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The protonation mechanism of metallocenes and [1.1]metallocenophanes

1994; Elsevier BV; Volume: 472; Issue: 1-2 Linguagem: Inglês

10.1016/0022-328x(94)80210-6

1872-8561

Ulrich T. Mueller‐Westerhoff, Thomas J. Haas, Gerhard F. Swiegers, Thomas K. Leipert,

Asymmetric Hydrogenation and Catalysis

A study of the protonation and deuteration of [1.1]metallocenophanes has revealed several details of the mechanism of metallocene protonation. In [1.1]ferrocenophane, protonation at both metallocenes is immediately followed by elimination of dihydrogen. The resulting bis-ferrocenium ion is substitutionally inert and no longer takes part in protonation or deuteration. This convenient circumstance permits us to analyze the protonation mechanism in greater detail than monomeric ferrocenes would allow. The reaction of [1.1]ferrocenophanes with deuterated acids leads to partially or completely deuterated rings, depending on their structure and the reaction conditions. The experimental results on ferrocene-containing systems are not compatible with a reaction path in which the incoming electrophile binds first to the iron and then transfers to the ring. Substitution must rather occur by exo-attack on the ring, followed by transfer of the proton to the metal and back to either one of the rings. For protonated ferrocene, a rapid equilibrium between ring- and metal-protonated species exists; deuterated acids lead to rapid and complete H/D exchange. In contrast to the protonation of ferrocene, the protonation of ruthenocene occurs at the metal only, without any participation of the Cp rings. Consequently, ruthenocene reacts with deuterated acids without H/D exchange. Studies on [1.1]ferroceno-ruthenocenophane demonstrate the differences in the protonation of these two metallocenes.