Titanium and Zirconium Ferrocene-Substituted Enolates and Their Reaction Products with Benzaldehyde and Acetophenone: Structural and Kinetic Studies of the Aldol Condensation Pathway

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 a in the solid state. When the deprotonation of 1 is carried out in the presence of 18-crown-6, the naked enolate 5, [(CH(2)COcp)(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)(2)MCl(2) (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 H-1 and C-13 NMR, and an X-ray crystal structure of 7 was obtained; compound 8 was characterized by H-1 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: Delta H-double dagger = 44.4 +/- 1.7 kJ mol(-1), Delta S-double dagger greater than or equal to -150 +/- 6 J mol(-1) K-1, and Delta G(298)(double dagger) = 89.0 +/- 2.4 kJ mol(-1). A similar study with 4-fluoroacetophenone gave Delta H-double dagger = 33.0 +/- 4.6 kJ mol(-1), Delta S-double dagger greater than or equal to -189 +/- 15 J mol(-1) K-1, and Delta G(298)(double dagger) = 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 rho = 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.