Palladium-catalyzed hydrogenation reactions are important in industrial chemistry and fine-chemicals manufacture, but precious metal catalysts, such as palladium, are costly. Researchers at Tufts University's School of Arts and Sciences and School of Engineering in Medford, Massachusetts, recently reported on the arrangement of individual atoms in a metal alloy and their ability to catalyze hydrogenation reactions.
The Tufts scientists scattered single atoms of palladium less than half a nanometer wide onto a copper support. For this research, the Tufts team heated small amounts of palladium to almost 1000 °C. At that temperature, individual atoms embedded themselves on the copper surface. A scanning tunneling microscope enabled the team to see how these single atoms dispersed in the copper and how molecular hydrogen could then dissociate at individual, isolated palladium sites and spill over onto the copper surface layer, according to the Tufts release.
Specifically, the researchers used desorption measurements in combination with high-resolution scanning tunneling microscopy to show that the individual, isolated palladium atoms in a copper surface substantially lowered the energy barrier to both hydrogen uptake on and subsequent desorption from the copper metal surface. The hydrogen dissociation at the palladium atom sites and weak binding to copper allowed for very selective hydrogenation of styrene and acetylene as compared with pure copper or palladium metal alone (1).
A team of University of Arkansas (US) researchers reported on using visible-light photocatalysis with a ruthenium catalyst to produce a building block in pharmaceutical synthesis. Specifically, the researchers reported on a visible-light-mediated intermolecular [3+2] cycloaddition of mono- and bicyclic cyclopropylamines with olefins catalyzed by [Ru(bpz)3](PF6)2•2 H2O to produce aminocyclopentane derivatives in good yields. Saturated 5,5- and 6,5-fused heterocycles were obtained in synthetically useful yields and diastereoselectivity (2).
Heterocyclic compounds are important in pharmaceutical applications, and researchers at the California Institute of Technology (Caltech) recently reported on an advance in the synthesis of such compounds. They reported on their work in the enantioselective construction of quaternary N-heterocycles by palladium-catalyzed decarboxylative allylic alkylation of lactams (3).
"We think it's going to be a highly enabling reaction, not only for preparing complex natural products, but also for making pharmaceutical substances that include components that were previously very challenging to make," said Brian Stoltz, professor of chemistry at Caltech, in a Jan. 13, 2012, university press release. "This has suddenly made them quite easy to make, and it should allow medicinal chemists to access levels of complexity they couldn't previously access.
Specifically, the researchers reported on the highly enantioselective palladium-catalyzed decarboxylative allylic alkylation of lactams to form 3,3-disubstituted pyrrolidinones, piperidinones, caprolactams and structurally related lactams. The researchers assert that the synthesis provides a new approach for the asymmetric synthesis of such structures, an important development given the prevalence of quaternary N-heterocycles in biologically active alkaloids and pharmaceutical agents. The researchers reported that the catalysis provided enantiopure quaternary lactams that intercept synthetic intermediates previously used in the synthesis of the Aspidosperma alkaloids, quebrachamine and rhazinilam, but that were previously produced by chiral auxiliary approaches or as racemic mixtures (3).