Cycle, catalytic

Sdietne 7.12. Catalytic cycle for 1,4-additions of R2Zn reagents. 

The presence of two ligands in the active catalyst is proposed on the basis of the optimum ligand-to-copper ratio of 2 and the nearly identical selectivities of monodentate and bidentate phosphoramidites in the lj4-addition of Et22n to 2-cydohexenone [45]. 

The observation of nonlinear effects both with dialcone and with cydobexe-none further supports this catalyst stoidiiometry. The nonlinear effects can be explained by the involvement of diastereomeric complexes L2CuR with two diiral ligands bound to copper (Fig. 7.2) [45]. 

The X-ray structure of the Cul complex 21 of pbospboramidite 14 provides additional insight into a possible medianism for stereocontrol (Fig. 7.3). The formation of the L CuEt-enone complex involves substitution of the iodide in 21 for the alkyl moiety and of one of the ligands for the coordinated enone. Coordination of R2nX results in the bimetallic intermediate 19 (Fig. 7.3). The absolute configuration of the two pbospboramidite ligands and the pseudo-C2-symmetric arrangement dictate the formation of (S)-3-etbyl-cydobexanone. 

6 Mechanism of the Mizoroki-Heck Reaction when the Ligand is an N-Heterocyclic Carbene 

PdX2(Cb)2 (X = halide, acetate) precursors may be formed from a Pd(II) salt (e.g. Pd(OAc)2) and A-heterocyclic azolium salts which are deprotonated into the NHC ligand [Ip, 64, 66a—c]. They are also generated in situ when iV-heterocyclic azolium salts are used as ionic liquid solvents [66d,e]. Isolated stable NHC-ligated Pd(0) complexes [67] are also used as catalysts in Mizoroki-Heck reactions [68]. 

The oxidative addition of aryl halides to Pd°(Cb)2 complexes has been reported and the complexes rran5-ArPdX(Cb)2 (Cb = c cto-C NR CR 2, X = I, R = R = Me X = Cl, R = r-Bu, R = H Chart 1.1, left) formed in the reaction have been isolated and characterized [68a, 69, 70]. The aryl-palladium(II) complexes are always ligated by two carbene ligands irrespective of their bulldness. 

DPT calculations reported by Green et al. [72] in 2005 on Pd(0) complexes ligated by two C=C unsaturated carbenes close to Cb support the dissociative mechanism with this assumption the bulkier the substituent on the N atoms is, the lower the dissociation energy of the biscarbene-Pd(0) complex is. 

In other words, the involvement of a monoligated Pd (Cb) as the active species is not a guarantee for a fast oxidative addition, because Pd (Cb) is always generated at low concentration in its endergonic equilibrium with the nonreactive Pd (Cb)2 complex. 

7 Copper-catalyzed Enantioselective Conjugate Addition Reactions of Organozinc Reagents 

CCl2=CCl2 --------------- CCl2=C(CI)Pd(CI)(PPhi3)2 --------- CCl2=C(CI)Pd(OMe)(PPh3)2 

HBpin I benzene CgHsMea pinB j -Bpin 150 C Bpin 32 

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A well-understood catalytic cycle is tliat of the Wilkinson alkene hydrogenation (figure C2.7.2) [2]. Like most catalytic cycles, tliat shown in figure C2.7.2 is complex, involving intennediate species in tire cycle (inside tire dashed line) and otlier species outside tire cycle and in dead-end patlis. Knowledge of all but a small number of catalytic cycles is only fragmentary because of tire complexity and because, if tire catalyst is active, tire cycle turns over rapidly and tire concentrations of tire intennediates are minute thus, tliese intennediates are often not even... 

Figure C2.7.2. Catalytic cycle (witliin dashed lines) for tire Wilkinson hydrogenation of alkene [2]. Values of rate and equilibrium constants are given in [2]...

Techniques such as NMR spectroscopy (section B1.12) and IR spectroscopy (section B1.2) are useful in such experiments. Furtlieniiore, tlieory (section B3.1) has proceeded to tlie point of being successful in predicting some simple catalytic cycles. 

Figure C2.7.4. Catalytic cycle for hydrogenation of methyl-(Z)-a-acetamidocinnamate tire rate constants were measured at 298 K S is solvent [8],...

Figure C2.7.8. Catalytic cycle indicating the working of tire enzyme haloalkane dehalogenase [11],...

Vonrhein et al. 1995] Vonrhein, C., Schlauderer, G.J., Schulz, G.E. Movie of the structural changes during a catalytic cycle of nucleoside monophosphate kinases. Structure 3 (1995) 483-490. 

Scheme 2.6. Catalytic cycle for a Lewis-acid catalysed Diels-Alder reaction.

When exclusively considering Lewis-add catalysis, the literature on ligand effects can be divided into studies describing quantitatively the effect of ligands on rates and equilibria of the individual steps in the catalytic cycle on one hand, and studies focused on the enantioselectivity of the reaction on the other. Interestingly, in the majority of the former investigations, aqueous media are employed. 

The most useful reaction of Pd is a catalytic reaction, which can be carried out with only a small amount of expensive Pd compounds. The catalytic cycle for the Pd(0) catalyst, which is understood by the combination of the aforementioned reactions, is possible by reductive elimination to generate Pd(0), The Pd(0) thus generated undergoes oxidative addition and starts another catalytic cycle. A Pd(0) catalytic species is also regenerated by /3-elimination to form Pd—H which is followed by the insertion of the alkene to start the new catalytic cycle. These relationships can be expressed as shown. 

Palladation of aromatic compounds with Pd(OAc)2 gives the arylpalladium acetate 25 as an unstable intermediate (see Chapter 3, Section 5). A similar complex 26 is formed by the transmetallation of PdX2 with arylmetal compounds of main group metals such as Hg Those intermediates which have the Pd—C cr-bonds react with nucleophiles or undergo alkene insertion to give oxidized products and Pd(0) as shown below. Hence, these reactions proceed by consuming stoichiometric amounts of Pd(II) compounds, which are reduced to the Pd(0) state. Sometimes, but not always, the reduced Pd(0) is reoxidized in situ to the Pd(II) state. In such a case, the whole oxidation process becomes a catalytic cycle with regard to the Pd(II) compounds. This catalytic reaction is different mechanistically, however, from the Pd(0)-catalyzed reactions described in the next section. These stoichiometric and catalytic reactions are treated in Chapter 3. 

All these intermediate complexes undergo various transformations such as insertion, transmetallation, and trapping with nucleophiles, and Pd(0) is regenerated at the end in every case. The regenerated Pd(0) starts the catalytic cycle again, making the whole process catalytic. These reactions catalyzed by Pd(0) are treated in Chapter 4. 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Perfluorinated ethers and perfluorinated tertiary amines do not contribute to the formation of ground level ozone and are exempt from VOC regulations (32). The commercial compounds discussed above have an ozone depletion potential of zero because they do not contain either chlorine or bromine which take part in catalytic cycles that destroy stratospheric ozone (33). 

N—Fe(IV)Por complexes. Oxo iron(IV) porphyrin cation radical complexes, [O—Fe(IV)Por ], are important intermediates in oxygen atom transfer reactions. Compound I of the enzymes catalase and peroxidase have this formulation, as does the active intermediate in the catalytic cycle of cytochrome P Q. Similar intermediates are invoked in the extensively investigated hydroxylations and epoxidations of hydrocarbon substrates cataly2ed by iron porphyrins in the presence of such oxidizing agents as iodosylbenzene, NaOCl, peroxides, and air. 

Catalysis (qv) refers to a process by which a substance (the catalyst) accelerates an otherwise thermodynamically favored but kiaeticahy slow reaction and the catalyst is fully regenerated at the end of each catalytic cycle (1). When photons are also impHcated in the process, photocatalysis is defined without the implication of some special or specific mechanism as the acceleration of the prate of a photoreaction by the presence of a catalyst. The catalyst may accelerate the photoreaction by interaction with a substrate either in its ground state or in its excited state and/or with the primary photoproduct, depending on the mechanism of the photoreaction (2). Therefore, the nondescriptive term photocatalysis is a general label to indicate that light and some substance, the catalyst or the initiator, are necessary entities to influence a reaction (3,4). The process must be shown to be truly catalytic by some acceptable and attainable parameter. Reaction 1, in which the titanium dioxide serves as a catalyst, may be taken as both a photocatalytic oxidation and a photocatalytic dehydrogenation (5). 

Donation of a proton to the reactant often forms a carbenium ion or an oxonium ion, which then reacts ia the catalytic cycle. For example, a catalytic cycle suggested for the conversion of phenol and acetone iato bisphenol A, which is an important monomer used to manufacture epoxy resias and polycarbonates, ia an aqueous mineral acid solution is shown ia Figure 1 (10). 

Fig. 1. Catalytic cycle for synthesis of bisphenol A from phenol and acetone in the presence of a dissociated mineral acid (10).

The kinetics of reactions cataly2ed by very strong acids are often compHcated. The exact nature of the proton donor species is often not known, and typically the rate of the catalytic reaction does not have a simple dependence on the total concentration of the acid. However, sometimes there is a simple dependence of the catalytic reaction rate on some empirical measure of the acid strength of the solution, such as the Hammett acidity function Hq, which is a measure of the tendency of the solution to donate a proton to a neutral base. Sometimes the rate is proportional to (—log/ig)- Such a dependence may be expected when the slow step in the catalytic cycle is the donation of a proton by the solution to a neutral reactant, ie, base but it is not easy to predict when such a dependence may be found. 

Wilkinson Hyd.rogena.tion, One of the best understood catalytic cycles is that for olefin hydrogenation in the presence of phosphine complexes of rhodium, the Wilkinson hydrogenation (14,15). The reactions of a number of olefins, eg, cyclohexene and styrene, are rapid, taking place even at room temperature and atmospheric pressure but the reaction of ethylene is extremely slow. Complexes of a number of transition metals in addition to rhodium are active for the reaction. 

The Wilkinson hydrogenation cycle shown in Figure 3 (16) was worked out in experiments that included isolation and identification of individual rhodium complexes, measurements of equiUbria of individual steps, deterrnination of rates of individual steps under conditions of stoichiometric reaction with certain reactants missing so that the catalytic cycle could not occur, and deterrnination of rates of the overall catalytic reaction. The cycle demonstrates some generally important points about catalysis the predominant species present in the reacting solution and the only ones that are easily observable by spectroscopic methods, eg, RhCl[P(CgH 2]3> 6 5)312 (olefin), and RhCl2[P(CgH )2]4, are outside the cycle, possibly in virtual equiUbrium with... 

The catalytic cycle (Fig. 5) (20) is well estabUshed, although the details of the conversion of the intermediate CH COI and methanol into the product are not well understood the mechanism is not shown for this part of the cycle, but it probably involves rhodium in a catalytic role. The CH I works as a cocatalyst or promoter because it undergoes an oxidative addition with [Rh(CO)2l2]% and the resulting product has the CO ligand bonded cis to the CH ligand these two ligands are then poised for an insertion reaction. 

Fig. 18. Schematic representation of the catalytic cycle for ammoxidation of propylene and related reactions. and M2 represent the two metals in a...

This catalytic cycle is related to some stereoselective industrial catalysis. 

In terms of cost, the effectiveness of the catalytic cycle in the ring closure makes this process economical in palladium. The first three steps in the reaction sequence -- ring opening of an epoxide by a Grignard reagent, converison of an alcohol to an amine with inversion, and sulfonamide formation from the amine — are all standard synthetic processes. 

Scheme 15.1 Catalytic cycle for the conversion of a diselenide to a selenenic acid...

Many enzymes require metal ions for maximal activity. If the enzyme binds the metal very tightly or requires the metal ion to maintain its stable, native state, it is referred to as a metalloenzyme. Enzymes that bind metal ions more weakly, perhaps only during the catalytic cycle, are referred to as metal activated. One role for metals in metal-activated enzymes and metalloenzymes is to act as electrophilic catalysts, stabilizing the increased electron density or negative charge that can develop during reactions. Among the enzymes that function in this... 

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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