Catalyst Poisoning To Enhance Selectivity
Usually, catalyst poisoning is undesirable as it leads to a loss of usefulness of expensive noble metals or their complexes. However, poisoning of catalysts can be used to improve selectivities of reactions.
In the classical "Rosenmund reduction" of acyl chlorides to aldehydes, the palladium catalyst (over barium sulfate or calcium carbonate) is poisoned by the addition of sulfur or quinoline. This system reduces triple bonds faster than double bonds allowing for an especially selective reduction. Lindlar's catalyst is another example — palladium poisoned with lead salts.
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