Molecular Crystals with Moving Parts: Synthesis, Characterization, and Crystal Packing of Molecular Gyroscopes with Methyl-Substituted Triptycyl Frames

We report a highly convergent synthesis for the preparation of molecular gyroscopes consisting of
para-phenylene rotors linked by triple bonds to methyl-substituted triptycenes acting as pivots
and encapsulating frames. The desired 1,4-bis[2-(2,3,6,7,12,13-hexamethyl-10-alkyl-9-triptycyl)ethynyl]benzenes were prepared from 2,3-dimethyl-1,3-butadiene using Diels−Alder cycloadditions
and Pd(0)-catalyzed coupling as the key reactions. The main challenge in the synthesis came about
in the preparation of 9-alkynyl-triptycenes by Diels−Alder reaction of benzynes and 9-alkynyl-2,3,6,7-tetramethylanthracenes. These reactions occurred with chemical yields and regioselectivities
that were strongly influenced by steric and electronic effects of substituents at C10 of the anthracene
core. Anthracenes with methyl, propyl, and phenyl substituents were utilized to complete the
synthesis of their corresponding molecular gyroscopes, and their solid-state structures were
determined by single-crystal X-ray diffraction analysis. Examination of these results indicated that,
as expected, the bulky triptycyl groups encourage crystallization motifs that create more free volume
around the phenylene rotor, as needed to facilitate fast gyroscopic motion in the solid state.