Transition-state energy determination

The preferential stabilization of a particular TS does not need to mirror the relative energies of ground state conformers. According to the Curtin-Hammett principle, the relative transition state energies determine the nature and the ratio of products, as long as reactive conformers can quickly equilibrate. In other words, even if A is the more stable conformer in the ground state, TS may still be the more stable transition state. If TSg is the... 

In summary, it seems that for most Diels-Alder reactions secondary orbital interactions afford a satisfactory rationalisation of the endo-exo selectivity. However, since the endo-exo ratio is determined by small differences in transition state energies, the influence of other interactions, most often steric in origin and different for each particular reaction, is likely to be felt. The compact character of the Diels-Alder activated complex (the activation volume of the retro Diels-Alder reaction is negative) will attenuate these eflfects. The ideas of Sustmann" and Mattay ° provide an attractive alternative explanation, but, at the moment, lack the proper experimental foundation. 

Further studies by Garcia, Mayoral et al. [10b] also included DFT calculations for the BF3-catalyzed reaction of acrolein with butadiene and it was found that the B3LYP transition state also gave the [4+2] cycloadduct, as happens for the MP2 calculations. The calculated activation energy for lowest transition-state energy was between 7.3 and 11.2 kcal mol depending on the basis set used. These values compare well with the activation enthalpies experimentally determined for the reaction of butadiene with methyl acrylate catalyzed by AIGI3 [4 a, 10]. 

The density functional theory calculations of primary 14C KIE and secondary deuterium kinetic isotope effects (SKIE)220 did not reproduce satisfactorily all the experimentally determined 14C KIE and deuterium (4,4-2H2)- and 6,6-2H2-SKIE, though the non-local DFT methods provide transition state energies on a par with correlated molecular orbital theory221. 

Transition state energies have been determined by computation (PM3 and AMI) for the reaction of norbomadiene 74a (X=CH2) and 7-isopropylidenenorbomadiene 74b (X is C=CMe2) with the 1,3 dipoles 23 formed from ring-opening of the A -phenyl and A -benzyl derivatives of aziridine 22 (see, Table 1). These data demonstrate the preference for formation of exo,exo-isomers 75 with norbomadiene in the A -benzyl series, however the energy difference between the transition states for the A -phenyl series is much closer and accords with the drop in stereoselectivity. Introduction of the isopropylidene substituent into the 7-position of the dipolarophile favours formation of the bent-frame isomers 76, especially in the A -phenyl series. These predictions accord well with the stereoselectivities observed experimentally. 

Since the two resulting antipodes have the same thermodynamic stability, the asymmetric synthesis must be kinetically controlled. Therefore, the factor controlling the enantiomeric excess must be the difference between the energies of the two diastereomeric transition states leading to one or to the other antipode respectively. Since the process leading from the olefins to the aldehydes is a multistep one, if more steps involving diastereomeric intermediates exist, the two diastereomeric transition states which determine the type of enantiomeric excess in the products must be identified. 

The simplest case which can be used to explore the factors influencing the difference in the energies of the diastereomeric transition states, which determine asymmetric induction, is the hydroformylation of (Z)-2-butene with the Rh/(—)-DIOP or Pt/ (—)-DIOP catalytic system. In this case, the asymmetric induction cannot be connected with enantioface discrimination in the step leading to the Tt-complex because this olefin has no enantiofaces 68). In the first step of the reaction it is assumed that a Tt-complex is formed by interaction between substrate and catalyst. This Tt-complex, depending on its geometry, can exist in two different conformations arising from the rotation of the olefin around the metal-olefin Tt-system-bond axis. 

While identifying the rate determining parameters of the catalytic trans formation schemes under consideration, we understand better the reason for possible energy correlations in catalysis, the Broensted Polanyi relations for the transition state energies of surface transformations being typically apphed. Such an approach was intensively used in 1960s—1970s to... 

The functional form of the triggers ate based on transition state, as determined by the quantum mechanical calculation and their numerical values are parameterized to satisfy the macroscopically determined rate constant and activation energy. Local equilibration at the end of the reaction helps in maintaining the correct heat of reaction and structure. For the vahdation of the algorithm, it has been implemented to study proton transport in bulk water. In bulk water the two components of the total diffusivity were found to be uncorrelated. 

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