Nickel borides alkynes, selectivity

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]. 

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. 

On balance, palladium offers the best combination of activity and selectivity at reasonable cost, and for these reasons has become the basis of the most successful commercial alkyne hydrogenation catalysts to date. Because of their inherently high activity, these catalysts contain typically less than 0.5 % (by weight) of active metal-to preserve selectivity at high alkyne conversion. Despite the prominence of these catalysts, other active metals are used in fine chemicals applications. Of particular utility is the nickel boride formulation formed by the action of sodium borohydride on nickel(II) acetate (or chloride). Reaction in 95 % aqueous ethanol solution yields the P2-Ni(B) catalyst and selectivity in alkyne semi-hydrogenation has been demonstrated in the reaction of 3-hexyne to form cw-3-hexene in 98 % yield [15,16] ... 

Nickel boride (P — 2) generated from nickel acetate and NaBH4 in ethanol exhibits notable selectivity in reduction depending on the substitution pattern of the alkene " ". Sodium borohydride with Co(II) has also proved to be of general utility for the reduction of various alkenes and alkynes (equation... 

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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