Stable carbonium ions. XXXI. p-Anisonium and methylphenonium ion-formation via aryl participation in strong acid solution

Looking for a Paradigm for the Reactivity of Phenonium Ions

“…The β‐stabilization effect of the aryl group in phenethyl cations and the attending formation of a σ‐bridged intermediate, the phenonium ion, was first evidenced by Cram1 through pioneering studies on the stereochemical course of the solvolysis of optically active phenethyl sulfonates. The role of such an intermediate has since been explored from both the experimental2 and computational points of view 3. Recent work favours a continuum of situations in between the phenethyl cation I and the σ‐bridged structure III , and the possible role of a weak π complex II (see Scheme ) 4.…”

“…Substituents on the ring and on the phenethyl moiety clearly affect the structure and reactivity of the intermediate, as do the medium characteristics. Apart from the initial experiments, most mechanistic studies have been carried out under conditions quite different to those suitable for preparative purposes, for example, in the gas phase5 or in superacids,2 a fact that has contributed to the slow development of synthetic processes via this intermediate. Application of this intermediate in organic synthesis for carbon–carbon or carbon–heteroatom bond formation was reported in the 90s by Olah and co‐workers 6,7,10.…”

Formation and mechanism for reactions of ring‐substituted phenonium ions in aqueous solution

“…The strong stabilization of phenonium ion H‐4 + by a 4‐MeO substituent enables the generation of MeO‐4 + from MeO‐3‐Cl by ionization using SbF 5 in SO 2 (Scheme A). The NMR spectrum of MeO‐4 + is unexceptional and shows the expected three types of protons: four cyclopropyl −H at 3.47 ppm; the −OCH 3 at 4.25 ppm, and protons from the A 2 B 2 quartet at 7.47 and 8.12 ppm . One important question is whether the phenonium ion should be treated as a classical spiro[2.5]oct‐5,7‐diene‐4yl carbocation H‐4 + , or if there is also a substantial contribution from the nonclassical resonance structure H‐5 + (Scheme B), where only two bonding electrons are used in a three‐centered bond to cyclopropyl carbon atoms .…”

“…The thermodynamically favorable isomerization of 2,4,6 trimethyl phenonium ion in the superacidic medium SbF 5 /SO 2 to the 1‐(2,4,6 trimethylphenyl)ethyl carbocation has been observed . A driving force of = −12 kcal/mol for isomerization of H‐4 + to H‐7 + (Scheme ) in a solvent with a relative permittivity of 40 was determined by quantum chemical calculations .…”

Formation of synthetically relevant CF3-substituted phenonium ions in superacid media

“…1a ), considerable interest has been focused on phenonium ions. The first observations of the anthrylethyl bridged cation and p -anisonium ion respectively by Eberson, Winstein 2 and Olah 3 ( Fig. 1b ) triggered numerous studies to understand the stability/reactivity of these original cations.…”