Reaction mechanisms, theoretical study

Goodman concluded that QMT dominated the 1,2-H shifts of MeCCl or MeCBr at low temperatures. At higher temperatures, the classical (activated) mechanism with a = 4.9 kcal/mol, became important, but even at 298 K, QMT was considered to account for >85% of the reaction.60 Theoretical studies supported the incursion of QMT in these 1,2-H rearrangements, accounting for both the KIE and entropic peculiarities.63 However, calculations implied that the classical mechanism overtook QMT at quite low temperatures (>200 K), so that the 1,2-H shift would be very largely classical at ambient temperature.63 It was also deemed possible that QMT could be important in 1,2-C shifts.63... 

An important improvement in the catalysis of olefin epoxidation arose with the discovery of methyltrioxorhenium (MTO) and its derivatives as efficient catalysts for olefin epoxidation by Herrmann and coworkers [16-18]. Since then a broad variety of substituted olefins has been successfully used as substrates [103] and the reaction mechanism was studied theoretically [67, 68, 80]. 

The preference for the [3+2] mechanism does not provide in itself an explanation for the high enantioselectivity observed in these reactions. The theoretical studies undertaken to discern the mechanism were carried out at quantum mechanical level on a model achiral system. To consider the asym-... 

The microscopic level is that of fundamental studies, and deals with the details of adsorption on surfaces, reaction mechanisms, theoretical modeling, and surface science. 

The originally proposed stereochemical model by Hajos and Parrish" was rejected by M.E. Jung and A. Eschenmoser. They proposed a one-proline aldolase-type mechanism involving a side chain enamine. The most widely accepted transition state model to account for the observed stereochemistry was proposed by C. Agami et al. suggesting the involvement of two (S)-(-)-proline molecules. " " Recently, K.N. Houk and co-workers reexamined the mechanism of the intra- and intermolecular (S)-(-)-proline catalyzed aldol reactions. Their theoretical studies, kinetic, stereochemical and dilution experiments support a one-proline mechanism where the reaction goes through a six-membered chairlike transition state. 

Restrepo-Cossio, A. A., Cano, H., Mari, F., Gonzalez, C. A. Molecular modeling of the Wittig reaction. 6. Theoretical study of the mechanism of the Wittig reaction ab initio and MNDO-PM3 treatment of the reaction of unstabilized, semistabilized and stabilized ylides with acetaldehyde. Heteroatom Chem. 1997, 8, 557-569. 

A number of books [17] and reviews [18] provide a description of cytochrome P450. Also see recent papers devoted to its reactions [19], theoretical studies [20], and mechanism of action [21], as well as certain biochemical investigations [22]. 

Even when the cyanide group is not directly involved in the reaction, it can have a considerable influence on the reaction mechanism. A study of kinetic deuterium isotope effects on a series of Cope rearrangementsshowed that rearrangement of the 3,3-dicyanohexa-1,5-diene (132) had the /rn/te ratio at C(4) (i.e. bond breaking) greater than the A d/ h ratio at C(6) bond making) rather than the reverse, which was observed with other substituents. A theoretical studyof the reaction indicated that this reaction is... 

Rhodium-catalyzed carbonylation of methanol is known as the Monsanto process, which has been studied extensively. From the reaction mechanism aspect, the study of kinetics has proved that the oxidative addition of methyl iodide to the [Rh(CO)2l2] is the rate-determining step of the catalytic cycle. It was also observed that acetyl iodide readily adds to [Rh(CO)2l2], indicating that the acetyl iodide must be scavenged by hydrolysis in order to drive the overall catalytic reaction forward. An alternative to sequential reductive elimination and the hydrolysis of acetyl iodide is the nucleophilic attack of water on the Rh acetyl complex and the production of acetic acid. The relative importance of these two alternative pathways has not yet been fully determined, although the catalytic mechanism is normally depicted as proceeding via the reductive elimination of acetyl iodide from the rhodium center. The addition of iodide salts, especially lithium iodide, can realize the reaction run at lower water concentrations thus, byproduct formation via the water gas shift reaction is reduced, subsequently improving raw materials consumption and reducing downstream separation. In addition to the experimental studies of the catalytic mechanism, theoretical studies have also been carried out to understand the reaction mechanism [17-20]. 

The reactions conducted with ligand-less cationic systems are proposed to occur through the rhodium(I) boryl complexes shown in Scheme 16.12, but fewer data are available to support this mechanism. Theoretical studies have implied that insertion of the olefin into the metal-boryl linkage occurs and that this step is followed by oxidative addition of the borane and reductive elimination to form the C-H bond of the product and the starting rhodiiun(I) boryl complex. 

Sunoj and coworker have identified the role of protic co-catalysts such as water, methanol, and formic acid in the MBH/aza-MBH reaction by theoretical studies [17bj. They found that the protic co-catalysts have a profound influence in decreasing the activation barriers associated with the key elementary steps due to the improved stabilization of the proton transfer transition state through a relay mechanism. 

The photochemical isomerization of various spiropyran molecules in different media (gas phase, solution, mono-layers, polymers) has been extensively experimentally [1-3, 20-30] and theoretically [31, 32] studied in order to understand the reaction mechanism. Less studied, however, are the effect of protonation, charge, and external fields on the isomerization pathway of this electrocyclic reaction. It was recently reported that the rate of isomerization of the SP form is considerably increased by protonation of the... 

It should be noted that formation of trans-product can be achieved in an anti-addition reaction through the outer-sphere mechanism. Theoretical studies have demonstrated that syn-addition and anti-addition reactions may start from the same 7i-complex, and direction of the multiple bond activation depends on the polarity of solvent [17, 18]. Relative reactivity in the inner-sphere and outer-sphere mechanisms contributes to the overall -/Z- selectivity of the addition reaction to alkynes (stereoselectivity issue). In some cases it is possible to switch the direction of C-Het bond formation by finding a suitable ligand [19]. In case of alkenes syn-addition and a f -addition processes do not necessarily result in different stereochemistry (unrestricted rotation around the single C-C bond in the product). Occurrence of these mechanisms for the N [20, 21], P [22, 23], O [24-26], S, Se [27, 28] heteroatom groups and application of different metal catalysts are discussed in detail in the other chapters of this book. Stereochemical pathways of nucleometallation and development of enantioselective catalytic procedures were reviewed [29]. In this chapter we focus our attention on the mechanism of irmer-sphere insertion reaction involving double and triple carbon-carbon bonds. 

In the first chapter, devoted to thiazole itself, specific emphasis has been given to the structure and mechanistic aspects of the reactivity of the molecule most of the theoretical methods and physical techniques available to date have been applied in the study of thiazole and its derivatives, and the results are discussed in detail The chapter devoted to methods of synthesis is especially detailed and traces the way for the preparation of any monocyclic thiazole derivative. Three chapters concern the non-tautomeric functional derivatives, and two are devoted to amino-, hydroxy- and mercaptothiazoles these chapters constitute the core of the book. All discussion of chemical properties is complemented by tables in which all the known derivatives are inventoried and characterized by their usual physical properties. This information should be of particular value to organic chemists in identifying natural or Synthetic thiazoles. Two brief chapters concern mesoionic thiazoles and selenazoles. Finally, an important chapter is devoted to cyanine dyes derived from thiazolium salts, completing some classical reviews on the subject and discussing recent developments in the studies of the reaction mechanisms involved in their synthesis. 

The first systematic theoretical study on dihydro-1,2,4-triazines was recently carried out (98JOC5824) the stabilities of all the possible unsubstituted dihydro-1,2,4-triazines were calculated using various theoretical methods, all reliable calculation methods consistently show that the 2,5-dihydro isomer 98 is the most stable. This is in perfect agreement with the experimental observations all the synthetic methods used for the preparation of dihydro-1,2,4-triazines result in 2,5-dihydro isomer 98, provided the structures of the reactants and the reaction mechanism allow its formation. Thus, although Metze and Scherowsky (59CB2481) claimed the formation of 1,2-dihydro-1,2,4-triazine 92 (R = = Ph) in the reduction... 

Because of the complexity of the pathway, the sensitivity of the reagents involved, the heterogeneous nature of the reaction, and the limitations of modern experimental techniques and instrumentation, it is not surprising that a compelling picture of the mechanism of the Simmons-Smith reaction has yet to emerge. In recent years, the application of computational techniques to the study of the mechanism has become important. Enabling theoretical advances, namely the implementation of density functional theory, have finally made this complex system amenable to calculation. These studies not only provide support for earlier conclusions regarding the reaction mechanism, but they have also opened new mechanistic possibilities to view. 

Orlova et al. (2003) theoretically studied the mechanism of the firefly bioluminescence reaction on the basis of the hybrid density functional theory. According to their conclusion, changes in the color of light emission by rotating the two rings on the 2-2 axis is unlikely, whereas the participation of the enol-forms of oxyluciferin in bioluminescence is plausible but not essential to explain the multicolor emission. They predicted that the color of the bioluminescence depends on the polarization of the oxyluciferin molecule (at its OH and O-termini) in the microenvironment of the luciferase active site the... 

A wide range of nitroxidcs and derived alkoxyamincs has now been explored for application in NMP. Experimental work and theoretical studies have been carried out to establish structure-property correlations and provide further understanding of the kinetics and mechanism. Important parameters are the value of the activation-deactivation equilibrium constant K and the values of kaa and (Scheme 9.17), the combination disproportionation ratio for the reaction of the nilroxide with Ihe propagating radical (Section 9.3.6.3) and the intrinsic stability of the nitroxide and the alkoxyamine under the polymerization conditions (Section 9.3.6.4). The values of K, k3Cl and ktieact are influenced by several factors.11-1 "7-"9 ... 

How deeply one wishes to query the mechanism depends on the detail sought. In one sense, the quest is never done a finer and finer resolution of the mechanism may be obtained with further study. For example, the rates and mechanisms of electron transfer reactions have been studied experimentally and theoretically since the 1950s. but the research continues unabated as issues of ever finer detail and broader import are examined. The same can be said of other reactions—nucleophilic substitution, hydrolysis, etc. 

Continuous emulsion polymerization systems are studied to elucidate reaction mechanisms and to generate the knowledge necessary for the development of commercial continuous processes. Problems encountered with the development of continuous reactor systems and some of the ways of dealing with these problems will be discussed in this paper. Those interested in more detailed information on chemical mechanisms and theoretical models should consult the review papers by Ugelstad and Hansen (1), (kinetics and mechanisms) and by Poehlein and Dougherty (2, (continuous emulsion polymerization). 

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