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Pure Appl. Chem., 2006, Vol. 78, No. 2, pp. 501-509

Rhodium-catalyzed asymmetric hydrogenation using self-assembled chiral bidentate ligands

James M. Takacs, Kittichai Chaiseeda, Shin A. Moteki, D. Sahadeva Reddy, Di Wu and Kusumlata Chandra

Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588, USA

Abstract: The chirality-directed self-assembly of bifunctional subunits around a structural metal - typically, zinc(II) - is used to form a heteroleptic complex in which a second set of ligating groups are suitably disposed to bind a second metal, forming a heterobimetallic catalyst system. We find that subtle changes in the structural backbone (i.e., ligand scaffold) of such chiral bidentate self-assembled ligands (SALs) can be used to manipulate the ligand topography and chiral environment around catalytic metal; thus, the scaffold can be optimized to maximize asymmetric induction. Using this combinatorial strategy for ligand synthesis, a preliminary study was carried out in which a library of 110 SALs was evaluated in the rhodium-catalyzed asymmetric hydrogenation of a simple N-acyl enamide. The level of enantioselectivity obtained varies from near racemic to greater than 80 % ee as a function of the ligand scaffold, with the possibility of further improvement yet to be explored.