Robert L. Augustine

Enantioselective heterogeneous catalysis I. A working model for the catalyst:modifier:substrate interactions in chiral pyruvate hydrogenations

Abstract The room temperature and atmospheric pressure hydrogenation of ethyl pyruvate over Pt/Al2O3 catalysts modified by varying amounts of dihydrocinchonidine was examined. Data were obtained which showed that the hydrogenation occurred on the corner atoms and adatoms on the Pt crystallites in the catalyst. The formation of (S) ethyl lactate was observed when very low concentrations of the alkaloid modifier were used while at higher modifier concentrations the (R) lactate was produced. The formation of the (R) lactate was accompanied by an increase in the hydrogenation rate. A working hypothesis was formulated to explain these results and to serve as a model for the design of future experiments. This model suggested that the initial adsorption of the dihydrocinchonidine takes place on the face atoms adjacent to the corner atoms on the metal crystallite. This will place the chiral portion of the alkaloid close to the corner atom active site. Enantioselective pyruvate adsorption would be facilitated by a hydrogen bond between the C9 OH of the alkaloid and the ethoxy oxygen of the pyruvate and hydrogenation of the keto group will lead to (S) lactate formation. In order to place the chiral portion of the modifier near an adatom it is proposed that the alkaloid is adsorbed in an edge-on manner on the face near the adatom active site. In this way the modifier can interact directly with the pyruvate to change its adsorption characteristics. This will lead to an increase in reaction rate and the formation of the (R) lactate.

Heterogeneous catalysis in organic chemistry. Part 10. Effect of the catalyst support on the regiochemistry of the heck arylation reaction

Abstract Data have been presented which show that dispersed metals can be considered as viable catalysts for synthetically useful organometallic reactions. In addition, the material used as the catalyst support has been shown to influence the regiochemistry of the reaction. In the Heck arylation an acidic support, such as silica, withdraws electrons from the palladium and this leads to predominant β enol ether formation. Palladium supported on a basic support, such as magnesia, which is electron donating, produces primarily the α product. Electron withdrawal also increases the rate of the reaction. As with a number of other synthetically useful reactions the heterogeneously catalyzed Heck arylation takes place on the coordinately unsaturated corner atoms and adatoms on the metal surface. These single atom active sites are the only ones having a sufficient number of orbitals available for interaction with both the aryl species and the enol ether at the same time, a procedure needed for the reaction to take place. A mechanism for the reaction on a tetrahedral corner atom is proposed. This is based on the Frontier Molecular Orbitals of this site and the corresponding mechanism established for the homogenously catalyzed reaction.

Effect of catalyst pretreatment on the oxidation of carbon monoxide over coprecipitated gold catalysts

Au/Co3O4, Au/Fe2O3 and Au/CuO and the corresponding silver containing catalysts were prepared by coprecipitation and subjected to various heat and oxygen pretreatments prior to being used to pro-mote carbon monoxide oxidations. The most active catalysts were those which were heated in oxygen before use. The relative activities of these catalysts depended on the degree of heating and oxygen exposure before use. Reduction of the gold containing precipitates with formaldehyde gave species with markedly diminished activities. The unreduced gold containing catalysts were more active than the silver species. These catalysts were also subjected to alternate pulses of O2, CO and O2 at varying temperatures to determine the type of interaction taking place on the catalyst surface. These data indicate that carbon monoxide oxidation takes place primarily by the interaction of carbon monoxide with adsorbed oxygen and to a lesser extent by the reverse.

Anchored homogeneous catalysts: the role of the heteropoly acid anchoring agent

Heteropoly acids (HPAs) are used to anchor catalytically active homogeneous complexes to supports by an interaction between the HPA and the metal atom of the complex. In an attempt to determine the nature of the HPA/metal bonding several substrates were hydrogenated using chiral and achiral complexes anchored to alumina by the commercially available Keggin HPAs, phosphotungstic acid (PTA), phosphomolybdic acid (PMA), silicotungstic acid (STA) and silicomolybdic acid (SMA). It was found that the nature of the HPA can have a significant influence on both the activity and selectivity of reactions run using these HPA anchored homogeneous catalysts. The results obtained indicate that there is a direct interaction between the HPA and the metal atom of the active organometallic complex. While these data do not unequivocally establish that there is a bond between the metal atom and an oxygen on the surface of the HPA, the extent to which the activity and selectivity change when different HPAs are used as anchors is more like what would be expected when one uses a series of ligands attached to the metal atom rather than changing the counterions associated with a charged organometallic species.

Selective heterogeneously catalyzed hydrogenations

Abstract A number of catalytic hydrogenations have been discussed with emphasis on reaction selectivity. These reactions were examples of chemoselectivity, regioselectivity, stereoselectivity, diastereoselectivity and enantioselectivity. Factors such as catalyst selection, reaction conditions and the steric nature of the substrate were discussed in terms of their contribution to the selectivity of particular hydrogenations.

Heterogeneous catalysis in organic chemistry Part 8.: The use of supported palladium catalysts for the Heck arylation

Abstract It has been found that the Heck arylation proceeds smoothly over supported Pd catalysts to give the same reaction products as are obtained with the more commonly used homogeneous catalysts. The effect of reaction variables on the regioselectivity of the process is about the same for both the homogeneous and heterogeneously catalyzed reactions, so the use of the supported metal catalyst in such reactions appears to be straightforward. Such materials should be considered as viable alternatives for the commonly used homogeneous catalytic species.

Enantioselective heterogeneous catalysis. 2. Examination of the formation of the individual (R) and (S) lactates in the cinchonidine modified platinum hydrogenation of pyruvate

The formation of the individual (R) and (S) lactates was measured during the dihydrocinchonidine modified platinum catalyzed hydrogenation of ethyl pyruvate at atmospheric pressure in a methyl acetate solvent. It was found that the product ee increased during the initial phase of the reaction. Plotting the amount of each enantiomer formed in one minute intervals against reaction time showed clearly that, as previously suggested, this reaction proceeds through an initial stage characterized by a significant increase in ee followed by a period of essentially stable ee. When (R) methyl lactate was added as a co-modifier with the dihydrocinchonidine, the production of (R) ethyl lactate decreased somewhat and the incremental changes observed during the first ten minutes of the reaction were less than those seen in the absence of the (R) methyl lactate co-modifier. With (S) methyl lactate as the co-modifier, the production of (R) ethyl lactate decreased significantly and the incremental changes were markedly different from those observed in the absence of a co-modifier. The production of (S) ethyl lactate was affected only slightly by the presence of either (R) or (S) methyl lactate as the co-modifier.

A facile technique for the surface characterization of dispersed metal catalysts

Abstract The product compositions obtained from a single-turnover olefin hydrogenation process were used to determine the relative number of the different types of active sites present on a series of Pt/ CPG (Controlled Pore Glass) catalysts. It was shown that during this procedure each site reacted only once per turnover sequence so the product compositions could be related directly to the densities of the various types of sites present. The amount of alkane formed was related to the number of 3 M sites present while the extent of isomerization occurring and the amount of stable half-hydrogenated species present indicated the density of 3 MH and/or 2 M sites. The 1 M sites present were determined by subtracting from the total amount of hydrogen required to saturate the metal surface that which was present on the 3 M and 2 M sites.

Organic Functional Group Hydrogenation

Abstract The utility of catalytic hydrogenation in the synthesis of organic compounds is evident from the number of monographs on the subject published in recent years [l-41. Unlike other synthetically useful reactions, however, the conditions used for a given hydrogenation are generally chosen on the basis of an empirical correlation and not on a knowledge of the mechanism of the reaction. One reason for this is that most of the mechanistic studies made on catalytic hydrogenation do not readily lend themselves to extrapolation to synthetic ally useful reactions.

Heterogeneous catalysis in organic synthesis. Part 9. Specific site solvent effects in catalytic hydrogenations

The hydrogenation of 4-methyl-1-cyclohexene was run in several different solvents over a series of single turnover (STO) characterized 8.06% Pt/SiO2 catalysts. From the reaction rate data specific site turnover frequencies (TOFs) were obtained for each of the three STO saturation sites in each solvent. In all solvents it was found that the 3MR sites are the most reactive. Taking the heptane data as the standard, it was found that ethyl acetate adsorbed somewhat on the 3MI and 3MH sites but does not affect the reactivity of the 3MR sites. In THF and ethanol the specific site TOFs were all decreased with ethanol having more of an effect than THF. These data provide a good example of the importance of the competitive adsorption of the solvent in a heterogeneously catalysed reaction.