Controlling the Scholl Reaction

Guidelines for the application of the Scholl reaction were developed. Labeling experiments demonstrate
that the Scholl reaction fails in small, unsubstituted oligophenylenes (e.g., o-terphenyl) due to
oligomerization of the products (e.g., triphenylene). Incorporation of suitably placed blocking groups
(e.g., t-butyl) suppresses oligomerization. The well-established directing group effects in electrophilic
aromatic substitution predict the outcome of Scholl reactions of substituted substrates. Activating o,p-directing groups (e.g., MeO) direct bond formation o,p, either intramolecularly or intermolecularly.
Deactivating o,p-directing groups (e.g., Br) also direct bond formation o,p but yields are lower. Deactivating
m-directors (e.g., NO2) suppress reaction. MoCl5 and PhI(OOCCF3)2/BF3·Et2O are general and effective
reagents for the Scholl oxidation. Calculations (B3LYP/6-31G(d)) predict the Scholl reaction in
alkoxyarenes to proceed via arenium cations, not radical cations. Suzuki−Miyaura couplings were used
to generate 12 substituted o-terphenyl derivatives.