Titanocene-Based Acetophenone Enolates: Synthesis and Structural Characterization of Mono, Bis, and Cationic Forms of Titanium Enolates and Some Aspects of Their Reactivity

The reaction of [cp2TiCl2] with potassium enolates derived from 2-MeOC6H4-COMe, 2, 4, 6-Me3C6H2COMe, and KH led to the metal–enolates [2-MeOC6H4C(CH2)OTi-(Cl)cp2](1), [2, 4, 6-Me3C6H2C(CH2)CTi(Cl)cp2] (2), and [{2-MeOC6H4C(CH2) 0}2Ticp2] (3) where the enolate is η1-O-bonded to the metal and the o-methoxy group does not interact with the metal. In order to increase the acidity and the template properties of the metal, complex 2 was ionized with CF3SO3Ag and AgBF4. The cationic form of 2 readily reacts with cyclohexanone to give [2, 4, 6-Me3C6H2C(O)CH=C6H10] (6) and a titanium oxo species. The cationic form, however, was isolated as an isocyanide adduct with ButNC, [2, 4, 6-Me3C6H2C(CH2)OTi-(CNBut)cp2]+X−(X− = CF3SO3, 8; X− = BF4, 9). The increased reactivity of the enolato functionality in the cationic form of 2 is demonstrated in the reaction with acetonitrile and benzonitrile, where the binding of the nitrile prompted attack by the nucleophilic enolate on the nitrile. The reaction led to β-keto enamine derivatives N, O-bonded to the [cp2Ti]2+ fragment, [2-MeOC6H4C(O) CH=C(Me) NHTicp2]+CF3SO3-(13) and [2, 4, 6-Me3C6H2C(O)CHC(R)-NHTicp2]+CF3SO3-(R = Me, 14; R = Ph, 15). Crystallographic details: 1 is orthorhombic, space group P212121, a = 7.690(1) Å, b = 14.113(2) Å, c = 15.685(2) Å, a = β = γ = 90°, Z = 4, and R = 0.034; 3 is orthorhombic, space group Aba2, a = 9.472(1) Å, b = 20.257(2) Å, c = 12.378(1) Å, α = β = γ = 90°, Z = 4, and R = 0.045; 14 is monoclinic, space group P21/n, a = 7.765(6) Å, b = 18.386(3) Å, c = 17.677(3) Å, α = γ = 90°, β = 100.6(3)°, Z = 4, and R = 0.088.