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

Abstract

AbstractAn 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 C11a′–C11′ 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.