Catalytic hydrogenation with chiral transition metal complexes

1 Catalytic hydrogenation with chiral transition metal complexes  

Only a brief overview can be given of the most important methods in this domain, since this is certainly one of the most widely exploited means of direct asymmetric synthesis, with many industrial applications. 

Catalytic reduction with transition metal complexes lends itself well to asymmetric synthesis because of the rigid structure of these (generally) octahedral complexes and the resulting high degree of order in the transition state. The most widely used metals have been 

The complexes (72) and (73) are octahedral and, as described in section 6.1.2, can exist in two enantiomeric forms, A and A. In this case, the / -BINAP forms exclusively the R-A diastereomer, and the S-BINAP the S-A. A catalytic quantity (1 mol % or even less) promotes the asymmetric hydrogenation of a wide variety of alkenes bearing nearby polar groups, such as amides, alcohols, esters, etc., which are necessary for high e.e.s. An important application of this is in the synthesis of amino acids by asymmetric reduction of amidoacrylates (74). 

This reaction is the key step in the industrial synthesis of (L)-DOPA, the anti-Parkinson s disease agent. In this case the hydrogenation is controlled by DIPAMP, a ligand chiral at phosphorus whose preparation was described in section 1.7. 

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A chiral diphosphine ligand was bound to silica via carbamate links and was used for enantioselective hydrogenation.178 The activity of the neutral catalyst decreased when the loading was increased. It clearly indicates the formation of catalytically inactive chlorine-bridged dimers. At the same time, the cationic diphosphine-Rh catalysts had no tendency to interact with each other (site isolation).179 New cross-linked chiral transition-metal-complexing polymers were used for the chemo- and enantioselective epoxidation of olefins.180... 

Some general reviews on hydrogenation using transition metal complexes that have appeared within the last five years are listed (4-7), as well as general reviews on asymmetric hydrogenation (8-10) and some dealing specifically with chiral rhodium-phosphine catalysts (11-13). The topic of catalysis by supported transition metal complexes has also been well reviewed (6, 14-29), and reviews on molecular metal cluster systems, that include aspects of catalytic hydrogenations, have appeared (30-34). 

Asymmetric synthesis (i) has gained new momentum with the potential k use of homogeneous catalysts. The use of a transition metal complex with chiral ligands to catalyze a synthesis asymmetrically from a prochiral substrate is beneficial in that resolution of a normally obtained racemate product may be avoided. In certain catalytic hydrogenations of olefinic bonds, optical purities approaching 100% have been attained (2,3,4,5) hydrogenations of ketones (6,... 

The development of the catalytic hydrogenation system based on RhCl(PPh3)3 and methods for the resolution of optical isomers of tertiary phosphines occurred around the same time (1965), and this led to the possibility of asymmetric catalytic hydrogenation of prochiral unsaturated substances with C=C, C=0, and C=N bonds using transition metal complexes with chiral phosphine ligands. Such tertiary phosphines are of three types ... 

Especially noteworthy is the field of asymmetric catalysis. Asymmetric catalytic reactions with transition metal complexes are used advantageously for hydrogenation, cyclization, codimerization, alkylation, epoxidation, hydroformylation, hydroesterification, hydrosilylation, hydrocyanation, and isomerization. In many cases, even higher regio- and stereoselectivities are required. Fundamental investigations of the mechanism of chirality transfer are also of interest. New chiral ligands that are suitable for catalytic processes are needed. 

This year has again emphasized the growing importance of organo-transition metal complexes in organic synthesis. In catalysed reactions the major advances have been in asymmetric catalysis with the first reports of chiral induction in catalytic epoxidation and further reports on improved catalysts for asymmetric hydrogenation and allylic alkylation. The formation of carbon-carbon bonds continues to attract attention, and several novel and potentially useful synthetic applications of organometallic complexes have been reported. 

Hydrosilanes undergo addition to carbon-carbon multiple bonds under catalysis by transition metal complexes. Nickel, rhodium, palladium, and platinum were used as catalytically active metals. By incorporating chiral ligands into the metal catalyst, the hydrosilylation can be performed analogously to other addition reactions with double bonds, for example, asymmetric hydrogenation to obtain optically active alkylsilanes. 

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