Catalytic Mechanisms bimolecular

This proposed catalytic mechanism (Chong and Sharpless, 1977) requires four reaction steps (3 bimolecular and 1 unimolecular), which take place on a molybdenum metal center (titanium and vanadium centers are also effective), to which various nonreactive ligands (L) and reactive ligands (e g., O-R) are bonded. Each step around the catalytic cycle is an elementary reaction and one complete cycle is called a turnover. 

Such redox reactions are frequently catalysed by platinum [3], other noble metals [232], silver [126-128], and carbons [233] which are all electronconducting solids. This fact points to a simple catalytic mechanism whereby the electron is transferred from Redi to Ox2 through the solid phase, as depicted in Fig. 19. In contrast to other bimolecular catalytic mechanisms (Sect. 1.5.3), the two reactants do not need to occupy neighbouring sites. Since the catalytic rate depends upon the coupled transfers of an electron from Red] to the solid and from the solid to Ox2, the kinetics are best treated in electrochemical terms. 

A similar, but bimolecular, photoinduced reaction was observed on the basis of the nickel complex (28), p-toluene thiolate, and thioanisole reactants to generate methane and disulfide. The thiyl radical and Ni(I) complex was prepared by the photolysis of the Ni(II) complex (28) and j -toluene-thiolate anion in acetonitrile solution. Upon irradiation (A, = 350 nm) of the mixture of complex (28), j -toluene-thiolate ion, and thioanisole in acetonitrile under argon, gas chromatography-mass spectral analysis showed the formation of methane, ditolyl disulfide (TolS)2, and a mixed disulfide TolSSPh. The proposed catalytic mechanism is depicted in... 

On the other hand, Hurst s group proposed another mechanism of water oxidation catalyzed by 2, based on structural [102], kinetic [101], and isotopic distribution data of O2 evolution [103]. Their mechanism involves a Ru ORu intermediate with a terminal ruthenyl O atom on each center that is considered to be either an O2 evolving species or its intermediate precursor. They proposed two pathways for a catalytic mechanism the elements of H2O are added as OH and H onto the adjacent terminal ruthenyl O atoms [as pathway (i). Fig. 14a], and OH and H are added to a bpy ring and one of the ruthenyl O atoms for incorporation of two water molecules [as pathway (ii). Fig. 14b] [103]. Their isotopic distribution data can be quantitatively accounted for by two pathways. It excludes a direct 0-0 bond formation either by the intramolecular coupling of Ru = O groups within a single complex or by the intermolecular mechanism by bimolecular reactions of the complexes. 

For example, in the presence of alkynes highly substituted pyrroles 54 were obtained in 56-88% yields [65], whereas working with a bimolecular amount of imine P-lactams 55 were obtained in 27-66% isolated yields [66]. The reaction can also be performed in the presence of two differently substituted imines, that is, N-aryl/alkylimines and N-tosylimines [67], regioselectively yielding imidazoles 56. It is worth noting that no products incorporating two of the same imine are observed. This selectivity is believed to result from the catalytic mechanism. In particular, the N-tosylimine is not sufficiently nucleophilic to interact with the acid chloride for... 

The proposed mechanism for the DD process is not intended to represent that of any actual catalytic reaction, but to simulate a generic bimolecular reaction. Monte Carlo simulations of the reaction mechanism described by Eqs. (21)-(25) have shown the existence of IPTs exhibiting a rich variety of critical behavior. 

Schechter 55) proposed that the catalytic effect of hydroxyl groups on the epoxide-amine addition reaction involved a termolecular activated complex formed in the concerted reaction of amine, epoxide and hydroxyl. Smith 57) suggested a modified mechanism in which the same activated complex is formed in a bimolecular reaction between an adduct formed from epoxide (E) and the proton donor (HX), and the amine ... 

In the case of the nonisothermal first-order exothermic reaction heat is auto catalytic, for it raises the temperature and provokes an increase of reaction rate, yet is itself a product of the reaction. In the Gray-Scott scheme, B is plainly autocatalytic and its degeneration by the second reaction plays the role of the direct cooling in the non-isothermal case. This reaction appears in the chemical engineering literature in 1983,16 and is the keynote reaction in Gray and Scott s 1990 monograph on Chemical Oscillations and Instabilities.17 A justification of the autocatalytic mechanism in terms of successive bimolecular reactions is the subject of Chapter 12. 

The density of the vapour from red or yellow phosphorus is the same, and it corresponds with the mol. P4 hence, from the analogy between a vapour and a solute —1.10, 8—it might be inferred that the two varieties of phosphorus would become identical in a common solvent. If a soln. of yellow phosphorus in phosphorus tribromide—with a trace of iodine as catalytic agent—is kept between 170° and 190°, red phosphorus is gradually deposited. R. Schenck measured the cone, of the yellow phosphorus in soln. after the lapse of different intervals of time, and found the reaction to be bimolecular, but when allowance is made for the mechanical removal of the catalytic agent from the soln. by the precipitated red phosphorus, the reaction is unimolecular. 

Their controlled formation can be utilized to control the course of the chemical reaction. In this context the chiral discrimination of PET processes of a chiral electron acceptor and (pro)chiral electron donors is of special interest We have observed such a discrimination in case of the isomerization of 1,2-diary Icyclo-propanes [122] and, for the first time, in case of a bimolecular PET process, e.g. the dimerization of 1,3-cyclohexadiene in presence of (+) and (—) l,l -bi-naphthalene-2,2 -dicarbonitrile as chiral electron acceptors [123]. Experiments in the same field are undertaken by Schuster and Kim and have been published recently [124], So far the enantiomeric excesses are small (ca. 15% [124] in toluene at —65 °C) but future efforts will certainly give more information about the applicability of catalytic asymmetric PET reactions. Consequently, the conditions which govern the formation and the fate of radical ion pairs are of central importance both for a better understanding of the mechanism and for synthetic applications. 

Phosphonate ester 30 can also be considered as a mimic of the transition state for subsequent esterolysis and aminolysis of the 8-lactone. In fact, the antibody that promotes ring formation was shown to catalyze the stereoselective reaction between 29 and 1,4-phenylenediaminc.39 The kinetic mechanism of the bimolecular process involves random equilibrium binding of lactone and amine, and the observed turnover rate could be approximated from the measured difference between the binding of reactants and the TSA. Again, entropic factors are presumed largely responsible for the observed rate acceleration, with minimal contributions derived from specific catalytic groups at the active site. 

Ring expansion of aromatic compounds by carbene, carbethoxycarbene, chlorocarbene, and carbenoid is well known 256, 336, 351-356). Muller and co-workers reported the reaction of aromatic compounds with carbene generated from a catalytic decomposition of diazomethane with copper salts, and proposed a bimolecular two-step mechanism involving an inverse ylid for the reaction. Miller (336) proposed another bimolecular two-step mechanism for the reaction of benzene with alkylcarbenoids of aluminum. Baldwin and Smith (25) proposed a concerted mechanism for the reaction of aromatic compounds with carbethoxycarbene. Reaction of alkylbenzene with diethylzinc and ethylidene iodide gives 7-methylcyclohepta-l,3,5-triene derivatives in yield 369). The... 

In contrast, the reaction over H-Mor at 200 °C has different characteristics than that over S04/Zr02. H-Mor is inactive at 0°C, and at 200°C a short induction period is observed for the catalytic activity and product selectivity. Although the production of isopentane is predominant in the induction period, an increase in the activity is observed along with the formation of isobutane, butane, and hexanes. The main product is isobutane after the induction period, highlighting the effect of Br0nsted acid sites. The surface alkenes for the bimolecular mechanism... 

Dagonnier et al. (295) also investigated the inclusion of the catalyst temperature into the set of differential equations describing a catalytic reaction. These authors, however, specified an a priori variable, the surface temperature, the meaning of which is not well-defined. The reaction mechanism they present consists of bimolecular reactions as described by Eqs. (44)-(46) ... 

Thus, it can be reasonably supposed that, at low time factor, in the initial conditions (first hour on stream), just a few C4+ ions are formed. Most of the catalytic sites are free and available and the DIS is the prevailing reaction, either with a Sn2 or with a SnI mechanism. When the time factor increases, the other reactions also occur and the consequent increase of the number of R+ ions justifies the predominance of the Sn2 mechanism (m/p = 2.0). However, the catalyst deactivates with t-o-s and the amount of caUdytic sites available for the bimolecular disproportionation reaction decreases. Under these conditions the SnI mechanism should be more important. Expectedly, the observed value of the m/p ratio at 24 hours on stream is about 1.5. 

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