Ionic transition state

In reactions at the sulfur atom of a sulfinate ion to form a sulfone, of a sulfoxide to form R3S+—0, or of bisulfite ion to form a sulfonic acid, the fractionally positive sulfur becomes more positively charged in the poly-ionic transition states. Definitive experimental evidence... 

This is a reaction in which neutral molecules react to give a dipolar or ionic transition state, and some rate acceleration from the added neutral salt is to be expected53, since the added salt will increase the polarity or effective dielectric constant of the medium. Some of the rate increases due to added neutral salts are attributable to this cause, but it is doubtful that they are all thus explained. The set of data for constant initial chloride and initial salt concentrations and variable initial amine concentrations affords some insight into this aspect of the problem. 

This oxad reaction is first order in both complex and CH3I, and the entropy of activation is large and negative. It is believed that the reaction goes through an ionic transition state in which CH3 adds to a pair of electrons on Ir followed by the attachment of I- in the trans position. 

The above SKIE data were taken as evidence against an ionic transition state. Allylic cation-like species would result in much larger normal SKIE at C(4) in a polar medium than in non-polar media by approaching the maximum possible value for conversion of an sp3 C(4) of the ether to an sp2 carbon of an allyl cation. 

The mechanism schematized above is a summary of the current knowledge. The role of Asp102 has long been controversial [10], Indeed, the catalytic triad has been depicted as a charge-relay system, meaning that the activation of the serine residue involves a concerted transfer of two protons, i.e., from serine to histidine and then to aspartic acid. More recent studies have shown that aspartic acid remains ionized and serves to stabilize the ionic transition state [6] [14-16],... 

The activation parameters for the (bimolecular) addition and the (unim-olecular) heterolysis steps have been determined [28] for the case of Ri, R2, R3 = H or CH3 and the results are shown in Fig. 1. It is obvious that the heterolysis reaction is entropy controlled which is the consequence of the highly ionic transition state which leads to freezing of water molecules with the concomitant loss of entropy. 

Because many of them are nearly inert, ionic liquids have been used to stabilize highly polar or ionic transition states. Ionic liquids provide favorable media for the formation and stabilization of intermediates in reactions that proceed through charged intermediates. An example is the Baylis-Hillman reaction catalyzed by 1,4-diazabicyclo (222). octane (DABCO) (Scheme 8) (162). 

As seen in Tables 22—25, the Arrhenius preexponential factors Aa for the initiation step are very low, 10 in 7, 10 in 20, 10 " in 41 and 1in 44. These are very low values for bimolecular reactions for which values of about 10 ° are observed and also predicted by the Transition State Theory Thus step (a) belongs to a class of slow reactions , some of which might have ionic transition states . The activation entropies AS obtained from the Transition State Theory rate constant expression... 

The 14N/15N and secondary a- H/2H kinetic isotope effects (KIEs) for the >SN2 reaction between PhS and benzyldimethylphenylammonium ion at different ionic strengths in DMF at 0 °C indicate that the structure of the transition state changes markedly with the ionic strength of the medium.67 A more reactant-like, more ionic, transition state is found at the higher ionic strength. A further contribution from the same research group... 

The halogenation reaction of ethylene has been modeled by many researchers [170, 172-176], For chlorination in apolar solvents (or in the gas phase), the formation of two radical species requires the use of flexible CASSCF and MRCI electronic structure methods, and such calculations have been reported by Kurosaki [172], In aqueous solution, Kurosaki has used a mixed discrete-continuum model to show that the reaction proceeds through an ionic mechanism [174], The bromination reaction has also received attention [169,170], However, only very recently was a reliable theoretical study of the ionic transition state using PCM/MP2 liquid-phase optimization reported by Cammi et al. [176], These authors calculated that the free energy of activation for the ionic bromination of the ethylene in aqueous solution is 8.2 kcalmol-1, in good agreement with the experimental value of 10 kcalmol-1. 

It is interesting to consider why the more ionic transition state is found in methanol. The earlier transition state in methanol may occur because the SN2 transition state is solvated primarily at the partially charged sulfur atom (there would be little or no solvation of the a-carbon and the nitrogen atoms because these charges are sterically hindered to solvation). As a result, the transition state would be solvated by hydrogen bonding to the sulfur atom in methanol whereas it is only solvated by a much weaker ion-dipole interaction in DMF. This means the more ionic transition state found in methanol would be more stable in the methanol and a less ionic (a dipolar) transition state would be found in the dipolar aprotic solvent. This suggests the structure of the transition state depends on its stability in that solvent. 

The high negative entropies of activation can be explained in terms of the highly ordered arrangement of solvent molecules around the ionic transition state. As an indication of this, AS for reaction 37 (X = C1, Nu = trimethylsilylimidazole) was found to be — 41 eu107. 

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