Titanium and zirconium ferrocene-substituted enolates and their reaction products with benzaldehyde and acetophenone: Structural and kinetic studies of the aldol condensation pathway

Article abstract of DOI:10.1021/om00009a013

The reaction of acetyl- and propionylferrocene, [(RCOcp)(cp)Fe] (R = Me (1), Et (2)), with KH led to the isolation of the corresponding ion-pair enolates 3 and 4 in the solid state. When the deprotonation of 1 is carried out in the presence of 18-crown-6, the naked enolate 5, [(CH₂COcp)(cp)Fe]^-K(18-crown-6)⁺], was isolated. The potassium enolate 3 has been converted into the corresponding titanium and zirconium enolates via the reaction with (cp)₂MCl₂ (M = Ti, Zr), while only 4 was converted into the analogous zirconium enolate. The following metal enolates have been isolated in good yield and as crystalline solids: [(cp)-Fe(cpC(CH₂)O)M(cp)₂(Cl)] (M = Ti (6), Zr (7)) and [(cp)Fe(cpC(CHMe)O)Zr(cp)₂(Cl)] (8). Compounds 6 and 7 have been characterized by ¹H and ¹³C NMR, and an X-ray crystal structure of 7 was obtained; compound 8 was characterized by ¹H NMR. The aldol reaction of 6 and 7 with benzaldehyde led to the corresponding metal aldol derivatives [(cp)Fe(cpC(O)CH₂C(H)(Ph)O)M(Cp)₂(Cl)] (M = Ti (9), Zr (10)). For compound 9 the solid-state structure and solution data are reported. Complex 9 undergoes a facile Ti-Cl ionization, leading to the cationic complex [(cp)Fe(cpC(O)CH₂C(H)(Ph)O)Ti(cp)₂]⁺BPh ₄^- (11). In complex 11, the aldol fragment forms a metallacycle where both oxygens are bonded to titanium. This structure mimics the bond connectivity of the generally proposed transition state of aldol reactions. The isolation of structurally well-defined titanium and zirconium enolates allowed us to carry out a kinetic investigation into the reaction of 7 with acetophenone. The reaction was carried out in C₆D₆ at temperatures from 283 to 340 K. The reaction is second order (first order in each reactant), and the following activation parameters have been obtained: ΔH? = 44.4 ± 1.7 kJ mol^-1, ΔS? ≥ -150 ± 6 J mol^-1 K^-1, and ΔG?₂₉₈ = 89.0 ± 2.4 kJ mol^-1. A similar study with 4-fluoroacetophenone gave ΔH? = 33.0 ± 4.6 kJ mol^-1, ΔS? ≥ -189 ± 15 J mol^-1 K^-1, and ΔG?₂₉₈ = 89.2 ± 6.4 kJ mol^-1. The reaction rate at 320 K determined with 4-chloro-, 4-methyl-, and 4-nitroacetophenone allowed the determination of a Hammett plot with ρ = 0.42 ± 0.9. This value is implicit for a carbon-carbon bond-forming, rate-limiting step. Complexes 7 and 9 have been characterized by X-ray analysis.