Alkynes hydrogenation using Lindlar catalyst

In an internal alkyne, syn addition gives a cis product. For example, when hex-2-yne is hydrogenated using Lindlar s catalyst, the product is d.v-hcx-2-cnc. 

Reduction of an alkyne occurs in two stages first, addition of one mole of H2 to form an al-kene and then addition of the second mole of H2 to the alkene to form the alkane. In most cases, it is not possible to stop the reaction at the alkene stage. However, by careful choice of catalyst, it is possible to stop the reaction at the addition of one mole of hydrogen. The catalyst most commonly used for this purpose consists of finely powdered palladium metal deposited on solid calcium carbonate that has been specially modified with lead salts. This combination is known as the Lindlar catalyst. Reduction (hydrogenation) of alkynes over a Lindlar catalyst is stereoselective syn addition of two hydrogen atoms to the carbon—carbon triple bond gives a cis alkene ... 

Hydrogenation of alkynes may be halted at the alkene stage by using special catalysts Lindlar palladium is the metal catalyst employed most often Hydrogenation occurs with syn stereochemistry and yields a cis alkene... 

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. 

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. 

Ring-closing alkyne metathesis (RCAM) leads to alkyne macrocydes in excellent yields [36]. When combined with hydrogenation using a Lindlar-type catalyst, it is a convenient pathway to macrocydic Z-cydoalkenes. Fiirstner and coworkers utilized this strategy for numerous total syntheses of natural products [37], epothilone A being one of them [38]. 

Figure 16.32 represents the alkenylation of an acetylene according to the procedure developed by Cacchi. The coupling partners are an alkenyl triflate (prepared in analogy to Figure 13.23) and an alkyne. The coupling product of these compounds is a 1,3-enyne—in this case conjugated with a cyclohexenone. The C=C triple bond of such an enyne can be hydrogenated in a cis-selective fashion by using Lindlar s Pd catalyst (cf. Figure 17.81). In this way 1,3-dienes are formed that contain at least one cij-configured C=C double bond.

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 ... 

Catalytic hydrogenation of alkynes using Lindlar s catalyst. 

A The alkynyl hydrogen of propyne is acidic and hence the propyne will form an organometallic derivative. The Grignard derivative will react with carbon dioxide to give the carboxylic acid. Cis alkenes are formed by the catalytic hydrogenation of alkynes using a Lindlar catalyst. Hence a propynyl alcohol is a precursor to compound (b). This could be obtained by the addition of a propyne anion to propanone ... 

Palladium metal is too active a catalyst to allow the hydrogenation of an alkyne to stop after one equivalent of H2. To prepare a cis alkene from an alkyne and H2, a less active Pd catalyst is used— Pd adsorbed onto CaC03 with added lead(II) acetate and quinoline. This catalyst is called the Lindlar catalyst after the chemist who first prepared it. Compared to Pd metal, the Lindlar catalyst is deactivated or poisoned. ... 

A CaC03 supported Pd-Pb alloy catalyst was found to be more selective in alkyne hydrogenation than the Lindlar catalyst.23 Styrene was produced in over 95% selectivity by the hydrogenation of phenyl acetylene over this catalyst (Eqn. 16.12). Further hydrogenation to ethyl benzene was significantly less than that observed using Lindlar s catalyst. The Z (cis) alkene was formed in >99% selectivity at 100% conversion in the hydrogenation of 11-hexadecynyl acetate Eqn. 16,13).23... 

The commonest example of. v>w-addition is the heterogeneously catalysed hydrogenation of an alkene, using either Ni, Pt or Pd, or in the case of the reduction from an alkyne to an alkene, using the Lindlar catalyst. The reduction of an alkene can also be achieved using diimide. 

---