Nucleophilic constant optical activity

NMR time scale for all counterions studied, i.e., Br, I-, C104, CF3S03, including also Cl- at low temperature. The rate constant was estimated to be at least of the order of 103 second-1 at 30°C. In light of this result it is reasonable to postulate a ligand exchange process for the nucleophile-induced racemization of optically active silanes (discussed in Section V,F). 

Now each act of nucleophilic substitution (1) occurs at a rate that is in accord with the observed value of kex and (2) produces a molecule of inverted configuration, and the polarimeter monitors this aspect of the reaction. For, say, the first 1% of reaction, the polarimeter will indicate 98% of the original optical activity, as the solution now contains 2% of the racemic compound. This explains why the polarimetric rate constant is twice that for exchange of the radiolabel (see Hughes et al.2). 

On the other hand, when optically active 2-octyl triflate (197) was hydrolyzed in the solubilized state with surfactant concentrations greater than their CMC, the stereochemical course changed to net retention (ca. 48% for CTAB, and ca. 27% for NaLS)193 The rate constants, very much lower than those in the absence of micelles, indicate that the reaction proceeded in the micelles. The different behavior of (196) and (197), particularly in the presence of cationic micelles (CTAB), may be due to the greater efficiency of triflates in producing carbocations. The comparison of (196) and (197) then suggests that the solvation model of micellar stereochemical control applies to unassisted solvolyses whereas the double inversion mechanism operates with leaving groups which invite participation of a nucleophile. 

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