Friedel-crafts acetylation of bis(trimethylsilyl)- and bis(tributylstannyl)ferrocene: implications on the mechanisms of acylation and proton exchange of ferrocene derivatives

Article abstract of DOI:10.1021/ja00013a023

The first unequivocal examples of intermolecular Friedel-Crafts reactions of ferrocene derivatives proceeding via exo attack of the electrophile are reported. Treatment of 1,1′-bis(trimethylsilyl)-(5a) or 1,1′-bis(tributylstannyl)ferrocene (5b) with acetyl chloride in the presence of AlCl₃ affords a mixture of three isomeric acetylferrocenes, 1′-acetyl- (6), 2-acetyl-(7), and 3-acetyl-1-(trialkylsilyl and -stannyl)ferrocene (8). Acetylation of 3,3′-dideutero-1,1′-bis(trimethylsilyl)ferrocene (5aD₂) under identical conditions generates the corresponding dideuterated products 6aD₂-8aD₂. Both 6aD₂ and 7aD₂ contam 1.0 deuterium atom in each cyclopentadienyl ring whereas 8aD₂ contains 05 deutenum atom in the substituted ring and 1.5 deuterium atoms in the unsubstituted ring. This demonstrates that the products are formed via exo attack of the electrophile followed by an intramolecular, interannular proton transfer. The lack of scrambling of the deuterium label also suggests that protonation of ferrocenes could also occur through the exo attack of a proton rather than direct protonation at the metal center.