Influence of Carbocation Stability in the Gas Phase on Solvolytic Reactivity:  Beyond Bridgehead Derivatives

Abstract

The intrinsic gas-phase stability of bicyclic secondary carbocations has been determined by Dissociative Proton Attachment of chlorides and alcohols, respectively. From these data, Gibbs free energies for hydride transfer relative to 1-adamantyl (Delta(r)G degrees (8,exp)) are derived after application of appropriate leaving group corrections, and good agreement with theoretical values, (Delta(r)G degrees (8,comp)), calculated at the G2(MP2) or MP2/6-311G(d,p) level, is reached (Table 1). The relative rate constants for solvolysis (log(k/k(0))) of the bicyclic secondary derivatives correlate with the stabilities of the respective carbocations in the same manner as tertiary bridgehead derivatives, but simple monoderivatives and acyclic derivatives solvolyze faster than predicted on the grounds of the ion stabilities. The corresponding stabilities of cyclopropyl- and benzyl-substituted carbocations have been obtained by a combination of experimental and computational data available in the literature with computational methods. Correlation of the rate constants for solvolysis vs ion stabilities for these compounds reveals a trend similar to that observed for bridgehead derivatives, but with much more scatter, which is attributed to nucleophilic solvent participation and/or nucleophilic solvation.

Impact of the α‐Ferrocenyl Group on the Solvolytic Reactivity – Electrofugality – of Ferrocenylphenylmethyl Cations

“…Hydrogen ion affinities (HIA) and corresponding standard reaction Gibbs energies in vacuo and water relative to benzyl cation, calculated at B2PLYP-D3/may-cc-pVTZ level of theory (in kcal/mol). [22] R + (g) + H -(g) → R-H(g) (2) As the difference of HIA values equals to standard reaction enthalpy of corresponding isodesmic reaction (3), it is much more convenient to give HIA values relative to some referent carbocation. [22] R + (g) + H -(g) → R-H(g) (2) As the difference of HIA values equals to standard reaction enthalpy of corresponding isodesmic reaction (3), it is much more convenient to give HIA values relative to some referent carbocation.…”

Fragmentation reactions of radical cations

“…A clear dependence of the fragmentation rate on the C-S bond dissociation free energies (BDFEs) in the radical cation can be noted and, accordingly, the rate of C-S bond cleavage increases by increasing the stability of the carbocation leaving group in the order PhMe 2 C þ < Ph 2 MeC þ < Ph 3 C þ . 54 However, the influence of the BDFEs on the rate of fragmentation is rather low: for about 13 kcal/mol difference in BDFE the difference in DG # is of only 2-3 kcal/mol. The probable reason is that the intrinsic barrier for the C-S bond cleavage reaction is not constant but changes along the series increasing as, in the leaving carbocation, the phenyl groups progressively replace the methyl groups (cumyl < diphenylmethyl < triphenylmethyl) and the fragmentation requires therefore more extensive electron reorganization.…”

Towards mechanisms of bimolecular nucleophilic reactions in solution—probing the variation of the activation parameters in the reactions of aromatic compounds

“…[77][78][79] The activation parameters are widely used for characterizing the TS structures. [6,23,36,37,68,72,73] For example, it was found that solvolysis of benzyl-and benzhydryl halides follows the S N 2 and S N 1 mechanism, respectively, [80,81] and S N 2 reactions show more negative values of ΔS 6 ¼ . [82,83] The activation parameters are also useful in probing the elimination-addition (E-Ad N ) mechanism of S N V reactions.…”