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.

Computations at the CCSD(T)/6-311+G(d,p)//B3LYP/6-311+G(d,p) level of theory support long-held beliefs that allene dimerization to 1,2-dimethylenecyclobutane proceeds through diradical intermediates rather than a concerted (π)2(s) + (π)2(a) mechanism. Two diastereomeric transition states with orthogonal and skew geometries have been located for C2-C2 dimerization of allene, with predicted barriers of 34.5 and 40.3 kcal/mol, respectively. In dimerization, the outward-facing ligands rotate in a sense opposite to the forming C-C bond. Both transition states lead to nearly orthogonal (D(2)) singlet bisallyl (or tetramethyleneethane) diradical. This diradical has a barrier to planarization of 3.2 kcal/mol through a planar D(2h) geometry and a barrier to methylene rotation of 14.3 kcal/mol. Bisallyl diradical closes through one of four degenerate paths by a conrotatory motion of the methylene groups with a predicted barrier of 15.7 kcal/mol. The low barrier to planarization of bisallyl, and similar barriers for methylene rotation and conrotatory closure are consistent with a stepwise dimerization process which can still maintain stereochemical elements of reactants. These computations support the observation that racemic 1,3-disubstituted allenes, with access to an orthogonal transition state which minimizes steric strain, will dimerize more readily than enantiopure materials and by a mechanism that preferentially bonds M and P enantiomers.

Quaterrylene is prepared in a single reaction and high yield by Scholl-type coupling of perylene, utilizing trifluoromethanesulfonic acid as catalyst and DDQ or molecular oxygen as oxidant. Dissolution in 1 M triflic acid/dichloroethane with sonication yields the aromatic quaterrylene oxidative dication, which is characterized by its (1)H NMR spectrum.