Phenyl shifts in substituted arenes via ipso arenium ions

The isomerization of substituted arenes through ipso arenium ions is an important and general molecular rearrangement that leads to interconversions of constitutional isomers. We show here that the superacid trifluoromethanesulfonic acid (TfOH), ca. 1 M in dichloroethane (DCE), provides reliable catalytic reaction conditions for these rearrangements, easily applied at ambient temperature, reflux (84 °C), or in a microwave reactor for higher temperatures. Interconversion of terphenyl isomers in TfOH/DCE at 84 °C gives an ortho/meta/para equilibrium ratio of 0:65:35, nearly identical to values reported earlier by Olah with catalysis by AlCl(3). For the three triphenylbenzenes, TfOH-catalyzed equilibration strongly (>95%) favors the 1,3,5-triphenyl isomer. Equilibration of the three possible tetraphenylbenzenes gives a 61:39 mixture of the 1,2,3,5- and 1,2,4,5-substituted isomers. Under the reaction conditions explored, none of these structures undergoes significant Scholl cyclization. DFT calculations with inclusion of solvation support a mechanistic scheme in which all of the phenyl migrations occur among a series of ipso arenium ions. In every case studied, the preferred isomers at equilibrium are those that yield highly stable cations by the most exothermic, hence least reversible 1,2-H shift.