Scholl Cyclizations of Aryl Naphthalenes: Rearrangement Precedes Cyclization.

In 1910, Scholl, Seer, and Weitzenbock reported the AlCl3-catalyzed cyclization of 1,1'-binaphthyl to perylene. We provide evidence that this classic organic name reaction proceeds through sequential and reversible formation of 1,2'- and 2,2'-binaphthyl isomers. Acid-catalyzed isomerization of 1,1'-binaphthyl to 2,2'-binaphthyl has been noted previously. The superacid trifluoromethanesulfonic acid (TfOH), 1 M in dichloroethane, catalyzes these rearrangements, with slower cyclization to perylene. Minor cyclization products are benzo[k]fluoranthene and benzo[j]fluoranthene. At ambient temperature, the observed equilibrium ratio of 1,1'-binaphthyl, 1,2'-binaphthyl, and 2,2'-binaphthyl is <1:3:97. DFT calculations with the inclusion of solvation support a mechanistic scheme in which ipso-arenium ions are responsible for rearrangements; however, we cannot distinguish between arenium ion and radical cation mechanisms for the cyclization steps. Under similar reaction conditions, 1-phenylnaphthalene interconverts with 2-phenylnaphthalene, with the latter favored at equilibrium (5:95 ratio), and also converts slowly to fluoranthene. Computations again support an arenium ion mechanism for rearrangements.