Partial Reduction of Alkynes

Partial reduction of alkynes to Z-alkenes is an important synthetic application of selective hydrogenation catalysts. The transformation can be carried out under heterogeneous or homogeneous conditions. Among heterogeneous catalysts, the one that 

Reduction of Carbon-Carbon Multiple Bonds, Carbonyl Groups, and Other Functional Groups 

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Partial reduction of alkynes with sodium in liquid ammonia or bycatalytic hydrogenation. 

The partial reduction of alkynes provides methods that are both regio-and stereospecific. Dissolving metal reductions tend to give trans alkenes, whereas catalytic methods of reduction generate the cis alkenes (Scheme 3.3). A Lindlar catalyst (Pd/CaCOj + PbO, partially poisoned with quinoline) has been recommended for use in this context. 

Isolated carbon-carbon double bonds are not normally reduced by metal-ammonia reducing agents and therefore the partial reduction of alkynes is conveniently effected by these reagents. The procedure is highly selective and none of the saturated product is formed. Furthermore, the reduction is completely stereoselective and the only product from a disubstituted alkyne is the corresponding f -alkene (7.52). This method thus complements the formation of Z-alkenes by catalytic hydrogenation (see Section 2.6). 

Useful reduction processes include the partial reduction of alkynes ( H2, Lindlar catalyst), reduction of alkenes with tetradeuterohydrazine, and reduction of carbonyl compounds with sodium borodeuteride or lithium aluminium deuteride as in the following examples ... 

The methods described for H labelling can also be applied to tritiation, in particular reduction of alkenes with N2 H4, partial reduction of alkynes with H2 and Lindlar s catalyst, and reduction of mesylates (or tosylates) with LiApH4. 

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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