Late transition state

The significance of the concept incorporated in Hammond s postulate is that, in appropriate cases, it permits discussion of transition-state structure in terms of the reactants, inteimediates, or products in a multistep reaction sequence. The postulate indicates that the cases in which such comparison is appropriate are those in which the transition state is close in energy to the reactant, intermediate, or product. Chemists sometimes speak of early or late transition states. An early transition state is reactant-like whereas a late transition state is product-like. 

The soft-nucleophile-soft-electrophile combination is also associated with a late transition state, in which the strength of the newly forming bond contributes significantly to the stability of the transition state. The hard-nucleophile-hffld-elechophile combination inqilies an early transition state with electrostatic attraction being more important than bond formation. The reaction pathway is chosen early on the reaction coordinate and primarily on the basis of charge distributiotL... 

Chlorination in acetic acid is characterized by a large p value ( — 9 to —10), and the partial rate factor for toluene is 820. Both values indicate a late transition state, which would resemble the rr-complex intermediate. 

The transition state involves six partially delocalized electrons being transformed from one 1,5-diene system to another. The transition state could range in character from a 1,4-diradical to two nearly independent allyl radicals, depending on whether bond making or bond breaking is more advanced. The general framework for understanding the substituent effects is that the reactions are concerted with a relatively late transition state with well-developed C(l)—C(6) bonds. 

Figure 5-4. Potential energy surface with a late transition state.

This valence bond description leads to an interesting conclusion. Because the transition state occurs at the point where the initial and final state VB configurations cross, the transition state receives equal contributions from each. This is so whether the transition state is early or late. Thus, the nucleophile Y and the leaving group X possess about equal charge densities in the transition state. This conclusion means that an early transition state is not (in this sense) reactantlike , for a reactantlike transition state should have most of the charge on Y. Similarly, a late transition state is not necessarily productlike. This view is at variance with other interpretations. 

Can we check this model Do we have evidence that reactions of arenediazonium ions and nucleophiles that have early and late transition states yield (Z)- and (E)-azo compounds, respectively ... 

All these questions can be answered if we consider the transition states for the dissociation reactions, which are all very similar. The transition state structure for a given substrate geometry is essentially independent of the type of molecule and substrate. Thus the close packed surfaces as well as the stepped surfaces considered in Fig. 6.42 each form a group. Dissociation is furthermore characterized by a late transition state, in which the two atoms have already separated to a large extent and... 

Figure 14-9. Snapshots from the simulations of the early transition state mimic (left) and the late transition state mimic (right), indicating the Mg2+ ion direct coordination (green lines) and key hydrogen bonds and indirect Mg2+ coordination (dotted lines). For clarity, the water molecules are not shown...

Water exchange proceeds through a trigonal bipyramidal reactive intermediate [Li(H20)5]+, reached via a late transition state. In accordance with the above-mentioned experimental observation of a very fast exchange process, the activation barrier... 

Mechanistic details are very similar for DMSO and water exchange. The reaction proceeds through a distorted trigonal bipyramidal reactive intermediate [Li(DMSO)5]+ that is reached via a late transition state. The enthalpy profile (see Fig. 13) is in line with the experimentally observed very fast exchange process. The five-coordinate intermediate is computed to be 7.9kcalmol 1 less stable than [Li(DMS0)4]+ and free DMSO, while an overall activation barrier of only 8.4kcalmol 1 is computed. Obviously,... 

In this situation, it is evident that it is not crucial to determine whether the reaction proceeds via an early or a late transition state, since the outcome would be the same in both cases. This property is a result of the close similarity between the initial and final structures indeed, the allyl moiety undergoes a rotation of only 30° from its idealised initial geometry to form the -coordinated alkene complex. 

Fig. 24. Schematic comparison of energy and volume profiles (1) early transition state and (2) late transition state. 

Another example of the use of transition state pKa values has been provided by Pollack (1978). From the rate constants for the decarboxylation of substituted a,a-dimethylbenzoylacetic acids ([37] — [38]) and their anions, he calculated pK for reaction of the acids (Table A6.2). The values vary significantly with the phenyl substituent (p = +1.7), much more so than the p/(a values of the substrate acids (p = +0.2). This difference is consistent with the proton being much closer to the phenyl group in the transition state than in the initial state, and it may even denote a relatively late transition state (Pollack, 1978). However, from the pKa values of the reactant acids (approximately 3.4), the transition states (approximately 4.4), and the enol product (11.8) (Pruszynski et al., 1986), the Leffler index... 

It used to be postulated that the magnitude of the alpha-carbon KIE in an Sn2 reaction is an indication of the transition state symmetry. The expectation was that this KIE reaches maximal value for a symmetric transition state and drops back to unity for extremely early or late transition states. That expected behavior is marked by the solid line in Fig. 10.10. However, recent calculations of the nucleophilic substitution of chloride from methyl chloride by a broad variety of nucleophiles... 

There is an inverse kinetic isotope effect, k /k-Q = 0.45 0.1 (for decomposition of PtH2(Mc3P)2) compared with PtD2(Me3P)2) in THF at 21 °C. This supports the previous prediction of nearly complete H-H formation in a late transition state. 

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