Computational studies reactions

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. ... 

Many computational studies in heterocyclic chemistry deal with proton transfer reactions between different tautomeric structures. Activation energies of these reactions obtained from quantum chemical calculations need further corrections, since tunneling effects may lower the effective barriers considerably. These effects can either be estimated by simple models or computed more precisely via the determination of the transmission coefficients within the framework of variational transition state calculations [92CPC235, 93JA2408]. 

The deprotonation of 132 is favored at Ni and the coordination of 135 occurs preferentially at 82- A second entity of 135 coordinates at N3. A computational study of thiouracil derivatives of the tungsten(O) hexacarbonyl shows that the sulfur-bound thiouracil is serving as a ir-donor during the CO dissociation (Scheme 91) [99IC4715]. DFT calculations show that 137 is significantly stabilized with respect to the alternative reaction product 138. 

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]. 

A few computational studies focus on the saturated analog of 4//-l,4-oxazine, i.e., morpholine [98JCS(P2)1223, 00JCS(P2)1619, 00TL5077]. These cover the structure of lithium morpholide, cycloaddition reactions, and molecular complexes with genistein. 

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... 

The overall reaction is exothermic by -18.8 kcal/mol and proceeds via pre-complexation of the CO2. For further computational studies on dithianes, see Refs. [97BCJ2571, 99T5027]. 

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. 

Note that all the ions involved have already been produced by previous reactions and that apart from those of Reactions 30 and 33 the heats of reaction are small. Thus, with these reactions and similar ones, there is no reason why equilibria should not be maintained. Again, the need to treat a large number of reactions simultaneously to explain the results is stressed. A computer study of such a reaction scheme is now under way at AeroChem. 

The previously outlined mechanistic scheme, postulating reversible propagation and cyclization, was simplified by neglecting the de-cyclization because in the very short time of the studied reaction the extent of de-cyclization is negligible. The rate constants appearing in the appropriate differential equations were computer adjusted until the calculated conversion curves, shown in Fig. 7, fit the experimental points. The results seem to be reliable inspite of the stiffness of the differential equations. 

In the computer simulations It was necessary to study reaction sequences more complex than those studied by Barkelew, which consequently led to rate functions having double rather than single concentration dependence. Numerous results from both theoretical and computational analyses. Including the effects of e and Tr, have been described elsewhere (see especially Figure 8 of reference 1). 

For a computational study on the origin of the enantioselectivity in this reaction, see Dudding T, Houk KN (2004) Proc Natl Acad Sci USA 101 5770-5775... 

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