Schiff bases homogeneous hydrogenation

The use of supported (i.e., heterogenized) homogeneous catalysts offers another possibility for easy catalyst separation. New examples include polymer-anchored Schiff-base complexes of Pd(TT),446 PdCl2(PhCN)2 supported on heterocyclic polyamides,447 various Pd complexes supported on crosslinked polymers 448 sol-gel-encapsulated Rh-quatemary ammonium ion-pair catalysts,449 and zwitterionic Rh(T) catalysts immobilized on silica with hydrogen bonding.450... 

There are some reports on the asymmetric catalytic hydrogenation of C=N double bonds. Rhodium phosphine complexes do not have high activity, and the stereoselectivity is low. - Homogeneous hydrogenation of oximes, Schiff s bases and cyclic imines " have been reported. A cyano cobalt complex has been used for the homogeneous catalytic hydrogenation of oximes and Schiff s bases but the degree of asymmetric induction is unknown. 

The complexes of ruthenium of the formula RUCI3L2 Where L is benzimidazole (BzlH), 2-Methylbenzimidazole (2-MebzlH), 2-Ethylbenzimidazole (2-EtbzlH) and 2-propyl benzimidazole (2-PrbzlH) acts as homogeneous catalysts in dimethylformamide in the presence of sodiumborohydride for the reduction of Schiff bases to secondary amines. The reactions were carried out at 30 C and 1 atmosphere of hydrogen pressure. Kinetic studies reveal that the reaction has depencence on [Catalyst] and [Schiff base] in the concentration ranges studied. 

Like homogeneous catalysis, the removal of a-hydrogen of the amino acid fragment by OH ions, the local concentration of which is apparently high in the polymer phase, is probably the rate-determining step of heterogeneous racemization. Under similar conditions, the rate of a-amino acid racemization decreases in the sequence Ala = Ser>Phe>Nva>Lys>Val, and correlates with the rate of substrate racemization in the presence of Schiff bases and transamination of amino acids by pyridoxal phosphate. 

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