7-METHYL- AND 7-PHENYLCYCLOHEPTA-1,3,5-TRIENES FROM BENZVALENE VIA 3,3a,4,5,6,6a-HEXAHYDRO-4,5,6-METHENOCYCLOPENTAPYRAZOLES AND TETRACYCLO2,4.03,5>HEPTANES

Article abstract of DOI:10.1016/S0040-4020(01)80119-8

The addition of benzvalene (1) to diazomethane, diazoethane, 2-diazopropane, phenyldiazomethane, and diphenyldiazomethane afforded the 1-pyrazolines 2a-g in good yields.By means of competition experiments, the relative reactivities of benzvalene (1) and norbornene with regard to diazomethane and 2-diazopropane have been determined.The fact that benzvalene reacts about twice as fast as norbornene with both diazoalkenes cannot be rationalized on the basis of frontier orbital energies.On direct photolysis, the pyrazolines 2a-g were converted into the tetracyclo<4.1.0.0^2,4.0^3,5>heptanes 4a-g exclusively.These compounds gave the 1,3,5-cycloheptatrienes 5a,b,d,e,g in high yields on treatment with silver ions, thus providing better access to 7,7-dimethyl-(5d) and 7,7-diphenylcycloheptatriene (5g) than before.Surprisingly, the latter compound is in equilibrium with a substantial quantity of the norcaradiene form.The heat of reaction for the rearrangement of 4a to 5a has been determined, which allows to derive the heat of formation of tetracyclo<4.1.0.0^2,4.0^3,5>heptane (4a).