Pyracylenes: Facile Synthetic Access and Continuous Face-To-Face Antiaromatic π-Stacking

Pyracylene is a nonalternant hydrocarbon which has long fascinated chemists, but whose structure is exceedingly rare in experimental reports. Pyracylene's surprising absence arises from synthetic inaccessibility attributable to its antiaromatic character. Meanwhile, sterically unimpeded polycyclic antiaromatic hydrocarbons have become fascinating candidates for organic materials applications and for antiaromatic π-stacked assemblies. Herein, we report a facile synthesis of pyracylenes via Pd-catalyzed ring contraction of diborinic acids. So-formed pyracylenes were derivatized by bromination and subsequent Pd-catalyzed cross-coupling. All pyracylenes were isolated, fully characterized, and studied by UV-vis absorption spectroscopy, cyclic voltammetry, density functional theory (DFT) calculations, and X-ray crystallography. In the solid state, three pyracylenes formed unprecedented continuous, face-to-face, antiaromatic π-stacks with equidistant molecular spacings and 90° twist angles between monomers, wherein HOMOs of monomers aligned with phase-complementary LUMOs of their neighbors (and vice versa). Nucleus-independent chemical shift (NICS) calculations indicated that this arrangement leads to antiaromaticity reduction of monomers. Solid-state conductivities of the pyracylenes were established using time-resolved microwave conductivity measurements.