Selectivity alkyne hydrogenation over palladium

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... 

A final example of shape selective catalysis is the selective hydrogenation of alkynes, alkenes and other hydrocarbons over Pd complexes anchored to montmorillonite clay supports.29 In these studies, phosphines and amine ligands were first reacted with montmorillonite and then reacted with palladium (II) salts to form anchored Pd(II) complexes in the interlamellar spacings of the montmorillonite clay. 

Perhaps the best known alkyne semi-hydrogenation catalyst is that developed by Lindlar which comprises calcium carbonate-supported palladium, modified by addition of lead acetate and, often, quinoline to improve selectivity [51]. Selective hydrogenation of 1-bromo-ll-hexadecyne (Eq. 8) has been shown to occur in high yield and without hydrogenolysis of the carbon-bromine bond, over Lin-dlar s catalyst treated with aromatic amine oxides such as pyridine A-oxide... 

A catalyst known as Lindlar s catalyst (which you will meet again in Chapter 27) is used to reduce alkynes to alkenes, but does not easily reduce alkenes to alkanes. This requires rather subtle chemoselectivity alkenes are usually hydrogenated at least as fast as alkynes, so we need to be sure the reaction stops once the alkene has been formed. The Lindlar catalyst is a palladium catalyst (Pd/CaC03) deliberately poisoned with lead. The lead lessens the activity of the catalyst and makes further reduction of the alkene product slow most palladium catalysts would reduce alkynes all the way to alkanes. Best selectivities are obtained if quinoline is also added to the reaction, and alkyne to alkene reductions work with Pd/ BaS04 -I- quinoline too. Even so, Lindlar reactions often have to be monitored carefully to make sure that over-reduction is not taking place. 

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