Static and Dynamic Stereochemistry of a Chiral, Doubly Bridged 9,10-Diphenylanthracene from a Stereospecific Polycyclic Aromatic Dicarbonyl Coupling

The reductive coupling of dibenzo[b,f] suberone (5) with a low-valent titanium reagent generated from TiCl₄, Zn, and pyridine in THF gave two dimeric hydrocarbons, the tetrahydrobis(benzocyclohept)anthracene 6 and the tetrahydrobi(dibenzocycloheptenyl) 7. The same reaction with reagents generated from TiCl₃or TiCl₄with LiAlH₄in THF gave only 7. Analogous couplings of dibenzo[b,f]tropone (4) gave bi(dibenzo[a,d]cycloheptenyl) (9) and sy n-bis (dibenzo [a,d]cycloheptenylidene) (3). X-ray cryystallography showed 6 to be a chiral, syn-bridged 9,10-diphenylanthracene. Molecular mechanics calculations (MM2-85) predicted a very similar syn-bridged structure to be the lowest energy conformation and a centrosymmetric anti form to be 1.39 kcal/mol higher in energy. 6 could be resolved into enantiomers by chromatography at 5 °C on swollen, microcrystalline triacetylcellulose with ethanol as eluent. Analysis of the CD spectrum in terms of the coupled oscillator model led to the assignment of (-)-6 to the S,S configuration. Thermal racemization, monitored by the CD spectrum, in the temperature range 20–50 °C gave AG* = 22.6 kcal/mol for the ring inversion. Analysis of the ¹H NMR spectrum showed the conformations of the CH₂-CH₂bridges in solution to be similar to those in the crystal, and simulation of the exchange-broadened spectra in the temperature range 144–169 °C gave ΔG*inv = 22.9 kcal/mol. No resonances due to an anti form could be observed in the NMR spectra.