Selective hydrogenation of conjugated diene and triene compounds is an important process in pharmaceutical and industrial fields. Transition metals and their complexes have been extensively employed in such reactions due to their high reactivity and selectivity. Best known examples are rhodium-based Wilkinson’s catalyst and various palladium complexes, which are used in many reactions such as hydrogenation, isomerization, decarboxylation, etc. Our research group previously reported that the thiosulfate synthesis method instead of the Brust-Schiffrin method could generate catalytically active Pd nanoparticles (PdNP) capped with a lower density of alkanethiolate ligands. These unsupported PdNP catalysts exhibited an excellent selectivity towards the isomerization of allylic alcohols to carbonyls in organic solvents. PdNP were also found to be selective for the isomerization of terminal alkenes to internal alkenes, which was somewhat different from the traditional Pd-based catalysts. Since PdNP catalysts might offer several advantages such as less metal-ligand dissociation and easier separation over the currently available organometallic catalysts, it is necessary to further explore the catalytic selectivity of PdNP and discover novel synthetic routes for functional materials and fine chemicals. Herein, we report the synthesis and catalytic investigation of PdNP capped with octanethiolate ligands (PdNP-C8). These PdNP-C8 were characterized by thermogravimetric analysis (TGA), proton NMR, transmission electron microscopy (TEM), IR, and gas adsorption using the Brunauer-Emmett-Teller (BET) model. The partially poisoned characteristic of PdNP-C8 allowed selective hydrogenation of terminal conjugated dienes into internal monoenes and trienes into dienes. The mechanistic studies suggested two major routes for the conjugated diene hydrogenation, which involve either 1,2-addition or 1,4-addition of hydrogen. The major product generated from the catalytic reaction of dienes using PdNP-C8 was the 1,4-addition hydrogenation product, a thermodynamic product. An equilibrium between 1,2 and 1,4-addtion product was also observed.

Selective hydrogenation of dienes and trienes using partially poisoned palladium nanoparticle catalysts

253rd National Meeting (2017)

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