Uncovering Alternative Reaction Pathways Taken by Group 4 Metallocene Cations: Facile Intramolecular CH Activation of Cp−(Dimethylamino)alkyl Substituents by a Methylzirconocene Cation

Abstract

Alkyl- and aryllithium reagents add cleanly to the electrophilic carbon center C6 of 6-(N,N-dimethylamino)fulvenes to yield the corresponding substituted cyclopentadienyllithium systems Li[C5H4−CR1R2NMe2]. Subsequent treatment with ZrCl4·2THF gives the corresponding Cp-functionalized zirconocene dichlorides. These were reacted with methyllithium to give the (C5H4CR1R2NMe2)2Zr(CH3)2 complexes 11a (R1 = R1 = CH3) and 11b (R1 = CH3, R2 = Ph), respectively. Treatment of 11 with tris(pentafluorophenyl)borane was carried out to generate the corresponding alkylmetallocene cations (12), which turned out to be unstable under the reaction conditions applied (−20 °C) with regard to liberation of 1 equiv of methane by CH activation at a methyl group adjacent to nitrogen and formation of the spiro-metallocene complex systems 13. CH activation may be a major reaction pathway open to alkylzirconocene cation systems under suitable reaction conditions.