Semihydrogenation of Alkyne

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. 

A more challenging task is the selective partial hydrogenation (semihydrogenation) of alkynes to yield alkenes. This is a selectivity problem similar to the hydrogenation of dienes in that that the alkyne is hydrogenated preferentially in the presence of an alkene. The possibility of the formation of geometric isomers from nonterminal acetylenes raises the problem of stereoselective semihydrogenation. 

Some of the supported (heterogenized) metal complex catalysts discussed above show good performance in the selective semihydrogenation of alkynes.446-449... 

As catalytic semihydrogenation of alkynes to Cis-alkenes is not only a very important synthetic operation (ref. 1) but also of industrial interest, it is a challenging task for both synthetic and catalytic chemists. For instance, the importance of the problem is illustrated by numerous recent publications on different aspects of the selective hydrogenation of many compounds related to the propargyl alcohol structure (refs. 2-7). In this respect, 1,4-butenediol, obtained by the liquid-phase semihydrogenation of 1,4-butynediol, is a raw material for insecticides and Vitamin Bg (refs. 2,8,9). Furthermore, the total and selective liquid-phase hydrogenation of this compound is one of the procedure for making butanediol, the top 95 chemical produced in the United States (refs. 10,11), whose major use is in the manufacture of polyesters. 

A Pd/Zeolite A catalyst that was treated with diphenyldiethoxysilane was effective for the semihydrogenation of alkynes, particularly disubstituted acetylenes.30 Hydrogenation of 3-nonyne gave 3-nonene in 97% yield over this catalyst (Eqn. 16.19). Using the non-silanized catalyst the alkene was formed only in 40% yield. ... 

Treatment of Ni(OAc)2 with ethanolic Na-borohydride affords the same type of catalyst for the semihydrogenation of alkynes" . 

Reductions. Acenaphthoquinonediimine ligated palladium complexes and a polymer-bound palladaisoindoline have found use as catalysts for semihydrogenation of alkynes and reduction of several common unsaturated compounds (including the nitro group), respectively. 

Transfer hydrogenation. Saturation of double bonds in the presence of Pd/C shows selectivity correlating with steric hindrance. Semihydrogenation of alkynes with this system is reported. 

Semihydrogenation of alkynes. Formic acid is a hydrogen source for the Pd(0)-catalyzed transfer reduction of the triple bond to afford the (Z)-alkene with a selectivity of 89-98%. The reducing system also contains triethylamine. 

Semihydrogenation of alkynes The surface of silica gel is covered with hydrosilane groups on reaction with trichlorosilane. The solid-supported silane effects hydrogenation of alkynes to (Z)-alkenes selectively. 

Reductions. The reagent system can be used to selectively hydrogenate 1 -alkenes at 0° in MeOH in the presence of disubstituted alkenes and conjugated acid derivatives. At 65°, disubstituted alkenes are saturated, leaving trisubstituted alkenes untouched. Semihydrogenation of alkynes is also achieved. ... 

In 1989 Trost and Braslav reported the first semihydrogenation of alkynes nsing a hydrogen donor in the presence of a homogeneous palladium catalyst.In the presence of 2.5 mol % of Pd2(dba)3-CHCl3 and 10 mol % of tii-o-tolylphosphine, alkynes are reduced smoothly by simple stirring wilh a mixture of tetramethyldihydrosiloxane (TMDHS) and... 

The semihydrogenation of alkynes is a valuable methodology giving access to (Z)-alkenes. Several heterogeneous catalysts have been shown to be efficient, the most famous being Lindlar s catalyst (Pb-poisoned Pd). However, this methodology suffered severe drawbacks such as Z/E isomerization, low selectivity, and... 

---