Protein-bound rhodium hydrogenation

Figure 7.6. Synthetic sequence for the preparation of a protein bound rhodium hydrogenation catalyst.

The NP2 unit and the resultant achiral [Rh(NP2)(NBD)] moiety can also be attached easily at a specific site in a protein. The protein structure then provides the chirality required for enantioselective hydrogenation. Thus, hydrogenation of a-acetamidoacrylic acid to A/ -acetylalanine catalyzed by [Rh(NP2)(NBD)] bound to avidin at RT and 1.5 atm of H2 showed —40% S enantiomeric excess. Although these hydrogenation results with avidin are modest, it does demonstrate that asymmetric synthesis is accomplished by the -phosphine rhodium catalyst attached covalently to a protein. 

Other diphosphino-complexes of rhodium(i) include those derived from (l/ ,2i )-bis(methylamino)cyclohexane, D-glucose, and D-l,6-anhydro-glucose all are capable of catalysing the hydrogenation of N -acetamidoacry-lic acids to a-amino-acids in optical yields of between 70 and 90%. A preliminary study on the use of protein-diphosphinerhodium(i) complexes has been carried out, but here optical yields of only 40% have been realized so far. An insoluble polymer-bound Rh -DIOP catalyst has also been developed. Studies of the hydrogenation of some (E)- and (Z)-A -acylaminocinnamic acids in the presence of Rh -DIOP suggest that some (E) - (Z) isomerization occurs prior to hydrogen addition. 

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