Hydrogenation, catalytic, alkene chiral ligands

A more versatile method to use organic polymers in enantioselective catalysis is to employ these as catalytic supports for chiral ligands. This approach has been primarily applied in reactions as asymmetric hydrogenation of prochiral alkenes, asymmetric reduction of ketone and 1,2-additions to carbonyl groups. Later work has included additional studies dealing with Lewis acid-catalyzed Diels-Alder reactions, asymmetric epoxidation, and asymmetric dihydroxylation reactions. Enantioselective catalysis using polymer-supported catalysts is covered rather recently in a review by Bergbreiter [257],... 

On the other hand, the preparation of TV-acetyl- and iV-benzoylferrocenylalanines 102 and 103 by enantioselective catalytic hydrogenation of the Z-alkenes 100 and 101 has been reported (Scheme 29).[6S1 In this case, a comparative study showed that among different chiral ligands for Rh, (+)- or (—)-Norphos were the ones giving the highest enantioselec-tivities (up to 94% ee) in the hydrogenation of 100. 

The use of ionic hquids in asymmetric catalysis was reported even later, beginning with Chauvin s report on a catalytic asymmetric hydrogenation and hydroformylation of alkenes in 1995 [125]. Since then, enantioselective catalysis in ionic hquids has attracted remarkable interest as an approach to the facile recycling of expensive chiral ligands and catalysts, and a range of enantioselective catalyhc transformahons have been examined in ionic liquids [126]. In many cases, ionic hquids have a beneficial effect on the achvihes and enanhoselectivities, and demonstrate facile recovery and reusabihty of the ionic solvent-catalyst systems. The reader is referred to Chapter 7 for an excehent review on the development of enanhoselechve catalysis in ionic hquids. 

Within the last three decades, chemists have discovered ways to embed transition metal hydrogenation catalysts in chiral molecules with the result that hydrogen can be delivered to only one face of the alkene. In catalytic reductions where a new chiral center is formed, a large enantiomeric excess of one enantiomer may be formed, and the reaction is said to be enantioselective. The most widely used of these chiral hydrogenation catalysts involve the chiral ligand... 

Because of the chiral ligand, the activation energy for the transition state in the syn addition to the alkene (Step 2 of the catalytic cyde) is different depending on which side of the alkene the metal complex approaches (the two transition states are diastereomers). This difference in activation energy means that approach to one side of the alkene is favored and results in an excess of one enantiomer of the product. Note that this reaction is not a normal Heck reaction in that it forms a carbon-carbon bond to the more substituted carbon and the double bond shifts. Attack at the other carbon, because of the requirement for syn elimination, cannot lead to a normal Heck product therefore, the reaction reverses. The attack takes place at the more substituted carbon less often, but in this case, there is a hydrogen that can undergo elimination. 

Abstract Chiral ferrocenyl phosphine ligands are certainly one of the most developed and successful classes of chiral ligands used in asymmetric catalysis. The literature describing their synthetic and coordination chemistry, as well as their metal-mediated applications in the field of catalysis, is extremely rich and varied. Moreover, they represent a rare example in which enantioselective chemical catalysts were used in industrial processes. The present chapter provides an account of the planar-chiral ferrocene ligands developed in the Authors laboratory, including their coordination chemistry with various metals as well as their use in different asymmetric catalytic reactions (allylic substitution, Suzuki coupling, methoxycarbonylation of alkenes, hydrogenation of ketones). 

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