Hydrogenation with polymer-bound phosphines

The homogeneous chiral phosphine/DPEN-Ru catalyst can be immobilized by use of polymer-bound phosphines such as polystyrene-anchored BINAP (APB-BINAP) [57, 98], Poly-Nap [99], and poly(BINOL-BINAP) [100], poly(BINAP) [101]. These complexes hydrogenate T-acetonaphthone and acetophenone with S/C of 1000-10 000 under 8 10 atm H2 to give the corresponding secondary alcohols in 84-98% e.e. The recovered complexes are repeatedly used without significant loss of reactivity and enantioselectivity. Immobilization allows the easy separation of catalyst from reaction mixture, recovery, and reuse. These advantages attract much attention in combinatorial synthesis. 

Much activity in this field, however, focuses on Ru- or Rh-catalyzed asymmetric hydrogenation of acylaminoacrylate with polymer-bound chiral phosphines to afford chiral amino acids. As early as 1976, Stille reported asymmetric catalytic hydrogenations of these compounds using an acrylate-styrene copolymer-bound DlOP [2,3-0-isopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphine)butane] and other chiral... 

A library of 63 peptides, each containing one of the two phosphine-substituted amino acids 8 or 9, was prepared by Gilbertson and coworkers in a parallel synthesis on polymer beads [13]. After complexation of all peptides with rhodium, the obtained polymer-bound complexes were employed in the asymmetric hydrogenation of 10 (Fig. 5). 

The solution to this problem has been to attach these catalysts to polymer supports. The ideal polymer-bound catalyst must satisfy a formidable list of requirements. It should be easily prepared from low cost materials. The support must be compatible with the solvent system employed, and be chemically and thermally stable under the reaction conditions. The catalyst should show minimal losses in reaction rate or selectivity when bound to the support, and should be able to be recycled many times without loss of activity. Finally, the interactions between the catalytic site and the support must be either negligible or beneficial. The development of polymer supported rhodium-phosphine catalysts for the asymmetric hydrogenation of amino acid precursors illustrates the incremental process which has led to supports which approach the ideal support. 

With phosphine-functionalized poly(styrene divinylbenzene) as a support, it was possible to generate the polymer-bound cluster [Fe2Pt(CO)8(Ph2P P)2] [P = poly(styrene-divinylbenzene)] which catalyzed ethylene hydrogenation under mild conditions.l )... 

Since the researcher normally looks to the chemistry of soluble complexes in designing polymer-bound catalysts, it is notable that some areas that have proven fruitful in homogeneous catalysis have been omitted from investigations using polymer-bound catalysts. One of these areas concerns the reactions of arenes. Benzene, for example, may be hydrogenated with homogeneous cobalt phosphite and ruthenium phosphine complexes, but the corresponding supported versions are not reported. Aryl halides may be carboxylated in the presence of a soluble palladium catalyst ... 

Hydrogenation of iV-acyl a-amino acids in 87-91% ee has been accomplished with Rh bound to copolymers (43) with chiral phosphine ligands, as shown in Scheme 16. The same % ee was obtained with Rh catalysts prepared from analogous soluble ligands. Hydrophilic poly(2-hydroxy-ethyl methacrylate) and poly(iV,iV-dimethylacrylamide) provided polar environments for the hydrogenations. The chiral polymeric catalyst can be recycled. Earlier studies with polymers based on chiral biphosphine monomer (44) showed that polar comonomers were required for high % ee catalysis. ... 

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