Reactions mechanisms, computational studies

Varying the substituent X in the alkyne branches from NO2 to NMei increases the cyclization rate 29,000 times, the same variation of Y in allene branch leads to 18,000 times increase in the rate, although normally the electronic effect of substitution is much lower, maximum acceleration at 60°C is 2.6 for the change from F to NO2 [424, 428], This may indicate a change in the reaction mechanism. Computational studies [429,430] for the compounds with amino substituents indicated that the intermediate does not have any diradical character 3.974, but... 

The activation energies were computed to 3.0 (toward 183), 0.3 (toward 182), and 21.8 kcal/mol (toward 184) at the B3-LYP/6-31G level, and thus the mechanism leading to 182 is the preferred one. The transition states of all three reactions belong to concerted but asynchronous reaction paths. The transacetalization of 177 with acylium cations results in the formation of the thermodynamically stabilized 187 (Scheme 121) [97JCS(P2)2105]. 186 is less stable than 187, and 185 is destabilized by 32.5 kcal/mol. Moreover, transacetalization of 177 with sulfinyl cations is not a general reaction. Further computational studies on dioxanes cover electrophilic additions to methylenedioxanes [98JCS(P2)1129] and the influence... 

S.-F. Wu, R.D. Levine, Quantum mechanical computational studies of chemical reactions ... 

S.-F. Wu, B. R. Johnson, and R. D. Levine, Quantum mechanical computational studies of chemical reactions III. Collinear A + BC reaction with some model potential energy surfaces, Mol. Phys. 25 839 (1973). 

More specifically, calculations have suggested that the ruthenium-alkyl complex in Equation 6.53 reacts with arene to exchange covalent ligands by a process closely related to a a-bond metathesis mechanism. Computational studies of the reactions of a simple iridium-alkyl and alkoxo complex with alkanes to generate new metal-alkyl complexes have also suggested that a mechanism is followed that involves many of the characteristics of a classic a-bond metathesis transition state. However, calculations of the mechanisms of these two processes imply that the transition state contains some degree of M-H bonding. 

The mechanism of catalyzed reaction was investigated comprehensively using 0-exchange experiments, kinetic analysis of the reaction, and computational studies. 

To fully elucidate the mechanism of the reaction a computational study using the M06 functional was undertaken. Most of the overall mechanism, such as the involvement of a P-hydride elimination, a migratory insertion, and some details on the structure of the active catalyst, had been determined experimentally. Therefore, the main challenge to be solved by this computational study was to confirm the mechanistic proposal and clarify the fine details such as the distinction between the two pathways in which either the hydride (pathway a) or the amine (pathway b) are located trans to the alkoxide. The full calculated, catalytic cycle is shown in Scheme 8.12. 

Using MMd. calculate A H and. V leading to ATT and t his reaction has been the subject of computational studies (Kar, Len/ and Vaughan, 1994) and experimental studies by Akimoto et al, (Akimoto, Sprung, and Pitts. 1972) and by Kapej n et al, (Kapeijn, van der Steen, and Mol, 198.V), Quantum mechanical systems, including the quantum harmonic oscillator, will be treated in more detail in later chapters. 

A recent paper by Singh et al. summarized the mechanism of the pyrazole formation via the Knorr reaction between diketones and monosubstituted hydrazines. The diketone is in equilibrium with its enolate forms 28a and 28b and NMR studies have shown the carbonyl group to react faster than its enolate forms.Computational studies were done to show that the product distribution ratio depended on the rates of dehydration of the 3,5-dihydroxy pyrazolidine intermediates of the two isomeric pathways for an unsymmetrical diketone 28. The affect of the hydrazine substituent R on the dehydration of the dihydroxy intermediates 19 and 22 was studied using semi-empirical calculations. ... 

Further computational studies on adenines and adenosines concern the reaction mechanism of ribonuclease A with cytidyl-3,5 -adenosine [99BP697] and the molecular recognition of modified adenine nucleotides [99JMC5338]. 

As is common in heterocyclic chemistry, many studies concern tautomeric equilibria. While quantum chemical calculations are straightforward for the question of the most stable isomer, experiments are sometimes very demanding. Therefore, quantum chemistry can easily provide answers that may require substantial experimental effort. Comparatively few studies concern the investigation of entire reaction paths. This is much more demanding than computing a limited number of tautomers, of course, but usually provides a very detailed picture of the reaction mechanism. In certain cases, it was only possible to judge the nature of a chemical reaction on the basis of quantum chemical calculations. 

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. 

Palladium(II) complexes provide convenient access into this class of catalysts. Some examples of complexes which have been found to be successful catalysts are shown in Scheme 11. They were able to get reasonable turnover numbers in the Heck reaction of aryl bromides and even aryl chlorides [22,190-195]. Mechanistic studies concentrated on the Heck reaction [195] or separated steps like the oxidative addition and reductive elimination [196-199]. Computational studies by DFT calculations indicated that the mechanism for NHC complexes is most likely the same as that for phosphine ligands [169], but also in this case there is a need for more data before a definitive answer can be given on the mechanism. 

Transient computations of methane, ethane, and propane gas-jet diffusion flames in Ig and Oy have been performed using the numerical code developed by Katta [30,46], with a detailed reaction mechanism [47,48] (33 species and 112 elementary steps) for these fuels and a simple radiation heat-loss model [49], for the high fuel-flow condition. The results for methane and ethane can be obtained from earlier studies [44,45]. For propane. Figure 8.1.5 shows the calculated flame structure in Ig and Og. The variables on the right half include, velocity vectors (v), isotherms (T), total heat-release rate ( j), and the local equivalence ratio (( locai) while on the left half the total molar flux vectors of atomic hydrogen (M ), oxygen mole fraction oxygen consumption rate... 

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