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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).
In many cases, highly empirical modifications involve selective "poisons". Thus, a carefully chosen catalyst can be used to hydrogenate some functional groups without affecting others, such as the hydrogenation of alkenes without touching aromatic rings, or the selective hydrogenation of alkynes to alkenes using 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.
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.
The so-called "canopy catalysts" containing tripodal ligands are particularly active and easy to prepare. [7] [8] Thorough experimental and computational studies showed that metallatetrahedranes were isolable but dynamic species within the catalytic cycle. [9] Alkyne metathesis catalyst have also been developed using rhenium(V) complexes. [10]
Palladium on carbon is a common catalyst for hydrogenolysis. Such reactions are helpful in deprotection strategies. Particularly common substrates for hydrogenolysis are benzyl ethers: [5] Other labile substituents are also susceptible to cleavage by this reagent. [6]
Hydrogen without or with a suitable catalyst; e.g. a Lindlar catalyst; Sodium amalgam (Na) Sodium-lead alloy (Na + Pb) Zinc amalgam (Zn) (reagent for Clemmensen reduction) Diborane; Sodium borohydride (Na BH 4) Ferrous compounds that contain the Fe 2+ ion, such as iron(II) sulfate; Stannous compounds that contain the Sn 2+ ion, such as tin(II ...