Lindlar palladium catalysts

In subsequent equations, we will not specify the components of the Lindlar palladium catalyst in detail but will simply write Lindlar Pd over the reaction arrow. 

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... 

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 s catalyst metallic palladium deposited on calcium carbonate and is poisoned with lead acetate and quinoline. 

Z)-Methyl styrene was easily obtained by hydrogenation of 1-phenyl-1-propyne using Lindlar s catalyst (5% palladium on calcium carbonate, poisoned with lead) in //-hexane under an atmospheric pressure of hydrogen. The mixture, containing 90% of (Z)-methyl styrene and 10% of the overreduced alkane, was used without further purification. 

The reaction does not appear to be satisfactory with aliphatic aldehydes and also with some aryl aldehydes. Like platinum, coupled with various substances, palladium is also used to catalyse several organic reactions. One such combination is known as Lindlar s catalyst. 

Palladium catalysts are more often modified for special selectivities than platinum catalysts. Palladium prepared by reduction of palladium chloride with sodium borohydride Procedure 4, p. 205) is suitable for the reduction of unsaturated aldehydes to saturated aldehydes [i7]. Palladimn on barium sulfate deactivated with sulfur compounds, most frequently the so-called quinoline-5 obtained by boiling quinoline with sulfur [34], is suitable for the Rosenmund reduction [i5] (p. 144). Palladium on calcium carbonate deactivated by lead acetate Lindlar s catalyst) is used for partial hydrogenation of acetylenes to cw-alkenes [36] (p. 44). 

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. 

Among the important reagents for which preparative procedures are given are 2,2 -bipyridine (by nickel directed and catalyzed dehydrogenation of pyridine p. 5), formamidine acetate (p. 39), phenyltrichloromethylmercury (p. 98), and trimethyl- and triethyloxonium fluoroborate (pp. 120, 113). The preparation of palladium catalyst ( Lindlar ) for the selective reduction of acetylenes is described (p. 89), as is the use of di-phenyliodonium-2-carboxylate, as a precursor of benzyne in the synthesis of 1,2,3,4-tetraphenylnaphthalene (p. 107). 

The preparation of Lindlar s catalyst (palladium on calcium carbonate moderated by treating with lead acetate and quinoline)31 has been described in detail it is used for effecting the partial reduction of an acetylenic bond to an olefin. 

With most hydrogenation catalysts of the platinum and nickel groups (Sections 4.2.61, 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... 

Hydrogenation of an alkyne can be stopped at the alkene stage by using a poisoned (partially deactivated) catalyst made by treating a good catalyst with a compound that makes the catalyst less effective. Lindlar s catalyst is a poisoned palladium catalyst, composed of powdered barium sulfate coated with palladium, poisoned with quinoline. Nickel boride (Ni2B) is a newer alternative to Lindlar s catalyst that is more easily made and often gives better yields. 

X You will meet Lindlar s catalyst in Chapter 31 but we will mention it now because of its special chemoselectivity. Unlike the other hydrogenations we have described, the Lindlar catalyst will hydrogenate alkynes to alkenes, rather than alkenes to alkanes. This requires rather subtle chemoselectivity alkenes are usually hydrogenated at least as easily as alkynes, so we need to be sure the reaction stops once the alkene has been formed. The Lindlar catalyst is a palladium catalyst (Pd/CaC03) deliberately poisoned with lead. The lead lessens the activity of the catalyst and makes further reduction of the alkene product slow most palladium catalysts would reduce... 

This pure Z-alkene was needed for studies on the mechanism of a rearrangement reaction. In Chapter 24 you met catalytic hydrogenation as a means of reducing alkenes to alkanes, and we introduced Lindlar s catalyst (palladium and lead acetate on a support of calcium carbonate) as a means of controlling chemoselectivity so that alkynes could be reduced to alkenes. What we did not empha-... 

Palladium catalysts have been shown to be selective in this hydrogenation. Hanika et al. studied the hydrogenation of 1,5-COD with 0.56% Pd-y-Al203 and two Lindlar catalysts, Pd-CaC03 (Farmakon, 5% Pd) and Pd-CaC03 (Engelhard), as catalysts in heptane at 27°C and 1 atm H2.73 The kinetic constants have been determined accord-... 

In a patent dealing with the selective hydrogenation of alkynols, use of palladium catalysts in combination with lower aliphatic amines such as butylamine, ethanolamine, and ethylenediamine, or in liquid ammonia was claimed to be more effective than use in the presence of higher amines, and superior to Lindlar catalyst in both activity and selectivity.43 Thus, linalool was obtained almost quantitatively by hydrogenation of 3,7-dimethyl-6-octen-l-yn-3-ol over Pd-CaC03 in the presence of butylamine (eq. 4.13). 

If Lindlar s catalyst is used in the reaction, the product is a cis compound. Lindlar s catalyst is a partially deactivated catalyst consisting of barium sulfate, palladium and quinoline. 

However, by partially poisoning the palladium catalyst supported on calcium carbonate with lead acetate and quinoline (the Lindlar catalyst ) it is possible to reduce alkynes to... 

CATALYSTS Chlorotrisftriphenylphos-phine)rhodium(I). (S)-a-(R)-2-Di-phenylphosphinoferrocenyl ethyldl-methylamine. Hydridotrisftriisopropyl-phosphine)rhodium(I). Iridium black. 2,3-0-Isopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino)butane. Lindlar catalyst. Nickel-Alumina. Palladium catalysts. Raney nickel. Rhodium oxide-Platinum oxide. 

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