Terminal alkyne, semihydrogenation

Simple Pd chloride in dimethylformamide selectively reduces 1-heptyne and 2-pentyne nearly quantitatively . Palladium-borohydride-on-carbon is a selective catalyst for triple bond semihydrogenation, easier to prepare than the deactivated Pd-on-Ca carbonate catalyst . A green complex of Pd and salicylidene ethylene diamine (salen) is an active catalyst for the selective hydrogenation of triple bonds including terminal alkynes, in the presence of alkenes ". 

It was shown that with a Pd/C catalyst in the liquid phase terminal triple bonds were saturated faster than internal ones, and both hydrogenated faster than terminal or internal double bonds in competitive processes (Eqn. 16.5). Further, alkene isomerization generally does not take place over palladium catalysts when alkynes are present. This selective hydrogenation depends on the stronger adsorption of an alkyne compared to an alkene. It is also possible that steric factors can influence the selectivity in the competitive semihydrogenation of an acetylene and an olefmic group in the same molecule. When the double bond and the triple bond are c/s to each other as in 7, selective adsorption of the acetylene... 

Reduction ofAlkynes. Alkynes are selectively reduced to (Z)-alkenes by a reduction catalyst prepared from NaH, t-CsHnOH, and Pd(OAc)2 (6 2 1) in THE The reactions, carried out in the presence of quinoline under near atmospheric pressure of H2, are self-terminating at the semihydrogenated stage, and are more selective than the corresponding reductions catalyzed by Lindlar s catalyst. Omitting the f-CsHnOH gave a catalyst that effected complete reduction. 

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