Preassociation , with

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... 

A third member of the bimolecular then unimolecular reaction class is a variant of the previous mechanism. In this case, the conjugate base of biotin reacts with bicarbonate to produce an addition intermediate that then reacts with ATP (Scheme 23). It is likely that the phosphorus of the terminal group of ATP would preassociate with an oxygen of bicarbonate. In particular, if the anionic center of bicarbonate associates with a cation, the 7r-electron density of bicarbonate would align with the phosphorus of the terminal phosphate of ATP. The addition of the conjugate base of a urea to a carboxylate is an appropriate model for this mechanism. The intermediate should be very reactive toward ATP based on the observation that the conjugate base of a carbonyl hydrate reacts rapidly with an internal phosphate ester (59). 

If in the first reaction step an unstable carbocation is formed, it will react with a neighboring nucleophilic molecule faster than it can diffuse away. Such a fast reaction can be a preassociation with a nucleophile, a fast formation of an intimate (or contact ) ion pair. The asterisk indicates a shortlived intermediate and the subscript int indicates an ion pair intermediate ... 

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... 

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... 

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. 

X 10 ° s for 75f. These ions are predicted to react with solvent H2O too rapidly for efficient trapping by non-solvent nucleophiles, so other inefficient trapping mechanisms such as preassociation can compete. The transition... 

Scheme 41 and Fig. 4 provide an example of a case in which preassociation can account for all of the observed N3 trapping. In this case trapping by Nj amounts to only 13% of reaction products at 0.475 M N3. The calculated value of ks of 2.1 x 10 s for 75b, obtained from the correlation of logS with AE described above, was used in the fit. It was assumed that the k had the same value, and was fixed at 10 °s. Since the fit had very little... 

Takeuchi and co-workers showed that the ratio of N- to C-substitution on the apparent intermediate 75a by a series of aromatics in arene/TFA mixtures was sensitive to the structure of the precursor to 75a (phenyl azide, N-phenylhydroxylamine or O-trifluoroacetyl-N-phenylhydroxylamine) even though the decomposition rates of these precursors were unaffected by the identity of the arene. This result is consistent with the characterization of 75a from N3 and Br trapping studies as a short-lived intermediate that will usually react through ion pair or preassociation processes that may be leaving-group dependent. ... 

Reactions of nitrenium ions with lifetimes in aqueous solution ns. It is clear from the work presented to date that these species react predominately by ion-pair or preassociation mechanisms, but the detailed processes are far from clear. The possible transition to a true bimolecular substitution mechanism (Sn2) has also not been systematically investigated. 

I think Dr. Tobe s paper clearly suggests that concepts developed in connection with reactions in nonaqueous solvents, preassociation, and its importance for reaction, may have important applications in studying reactions in aqueous solution. In that context I would like to offer a few thoughts on the more or less classic reaction of water with the chloropentamminecobaltic ion. 

MeO N Me -catalyzcd raethanolysis of phenyl acetate is accelerated by the addition of alkaline-earth metal salts. The kinetics are consistent with a reaction scheme [Eq. (1)] involving preassociation of MeO ion with the metal cation (Km) and independent contributions to the overall rate from free (ko) and cation-paired (Icm) MeO [3,4]. 

---