Esa Toukoniitty

Asymmetric Heterogeneous Catalysis: Science and Engineering

Asymmetric heterogeneous catalysis is a vivid branch of catalysis, remaining, however, largely a domain of organic chemists. The view towards asymmetric heterogeneous catalysis adopted in this review is mainly from catalytic science and engineering. Not only reaction mechanisms, but also catalytic properties, kinetic regularities, as well as chemical engineering aspects, are covered with the main focus on recent developments.

Solvent effects in enantioselective hydrogenation of 1-phenyl-1,2-propanedione

Abstract Solvent effects in enantioselective hydrogenation of 1-phenyl-1,2-propanedione ( A ) were investigated in a batch reactor over a cinchonidine modified Pt/Al 2 O 3 catalyst. The effect of different solvents, binary solvent mixtures and solvent dielectric constant on regio- and enantioselectivity as well as on the hydrogenation rate were studied. The hydrogen solubility in different solvents and the dielectic constants of solvent mixtures were measured. The highest enantiomeric excesses (ee) of ( R )-1-hydroxy-1-phenylpropanone ( B ) (65%) were obtained in toluene. The ee decreased non-linearly with an increasing solvent dielectric constant being close to zero in methanol. The role of the reactant conformation in different solvents was evaluated by quantum chemical calculations and the role of the Open(3) conformer of the modifier, cinchonidine was discussed. The dependence of ee on the dielectric constant could not solely be attributed to the abundance of the Open(3) conformer of cinchonidine in the liquid phase. A possible involvement of additional factors was proposed and discussed. The non-linear dependence of the ee on the dielectric constant was included in a kinetic model to describe quantitatively the variation of the ee in different solvents.

Liquid-phase hydrogenation of citral over an immobile silica fibre catalyst

A new knitted silica fibre catalyst was introduced into liquid-phase hydrogenation of organic compounds. The fibre catalyst combines the advantages of catalyst slurries and large catalyst particles: diffusional limitations are suppressed and the catalyst is immobile. Nickel-impregnated silica fibres were used in the hydrogenation of citral to citronellal and citronellol in a tubular glass reactor where the catalyst was packed in two layers. The liquid phase was recirculated while the gas phase was flowing concurrently through the reactor to the vent. The highest selectivities to the desired product, citronellol, exceeded 90%. The catalyst performance was competitive with a conventional crushed nickel catalyst.

Batchwise and continuous enantioselective hydrogenation of 1-phenyl-1,2-propanedione catalyzed by new Pt/SiO2 fibers

Abstract A new knitted silica fiber was investigated as a support material in enantioselective hydrogenation of 1-phenyl-1,2-propanedione in a pressurized batch reactor. The active metal was platinum and the catalyst was modified in situ with (−)-cinchonidine. The catalysts were prepared by impregnation method using hexachloroplatinic acid as metal precursor. The main experimental parameters were the support calcination temperature and the metal loading of the catalyst. The optimum catalyst dispersion, BET specific surface area and mean Pt particle size for enantio-differentiation were measured. A comparison of alumina and silica supports revealed that the main parameters in the enantio-differentiation were the mean metal particle size and the metal dispersion. Optimal metal particle size and dispersion were detected which optimize the enantioselection. Experiments with continuously operated fixed bed reactor demonstrated that continuous hydrogenation, over the new knitted silica fiber catalyst, gives equally high enantiomeric excesses compared to the batch operation. Continuous operation can be used as tool to study catalyst deactivation and reaction mechanisms in enantioselective hydrogenation.

Hydrosilylation of cinchonidine and 9-O-TMS-cinchonidine with triethoxysilane: application of 11-(triethoxysilyl)-10,11-dihydrocinchonidine as a chiral modifier in the enantioselective hydrogenation of 1-phenylpropane-1,2-dione

The detailed synthesis and characterization of (−)-11-(triethoxysilyl)-10,11-dihydrocinchonidine (4), a starting material for the immobilization of (−)-cinchonidine on silica based supports, is described. Compound 4 together with its precursors 9-O-(trimethylsilyl)cinchonidine (2) and 9-O-(trimethylsilyl)-11-(triethoxysilyl)-10,11-dihydrocinchonidine (3) were employed as chiral modifiers in the hydrogenation of a prochiral diketone, 1-phenylpropane-1,2-dione, over a heterogeneous Pt/Al2O3 catalyst using cinchonidine (1) as a reference modifier. The unexpected enhancement of ee induced by 4, demonstrating the positive effect of distal modifier substitution, is discussed in the light of molecular modeling and NMR studies.

Modeling of the enantioselective hydrogenation of 1-phenyl-1,2-propanedione over Pt/Al2O3 catalyst

Abstract A kinetic model was developed for the enantioselective hydrogenation of 1-phenyl-1,2-propanedione based on parallel racemic and enantioselective routes in the presence of cinchonidine. The Langmuir–Hinshelwood type of competitive adsorption approach was used in the model, which was combined with a batch reactor model. The proposed model could sufficiently describe the observed kinetic results.

Effect of cinchonidine and dissolved oxygen in continuous enantioselective hydrogenation of ethyl pyruvate

Continuous hydrogenation of ethyl pyruvate was carried out in a fixed bed reactor over a Pt/SiO2 fiber catalyst with step changes in cinchonidine and oxygen feed. Presence of cinchonidine always led to a rate deceleration and appearance of enantioselectivity. Trace amounts of O2 induced overall rate acceleration. A mechanism is proposed which includes ethyl pyruvate hydrogenation and decomposition.

A combined NMR, DFT, and X-ray investigation of some cinchona alkaloid O-ethers.

Structures and conformational behavior of several cinchona alkaloid O-ethers in the solid state (X-ray), in solution (NMR and DFT), and in the gas phase (DFT) were investigated. In the crystal, O-phenylcinchonidine adopts the Open(3) conformation similar to cinchonidine, whereas the O-methyl ether derivatives of both cinchonidine and cinchonine are packed in the Closed(1) conformation. Dynamic equilibria in solutions of the alkaloids were revealed by combined experimental-theoretical spin simulation/iteration techniques for the first time. In the (1)H NMR spectra in CDCl3 and toluene-d8 at room temperature, Closed(1) conformation was observed for the O-silyl ethers as a separate set of signals. For O-methyl ether derivatives Closed(1) could be separated only at -30 degrees C in CDCl3 or toluene-d8 and for O-phenylcinchonidine at -70 degrees C in CDCl3/CD2Cl2. The ratio between the Closed(2) and Open(3) conformers was estimated by analyzing the vicinal coupling constant (3)J(H9,H8) at ambient and low temperatures. The observed conformational equilibria of O-(tert-butyldimethylsilyl)cinchonidine in CDCl 3 and toluene-d8 are in good agreement with the theoretically estimated equilibrium populations of the conformations according to Boltzmann statistics. The conformational equilibria of four cinchona alkaloid O-ether solutes in CDCl3 and toluene-d8 are discussed in the light of their relevance to the mechanism of 1-phenyl-1,2-propanedione (PPD) hydrogenation over cinchona alkaloid modified heterogeneous platinum catalysts. It was demonstrated that the conformation found to be abundant in the liquid phase has no direct correlation with the enantioselectivity of the PPD hydrogenation reaction.

Experimental and Theoretical Analysis of Asymmetric Induction in Heterogeneous Catalysis : Diastereoselective Hydrogenation of Chiral α-Hydroxyketones over Pt Catalyst

Assessing the origin of asymmetric induction in heterogeneously catalyzed hydrogenation is a challenging task. In this work, hydrogenation of a chiral compound, (R)-1-hydroxy-1-phenyl-2-propanone [(R)-PAC], in toluene over cinchonidine modified and unmodified Pt/Al(2)O(3) was studied. To reveal the detailed reaction mechanism and the origin of stereoselectivity in the Pt-catalyzed hydrogenation of the CO double bond, the structures and energies of several adsorption modes of (R)-PAC as well as whole reaction paths for hydrogenation were investigated on Pt(111) by density functional theory (DFT). In agreement with experimental results, the theoretically obtained potential energy profiles for the studied hydrogenation mechanisms implied that (1R,2S)-1-phenyl-1,2-propanediol is formed in excess with respect to the other diastereomeric product diol, (1R,2R)-1-phenyl-1,2-propanediol. Generally, if the elementary hydrogen addition step was thermodynamically more favorable on one of the two diastereotopic faces, it was also kinetically preferred on the same face, and vice versa. Pairwise addition of hydrogen was the most energetically favorable mechanism. Adsorption and hydrogenation of other structurally similar chiral alpha-hydroxyketones, (R)-3-hydroxy-2-butanone and (R)-2-hydroxy-1-cyclohexanone, were also studied computationally on Pt(111). The results showed that cluster model DFT calculations can be used to assess (dia)stereoselectivity in metal-catalyzed hydrogenation of even such complex organic molecules as studied here.

Catalyst selection and solvent effects in the enantioselective hydrogenation of 1-phenyl-1,2-propanedione

1-Phenyl-1,2-propanedione was hydrogenated at 5 bar and 25°C over Pt/Al2O3 catalysts in the presence of cinchonidine. The effect of four different solvents, ethyl acetate, toluene, ethanol and dichloromethane in the enantioselective hydrogenation of the dione was tested. The highest enantiomeric excess of (R)-1-hydroxy-1-phenyl-2-propanone, 65%, was obtained in dichloromethane. Also toluene and ethyl acetate gave relatively high enantiomeric excesses. In ethanol, which highly interacts with Pt surface, the enantiomeric excess was only 12%. Two different alumina supports and Pt-precursors were investigated using ethyl acetate as solvent. The highest enantiomeric excesses of (R)-1-hydroxy-1-phenyl-2-propanone were obtained with α-Al2O3 supported Pt catalyst prepared from hydrogen hexachloroplatinate(IV)hydrate presursor. The highest Pt loadings in the catalyst preferred enantiodifferentiation. Higher enantiomeric excesses were obtained by increasing the mean metal particle size of Pt.