Emília Tálas

The role of cinchona alkaloids in enantioselective hydrogenation reactions: Are they modifiers or hosts involved in supramolecular heterogeneous catalysis?

Abstract In this study experimental evidences are summarized supporting the modifier–substrate interaction taking place in the liquid phase in the enantioselective hydrogenation of α-keto esters and related compounds. The results indicate that the catalytic system cinchona alkaloids-supported platinum (or palladium) can effectively be used in enantioselective hydrogenation for prochiral substrates, in which the prochiral group is part of a conjugated double bond system. It is considered that the above catalytic system is the first example of a new class of heterogeneous catalytic reactions with the involvement of supramolecular catalysis.

Preparation of alumina supported tin-platinum catalysts by reaction of tin tetraethyl with hydrogen adsorbed on platinum. Study of the formation and decomposition of the primary surface complex

In this work surface reactions involved in the preparation of alumina supported tin-platinum catalysts with metal-metal interaction were studied. The preparation of these catalysts is based on surface reaction between tin tetra-alkyls and hydrogen adsorbed on platinum. In the above reaction a Primary Surface Complex (PSC (I)) is formed, which has a twodimensional layer of -SnR3 moieties at the platinum surface. PSC (I) can be decomposed in hydrogen atmosphere with formation of bimetallic surface entities. The fate of the PSC (I) strongly depended on (i) the presence or absence of gas phase hydrogen, (ii) the dispersion of platinum, (iii) the time interval between its formation and decomposition. In addition to the main reactions involved in this preparation side reactions with partial dehydrogenation of the alkyl groups of the -SnR3 moieties were also evidenced. Upon using this approach to prepare tailor made supported bimetallic catalysts these side reactions should be avoided as they are responsible for the formation of surface carbon on the platinum sites.

Natural alkaloids and synthetic relatives as chiral templates of the Orito's reaction.

The enantioselective hydrogenation of methyl or ethyl pyruvate over cinchona-platinum catalyst system (Oritos reaction) is one of the most intensively studied heterogeneous catalytic asymmetric hydrogenation reactions. Studies aiming at systematic changes of the chiral template have played a crucial role in creating hypotheses for the mechanism of Oritos reaction. It is very important to clarify which structural unit of the alkaloid takes part in the enantiodifferentiation, and learn about the role of the different structural units of chiral templates. In this article, we made an attempt to describe the behavior of natural alkaloids, their synthetic derivatives, and analogues as chiral templates in the heterogeneous catalytic asymmetric hydrogenation of activated ketones.

Synthesis and resolution of naphthyl-Tröger's base

Abstract A naphthyl analogue of Trogers base {8 H ,16 H -7,15-methanodinaphtho[2,1- b ][2′,1′- f ][1,5]-diazocine (NTB)} was prepared and successfully resolved using (−)- and (+)-di- p -toluoyl-tartaric acid. Enantiomers obtained show extremely high specific rotations related to (i) the rigid [1,5]-diazocine skeleton with molecular asymmetry and (ii) the presence of condensed aromatic rings, similar to helicenes.

Reactions of methylcyclopentane on Pt/Al2O3 and Sn-Pt/Al2O3 catalysts

Abstract Reaction of methylcyclopentane has been studied on different Sn-Pt/Al2O3 catalysts at 520 °C. Catalysts were prepared by using controlled surface reactions creating either tin-platinum or tin-alumina interactions exclusively. Based on the analysis of the yield-conversion dependences it was established that in addition to the consecutive routes for the formation of methylcyclopentadiene and benzene, direct routes should also exist. Formation of benzene can be strongly hindered by simultaneous introduction of metallic tin into platinum and ionic tin into alumina.

Enantioselective Hydrogenation of α-Keto Esters Over Pt/Al2O3 Catalyst: Kinetic Aspects of the Rate Acceleration Effect Induced by Addition of Cinchonidine

Abstract The enantioselective hydrogenation of ethylpyruvate catalysed by Pt/Al 2 O 3 modified with cinchonidine has been investigated with the aim to gain information about the relevant interactions occurring in this complex catalytic system. Transient kinetic experiments with separate injection of the modifier (cinchonidine) and substrate (ethylpyruvate) into the reactor revealed that the performance of this catalytic system depends significantly on the contacting of the different interacting species. Highest initial hydrogenation rates were measured when cinchonidine was injected into the reactor. In contrast, lowest initial rates were measured with simultaneous introduction of all reaction components prior to the addition of hydrogen. This behaviour is attributed to undesired side reactions of the substrate and the cinchonidine. The experimental results indicate that the interactions occurring in the liquid phase have to be taken into account to explain the global catalytic behaviour of this system.

Enantioselective hydrogenation of ethyl pyruvate over cinchonidine–Pt/Al2O3 catalysts containing anchored silicium organic moieties

Abstract The enantioselective hydrogenation of ethyl pyruvate was investigated over a catalytic system consisting of a Pt/Al 2 O 3 catalyst containing silicium organic moieties anchored to the support and cinchonidine, the chiral template molecule. The –Si(CH 3 ) 2 R moieties (where R=–CH 3 or –C 8 H 17 ) were anchored to the alumina via surface reaction of silicium organic compounds and the surface OH groups of the support. The introduction of –Si(CH 2 ) n R moieties onto the alumina significantly decreased both the rate and the enantioselectivity of the hydrogenation reaction. The decrease of the enantioselectivity indicates that the enantio-differentiation step is more complex than it is predicted by the existing models. The results also show that the enantio-differentiation step cannot be exclusively attributed to the interaction between the half-hydrogenated substrate and cinchonidine taking place at the platinum surface.

Qualitative determination of the efficiency of re-reduction in supported platinum catalysts. Development of a new method

Abstract A simple test reaction has been developed for the qualitative determination of the efficiency of platinum re-reduction or to show the presence of adsorbed oxygen on supported Pt catalysts. The method is based on the surface reaction taking place between tin tetraethyl and unreduced platinum or adsorbed oxygen. The formation of ethylene in the above reaction indicates that supported platinum catalyst contains either unreduced platinum or adsorbed oxygen.

Specific behavior of the p-aminothiophenol--silver sol system in their Ultra-Violet-Visible (UV-Visible) and Surface Enhanced Raman (SERS) spectra.

The UV-Visible and Surface Enhanced Raman Spectroscopy (SERS) behavior of silver sol (a typical SERS agent) were studied in the presence of different bifunctional thiols such as p-aminothiophenol, p-mercaptobenzoic acid, p-nitrothiophenol, p-aminothiophenol hydrochloride, and 2-mercaptoethylamine hydrochloride in diluted aqueous solution. Our results confirm that the p-aminothiophenol induced aggregation of citrate stabilized silver colloid originates from its electrostatic nature, as well as the azo-bridge formation cannot be the reason of the observed time dependent UV-Visible spectra. Based on our parallel SERS and electrospray ionization mass spectrometry measurements, we have concluded that certain amount of oxidized form of the probe molecule has to be present for the so-called b2-mode enhancement in the SERS spectrum of p-aminothiophenol. Our findings seem to support the idea that the azo-bridge formation is responsible for the b2-mode enhancement in the SERS spectrum of p-aminothiophenol.

Modification of NI/Al2O3 Hydrogenation Catalysts with Lead by Using controlled surface reactions

Abstract In the present work alumina supported nickel catalysts modified by lead were used for the selective hydrogenation of acrylonitrile to propionitrile. The unmodified nickel catalyst was not selective in this reaction. The modification of the Ni/Al 2 O 3 catalyst, i.e. the preparation of Pb-Ni/A1 2 O 3 catalysts was carried out by using Controlled Surface Reactions (CSRs). CSRs applied in this work are based on the reactivity of hydrogen adsorbed on the nickel. This hydrogen reacts selectively with tetraethyl lead resulting in bimetallic surface entities. Experimental results obtained both in the surface reactions Involved in the modification of the catalyst and in the study of selective hydrogenation of acrylonitrile strongly indicate that catalyst modification via CSRs is a powerful tool to design selective hydrogenation catalysts.