Preassociation

The preassociation mechanism is more efficient than the trapping mechanism because it generates an intermediate which immediately reacts by an ultrafast proton transfer (in the pre-association complex, Int) and thus avoids the diffusion-controlled step bringing the catalyst and intermediate together. This mechanism is sometimes called a spectator mechanism because, although the catalyst is present in the transition structure, it is not undergoing any transformation [10]. 

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An elaboration of the ion-pair concept includes an ion sandwich in which a preassociation occurs between a potential nucleophile and a reactant. Such an ion sandwich might be a kinetic intermediate which accelerates dissociation. Alternatively, if a caibocation were quite unstable, it might always return to reactant unless a nucleophile was properly positioned to capture the caibocation. 

We will give a brief summary of the general principles of diffusion and preassociation in electrophilic substitutions before discussing the values in Table 3-1 (for more comprehensive reviews see North, 1964 and Ridd, 1978). A precursor A (HN02 in... 

The fact that the binding is accommodated by a single-step reversible interaction does not prove that the interaction in the upper panel is between L and a preassociated RG. However, it implies that if there is an additional step, i.e.. 

An excellent review on organic photochemistry in organized media, including aqueous solvent, has been reported.178 The quantum efficiency for photodimerization of thymine, uracil, and their derivatives increased considerably in water compared with other organic solvents. The increased quantum efficiency is attributed to the preassociation of the reactants at the ground state. 

A number of mechanistic pathways have been identified for the oxidation, such as O-atom transfer to sulfides, electrophilic attack on phenols, hydride transfer from alcohols, and proton-coupled electron transfer from hydroquinone. Some kinetic studies indicate that the rate-determining step involves preassociation of the substrate with the catalyst.507,508 The electrocatalytic properties of polypyridyl oxo-ruthenium complexes have been also applied with success to DNA cleavage509,5 and sugar oxidation.511... 

Davies, M. S. Thomas, D. S. Hegmans, A. Berners-Price, S. J. Qu,Y. Farrell, N. A Comparison of the kinetics of formation of 1,4-interstrand crosslinks by the trinuclear clinical agent BBR3464 Preassociation and evidence of conformational flexibility J. Am. Chem. Soc. submitted for publication. 

Fluorescence quenching may be dynamic, if the photochemical process is the result of a collision between the photoexcited indicator dye and the quencher species, or static, when the luminophore and the quencher are preassociated before photoexcitation of the former20. It may be easily demonstrated that dynamic quenching in isotropic 3-D medium obeys the so-called Stem-Volmer equation (2)21 ... 

The nitrosation step (equation 26) was studied in the last ten years in more detail with respect to diffusion-controlled preassociation phenomena and the steps following the formation of the N-nitrosoanilinium ion in the step in equation 26. Besides the case of the diazotization of very reactive aniline derivatives where the formation of the nitrosyl... 

In order to accelerate the reaction, acid catalyst is employed. In the presence of acid catalyst, carbonyl compounds preassociate with the acid followed by proton... 

When a nucleophile approaches a C=0 or a C=N bond, there is a transfer of electron density to the oxygen or nitrogen. The oxygen or nitrogen then becomes more basic as the reachon progresses. If there is a H—Y bond that is preassociated to the C=0 or C=N, the addihon of the nucleophile will cause an increase in the electron density at oxygen, and this will perturb the H—Y bond. 

Inter- and intramolecular hydrogen bondings are also important for the formation of epoxytwinol A (71), which would be produced by the quite rare formal [4 -I- 4] cycloaddition reaction. Theoretical calculations suggest that preassociation... 

X is strongly electron withdrawing (ct > 0.34). These substrates react with azide ion to form X-2-N3 exclusively by a preassociation reaction mechanism. The minimum observed selectivity of ( az/ s)obsd = consistent either with... 

The yield of the nucleophilic substitution product from the stepwise preassociation mechanism k[ = k. Scheme 2.4) is small, because of the low concentration of the preassociation complex (Xas 0.7 M for the reaction of X-2-Y). Formally, the stepwise preassociation reaction is kinetically bimolecular, because both the nucleophile and the substrate are present in the rate-determining step ( j). In fact, these reactions are borderline between S l and Sn2 because the kinetic order with respect to the nucleophile cannot be rigorously determined. A small rate increase may be due to either formation of nucleophile adduct by bimolecular nucleophilic substitution or a positive specific salt effect, whUe a formally bhnole-cular reaction may appear unimolecular due to an offsetting negative specific salt effect on the reaction rate. 

The change from a stepwise preassociation mechanism through a triple ion intermediate to an uncoupled concerted reaction occurs as the triple ion becomes too unstable to exist in an energy well for the time of a bond vibration ( 10 s). The borderline between these two reaction mechanisms is poorly marked, and there are no clear experimental protocols for its detection. These two reaction mechanisms cannot be distinguished by experiments designed to characterize their transition states, which lie at essentially the same position in the inner upper right hand corner of Figure 2.3. Only low yields of the nucleophilic substitution product are obtained from both stepwise preassociation and uncoupled concerted reactions, because for formation of the preassociation complex in water is small... 

An important question is whether nucleophilic substitution at tertiary carbon proceeds though a carbocation intermediate that shows a significant chemical barrier to the addition of solvent and other nucleophiles. The yield of the azide ion substitution product from the reaction of 5-Cl is similar to that observed for the reactions of X-2-Y when this product forms exclusively by conversion of the preassociation complex to product. Therefore the carbocation 5 is too unstable to escape from an aqueous solvation shell and undergo diffusion-controlled trapping by azide ion. This result sets a lower limit of w fcj > -d 1.6 x 10 ° s (Scheme 2.4) " for addition of solvent to the ion pair intermediate 5" C1 . 

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