The partial hydrogenation of alkynes

Alkynes readily undergo catalytic hydrogenation to yield first the corresponding alkene and then the alkane. 

With most hydrogenation catalysts of the platinum and nickel groups (Sections 4.2.67, p. 459 and 4.2.50, p. 450), a mixture of products is obtained even if an attempt is made to stop the reaction at the half-way stage. An alkene may only be obtained in good yield if particular attention is paid to the selection of a deactivated hydrogenation catalyst. An early, highly effective formulation of a deactivated palladium catalyst is Lindlar s catalyst (Section 4.2.54, p. 453), but palladium-on-barium sulphate in the presence of quinoline, or palladium-on-calcium carbonate, is also recommended. In these cases it is advisable to 

Although these catalytic partial hydrogenations of alkynes may well be regarded as the procedure of choice for (Z)-alkenes,25 other catalytic systems have been explored. These include a sodium hydride-sodium alkoxide-nickel(n) acetate reagent,26 and a sodium borohydride-palladium chloride-polyethylene glycol system.27 Diisobutylaluminium hydride (DIBAL) has also been used for the conversion of alkynes into (Z)-alkenes.28 ( )-Alkenes are formed when the internal triple bond is reduced with sodium in liquid ammonia.29 

(1) See Section 4.2.54, p. 453 a suitable preparation is available from, for example, Fluka-A.G. or Aldrich. 

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Since that time several other metal-solvent systems have been found useful, effective, and selective in the partial hydrogenation of alkynes. The mechanism of the transformation depends on the nature of the solvent and the substrate. Detailed studies were carried out with the Na-HMPA-tert-BuOH system,197 resulting in the suggestion of the mechanism depicted in Scheme 11.10. 

Lindlar catalyst Acts as a catalyst for the partial hydrogenation of alkynes to yield cis alkenes (Section 8.5). 

In 1973, Brown and Ahuja introduced an interesting alternative method to Lindlar reduction that uses P-2 nickel in the presence of ethylenediamine for the partial hydrogenation of alkynes to Z-alkenes [33]. The nickel catalyst can be generated in situ through the reduction of nickel acetate by NaBHa, which makes it straightforward to control the exact catalyst loading for the reaction. 

Woelk and Bargon have studied the catalytic hydrogenation of alkynes in scCOj [34], using catalysts based on transition metal colloids, carried out in the TCA setup. Under mild experimental conditions (15 bar hydrogen partial pressure, at 323 K) they have found extremely high TOFs (as high as 4 X 10 h" ... 

Preparation of cis-alkenes Lindlar s catalyst, which is also known as poisoned catalyst, consists of barium sulphate, palladium and quinoline, and is used in selective and partial hydrogenation of alkynes to produce c/s-alkenes. Hydrogen atoms are delivered simultaneously to the same side of the alkyne, resulting in syn addition (cw-alkenes). Thus, the syn addition of alkyne follows same procedure as the catalytic hydrogenation of alkyne. 

As was discussed in Sections 11.1.5 and 11.2.3, the stereoselective partial hydrogenation of alkynes to either cis or trans alkenes is of key importance. Chemical reductions can also be applied to achieve both selective transformations. 

A is the reactant, P the desired product, and S an undesired by-product. ki and 2 are the reaction rate constants (s ). An example of such a reaction is the partial hydrogenation of an alkyne (A) to an alkene (P) and further to an alkane (S). Table 3 lists the variables used in the simulations. 

Isolated carbon-carbon double bonds are not normally reduced by metal-ammonia reducing agents and therefore the partial reduction of alkynes is conveniently effected by these reagents. The procedure is highly selective and none of the saturated product is formed. Furthermore, the reduction is completely stereoselective and the only product from a disubstituted alkyne is the corresponding f -alkene (7.52). This method thus complements the formation of Z-alkenes by catalytic hydrogenation (see Section 2.6). 

A strong inhibitor is a catalyst poison, e.g. sulfur for Ni hydrogenation catalysts. For example, partially sulfided Ni catalysts are applied for the selective hydrogenation of alkynes (lower activity than Lindlar s catalyst, reaction temperature 200-250 °C, continuous feed of 1 ppm H2S to the reactant). 

Pd NPs prepared in CN-functionalized ILs were active for the selective hydrogenation of alkynes [28]. In the presence of Ibar Hj, 3-hexyne was partially hydrogenated to the ds-alkene with 92% selectivity at full conversion. Alkanes as the major products could be obtained on increasing the hydrogen... 

Noting that cis alkenes are intermediates in the hydrogenation of alkynes leads us to con sider the possibility of halting hydrogenation at the cis alkene stage If partial hydrogena tion of an alkyne could be achieved it would provide us with methods for preparing... 

Among the tasks remaining is the replacement of the C-16 hydroxyl group in 16 with a saturated butyl side chain. A partial hydrogenation of the alkyne in 16 with 5% Pd-BaS04 in the presence of quinoline, in methanol, followed sequentially by selective tosylation of the primary hydroxyl group and protection of the secondary hydroxyl group as an ethoxyethyl ether, affords intermediate 17 in 79% overall yield from 16. Key intermediate 6 is formed in 67 % yield upon treatment of 17 with lithium di-n-butylcuprate. 

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

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... 

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. 

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