Homogeneous hydrogenation ruthenium

There is much current excitement and activity in the field of homogeneous hydrogenation using ruthenium catalysts. This is reflected in the recent, explosive increase in the number of research publications in this area, now rivaling those for rhodium catalysts (Fig. 3.1). Meanwhile, the price of rhodium metal has risen dramatically, becoming about ten times that of ruthenium, on a molar basis. The number of reports on the use of osmium catalysts has remained low, partly because of the higher price of osmium compounds - about ten times that of ruthenium - and partly because the activity of osmium catalysts is often lower. 

Whilst the metals of the cobalt group have provided valuable mechanistic information on the mechanism of homogeneous hydrogenation of arenes, there is little doubt that ruthenium forms the most active and versatile catalysts. 

Keywords Asymmetric Hydrogenation m Carbon Dioxide m Carbonylation m Dimethylformamide Enantioselectivity m Formic Acid m Homogeneous Hydrogenation n Palladium Catalysts Radical Reactions m Ruthenium Catalysts m Supercritical Fluids m Solvent Replacement... 

Many catalysts, certainly those most widely used such as platinum, palladium, rhodium, ruthenium, nickel, Raney nickel, and catalysts for homogeneous hydrogenation such as tris(triphenylphosphine)rhodium chloride are now commercially available. Procedures for the preparation of catalysts are therefore described in detail only in the cases of the less common ones (p. 205). Guidelines for use and dosage of catalysts are given in Table 1. 

Table I. Hydrogenation of Unsaturated Organic Substrates Using Homogeneous Chiral Ruthenium and Rhodium Catalysts...

Asymmetric catalysis has been most prevalent in the area of homogeneous hydrogenations. As previously stated, producing considerable amounts of a single enantiomer or diastereoisomer from a small amount of chiral catalyst has a huge industrial impact. Natural and unnatural amino acids, particularly L-dopa (12), have been produced by this method.17-21 Catalysts based on rhodium and ruthenium have enjoyed the most success. 

In the following section, we discuss the basic mechanism of homogeneous hydrogenation by Wilkinson s catalyst, RhCl(PPh3)3. Many other complexes of rhodium as well as complexes of other metals such as ruthenium, platinum, lutetium, etc. have also been used as homogeneous, laboratory-scale, hydrogenation catalysts. The mechanisms in all these cases may differ substantially. 

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 low-temperature hydride reductions of [Ru(CO)2(P—P)2][SbFg]2 (PP = PPh2(CH2) PPh2 n = 1, dppm, n = 2, dppe) have been undertakento elucidate the mechanism of the homogeneous hydrogenation of carbon monoxide to produce organic products for the petrochemical industry catalysed by ruthenium formyl... 

This mechanism is remarkably similar to that proposed by Halpern, Harrod, and James (42) for the homogeneous hydrogenation of olefins at certain ruthenium complexes. 

The discovery of the chiral atropisomeric ligand, BINAP, greatly expanded the number of asymmetric homogeneous hydrogenation catalyses. Rhodium and ruthenium complexes that contain BINAP and similar ligand systems have demonstrated an amazing versatility in the reduction of a wide variety of substrate classes in excellent stereoselectivities and reactivities. (-)-Menthol, a variety of... 

A review of homogeneous hydrogenation by Ru catalysts was published in 1970 and the synthesis and properties of ruthenium-hydride complexes known prior to 1977 have been reviewed . An up to date review of ruthenium-hydride complexes appeared in 1984 . We concern ourselves here with hydrogenation reactions that involve Ru(II) species as catalyst precursors or catalytic intermediates. 

Ruthenium catalysts require an induction period at low P and are commonly used at elevated P (20 X 10 kPa). Anionic Ru complexes, 3 and 4, catalyze homogeneous hydrogenation of carbonyl derivatives in THF at 85°C . 

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