Alkynes complete hydrogenation

Alkynes are completely hydrogenated yielding alkanes in the presence of the customary metal hydrogenation catalysts... 

The complete hydrogenation of alkynes to alkanes occurs quite readily under the usual alkene hydrogenation conditions. This may or may not involve an intermediate alkene 81... 

Alkenes and alkynes add hydrogen much more readily than does nitrogen. For example, ethene reacts rapidly and completely with hydrogen at ordinary pressures and temperatures in the presence of metal catalysts such as nickel, platinum, palladium, copper, and chromium ... 

The reduction of alkynes may be carried out with sodium dissolving in liquid ammonia. Subsequent protonation of the dianion gives the trans-alkene. On the other hand, catalytic reduction gives the r /A-alkene. An illustration of alkyne chemistry involves the preparation of intermediates for the synthesis of vitamin A shown in Scheme 3.28. Complete hydrogenation to an alkane takes place over a platinum catalyst. 

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. 

Radical-mediated silyldesulfonylation of various vinyl and (a-fluoro)vinyl sulfones 21 with (TMSlsSiH (Reaction 25) provide access to vinyl and (a-fluoro)vinyl silanes 22. These reactions presumably occur via a radical addition of (TMSlsSi radical followed by /)-scission with the ejection of PhS02 radical. Hydrogen abstraction from (TMSlsSiH by PhS02 radical completes the cycle of these chain reactions. Such silyldesulfonylation provides a flexible alternative to the hydrosilylation of alkynes with (TMSlsSiH (see below). On oxidative treatment with hydrogen peroxide in basic aqueous solution, compound 22 undergoes Pd-catalyzed cross-couplings with aryl halides. 

In an effort to apply the cooperative principles of metalloenzyme reactivity, involving a combination of metal-ligand and hydrogen bonding, we have reported a ruthenium catalyst incorporating imidazolyl phosphine ligands that efficiently and selectively hydrates terminal alkynes (5). We subsequently found that application of pyridyl phosphines to the reaction resulted in a >10-fold rate enhancement and complete anti-Markovnikov selectivity, even in the... 

Oxidative addition of molecular hydrogen was considered to be involved in the alkyne hydrogenations catalyzed by [Pd(Ar-bian)(dmf)] complexes (4 in Scheme 4.4) [41, 42]. Although the mechanism was not completely addressed, 4 was considered to be the pre-catalyst, the real catalyst most likely being the [Pd(Ar-bian)(alkyne)] complex 18 in Scheme 4.11. Alkyne complex 18 was then invoked to undergo oxidative addition of H2 followed by insertion/elimination or pairwise transfer of hydrogen atoms, giving rise to the alkene-complex 19. 

Complete reduction of alkynes to alkanes is easily accomplished by catalytic hydrogenation, especially using palladium [386, 387], platinum oxide and active nickel catalysts [559]. 

The selectivity values observed in the first stage of the reaction, summarised in Table 23, show similar trends from metal to metal to those observed in alkyne hydrogenation. Again, palladium is almost completely... 

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