Peterson Olefination: Unexpected Rearrangement in the Overcrowded Polycyclic Aromatic Ene Series

An attempted synthesis of the angularly annelated 9-(11'-benzo[a]fluoren-11'-ylidene)-9H-fluorene (3) through a Peterson olefination reaction between (9H-fluoren-9-yl)trimethylsilyl anion (5a) and 11H-benzo[a]fluoren-11-one (6) led to the linearly annelated 9-(11'-benzo[b]fluoren-11'-ylidene)-9H-fluorene (4), due to an unexpected rearrangement. The proposed mechanism of the rearrangement is illustrated. The core of the mechanism is an intramolecular Haller-Bauer cleavage of the naphthyl C-11a'-C-11' bond in the beta-oxysilane anion 11 (formed from 5a and 6) to give the 1-naphthyl anion (E)-12, followed by E/Z isomerization to (Z)-12 and by proton migration to give the 3-naphthyl anion (Z)-14. The intramolecular nucleophilic addition of the naphthyl anion at C-3' of (Z)-14 to the carbonyl carbon gives the beta-oxysilane anion 15, a benzo[b]fluorenylidene derivative. The mechanism is supported by the results of DFT calculations. The synthesis of 3 was achieved by application of Barton's double extrusion diazo-thione coupling method.