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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 alkynes to alkenes (i.e. without further reduction into alkanes). It is named after its inventor Herbert Lindlar, who discovered it in 1952.
Poisoning refers specifically to chemical deactivation, rather than other mechanisms of catalyst degradation such as thermal decomposition or physical damage. [1] [2] Although usually undesirable, poisoning may be helpful when it results in improved catalyst selectivity (e.g. Lindlar's catalyst).
Herbert Lindlar-Wilson (15 March 1909 – 27 June 2009), better known as Herbert Lindlar, was a British-Swiss chemist. He is known in particular through the development of his catalyst for hydrogenation , as the Lindlar catalyst bears his name.
After ring closure the new triple bond is stereoselectively reduced with hydrogen and the Lindlar catalyst in order to obtain the Z-alkene (cyclic E-alkenes are available through the Birch reduction). An important driving force for this type of reaction is the expulsion of small gaseous molecules such as acetylene or but-2-yne.
Phenylacetylene is a prototypical terminal acetylene, undergoing many reactions expected of that functional group. It undergoes semihydrogenation over Lindlar catalyst to give styrene. In the presence of base and copper(II) salts, it undergoes oxidative coupling to give diphenylbutadiyne. [6]
This compound was reduced to the allyl alcohol 25 using the Lindlar catalyst and lithium aluminium hydride removed the remaining amide group in 26. An allylic rearrangement to alcohol 27 (isostrychnine) was brought about by hydrogen bromide in acetic acid followed by hydrolysis with sulfuric acid.
When it is finely divided, as with palladium on carbon, palladium forms a versatile catalyst; it speeds heterogeneous catalytic processes like hydrogenation, dehydrogenation, and petroleum cracking. Palladium is also essential to the Lindlar catalyst, also called Lindlar's Palladium. [47]
The reaction, a hydrogenolysis, is catalysed by palladium on barium sulfate, which is sometimes called the Rosenmund catalyst. Barium sulfate has a low surface area which reduces the activity of the palladium, preventing over-reduction. However, for certain reactive acyl chlorides the activity must be reduced further, by the addition of a poison.