Abstract
Based on the intramolecular core-shell strategy, dihydropyracylene with two spiro(dibenzocycloheptatriene) units was designed, for which theoretical calculations predicted a very weak C-C bond with a bond length beyond 1.8 A. This bond is expanded by the forced adoption of an eclipsed conformation and by angle strain through a scissor effect. The highly strained hydrocarbon was isolated as a thermally stable compound with no signs of diradical contribution because the weak C-C bond (core) is protected by the shape-persistent fused-ring structure (shell). A Raman shift corresponding to the C-C stretching vibration (587 cm-1) is much different from that for ethane (993 cm-1). The bond length determined by X-ray [1.806(2) A] is greater than both the shortest non-bonded intramolecular C…C contact [1.80(2) A] and theoretically predicted limit for C-C bond (1.803 A). The long-assumed linear correlation between bond length and bond-dissociation energy for covalent bonding is invalid in greater bond lengths.
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