Alkynes selective hydrogenation

Partial reduction of alkynes to Z-alkenes is an important synthetic application of selective hydrogenation catalysts. The transformation can be carried out under heterogeneous or homogeneous conditions. Among heterogeneous catalysts, the one that... 

Three new methods for the conversion of alkynes to (Z)-alkenes were reported, although Lindlar semi-hydrogenation still remains as the most convenient method. Copper (I) hydride reagent could reduce alkynes to (Z)-alkenes as shown in Scheme 3 [12]. Yoon employed nickel boride prepared on borohy-dride exchange resin for selective hydrogenation of alkynes to (Z)-alkenes (Scheme 4) [13]. 

The complex [Rh(COD)L L2]+, where L1 = PPh3 and L2 = pyridine, and a neutral benzoate complex, Rh(COD)(PPh3)(OCOPh), also effect highly selective hydrogenation of 1-alkynes to 1-alkenes as well as reduction of 1-alkenes and ketones to alcohols (139) the one equivalent of base required may be related to monohydride formation [Eq. (25)]. The bisphosphine complexes also catalyze reduction of styrene oxide to 2-phenylethanol and phenylacetaldehyde (140) ... 

Alkynes are hydrogenated to cis olefins with the same catalytic systems, and subsequently undergo hydrogenation to yield the corresponding alkanes [7, 42, 45, 47, 49, 59, 93]. For example, Jordan et al. reported the selective hydrogenation of 3-hexyne into cis 3-hexene with a TOF of 25 IT1 [25], and cis 3-hexene is... 

A selective hydrogenation catalyst for alkynes was obtained with the PdCl2 complex of such immobilized pyridine. Diphenylacetylene was hydrogenated under 0.44 MPa H2 in ethanolic solution. At full conversion, the following selec-tivities were observed cis-stilbene 80.7%, trans-stilbene 16.1%, and only 3.2% 1,2-diphenylethane [90]. 

Alkenes are obtained from selective hydrogenation of alkynes (see Section 5.3.1), and the reaction of a phosphorus ylide (Wittig reagent) with an aldehyde or a ketone (see Section 5.3.2). 

These reactions are most common for polar double bond as reactants (carbonyls and imines) than for non-polar substrates (alkene and alkyne). Hence, hydrogen-transfer processes are a very interesting option in order to perform polar double bond hydrogenation since they allow mild conditions, high selectivity,... 

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. 

One important application of selective hydrogenation of alkynes is their removal from the industrial steam cracker products. These can contain several percents of alkynes as byproducts. They are particularly unwelcome in that they poison the catalysts used for the downstream polymerization of the olefins. Selective hydrogenation of these steam cracker cuts has two advantages. It removes acetylenes and converts them to desired alkenes, thereby increasing the overall yields (see Section 11.6.1). 

The observation was a significant finding since at the time, when the only synthetic method to reduce alkynes selectively was their conversion by heterogeneous catalytic hydrogenation (Raney nickel) to cis alkenes. The dissolving-metal reduction provided easy access to high-purity trans alkenes since the latter do not readily react further under the conditions used. The efficient reduction of 1-alkynes in this system requires the presence of ammonium ion.196... 

Unsaturated aldehydes.4 A nickel boride catalyst similar to P-2 nickel boride is obtained by reaction of NiCl2 and excess NaBH4 in C2H5OH. It effects selective hydrogenation of a,P-alkynal acetals to the (Z)-a,p-alkenal acetals. 

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. 

Reduction of an alkyne with hydrogen on a metal catalyst gives the corresponding alkane. By selectively poisoning the catalyst it is possible to reduce an alkyne to an alkene. Once again, the reaction is stereoselective, adding both hydrogen atoms from the same side of the C—C bond to form the c/s-alkene. 

Catalyzed hydrogenation of alkynes, alkenynes, and alkadienes.1 This catalyst effects highly c/s-selective hydrogenation of triple bonds of alkynes and alkenynes, with easy recovery of the complex by filtration. It also effects only 1,2-addition in hydrogenation of even hindered 1,4-substituted 1,3-butadienes. 

A final example of shape selective catalysis is the selective hydrogenation of alkynes, alkenes and other hydrocarbons over Pd complexes anchored to montmorillonite clay supports.29 In these studies, phosphines and amine ligands were first reacted with montmorillonite and then reacted with palladium (II) salts to form anchored Pd(II) complexes in the interlamellar spacings of the montmorillonite clay. 

Styrene and 1-hexene have been selectively hydrogenated as well as substituted acetylenes, alkyne diols, stilbene and other unsaturated hydrocarbons with these palladium montmorillonites. A size selectivity was invoked to explain the enhanced hydrogenation activity of certain clay catalysts presumably due to the differences in interlamellar spacings of the clay which will depend on degree of hydration, concentration of Pd(II) complex, dielectric constant of the solvent used to disperse the reactants and other factors. 

Both of these reactions have very important industrial uses (Section 14.3.9). In order to obtain alkene streams of sufficient purity for further use, the products of steam-cracking or catalytic cracking of naphtha fractions must be treated to lower the concentration of alkynes and alkadienes to very low levels (<5ppm). For example, residual alkynes and dienes can reduce the effectiveness of alkene polymerisation catalysts, but the desired levels of impurities can be achieved by their selective hydrogenation (Scheme 9.4) with palladium catalysts, typically Pd/A Os with a low palladium content. A great deal of literature exists,13,37 particularly on the problem of hydrogenating ethyne in the presence of a large excess of... 

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