Ferrocenes protonation

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... 

Proton-coupled intramolecular electron transfer has been investigated for the quinonoid compounds linked to the ferrocene moiety by a 7r-conjugated spacer, 72 (171) and 75 (172). The complex 72 undergoes 2e oxidation in methanol to afford 74, which consists of an unusual allene and a quinonoid structure, with the loss of two hydrogen atoms from 72 (Scheme 2). The addition of CF3SO3H to an acetonitrile solution of 74 results in two intense bands around 450 nm, characteristic of a semi-quinone radical, and a weak broad band at lOOOnm in the electronic... 

EI-EPR has also been applied in powder samples, where single crystal-like EPR spectra can be obtained for B0 observer fields which correspond to the orientations of principal values of ligand hfs tensors37. This is illustrated in Fig. 15 for a powder sample of dibenzene vanadium diluted into polycrystalline ferrocene. The hfs tensors of the twelve geometrically equivalent benzene protons are not coaxial with the g and the... 

Fig. 47. Temperature dependence of the high field part of the two-dimensional proton ENDOR spectrum of V(bz)2 diluted into ferrocene. (From Ref. 78)...

Fig. 12 A plot of the inverse Fe-N distances in four ferrocene amines against the differences of the redox couples of ligands in their free and protonated forms.

As the latter were not easily accessible by chemical synthesis at that time, new methods of preparing these ferrocene derivatives were developed and introduced in 1969. It was then proved that the U-4CRs of chiral a-ferrocenyl-alkylamines can form diastereomeric a-aminoacid derivatives stereo-selectively, and it was further shown that after the reaction the a-ferrocenyl groups of the products can be replaced by protons, thus resynthesizing the chiral a-ferrocenyl-alkylamines simultaneously." Later, the development of this ferrocene chemistry was given up since such syntheses cannot form the products in sufficient quantity and stereoselective purity. ... 

By coordinating to arenes, transition metals can facilitate ring lithiation by decreasing the electron density in the ring and acidifying the ring protons. We shall consider briefly the two most important metal-arene complexes in this regard—arenechromium tricarbonyls and ferrocenes. 

For the [Pdltriphosphinejlsolvent)] " " complexes, the metallocarboxylic acid formed in step 3 of Sch. 2 is not ready to undergo C—O bond cleavage. In order for this reaction to occur, an additional electron transfer, solvent loss, and a second protonation have to occur. Of particular interest in this sequence is the loss of a weakly coordinated solvent molecule (step 5), to produce a vacant site on the metal for water to occupy as the C—O bond of CO2 is broken to form coordinated CO and coordinated water [34, 35]. This C—O bond cleavage reaction is the slow step in the catalytic cycle for these catalysts at low acid concentrations, and a vacant coordination site is required for this reaction to occur. C—O bond cleavage is also the slow step for Fe(porphyrin) catalysts at low acid concentrations (H+, Mg +, or CO2) [37-39]. In this case, a vacant coordination site is not required. However, the potentials at which catalysis occurs in this case (approximately —2.0 V vs. ferrocene/ferrocen-ium) is much more negative than those... 

In addition to the nature of the cation at the adsorption site, the superhyperfine tensor can also give information on neighboring atoms further away. For example, OJ adsorbed on MgO exhibits a superhyperfine tensor ascribed to the presence of a nearby proton, presumably as a hydroxyl group (68) and this has been confirmed by isotopic labeling with deuterium (see Section IV,A). Superhyperfine tensors indicating the presence of nearby protons have also been reported for O J adsorbed on ferrocene deposited on porous Vycor glass (PVG) (120) and for alkylperoxy radicals supported on Ti02 (90). 

The silylated product exhibits an interesting H NMR spectrum in that the resonance of the SiMe3 protons occurs at higher fields (r = 10.2 ppm) than in the ferrocene analogs. 

Another interesting comparison of Group VIII metallocenes concerns their ease of protonation in boron trifluoride hydrate (14). It appears that the proton is attached directly to the metal atom, and that ruthenocene and osmocene are protonated to a lesser extent than ferrocene. 

---