybiphenyl phosphine ligand. This reaction has not yet been applied to the enantioselective synthesis of a-tocopherol because the cyclization step with the phenol is not enantiospecific. 

Togni and co-workers have used the convergent methodology to link phosphine-containing chiral ferrocene ligands on the cyclophosphazene core to obtain dendrimeric structures of the type 37 (Fig. 21) (201). The reaction with the cyclophosphazene end occurs by the replacement of the P-Cl bond and by the formation of the P-0 bond. The dendrimers contain twelve and sixteen ferrocene moieties respectively. The phosphine units present can coordinate to Rh(I) to afford metallic dendrimers, which have been shown to be excellent catalysts for the enantioselective hydrogenation of dimethyl itaconate. The product... 

Another important use for Wilkinson s catalyst is in the production of materials that are optically active (by what is known as enantioselective hydrogenation). When the phosphine ligand is a chiral molecule and the alkene is one that can complex to the metal to form a structure that has R or S chirality, the two possible complexes will represent two different energy states. One will be more reactive than the other, so hydrogenation will lead to a product that contains predominantly only one of the diastereomers. 

Usually, a catalyst has to be synthesized or conditioned prior to its use in a catalytic reaction. However, there is an alternative to such an isolated or preformed catalyst, the so-called in-situ catalyst. The in-situ catalyst is prepared by mixing the transition metal compound (the procatalyst) and the ligand (the cocatalyst) in the solvent in which the reaction is to be carried out [79]. The use of in-situ catalysts is most appropriate in enantioselective hydrogenation reactions with Wilkinson-type catalysts. The optically active phosphines needed for optical induction have to be synthesized in multi-step syntheses [80, 81]. It is most convenient to combine them directly with the Rh-containing procatalysts. 

There are many other enantioselective reactions catalyzed by chiral metal complexes. These include complexes of various chiral phosphines, tartrate ion, and others as ligands. Chiral cyclopentadienyl metal complexes (32) also are used. Chiral ligands are not limited to those containing an asjonmetric carbon. [RuCbinap)] " is a catalyst for enantioselective hydrogenation. In some cases such as [Rh(dipamp)] and [Rh(pnnp)] well defined complexes are used. In many cases the reaction is carried out in the presence of a chiral ligand and a metal compound. The reaction involving... 

The hydrogenation of (Z)-a-N-acetamidocinnamic acid to give N-acetylphenylalanine is shown in Scheme 1, top. Rh compounds of the Wilkinson-type catalyze this reaction. With triphenylphosphine as a ligand, the product is formed as a racemic mixture in an achiral catalysis. If triphenylphosphine is replaced by an optically active phosphine, N-acetylphenylalanine is obtained in optically active form in an enantioselective catalysis. In 1968, pioneering studies used optically active phosphines of the Homer-type, such as PMePrPh as ligands [1,2]. Optical inductions, however, remained low with catalysts containing these monodentate phosphines. 

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