R. Kuwano et Y. Ito, Asymmetric hydrogenation of 1,4,5,6-tetrahydropyrasine-2-(N-tert-butyl)carboxamide catalyzed by trans-chelating chiral diphosphine-rhodium complexes, J ORG CHEM, 64(4), 1999, pp. 1232-1237
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.