Semi-hydrogenation

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]. 

A similar H2 activation mechanism was proposed for the [Pd(NN S)Cl] complexes (5 in Scheme 4.5) in the semi-hydrogenation of phenylacetylene [45] after formation of the hydride 14 (Scheme 4.9), coordination of the alkyne occurs by displacement of the chloride ligand from Pd (15). The observed chemos-electivity (up to 96% to styrene) was indeed ascribed to the chloride anion, which can be removed from the coordination sphere by phenylacetylene, but not by the poorer coordinating styrene. This was substantiated by the lower che-moselectivities observed in the presence of halogen scavengers, or in the hydrogenations catalyzed by acetate complexes of formula [Pd(NN S)(OAc)]. Here, the acetate anion can be easily removed by either phenylacetylene or styrene. 

Metallocene complexes of early transition metals [Cp2MR R2] (R1, R2 = H, alkyl, M = Zr, Ti, Hf) are active and selective catalysts in semi hydrogenation of dienes.431 Two ruthenium-carbene complexes display high activity in alkene hydrogenation, which may be further enhanced by the addition of HBF4-OEt2.432 A turnover number of 12,000 h 1 was obtained. 

Step 3 Semi-hydrogenation of the triple bond with Lindlar s catalyst gives the (Z)-alkene. 

The semi-hydrogenated acetate coupled product was dissolved in a 1% solution of sodium hydroxide in methyl alcohol. The solution was allowed to stand about 12 hours at room temperature under a blanket of nitrogen. The hydrolysis was complete as shown by the total absence of ester bands in an infrared analysis. 

De-ethanolation of the Semi-Hydrogenated, Hydrolyzed Coupling Product, Preparation of Vitamin A. 

Self-Poisoning and Aging of Pd-Ag/Al203 in Semi-Hydrogenation of 1,3-Butadiene Effects of Surface Inhomogeneity Caused by Hydrocarbonaceous Deposits... 

The selective semi-hydrogenation of ethyne in ethene is also an industrial process of vital importance, used in both laboratory practice and relevant to fine chemicals and polymer production [445,462]. The reaction is generally performed over low loaded Pd catalysts. Systematic investigations have been performed over Pd-Ag, Pd-Cu and Pd-Au supported catalysts that are superior to mono-metallic Pd catalysts [445,461,463]. The presence of Au decreases the carbon coverage and improves the ethene selectivity [445]. 

Lindlar semi-hydrogenation to C1S vinyl alcohol 23, subsequent acetylation, and Pd-mediated rearrangement of the tertiary allylic acetate [64] provided access to 24 as a ca. 85 15 (E/Z)-mixture. Notably, full conversion in the preceding hydrogenation reaction is important, since the acetylenic acetate is a strong catalyst poison for the allylic rearrangement. 

Conversion of the chloro ketone E to (+)-punaglandin 4 (151) is summarized in Figure 6.11. Treatment of E with the dianion derived from propyne gave J, which was alkylated with 1-iodopentane. Semi-hydrogenation of the product afforded K, which was converted to L. The aldol reaction between I and L was problematic, and yielded the desired M as the minor product. The desired aldol product M was obtained in 25% yield after chromatographic purification. Punagladin 4 (151) was obtained in 24% yield based on M. The usefulness of lipase in enantioselective synthesis is well illustrated in the present synthesis of punaglandin 4 (151).12... 

The selective semi-hydrogenation of alkynes, is a particularly important reaction in the context of fine chemieals manufaeture. The acetylenic group, RC CR, readily participates in substitution reactions enabling the formation of new carbon-carbon bonds, for example, and selective hydrogenation, leading to alkene or alkane species, further enhances the synthetic utility of the alkynes and has been exploited in the synthesis of biologically active compounds, e.g. insect sex pheromones (pest control) and vitamins [1-3]. 

On balance, palladium offers the best combination of activity and selectivity at reasonable cost, and for these reasons has become the basis of the most successful commercial alkyne hydrogenation catalysts to date. Because of their inherently high activity, these catalysts contain typically less than 0.5 % (by weight) of active metal-to preserve selectivity at high alkyne conversion. Despite the prominence of these catalysts, other active metals are used in fine chemicals applications. Of particular utility is the nickel boride formulation formed by the action of sodium borohydride on nickel(II) acetate (or chloride). Reaction in 95 % aqueous ethanol solution yields the P2-Ni(B) catalyst and selectivity in alkyne semi-hydrogenation has been demonstrated in the reaction of 3-hexyne to form cw-3-hexene in 98 % yield [15,16] ... 

Literature on the use of promoters is voluminous, and all claim enhancement of semi-hydrogenation selectivity. One of the more successful commercial catalysts for ethyne conversion to ethene, uses a silver promoted alumina-supported palladium catalyst [23]. Other promoting metals have been used, including rhodium and gold [24,25], copper [26-28], zinc (shown to inhibit oligomerization) [29-... 

Perhaps the best known alkyne semi-hydrogenation catalyst is that developed by Lindlar which comprises calcium carbonate-supported palladium, modified by addition of lead acetate and, often, quinoline to improve selectivity [51]. Selective hydrogenation of 1-bromo-ll-hexadecyne (Eq. 8) has been shown to occur in high yield and without hydrogenolysis of the carbon-bromine bond, over Lin-dlar s catalyst treated with aromatic amine oxides such as pyridine A-oxide... 

The semi-hydrogenation of the perfluoroacetylenic ester to the c/j-acrylate (Eq. 10) was accomplished with 75-85% yield [55],... 

The use of secondary modifiers, e. g. quinoline, and the choice of solvent also play important roles in directing semi-hydrogenation selectivity. For example, in the hydrogenation of 1-octyne over a series of Pd/Nylon-66 catalysts metal loading had no effect on selectivity when the reaction was performed in n-heptane as solvent. When the same experiment was conducted in n-propanol, however, an inverse relationship between selectivity and catalyst metal loading was observed [56], This effect has been interpreted as a polar solvent-induced modification of the Pd active sites, which alters the relative adsorption behavior of the alkyne and alkene species [57], Modification by addition of quinoline is reported to benefit the selective production of a cij-vitamin D precursor from the related disubstituted alkyne [58] ... 

Similarly, the effect of quinoline addition has been found to benefit the selectivity of other palladium formulations, as demonstrated in the semi-hydrogenation of an alkyne diester [59] ... 

As mentioned earlier, the particular geometric arrangement around the alkyne triple bond can also play a great part in controlling semi-hydrogenation selectivity. In molecules with both alkenic and alkynic functionality it is possible to preserve the original alkene group only if its approach to the catalyst surface can be restricted in some way. This effect was demonstrated in the reaction shown by Eqs (17) and (18) [67] ... 

In this case, semi-hydrogenation proceeded to 86 % conversion with 78 % yield of the diene and 8 % of the propylcyclohexene over-hydrogenated product. 

Although palladium occupies the dominant position in semi-hydrogenation catalysts, it is by no means the only metal suitable for formulation into a viable catalyst. Mention has already been made of the nickel boride alternatives, with or without copper promotion, for example. Other examples include the skeletal catalyst Raney nickel [69], alumina-supported nickel [70], and aluminum phosphate-supported nickel [71] (Eqs 21 and 22) ... 

Fig. 6. Hydrogenation of buta-1,3-diene semi-hydrogenation selectivities (Ss(l) as a function of the conversion (50-100%) on RPd3 catalysts triangles, CePd3 solid circles, LaPd3 open squares, PrPd3 open circles, NdPd3 solid squares, SmPd3 stars, Pd/pumice (H) (Sim et al. 1991).

An inherent problem in the usage of phenolic lipids, particularly in surface coatings, is the discolouration which can impair products. Apart from colourants arising from the solvent action of CNSL on the shell in the industrial process, the dihydric phenols In CNSL notably the minor component 2-methylcardol (ref. 200) more than cardol appear to be the cause of this deterioration rather than the monohydric member, cardanol. The usage of purer cardanol, or the less unsaturated material by semi-hydrogenation or chemical reduction, as well as the Incorporation of an antioxidant are methods for colour stabilisation (ref. 277). Antioxidant applications and pharmaceutical uses of CNSL and its component phenols are referred to in the next section. 

On surfaces covered with hydrogen, hydrogenation and isomerization start with the formation of a semi-hydrogenated intermediate ... 

It is reported, (3-6) that a-phase hydride with a lower hydrogen content does not attack the double bond and is therefore the most selective catalyst of the semi-hydrogenation of triple bond, whereas p-phase Pd-H is active in the saturation of the C=C double bond. 

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