Metallocenes, Strong Electron Donors. A Mechanistic Review

The great stability of a-metallocenyl carbocations is well known. There is general agreement that the electrons from the region between the two cyclopentadienyl rings are most effective in accounting for this stability. Electron transport may involve conjugation with the 7t-system of the pentadienyl rings or direct participation of metal electrons. The study of the secondary a-deuterium kinetic isotope effect (a-D KIE) can help, under certain circumstances, in solving the problem. We have recently determined the a-D KIEs in acetolysis and for- molysis of dideuterioferrocenylmethyl benzoate and found that, in the presence of LiClO.^ (which prevents reversion from the solvent- separated to contact ion-pairs), the ratios k^lkD at 25 °C are 1.53 ± 0.02 (acetolysis) and 1.48 ± 0.03 (formolysis). The solvolyses exhibited a special salt effect, indicating the presence of solvent- separated ion-pairs and the return to contact ion-pairs. The high values of the KIE strongly suggest that both solvolyses are limiting dissociation processes with a carbenium ion-like transition state, which is stabilized mainly by conjugation with the 7t-system of the pentadienyl rings. For both solvolyses, the ratios of Arrhenius pre-exponential factors AR / AD are significantly less than unity; whether tunnelling plays a role in causing this effect is discussed. Consistent with a dissociative (Sjjl) mechanism, in similar solvolyses a general tendency has been observed for faster reactions in solvents of higher ionizing power. However, we observed even a somewhat lower solvolysis rate in formic acid than in acetic acid, though the ionizing power of formic acid is much higher than that of acetic acid. This phenomenon is, at least partially, explained by the reaction being of the protonated ester, or protonate