Nanoparticulate Rh and Pd Catalysts

Aldrich ChemFiles 2007, 7.5, 13.

The mild hydrogenation of a variety of arenes—such as benzene, mono- and disubstituted benzenes, naphthalene, and quinoline— has been achieved by using rhodium nanoparticles entrapped in a highly porous and fibrous boehmite (Al(O)OH) matrix (Table 1).1 The Rh/boehmite system has performed very favorably when compared to other commonly used hydrogenation catalysts such as Rh/Al2O3, with reductions occurring rapidly at room temperature under H2 at atmospheric pressure. It also offers the added advantages of an effortless catalyst recovery by simple filtration after the hydrogenation is completed and the ability to perform the reaction without added solvent. The recovered catalyst has been reused ten times without a noticeable loss of activity.

Table 1.

Aluminum hydroxide entrapped palladium nanoparticles (Pd/Al(O)OH) serve as a versatile heterogeneous catalyst that can be applied to a variety of reaction paradigms with low catalyst loadings. The easily recyclable catalyst is active in many organic solvents as well as in water.

As shown in Table 2, palladium-catalyzed aerobic oxidation of a variety of aryl- and alkylcarbinols using Pd/Al(O)OH occurs at high yields to give ketones or aldehydes, often with superior results relative to existing heterogeneous catalysts (e.g., Pd/C, Pd/Al2O3).2 Diols are also viable substrates for the oxidation, leading to lactone products.

Table 2.

Moreover, the catalyst is also effective in alkene hydrogenation, with reactions proceeding rapidly at room temperature. As illustrated in Scheme 1, both processes, catalytic oxidation and catalytic reduction, were employed in the one-pot conversion of cholesterol to cholestan-3-one using the same Pd catalyst.

Scheme 1.

This catalyst demonstrates enormous promise in the selective a-alkylation of ketones with primary alcohols (Scheme 2).3 Reactions performed in the presence of oxygen provide conjugated enone adducts, while anaerobic conditions result in the preferential formation of ketones. Importantly, since no additives (e.g., strong bases) are required to achieve the alkylation, there are no salt byproducts to remove after reaction work-up and the catalyst can be recycled by a simple filtration of the reaction mixture.

Scheme 2.

Finally, the catalyst, in conjunction with lipase, has been applied to the dynamic kinetic resolution (DKR) of primary amines to give acylated amines with excellent yields and selectivities (Scheme 3).4 DKR is a powerful method for converting a racemic mixture into a single enantiomer. Racemic benzylic amines undergo DKR with a low Pd catalyst loading (1 mol %), and either ethyl acetate or ethyl methoxyacetate can function as acyl donors with similar yields. The latter donor allows for a reduced enzyme loading. Aliphatic amines can also participate in a high-yielding, highly selective DKR at increased Pd loadings.

Scheme 3.

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  1. Park, I. S. et al. Chem. Commun. 2005, 5667.
  2. Kwon, M. S. et al. Org. Lett. 2006, 7, 1077.
  3. Kwon, M. S. et al. Angew. Chem., Int. Ed. 2005, 44, 6913.
  4. Kim, M.-J. et al. Org. Lett. 2007, 9, 1157.

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