Hydrogenation of alkynes to -alkene

Hydrogenation of alkynes to alkenes using the Lindlai catalyst is attractive because it sidesteps the regioselectivity and stereoselectivity issues that accompany the dehydration of alcohols and dehydrohalogenation of alkyl halides. In tenns of regioselectivity, the position of the double bond is never in doubt—it appears in the carbon chain at exactly the sane place where the triple bond was. In tenns 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. 

Hydrogenation of alkynes to alkenes (Section 9.9) 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. 

Scheme 3.5 Hydrogenation of alkynes to alkenes catalyzed by [RuH 2-H2)(P(CH2CH2PPh2)3)]BPh4 ([Ru] =...

The stereoselective hydrogenation of alkynes to alkenes can be effected by a wide variety of homogeneous catalysts. The appropriate choice of catalyst and reaction conditions allows the selective formation of either the (Z)- or the (l )-a1-kene. Most of the catalysts display a very high chemoselectivity, as they are not reactive towards reducible functional groups such as carbonyl, ester, and double bonds. Many of the details related to catalyst behavior and intricate mechanistic details concerning semihydrogenation of alkynes have often not been unraveled, and will remain a topic of research for the coming years. 

Iron catalysts have found only limited use in usual hydrogenations, although they play industrially important roles in the ammonia synthesis and Fischer-Tropsch process. Iron catalysts have been reported to be selective for the hydrogenation of alkynes to alkenes at elevated temperatures and pressures. Examples of the use of Raney Fe, Fe from Fe(CO)5, and Urushibara Fe are seen in eqs. 4.27,4.28, and 4.29, respectively. 

Excellent examples of the use of nitrogen bases as catalyst poisons are seen in the selective hydrogenation of alkynes to alkenes (see Chapter 4). Quinoline is probably the base that has been most often employed for this purpose. In this selective hydrogenation, the nitrogen base effectively inhibits the hydrogenation of alkenes to alkanes... 

On the other hand, hydrogenations under mild conditions, in particular those at ordinary temperature and pressure, are advantageous for monitoring the extent of conversion of substrate exactly and thus achieving selective hydrogenation successfully, as in selective hydrogenation of alkynes to alkenes and in selective hydrogenation of the carbon-carbon double bond of unsaturated carbonyl compounds. 

The iron cluster [ ( ri -C5H5)Fe( jL3-CO) 4] catalyzes the selective hydrogenation of alkynes to alkenes at 100-130 °C and 6.8-68 atm of hydrogen. Terminal alkynes are selectively hydrogenated to alkenes even in the presence of alkenes or internal alkynes. ... 

Hydrogenation of alkynes to alkenes yields the cis (or Z) alkene by syn addition to the triple bond. 

Pathways corresponding to those of Scheme 3 are applicable for hydrogenation of alkynes by monohydride catalysts, the key intermediate now is a vinyl rather than an alkyl (10). Some rhodium complexes effective for hydrogenation of alkynes to alkenes were mentioned earlier and are listed in Table 3 with other rhodium catalysts. 

The [MH(H2)(pp3)]+ system, which was studied extensively by Bianchini and coworkers,94 105 highlights the importance of differences in M-H2 bond strengths (Os > Fe > Ru) on the selective hydrogenation of alkynes to alkenes. The Fe congener is one of the most stable H2 complexes and reactions with alkynes under 1 atm ofH2 produce only alkenes (with one exception) via the mechanism in Eq. (9.54), where the H2 remains attached while the alkyne coordinates. In contrast, the complex [RuH(H2)(pp3)]+ catalyzes dimerization of 1-alkynes to nearly 100% Z-l,4-disubstituted enynes.107... 

The selective hydrogenation of alkynes to alkenes has an industrial application in the hydrogenation of low concentrations of alkynes and dialkenes in alkene streams obtained from the steam cracking of petroleum fractions. The impurities are selectively hydrogenated over a Pd catalyst to produce pure alkenes. 

The answer is B. Lindlar catalyst is a combination catalyst which does selective or restrictive hydrogenation of alkynes to alkenes. Furthermore, the product is a cis-alkene. 

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