Gábor London

Palladium on Polydopamine: Its True Potential in Catalytic Transfer Hydrogenations and Heck Coupling Reactions

The application of Pd–polydopamine and magnetic Fe3O4@Pd–polydopamine catalysts in catalytic transfer hydrogenation reactions and the Heck arylation is reported. The reduction of a wide range of aromatic nitro‐compounds bearing both electron‐donating and ‐withdrawing substituents to the corresponding anilines could be efficiently performed, although the reduction of carbonyl compounds was found to be less general. In the latter case, only aromatic ketones could be reduced to the corresponding alcohols, whereas aldehyde substrates were unaffected, which may be owing to their reaction with the catalyst support leading to catalyst deactivation. By using magnetic Fe3O4@Pd–polydopamine system, facilitated catalyst recovery and reuse for five consecutive cycles without considerable loss of activity in nitro‐group reduction. The efficiency of the catalyst in Heck reactions was comparable to that in transfer hydrogenation, however, no catalytic activity was observed upon reuse in this case, likely as a result of metal leaching. We also explored tandem Heck reaction/catalytic transfer hydrogenation sequences, however, the two reactions showed limited compatibility under the applied conditions.

Effect of Particle Restructuring During Reduction Processes Over Polydopamine-Supported Pd Nanoparticles

The effect of catalyst restructuring on the polydopamine-supported Pd catalyzed transfer hydrogenation of ethyl 4-nitrobenzoate and the catalytic hydrogenation of (E)-2-methyl-2-butenoic acid is reported. Transmission electron microscopy investigation of different catalyst pre-treatment and reaction conditions revealed high catalytic activity in both reactions unless drastic aggregation of the active metal occurred. In the transfer hydrogenation reaction aggregation was primarily dependent on the H-source used, while in the catalytic hydrogenation additives in combination with the reductive environment led to extensive Pd aggregation and thus decreased catalytic activity. The enantioselective hydrogenation of (E)-2-methyl-2-butenoic acid showed increased enantioselectivity and decreased conversion with increased particle size.

Cyanation of aryl bromides with K4[Fe(CN)6] using polydopamine supported Pd nanoparticle catalysis: formation of magnetite during the reaction

We report the application of polydopamine (PDA) supported Pd nanoparticles in the cyanation of aryl halides with inexpensive and non-toxic K4[Fe(CN)6]. We found that Pd/PDA is an efficient catalyst in the reaction of electron poor aryl halides, however, in other cases the addition of tetrabutyl ammonium bromide was necessary for conversion. The main limitation of the system is suggested to be catalyst deactivation due to Pd aggregation, polymer degradation/functionalization under the high temperature required for catalysis. In several cases we found the formation of magnetite upon reaction, suggesting that K4[Fe(CN)6] is giving away all its six CN groups in cyanation reaction.

Application of hydroxyproline derivatives in enantioselective α-amination reactions in organic and aqueous environments: a structure-activity relationship study

We examined the activity of a series of L-hydroxyproline derivatives in enantioselective α-amination reaction between diethyl azodicarboxylate and propanal both in organic and aqueous media. In organic media most of the catalysts showed high activity and enantioselectivities comparable to that accessible with L-proline that is among the best catalysts in the reaction. The catalysts showed good activity under aqueous conditions as well; however, only low enantioselectivities were obtained in this case, primarily due to the racemisation of the product under the reaction conditions. Thus, the attempted achiral acid/base additive-driven stereocontrol was not feasible on a practical level.

Exploring Pd/Al2O3 Catalysed Redox Isomerisation of Allyl Alcohol as a Platform to Create Structural Diversity

We report our results on exploiting the different reactivities present in the catalytic cycle of the Pd/Al2O3 catalyzed redox isomerization of allyl alcohol. We show that the reactivity of allyl alcohol derived acrolein and enol can be involved in further cascade reactions leading to a diverse set of products. While the oxidation product acrolein can react via Michael and oxa-Michael reactions, the isomerization product enol can be readily involved in aldol condensation processes. Salicylaldehydes, that are able to react on their electrophilic carbonyl and nucleophilic OH-groups with allyl alcohol derived enol and acrolein, respectively, are used to explore conditions where the structure of the product heterocycles can be controlled.Graphical Abstract