Catalytic Mechanisms initiation step

Polymerization Mechanism. The mechanism that accounts for the experimental observations of oxidative coupling of 2,6-disubstituted phenols involves an initial formation of aryloxy radicals from oxidation of the phenol with the oxidized form of the copper—amine complex or other catalytic agent. The aryloxy radicals couple to form cyclohexadienones, which undergo enolization and redistribution steps (32). The initial steps of the polymerization scheme for 2,6-dimethylphenol are as in equation 6. 

The term Knoevenagel reaction however is used also for analogous reactions of aldehydes and ketones with various types of CH-acidic methylene compounds. The reaction belongs to a class of carbonyl reactions, that are related to the aldol reaction. The mechanism is formulated by analogy to the latter. The initial step is the deprotonation of the CH-acidic methylene compound 2. Organic bases like amines can be used for this purpose a catalytic amount of amine usually suffices. A common procedure, that uses pyridine as base as well as solvent, together with a catalytic amount of piperidine, is called the Doebner modification of the Knoevenagel reaction. 

The hydrocarbon catalytic cracking is also a chain reaction. It involves adsorbed carbonium and carbenium ions as active intermediates. Three elementary steps can describe the mechanism initiation, propagation and termination [6]. The catalytic cracking under supercritical conditions is relatively unknown. Nevertheless, Dardas et al. [7] studied the n-heptane cracking with a commercial acid catalyst. They observed a diminution of the catalyst deactivation (by coking) compared to the one obtained under sub-critical conditions. This result is explained by the extraction of the coke precursors by the supercritical hydrocarbon. 

Here, L is a mobile ligand which can leave the metal site (M) open briefly for reaction with A in the initial step of the catalytic cycle. The transformation of the M A complex into products completes the cycle. The equilibrium in step (1) lies far to the left in most cases, because the ligands protect the metal centers from agglomeration. Thus, the concentration of M is very small, and the total concentration of catalyst is cMr = cm a + cm l- The rate law which arises from this mechanism is... 

The Fe(III)/S(IV) reaction has long been of interest because of its importance in the catalytic autoxidation of S(IV). The latter reaction is known to have a complex chain mechanism, and the production of SOr radicals has been considered to be the essential chain-initiating step. It is also widely believed that the direct oxidation of S(IV) by Fe(III) is the source of SO -. There is little agreement among the various papers published on the direct reaction of Fe(III) with S(IV) with regard to its mechanism, and much of this disagreement can be traced to the potential for Fe(III) to bind several S(IV) ligands under the typical conditions of excess S(IV). 

Two-Step (Push-Pull, Ping-Pong) Mechanisms Two-step mechanisms are typical of chemical catalytic processes, as opposed to redox catalysis processes, that are discussed and exemplified in Section 6.2. The first step following the generation at the electrode of the active form of the catalyst, Q, is the formation of an adduct, C, with the substrate A (Scheme 2.11). C requires an additional electron transfer to regenerate the initial catalyst, P. There are then two main possibilities. One is when C is easier to reduce (or oxidize in oxidative processes) than P. The main route is then a homogeneous electron... 

Catalyst particle nucleation in the initial stages and their subsequent growth play an important role in catalytic mechanisms. In a model Pt/alumina catalyst, the general view is that the formation of particles is a stepwise process incorporating the following steps (Wynblatt and Gjostein 1975, Cottrell 1971) individual metal atoms (called monomers) transform to two-dimensional islands, which subsequently transform to three-dimensional clusters. These clusters eventually transform into finite-sized particles. 

Metal Hydrides. The simplest reactions in this group are the various catalytic reduction reactions of carbon monoxide. Methane or higher hydrocarbons, methanol or higher alcohols, and a variety of other oxygenated organic compounds may be formed, depending upon the catalyst and reaction conditions (23). There is little evidence about the mechanism of these reactions, but the initial step in every example is probably a carbon monoxide insertion into a metal hydride, followed by reduction reactions. 

TIs also inhibit the reverse transcriptase enzyme s ability to perform one of the initial steps in HIV replication. The NNRTIs, however, directly inhibit the active (catalytic) site on this enzyme, whereas zidovudine and other NRTIs serve as false substrates that take the place of the substance (thymidine) normally acted on by this enzyme (see Reverse Transcriptase Inhibitors Mechanism of Action ). Hence, NNRTIs provide another way to impair one of the key steps in HIV replication, and these drugs can be used along with other agents (NRTIs, protease inhibitors) to provide optimal benefits in preventing HIV replication and proliferation (see the next section). 

In principle, L-proline acts as an enzyme mimic of the metal-free aldolase of type I. Similar to this enzyme L-proline catalyzes the direct aldol reaction according to an enamine mechanism. Thus, for the first time a mimic of the aldolase of type I was found. The close relation of the reaction mechanisms of the aldolase of type 1 [5b] and L-proline [4] is shown in a graphical comparison of both reaction cycles in Scheme 3. In both cases the formation of the enamines Ila and lib, respectively, represents the initial step. These reactions are carried out starting from the corresponding ketone and the amino functionality of the enzyme or L-proline. The conversion of the enamine intermediates Ha and lib, respectively, with an aldehyde, and the subsequent release of the catalytic system (aldolase of type I or L-proline) furnishes the aldol product. 

Stereoselectivity in homogeneous catalytic reactions is well documented. Oftentimes mechanisms can be postulated regarding individual steps in contrast to heterogeneous systems. A recent detailed investigation of the stereoselective formation of C-C bonds catalyzed by (C5H5)2ZrCl2 has been reported by Hoveyda and Xu.12 The initial step in these reactions is the addition of an alkyl Grignard reactant to an unactivated olefin followed by further bond formation as in Equation (Eq.) [I] 12... 

Without a doubt, a complete picture of the dynamics of dissociative chemisorption and the relevant parameters which govern these mechanisms would be incredibly useful in studying and improving industrially relevant catalysis and surface reaction processes. For example, the dissociation of methane on a supported metal catalyst surface is the rate limiting step in the steam reforming of natural gas, an initial step in the production of many different industrial chemicals [1]. Precursor-mediated dissociation has been shown to play a dominant role in epitaxial silicon growth from disilane, a process employed to produce transistors and various microelectronic devices [2]. An examination of the Boltzmann distribution of kinetic energies for a gas at typical industrial catalytic reactor conditions (T 1000 K)... 

Murai et al. [14] found that Ru3(CO)12 shows a high catalytic activity for the intramolecular hetero-P-K-type reaction of yne-aldehydes (Eq. 4). A variety of substituents on the acetylenic moiety can be tolerated, and the application to cyclohexane-fused bicyclic systems is also feasible. Although the mechanism of this catalysis remains elusive, two pathways have been proposed as the initial step for the reaction in Eq. (4) via the oxidative cyclization of yne-aldehydes to a ruthenium center, leading to a metallacycle 3, or via the oxidative addition of an aldehyde C-H bond to ruthenium, leading to 4. 

The proposed free radical chain mechanism for this reaction is given in Scheme 3. The striking catalytic effect of the metal ions such as Cu2+ and Fe3+ is attributed to their ability to accept an electron from the enamine in the chain initiation step. The autooxidation of the SchifFs bases of a,/ -unsaturated ketones is thought to proceed similarly via the enamine form of the SchifFs bases. 

Based on the catalytic results, in combination with the information obtained by 27A1 Mas-NMR and FT-IR, the following reaction mechanism (32) has been proposed. The initial step is chemisorption of the sec-alcohol on a Lewis-acid site, consisting of coordinatively unsaturated Al. This results in the formation of surface alkoxide. The coordinative interaction of the carbonyl of the ketone with the same aluminium center allows the formation of a six-membered transition state in which hydride transfer can occur (Fig. 15.5). 

In the case of AcrHt-Bu, the mechanism of the catalytic four-electron reduction of O2, accompanied by the oxygenation of Bu, is modified, as shown in Scheme 30 (177). The initial ET from AcrHt-Bu to the Co(III)2 complex results in the homoly-tic C9—C bond cleavage to produce f-Bu and AcrH (178, 179). Since the homolytic C9—C bond cleavage is also the catalytic rate-determining step, the... 

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