Palladium, barium sulfate, quinoline, catalyst hydrogenation

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). 

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. 

A heterogeneous catalyst for the hydrogenation of alkynes to cis alkenes. In its most common form, it consists of a thin coating of palladium on barium sulfate, with quinoline added to decrease the catalytic activity, (p. 406)... 

Alternative catalysts such as palladium-on-barium sulfate (poisoned by synthetic quinoline) (24), "P-2" nickel boride (with ethylenediamine) (25), and other nickel catalysts (19c) can be used in place of Lindlar catalyst. However, in our hands selective hydrogenation of triple bonds to Z olefins proceeds with the greatest stereoselectivity with Lindlar catalyst. Palladium-on-barium sulfate (in ethanol with quinoline) can give considerable over-reduction and isomerization to the E isomer (22a). Use of "P-2" nickel boride as the catalyst at room temperature usually gives a. 2% of the J5 isomer (e.g. 23). 

In contrast to Lindlar catalyst we have found that the hydrogenation of an alkyne over ethylenediamine-poisoned "P-2" nickel boride or quinoline-poisoned palladium-on-barium sulfate always gives a minor amount of the saturated hydrocarbon in addition to the olefin. The ratio of saturated hydrocarbon to olefin (about 0.01) also is nearly constant throughout the hydrogenation until the alkyne is consumed, and then it increases. Further reaction of the alkene on the catalyst surface before desorption would explain these results. 

The Intermediate In this synthesis of disparlure, (Z)-2-tridecenol, Is obtained by hydrogenation of 2-trldecynol In the presence of a palladium catalyst poisoned with barium sulfate and quinoline. Oxidation of (Z)-2-tridecenol with tert-butyl hydroperoxlde/tItanium tetralsopropoxlde/D (-)-diethyl tartrate gives (2R, 3S)-epoxytrldecanol In enantiomeric excess reported to be as much as 98 percent after recrystalllzatlon. 

In the second step, the triple bond in 63 is selectively reduced to the cz -alkene using the Lindlar catalyst to form 64. In this case, the Lindlar catalyst is a poisoned heterogeneous palladium catalyst on barium sulfate. The deactivation of the catalyst with quinoline is responsible for the selective hydrogenation to the alkene and not through to the alkane. The reason for the highly stereoselective reduction with syn-addition to the cw-alkene is that one face of the triple bond is shielded by the catalyst surface. 

Cram and Allinger employed in place of the Lindlar catalyst a palladium-on-barium sulfate catalyst poisoned by synthetic quinoline (quinoline from coal tar was unsatisfactory). Thus on hydrogenation of dimethyl 5-decynedioate (I) in methanol the mildly exothermal reaction ceased abruptly after 20 min. with uptake... 

Controlled hydrogenation, in a study of the controlled hydrogenation of acetylenes to r/.s-olcfins. Cram and Allinger38 found palladium on barium sulfate with a trace of pure quinoline (synthetic) to be superior to the Lindlar catalyst in reproducibility and ease of preparation (a sample of quinoline from coal tar was found to be unsuitable for this purpose). In one example 19.2 g of dimethyl 5-decynedioate (1) was hydrogenated with use of 0.4 g. of 5% palladium on barium sulfate and 0.4 g. of synthetic quinoline in 100 ml. of methanol. The mildly exothermal reaction ceased abruptly after 20 min., with absorption of exactly one equivalent of hydrogen. 

Rosenmund Reaction. A catalyst poison prepared from quinoline and sulfur is useful for controlling the reaction of p-naphthoyl chloride (19) with hydrogen gas and palladium on barium sulfate catalyst. If control of the reaction is not maintained by catalyst poisoning to reduce activity, further reduction beyond the desired /3-naphthaldehyde product (20) is often observed (eq 13). ... 

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