A Lindlar catalyst is a heterogeneous catalyst that consists of palladium deposited on calcium carbonate or barium sulfate and treated with various forms of lead. The lead additive serves to deactivate the palladium sites. A variety of "catalyst poisons" have been used including lead acetate and lead oxide. The palladium content of the catalyst is usually 5% by weight. The catalyst is used for the hydrogenation of alkynes to alkenes (i.e. without further reduction into alkanes).Thus if a compound contains a double bond as well as a triple bond, only the triple bond reduces to a double bond as opposed to both bonds being reduced down to a single bond.
As described by its inventor Herbert Lindlar, the catalyst is prepared by reduction of palladium chloride in a slurry of calcium carbonate followed by adding lead acetate. By this approach, one obtains a catalyst with a large surface area. Further deactivation of the catalyst with quinoline enhances its selectivity, preventing formation of alkanes. An example of alkyne reduction is the reduction of phenylacetylene to styrene. A recent description on how the poisons act on the selectivity of the Lindlar catalyst identifies a complementary role of the poisons while Pb reduces the amount of hydrogen content in the bulk of Pd, quinoline blocks the formation of oligomers.
Alkyne reduction is stereoselective, occurring via syn addition to give the cis-alkene. An example of commercial use is the organic synthesis of vitamin A which involves an alkyne reduction with the Lindlar catalyst.
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