FRIEDEL CRAFTS ACETYLATION OF BIS(TRIMETHYLSILYL)FERROCENE AND BIS(TRIBUTYLSTANNYL)FERROCENE - IMPLICATIONS ON THE MECHANISMS OF ACYLATION AND PROTON-EXCHANGE OF FERROCENE DERIVATIVES

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 contain 1.0 deuterium atom in each cyclopentadienyl ring whereas 8aD2 contains 0.5 deuterium atom in thc 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.