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Synthesis of (s-trans4-Butadiene)tantalocene Cation and Its CC-Coupling Reactions with Ketones, Nitriles, and Alkynes

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journal contribution
posted on 25.11.1998, 00:00 by Hans Christian Strauch, Gerhard Erker, Roland Fröhlich
4-Butadiene)(η5-cyclopentadienyl)tantalum dichloride (6), synthesized by treatment of CpTaCl4 (5) with (butadiene)magnesium (7), was treated with 2 molar equiv of sodium cyclopentadienide to yield Cp3Ta(butadiene) (9). The spectroscopic analysis revealed the (η5-Cp)21-Cp)(η2-C4H6)Ta structure of 9. Treatment of 9 with 1 molar equiv of the salt [(Cp2Zr−CH3)+(CH3B(C6F5)3)-] (11) resulted in the clean transfer of a cyclopentadienide ligand from tantalum to zirconium with formation of the neutral zirconium compound Cp3Zr−CH3 (12) and the cation Cp2Ta(butadiene)+ (10; with CH3B(C6F5)3- counteranion). The spectroscopic characterization and an X-ray crystal structure analysis has revealed that 10 is a cationic (s-trans4-butadiene)bis(η5-cyclopentadienyl)tantalum complex. It reacts cleanly with a variety of ketones at 60 °C to form the respective 7,7-disubstituted tetrahydrotantalaoxepines 13. These cationic seven-membered metallacyclic σ-allyl complexes undergo a rapid topomerization process on the NMR time scale (ΔGtop ≈ 12 kcal/mol-1). Organic nitriles add similarly at 60 °C to the (s-trans4-butadiene)tantalocene cation (10) to yield the respective 7-substituted 3,4-dihydrotantala-6H-azepines (14) that exhibit a similar dynamic NMR behavior due to conformational equilibration. Two examples of the seven-membered σ-allyl metallacyclic cation complexes 14 were characterized by X-ray crystal structure analyses (14a, R = CH3, and 14d, R = CH2-p-C6H4CH3). In contrast, 2-butyne adds to the (s-trans4-butadiene)tantalocene cation in a 1:1 ratio to yield the corresponding metallacyclic (π-allyl)metallocene cation complex 15a (characterized by X-ray diffraction). The addition of 1-pentyne gave a 60:40 mixture of the respective regioisomers 15b and 15c, exhibiting the n-C3H7 substituent at the ring position C5 and C6, respectively.