Alkynes Lindlar catalyst hydrogenation

Lindlar catalyst (Section 9 9) A catalyst for the hydrogenation of alkynes to as alkenes It is composed of palladium which has been poisoned with lead(II) acetate and quino line supported on calcium carbonate... 

The product i n this case is a cis-disubstituted alkene, so the fi rst question is, " What is an immediate precursor of a cis-disubstituted alkene " We know that an alkene can be prepared from an alkyne by reduction and that the right choice of experimental conditions will allow us to prepare either a trans-disubstituted alkene (using lithium in liquid ammonia) ora cis-disubstituted alkene (using catalytic hydrogenation over the Lindlar catalyst). Thus, reduction of 2-hexyne by catalytic hydrogenation using the Lindlar catalyst should yield cis-2-hexene. 

Alkynes can be reduced to yield alkenes and alkanes. Complete reduction of the triple bond over a palladium hydrogenation catalyst yields an alkane partial reduction by catalytic hydrogenation over a Lindlar catalyst yields a cis alkene. Reduction of (he alkyne with lithium in ammonia yields a trans alkene. 

Lindlar catalyst (Section 8.5) A hydrogenation catalyst used to convert alkynes to cis alkenes. 

The used Pd/ACF catalyst shows a higher selectivity than the fresh Lindlar catalyst, for example, 94 1% versus 89 + 2%, respectively, at 90% conversion. The higher yield of 1-hexene is 87 + 2% with the used catalyst versus 82 + 3% of the Lindlar in a 1.3-fold shorter reaction time. Higher catalyst activity and selectivity is attributed to Pd size and monodispersity. Alkynes hydrogenation is structure-sensitive. The highest catalytic activity and alkene selectivity are observed with Pd dispersions <20% [26]. This indicates the importance of the Pd size control during the catalyst preparation. This can be achieved via the modified ME technique. 

Hydrogenation of alkynes to alkenes using the Lindlar catalyst is attractive because it sidesteps the regioselectivity and stereoselectivity issues that accompany the dehydration of alcohols and dehydrohalogenation of alkyl halides. In terms of regioselectivity, the position of the double bond is never in doubt—it appears in the carbon chain at exactly the same place where the triple bond was. In terms of stereoselectivity, only the cis alkene forms. Recall that dehydration and dehydrohalogenation normally give a cis-trans mixture in which the cis isomer is the minor product. 

Reactions of internal alkynes may be performed in the presence of terminal alkynes if the terminal alkyne is first converted to an alkynylsilane. In the example depicted in Figure Si5.3, hydrogenation of a silylated diyne using the Lindlar catalyst partially reduces the non-silylated alkyne and leaves the sterically hindered alkynylsilane intact. 

Alkynes are hydrogenated all the way to alkanes if the usual heterogeneous catalysts (Pd, Pt, Raney Ni) are used. If a suitable deactivated catalyst is used, however, it is possible to stop these reactions after monohydrogenation. The so-called Lindlar catalyst is a commonly used deactivated catalyst of this type (Figure 17.81). To prevent an overhydrogenation, it is still necessary to monitor the rate of hydrogen consumption and to interrupt the reaction after one equivalent of hydrogen gas has been absorbed even when the deactivated catalyst is used. The... 

Alkynes normally react with two equivalents of hydrogen to produce alkanes. However, it is also possible to react an alkyne with only one equivalent of hydrogen and stop the reaction at the alkene stage if a special catalyst, called Lindlar catalyst, is used. The Lindlar catalyst is a deactivated form of palladium that is less reactive than normal catalysts. This method provides good yields of the cr. v-alkene, resulting from syn addition of hydrogen ... 

X You will meet Lindlar s catalyst in Chapter 31 but we will mention it now because of its special chemoselectivity. Unlike the other hydrogenations we have described, the Lindlar catalyst will hydrogenate alkynes to alkenes, rather than alkenes to alkanes. This requires rather subtle chemoselectivity alkenes are usually hydrogenated at least as easily 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... 

In contrast to Lindlar catalyst we have found that the hydrogenation of an alkyne over ethylenediamine-poisoned "P-2" nickel boride or quinoline-poisoned palladium-on-barium sulfate always gives a minor amount of the saturated hydrocarbon in addition to the olefin. The ratio of saturated hydrocarbon to olefin (about 0.01) also is nearly constant throughout the hydrogenation until the alkyne is consumed, and then it increases. Further reaction of the alkene on the catalyst surface before desorption would explain these results. 

Lindlar catalyst Acts as a catalyst for the partial hydrogenation of alkynes to yield cis alkenes (Section 8.5). 

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