Protonation of ferrocene

RT, the protonation equilibrium constant is 203 M for RhCKCOCAsPhj) it is 75 M. Similar K values are found for Ir and Rh complexes with phosphine and halide ligands. Finally, ruthenocene is half-protonated in H SO with an acidity function, Hg, of —5.7, whereas half-protonation of ferrocene requires an Hg of —7.7. 

This situation affects all aspects of Chemistry. For example, ths frontier orbital theory of reaction mechanisms (57) stresses the symmetry properties of the highest occupied MO (HOMO) and of the lowest occupied MO (LUMO) of the reacting species in the point group of the transition state. Suppose we are discussing the protonation of ferrocene the question arises, what is the HOMO of the ferrocene molecule Is it the orbital given by Koopmans sequence of eigenvalues, or is it the 2g orbital whose energy on protonation may follow the ionization pattern and be hi er than that of the protonated orbital ... 

Coupled-cluster calculations on ferrocene and its protonated derivatives Towards the final word on the mechanism of protonation of ferrocene ... 

However, electrophilic aromatic substitution on ferrocene presents two complications (1) its susceptibility to one-electron oxidation, which precludes halogenation, nitration, or direct (H SO ) sulfonation and (2) competition between the Fe and the ring as the initial site of electrophilic attack. It is now clear from H NMR spectroscopy of the product of the protonation of ferrocene with a superacid at -122 °C that protonation of ferrocene initially occurs endo. The detailed structure of the protonation product has not been unambiguously established, but calculations imply that the proton is associated at least as much with the cyclopentadienyl carbons as with the iron. ... 

The density functional approach has been used in an investigation of the protonation of ferrocene, ruthenocene, and osmocene in the gas phase, and the general conclusion is that the addition of a proton to the carbon atoms in the cyclopentadienyl ring is favored in ferrocene, whereas metal protonation is favored with ruthenocene and osmocene. The results obtained from these calculations were used in the interpretation of electrophilic substitution reactions of metallocenes. The basicity of the ligand group l,l -bis(diphenylphosphino)metallocene has also been examined. This is an important aspect for catalysis because it has proved difficult to obtain results which... 

Initial protonation of iron in protodesilylation of trimethylsilylferrocene was not, however, favoured as a mechanism by Marr and Webster689, who measured rates by the spectroscopic method using hydrochloric acid in 20 vol. % aqueous methanol (Table 235) and found that the rate of desilylation of the ferrocene compound was little more than that for the 4-methoxyphenyl and 2,4-dimethyl compounds. The similarity of the spread of rates in the different media and the similar activation energies and entropies were considered as evidence that the transition states for reaction of all three compounds were similar. The lower activation energy obtained for the 4-methoxyphenyl relative to the ferrocene compound may arise from the different media involved the difference in entropy seems, however, to be rather larger than one might have expected even allowing for the solvent differences. 

Cunningham AF Jr (1994) Sequential Friedel-Crafts diacetylation of ferrocene interannular proton transfers as a mechanistic probe. Organometallics 13 2480-2485... 

Reaction of the heteroannular diol (30) with phenylisocyanate or p-methoxyphenylisocyanate at room temperature gave the expected urethans, but use of these isocyanates at elevated temperature or use of p-nitrophenylisocyanate at room or elevated temperature gave the bridged amine (31 R = H, OCHs, and N02)18 The NMR spectrum of 31 shows a triplet for the ferrocene ring protons.19 This triplet is attributed to the direct inductive influence on the a-ring proton of the bridge nitrogen atom. 

Chiral bis(oxazolines) 51 with an oxalylic acid backbone were used for the Ru-catalyzed enantioselective epoxidation of tran5-stilbene yielding franx-l,2-diphenyloxirane in up to 69% ee [24]. The asymmetric addition of diethylzinc to several aldehydes has been examined with ferrocene-based oxazoline ligand 52 [25], resulting in optical yields from 78-93% ec. The imide 53 derived from Kemp s triacid containing a chiral oxazoline moiety was used for the asymmetric protonation of prochiral enolates [26]. Starting from racemic cyclopentanone- and cyclohexanone derivatives, the enantioenriched isomers were obtained in 77-98 % ee. 

Shielding Values (ppm) of Ring Protons of Alkylferrocenes from Ferrocene" ... 

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