Clocks for reactions of ion pairs

We have focused on determining partition rate constant ratios for a variety of reactions of ion pairs, and of absolute rate constants from these ratios. This has been accomplished by use of one of the rate constants from this product ratio as a clock for the second reaction. 

Values of k.v, = 5 X 109 M-1 s 1 have been determined for diffusion-controlled addition of azide ion to a variety of ring-substituted benzhydryl24 and a-substituted 4-methoxybenzyl25 carbocations. This rate constant have seen extensive use as a clock to determine absolute rate constants for addition of a variety of nucleophiles to benzylic carbocations.5 Relative rate constants for addition of azide ion 

Separation of an ion-pair to free ions, with rate constant fc d, has been used as a clock for other reactions of the ion pair. Estimates of fc d can be obtained from the simple relationship between the association constant for ion-pair formation (Xas, equation (2)) and the rate constant for diffusion-controlled encounter (fcd = 5 X 109 M-1 s-1) of anions and carbocations. 

Ionic complexes in water are generally are weak. For example, the association constant for formation of complexes between stable monocations and monanions ions in water are typically 0.1 M-1.26 Values of Xas (M-1) for association of nucleophilic anions and neutral electrophilic substrates for solvolysis have been estimated from the limiting rate constant ratios determined for stepwise nucleophilic 

The descending nucleophile selectivity (7 az// s)obsd (M 1) on the left-hand limb of Fig. 1 for stepwise solvolysis of R-X is due to the increase in ks (s-1), with decreasing stability of the carbocation intermediate, relative to the constant value of /taz (M-1 s-1) for the diffusion-limited addition of azide anion. The lifetime for the carbocation intermediate R+ eventually becomes so short that essentially no azide ion adduct forms by diffusion-controlled trapping, because addition of solvent to R+ occurs faster than escape of the carbocation from the solvent cage followed by addition of azide ion (k s k-d). Now, the nucleophile adduct must form through a preassociation mechanism, where the azide anion comes together in an association 

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A Global scheme for solvolysis 2 Clocks for reactions of ion pairs 3 Addition of solvent to carbocation-anion pairs i Protonation of a carbocation-anion pair 11 Isomerization of ion pair reaction intermediates Reorganization of ion pairs in water 13 Internal return of isotopically labeled ion pairs Racemization of ion pairs 22 Concluding remarks 24 Acknowledgements 24 References 24... 

If ET intermediates play any role in representative aldehyde or ketone Wittig reactions, they are too short-lived for detection by the fastest available radical or radical anion clocks. This is conceivable if the geometry of the radical ion pair resembles that of an oxaphosphetane with partially developed bonds (223c). Such a scenario fits within the broad definition of a four-center mechanism and allows little (if any) distinction between the geometry of stereochemistry-determining TS that invoke ET versus those that do not. More precise distinctions may have theoretical significance, but they will not influence the stereochemical issues that are of concern in this review. 

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