And hydrogenation of alkynes

Hydrogenation of alkynes with this new catalyst gives the Z-alkene as the major product, with only trace amounts of the )-alkene. This selective catalyst is called the Lindlar catalyst and hydrogenation of alkynes using it is called Lindlar hydrogenation or Lindlar reduction, named for its discoverer, H. Lindlar (Switzerland). This catalyst has been supplanted by another version that is easier to prepare and less prone to the oxidation of the lead carbonate and lead oxide constituents. It is now the standard and is formally called the Rosenmund catalyst, but it is commonly referred to as a Lindlar catalyst. The new catalyst is composed of palladium-on-barium sulfate (BaS04) and it... 

The material on the addition of hydrogen halides and halogens and hydrogenation of alkynes is deleted. 

Substituents affect the heats of hydrogenation of alkynes m the same way they affect alkenes Compare the heats of hydrogenation of 1 butyne and 2 butyne both of which give butane on taking up two moles of H2... 

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... 

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... 

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. 

Concerning consecutive reactions, a typical example is the hydrogenation of alkynes through alkenes to alkanes. Alkenes are more reactive alkynes, however, are much more strongly adsorbed, particularly on some group VIII noble metal catalysts. This situation is illustrated in Fig. 2 for a platinum catalyst, which was taken from the studies by Bond and Wells (45, 46) on hydrogenation of acetylene. The figure shows the decrease of... 

The increase of selectivity in consecutive reactions in favor of the intermediate product may be sometimes extraordinarily high. Thus, for example, in the already cited hydrogenation of acetylene on a platinum and a palladium catalyst (45, 46) or in the hydrogenation or deuteration of 2-butynes on a palladium catalyst (57, 58), high selectivities in favor of reaction intermediates (alkenes) are obtained, even though their hydrogenation is in itself faster than the hydrogenation of alkynes. 

Baruwati, B., Polshettiwar, V. and Varma, R.S. (2009) Magnetically recoverable supported ruthenium catalyst for hydrogenation of alkynes and transfer... 

Hydrogenation of alkynes in the presence of P-2 catalyst causes syn addition of hydrogen to take place, and the alkene that is formed from an alkyne with an internal triple bond has the (Z) or cis configuration. 

The hydrogenation of alkynes is a very interesting reaction, since the selectivity toward the partially or the fully reduced product allows the in-situ comparison of the ability of a catalyst to reduce C=C versus C=C bonds. This is perhaps the area in which duster catalysis has been most extensively developed, as recently reviewed by Cabeza [27], Adams and Captain [4], and Dyson [28]. A good number of metal clusters have been employed as catalyst precursors in alkyne hydrogenation, the majority of them containing ruthenium. 

More recently, Adams and coworkers have provided a very interesting case of heteronuclear clusters that are very active for the hydrogenation of alkynes [4, 54, 55]. The high-nuclearity layer-segregated Pt-Ru complex [Pt3Ru6(CO)21(jU3-H)(/ -H)j], consisting of three stacked triangular layers of metal atoms with an... 

---