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.

Formation and growth of palladium nanoparticles inside porous poly(4-vinyl-pyridine) monitored by operando techniques: The role of different reducing agents

“…As discussed in the introduction, alcohols are often used as solvents in reactions catalyzed by Pdbased catalysts [33], or they may be used even as reactants [34][35][36][37][38][39][40][41]. Therefore, it is important to clarify if they are innocent toward the catalyst, or if they may react with it and eventually lead to its final deactivation.…”

“…In other cases, alcohols are used as reactants. For example, alcohol hydrogenation, oxidation and cross-coupling reactions are also catalyzed by palladium-based catalysts, either homogeneous or heterogeneous [34][35][36][37][38][39][40][41]. In this respect, the homogeneous Pd(OAc)2/pyridine system is by far one of the most efficient and selective catalyst for oxidation of alcohols to aldehydes.…”

Hydrogen (H2) sensing and catalysis with organic-stabilized Pd and Pd alloy nanoparticles.

“…51,67 Shon and co-workers recently described the same selectivity with alkanethiolate-capped Pd NPs prepared from Bunte salts. [130][131][132][133] With 12x C8NH 2 Pd NPs, the peak for allyl alcohol also decreased (Frame B), but two peaks appeared at 1.26 and 3.12 min, corresponding to the aldehyde and alcohol, respectively, in about a 50:50 ratio. Figure 6.3 shows the percent hydrogenation, percent isomerization, and total conversion of allyl alcohol at different times when 12x C8NH 2 Pd NPs were used as catalysts.…”

“…In comparison, the anti-Markovnikov addition of Pd−H to the π-bond produces a linear Pd-alkyl intermediate and increases the hydrogenation of allyl alcohol (route b). 133 A major difference of the π-allyl Pd hydride mechanism shown in Frame B is that the reaction does not require the presence of H 2 gas, because the Pd−H adduct is generated from the oxidative addition of Pd to the C−H bond α to the OH group. In both, Type I and II mechanisms, steric and electronic effects may play an important role in the formation of key intermediates such as the σ-bonded complexes or the π-allyl derivatives.…”

“…133 Shon and co-workers showed the isomerization of allyl alcohol to the corresponding carbonyl compound with a relatively high selectivity (95%) compared to other Pd-based catalysts (<75%). [130][131][132][133] In our research we are the first to compare the catalytic activity of various alkylamine-, alkanethiolate-, and mixed ligand organicprotected Pd and PdAg NPs for the hydrogenation and isomerization of allyl alcohol in the presence of hydrogen gas.…”