Reaktionen von Carbokationen mit π‐Nucleophilen: polarer Mechanismus statt Outersphere‐Elektronentransfer

Abstract

Professor Puul von Rug& Schleyev zuin 65. Geburtstug gewidmetIn vielen Bereichen der Organischen Chemie ist die Bedeutung von Elektronentransferprozessen erkannt worden. Zahlreiche Reaktionen, die fruher durch ,,Verschiebungen von Elektronenpaaren" nach der wohlbekannten ,,Curved-arrow"-Methode beschrieben worden sind, versteht man nun als Prozesse, bei denen zunachst durch einen Elektronentransferschritt ein Radikalionen-Paar entsteht, das dann unter Bildung einer neuen Bindung reagiertr1I.Infolgedessen mussen auch fur die Reaktionen von kationischen Elektrophilen R + rnit neutralen n-Nucleophilen zwei Mechanismen in Betracht gezogen werden. Beim polaren Mechanismus (Schema 1, oberer Weg) nahert sich das Carbokation

Rates and Equilibria of the Reactions of Tertiary Phosphanes and Phosphites with Benzhydrylium Ions

“…Multiplication of the slope of the N / E 1/2 correlation with an averaged value of s = 0.65 (for the employed phosphorus compounds) results in an effective slope of −4.17 for the corresponding log k / E 1/2 correlations. This value is much smaller than the slope, which was found for the linear correlation between the E parameters of benzhydrylium ions and their reduction potentials in acetonitrile ( E = 14.09 E ° red −0.279),45 in agreement with an earlier observation that the rates of carbocation nucleophile combinations are affected more strongly by variation of the reduction potential of the electrophile than of the oxidation potential of the nucleophile 46…”

Role of Electron-Transfer Processes in Reactions of Diarylcarbenium Ions and Related Quinone Methides with Nucleophiles

“…It has previously been discussed 13 that the reactions of carbocations with π-systems cannot proceed via outer-sphere electron transfer because the observed free energies of activation for these reactions are much smaller than those expected for electron transfer (Figure 3). With the reduction potentials determined in this work, this analysis can now be generalized.…”

“…Previous kinetic investigations have shown that the reactivity range of the benzhydrylium ions and quinone methides used as reference electrophiles covers 24 orders of magnitude corresponding to reaction times of 1 s vs 10 16 years. , It has now been determined that the reduction potentials of these compounds , extend over 1.64 V, corresponding to 158 kJ mol -1 (Table ).

2 Correlation of the empirical electrophilicity parameters E with the one-electron reduction potentials E ° red (vs SCE, AN, 298 K) of benzhydrylium ions and structurally analogous quinone methides.

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“…d From ref , calculated at the B3LYP/6-31G(d,p) level of theory according to eq 2 with X - = CH 3 - . e From ref . f From ref .…”

“…Previous kinetic investigations have shown that the reactivity range of the benzhydrylium ions and quinone methides used as reference electrophiles covers 24 orders of magnitude corresponding to reaction times of 1 s vs 10 16 years. 2,3 It has now been determined that the reduction potentials of these compounds 13,14 extend over 1.64 V, corresponding to 158 kJ mol -1 (Table 1).…”