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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
Author(s) -
Dunitz Jack D.,
Krüger Carl,
Irngartinger Hermann,
Maverick Emily F.,
Wang Yu,
Nixdorf Matthias
Publication year - 1988
Publication title -
angewandte chemie international edition in english
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 0570-0833
DOI - 10.1002/anie.198803871
Subject(s) - antiaromaticity , tetra , steric effects , valence (chemistry) , chemistry , crystallography , crystal structure , ring (chemistry) , molecule , tautomer , crystal (programming language) , state (computer science) , stereochemistry , aromaticity , mathematics , medicinal chemistry , organic chemistry , algorithm , computer science , programming language
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.