Synthesis, structure, and reactivity of tantalum and tungsten homoenolate complexes

Article abstract of DOI:10.1021/om060264f

Preparation and characterization of homoenolate complexes of tantalum, Cp*Cl₃Ta(CH₂CR¹₂C(=O)-OR ²) (1a: R¹ = H, R² = Et; 1b: R¹ = R² = Me; 1c: R¹ = Me, R² = C₆H ₄CH₃-4), and tungsten, (Xyl-N=)Cl₃W(CH ₂CH₂C(=O)OEt) (7), using zinc homoenolate reagents are described. Intramolecular coordination of the carbonyl moiety to the metal center in these complexes was confirmed by their NMR and IR spectroscopy together with X-ray analyses of 1a and 1b. The insertion reaction of isocyanide into the metal carbon bond of 1a and 7, respectively, resulted in the formation of a diazametallacycle, Cp*Cl₃Ta-[N(Xyl)-C(=C=N-Xyl)-C(CH ₂CH₂CO₂Et)=N-Xyl] (3a), which possesses a metallacyclic structure with an exocyclic ketene-imine moiety, and an η²-iminoacyl tungsten complex, (Xyl-N=)W{C(-N-Xyl)-CH ₂CH₂CO₂Et}(CNXy])Cl₃ (8). The β-proton of the homoenolate moiety of la was selectively deprotonated by KN(SiMe₃)₂ to afford an η⁴-ethyl acrylate complex, Cp*Cl₂Ta(η⁴-ethyl acrylate) (4a). In the case of complex 1b, in which the β-positions were protected by dimethyl substituents, reaction with the dilithium salt of diazadiene afforded a tantalalactone complex, Cp*(η⁴-p-MeOC₆H ₄-DAD)Ta(CH₂-CMe₂C(=O)O) (5). The addition of Al(C₆F₅)₃ to 5 afforded a novel zwitterionic complex, Cp*(p-MeOC₆H₄-DAD)Ta(CH₂CMe ₂C(=O)O-Al(C₆F₅)₃) (6), in which Al(C₆F₅)₃ coordinated to the exocyclic carbonyl oxygen of the tantalalactone.