Enhanced polarization of the transition state

The effect of water molecules on pericyclic reactions can also be compared with the effects of Lewis acids on these reactions. The enhanced polarization of the transition state in these reactions would lead to stronger hydrogen bonds at the polar groups of the reactants, which will result in a substantial stabilization of the transition states in the same way Lewis acids do. A computer-simulation study on the Diels-Alder reaction of cyclopentadiene by Jorgensen indicated that this effect contributes about a factor of 10 to the rates.7... 

Theoretical work by the groups directed by Sustmann and, very recently, Mattay attributes the preference for the formation of endo cycloadduct in solution to the polarity of the solvent Their calculations indicate that in the gas phase the exo transition state has a lower energy than the endo counterpart and it is only upon introduction of the solvent that this situation reverses, due to the difference in polarity of both transition states (Figure 1.2). Mattay" stresses the importance of the dienophile transoid-dsoid conformational equilibrium in determining the endo-exo selectivity. The transoid conformation is favoured in solution and is shown to lead to endo product, whereas the cisoid conformation, that is favoured in the gas phase, produces the exo adduct This view is in conflict with ab initio calculations by Houk, indicating an enhanced secondary orbital interaction in the cisoid endo transition state . 

If stabilization of the transition state (TS) is more effective than that of the ground state (GS), this results in enhancement of reactivity as a result of a decrease in the activation energy (Fig. 3.5), because of electrostatic (dipole-dipole type) interactions of polar molecules with the electric field. 

This high sensitivity to polar effects of the homolytic alkylation of protonated heteroaromatic bases has been interpreted in terms of the transition state.This would be similar to a w-complex in which an enhanced contribution of polar forms (5) would explain the high sensitivity to polar influence. 

Recently, it has been postulated that the increase of the polarity of the reaction system (i.e., either development or increase of dipole moment) from the ground state (GS) towards the transition state (TS) can result in an acceleration of the reaction due to a stronger interaction of microwaves with the reagents during the course of the reaction. Thus, non-thermal microwave effect can be expected for the reaction with polar mechanisms when the stabilization of the transition state (TS) is more effective than that of the ground state (GS), which results in an enhancement of the reactivity as a result of the decrease in activation energy (Fig. 2.3) [36]. 

State is highly sensitive to the medium. There is a change of polarity of the reaction states from initial to transition state with a volume decrease known as electro-strictive shrinkage. Electrostriction can be critical to rate enhancement under pressure. In terms of kinetic parameters it is related to the pressure dependence of the dielectric constant (dins/dp). Some representative values of this quantity are listed in Table 10.15 [50]. 

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