Rhodium-catalyzed asymmetric hydrogenation of functionalized olefins using monodentate spiro phosphoramidite ligands.
Novel chiral monodentate phosphorus ligands, SIPHOS, were conveniently synthesized from 1,1'-spirobiindane-7,7'-diol. The Rh complexes of SIPHOS can catalyze the hydrogenation of alpha-dehydroamino esters in mild conditions, providing alpha-amino acid derivatives in up to 99% ee. Enamides and beta-dehydroamino esters can also be hydrogenated in good to excellent enantioselectivities (up to 99% and 94% ee, respectively). The SIPHOS ligand with smaller alkyl groups on the N-atom afforded higher enantioselectivity. The X-ray analysis of single crystal showed that the structure of Rh/SIPHOS catalyst is [Rh(COD)((S)-SIPHOS-Me)(2)](+), which clarified the configuration of the catalyst with the monodentate chiral phosphorus ligand in Rh-catalyzed asymmetric hydrogenation. A positive nonlinear effect in the relationship of the optical purities of ligand and product was observed in the hydrogenation of dehydroamino acid derivatives. The kinetic study of hydrogenation showed that the reaction is zero order in the concentration of substrate and first order in the concentration of Rh catalyst and the hydrogenation pressure. The rate of hydrogenation decreased when the Rh/L ratio changed from 1:1 to 1:4.
Fu
Yu
Y
State Key Laboratory and Institute of Elemento-organic Chemistry, Nankai University, Tianjin 300071, China.
Guo
Xun-Xiang
XX
Zhu
Shou-Fei
SF
Hu
Ai-Guo
AG
Xie
Jian-Hua
JH
Zhou
Qi-Lin
QL