A Lindlar catalyst is a heterogeneous catalyst consisting of palladium deposited on calcium carbonate or barium sulfate then poisoned with various forms of lead or sulfur. It is used for the hydrogenation of to (i.e. without further reduction into alkanes). It is named after its inventor Herbert Lindlar, who discovered it in 1952.
Synthesis
Lindlar catalyst is commercially available but can also be created by reducing palladium chloride in a slurry of calcium carbonate (CaCO
3) and adding
lead acetate.
A variety of other "catalyst poisons" have been used, including lead oxide and
quinoline.
The palladium content of the supported catalyst is usually 5% by weight.
Catalytic properties
The catalyst is used for the
hydrogenation of
alkynes to
(i.e. without further reduction into
alkanes). The lead serves to deactivate the palladium sites, and further deactivation of the catalyst with
quinoline or 3,6-dithia-1,8-octanediol enhances its selectivity, preventing formation of
. Thus if a compound contains a double bond as well as a triple bond, only the triple bond is reduced.
An example being the reduction of
phenylacetylene to
styrene.
Alkyne hydrogenation is stereospecific, 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. These catalysts are also used in the synthesis of dihydrovitamin K1.[Fritz Weber, August Rüttimann "Vitamin K" Ullmann's Encyclopedia Of Industrial Chemistry, 2012, Wiley-VCH, Weinheim. ]
See also
-
Rosenmund reduction, a reduction using palladium on barium sulfate, poisoned with sulfur compounds.
-
Urushibara Nickel, a nickel based catalyst used to hydrogenate alkynes to alkenes.
