Asymmetric Hydrogenations of Olefins

Wide variations in stereoselectivity are possible between the and Z isomers (79). In hydrogenation of several ( )- and (Z)-a-acylaminocinnamic acid derivatives, the Z isomers gave greater enantiomeric excesses at 15-100 times the rate of reduction of the isomer, but in all cases the 5 enantiomer was formed in greater excess (//7). The greater effectiveness of Z-olefins is general If8). 

Stereoselectivity may be influenced strongly by both temperature and pressure. In general, the optical yield is decreased with increasing pressure, and at high pressures (50atm) the predominant product chirality actually has been 

The origin of the remarkable stereoselectivities displayed by chiral homogeneous catalysts has occasioned much interest and speculation. It has been generally assumed, using a lock-and-key concept, that the major product enantiomer arose from a rigid preferred initial binding of the prochiral olefin with the chiral catalyst. Halpren 48) on the basis of considerable evidence, reached the opposite conclusion the predominant product enantiomer arises from the minor, less stable diastereomer of the olefin-catalyst adduct, which frequently does not accumulate in sufficient concentration to be detected. The predominant adduct is in essence a dead-end complex for it hydrogenates at a much slower rate than does the minor adduct. 

Homogeneous catalysts may also be effective in the hydrogenation of sulfur-containing compounds. (Z)-2-Benzamide(acetamido)-3-(2-thienyl)-2-propenoic acid was reduced in 100% yield and 78% enantiomeric excesses over Rh(I)-DlOP catalysts (25), 

Augusline, Organic Reactions in Steroid Chemistry (J. Fried and J. A. Edwards, eds.), Vol. 1. Van Nosirand Reinhold, New York, 1972. 

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In addition, several S/S ligands were also investigated for the asymmetric hydrogenation of olefins. In 1977, James and McMillan reported the synthesis of various disulfoxide ligands, which were applied to the asymmetric ruthenium-catalysed hydrogenation of prochiral olefinic acid derivatives, such as itaconic acid. These ligands, depicted in Scheme 8.16, were active to provide... 

In summary, the asymmetric hydrogenation of olefins or functionalized ketones catalysed by chiral transition metal complexes is one of the most practical methods for preparing optically active organic compounds. Ruthenium and rhodium-diphosphine complexes, using molecular hydrogen or hydrogen transfer, are the most common catalysts in this area. The hydrogenation of simple ketones has proved to be difficult with metallic catalysts. However,... 

Iridium-Catalyzed Asymmetric Hydrogenation of Olefins with Chiral N,P and C,N Ligands 33... 

Iridium-Catalyzed Asymmetric Hydrogenation of Olefins with Chiral N,P and C,N Ligands 55 Table 4 Asymmetric hydrogenation of a-substltuted a,(3-unsaturated amides with ligand 24... 

In asymmetric hydrogenation of olefins, the overwhelming majority of the papers and patents deal with hydrogenation of enamides or other appropriately substituted prochiral olefins. The reason is very simple hydrogenation of olefins with no coordination ability other than provided by the C=C double bond, usually gives racemic products. This is a common observation both in non-aqueous and aqueous systems. The most frequently used substrates are shown in Scheme 3.6. These are the same compounds which are used for similar studies in organic solvents salts and esters of Z-a-acetamido-cinnamic, a-acetamidoacrylic and itaconic (methylenesuccinic) acids, and related prochiral substrates. The free acids and the methyl esters usually show appreciable solubility in water only at higher temperatures, while in most cases the alkali metal salts are well soluble. 

We note that there are NMR-based kinetic studies on zirconocene-catalyzed pro-pene polymerization [32], Rh-catalyzed asymmetric hydrogenation of olefins [33], titanocene-catalyzed hydroboration of alkenes and alkynes [34], Pd-catalyzed olefin polymerizations [35], ethylene and CO copolymerization [36] and phosphine dissociation from a Ru-carbene metathesis catalyst [37], just to mention a few. 

As in asymmetric hydrogenation of olefins and ketones, chiral diphosphine-Rh or -Ir complexes have frequently been used as catalysts [ 1,162,335]. Recently, a chiral titanocene catalyst... 

Bidentate oxazoline-imidazolylidene ligands, in which both units are linked by a chiral paracyclophane, have been studied in Bolm s group [129]. In this case, the planar chirality of the pseudo-orfho-paracyclophane is combined with the central chirality of an oxazoline (Scheme 48). Compounds 70 were tested in the asymmetric hydrogenation of olefins displaying moderate selectivity (ee s of up to 46% for dimethylitaconate in the presence of 70b). 

Reduction. Successful asymmetric hydrogenations of olefin double bonds mediated by chiral phosphines have been reported (16) and the factors crucial for effective asymmetric induction in related systems have been discussed (17, 18). These reductions require functionality proximate to the double bond for any degree of success. 

All of the above examples involve an extra coordinating group such as ena-mide, acid, or ester in the substrate. This is necessary for optimum coordination to the metal. Asymmetric hydrogenation of olefins without functional groups is an emerging area [80]. 

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