Energy steric/electronic barriers

Finally, in case 3 (Figure 3), Estel and E are both of similar and substantial magnitude. The energy minima fall in the neighborhood of 0 = 45°, 135°, 225° and 315°, and in systems with suitable barriers and substituents the passages past both the steric and the 7r-electronic barriers can be followed by NMR spectroscopy. 

Similarly, a reaction that is allowed is simply one that does not have such an electronic barrier. This does not automatically mean, however, that the reaction will be favorable. Steric interactions or other factors could make the reaction quite slow. All we know is that no additional barrier due to electronic factors contributes to the overall activation energy. 

Overall, steric and electronic factors, which are seen to be small, are found to work in opposite directions and, to some degree, cancel each other out. Consequently, the intrinsic free activation barriers and reaction free energies (AG nt, AG nt), respectively, span a small range for catalysts I-IV and differ by less than l.Okcalmol-1. Thus, oxidative coupling represents the one process (beside allylic isomerization, cf. Section 5.3) among all the critical elementary steps of the C8-cyclodimer channel, that is least influenced by electronic and steric factors. 

Estimated Electronic and Steric Contributions to Intrinsic Activation Barriers and Reaction Energies for Oxidative Coupling via the Most Feasible Pathway along... 

Among catalysts I-IV, which predominantly catalyze the generation of cyclodimer products, the overall lowest intrinsic free-energy barrier of 20.5 kcal mol-1 (AG nt) for 2a -> 8a appears for catalyst IV with L = P(OPh)3, where both electronic and steric factors are seen to assist the formation of VCH to a similar amount. Reductive elimination involves a higher intrinsic barrier (AG nt) for catalysts bearing moderately bulky, donor phosphines,... 

In addition to the electronic difference between PR3 and PH3, bulkier ligands on the phosphine can change the reaction through their steric effect. Using the R = Bu on the anthraphos system, Haenel et al. calculated the available molecular surface (AMS) around the metal center as a measure of the space available to the alkane (13b). They correlated the AMS to the relative reactivities of the catalysts and the results show that two bulky tert-butyl groups on each P certainly limit the access to the metal center, and thus, may reduce the reactivity. Other theoretical studies on the pincer complexes showed that this steric contribution/ limitation plays a less important role than the activation barriers introduced by the catalyst itself (22), where the increase in energy barrier induced by the bulky 4Bu is smaller than the original barriers calculated... 

One of the main aims of such computations is the prediction and rationalization of the optoelectronic spectra in various steric and electronic environments by either semiempirical or ab initio methods or a combination of these, considering equilibrium structures, rotation barriers, vibrational frequencies, and polarizabilities. The accuracy of the results from these calculations can be evaluated by comparison of the predicted ionization potentials (which are related to the orbital energies by Koopman s theorem) with experimental values. 

Steric factors are often responsible for skeletal isomerization in ion-radical states. The simple example in Scheme 6.31 illustrates the effect of steric congestion on activation energy of this kind of isomerization and depicts the transition of 2,2,3,3-tetramethylmethylenecyclopropane into 1,1,2,2-tetramethyltrimethylenemethane cation-radical. The rearrangement is brought about by one-electron oxidation of the substrate and represents an entirely barrierless process. Interestingly, methylenecy-clopropane (bearing no methyl groups) is protected from such a spontaneous collapse by a barrier of 7.4 k J mol l (Bally et al. 2005). 

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