Synthesis of 13,14-dialkylpicenes and hindered rotation of the alkyl groups as studied by dynamic NMR spectroscopy and molecular mechanics calculations

Abstract

The synthesis of four 13,14-dialkylpicenes is reported. NMR spectra at various temperatures and molecular mechanics calculations indicate that while the α-carbon atoms are near the aromatic plane, the remaining carbon atoms are above or below the plane, the two substituents being trans in that way. There is a barrier to rotation of alkyl groups through the plane of the ring to interconvert up–down and down–up conformations of 11–12 kcal mol^–1, which is dependent on the substituent.