Figure 1: BABIBOP ligands. Structure of ligands used for purchase and in the literature.
Transition-metal-catalyzed reactions have a history of using chiral phosphorus ligands.1 Ligands such as BINAP, BINOL, and BIPHEP have performed well in asymmetric transition-metal-catalyzed reactions.2,3 While these ligands offer a powerful tool for the synthetic chemist, they are not without their limitations.4,5 A new series of bisphosphorus chiral ligands (BABIBOPs), which possess chirality at the phosphorus center as opposed to the biaryl axis, have been recently catalogued for efficient asymmetric hydrogenation ability.6,7
The Senanayake and Kozlowski groups have successfully employed the use of these ligands in their copper-catalyzed asymmetric hydrogenations of ketones (Scheme 1).6
Scheme 1: Asymmetric Hydrogenation of a 2-Aryl-1-Tetralone
DFT calculations performed in this study suggest a dimeric copper hydride transition state, and ligand coordination modes similar to BINAP- and BINOL-type structures. The Tang group has taken these BABIBOP ligands and modified the 2 and 2’ positions (Scheme 2),7 which we also offer as alkylated BABIBOP ligands (see “Product Availability” below). These modified ligands are useful in the asymmetric hydrogenation of ß-ketoesters, affording ß-hydroxyesters in good yields and high ee’s.7
Scheme 2: Asymmetric Hydrogenation Reactions of ß-Ketoesters
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