Synthesis, Structure, and Reactivity of Tantalum and Tungsten Homoenolate Complexes

Preparation and characterization of homoenolate complexes of tantalum, (1a: R1 = H, R2 = Et; 1b: R1 = R2 = Me; 1c: R1 = Me, R2 = C6H4CH3-4), and tungsten, (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, (3a), which possesses a metallacyclic structure with
an exocyclic ketene−imine moiety, and an η2-iminoacyl tungsten complex, (Xyl-N)W{C(N-Xyl)CH2CH2CO2Et}(CNXyl)Cl3 (8). The β-proton of the homoenolate moiety of 1a 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, (5). The addition of Al(C6F5)3 to 5 afforded a novel zwitterionic complex, (6), in which Al(C6F5)3 coordinated to the exocyclic
carbonyl oxygen of the tantalalactone.