Homogeneous Hydrogenation of Alkenes

Remarkable advances have been achieved in the homogeneous asymmetric hydrogenation of prochiral alkenes by using chiral complexes of Rh and Ru, which 

In the following sections, efficient asymmetric hydrogenation of various alkenes using Rh and Ru catalysts are illustrated by selecting only a few representative examples for each type of the hydrogenation. 

Esters of -branched amino acids are obtained by highly enantioselective hydrogenation of a-acylaminoacrylatcs bearing different substituents at the ff-carbon using Rh complexes of Me-DuPHOS (XI) and Me-BPE (XV). The D-a//o-isoleucine derivative 17 with 98.2% ee is obtained from the (Z)-enamide 16, and the D-isoleucine 

Asymmetric hydrogenation offers a useful synthetic route to chiral amines. Although the mechanism is unknown, only the (7%/V-acetyl-1 -arvlalkylamine 21 with 95% ee was obtained by the hydrogenation of a mixture of ( )- and (Z)-enamides 20a and 20b using Rh-Me-DuPHOS (XI). The A-acetvl enamines 20a,b are prepared by the reduction of oximes with Fe powder in acetic anhydride [21]. Also the acetamide 23 was obtained from 22 [22]. 

The amine 25 was prepared with excellent enantioselectivity by the hydrogenation of 24 with Rh—H8-BDPAB (XXXV) [23]. Beside these ligands, few suitable ligands are known for the hydrogenation of these substrates. 

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Early transition-metal complexes have been some of the first well-defined catalyst precursors used in the homogeneous hydrogenation of alkenes. Of the various systems developed, the biscyclopentadienyl Group IV metal complexes are probably the most effective, especially those based on Ti. The most recent development in this field has shown that enantiomerically pure ansa zirconene and titanocene derivatives are highly effective enantioselective hydrogenation catalysts for alkenes, imines, and enamines (up to 99% ee in all cases), whilst in some cases TON of up to 1000 have been achieved. 

As recently recognized by the Nobel Chemistry award committee, the conceptualization, development, and commercial application of enantioselective, homogeneous hydrogenation of alkenes represents a landmark achievement in modem chemistry. Further elaboration of asymmetric hydrogenation catalysts by Noyori, Burk, and others has created a robust and technologically important set of catalytic asymmetric synthetic techniques. As frequently occurs in science, these new technologies have spawned new areas of fundamental research. Soon after the development of... 

It has been shown experimentally that attack by strong nucleophiles also occurs regio-selectively at this C atom, stereo selectively from the face opposite to the metal [287]. Since the alkyl group a bonded to the metal is very carbanion-like, it is susceptible to protonation by acids, yielding an alkane. The overall reaction provides the mechanism for homogeneous hydrogenation of alkenes. It may be extended to hydrogenation of C=N and C=0 pi bonds. 

The homogeneous hydrogenation of alkenes is explained by two mechanisms. The first is the dihydride mechanism, in which the dihydride 1 is formed by oxidative addition of H2, and the hydrogenation proceeds by the insertion of alkene to the metal hydride bond, followed by reductive elimination (Scheme 10.1). The other hydrogenation is explained by the formation of the monohydride 2 (Scheme 10.2). Insertion of alkenes... 

Geoffrey Wilkinson Homogeneous hydrogenation of alkenes by (Ph3P)3RhCl (Wilkinson s catalyst)... 

By contrast with the radical systems, this mechanism is the most common and most studied. It is also historically important for giving the first practical demonstration of catalytic homogeneous hydrogenation of alkenes. 

Chlorohydridotris(triphenylphosphine)ruthenium(ll) was the first complex in which homogeneous hydrogenation of alkenes was shown to follow the alkyl route." It can be prepared from dihydrogen and [RuCl2(PPh3)3] in the presence of base (equation 41). Most other alkyl route catalysts are also monohydrido complexes. They are usually specific for terminal alkenes. The behavior of several exo-nirfo-dicarbaborane complexes of rhodium has been reviewed." ... 

The complex has enjoyed relatively little use in organic synthesis. For iridium-catalyzed homogeneous hydrogenation of alkenes, Crabtree s iridium complex ((1,5-Cycloocta-diene)(tricyclohexylphosphine)(pyridine)iridium(I) Hexafluoro-phosphate) is generally preferred, although this readily prepared Ir complex is active. It is more reactive than its rhodium counterpart in the catalytic isomerization of butenyl- to allylsilanes. ... 

Brown, J. M., Giernoth, R. New mechanistic aspects of the asymmetric homogeneous hydrogenation of alkenes. Current Opinion in Drug Discovery Development 2000, 3, 825-832. 

A very special application of homogeneous hydrogenation of alkenes in aqueous solution is the catalytic modification of lipid membranes, either in model systems (liposomes) or in living cells (cf. Section 6.15) [77-80],... 

The inspiration for these catalysts came from the work of Wilkinson in London in the 1960s, who showed that RhClfPPhjjj (known as Wilkinson s catalyst) promoted homogeneous hydrogenation of alkenes (i.e. during the reaction there is only one, solution, phase). Wilkinson s catalyst is a 16-electron complex of Rh(I), and it works as a catalyst because it can easily lose one of the phosphine ligands to form a 14-electron complex. This undergoes addition of Hj, giving a 16-electron Ru(III) complex. 

The use of Rh for hydrogenation started with the discovery by Wilkinson [15] that phosphine complexes were active for the homogeneous hydrogenation of alkenes in the 1960s. Very soon after, the use of chiral phosphines was tried, culminating in... 

The homogenous hydrogenation of alkenes is one of the most studied reactions the results of these studies have been very important for the understanding of the activation of small molecules by metal complexes. 

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