STEREOCHEMISTRY OF DEWAR BENZENES - MOLECULAR-STRUCTURES INFLUENCED BY ELECTRONIC AND STERIC EFFECTS

The molecular structures of the four Dewar benzene derivatives 4–7 were determined by means of X‐ray diffraction. The central C1–C4 bonds are extremely long. The carboxylate groups on the bridgehead carbon atoms of 4, 5, and 7 are in optimal orientation for an electronic interaction with the central bonds having high p‐character. Therefore the central bonds in 4, 5, und 7 (1.602–1.612 Å) are even more elongated than in 6 (1.563 Å). Repulsion between tert‐butyl groups causes bond lengthening and structural deformations. The sp2‐hybridized carbon atoms are pyramidalized. From structural comparison of Dewar benzene derivatives a strong correlation between the length of the central bond and the folding angle of the four‐membered rings can be derived. A new rule has been established for the thermal stability of Dewar benzene derivatives. Copyright © 1990 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim