Acceptorless Amine Dehydrogenation and Transamination Using Pd-Doped Hydrotalcites

Abstract

Voutchkova -Kostal, Adelina (2018): Acceptorless amine dehydrogenation and transamination using Pd-doped layered double hydroxides. ChemRxiv. Preprint.The synthesis, characterization, and activity of Pd-doped layered double hydroxides (Pd-LDHs) for for acceptorless amine dehydrogenation is reported. These multifunctional catalysts comprise Brønsted basic and Lewis acidic surface sites that stabilize Pd species in 0, 2+, and 4+ oxidation states. Pd speciation and corresponding cataytic performance is a strong function of metal loading. Excellent activity is observed for the oxidative transamination of primary amines and acceptorless dehydrogenation of secondary amines to secondary imines using a low Pd loading (0.5 mol%), without the need for oxidants. N-heterocycles, such as indoline, 1,2,3,4-tetrahydroquinoline, and piperidine, are dehydrogenated to the corresponding aromatics with high yields. The relative yields of secondary imines are proportional to the calculated free energy of reaction, while yields for oxidative amination correlate with the electrophilicity of primary imine intermediates.Reversible amine dehydrogenation and imine hydrogenation determine the relative imine:amine selectivity.Poisoning tests evidence that Pd-LDHs operate heterogeneously, with negligible metal leaching; catalysts can be regenerated by acid dissolution and re-precipitation. a The synthesis, characterization, and activity of Pd-doped layered double hydroxides (Pd-LDHs) for for acceptorless amine dehydrogenation is reported. These multifunctional catalysts comprise Brønsted basic and Lewis acidic surface sites that stabilize Pd species in 0, 2+, and 4+ oxidation states. Pd speciation and corresponding cataytic performance is a strong function of metal loading. Excellent activity is observed for the oxidative transamination of primary amines and acceptorless dehydrogenation of secondary amines to secondary imines using a low Pd loading (0.5 mol%), without the need for oxidants. N-heterocycles, such as indoline, 1,2,3,4-tetrahydroquinoline, and piperidine, are dehydrogenated to the corresponding aromatics with high yields. The relative yields of secondary imines are proportional to the calculated free energy of reaction, while yields for oxidative amination correlate with the electrophilicity of primary imine intermediates. Reversible amine dehydrogenation and imine hydrogenation determine the relative imine:amine selectivity. Poisoning tests evidence that Pd-LDHs operate heterogeneously, with negligible metal leaching; catalysts can be regenerated by acid dissolution and re-precipitation.

Microwave‐Assisted Decarbonylation of Biomass‐Derived Aldehydes using Pd‐Doped Hydrotalcites

“…Pd‐HTs were synthesized by using a previously described mesoscale continuous‐flow method (Figure S1 in the Supporting Information), which yielded materials with reproducible elemental composition, surface area, and acid–base properties. Four Pd‐HT catalysts ( A , B , C , and D ) were synthesized with nominal loadings of 5, 1, 0.5, and 0.1 mol % Pd, respectively, by varying the metal precursor concentrations (Table S1 in the Supporting Information).…”

“…The above literature precedents suggest that potential improvements in catalytic activity could be achieved by using basic and tunable supports that afford high metal dispersion. Given our prior work on a class of palladium catalysts incorporating tunable hydrotalcite (HT) solid bases, we investigated whether these materials offer advantageous activity and selectivity for the decarbonylation of biomass‐derived and synthetic aldehydes. HTs are a subset of layered double hydroxides (LDHs) composed of metal hydroxide sheets sandwiched by weakly bound anions and water .…”

Selective Acceptorless Dehydrogenation of Primary Amines to Imines by Core–Shell Cobalt Nanoparticles

“…A few heterogeneous systems based on noble metals have been developed for this process. For example, Pd nanoparticles supported on basic hydrotalcites allow the selective synthesis of secondary imines via acceptorless dehydrogenation . In addition, a non‐noble metal iron‐based catalyst was used to prepare several primary amines via acceptorless dehydrogenation, which is active only in the presence of a strong base ( t ‐BuOK) .…”

Selective Acceptorless Dehydrogenation of Primary Amines to Imines by Core–Shell Cobalt Nanoparticles

“…After 12 h, the imine was obtained in 96 % yield. Importantly, there is no evidence for the formation of the N‐alkylated secondary amine and tertiary amine, resulting from the further hydrogenation of the imine using the released hydrogen, which are detected in reactions employing other heterogeneous and homogeneous catalysts . The imine is stable beyond 12 h, implying the reverse hydrogenation is not possible.…”

“…A few heterogeneous systems based on noble metals have been developed for this process. For example, Pd nanoparticles supported on basic hydrotalcites allow the selective synthesis of secondary imines via acceptorless dehydrogenation . In addition, a non‐noble metal iron‐based catalyst was used to prepare several primary amines via acceptorless dehydrogenation, which is active only in the presence of a strong base ( t ‐BuOK) .…”