Catalysts for alkene hydrogenation

Both objectives have been met by designing special hydrogenation catalysts. The most frequently used one is the Lindlar catalyst, a palladium on calcium carbonate combination to which lead acetate and quinoline have been added. Lead acetate and quinoline partially deactivate ( poison ) the catalyst, making it a poor catalyst for alkene hydrogenation while retaining its ability to catalyze the addition of H2 to the triple bond. 

Platinum and palladium are the most common catalysts for alkene hydrogenations. Palladium is normally used as a very fine powder supported" on an inert material such as charcoal (Pd/C) to maximize surface area. Platinum is normally used as PtC, a reagent called Adams catalyst after its discoverer, Roger Adams. 

Most low-valence metal complexes are generally deactivated by air and sometimes also by water. Carbon monoxide, hydrogen cyanide, and PH3 frequently act as poisons for these catalysts. Poisoning by strongly co-ordinating molecules occurs by formation of catalytically inert complexes. An example is the poisoning of Wilkinson s catalyst for alkene hydrogenation ... 

The trinuclear cluster [(/i-H)2Ru3(/i3-0)(C0)5(DPPM)2] is also an efficient catalyst for alkene hydrogenation reaction, for which Bergounhou proposed the catalytic Scheme 73.38... 

Hydrogenation catalysts from non-platinum group have been also reported. For example, some organolanthanides of formula [ (r/5-C5Me5)2MH)2] are active catalyst for alkene hydrogenation.384 It has been proposed on the basis of kinetic studies that first the dimer dissociates according to (Equation (18)),... 

The First Catalysts for Alkene Hydrogenation Mechanistic Considerations... 

The [IrH2(NCMe)3(P Pr3)]BF4 complex is an active catalyst for alkene hydrogenation, in acetone soluhon, at 298 K and 1 atm of hydrogen. Thus, under these conditions, cyclohexene was hydrogenated with a TOF of 1.1 min" . This complex has also been very useful in the mechanistic investigation of alkene hydrogenation. 

The complexes [RCo(CO)2 P(OMe)3 2] (R = Me and MeCO) show a marked catalytic activity (450 turnovers h ) for hydrogenation of terminal alkenes under ambient conditions, while more-hindered alkenes are reduced slowly. Cobalt complexes such as [CoH2(PR3)3] and [CoHN2(PPh3)3] are also active catalysts for alkene hydrogenation. 

Several complexes of Pd° and Pd containing PR3, DMSO, etc. catalyze hydrogenation of monoal-kenes. " Salen complexes of Pd and Ni are employed as catalysts for alkene hydrogenation. The cluster complex of palladium [Pd5(PPh)2], is a catalyst for the hydrogenation of alkenes, dienes, alkynes and many other kinds of unsaturated compound. ... 

While various borohydride nickel ratios have been used in these nickel boride preparations maximum P-2 catalytic activity was observed with a 2 1 BH4 Ni ratio.However, when the reduction was run under a hydrogen atmosphere using a 4 1 BH4 Ni ratio, a hydrogenated nickel boride catalyst, the P-3 nickel boride, was obtained.27 This catalyst was somewhat more active than the P-2 catalyst for alkene hydrogenation but it induced signifieantly more double bond isomerization during the reaction. 

The borohydride reduction of nickel decanoate in cyclohexane also produced a reversed micelle stabilized nickel boride cateilyst.37 The most active catalyst was formed using a 3 1 BH4 Ni ratio. This gave a catalyst that was about as active as the P-2 catalyst for alkene hydrogenation. The use of other BH4 Ni ratios gave less active catalysts. 

The treatment of solutions of platinum metals with aqueous borohydride results in the formation of finely divided black precipitates that are active catalysts for alkene hydrogenations. The platinum black obtained in this way was twice as active as that obtained by the hydrogenation of platinum oxide. The borohydride reduced rhodium black is even more active. While the borohydride reduction of base metals gives the corresponding metal borides, there is little, if any, boron incorporated into these platinum metal blacks. Analysis of the borohydride reduced palladium found that while the palladium boron ratio in the bulk was 10 1, less than 1% of the surface was boron.59 7, 5 small amount of boron, however, can impart a significant difference in catalytic activity to this catalyst as compared with other, more common, palladium catalysts. The most striking difference is the inability of the borohydride reduced palladium to promote the hydrogenolysis of activated C-0 and C-N bonds, a reaction that takes place readily over standard palladium catalysts. 

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