- The β-effect with vinyl cations: Kinetic study of the protiodemetalation of silyl-, germyl-, and stannylalkynes
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The relative magnitude of the hyperconjugative stabilization of vinyl cations by adjacent C-M bonds (M = Si, Ge, Sn; the β-effect) has been examined by measuring the rate constants for the protonation and subsequent protiodemetalation of group 14 metalated (trimethylsilyl)-acetylenes (R3MC≡CSiMe3). The relative β-effect arising from the second-order rate constants Sn ? Ge > Si (maximum kM/kSi = 108, 5 × 102, 1, respectively) follows the same order as that reported for simple carbenium ions. The product ratio from the protonation of Ph3GeC≡CSiMe3 was found to be particularly sensitive to acid concentration and strength, leading to loss of Ph3Ge preferentially with weaker acids. With tin groups, the rate of destannylation decreased with increasing steric bulk, unlike the corresponding situation with silyl groups. The origins of both these observations may be attributed to nucleophilic interaction at the metal center during protonation.
- Dallaire, Carol,Brook, Michael A.
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p. 2332 - 2338
(2008/10/08)
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- Electron-Transfer Activation in Electrophilic Mechanisms. Cleavage of Alkylmetals by Mercury(II) Complexes
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The disappearance of the transient charge-transfer (CT) absorption bands coincides with the electrophilic (SE2) cleavage of homologous series of alkyltin compounds by various mercury(II) halides, cyanide, and carboxylates.The second-order kinetics for HgCl2 cleavage afford rate constants which vary in a rather unaccountable way with the structure of the alkyltin compound and with the polarity of the solvent.Furthermore, the relative reactivities of these alkyltin compounds in the analogous electrophilic cleavage by I2 or Br2 show poor correlations with HgCl2 cleavages, in different solvents.However, the description of the activation process as an electron transfer in the precursor complex, e.g., -> +HgCl2->, stems from the CT transition energy and leads to a linear free energy relationship in which the activation free energy is equal to the driving force for the formation of the ion pair.The latter is readily dissected by eq 18 into separate changes in electronic, steric and solvation energies.With this mechanistic formulation, the reactivities of various alkyltin compounds follow a remarkably simple linear correlation with the ionization potentials and the solvent effects, in the comparison with I2 and Br2 cleavages.Moreover, the reactivities of the various mercury(II) derivatives relate directly to differences in their electron affinities.
- Fukuzumi, S.,Kochi, J. K.
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p. 7290 - 7297
(2007/10/02)
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