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)‐9 H ‐fluorene ( 3 ) through a Peterson olefination reaction between (9 H ‐fluoren‐9‐yl)trimethylsilyl anion ( 5a ) and 11 H ‐benzo[ a ]fluoren‐11‐one ( 6 ) led to the linearly annelated 9‐(11′‐benzo[ b ]fluoren‐11′‐ylidene)‐9 H ‐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 β‐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 β‐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.