Catalysis, homogeneous

A homogeneous catalyst exists in solution with the reaction mixture. All homogeneous catalysts are gases, liquids, or soluble solids. 

A thoroughly studied example of homogeneous catalysis is the hydrolysis of an organic ester (RCOOR ), a reaction introduced in Section 15.4  

Here R and R are hydrocarbon groups, R—C—OH is a carboxylic acid, and R —OH is an alcohol. The reaction rate is low at room temperature but can be increased greatly by adding a small amount of strong acid, which provides H ion, the catalyst in the reaction strong bases, which supply 0H ions, also speed ester hydrolysis, but by a slightly different mechanism. 

Many digestive enzymes, which catalyze the hydrolysis of proteins, fats, and carbohydrates during the digestion of foods, employ very similar mechanisms. The difference is that the acids or bases that speed these reactions are not the strong inorganic reagents used in the lab, but rather specific amino-acid side chains of the enzymes that release or abstract H ions. 

In homogeneous catalysis the reactants and the catalyst are dispersed in a single phase, usually liquid. Acid and base catalyses are the most important types of homogeneous catalysis in liquid solution. For example, the reaction of ethyl acetate with water to form acetic acid and ethanol normally occurs too slowly to be measured. 

The reaction, however, can be catalyzed by an acid. Often a catalyst is shown above the arrow in a chemical equation  

CH3COOC2H5 -b H,0 CH3COOH -h C2H5OH In the presence of acid, the rate is faster and the rate law is given by rate = [CHsCOOCsHsliH- ] 

Because in magnitude, the rate is determined solely by the catalyzed portion of the reaction. 

The term homogeneous catalysis also covers simple acid catalysts and non-organometallic catalysis, such as the decomposition of H2O2 by Fe. Catalytic mechanisms are considerably easier to study in homogeneous systems, where 

The Organomemlltc Chemisrry of the TTansUion Metah, Fourth Edition, by Robert H. Crabtree Copyright W.S John Wiley Sons, Inc. 

A typical reaction (Eq. 9.1) that is catalyzed by many transition metal complexes is the isomerization of allylbenzene (the substrate) into propenylbenzene (the product). Normally, the substrate for the reaction will comdinate to the metal complex that serves as catalyst. The metal then brings about the rearrangement, and the product dissociates, leaving the metal fragment free to bind a new molecule of substrate and participate in the catalytic cycle once again. 

Each time the complete catalyst cycle occurs, we consider one catalytic turnover (one mole of product formed per mole of catalyst) to have been completed. The catalytic rate can be conveniently given in terms of the turnover frequency (TOF) measured in turnovers per unit time (often per hour) the lifetime of the catalyst before deactivation is measured in terms of total turnovers. 

In a stoichiometric reaction, the passage through M.TS would be the slow, or rate-determining, step. In a catalytic reaction the cyclic nature of the system means that the rates of all steps are identical. On a circular track, on average the same number of Crains must pass each point per unit lime. The slow step in a catalytic process is called the turnover limiting step. Any change that lowers the barrier for this step will increase the turnover frequency (TOF). Changes in 

We first consider the case where the rate-determining step is the forward homogeneous electron transfer step (rate constant ke). The governing equations are 

Conversion into a dimensionless formulation follows the same principle and notations as in the preceding sections with, in addition, the following definitions  

Resolution of the problem may therefore be pursued looking only at the fate of q and a. 

A simple situation is reached if the excess is big enough for a to remain unconditionally equal to y whatever t and c. More precisely, this situation is reached when Xe/y — 0, ensuring that the consumption of a is negligible. Equation (6.77) then becomes 

When Xe — 0, we are back to the reversible Nernstian wave. When, conversely, Xe — oo (pure kinetic conditions), 

High gas pressure NMR investigations in homogeneous catalysis have focused mainly on both on-line monitoring of chemical reactions and stabilization and identification of intermediates involved in a catalytic cycle. The following sections address these high gas pressure studies according to the gas used. 

In contrast to heterogeneous catalysis, industrial applications of homogeneous catalysis are relatively scarce, largely being restricted to the speciality and pharmaceutical sectors. Homogeneous catalysts have been well researched, since their catalytic centres can be relatively easily 

The most widely used homogeneous catalysts are simple acids and bases which catalyse well-known reactions such as ester and amide hydrolysis, and esterification. Such catalysts are inexpensive enough that they can be neutralized, easily separated fi om organic materials, and disposed of. This, of course, is not a good example of green chemistry and contributes to the huge quantity of aqueous salt waste generated by industry. 

A catalyst that is present in the same phase as the reactants in a reaction mixture is called a homogeneous catalyst. Examples abound both in solution and in the gas phase. Consider, for example, the decomposition of aqueous hydrogen peroxide, H202(uq), into water and oxygen  

In the absence of a catalyst, this reaction occurs extremely slowly. Many substances are capable of catalyzing the reaction, however, including bromide ion, which reacts with hydrogen peroxide in acidic solution, forming aqueous bromine and water (T FIGURE 14.22)  

NaBr catalyst about to be added to reaction mixture 

A FIGURE 14.22 Homogeneous catalysis. Effect of catalyst on the speed of hydrogen peroxide decomposition to water and oxygen gas. 

If this were the complete reaction, bromide ion would not be a catalyst because it undergoes chemical change during the reaction. However, hydrc en peroxide also reacts with the Br2( q) generated in Equation 14.30  

Why does the solution in the middle cylinder have a brownish color  

Where are the intermediates and transition states in this diagram  

Enantioselective catalysis that rivals enzymes in selectivity is a major development in homogeneous catalysis. As a result, many earlier processes in the pharmaceutical and perfumery industries are being replaced by more elegant syntheses using soluble catalysts in which handedness is introduced in the critical step of the process, thus avoiding the costly separation of racemic mixtures. In view of its importance in organic synthesis, enantioselective (or asymmetric) catalysis was briefly introduced in Chapter 6 and is again considered as a powerful synthetic tool in Chapter 9. 

Noyori and Kitamura (1989), Parshall and Ittel (1992), Gates (1992), Chan (1993), Akutagawa (1995). 

Homogeneous catalysts and the reaction components of these catalyst-catalyzed reactions remain in the same phase. Thus, the homogeneous catalysis requires both the catalyst and the reactants or the reaction components to be present in the same phase. For example, the net reaction between acetone and bromine in an acidic aqueous medium is expressed as the following  

Homogeneous Microheterogeneous (micellar) Heterogeneous Phase transfer Asymmetric 

Nucleophilic Electrophilic Specific base Specific acid General base General acid FIGURE 2.3 Representation of varions types of catalysis and snbcatalysis. 

The observed rate law for the reaction shows that the k3-step cannot be the rate-determining step. The rate of bromination of acetone shows substantial amount of deuterium primary kinetic isotope effect (kH/k 7 where kn and k represent rate constants for the reaction of Br2 with CH3COCH3 and CD3COCD3, respectively), which imphes k2-step as the rate-determining step. 

There are three general types of catalysis, depending on the nature of the rate-increasing substance heterogeneous catalysis, homogeneous catalysis, and enzyme catalysis. 

In heterogeneous catalysis, the reactants and the catalyst are in different phases. The catalyst is usually a solid, and the reactants are either gases or liquids. Heterogeneous catalysis is by far the most important type of catalysis in industrial chemistry, especially in the synthesis of many important chemicals. Heterogeneous catalysis is also used in the catalytic converters in automobiles. 

At high temperatures inside a car s engine, nitrogen and oxygen gases react to form nitric 

When released into the atmosphere, NO rapidly combines with O2 to form NO2. Nitrogen dioxide and other gases emitted by automobiles, such as carbon monoxide (CO) and various unbumed hydrocarbons, make automobile exhaust a major source of air pollution. 

There are two areas of catalysis depending on the whether the catalyst belongs to the same phase as the principal constituents or a separate phase (usually a solid). The first case constitutes homogeneous catalysis, mainly in liquid phase. The second is heterogeneous catalysis for reactions between catalyzed gases by a solid. 

In homogeneous catalysis, all the reactants and the catalyst belong to the same phase. The most common are catalysts in liquid medium. 

A very general mechanism of homogeneous catalysis has been proposed by 

Catalyzed by a substance C, Hertzfeld proposed the following mechanism  

It can be seen that the sum of the three steps [2.R9a], [2.R9b] and [2.R9c] gives back the overall reaction. 

Catalytic reactions are those accelerated in the presence of a foreign compound (the catalyst), usually present in a small amount. When the catalyst and the reactant are in the same (gas or liquid) phase, the process is called homogeneous catalysis, and when they are in different phases, it is called heterogeneous catalysis. 

According to the extensive information on homogeneous catalytic reactions, the catalyst is a compound directly involved in a reaction, but in contrast to other compounds it is not consumed in the course of the reaction. 

In reality, however, for some reason or other, a part of the catalyst becomes lost in the reaction and its regeneration is incomplete. The definition catalyst regeneration is used in the sense that the catalyst is not consumed for formation of the reaction product. 

In spite of the great theoretical and practical importance of homogeneous catalytic reactions and the large amount of research devoted to their study, the chemical mechanisms of such reactions have not been sufficiently studied. The most common type of a homogeneous catalytic reaction seems to be one the course of which is controlled by the generation of an active intermediate in the presence of a catalyst. This suggests that the interaction of catalyst (K) with one of the initial species yields an active intermediate (X) converted further to form the reaction product (C) with subsequent regeneration of the catalyst. 

This scheme can be represented in a very general form by a combination of processes A + K- X -f,  

Ammonium salts have also been used as catalysts in the synthesis of t-BuNH2 from NH3 and isobutene in water (Eq. 4.7). The co-produced t-BuOH can be recycled [97]. 

Hegedus et al. have thoroughly studied the homogeneous hydroamination of olefins in the presence of transition metal complexes. However, most of these reactions are either promoted or assisted, i.e. are stoichiometric reactions of an amine with a coordinated alkene [98-101] or, if catalytic, give rise to the oxidative hydroamination products, as for example in the cyclization of o-allylanilines to 2-alkylindoles [102, 103], i.e. are relevant to Wacker-lype chemistry [104]. 

The first transition metal atalyzed hydroamination of an olefin was reported in 1971 by Coulson who used rhodium(I), rhodium(lll) or iridium(III) catalysts (Eq. 4.8) [105,106]. 

This reaction is restricted to ethylene and to secondary amines of high basicity (nude-ophUicity) and low steric bulk (MeaNH, pyrrolidine, piperidine). No high molecular weight products are formed. However, the same catalysts [107,108] as well as PdQa [108] also exhibit some activity for the hydroamination of ethylene with PhNHa (Eq. 4.9). 

The reaction can be made selective for the synthesis of N-ethylaniline (150°C, 10 bar) but at the expense of catalytic activity [107, 108]. 

M because only a small fraction of the metal is likely to be on the loop at any given time. Even if a species appears to be an intermediate we still cannot be sure it is not M.S, an off-loop species. If a species builds up steadily during the reaction it might be a catalyst deactivation product M , in which case the catalytic rate will fall as [M rises. Two excellent reviews are available on the determination of mechanism in catalytic reactions. -  

In order for a complex to function as a hydrogenation catalyst, it is necessary for it to be able to bond to hydrogen and to the alkene. This requires the complex to be able to add hydrogen in an oxidative 

Some disagreement exists regarding the structure of the transition state (whether H or P j 3 is trans to the alkene) and whether or not solvent molecules occupy sites that are apparently vacant. In spite of some uncertainty regarding these details, the major issues regarding the catalyzed hydrogenation of alkenes using Wilkinson s catalyst are fairly well understood. 

Another important use for Wilkinson s catalyst is in the production of materials that are optically active (by what is known as enantioselective hydrogenation). When the phosphine ligand is a chiral molecule and the alkene is one that can complex to the metal to form a structure that has R or S chirality, the two possible complexes will represent two different energy states. One will be more reactive than the other, so hydrogenation will lead to a product that contains predominantly only one of the diastereomers. 

I FIGURE 22.9 A plausible mechanism for hydrogenation of 1,3-butadiene catalyzed by Co(CN)5 

As shown earlier, Co(CN)53 has the ability to split hydrogen molecules as a result of an oxidative addition reaction. 

With the recently developed [py3RhCl3]-NaBH4 catalyst the double bonds of A -3-keto steroids are reduced quite readily but mixtures of 5a- and 5j9-products are obtained.  

A solution of 0.2 g of cholestenone and 0.47 g of ( 3P)3RhCl in 150 ml of acetone is stirred under a hydrogen atmosphere for 3 days. The solvent is evaporated and the residue separated by thin layer chromatography to afford 5a-cholestan-3-one in 25-35% yield.  

The hydroamination of allene with morpholine or allylamines has been attempted with palladium-based catalysts. Usually, a mixture of 1 1 telomers (hydroamination products) and 1 2 telomers is obtained, the latter being the major [308, 309] or only 

These reactions have been improved (yields, reaction rates, and selectivity) to give only allylic amines by using l,T-bis(diphenylphosphino)ferrocene and acetic acid instead of PPhj and EtjNHI, respectively (Eq. 4.91) [312]. 

An analogous catalytic system has been applied to the IH of y- and 5-allenic amines which cyclize smoothly in the 5-Exo-Trig or 6-Exo-Trig mode, giving vinylpyrrolidines and vinylpiperidines, respectively (Eq. 4.92) [313]. 

IH of allenic amines also occurs in the presence of silver salts. IH of a-allenic amines proceeds in good yields in the presence of AgBp4 and provides a useful method for 3-pyrrolines synthesis via Endo-Trig processes (Eq. 4.93) [314]. 

Although less efficient (TOP = 0.04 h ), similar IH ofofy- and 5-allenic amines in the presence of AgNOj give 2-alkenylpyrrolidines and 2-alkenylpiperidines, respectively (5 [or 6]-Exo-Trig processes) [315]. These reactions have been applied to the synthesis of ( )-pinidine [316] and -) R) coniine [317]. 

showing the products that can be obtained from butadiene with various nickel catalysts (not shown). Polymers are obtained when allylnickel(II) complexes are used as catalysts and cyclic dimers and the all-/ram-trimer are the product when nickel(O) is the catalyst precursor. Linear dimerisation requires the presence of protic species. Ligands are useful for the fine-tuning of the microstructures of polymers and oligomers. Cyclooctadiene is a commercial product, which is converted mainly to cycloctene. The all-c/.v-trimer is a commercial product as well, but this is made with the use of a titanium catalyst. 

All catalytic reactions appear to Involve the formation of intermediate compounds of the catalyst and the substance undergoing the reaction (the substrate, S), the sequence of reactions being S + Cat = X = Product + Cat 

Many ions catalyze homogeneous reactions. The hydronium ion, H30, and the 

Some catalytic reactions proceed at a reduced rate in the absence of catalyst. For a reaction, A = Products, the rate equation then will be 

Some reactions will not proceed at all without catalyst, for instance some enzyme reactions. Then the rate equation is 

Enzymes also are homogeneous catalysts, although they are sometimes attached to solid surfaces without degradation. They possess a different form of rate equation, for which the development may be found in problem P2.03.02. Their behavior Is especially sensitive to temperature and to substrate concentration. 

Most extensively, as well as most impressively, studied is the organolantha-nide-catalyzed hydroamination/cyclization of N-unprotected aminoolefins by 

Achiral precatalysts of type A form racemic products. However, high enan-tioselectivity is obtained by Cl-symmetric chiral precatalysts B. The turnover 

The overall high enantioselectivities are increased at lower temperature and can attain up to 74% ee. Epimerization is observed during the initiating rapid protonolysis reaction, but the optical purity of the precatalyst does not influence the configuration and optical purity of the product. 

As expected, the chiral precatalysts also initiate diastereoselective processes ( 95%) [294c]. 

The same type of precatalysts catalyze the regiospecific hydroamination/ cyclization of aliphatic and aromatic aminoalkynes RC=(CH2) NH2 [295]. The mechanistic scenario parallels that of the corresponding aminoolefin cyclization. However, the cyclization of the aminoalkynes is 10-100 times more rapid, and a rather contrary effect of the cyclopentadienyl substitution on N, was observed. 

Although the chemistry described in the foregoing deals with the oxidative addition of low-valent transition metal complexes to terminal B—H bonds, we now describe the formal addition of the same reagents to the 

The structure of the product has been confirmed by an X-ray diffraction study 92). The iridium analog was also prepared in a similar fashion. 

Although the hydridorhodacarborane is formally a rhodium (III) derivative, it functions as a facile catalyst in alkenc isomerization, hydrogenation, hydroformylation, and hydrosilylation reactions 80). This catalyst system is extremely stable and may be recovered quantitatively from alkene isomerization and hydrogenation reactions. In addition to these reactions, the hydridorhodacarborane is very effective in the catalysis of deuterium exchange at terminal BH positions 59). These discoveries may soon lead to industrially useful metallocarborane catalysts. 

Callahan, K. P., and Hawthorne, M. F., Pure Appl. Chem. 39, 475 (1974) and references therein. 

Grimes, R. N., Carboranes. Academic Press, New York, 1970, and references therein. 

Alkanes and arenes can also be activated to other reactions by platinum complexes in aqueous solution (57,58). For arenes in the presence of H2PtCl5, reduction from Pt(IV) to Pt(II) occurs and the arene undergoes chlorination. The reaction is catalyzed by platinum(II) (59). Similarly, if a platinum(IV) catalyst such as HjPtClg is used, chloroalkanes are formed from alkanes. As an example, chloromethane is formed from methane (Eq. 23) (60-62). Linear alkanes preferentially substitute at the methyl 

At 120°C, a mixture of PtC and PtCl4 in the presence of oxygen can be used for the oxidation of alkanes to alcohols (70). The substrate p-toluenesulfonic acid is oxidized sequentially at the side-chain functionality, first to the alcohol and then to the aldehyde (Eq. 25). For ethylbenzene, 

A mixture of PtCl4 and metallic Pt with oxygen in aqueous media can be used to oxidize ethane, propane, ethers, and esters. This combination acts sequentially, whereby the initial cleavage of an unactivated C-H bond is induced by reaction with PtCl4 . The subsequent oxidation step with oxygen is catalyzed by the metallic Pt. For ethane this sequence of 

PtCl - + C4H,4 —+ PtHCl - + CjHis PtClj + CjHis — PtCl + CjHij + HCI PtCl - CftH,3 PtCl - C4H,3a PtHCll — PICI4 HCI 2PtCl — PtCI + PtClj  

Palladium(II) catalysts, such as PdCl4 in trifluoroacetic acid solvent, have been used for the conversion of methane to methanol (7i). The system uses hydrogen peroxide as oxidant the overall reaction involving the addition of one mole of water is shown in Eq. 26. The function of the 

This is a fundamentally important aspect of DENs, particularly with regard to their catalytic properties however, there are presently no reliable characterization methods for evaluating particle composition distributions. One method that has been appUed to PdAu [32] and PtPd [30] DENs, as well as dendrimer templated PtAu [37] is to collect single particle EDS spectra from several (15-20) nanoparticles. These experiments indicate that individual particle composition distributions may vary widely, but the difficulty obtaining data from the smallest particles may skew the results somewhat. 

Advantages mild reaction conditions, high activity and selectivity, better mechanistic understanding, etc. 

Drawbacks difficult and expensive separation of the soluble catalyst from the product. 

The advantages of homogeneous catalysis outweigh the drawbacks, and therefore, the interest in homogeneous catalysis is growing. In view of this, some basic information on homogeneous catalysis is presented in the following. 

The major defining characteristics for a homogeneous catalyst are (i) activity that is quantified by turnover number (TON) or frequency and (ii) selectivity. 

The subject matter covered in this chapter is more extensively reviewed than most and each sub-section is preceded by mention of reviews dealing specifically with the subject in question. Other reviews of a more general nature are given in refs. 1—10. 

Interest in the olefin disproportionation (metathesis) reaction catalysed by transition-metal compounds has increased markedly during the present period and although a number of features of such reactions remain unexplained or are topics of controversy, sufficient evidence has now been obtained to suggest that carbene intermediates are involved in most catalytic systems. The exact nature of the catalytic sites remains obscure but the recent discovery of one-component metathesis catalysts may enable more rapid advances to be made in this direction. 

The rate of a chemical reaction may be enhanced in many ways. These methods include  

The first two methods are not catalytic since they are based on adding energy to the system. The third involves gross alteration of the reaction medium. Only the fourth, in which trace quantities of reagent markedly affect the reaction rate, is catalysis. 

Conventionally a catalyst is defined as a substance that alters the speed of a chemical reaction without undergoing any chemical changes 

A case where a catalyst accelerates reaction is the commercial process for sulfuric acid. The first step is the oxidation of SOg to SO3 uncatalyzed this proceeds via the slow termolecular reaction 

In the presence of NO a two-step pathway may be followed (the termolecular NO-O2 reaction has zero activation energy)  

As may be seen from the data of Table 5.4, the interaction of neutral nucleophiles (such as NH3,H20) with the carbonyl reactants corresponds to the repulsion potential. Detailed ab initio (STO-3G and 4-3IG) calculations [94, 102-104] showed that as the water and the formaldehyde molecules draw nearer along the reaction path (of the type depicted in Fig. 5.7), the repulsion between them increases and that the dipolar structures XXXIIa, in contrast to the ions XXXII XXXIIc, do not belong to the minima regions on the PES and do not conform to the bound structures  

Stabilization of the ionic tetrahedral intermediates manifests the specific acid (XXXIIb) and base (XXXIIc) catalysis of the nucleophilic substitution reactions. Energetically favorable proves to be a two-step formation of the tetrahedral intermediate with prior protonation of the substrate (specific acid catalysis) or deprotonation of the nucleophilic reactant (specific base catalysis). The chief factor which helps to overcome the repulsion potential and provides for the exothermicity of the formation of the intermediates XXXIIb, XXXIIc is the drawing together of the levels of the frontier orbitals of reactants and the effective mechanism of charge transfer (Fig. 5.5). 

The calculations point to a possibility of a theoretical explanation of the effectiveness of the homogeneous catalysis. However, it should be emphasized 

According to the calculations [108], inclusion of four water molecules into the calculation of the formaldehyde hydration reaction is sufficient for the appearance of a local minimum whose structure corresponds to the dipolar form. When number of the solvent molecules reaches six, the minimum becomes rather deep (over 5 kcal/mol). Thus, dipolar intermediates may, in principle, exist in solution. 

This result, is apparently, of a more or less general significance for the reactions of nucleophilic substitution at the carbonyl carbon. Thus, quantum chemical calculations predict appreciable acceleration of ammonolysis reactions of esters [93] and carboxylic acids [108] when an additional ammonia molecule takes part in their catalysis. The structure, found by a nonempirical calculation [109], of a transition state in the concerted reaction of formation of formaldehyde XXXVII in the reaction  

If VCI3 used instead of TiCl, there is no Equatorial-Axial Shift... 

Three mechanisms have been proposed to explain metallocene-based homogeneous and Ziegler-Natta polymerization schemes. The Cossee-Arlman mechanism 

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Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

Halpern J 1978 Mechanistic aspects of homogeneous catalysis Trans. Am. Crystallogr. Assoc. 14 59-70... 

Parshall G D and Ittel S D 1992 Homogeneous Catalysis 2nd edn (New York Wiley)... 

A concise summary of chemistry of technologically important reactions catalysed by organometallic complexes in solution. Cornils B and Herrmann W A (eds) 1996 Applied Homogeneous Catalysis with Organometallio Compounds (Weinheim VCH) A two-volume, multiauthored account with emphasis on industrial applications. 

Khan, M. M. T. 1974, Homogeneous Catalysis by Metal Complexes, Vol. II, Activation of Alkenes and Alkynes, Academic Press New York - London... 

The sonochemistry of solutes dissolved in organic Hquids also remains largely unexplored. The sonochemistry of metal carbonyl compounds is an exception (57). Detailed studies of these systems led to important mechanistic understandings of the nature of sonochemistry. A variety of unusual reactivity patterns have been observed during ultrasonic irradiation, including multiple ligand dissociation, novel metal cluster formation, and the initiation of homogeneous catalysis at low ambient temperature (57). 

R. S. Dickson, Homogeneous Catalysis with Compounds of Rhodium and Iridium, Reidel, Dordrecht, The Netherlands, 1985. 

D. Forster and J. F. Roth, eds.. Homogeneous Catalysis 11 (Advances ia Chemistry Series 132), American Chemical Society, Washiagton, D.C. 1974 ... 

The mechanism and rate of hydrogen peroxide decomposition depend on many factors, including temperature, pH, presence or absence of a catalyst (7—10), such as metal ions, oxides, and hydroxides etc. Some common metal ions that actively support homogeneous catalysis of the decomposition include ferrous, ferric, cuprous, cupric, chromate, dichromate, molybdate, tungstate, and vanadate. For combinations, such as iron and... 

Gas Phase. The decomposition of gaseous ozone is sensitive not only to homogeneous catalysis by light, trace organic matter, nitrogen oxides. 

The low temperature limitation of homogeneous catalysis has been overcome with heterogeneous catalysts such as modified Ziegler-Natta (28) sohd-supported protonic acids (29,30) and metal oxides (31). Temperatures as high as 80°C in toluene can be employed to yield, for example, crystalline... 

Metallacarboranes. These are used in homogeneous catalysis (222), including hydrogenation, hydrosilylation, isomerization, hydrosilanolysis, phase transfer, bum rate modifiers in gun and rocket propellants, neutron capture therapy (254), medical imaging (255), processing of radioactive waste (192), analytical reagents, and as ceramic precursors. 

G. W. ParshaU, Homogeneous Catalysis, WUey-Interscience, New York, 1980. 

G. N. Schrauzer, ed.. Transition Metals In Homogeneous Catalysis, Marcel Dekker, Inc., New York, 1971. 

These appHcations are mosdy examples of homogeneous catalysis. Coordination catalysts that are attached to polymers via phosphine, siloxy, or other side chains have also shown promise. The catalytic specificity is often modified by such immobilization. Metal enzymes are, from this point of view, anchored coordination catalysts immobilized by the protein chains. Even multistep syntheses are possible using alternating catalysts along polymer chains. Other polynuclear coordination species, such as the homopoly and heteropoly ions, also have appHcations in reaction catalysis. 

G. W. ParshaH, Homogeneous Catalysis The applications and Chemistry of Catalysis by Soluble Transition Metal Complexes,Johm. Wiley Sons, Inc., New York, 1980, 240 pp. An excellent treatment of catalysis by coordination compounds. 

The most numerous cases of homogeneous catalysis are by certain ions or metal coordination compounds in aqueous solution and in biochemistry, where enzymes function catalyticaUy. Many ionic effects are known. The hydronium ion and the hydroxyl ion OH" cat-... 

Significant characteristics of homogeneous catalysis are that they are highly specific and proceed under relatively mild conditions— again in contrast to solid catalysis, which is less discriminating as to reaction and may require extremes of temperature and pressure. A problem with homogeneous operation is the difficulty of separating product and catalyst. 

T. C. Bruice and S. I Benkovic, Bioorganic Mechanisms, Vol. 1, W. A. Benjamin, New brk, 1966, pp. 1-258 W. P. Jencks, Catalysis in Chemistry and Enzymology, McGraw-Hill, New York, 1969 M. L. Bender, Mechanisms of Homogeneous Catalysis from Protons to Proteins, Wiley-Interscience, New York, 1971 C. Walsh, Enzymatic Reaction Mechanisms, W. H. Freeman, San Francisco, 1979 A. Fersht, Enzyme Structure and Mechanism, 2nd ed., W. H. Freeman, New York, 1985. 

M. L. Bender, Mechanisms of Homogeneous Catalysis from Protons to Proteins. Wiley-Interscience, New York, 1971. 

Catalytic hydrogenation of tnfluoroacetic acid gives tnfluoroethanol in high yield [73], but higherperfluorocarboxybc acids and their anhydndes are reduced much more slowly over rhodium, iridium, platinum, or ruthenium catalysts [7J 74] (equation 61) Homogeneous catalysis efficiently produces tnfluoroethanol from tnfluoroacetate esters [75] (equation 61)... 

Homogeneous catalysis by lin compounds is also of great indusirial importance. The use of SnCU as a Friedel-Crafts catalyst for homogeneous acylation, alkylation and cyclizaiion reactions has been known for many decades. The most commonly used industrial homogeneous tin catalysis, however, are the Sn(ll) salts of organic acids (e.g. acetate, oxalate, oleale, stearate and ocToate) for the curing of silicone elasloniers and, more importantly, for the production of polyurethane foams. World consumption of tin catalysts for the.se Iasi applications alone is over 1000 tonnes pa. 

Hydrido complexes of all three elements, and covering a range of formal oxidation states, are important because of their roles in homogeneous catalysis either as the catalysts themselves or as intermediates in the catalytic cycles. 

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