Dendritic MonoPhos: synthesis and application in Rh-catalyzed asymmetric hydrogenation
摘要:
A new class of dendritic monodentate phosphoramidite ligands were synthesized through substitution of the dimethylamino moiety in MonoPhos by the Frechet-type dendritic wedge and applied in the asymmetric hydrogenation of alpha-dehydroamino acid esters and dimethyl itaconate. High enantioselectivities (up to 97.9% ee) and catalytic activities (up to 4850 h(-1) TOF) were achieved, which are better or comparable to those obtained from MonoPhos. The third generation dendrimer catalyst gave the slightly lower catalytic activity relative to the lower generation ones. The steric shielding by the dendrimer could stabilize the rhodium complex against decomposition caused by hydrolysis in a protic solvent. The inactive catalysts (RhL3 and RhL4) were activated by addition of a free metal precursor Rh(COD)(2)BF4, and showed high enantioselectivities and catalytic activities. (c) 2006 Elsevier Ltd. All rights reserved.
article we demonstrate how asymmetric total synthesis of (S)-rivastigmine has been achieved using direct asymmetric reductive amination as the key transformation in four steps. The route started with readily available and cheap m-hydroxyacetophenone, through esterification, asymmetric reductive amination, N-diphenylmethyl deprotection and reductive amination, to provide the final (S)-rivastigmine in