Abstract
Summary On the basis of the intramolecular core-shell strategy, we designed dihydropyracylene with two spiro(dibenzocycloheptatriene) units, for which theoretical calculations predicted a very weak C–C bond with a bond length around 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 ) was very different from that for ethane (993 cm −1 ). The bond length determined by X-ray (1.806(2) A) was greater than the shortest non-bonded intramolecular C … C contact (1.80(2) A). The assumed limit for a C–C bond (1.803 A) by supposing linear correlation between bond length and bond-dissociation energy for covalent bonding was proven to be invalid.
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