Kornél Szőri

Palladium Nanoparticle–Graphene Catalysts for Asymmetric Hydrogenation

We report for the first time the application of palladium nanoparticle-graphene (Pd/Gn) catalysts in the asymmetric hydrogenation of aliphatic α,β-unsaturated carboxylic acids using cinchonidine as chiral modifier. Pd/Gns were prepared by deposition–precipitation from the aqueous phase over graphite oxide and subsequent simultaneous reduction of both the support and the metal precursor with NaBH4. The materials obtained were characterized by ICP optical emission spectroscopy, X-ray diffraction spectroscopy, Raman spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. We demonstrate that the Pd/Gns modified by cinchonidine can act as efficient catalysts in the asymmetric hydrogenation of α,β-unsaturated carboxylic acids for producing optically enriched saturated carboxylic acids.Graphical Abstract

The first case of competitive heterogeneously catalyzed enantioselective hydrogenation of ketones

In competitive racemic hydrogenation of methyl benzoylformate (MBF) + ethyl pyruvate (EP) binary mixture over Pt/Al(2)O(3): k(MBF) > k(EP), but in competitive enantioselective hydrogenation k(MBF) < k(EP); the phenomenon verified for the first time is dependent on the adsorption strength of the surface complexes of various compositions (MBF-Pt, EP-Pt, MBF-CD-Pt, EP-CD-Pt, CD = cinchonidine).

A novel asymmetric heterogeneous catalytic reaction: hydrogenation of ethyl 2-acetoxyacrylate on cinchonidine modified Pd and Pt catalyst

SummaryThe transformations of a-ICN and b-ICN were studied in the presence of hydrogen in AcOH over Pt-alumina catalyst under the conditions of the enantioselective hydrogenation of EtPy (hydrogen pressure 1-30 bar, temperature 298-323 K). It was established that the quinoline skeleton of the alkaloids is hydrogenated even under mild experimental conditions, whereas hydrogenolysis of the oxazacycloalkane structure only takes place at hydrogen pressures exceeding 1 bar. ESI-MS-MS, HPLC-ESI-ion-trap MS and NMR made possible the identification of several hydrogenated cinchona alkaloid derivatives with so far unknown structures. According to these experimental results, the conformation of isocinchona alkaloids remains unchanged under the conditions of the enantioselective hydrogenation of activated ketones, making them suitable for utilization as chiral modifiers of well-defined conformation.

The enantioselective hydrogenation of 5,6-dihydro-2H-pyran-3-carboxylic acid over a cinchona alkaloid-modified palladium catalyst: asymmetric synthesis of a cockroach attractant

A novel application of a cinchona-modified supported Pd catalyst is presented. The key step in the asymmetric synthesis of the cockroach attractant methyl (+)-tetrahydro-2H-pyran-3-carboxylate was the enantioselective hydrogenation of 5,6-dihydro-2H-pyran-3-carboxylic acid over 5% Pd/Al2O3 modified by cinchonidine, which afforded the saturated product in up to 89% optical purity.

Pd/Al2O3-catalysed redox isomerisation of allyl alcohol: application in aldol condensation and oxidative heterocyclization reactions

The application of the Pd/Al2O3 catalyst in allyl alcohol isomerization and subsequent aldol condensation and heterocyclization reactions is described. The activity of Pd/Al2O3 in these transformations is suggested to be due to the participation of the Lewis acidic sites of the support in the activation of the alcohol towards oxidative dehydrogenation by the metal and subsequent hydride transfer. The resulting enol(ate)/aldehyde could undergo further reactions promoted by the acid–base properties of the support. In the aldol condensation reactions of the isomerization product, electron poor aromatic aldehydes and heteroaromatic aldehydes showed the highest activity, while aromatic aldehydes bearing electron donating substituents reacted after transformation to the corresponding N-tosyl imines. 1,2-Disubstituted aromatics gave heterocyclic products in one-pot multistep reaction sequences.

Sonochemical hydrosilylation of 2-substituted cyclohexanones in the presence of Wilkinson complex.

The sonochemical hydrosilylation of alkyl substituted cyclohexanones catalyzed by Rh (PPh3)3Cl is described. The application of ultrasonic irradiation during the reaction highly enhances the reaction rates and produces higher yields than those obtained in silent reactions. The stereoselectivity of the reaction has not been changed, the isomeric distribution, i.e. the cis/trans ratio of the 2-substituted cyclohexanols formed, is very close to those obtained in conventional stirred reactions.

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