Phenyl Shifts in Substituted Arenes via Ipso Arenium Ions

Abstract

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.

Unsymmetric Twofold Scholl Cyclization of a 5,11‐Dinaphthyltetracene: Selective Formation of Pentagonal and Hexagonal Rings via Dicationic Intermediates

“…Rathore and co-workers have demonstrated that ortho-terphenyl derivatives such as 7 undergo efficient Scholl cyclizations in the presence of DDQ/MeSO 3 Ht hrough the generation of radical cations such as 7C + (Figure 4). [6f] Consistent with the reported results for 7, [6f] cyclization or aryl migration [15] was not observed for 3 in acidic media using MeSO 3 H, implying that it should be unlikely that the cyclization of 3 proceeds by the arenium ion pathway.…”

Synthesis of Novel Pyridyl-Based Schiff Base and Its Coordination Behaviour with Ruthenium(II) and Zinc(II)

“…Many reports highlighted the importance of temperature and/or the presence of acid to catalyze the process [18][19][20][21][22]. In present case, the pyridine-2-carboxaldehyde (99 %) contains 0.5 % of the free acid and it is possibly the source of H + that catalyze the isomerization of the methyl group (steps 1, 2 and 3 in Scheme-II).…”

“…Isomerization of alkyl and aryl groups in arene rings has been reported in literatures [18][19][20][21] and such reaction has been used in many industrial processes including the conversion of mixed xylene to p-xylene [22]. Many reports highlighted the importance of temperature and/or the presence of acid to catalyze the process [18][19][20][21][22].…”

Labelling and Comparison of Isomeric Tree-Like Polyphenyl Systems

“…Details of these systems can be found in [1,3,4,8,10,13,14,16,17]. In what follows, we simply refer to them as A, B, C, D, E, and F. Similarly, our discussion in Section 3 implies that ( ) ( ).…”