Catalysts Lindlar catalyst

Hydrogenation Copper chromite (Lazier catalyst). Copper chromium oxide (Adkins catalyst). Lindlar catalyst (see also Lithium ethoxyacetylide, Malealdehyde, Nickel boride). Nickel catalysts. Palladium catalysts. Palladium hydroxide on carbon. Perchloric acid (promoter). Platinum catalysts. Raney catalysts, Rhenium catalysts. Rhodium catalysts. Stannous chloride. Tributylborane. Trifluoroicetic acid, Tris (triphenylphosphine)chlororhodium. 

Historically, the first synthesis of (Z)-isomers of carotenoids became possible after the introduction of a special palladium catalyst (Lindlar catalyst) for the partial hydrogenation of carbon-carbon triple bonds [4]. Thus (15Z)-P,P-carotene [(15Z)-3] [5] and (11Z,1 l Z)-p,13-carotene [(11Z,1 rZ)-3] [6-8] were obtained as intermediates in the synthesis of the (all- 3 target carotene. This partial hydrogenation of triple bonds, proceeding stereospecifically as a syn addition, is still a valuable method for the synthesis of disubstituted double bonds in the (Z)-configuration. The various reactions used to form carbon-carbon double bonds in the construction of the carotenoid skeleton generally proceed with variable stereoselectivity and result in ( 7Z)-mixtures (Chapter 2 Parts I, HI and IV). 

Many monoterpenes are desired fragrances in perfumery and flavors in food. They are produced on a larger scale from acetone (C3) and ethyne (acetylene C2) involving repetitive synthetic steps (Fig. 5). Initially, acetone is ethynylated by acetylene in the presence of a base (sodium hydroxide, amines with sodium carbonate) yielding 3-butyn-2-ol (C5) which is partially hydrogenated in the presence of deactivated catalysts (Lindlar catalysts) to 2-methyl-3-buten-2-ol. This can be converted to the key intermediate 6-methyl-5-hepten-2-one (Cg) via two pathways, either by transetherification with methylpropenylether and subsequent oxa-CoPE rearrangement, or by transesterification with methyl acetoacetate and subsequent Carroll decarboxylation. 

Lindlar Catalyst ( Pd/BaS04/ quinoline)- partially poisoned to reduce activity will only reduce the most reactive functional groups. 

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... 

The structure of quinoline is shown on page 460 In sub sequent equations we will simply use the term Lindlar Pd to stand for all of the components of the Lindlar catalyst... 

Hydrogenation of alkynes may be halted at the alkene stage by using special catalysts Lindlar palladium is the metal catalyst employed most often Hydrogenation occurs with syn stereochemistry and yields a cis alkene... 

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. 

Another example of this preference is found in the enantiospecific syntheses of tritium-labeled leukotrienes(/i). Commercially available 3-nonyn-l-ol was converted to its phosphorane (16) and Wittig-coupled with the unsaturated aldehyde (17) to afford 18, which was reduced over Lindlar catalyst to give 19. 

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. 

Name the following alkynes, and predict the products of their reaction with (i) H2 in the presence of a Lindlar catalyst and (ii) HsO" " in the presence of HgSC ... 

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. 

Lindlar catalyst, 268 Line-bond structure, 9 1—>4 Link, 997... 

Olefins, reaction with nitrones, 46,130 cjs-Olefins f lom disubstituted acetylenes by selective reduction v ith modi fied palladium (Lindlar) catalyst, 46,92... 

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-... 

For a study of the detailed structure of Lindlar catalysts, which were shown to consist of seven distinct chemical phases, see Schlogl, R. Noack, K. Zbinden, H. Reller, A. Helv. 

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