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

## TL;DR

This paper presents a new method to synthesize pyracylenes and reveals their unique antiaromatic π-stacking behavior in the solid state.

## Contribution

A facile Pd-catalyzed synthesis of pyracylenes and the discovery of continuous face-to-face antiaromatic π-stacking in the solid state.

## Key findings

- Pyracylenes were synthesized via Pd-catalyzed ring contraction of diborinic acids.
- Three pyracylenes formed continuous face-to-face antiaromatic π-stacks with equidistant spacing and 90° twist angles.
- NICS calculations showed reduced antiaromaticity in the stacked pyracylene structures.

## Abstract

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.