A-ferrocenyl carbonium ion

Ware, J. C., and T. G. Traylor Resonance stabilization of a-ferrocenyl carbonium ions. Tetrahedron Letters [London] 1965, 1295. 

The very strong electron-donating power of the ferrocene nucleus is clearly demonstrated in the extraordinary stability of the a-ferrocenyl carbonium ion (X). Questions have been raised concerning the mode of stabilization of such ions, i.e., (1) whether the77-orbitals of the ring overlap the empty p-orbital (resonance), (2) whether the f-orbitals on iron participate or overlap the empty p orbital, and (3) whether there is ring movement. 

The synthesis of the required acids was effected as outlined below. Condensation of the cyclic ketone (XXVI) with- dimethylsulfonium methylide 10 ) afforded a mixture of exo and endo aldehydes (XXVIII and XIX) on acid work-up of the reaction. The very great ease with which the intermediate epoxide (XXVII) undergoes acid catalyzed isomerization is not surprising in view of the very great stability of a-ferrocenyl carbonium ions previously noted. 

In the 1980s, the successful synthesis of elastomeric polyamides 120 of high molecular weight (d/n =10,000-18,000) was reported from the polycondensation of l,l -bis(/ -aminoethyl)ferrocene with diacid chlorides (Scheme 13). Also, polyureas 121 were prepared from the same ferrocene monomer and diisocyanates, and polyesters and polyurethanes were prepared from l,l -bis(/ -hydroxyethyl)ferrocene. However, the latter materials had much lower molecular weights and were characterized only by scanning electron microscopy. X-ray, and IR analyses. The introduction of ferrocenes in which the functional groups are separated from the cyclopentadienyl ring by at least two methylene units was crucial in order to reduce steric effects and to avoid the instability found previously in polymers of a-functionalized ferrocene due to the a-ferrocenyl carbonium ion stability. 

An essential characteristic of ferrocene chemistry is the stabilization of ferrocenyl carbonium ions. These carbocations are mesomers of the corresponding hexahapto fulvene complexes [FeCp(r -fulvene)]+. They are even more stable than the trityl cation PhsC". The stabilization of the a-ferrocenyl carbonium ions explains the acetolysis of vinylferrocene, the hydrolysis of the acetate formed, the ease of nucleophilic substitution in a position, and the OH" abstraction from the a-ferrocenyl alcohols. This stabilization is still enhanced by going down in the iron column of the periodic table, because the size of the d orbital increases, which facilitates their insertion with the carbocation and accelerates the solvolysis of acetates (Os > Ru > Fe). 

This reaction is unusual in comparison to the reaction of benzene and 1,2-dichloroethane, which gives dibenzyl. Therefore, the formation of 1,1-diferrocenylethane distinctly shows that a hydride ion rearrangement takes place in going from [6-3] to [6-4]. The rearrangement might result from the remarkable stability of the a-ferrocenyl carbonium ion. 

Cais, M. E. Dannerberg, T. T Eisenstandt, A. Levenberg, M. L Richards, T. H. Nuclear Magnetic Resonance Spectra of Ferrocenyl Carbonium ions. Tetrahedron Lett., 1966, 1695-1701. 

The reaction of ferrocene and formaldehyde in either concentrated sulfuric acid or liquid hydrogen fluoride, followed by reduction, produces a compound containing two ferrocenyl and two methylene groups (57, 98, 123). After several incorrect assignments had been proposed for the structure of this condensation product, Rinehart and coworkers showed by an unequivocal synthesis that the product was 1,2-diferrocenylethane (XIX) (104). The mechanism of the reaction presumably involves the initial formation of ferrocenylcarbinol (XX) followed by ionization in the strongly acidic medium to the ferrocenylmethyl-carbonium ion (XXI). Conversion to radical ion XXII followed by dimerization and subsequent reduction produces the product. 

A number of observations have been made which qualitatively suggest that carbonium ions adjacent to metallocene systems possess unusual stability. Ferro-cenecarboxaldehyde, for example, is soluble in dilute hydrochloric acid (5), ferrocenyl carbinols such as ferrocenyl phenyl carbinol form ethers with great ease (124), and ferrocenylmethylcarbinol can be dehydrated to vinylferrocene under exceedingly mild conditions (114). The concept of stabilizations of this type has also been used to explain certain anomalous ring substitution reactions. 

One of the most fascinating areas of metal-cyclopentadienyl chemistry in recent years concerns the formation and relative stabilities of metal-locenyl-carbonium ions. The ability of iron to stabilize cationic centers in certain ferrocene compounds was noted by Weliky and Gould 243) in 1957. In 1959 Richards and Hill 218y 214, ) reported the results of some kinetic studies relating to the relative rates of solvolysis of metal-locenylmethylcarbinyl acetates. They determined that these solvolyses proceeded via a carbonium ion mechanism and found that the ferrocenyl acetate, for example, solvolyzed nearly seven times faster than did tri-phenylmethyl acetate under the same conditions. 

As a stabilizing group for carbonium ions, a single ferrocenyl nucleus is almost as effective as three phenyl substituents, and this stabilization increases progressively as the metal atom is changed to ruthenium and then osmium. 

A ferrocenyl substituent attached to the carbonium ion centre of a cyclopropylmethyl cation provides stabilization of such a magnitude that homoallylic delocalization cannot be detected by spectroscopic methods, and quenching yields unrearranged products. Somewhat related is the observation of optically active bridgehead... 

The stabilization of carbonium ions adjacent to a ferrocenyl group was noted on p. 285. Compared with the a-ferrocenyl group the stabilization arising from an a-... 

Richards and Hill have studied in some detail the reactivity of a-carbonyl acetates [295,296]. The ease with which the a-acetoxy ferrocenyls undergo acid catalysed deacetylation provides strong evidence for carbonium ion stabilization. 

All these postulated mechanisms of stabilization of the a-carbonium ion can account for the stereospecificity of the solvolysis of the isomers, 4.31 and 4.32. Of course, the carbonium ions will always interact simultaneously with both the metal and the ring and the actual electron distribution in the carbonium ions may be better described by combination of say (a) and (6). It may well be that the carbonium ions Jt-bond more strongly to the ring carbon than they bond to the metal, whereas in the ferrocenyl phosphonium cations the phosphorus might form a stronger bond with the iron, using its vacant 2d orbitals, than with the ring. 

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