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

Highly enantioselective hydrogenation of 1,4,5,6-tetrahydropyrazine-2- (N-tert-butyl)carboxamide (2) was accomplished by a rhodium complex coordinated with a chiral diphosphine TRAP ligand, which is possible to chelate to a transition 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 acid derivative (3) with (S) configuration, while the hydrogenation with (R,R)-(S,S)-MeTRAP-rhodium catalyst provided (R)-3 with up to 85% ee. ³¹P NMR studies of behavior of i-Bu- and MeTRAP-rhodium catalysts during the reaction suggest that the asymmetric hydrogenation of 2 with TRAPs may involve two competitive reaction pathways, giving their respective enantiomeric products 3.