Buchler Glossary

Chiral Ligands

A chiral ligand is a specially adapted ligand used for asymmetric synthesis. This ligand is an enantiopure organic compound which combines with a metal center by chelation to form an asymmetric catalyst. Especially Cinchona alkaloids have been proven to provide access to highly enantioselective catalysts. Most recently novel chiral Cinchona alkaloid-based NNP tridentate ligands have been developed for asymmetric hydrogenation (https://doi.org/10.1021/acs.orglett.1c04101; Org. Lett. 2022, 24, 415−419). These Iridium-based chiral catalysts converts prochiral ketones including aromatic, heteroaryl, and alkyl ketones in high conversion and extremely high enantioselectivities (96.1−99.9% ee) into enantiopure secondary alcohols. The performance of these chiral ligands is amazing, its potential in the synthesis of biologically active molecules, versatile intermediates in pharmaceuticals and agriculture is huge. Even the asymmetric hydrogenation of methyltertbutyl ketone, which is among the most challenging ketone substrate, can be obtained in an enantioselectivity of 97.3%.

One of the most famous enantioselective reactions using Cinchona alkaloid ligands is the Sharpless asymmetric dihydroxylation. High conversions and enantioselectivities can be achieved by the use of chiral Dihydroquinine (DHQ) or Dihydroquinidine (DHQD) ligands. The mixture containing (DHQ)2-PHAL is called AD-mix-α, and the mixture containing (DHQD)2-PHAL is called AD-mix-β. Similar results can be obtained for the same chiral ligands in the aminohydroxylation. (DHQD)2-PHAL  Dihydroquinidine 1,4-phthalazinediyl diether as well as  (DHQ)2-PHAL  Dihydroquinine 1,4-phthalazinediyl diether are Cinchona alkaloid dimers widely used as chiral ligands in asymmetric synthesis. (DHQD)2PHAL has been found to be an excellent accelerating ligand for the copper(I)-catalyzed azide–alkyne cycloaddition reaction under air. The procedure is simple, efficient, environmentally-friendly and the products can be isolated in excellent yields with high enantiomeric purity.

More detailed information can be found in the free of charge chiral catalyst search data base.

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