Nucleophiles, partitioning of carbocations between addition and

Nuclear motion, the principle of least, and the theory of stereoelectronic control, 24, 113 Nucleophiles, partitioning of carbocations between addition and deprotonation. 35, 67 Nucleophilic aromatic photosubstitution, 11,225 Nucleophilic catalysis of ester hydrolysis and related reactions, 5,237 Nucleophilic displacement reactions, gas-phase, 21, 197... 

Nucleophiles, partitioning of carbocations between addition and deprotonation, 35, 67... 

How Does Structure Determine Organic Reactivity Partitioning of Carbocations between Addition of Nucleophiles and Deprotonation... 

Partitioning of carbocations between addition of nucleophiles and deprotonation, 35, 67 Perchloro-organic chemistry structure, spectroscopy and reaction pathways, 25, 267 Permutational isomerization of pentavalent phosphorus compounds, 9, 25 Phase-transfer catalysis by quaternary ammonium salts, 15, 267 Phosphate esters, mechanism and catalysis of nucleophilic substitution in, 25, 99 Phosphorus compounds, pentavalent, turnstile rearrangement and pseudoration in permutational isomerization, 9, 25... 

Partitioning of carbocations between addition of nucleophiles and deprotonation,... 

In summary, there now exists a body of data for the reactions of carbocations where the values of kjkp span a range of > 106-fold (Table 1). This requires that variations in the substituents at a cationic center result in a >8 kcal mol-1 differential stabilization of the transition states for nucleophile addition and proton transfer which have not yet been fully rationalized. We discuss in this review the explanations for the large changes in the rate constant ratio for partitioning of carbocations between reaction with Bronsted and Lewis bases that sometimes result from apparently small changes in carbocation structure. 

It is often difficult to understand at an intuitive level the explanation for the effect of changing substituents on the rate constant ratio kjkp for partitioning of carbocations between nucleophilic addition of solvent and deprotonation. In these cases, insight into the origins of the changes in this rate constant ratio requires a systematic evaluation of substituent effects on the following ... 

Fig. 4 Free energy reaction coordinate profiles that illustrate a change in the relative kinetic barriers for partitioning of carbocations between nucleophilic addition of solvent and deprotonation resulting from a change in the curvature of the potential energy surface for the nucleophile addition reaction. This would correspond to an increase in the intrinsic barrier for the thermoneutral carbocation-nucleophile addition reaction.

The results described in this review provide support for the following generalizations about the influence of thermodynamics and intrinsic kinetic barriers on the partitioning of carbocations between nucleophilic addition of aqueous solvents to form a tetrahedral adduct (ks) and proton transfer to these solvents to form an alkene (kp). 

The partitioning of simple tertiary carbocations, ring-substituted 1-phenylethyl carbocations, and cumyl carbocations between deprotonation and nucleophilic addition of solvent strongly favors formation of the solvent adduct. The more favorable partitioning of these carbocations to form the solvent adduct is due, in part, to the greater thermodynamic stability of the solvent... 

Table 1 Rate and equilibrium constants for partitioning of substituted a-methyl carbocations R (R2)CCH3+ between nucleophilic addition of solvent (ks) and deprotonation (kp) (Scheme 7)°... 

By contrast, the addition of a pair of ortho-methyl groups to the aromatic ring of X-[8+] to give X-[10+] has the following dramatic effects on partitioning of the carbocations between nucleophilic addition of solvent and proton transfer 27-41... 

The net effect of the two orrAo-methyl groups at Me-[10+] is a 4200-fold decrease in the rate constant ratio As/Ap for partitioning of the carbocation between nucleophilic addition of solvent and proton transfer.27... 

The results of studies of the acid-catalyzed hydration of oxygen-, sulfur-, seleno-and nitrogen-substituted alkenes and the relevance of this work to partitioning of the corresponding carbocation intermediates (Chart 1) between deprotonation and nucleophile addition was reviewed in 1986.70. We present here a brief summary of this earlier review, along with additional discussion of recent literature. 

The partitioning of ferrocenyl-stabilized carbocations [30] between nucleophile addition and deprotonation (Scheme 18) has been studied by Bunton and coworkers. In some cases the rate constants for deprotonation and nucleophile addition are comparable, but in others they favor formation of the nucleophile adduct. However, the alkene product of deprotonation of [30] is always the thermodynamically favored product.120. In other words, the addition of water to [30] gives an alcohol that is thermodynamically less stable than the alkene that forms by deprotonation of [30], but the reaction passes over an activation barrier whose height is equal to, or smaller than, the barrier for deprotonation of [30], These data require that the intrinsic barrier for thermoneutral addition of water to [30] (As) be smaller than the intrinsic barrier for deprotonation of [30] (Ap). It is not known whether the magnitude of (Ap — As) for the reactions of [30] is similar to the values of (Ap - As) = 4-6 kcal mol 1 reported here for the partitioning of a-methyl benzyl carbocations. 

By contrast, [31]-(brosylate) was proposed to show a large amount of reaction through a secondary carbocation intermediate,122 which partitions between nucleophile addition and deprotonation. We are uncertain of how to relate the partitioning of the putative carbocation intermediate of the reaction of [31]-(brosylate) to the partitioning of other carbocations discussed in this review. 

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