Concerted reaction alternative models

In each of the chapters of this text, we will explore the use of different models to explain and predict the structures and reactions of organic compounds. For example, we will consider alternative explanations for the hybridization of orbitals, the c,7c description of the carbon-carbon double bond, the effect of branching on the stability of alkanes, the electronic nature of substitution reactions, the acid-base properties of organic compounds, and the nature of concerted reactions. The complementary models presented in these discussions will give new perspectives on the structures and reactions of organic compounds. 

A study of debrominations of vtc-dibromides promoted by diaryl tellurides and din-hexyl telluride has established several key features of the elimination process the highly stereoselective reactions of e/7f/tro-dibromides are much more rapid than for fhreo-dibromides, to form trans- and cw-alkenes, respectively the reaction is accelerated in a more polar solvent, and by electron-donating substituents on the diaryl telluride or carbocation stabilizing substituents on the carbons bearing bromine. Alternative mechanistic interpretations of the reaction, which is of first-order dependence on both telluride and vtc-dibromide, have been considered. These have included involvement of TeAr2 in nucleophilic attack on carbon (with displacement of Br and formation of a telluronium intermediate), nucleophilic attack on bromine (concerted E2- k debromination) and abstraction of Br+ from an intermediate carbocation. These alternatives have been discounted in favour of a bromonium ion model (Scheme 9) in which the role of TeArs is to abstract Br+ in competition with reversal of the preequilibrium bromonium ion formation. The insensitivity of reaction rate to added LiBr suggests that the bromonium ion is tightly paired with Br. ... 

The formaldehyde disproportionation has been examined by semi-empirical MO methods (Rzepa and Miller, 1985). With the MNDO procedure, transfer of hydride from hydrate mono-anion to formaldehyde is exothermic by 109 kJ mol-1, and the transition structure [29], corresponding to near symmetrical transfer of hydride, lies 72 kJ mol -1 above the separated reactants. Inclusion of two water molecules, to model solvation effects, stabilizes reactants and transition structures equally. Hydride transfer from the hydrate dianion was found to have a less symmetrical transition structure [30] not unexpected for a more exothermic reaction, but the calculated activation energy, 213 kJ mol-1, is unexpectedly high. Semi-classical primary kinetic isotope effects, kH/kD = 2.864 and 3.941 respectively, have been calculated. Pathways involving electron or atom transfers have also been examined, and these are predicted to be competitive with concerted hydride transfers in reactions of aromatic aldehydes. Experimental evidence for these alternatives is discussed later. 

The models of concerted processes discussed above are only a crude approximation of the motion of a complex system of nuclei along the reaction coordinate. However, such an approximation apparently permits one to choose between the possible reaction mechanisms. The reliability of such a choice increases through a comparative examination of alternative reaction coordinates. 

The model calculations for both suggested reaction pathways are summarized in Fig. 4. In contrast to the experimental results the calculations predict that bicyclobutane 5 is more stable than cyclobutene 6. Obviously, the silyl substituents in the experiment destabilize 11 compared to 2. The preferred reaction path for the isomerization 5 -> 6 proceeds via a 1,2-H-shift. It involves, however, a significant barrier of 39.8 kcal mol". The highest point along the alternative multistep pathway, the transition state for the concerted ring opening of bicyclobutane 5 to form s-trans butadiene 10 is... 

A comparative ab initio study [22] on these two alternative reaction pathways concludes that a zwitterionic structure of the type suggested by Westheimer is not a stationary point on the explored potential energy surfaces for the systems H2O + (H0)2P(0)H and H2O + (H0)3P(0). Two types of critical points were found for these model systems. The first type corresponds to transition structures for the concerted addition of water to these phosphoryl compounds (Figure 3.1). 

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