Bifunctional catalysis mechanisms

However, due to the amphoteric character of alumina, acid catalysis due to OH surface groups or to A1 atoms can take place in a bifunctional catalysis mechanism [Eq. (13)] [24]. 

Other publications have also appeared describing condensation kinetics with heterogeneous catalysts by a designed experimental approach to optimize reaction conditions, stannous octanoate condensation catalysis and the role of bifunctional catalysis mechanisms. A bifunctional transition state , shown for compound 8, was postulated to explain the kinetics of heterofunctional condensation reactions catalysed by CI3CCOOH, Et2NOH, MeEtC=NOH or CH3COOH (equation 6). 

The concerted delivery of protons from OH and hydride from RuH found in these Shvo systems is related to the proposed mechanism of hydrogenation of ketones (Scheme 7.15) by a series of ruthenium systems that operate by metal-ligand bifunctional catalysis [86]. A series of Ru complexes reported by Noyori, Ohkuma and coworkers exhibit extraordinary reactivity in the enantioselective hydrogenation of ketones. These systems are described in detail in Chapters 20 and 31, and mechanistic issues of these hydrogenations by ruthenium complexes have been reviewed [87]. 

One of the systems was found to be very efficient catalyzing enantioface-selective hydrogen transfer reactions to aromatic and in particular to aliphatic ketones with up to 95% ee. Regarding the latter reaction these are unprecedented ee values. The reaction mechanism of these transformations is best described as a metal-ligand bifunctional catalysis passing through a pericyclic-like transition state. 

This reaction encompasses a number of interesting features (general Brpnsted acid/ Brpnsted base catalysis, bifunctional catalysis, enantioselective organocatalysis, very short hydrogen bonds, similarity to serine protease mechanism, oxyanion hole), and we were able to obtain a complete set of DFT based data for the entire reaction path, from the starting catalyst-substrate complex to the product complex. 

Noyori, R., Yamakawa, M. and Hashiquchi, S. Metal-Ligand Bifunctional Catalysis A Nonclassical Mechanism for Asymmetric Hydrogen Transfer between Alcohols and Carbonyl Compounds. J. Org. Chem. 2001, 66, 7931-7944. 

According to the available experimental data, it is impossible to distinguish between these mechanisms, but the second mechanism seems to be preferred [Scheme (7)] for, according to this Scheme, the reaction of amine addition proceeding through a cyclic transition state is completed in one step, whereas for the reaction to occur according to Scheme (2) or (6) it is additionally necessary to transfer the proton. Then, it is probable that the different mechanisms [Schemes (3) and (5)] may precede formation of one and the same transition state [Scheme (7)]. Note finally that the mechanism of bifunctional catalysis [Scheme (7)] is extremely popular in different reactions of nucleophilic substitution at the saturated carbon atom and reactions with participation of a carbonyl group32>. 

Dual-function catalysts possessing both metallic and acidic sites bring about more complex transformations. Carbocationic cyclization and isomerization as well as reactions characteristic of metals occurring in parallel or in subsequent steps offer new reaction pathways. Alternative reactions may result in the formation of the same products in various multistep pathways. Mechanical mixtures of acidic supports (silica-alumina) and platinum gave results similar to those of platinum supported on acidic alumina.214,215 This indicates that proximity of the active sites is not a requirement for bifunctional catalysis, that is, that the two different functions seem to operate independently. 

A more recent paper by Cunningham and Schmir242 on the hydrolysis of 4-hydroxybutyranilide XL in neutral and alkaline solution suggests that intramolecular nucleophilic attack by the neighbouring hydroxyl group is followed by bifunctional catalysis by phosphate or bicarbonate buffers of the conversion of the tetrahedral intermediate to products. A quantitative comparison was made between the effects of buffer on the hydrolysis of 4-hydroxybutyranilide and on the hydrolysis of 2-phenyliminotetrahydrofuran, since both reactions proceed via identical intermediates. The mechanism suggested243 states that the cyclisation of the hydroxyanilide ion yields an addition intermediate whose anionic form may either cleave to products or revert to reactant and whose neutral form invariably gives aniline and butyrolactone, viz. 

This review is concerned with a discussion of the reactions of hydrocarbons over bifunctional catalysts, primarily from the viewpoint of mechanism and kinetics. Some discussion will also be given of the structure and properties of typical bifunctional reforming catalysts, since this is somewhat helpful in understanding how the catalyst functions in promoting the various reactions. In addition, at appropriate places in the article, the practical application of the principles of bifunctional catalysis in commercial reforming processes will be considered. 

The chain length of n-alkanes has a marked influence on reactivities for hydroisomerization, and especially for hydrocracking. To a very small extent a methane and ethane abstracting mechanism, probably hydrogenolysis as predicted in a basic work on bifunctional catalysis (14), is found to be superimposed when lower carbon number feeds (C, Cg, Cg) are used. n-Hexane is excluded from ideal hydrocracking. On the Pt/Ca-Y-zeolite catalyst it is cracked via a mechanism that is mainly hydrogeno-lytic. 

Bronsted acid/base catalysis is the most common enzymatic mechanism, since nearly all enzymatic reactions involve a proton transfer. This means that nearly all enzymes have acidic and/or basic groups in their active site. In add catalysis, the substrate is protonated by one of the amino add residues at the active site (typically aspartic acid, glutamic acid, histidine, cysteine, lysine, or tyrosine). This residue itself must therefore be protonated at the readion pH (typically between pH 5 and 9), with a pKa just above this value. Conversely, in base catalysis, the pJCa of the deprotonating residue must be just below the physiological pH. Some enzymes can even carry out bifunctional catalysis, by protonating and deprotonating two different sites on the same substrate molecule simultaneously. 

Cyclodextrin bis-imidazole catalyzes enolization by a bifunctional mechanism in which the ImH+ is hydrogen-bonded to the carbonyl oxygen while the Im removes the neighboring methyl proton (cf. 50). As expected from this, there was a bell-shaped pH vs. rate profile for the process. In the transition state two protons will move simultaneously, as in the hydrolysis reaction described above. Thus we indeed have a powerful tool to determine the geometric requirements for simultaneous bifunctional catalysis, a tool that could be of quite general use. 

Unlike studies in which acid and base catalysts are distinct molecular species, most attempts to observe bifunctional catalysis when the two catalytic sites are enclosed within the same molecule (whether in the catalyst or in the substrate itself) have failed. First, Lienhard and Anderson (1967) studied acetone iodination catalysed by the monoanions and dianions of a number of dicarboxylic (or polycarboxylic) acids (oxalic acid, 1,1-cyclobutane dicarboxy-lic acid, succinic acid and cis-1,2,3,4-cyclopentanetetracarboxylic acid) for which mechanism (21) can be assumed. It was observed that the catalytic... 

Bell-shaped pH-rate profiles are obtained in lysozyme reactions (Rupley et al., 1967) which are consistent with direct involvement of two groups in the reaction. However, bell-shaped pH-rate constant profiles are also observed in the hydrolysis of benzaldehyde disalicyl acetals, and in the case of p-nitrobenzaldehyde o-carboxyphenyl p-carboxyphenyl acetal only one carboxyl group can participate. One should then take care in postulating bifunctional catalysis in the lysozyme reaction, since the observed kinetics and the rate enhancements are explicable in terms of a chemically simpler mechanism (general acid catalysis by glutamic acid-35 along with release of ground state... 

As it is generally the case with bifunctional catalysis processes, the balance between hydrogenating and acid functions determines for a large part the catalyst activity. This was quantitatively shown for series of bifunctional catalysts constituted by mechanical mixtures of a well dispersed Pt/Alumina catalyst and of mordenite samples differing by their acidity and their porosity (25). The balance between hydrogenating and acid functions was taken as nPt/nH+ the ratio between the number of accessible platinum atoms and the number of protonic sites determined by pyridine adsorption. 

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