Early-late transition states

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... 

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 ... 

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...

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. 

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... 

The terms early and late transition states are qualitative descriptions related to concepts such as those advanced by the Hammond principle. An early transition... 

The cis product must be formed through a transition state with a twistlike conformation to adhere to the requirements of stereoelectronic control. The fact that this pathway is not disfavored is consistent with other evidence that the transition state in enolate alkylations occurs early and reflects primarily the structural features of the reactant, not the product. A late transition state should disfavor the formation of the cis isomer because of the strain energy associated with the nonchair conformation of the product. 

Equation (82) predicts that for reactions with zero free energy change a = 0.5, while for exothermic reactions, a < 0.5 and for endothermic reactions, a > 0.5. Since according to both Marcus theory and the Bell— Evans-Polanyi model early transition states are related to exothermic reactions and late transition states to endothermic reactions, a may be interpreted as a relative measure of transition state geometry. However, in our view even this interpretation should be treated with a measure of healthy scepticism. Even if one accepts Marcus theory without reservation, the a... 

In Section 3 (p. 148) we saw that both early and late transition states for SN2 reactions of methyl derivatives are likely to have progressed about 50% in terms of their charge development. This proposal is quite contrary to the generally accepted view and in this section we wish to discuss how general this disparity between charge and geometric progression is likely to be. 

Proton abstraction from a model carbon acid, hydroxyacetaldehyde, by formate anion has been examined theoretically for the gas phase and for aqueous solution.152 The reaction shows an early transition state, whereas its enzymatic equivalent has a late transition state. Solvation brings the transition state foiward. The factors that contribute to producing the later transition state in enzymes are discussed. 

AMI and PM3 calculations reveal that epoxidations by DMDO and TFDO involve peroxide-bond cr at a very early stage and that TFDO is the most reactive dioxirane as the CF3 group in it stabilizes this cr level. In accord with previous calculations a spiro transition state is predicted. Furthermore, allene is predicted to be less reactive than alkenes toward epoxidation by DMDO.192 DFT calculations on the oxidation of primary amines by dimethyldioxirane predict a late transition state with a barrier of 17.7 kcal mol-1 which is drastically lowered by hydrogen bonding to the O—O bond to just 1.3 kcal mol-1 in protic solvents.193... 

Keumi et al. applied MNDO calculations to the jt and o complexes to explore the early and late transition states, respectively. Their data confirm that an early transition state favors attack at position 4 the later ones give attack at position 5. 

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