[7440-05-3] · H2O2Pd · Palladium(II) Hydroxide · (MW 140.44)
(catalyst for hydrogenolysis of benzyl groups and epoxides; used to reduce nitro compounds to amines and as dehydrogenation reagent)
Alternate Name: Pearlman catalyst.
Solubility: insol all organic solvents.
Form Supplied in: black powder, typically containing 20 wt % of Pd and 10-15 wt % of H2O.
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. If a filter aid is necessary, a cellulose-based material should be used if catalyst recovery is desired.
Palladium hydroxide on carbon has been used for difficult debenzylations when other supported palladium catalysts were not effective or satisfactory.1 For example, although para- and meta-substituted benzyl alcohols were easily hydrogenolyzed with 10% Palladium on Carbon, hydrogenolysis of the ortho-substituted substrate was only effective with 20% Pd(OH)2/C (eq 1).2 This catalyst also catalyzes hydrogenation and hydrogenolysis of other functional groups.
The transfer hydrogenolysis of a benzyl ether proceeded in
high yield without the hydrogenolysis of the chlorine or the hydrogenation of the alkene (eq 2).3 Evidence showed that an oxidative mechanism, rather than the usual reductive mechanism, might be involved in this case.
Deprotection of a benzyl carbamate and concomitant reduction of a double bond with the Pearlman catalyst has provided an amino acid in 82% yield (eq 3).4
Debenzylations of N-benzylated imidazole derivatives are very difficult, but in the case of the imidazole derivative in eq 4, hydrogenolysis proceeded to give 84% yield of the desired product. The N-N bond was not cleaved under the reaction conditions, but the formyl group was hydrogenolyzed (eq 4).5
An N-protected azete hydrochloride was deprotected in EtOH to provide the azete hydrochloride in 86% yield. The azete ring was stable under the reaction conditions. This intermediate was converted to d-coniceine in four steps (eq 5).6
Excellent regioselective opening of an epoxide has been accomplished with the Pearlman catalyst. The epoxide opening with Lithium Aluminum Hydride (LAH) gave no selectivity and a 50/50 mixture of the triol isomers was obtained. Hydrogenolysis with the Pearlman catalyst gave 99% selectivity. An equivalent weight of catalyst vs. substrate must be used to obtain the excellent 92% yield (eq 6).7
Deuterium studies showed that the ring opening proceeded with inversion of configuration at C-3 to give the deuterated product (1). This is a common observation with Pd catalysts.
The reduction of a variety of azides to amines was carried out under transfer hydrogenation conditions with hydrazine and Pearlman catalyst in refluxing MeOH. The yields ranged from 71 to 90%.8
An o-substituted dinitrostyrene has been reduced in THF/HOAc at 150 psi H2 and 50 °C to provide an indole in 75% yield. Interestingly, when the H2 pressure was lowered to atmospheric pressure, the reaction proceeded only to 30% conversion to give an unidentified intermediate (eq 7).9
The reagent has also been used as a dehydrogenation catalyst. The ketone shown in eq 8 was converted to the enone in 79% yield. trans-4-Phenyl-3-buten-2-one was obtained in 50% conversion in toluene from 4-phenylbutan-2-one, but no dehydrogenation was observed with propiophenone, indanone, a-tetralone, or cyclohexanone.10
Anthony O. King & Ichiro Shinkai
Merck Research Laboratories, Rahway, NJ, USA