Equilibrium Structure, Stabilized Transition State, or Disorder in the Crystal? Studies of the Antiaromatic Systems Tetra‐tert‐butyl‐s‐indacene and Tetra‐tert‐butylcyclobutadiene by Low‐Temperature Crystal Structure Analysis

The answers to the question posed in the title are different for the two compounds: In the case of 1 the X-ray crystal structure observed at 100 K is that corresponding to the transition state of the free molecule between the two possible valence tautomers. In contrast, the crystal of 2 contains molecules with rectangular arrangement of the four-membered ring in two orientations rotated through 90°. The transition state of 1 in the crystal is the energetically more favored state since it enables a better packing of the tert-butyl groups. These investigations disprove afresh the nearly square arrangement considered as plausible for the four-membered ring atoms of 2 on steric grounds.