Nickel borides alkene hydrogenation

As shown by the data in Fig. 12.4 these micelle stabilized catalysts were considerably more active for alkene hydrogenation than the P-2 nickel borides. The hydrogenation run using a preformed P-2 catalyst in a reversed micelle medium proceeded at a faster rate than the reaction run over P-2 nickel boride in... 

Three new methods for the conversion of alkynes to (Z)-alkenes were reported, although Lindlar semi-hydrogenation still remains as the most convenient method. Copper (I) hydride reagent could reduce alkynes to (Z)-alkenes as shown in Scheme 3 [12]. Yoon employed nickel boride prepared on borohy-dride exchange resin for selective hydrogenation of alkynes to (Z)-alkenes (Scheme 4) [13]. 

A catalyst that permits hydrogenation of an alkyne to an alkene is the nickel boride compound called P-2 catalyst. 

Sodium hydrogen telluride reacts with epoxides, in accordance with an S 2 displacement, giving rise to telluro-alcohols. These products are useful intermediates since they are easily converted into the corresponding alcohols and ketones by treatment with nickel boride followed by oxidation (reaction (a)) or to alkenes via the corresponding tosylates (reaction b)). ... 

Another highly active non-pyrophoric nickel catalyst is prepared by reduction of nickel acetate in tetrahydrofuran by sodium hydride at 45° in the presence of tert-amyl alcohol (which acts as an activator). Such catalysts, referred to as Nic catalysts, compare with P nickel boride and are suitable for hydrogenations at room temperature and atmospheric pressure, and for partial reduction of acetylenes to civ-alkenes [49]. 

Hydrogenation using Raney nickel is carried out under mild conditions and gives cis alkenes from internal alkynes in yields ranging from 50 to 100% [356, 357, 358, 359, 360]. Half hydrogenation of alkynes was also achieved over nickel prepared by reduction of nickel acetate with sodium borohydride (P-2 nickel, nickel boride) [349,361,362] or by reduction with sodium hydride [49], or by reduction of nickel bromide with potassium-graphite [363]. Other catalysts are palladium on charcoal [364], on barium sulfate [365, 366], on... 

Unsaturated aldehydes.4 A nickel boride catalyst similar to P-2 nickel boride is obtained by reaction of NiCl2 and excess NaBH4 in C2H5OH. It effects selective hydrogenation of a,P-alkynal acetals to the (Z)-a,p-alkenal acetals. 

When one component of a bimetallic alloy is leached out, a finely divided metal powder of high surface area results. One of the oldest of these so-called skeletal metal catalysts is Raney nickel10,11. Nickel boride is a more recently developed hydrogenation catalyst prepared by the reduction of nickel salts with sodium borohydride12-14. Bimetallic catalysts are often used to achieve selective saturation of a double bond in bifunctional unsaturated systems, e.g. in dienes. Amorphous metal alloys, a newly developed class of metal catalysts15,16, have also been applied in the hydrogenation of alkenes and dienes. 

Hydrogenation of an alkyne can be stopped at the alkene stage by using a poisoned (partially deactivated) catalyst made by treating a good catalyst with a compound that makes the catalyst less effective. Lindlar s catalyst is a poisoned palladium catalyst, composed of powdered barium sulfate coated with palladium, poisoned with quinoline. Nickel boride (Ni2B) is a newer alternative to Lindlar s catalyst that is more easily made and often gives better yields. 

TABLE 3.1 Rates of Hydrogenation of Alkenes over P-1 and P-2 Nickel Boride and Nic Catalysts... 

In contrast to Lindlar catalyst we have found that the hydrogenation of an alkyne over ethylenediamine-poisoned "P-2" nickel boride or quinoline-poisoned palladium-on-barium sulfate always gives a minor amount of the saturated hydrocarbon in addition to the olefin. The ratio of saturated hydrocarbon to olefin (about 0.01) also is nearly constant throughout the hydrogenation until the alkyne is consumed, and then it increases. Further reaction of the alkene on the catalyst surface before desorption would explain these results. 

The more ethanol in the reduction solvent, the greater the amount of BOj found in the resulting nickel boride precipitate.This ion is formed by the hydrolysis of borohydride but, since it is not very soluble in ethanol, it is not kept in solution in alcoholic solvents. The BO2 is apparently responsible for the decreased activity and increased selectivity of the P-2 nickel boride. 3,35 Vvt]ien the P-2 catalyst was isolated and washed with water before use, the resulting P-2W catalyst was more active than both the P-2 and P-1 nickel borides but it became less selective in alkene hydrogenation than the P-2 catalyst. ... 

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. 

Nickel boride catalysts have been considered viable alternatives to Raney nickel since they are non-pyrophoric, are easily prepared and the preparation procedure is reproducible. In addition these catalysts appear to be somewhat more active than W2 Raney nickel for the hydrogenation of alkenes and nitriles but both types of catalyst have about the same activity for carbonyl group hydrogenation. While nickel boride and Raney nickel are both more resistant to... 

By reduction of alkynes with sodium in liquid ammonia, the trans-alkene will predominate. On the other hand, almost entirely cis-alkene (as high as 98%) is obtained by hydrogenation of alkynes with specially prepared palladium called Lindlar s catalyst or a nickel boride called P-2 catalyst. 

A heterogeneous catalyst that permits hydrogenation of an alkyne to an alkene is the nickel boride compound called P-2 catalyst. The P-2 catalyst can be prepared by the reduction of nickel acetate with sodium borohydride ... 

Partial reduction of alkynes to Z-alkenes is another important application of selective hydrogenation catalysts. The transformation can be carried out under heterogeneous or homogeneous conditions. Among heterogeneous catalysts, the one which is most successful is Lindlars catalyst, which is a lead-modified palladium-CaC03 catalyst. A nickel-boride catalyst prepared by reduction of nickel salts with NaBILj is also useful. Rhodium catalysts have also been reported to show good selectivity. ... 

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