E- and Z-alkenes can be made by stereoselective addition to alkynes

In this last section of the chapter we shall leave elimination reactions to look at addition reactions. Alkynes react with some reducing agents stereoselectively to give either the Z double bond or the E double bond. Some of these reactions were described briefly in Chapter 24. 

Z selective reduction of alkynes uses Lindlar s catalyst 

The neasorrtftat catalytic hydrogenation often results fa sym addition of 1. hydrogen to alkenes was discussed in Chapter 24. 

This pure Z-alkene was needed for studies on the mechanism of a rearrangement reaction. In Chapter 24 you met catalytic hydrogenation as a means of reducing alkenes to alkanes, and we introduced Lindlar s catalyst (palladium and lead acetate on a support of calcium carbonate) as a means of controlling chemoselectivity so that alkynes could be reduced to alkenes. What we did not empha- 

You can compare this method of forming Z-alkenes directly with the Wittig reaction in these two syntheses of another insect pheromone, that of the Japanese beetle. 

In this case, the Wittig reaction is not entirely Z-selective, and it generates some -isomer. Lindlar-catalysed reduction, on the other hand, generates pure Z-alkene, 

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