[7440-05-3] · Pd · Palladium on Calcium Carbonate (Lead Poisoned) · (MW 106.42)
Alternate Name: Lindlar catalyst.
Solubility: insol most organic solvents; incompatible with water or acids.
Form Supplied in: dark gray to black powder, typically 5 wt% of palladium.
Analysis of Reagent Purity: atomic absorption.
Handling, Storage, and Precautions: can be stored safely in a closed container under air but away from solvents and potential poisons such as sulfur- and phosphorus-containing compounds. The reagent is pyrophoric in the presence of solvents. General precautions for handling hydrogenation catalysts should be followed. The catalyst must be suspended in the organic solvent under an atmosphere of N2. During filtration the filter cake must not be allowed to go dry.
Lead-poisoned palladium on calcium carbonate, also known as Lindlar catalyst, is mainly used for the selective conversion of alkynes to cis-alkenes.1 The catalyst has been used frequently in the presence of an amine. The hydrogenation in most cases stops sharply after one equiv of hydrogen uptake. For example, the reduction of 1,2-epoxydec-4-yne over Lindlar catalyst and quinoline gave (Z)-1,2-epoxydec-4-ene in 95% yield. The epoxide was stable under the reaction conditions.2 Terminal alkynes can be reduced selectively to 1-alkenes. The ease of reduction was established to be 1-alkyne > internal alkyne > 1-alkene.3 Conjugated enynes can be reduced smoothly to the corresponding dienes with excellent selectivity (eq 1).4
Alkynes in conjugation with one or two carbonyl functions are also reduced smoothly to the alkene. Lactone formation was observed when the hydroxyalkynoic acid shown in eq 2 was hydrogenated.5
The reduction of 1-octynyldiisopropoxyborane with Lindlar catalyst in 1,4-dioxane with a small amount of pyridine gave a 95:5 (cis:trans) ratio of vinyl boronates. High selectivity was also observed with a variety of 1-alkynyl-1,3,2-dioxaborinanes under the above optimized reaction conditions.6 It is important to note that other catalysts, such as Palladium on Barium Sulfate, Nickel Boride, and, more recently, Palladium on Poly(ethylenimine), have also been used to effect the reduction of alkynes to cis-alkenes with excellent results and are sometimes superior to Lindlar catalyst.7
Azides are readily hydrogenated to amines with this catalyst. As expected, alkenic groups are stable under the reaction conditions.8
Under catalytic transfer hydrogenation conditions, nitrobenzenes and azoxybenzenes are converted to hydrazobenzenes (eq 3).9
The catalyst has also been used in oxidation reactions. Methacrolein in MeOH was oxidized to give 93% yield of methyl methacrylate with a selectivity of 95% at 98% conversion.10
Anthony O. King and Ichiro Shinkai
Merck Research Laboratories, Rahway, NJ, USA