Transition state effective charges

The main features of the effect of structure on the site of attack are summarized in Table 3, and can be understood in.terms of a borderline 5n2 (59CRV737) transition state (48) which somewhat resembles an 5n1 transition state in charge distribution because C—O bond breaking runs ahead of Nu—C bond making. 

Mechanism Initial state Transition state Relative charge in transition state Rate effect of increase in solvent polarity... 

The negatively charged ring in the transition state and intermediate complex presumably exerts little or no inductive electron attraction on a substituent. So, as one might expect, the transition-state effect of an azine methoxy group can differ from its (conjugated)... 

Zeolite framework stabilization effects uniformly all transition states and charged transient intermediates, and does not effect the neutral intermediates. Similar effect of the zeolite framework has been described by Corma et al. for another reaction. 

Two recent determinations of activation volumes for Mel addition to rhodium(I) 8-diketonate complexes (197) could not discern which type operated. The values, along with those of AS - and the effects of solvent change, clearly indicated development of polar transition states (the charge separation involved in either 29 or 30 would fit) and the authors marginally favored 29. Interestingly, a complex of iridium(III) and Mel, 31, has been structurally characterized and reveals iodide-bonded Mel molecules (198). The Ir-I-C bond angles are 105.5° and 108.2° and although the interaction can be considered nucleophilic... 

Figure 3.2 Effect of solvent polarity on reaction rate, (a) Charge built up in the transition state, (b) Charge destroyed or dispersed in the transition state...

Most transition states involve charged intermediates, which are stabilized within the active site of an enzyme via ionic bonds in pockets or holes bearing a matching opposite charge. Such charges are derived from acidic or basic amino acid side chains (such as Lys, Arg, Asp, or Glu) ° or are provided by (Lewis acid-type) metal ions, typically Zn +. Computer simulations studies suggested that in enzymes electrostatic effects provide the largest contribution to catalysis [107]. As a prominent example, the tetrahedral intermediate of carboxyl ester hydrolysis is stabilized in serine hydrolases by the so-called oxyanion hole (Scheme 2.1). 

The mechanistic difference of these two cases Is well recognized by the observation that the transition state XII differs much from XIII since going from the Initial to transition state the charge Is developed at XII whereas it is despersed at XIII. This was further supported by the examination of the solvent effect on the rate of BCMO polymerization (12). 

Consideration of the effect of counterion on the p value for addition of poly(styryl)carbanions to DPE is also instructive. The p value increases as the size of the counterion increases, i.e., as the electrostatic interaction of the counterion with the negative charge decreases. This effect would be expected for any transition state structure, since the small lithium cation would be expected to minimize delocalization into the phenyl rings (see Sect. 2.2 of this review and Chap. 1 of [3]). Thus, with the cesium counterion, for any degree of charge transfer onto the 1,1-diphenylcarbon in the transition state, this charge would be more delocalized than for the corresponding lithium system. It would also be predicted that the p value for addition of butyllithium to styrene... 

It is now necessary to consider to what extent deviations from smooth correlations of k /kj) with AG or AG in proton transfer reactions reflect departures from the Hammond postulate. The Hammond postulate may be expected to apply to reasonably closely related three-centre reactions of the type considered above, and in particular to their simplified representations by empirical potential-energy surfaces. It does not follow that it applies generally to more complex reactions, and it is now well established that for concerted ionic reactions failure of bond-making and bond-breaking to remain properly in step can lead in the transition state to charge-localizations at atoms that bear no charge in the reactants or products, with the consequence that substituents at these atoms show no correlation between their effects on reaction rates and equilibria [123-126]. This behaviour has been best characterized in E2 j8-eliminations,... 

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