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.