Pd Nanoparticle-Catalyzed Isomerization vs Hydrogenation of Allyl Alcohol: Solvent-Dependent Regioselectivity

Abstract

Our previous work has shown that alkanethiolate-capped Pd nanoparticles generated from sodium S-dodecylthiosulfate are excellent catalysts for selective isomerization of various allyl alcohols to the carbonyl analogues. The present work focuses on understanding the mechanism and the regioselectivity of Pd nanoparticles in different environments. First, the presence of H₂ gas has turned out to be essential for the efficient catalytic isomerization reaction. This suggests that the mechanism likely involves the Pd-alkyl intermediate rather than the η³ π-allyl Pd hydride intermediate. Second, the Pd nanoparticles are found to convert allyl alcohol selectively to either propanal or 1-propanol depending on the type of solvent used for the catalytic reactions. The reaction pathway is most likely determined by steric hindrance, which is the result of the interaction between substrate and alkylthiolate ligands on Pd nanoparticles. Presumably, the conformation of alkylthiolate ligands changes upon the type of solvents, resulting in varying degree of available space close to the nanoparticle surface. In general, nonpolar or weakly polar solvents such as benzene and chloroform, respectively, promote the isomerization of allyl alcohol to propanal via the formation of the branched Pd-alkyl intermediate. On the other hand, polar protic solvents such as methanol and water foster the hydrogenation of allyl alcohol to 1-propanol involving the steric induced formation of a linear Pd-alkyl intermediate. Third, the use of sodium S-hexylthiosulfate instead of sodium S-dodecylthiosulfate for the synthesis of Pd nanoparticles results in nanoparticle catalysts with a lower regioselectivity toward isomerization over hydrogenation. This is due to the higher surface ligand density of hexanethiolate-capped Pd nanoparticles, which negatively impacts the formation of branched Pd-alkyl intermediate. The results clearly indicate that controlling the structure and surface density of alkanethiolate ligands around Pd nanoparticles can provide an opportunity to tune the activity and selectivity of nanoparticle catalysts. Lastly, the high stability of soluble nanocatalysts is demonstrated by recycling dodecanethiolate-capped Pd nanoparticles over 10 times for the isomerization reaction of allyl alcohol.