New Homochiral Ligands Bearing Nonstereogenic Chirotopic Centers. Lithiated N,N'-Dialkylureas as Chiral Bases and Sterically Crowded Phosphines for Asymmetric Catalysis.
The synthesis of new homochiral pseudo-C(2)()-symmetrical ligands of type 1 bearing a nonstereogenic chirotopic center is reported. Two such structural units are combined in the ureas 2 and 10, which proved to be useful for enantioselective deprotonation and alkylation of ketones (up to 88% ee). The monophosphine 3 induced a high enantiomeric excess in the Pd-catalyzed allylation of dimethyl malonate (up to 73% ee). Furthermore, we have shown that the achiral sterically crowded alkyl diphosphine 23 efficiently stabilized Pd(0) at high temperatures.
Graf
Claus-Dieter
CD
Fachbereich Chemie der Philipps-Universität Marburg, D-35032 Marburg, Germany, and Institut für Organische Chemie, LMU-München, Butenandtstrasse 5-13, D-81377 München, Germany.
Malan
Christophe
C
Harms
Klaus
K
Knochel
Paul
P