Lindlar s catalyst

Hydrogenation with Lindlar s catalyst gives cis-olefins... 

Dibromo-5,6,7,8-tetrafluoroquinoxaline (187) gave 5,6,7,8-tetrafluoroqui-noxaline (186) [H2, Pd/CaCOs (deactivated. Lindlar s catalyst), EtsN CH2CI2, 3 days 44%] or 5,6,7,8-tetrafluoro-l,2,3,4-tetrahydroquinoxaline (188) (H2, Pd/C, EtsN, CH2CI2, 2.5 days >95%) note the selective dehalogenation in both procedures. ... 

A keto thioester was also efficiently reduced to a keto aldehyde by the previously-mentioned Fukuyama hydrosilylation procedure using Lindlar s catalyst (Pd/CaC03/PbO) in acetone at room temperature.411... 

Pd(OH)2/C catalyst was also used for the reduction of azides.593-595 Ra-Ni596,597 and Pt02598 were also used for the reduction of azides. Lindlar s catalyst was used for the reduction in the presence of double bonds (Scheme 4 154) 599-601... 

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

It however failed to react with methylenetriphenylphosphorane. Attempted partial hydrogenation of 65 using Lindlar s catalyst or 10% palladised charcoal led to none of the expected [13]annulenone e.g. 68. This may be due to the potential instability of the [13]annulenone which like its lower vinylogue cyclopentadienone 5 may undergo ready self-condensation. 

A hydroboration-protonolysis procedure for the conversion of conjugated enynes to dienes is far superior to partial hydrogenation over Lindlar s catalyst, in terms of stereoselectivity and yields280. Ratovelomanana and Linstrumelle reported the synthesis of methyl a-eleostearate (equation 163) and methyl punicate by employing this strategy280. 

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. 

Reduction of enynes to (Z)-atkenes. Lindlar s catalyst is not useful as a hydrogenation catalyst for reduction of trienynes or of dienediynes. The best results can be obtained in CH3OH with zinc activated by successive treatment with Cu(OAc)2 (10%) and AgN03 (10%). This reduction results in conversion of the triple bond to a (Z)-double bond. The system does not reduce simple, nonactivated alkynes, and a-branched enynes are reduced slowly. The reduction is effected at 25° with (Z)-enynes, but temperatures of 45° are necessary for the (E)-isomers. Yields of pure tetraenes are 25-65%. 

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). 

Lindlar s catalyst (5% Pd on CaC03, poisoned with Pb, 250 mg) and the mixture was hydrogenated at 0.2 MPa for 35 min at rt. TLC showed the conversion of azide [R/0.48 (CHCI /MeOH 10 1)] into amine (Rf 0.21), and showed evidence of O- to JV-acetyl migration, but no evidence of an anomeric mixture. The catalyst was filtered off and washed with MeOH, and removal of the solvent gave the amine as colorless oil. The crude oil was used immediately for amino acid coupling. 

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. 

Lead tetraacetate oxidation, 242 Lead tetrafluoride, 452 Lindlar s catalyst, 132 Liquid deuterioammonia-O-deuterio-alcohol, 165... 

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