Erno Tfirst

Formation of multilayered tin organometallic surface species. Preparation of new type of supported Sn–Pt catalysts

Abstract In this study, a new aspect of anchoring of tin organic moieties onto platinum is described and discussed. The new approach resulted in substantial increase of the Sn/Pt ratio in tin modified Pt/SiO2 catalysts. In the earlier approach, adsorbed hydrogen was exclusively used for tin anchoring, resulting in a monolayer of tin organic moieties at the top of platinum. In the new approach, a large excess of either tin tetraethyl or hydrogen was used in the tin-anchoring reaction. The presence of coadsorbents, such as oxygen, also led to a substantial increase of the amount of tin anchored. When the primary formed –SnR3 surface entities were partially decomposed, the coordinatively formed unsaturated –SnR2 and –SnR surface species provided additional anchoring sites for the next layer of SnR4. In the presence of adsorbed oxygen, additional new types of landing sites were created to anchor SnR4 in the neighborhood of platinum. The above approach resulted in Sn–Pt/SiO2 catalysts with exclusive tin–platinum interaction and an Sn/Pt ratio ca. 2. Results obtained in this study also reveal that the formation of surface organometallic moieties takes place in a stepwise way, e.g. the buildup of tin organic moieties occurs layer-by-layer. The supported Sn–Pt bimetallic entities formed showed both high activity and selectivity in the hydrogenation of crotonaldehyde to crotylalcohol.

Enantioselective hydrogenation of α-keto esters over cinchona-PtAl2O3 catalyst. Kinetic evidence for the substrate-modifier interaction in the liquid phase

Abstract The hydrogenation of ethyl pyruvate was studied over cinchonidine- Pt Al 2 O 3 catalyst. Contrary to earlier results it has been found that the initial enantiomeric excess extrapolated to zero conversion is close to zero. Based on kinetic analysis the results are considered as indirect evidence for the substrate-modifier interaction taking place in the liquid phase. The above interaction leads to the formation of a weak substrate-modifier complex. The formation of the complex in the liquid phase is the key step to control both the rate acceleration and the induction of enantio-differentiation in the hydrogenation of α-keto esters in the presence of cinchona- Pt Al 2 O 3 catalysts. The character of interactions in the substrate-modifier complex is discussed. By using molecular modelling the possible form of the complex is also given.

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.

Enantio-differentiation over heterogeneous catalysts. The shielding effect model

This paper deals with the origin of enantio-differentiation over heterogeneous catalysts. A new model is proposed, in which the modifier provides a specific shielding effect. A prochiral molecule, due to the specific character of shielding, can adsorb onto the metal surface by its unshielded site resulting in enantio-differentiation. As emerges from computer modeling, quantum chemical and quantum mechanical calculations made on the catalytic system: cinchonidine—α-keto esters—Pt the shielding effect can be responsible both for the rate acceleration and induction of enantio-differentiation. This model is based on an earlier proposition, which suggest (i) the formation of a weak complex between the modifier and the substrate in the liquid phase and (ii) the hydrogenation of either the shielded or unshielded forms of α-keto ester over the Pt sites. The shielded form gives the optically active, while the unshielded one the racemic product. Further support for this model was obtained in kinetic experiments and kinetic modeling.

Theoretical study of some lower-lying water hexamer clusters

Abstract Lower-energy water hexamers are studied theoretically at the Hartree–Fock and correlated Moller–Plessett levels with focusing on theoretical spectra and many-body effects of pentacoordinated cage-type structures as possible structural units of the high-density liquid phase of liquid water.

A study of weakly interacting systems in localized representation, including the many-body effect

The method of the separated molecular orbitals (SMOs), as a version of the localized representation is discussed as a useful tool for studying weakly interacting systems. As to the Hartree-Fock level, the contributions of the atomic basis sets in some dimers and their counter-poise (CP) corrected systems are compared. The results suggest, that using SMOs the validity of the CP recipe can be demonstrated. The localized many-body perturbation theory (LMBPT) incorporating the method of SMOs makes it possible to study the van der Waals type systems at several levels of correlation. This allows using the SMO-LMBPT scheme for discussing not only the two-body but the many-body interaction energy quantities as well. As an example, the results obtained for the three-body interaction energies in some He-clusters are given. These results show that the three-body effects are negligible in comparison to the two-body ones, the relative significance of the charge-transfer components, on the other hand, are important.

Asymmetric synthesis of (S)-alkylamines via reductive transamination of ketones over carbon-supported palladium catalysts

Methylethylketone (MEK) and methoxyacetone (MEOAC) were transaminated with benzylamine (BzNH 2 ) or L-(-)-α-methylbenzylamine [L-(-)-α-MeBzNH 2 ] over Pd/C catalysts in the presence of chiral modifiers (L-alanine, L-alaninol, L-phenylalaninol, L-lysine and methyl-, t-butyl- and benzyl-esters of L-alanine) introduced either by incipient wetness impregnation or by equilibrated impregnation onto the catalyst surface. In the first step of the transamination the Schiff base was formed from the ketone and aralkylamine which was then hydrogenated to secondary amine in the second step. In the third step of the transamination the hydrogenolysis of the secondary amine resulting in primary alkylamine and a hydrocarbon was carried out. In the transamination of both MEK and MEOAC on Pd/C catalysts the highest enantiomeric excess was observed in cyclohexane (ee = - 20%-21%) using L-alanine alkyl esters or L-alaninol as modifier. In different solvents the ee of the corresponding primary amine increased in the order: MeOH < Dioxan < H 2 O < CH. Upon using (L)-(-)-α-MeBzNH 2 , as a chiral transaminating agent, (S)-methoxyisopropylamine was obtained, with ee value around 70%. The conformational analysis of N-(methoxy-2-propylidene)-methylbenzylamine strongly supports the validity of Prelogs rule in the enantio- or diastereoselective hydrogenation of Schiff base over heterogeneous catalysts.

Ab initio program for treatment of related systems. Transferable quantities of localized molecular orbitals

Abstract It is well known that transferability is an important property for the investigation of related systems. In cases when one can define quantities for molecular parts, which are additive and transferable, certain similarities of different molecules can be simply recognized. One-electron properties, as electric moments or kinetic energy contributions derivable from transferable/localized molecular orbitals, are especially useful for the above purpose. In this paper we demostrate, that these quantities are convenient for studying not only extended molecules, but weakly interacting systems as well.

Reductive transamination of methoxyacetone with benzylamine over Pd/SiO2 catalyst modified with anchored chiral compounds

Methoxyacetone was transaminated with benzylamine to methoxyisopropylamine over a Pd/SiO2 catalyst modified with L‐alaninol or L‐phenylalaninol covalently anchored to the surface of the support via an organo‐silicon spacer group. In the first step of transamination a Schiff base was formed from the ketone and benzylamine, and then it was hydrogenated in the second step on the chirally modified Pd/SiO2 catalysts to an asymmetric secondary amine (N‐benzyl‐methoxyisopropylamine). In the third step the hydrogenolysis of the asymmetric secondary amine resulting in methoxyisopropylamine and toluene was carried out over a 10 wt% Pd/C catalyst. The highest enantiomeric excess of (S)‐methoxyisopropylamine was observed in cyclohexane (ee = −20–21%) using anchored L‐alaninol as a chiral modifier.

A decomposition of the total energy at the HF-SCF level and at several levels of correlation: IV. A study of the interaction in H2O-AlH2OH + NH3 at the correlated level

Abstract The method of separated molecular orbitals (SMOs) combined with the localized many-body perturbation theory (LMBPT) has already proved useful for partitioning the energy at several levels of correlation in various weakly interacting systems. The energy decomposition into different contributions according to the interacting units is discussed for a zeolite model system in the present paper. The effect of basis set superposition is also considered, using the SMO–LMBPT scheme.