Synthesis and reactivity of trans-tricyclo[4.2.0.0(1,3)] oct-4-ene

The first synthesis of trans-tricyclo[4.2.0.0(1,3)]oct-4-ene (1), an etheny l bridged spirohexane, was accomplished in four steps starting from Carpino et al. gem-dichloro ketone 6. An X-ray crystal structure of 1 with one sub stituent was obtained to provide geometry data on this novel ring system an d to confirm the stereochemical assignment of the penultimate synthetic int ermediate. Tricyclo[4.2.0.0(1,3)]oct-4-ene is surprisingly stable. It react s with glacial acetic acid but only slowly at 145 degrees C; the products w ere isolated and identified. A unimolecular rearrangement takes place at el evated temperatures (165 degrees C and higher), presumably, via a biradical intermediate to afford tricyclo[4.2.0.0(1,5)]oct-3-ene (23). The structure of this 1,5-bridged bicyclo[2.1.0]pentane derivative was established by NM R and an X-ray crystal structure of its Diels-Alder adduct with isobenzofur an. Tricyclo[4.2.0.0(1,3)]oct-4-ene equilibrates with 23, so equilibrium co nstants and reaction rates were measured over a 20 degrees C temperature ra nge from 180 degrees C to 200 degrees C. The difference in the heats of for mation (Delta Delta H(f)degrees (23-1)) is -2.1 kcal/mol, which is in good agreement with ab initio (HF and MP2) calculations using the 6-31G(d) basis set (-1.9 (HF) and -1.4 (MP2) kcal/mol). Computations on trans-tricyclo[4. 2.0.0(1,3)]octane and spirohexane also were carried out, and the structures and energies were compared.