Asymmetric catalysis alkene hydrogenation

Asymmetry in metal-alkene coordination plays a critical role in asymmetric catalysis, with implications far beyond the scope of the present treatment. An instructive example is provided by catalytic asymmetric hydrogenation of enamides,... 

The use of ionic liquids has been successfully studied in many transition metal-catalyzed hydrogenation reactions, ranging from simple alkene hydrogenation to asymmetric examples. To date, almost all applications have included procedures of multiphase catalysis with the transition-metal complex being immobilized in the ionic liquid by its ionic nature or by means of an ionic (or highly polar) ligand. 

Cationic zirconocenes serve as useful reagents in such diverse fields as alkene polymerization, carbohydrate chemistry, asymmetric catalysis, and so on. Reagents that were originally developed for polymerization reactions (MAO, ansa-metallocenes, non-nucleophi-lic borate counterions) have now found use in organic synthesis and are being employed for carbometalation reactions, hydrogenation, and Diels—Alder catalysis. 

Unlike the impressive progress that has been reported with asymmetric catalysis in other additions to alkenes (i.e., the Diels-Alder cycloaddition, epoxidation, dihydroxylation, aminohydroxylation, and hydrogenation) so far this is terra incognita with nitrile oxide cycloadditions. It is easy to predict that this will become a major topic in the years to come. 

Asymmetric catalysis allows chemicals to be manufactured in their enantiomer-ically pure form and reduces derivatisation and multiple purification steps that would otherwise be required. The 2001 Nobel Prize was awarded for two of the most important asymmetric reactions hydrogenations and oxidations. A variety of ligands suitable for asymmetric reductions are available commercially including BINAP, Figure 3.16. A BINAP Rh complex is used in the commercial production of 1-menthol to enantioselectively hydrogenate an alkene bond (Lancaster, 2002). Ru BINAP complexes can be used in asymmetric reductions of carbonyl groups (Noyori, 2005 Noyori and Hashiguchi, 1997). 

Asymmetric catalysis, the introduction of chirality into non-chiral reactants through usage of a chiral catalysts, is an important aspect of asymmetric synthesis. The most extensively studied asymmetric catalysis reaction is that of hydrogenation of alkenes. In... 

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. 

Most reactions involving asymmetric catalysis are based around the conversion of a planar sp carbon atom into a tetrahedral sp carbon atom. This category of reactions includes asymmetric hydrogenation of alkenes and ketones, as well as the addition of other reagents to these groups as identified in Figure 1.1. 

Asymmetric Catalysis The corresponding RhL2+ catalysts were developed by Schrock and Osborn.Their most important application is asymmetric catalysis.Eq. 9.6 shows how the achiral alkene 9.13 can give two enantiomers 9.14, and 9.15 on hydrogenation. 

In 34 and 35, there is one more element of chirality in the phosphite or phosphinite moiety, such as the binaphthyl system [44]. These modifications could potentially influence the origin of the stereochemistry in the asymmetric catalysis process, and sometimes excellent results have been obtained (quantitative conversions and enantiomeric excess values higher than 99%) in the case of 34 (R = Ph, = H, R = Ph) that is better than other known catalyst systems for the asymmetric hydrogenation of unfunctionalized 1,1-disubstituted terminal alkenes [44]. 

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