Alkynes half-hydrogenation

The kind of surface site catalyzing alkyne half-hydrogenation has been the subject of some speculation. Two possibilities are shown in Fig. 2.2. Certainly such a site should fit the evidence Burwell and associates obtained from the hydrogenation of di-tert-butyl acetylene (2,2,5,5-tetramethyl-3-hexyne). 31 They proposed that the molecule dislocated a surface atom, pulling it up out of the plane of the surface (Fig. 2.3). That work concluded that hydrogenation... 

Half-hydrogenations of alkynes are usually accomplished over poisoned Pd catalysts, but other catalysts also may also be used.13 Unpoisoned Pd sometimes does not give high stereoselectivity.14 One of the most mentioned Pd cat-... 

The mechanism of half-hydrogenation of alkynes is not fully understood, but some details are recognized. For example, it has long been recognized that alkynes adsorb more strongly than alkenes. During the half-hydrogenation of... 

FIGURE 2.2 Mechanism of half-hydrogenation of alkynes showing two possible adsorption sites. 

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... 

A new catalyst, nickel P-2, easily prepared from Ni-acetate and NaBH, is non-pyrophoric and highly selective. It may serve for the half-hydrogenation of dienes, the reduction of alkynes to pure cis-olefins, and hydrogenolysis-free hydrogenation... 

The non-pyrophoric P-2 nickel catalyst can be easily prepared by treatment of nickel acetate tetrahydrate in alcohol with NaBH4 under Ng. It is highly selective Half-hydrogenation of conjugated and non-conjugated dienes, reduction of alkynes... 

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