SYNTHESIS AND DEGENERATE COPE REARRANGEMENT OF 2,6-DIPHENYLBARBARALANE

Article abstract of DOI:10.1016/S0040-4020(01)87599-2

The oxygen-sensitive 2,6-diphenylbarbaralanes 15 and 16 are synthesized from bicyclo<3.3.1>nonane-2,6-dione (9) via dibromide 14 in four steps with 46 and 54percent overall yield, respectively.Crystalline 2,6-diphenylbarbaralane (15) is yellow at room temperature and decolourizes reversibly on cooling.Accordingly, the UV spectrum of trans-decalin solutions of 15 changes reversibly in the temperature range 295-450 deg K.The bromodiphenylbarbaralane 16 is shown to exist as a single valence tautomer.The 13C chemical shifts of 16 serve as estimates of the analogous shifts in the non-exchanging valence tautomer of 2,6-diphenylbarbaralane (15) itself.The barrier for the degenerate Cope rearrangement of 15 in dichloromethane has been determined from 13C-NMR line width measurements in the temperature range of 169-233 deg K to be ΔG⁺⁺₂₀₀ = 21.6 kJ mol^-1.Hence phenyl substituents at the termini of the allylic parts of barbaralane accelerate the degenerate Cope rearrangement somewhat more than cyano groups.