Asymmetric hydrogenation of 1,4,5,6-tetrahydropyrasine-2-(N-tert-butyl)carboxamide catalyzed by trans-chelating chiral diphosphine-rhodium complexes

Highly enantioselective hydrogenation of 1,4,5,6-tetrahydropyrazine-2-(N-te rt-butyl)carboxamide (2) was accomplished by a rhodium complex coordinated with a chiral diphosphine TRAP ligand, which is possible to chelate to a tr ansition metal atom in a trans-manner. Of particular interest is that (R,R) -(S,S)-i-BuTRAP gave 97% ee of the corresponding piperazine-2-carboxylic ac id derivative (3) with (S) configuration, while the hydrogenation with (R,R )-(S,S)-MeTRAP-rhodium catalyst provided (R)-3 with up to 85% ee. P-31 NMR studies of behavior of i-Bu- and MeTRAP-rhodium catalysts during the reacti on suggest that the asymmetric hydrogenation of 2 with TRAPs may involve tw o competitive reaction pathways, giving their respective enantiomeric produ cts 3.