Stereochemistry of Dewar Benzenes – Molecular Structures Influenced by Electronic and Steric Effects

Abstract

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 A) are even more elongated than in 6 (1.563 A). 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.