Investigation of the Mechanism of the Intramolecular Scholl Reaction of Contiguous Phenylbenzenes

Two mechanisms of the Scholl reaction were investigated in the series 1, 2, ..., n-oligophenylbenzenes (n = 2, 3, 4, 6) at the B3LYP/6-31G(d) level of theory. A mechanism involving generation of a radical cation followed by C−C bond formation and dehydrogenation is unlikely on the basis of unfavorable energies of activation. A mechanism involving generation of an arenium cation followed by C−C bond formation and dehydrogenation is energetically feasible. An explanation for the facile polycondensation of hexaphenylbenzene to hexa-peri-hexabenzocoronene, where six new aryl−aryl bonds are formed, is provided. Kinetic simulations based on the calculated activation energies of the arenium cation mechanism predict that intermediates will not accumulate; this is supported by mass balance experiments. Reaction optimization studies suggest that PhI(O₂CCF₃)₂/BF₃·OEt₂ or MoCl₅ are superior to FeCl₃ or AlCl₃/CuCl₂. This is a full account of our work reported partially as a communication previously (Rempala, P.; Kroulík, J.; King, B. T. J. Am. Chem. Soc. 2004, 126, 15002−15003).