Carbocations with nucleophiles

Table 1 includes partitioning data only for carbocations that are sufficiently stable to form in the nucleophilic aqueous/organic solvents used in these experiments. For example, it is not possible to obtain values of ks for reaction of secondary aliphatic carbocations in water and other nucleophilic solvents, because the chemical barrier to ks is smaller than that for a bond vibration.83 The vanishingly small barriers for reaction of secondary carbocations with nucleophilic solvents results in enforced concerted mechanisms2-3 for the nucleophilic substitution and elimination reactions of secondary derivatives in largely aqueous solvents.83-84... 

We will deal more briefly with reactions of carbocations with nucleophiles other than water, and then consider correlations in which the nucleophile rather than (as hitherto) the carbocation is varied. Fig. 7 shows a plot of... 

It seems clear that for reactions of carbocations with nucleophiles or bases in which the structure of the carbocation is varied, we can expect compensating changes in intrinsic barrier and thermodynamic driving force to lead to relationships between rate and equilibrium constants which have the form of extended linear plots of log k against log K. However, this will be strictly true only for structurally homogeneous groups of cations. There is ample evidence that for wider structural variations, for example, between benzyl, benzhydryl, and trityl cations, there are variations in intrinsic barrier particularly reflecting steric effects which lead to dispersion between families of cations. 

It seems to the author that consideration of least motion effects in this fine detail with respect to the substrate is unlikely to give worthwhile insight into reactions in solution, particularly in solution in highly polar, and hence highly structured solvents. It is precisely in these solvents that many of the heterolytic reactions whose stereochemical outcome is held to support ALPH have been carried out. Most of these reactions are formally either the reaction of a delocalised carbocation with a nucleophile, or its microscopic reverse. The reactions of delocalized carbocations with nucleophiles have been studied extensively by Ritchie and co-workers, and the main conclusions of their work are particularly germane to considerations of least nuclear motion. 

Controiied/Living Polymerizations with Added Salts The two approaches discussed above are primarily useful in nonpolar solvents (like toluene and n-hexane) where the interactions of carbocations with nucleophiles are strong and favored. In relatively polar solvents like methylene chloride, these methods often fail to give controlled polymerizations, most likely because the interaction is weaker between the growing carbocations and nucleophiles [whether they are built-in (counteranions) or externally added (esters, etc.)], which facilitates dissociation of the carbocation. The effect of solvent in the latter system, however, is much weaker. 

Benzylic carbocations are also stabilized by complexation to chromium and a number of interesting reactions have been reported. Again, reaction of the carbocations with nucleophiles occurs from the exo face of the complex, relative to the metal. Carbocations are readily formed by treatment of benzylic alcohols with a strong acid, such as sulfuric acid, tetrafluoroboric acid, or borontrifluoride etherate. The cation can be trapped with water, alcohols, nitriles, and mono-or disubstituted amines to form alcohols, ethers, amides, and di- or trisubstituted amines respectively. Scheme 96 illustrates the formation of a benzylic carbocation followed by intramolecular trapping, resulting in a net inversion of stereochemistry. Benzylic acetates react with trimethyl aluminium introducing a methyl group from the opposite face of the metal. 

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