Carbocations, partitioning between addition

Carbocations, partitioning between addition of nucleophiles and deprotonation, 35, 67... 

The solvolysis of MeS-4-0(S)CPh in aqueous/trifluoroethanol at 25 °C is zero-order in [N ].42 Fig. 4A shows that formation of MeS-4-N3 occurs at the expense of MeS-4-OSolv but not MeS-4-S(0)CPh. The reaction of MeS-4-(3,5-dinitrobenzoate) under the same conditions proceeds through the liberated carbocation intermediate MeS-4+ which partitions between addition of solvent to form MeS-4-OSolv and azide anion to form MeS-4-Nj.14 Fig. 4B shows that the yields of MeS-4-Nj, calculated as a fraction of the sum of the yields of MeS-4-Nj and MeS-4-OSolv, from the reactions of MeS-4-0(S)CPh and MeS-4-(3,5-dinitrobenzoate) are identical. These data show that essentially 100% of the ion-pair intermediate of reaction of MeS-4-(3,5-dinitrobenzoate) undergoes separation to the free carbocation which is trapped by azide ion, while 14% of the reaction of MeS-4-0(S)CPh proceeds by this pathway and 86% proceeds by the competing isomerization reaction.42... 

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

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. 

The substitution of an a-oxygen by an a-sulfur or an a-selenium results in a decrease in ks/kp for partitioning of oxocarbenium ions between addition of solvent and deprotonation. The largest effect is observed for the a-selenium substitution. These changes are probably the result of several different effects on carbocation reactivity, each of which contributes to the observed substituent effect on kjkp. 

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... 

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... 

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