Transition-state energies for

In the stable trans-form the H atoms lie along the diagonal of the square. The energy of the cis-form, in which the atoms are positioned on one of the edges, is 3-5 kcal/mol higher than that of the trans-form [Smedarchina et al. 1989]. The transition state energies for trans-cis and... 

Another means of resolution depends on the difference in rates of reaction of two enantiomers with a chiral reagent. The transition-state energies for reaction of each enantiomer with one enantiomer of a chiral reagent will be different. This is because the transition states and intermediates (f -substrate... f -reactant) and (5-substrate... R-reactant) are diastereomeric. Kinetic resolution is the term used to describe the separation of enantiomers based on different reaction rates with an enantiomerically pure reagent. 

To account for the course of this reaction theoretical calculations of the coordination of ketomalonate 37 to copper(II) and zinc(II) have revealed that the six-membered ring system is slightly more stable than the five-membered ring system (Scheme 4.30). The coordination of 37 to catalyst (l )-39 shows that the six-membered intermediate is C2-symmetric with no obvious face-shielding of the carbonyl functionality (top), while for the five-membered intermediate (bottom) the carbonyl is shielded by the phenyl substituent. Calculations of the transition-state energy for the reaction of the two intermediates with 1,3-cyclohexadiene leads to the lowest energy for the five-membered intermediate this approach is in agreement with the experimental results [45]. 

The four different transition states in Fig. 8.10 were considered with BF3 as a model for the BLA catalyst in the theoretical calculations. It was found that the lowest transition-state energy for the BF3-catalyzed reactions was calculated to be 21.3 kcal mol for anti-exo transition state, while only 1.5 kcal mol higher in energy the syn-exo transition state, was found. The uncatalyzed reaction was calculation to proceed via an exo transition state having an energy of 37.0 kcal mol . The calculations indicated that the reaction proceeds predominantly by an exo transition-state structure and that it is enhanced by the coordination of the Lewis acid. 

The above mentioned conclusion is confirmed nicely by energetic measurements63 (Fig. 2). The experimentally derived transition state energy for OH loss from 168 is approximately 175 kcal mol-1. This is much too low for a direct a-cleavage process which would give rise to the formation of 170 and OH , the sum of the heats of formation of which is around 190 kcal mol-1. However, the transition... 

The formation of the adduct (112) undoubtedly reflects the higher energy of tetrafluorobenzyne, as compared with benzyne, and may also possibly result, in part, from a lower transition state energy for the formation of a cyclo-adduct with tetrafluorobenzyne. 

Although accurate kinetics have not been obtained experimentally, the AG transition-state energy for the above dehydrogenation step has been estimated as 10 1 kcal/mol (15). Since this measurement starts at the energy level of the 1,5-bridged cation, the actual barrier for the second step should be at least 1 kcal/mol lower, e.g. 9 1 kcal/mol. 

Pericas and coworkers173 studied the endo selective reactions of 1-alkoxy-l,3-butadienes and 1-alkoxy-l,3-octadienes with maleic anhydride. They found that the trans-2-phenyl-cyclohexan-l-ol and 3-exo-(neopentyloxy)isobornan-l-ol based chiral dienes induced the highest facial selectivities. The relative transition state energies for the formation of the different diastereomers were calculated using semi-empirical methods (AMI). 

Figure 4.10. Calculated chemisorption energies for CO (top panel) and for different atomic adsorbates (middle panel), together with transition state energies for dissociation reactions (bottom panel) which are shown as a function of the average energy of the d states projected onto the surfaces atoms to which the adsorbates form bonds. Adapted from Ref. [29].

Figure 4.22. Calculated transition state energies for N2 dissociation shown as a function of the dissociative N2 chemisorption energy for both close-packed and stepped metal surfaces. Adapted from Ref. [74].

The transition state energy for the reaction is lowered by 33 kj mol1 by cyclophilin Ak and by 27 kj mol 1 by FKBP-12 with corresponding rate increases of — 6 x 105- and 5 x hT-fold, respectively. 

Table 9.12 compares partial rate factors for substitution by phenyl radical with those for electrophilic bromination. Selectivity is clearly much lower for the radical substitution furthermore, for attacking phenyl radical, nearly all positions in the substituted benzenes are more reactive than in benzene itself, a finding that reflects the tendency for most substituents to stabilize a radical, and thus to lower transition state energy for formation of the cyclohexadienyl intermediate, when compared with hydrogen. The strong polar effects, which cause the familiar pattern of activation and deactivation in the electrophilic substitutions, are absent. One factor that presumably contributes to the low selectivity in radical attack is an early transition state in the addition step, which is exothermic by roughly 20 kcal mole-1.178... 

Calculations of the electron density on the carbon atoms in the title compound (11) show that position 2 (and 7) has the lowest electron density (Table I) and therefore can be expected to have the lowest transition state energy for the charge-controlled addition of the amide ion.13141819 These calculations agree with experiment upon dissolving 11 in KNH,/NH3 at — 40 C, the H- and 13C-NMR spectra unequivocally show the presence of only one rr-adduct, 2-aminodihydro-l,8-naphthyridinide (12)15 (see Tables II and III). Increasing the temperature of this solution from —40 to 10 C does not change the H- and 13C-NMR spectra 12 is still the sole [Pg.105]

By examination of the stereochemical consequences of decarboxylation, Cram and Haberfield8 obtained evidence for internal return of carbon dioxide to the carbanion, affecting the stereochemical outcome of these reactions. It is reasonable to consider that the barrier for the combination of the carbanion and carbon dioxide may be comparable to or lower than that for diffusion, in which case the reverse reaction will be a kinetically significant component in the overall rate of reaction. In such a case, a catalyst cannot deal with the direction of the reaction -if it lowers the transition state energy for the forward reaction, conservation of energy demands that it also lower the barrier for the reverse reaction. The energy for addition of the carbanion to carbon dioxide is also inherent. The reaction should occur readily if the reaction partners have reduced entropy. 

Hydrogen bonds appear to be essential in all enzyme-catalyzed reactions, although why they are essential and how they promote function is an open question. In recent years a specific hypothesis for their involvement in catalysis has emerged so-called low-barrier hydrogen bonds (LBHB) have been proposed to lower the transition state energy for many enzymatic reactions, including those of serine protease, citrate... 

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