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Resumo:
In order to determine the properties of the bicycloheptatrienyl anion (Ia) (predicted to be conjugatively stabilized by Hückel Molecular Orbital Theory) the neutral precursor, bicyclo[3. 2. 0] hepta-1, 4, 6-triene (I) was prepared by the following route.
Reaction of I with potassium-t-butoxide, potassium, or lithium dicyclohexylamide gave anion Ia in very low yield. Reprotonation of I was found to occur solely at the 1 or 5 position to give triene II, isolated as to its dimers.
A study of the acidity of I and of other conjugated hydrocarbons by means of ion cyclotron resonance spectroscopy resulted in determination of the following order of relative acidities:
H2S ˃ C5H6 ˃ CH3NO2 ˃ 1, 4- C5H8 ˃ I ˃ C2H5OH ˃ H2O; cyclo-C7H8 ˃ C2 H5OH; фCH3 ˃ CH3OH
In addition, limits for the proton affinities of the conjugate bases were determined:
350 kcal/mole ˂ PA(C5 H5-) ˂ 360 kcal/mole
362 kcal/mole ˂ PA(C5H7-, Ia, cyclo-C7H7-) ˂ 377 kcal/mole PA(фCH2-) ˂ 385 kcal/mole
Gas phase kinetics of the trans-XVIII to I transformation gave the following activation parameters: Ea = 43.0 kcal/mole, log A = 15.53 and ∆Sǂ (220°) = 9.6 cu. The results were interpreted as indicating initial 1,2 bond cleavage to give the 1,3-diradical which closed to I. Similar studies on cis-XVIII gave results consistent with a surface component to the reaction (Ea = 22.7 kcal/mole; log A = 9.23, ∆Sǂ (119°) = -18.9 eu).
The low pressure (0.01 to 1 torr) pyrolysis of trans-XVIII gave in addition to I, fulvenallene (LV), ethynylcyclopentadiene (LVI) and heptafulvalene (LVII). The relative ratios of the C7H6 isomers were found to be dependent upon temperature and pressure, higher relative pressure and lower temperatures favoring formation of I. The results were found to be consistent with the intermediacy of vibrationally excited I and subsequent reaction to give LV and LVI.