Catalyst for hydrogenation

Adams catalyst, platinum oxide, Pt02 H20. Produced by fusion of H2PtCl6 with sodium nitrate at 500-550 C and leaching of the cooled melt with water. Stable in air, activated by hydrogen. Used as a hydrogenation catalyst for converting alkenes to alkanes at low pressure and temperature. Often used on Si02... 

Nickel Arsenate. Nickel arsenate [7784-48-7] Ni2(As0 2 8H20, is a yellowish green powder, density 4.98 g/cm. It is highly iasoluble ia water but is soluble ia acids, and decomposes on heating to form As20 and nickel oxide. Nickel arsenate is formed by the reaction of a water solution of arsenic anhydride and nickel carbonate. Nickel arsenate is a selective hydrogenation catalyst for iaedible fats and oils (59). 

The alloy hydride has been investigated as a useful hydrogenation catalyst for a wide variety of substrates under mild conditions. It is however pyrophoric in air, and an experimental procedure has been developed to avoid this hazard. A related hydride, LaNi4.5Alo.5H5 has similar properties. 

Rhodium-catalyzed enantioselective hydrogenation of acctamido -cinnamic in water was also achieved using pyrphos bound to poly-acrylic acid as ligand.337 Roucoux described some Rh° nanoparticles which function as reusable hydrogenation catalyst for arene derivatives in a biphasic water-liquid system.338... 

Other advances over the past few years have been the development of (a) homogeneous hydrogenation catalysts for substrates normally not readily reduced, e.g., aromatics, isonitriles, and nitro compounds, and (b) a number of catalyst systems with unusual selectivity properties, e.g., with the capability of reducing a,/3-unsaturated aldehydes to the corresponding a,/3-unsaturated alcohols (see Sections II,B,2 and VII). 

Following Wilkinson s discovery of [RhCl(PPh3)3] as an homogeneous hydrogenation catalyst for unhindered alkenes [14b, 35], and the development of methods to prepare chiral phosphines by Mislow [36] and Horner [37], Knowles [38] and Horner [15, 39] each showed that, with the use of optically active tertiary phosphines as ligands in complexes of rhodium, the enantioselective asymmetric hydrogenation of prochiral C=C double bonds is possible (Scheme 1.8). 

Styrene yields a stable /6-arene complex (Scheme 2.1), which explains why neither 3 nor 7 is an effective hydrogenation catalyst for styrene and related substrates. The formation of such stable adducts is highly disadvantageous for rapid catalysis, but not for the exploration of organometallic chemistry. No similar stable complexes have been obtained from the catalyst 4 the faster catalytic rates seen for 4 may correlate with the presence of less stable intermediates in this case [30]. 

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. 

Ruthenium complexes are active hydrogenation catalysts for the reduction of dienes to monoenes. Both zerovalent and divalent ruthenium complexes containing various (alkene, diene and phosphine) ligands have been employed as catalysts that have met with different degrees of success. 

Transfer Hydrogenation Catalysts for Reduction of C-C Double and Triple Bonds... 

The next step in the use of transfer hydrogenation catalysts for recycling of the unwanted enantiomer is the dynamic kinetic resolution. This is a combination of two reaction systems (i) the continuous racemization of the alcohol via hydrogen transfer and (ii) the enantioselective protection of the alcohol using a... 

The development of chiral hydrogenation catalysts for unfunctionalized alkenes also allows enantioselective hydrogenation of functionalized olefins where the functionality in the molecule is remote from the double bond. A series of oxazoline-, imidazoline- and pyridine-derived catalysts have been screened for the hydrogenation of unsaturated derivatives of vitamin E (Scheme 30.3). Hy-... 

A selective hydrogenation catalyst for alkynes was obtained with the PdCl2 complex of such immobilized pyridine. Diphenylacetylene was hydrogenated under 0.44 MPa H2 in ethanolic solution. At full conversion, the following selec-tivities were observed cis-stilbene 80.7%, trans-stilbene 16.1%, and only 3.2% 1,2-diphenylethane [90]. 

Reduction of enynes to (Z)-atkenes. Lindlar s catalyst is not useful as a hydrogenation catalyst for reduction of trienynes or of dienediynes. The best results can be obtained in CH3OH with zinc activated by successive treatment with Cu(OAc)2 (10%) and AgN03 (10%). This reduction results in conversion of the triple bond to a (Z)-double bond. The system does not reduce simple, nonactivated alkynes, and a-branched enynes are reduced slowly. The reduction is effected at 25° with (Z)-enynes, but temperatures of 45° are necessary for the (E)-isomers. Yields of pure tetraenes are 25-65%. 

There is a preferential formation of 1,4- addition of H atoms in contrast to 1,2- addition which is commonly observed for conventional hydrogenation catalysts. For example, as discussed above in the case of MgO-catalyzed hydrogenation, 2-butene is preferentially formed while 1-butene is the main product over conventional hydrogenation catalysts. 

Hirai, H., Nakao Y., and Toshima, N., Colloidal Rh in poly(vinylpyrrolidone) as hydrogenation catalysts for internal olefins Chem. Lett., 7, 545, 1978. 

The homogeneous hydrogenation catalysts for polymer saturation can be classified into two types Ziegler-type (Ni, Co, Fe, Ti, Zr based) and noble metal (Rh, Ru, Pd) catalysts. 

The behavior shown in Schemes 2.22 and 2.23 suggests that these hydrido arene-iridium complexes might act as hydrogenation catalysts. Thus, the [(T -C6H6)IrH2(P Pr3)]BF4 complex has been shown to be an efficient hydrogenation catalysts for alkenes and alkynes, as well as some carbonyl groups [22]. 

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