171818-36-3Relevant academic research and scientific papers
Titanium and zirconium ferrocene-substituted enolates and their reaction products with benzaldehyde and acetophenone: Structural and kinetic studies of the aldol condensation pathway
Veya, Patrick,Cozzi, Pier Giorgio,Floriani, Carlo,Rotzinger, Fran?ois P.,Chiesi-Villa, Angiola,Rizzoli, Corrado
, p. 4101 - 4108 (1995)
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.
