Hydrogenation Lindlar catalyst

Both objectives have been met by designing special hydrogenation catalysts The most frequently used one is the Lindlar catalyst, a palladium on calcium carbonate combi nation to which lead acetate and quinoline have been added Lead acetate and quinoline partially deactivate ( poison ) the catalyst making it a poor catalyst for alkene hydro genation while retaining its ability to catalyze the addition of H2 to the triple bond... 

Lindlar catalyst (Section 9 9) A catalyst for the hydrogenation of alkynes to as alkenes It is composed of palladium which has been poisoned with lead(II) acetate and quino line supported on calcium carbonate... 

One of the important processes for manufacturing linalool is from the P-methylheptenone intermediate produced by the methods from petrochemical sources discussed earlier. For example, addition of sodium acetyUde to P-methylheptenone gives dehydrolinalool (4), which can be selectively hydrogenated, using a Lindlar catalyst, to produce linalool. 

Lindlar catalyst can be used for hydrogenation of l-[3-(2-phenylpyrazolo[1.5-a]pyridin-3-yl)propynoyl]-2-ethylpiperidine in ethyl acetate (38%) (Scheme 82 89EUP299209 92USP5102869) and l-(hetaryl)-4-alkynylpyrazole derivatives to the corresponding alkenes (96EUP703234). 

Hydrogenations with Lindlar catalysts frequently slow down or stop after absorption of 1 mol of hydrogen. Reduction of 4-hydroxydodec-2-ynoic acid stopped spontaneously when about 95% of the theoretical hydrogen had been absorbed 39). 

Amines (7a,12a), especially pyridine (75), have also been used as solvents in the hydrogenation of acetylenes. Hydrogenation of 3 over 5% Pd-on-BaS04 in pyridine gave df-cis-jasmanate (4) quantitatively (40). The authors comment that this combination for reduction of acetylenes was superior to the Lindlar catalyst in all cases examined. (See also Refs. 12 and 24 for similar conclusions.)... 

Acetylenic epoxides are reduced readily to the olehnic epoxide, provided the resulting epoxide is not allylic (27). In the latter case, one might surmise that hydrogenolysis could best be avoided by use of rhodium in a neutral nonpolar solvent (81) or a Lindlar catalyst (13). Reduction of l,2-epoxydec-4-yne over Lindlar catalyst gave (Z)-l,2-epoxydec-4-ene in 95% yield (69). Hydrogenation ceased spontaneously. 

Complete reduction to the alkane occurs when palladium on carbon (Pd/C) is used as catalyst, but hydrogenation can be stopped at the alkene if the less active Lindlar catalyst is used. The Lindlar catalyst is a finely divided palladium metal that has been precipitated onto a calcium carbonate support and then deactivated by treatment with lead acetate and quinoline, an aromatic amine. The hydrogenation occurs with syn stereochemistry (Section 7.5), giving a cis alkene product. 

The product i n this case is a cis-disubstituted alkene, so the fi rst question is, " What is an immediate precursor of a cis-disubstituted alkene " We know that an alkene can be prepared from an alkyne by reduction and that the right choice of experimental conditions will allow us to prepare either a trans-disubstituted alkene (using lithium in liquid ammonia) ora cis-disubstituted alkene (using catalytic hydrogenation over the Lindlar catalyst). Thus, reduction of 2-hexyne by catalytic hydrogenation using the Lindlar catalyst should yield cis-2-hexene. 

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. 

A hydrocarbon of unknown structure has the formula CgHjQ. On catalytic hydrogenation over the Lindlar catalyst, 1 equivalent of H2 is absorbed. On hydrogenation over a palladium catalyst, 3 equivalents of H2 are absorbed. 

Lindlar catalyst (Section 8.5) A hydrogenation catalyst used to convert alkynes to cis alkenes. 

Triple bonds can be reduced, either by catalytic hydrogenation or by the other methods mentioned in the following two sections. The comparative reactivity of triple and double bonds depends on the catalyst. With most catalysts, (e.g., Pd) triple bonds are hydrogenated more easily, and therefore it is possible to add just 1 mol of hydrogen and reduce a triple bond to a double bond (usually a stereoselective syn addition) or to reduce a triple bond without affecting a double bond present in the same molecule. A particularly good catalyst for this purpose is the Lindlar catalyst (Pd-CaCOs—PbO). An alternative catalyst used for selective hydrogena-... 

Redox-type reactions show by far the worst performance in meeting the golden atom economical threshold. Three reductions meet this criterion with (AE)min values of 1 hydrogenation of olefins using the Lindlar catalyst (1952), Noyori stereoselective hydrogenation reaction (1985), and Zincke disulphide cleavage reaction (1911) whereas, oxidations... 

The synthesis of Pd/ACF (0.42wt.% Pd) catalyst with monodispersed nanoparticles carried out at cuo = 3 is illustrated, as well as its catalytic performance in a liquid-phase hydrogenation of 1-hexyne in comparison with a traditional powdered Lindlar catalyst. 

Catalytic behavior of the synthesized material is superior in comparison with a traditional hydrogenation catalyst which is a powdered Lindlar catalyst (5%Pd-3.5%Pb/CaC03), as can be seen from Figure 9(a) and (b). 

The used Pd/ACF catalyst shows a higher selectivity than the fresh Lindlar catalyst, for example, 94 1% versus 89 + 2%, respectively, at 90% conversion. The higher yield of 1-hexene is 87 + 2% with the used catalyst versus 82 + 3% of the Lindlar in a 1.3-fold shorter reaction time. Higher catalyst activity and selectivity is attributed to Pd size and monodispersity. Alkynes hydrogenation is structure-sensitive. The highest catalytic activity and alkene selectivity are observed with Pd dispersions <20% [26]. This indicates the importance of the Pd size control during the catalyst preparation. This can be achieved via the modified ME technique. 

Industrial synthesis of vitamin A (Hoffman-La-Roche) goes through partial hydrogenation of an enyne (equation 161)277. A number of syntheses of pheromones, where the reduction of an enyne to a diene is the key step, have been devised. A few selected examples are given in Table 29278. During the total synthesis of endiandric acids, Nico-laou employed hydrogenation of a polyenyne intermediate with a Lindlar catalyst to generate an intermediate which underwent symmetry-allowed cyclizations to result in the natural product (equation 162)279. 

Lindlar catalyst is highly effective in preferential hydrogenation of triple bonds to cis double bonds. 

The last step, in the synthetic direction, would therefore be the catalytic hydrogenation of a triple bond to a double bond, a reaction that performed with Lindlar catalyst would assure the correct (Z)-configuration. 

On the other hand, some compounds slow down the uptake of hydrogen and may even stop it at a certain stage of hydrogenation. Addition of lead acetate to palladium on calcium carbonate makes the catalyst suitable for selective hydrogenation of triple to double bonds (Lindlar catalyst) [36] (Procedure 9, p. 206). 

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