Ferrocenes mercuration

Although the mercuration of ferrocene has been reported for many years, only recently has the mechanism of this reaction been elucidated. Recent studies indicate that the soft mercuric cation undergoes complexation to the Lewis-basic iron center prior to mercuration. This event is followed by the rate-limiting endo-attack of the cyclopentadienyl ring and subsequent product formation (Scheme 7).103... 

Another derivative that is a useful intermediate for synthesizing other derivatives is the mercury compound that can be prepared from ferrocene by reaction of mercuric acetate in a solution also containing chloride ion. 

Ferrocene, like thiophene, furan, and other so-called superaromatic systems, reacts readily with mercuric acetate to form mercurated derivatives. Nes-meyanov and coworkers first reported that ferrocene could be mercurated under relatively mild conditions in either ethyl ether-alcohol or benzene-alcohol solution (63). The acetoxymercuriferrocenes formed in this manner are usually treated with an alcoholic solution of an alkali metal halide. The resulting products, chloro-mercuriferrocene (XXVII) and l,l -di(chloromercuri)ferrocene (XXVIII), can be conveniently separated by extraction with n-butyl alcohol. 

N-Methylethanamide, AC01 Methyl ethyl ketone, AD11 Methyl ferrocene, ATL3 Methylformate-l-mercuric halide, AA56... 

Iminophosphoranes, mercuration, 2, 432 (Iminophosphoranyl)ferrocenes, synthesis, 6, 202 Imino-pyrrolate systems, with zirconium, 4, 1138-1139 Imino-pyrrolides, with Zr(IV) and Hf(IV), 4, 780 Imino-pyrrolyl ligands, in chromium complexes, 5, 355 Iminosilanes, reactions, 3, 440 Imprinted complexes, on oxide surface, 12, 810 Impurities, glovebox techniques, 1, 215 Inclusion chemistry... 

Facile mercuration of Fe(CO)3(r -C4H4) occurs with Hg(OAc)2 in acetic acid (30 min, r.t.) to give an equilibrium mixture of all possible products, including Fe(CO)3 t -C4(HgOAc)4, which were characterized by iodination (KI3).357 In acetic acid, disproportionation occurs, even with the tetra-substituted product. This system has a reactivity similar to that of ferrocene. 

The mercuration of metallocenes has continued to attract interest due to the potential use of the mercurated species in the synthesis of different metallocene derivatives. New mercurometallocenes, together with some mechanistic studies on their formation,have been reported over the past few years. Besides the monomercurated species, a detailed study has appeared on the permercuration of ferrocene and ruthenocene. For both metallocenes, the decamercurated species shown in equation (7) have been obtained in high yields. 

In several respects, the cylindrical molecule ferrocene (di( 7 -cyclopentadienyl)iron, FeCpj) is analogous to the planar molecule benzene (CgHg). Both ferrocene and benzene are electron-rich aromatic systems that undergo electrophilic substitution. Ferrocene reacts about 10 times faster than benzene in Friedel-Crafts acetylation and about 10 times faster in mercuration with Hg(OAc)2-... 

However, in contrast to benzene, ferrocene is sensitive to oxidation, and the ferrocenium cation, FeCpj, a paramagnetic 17-electron species, is readily formed in the presence of various oxidants. The ferrocenium cation is reluctant to undergo electrophilic substitution, and therefore reactions such as halogenation and nitration, which are important routes to substituted benzene derivatives, cannot be used for the synthesis of substituted ferrocenes. Only electrophilic substitution under nonoxidizing conditions (e.g., Friedel-Crafts acylation, Mannich reaction, borylation, lithiation or mercuration), and radical substitution are available as an entry into the chemistry of substituted ferrocenes. 

Three routes have been preferentially used to prepare ferrocene compounds that contain heteroelements directly attached to the metallocene unit borylation, mercuration, and lithiation. 

The metallation of ferrocene, for example lithiation and mercuration, has been the historically important route to the halogenated derivatives, Fc-X and fcX2-Scheme 5-3 summarizes the basic compounds that are useful intermediates for the synthesis of many ferrocenes bearing heteroatoms. 

Scheme 5-3. Useful intermediates for the synthesis of ferrocene derivatives containing heteroatoms (a) Lithiation (generally with nBuLi) (b) mercuration with Hg(OAc)2, followed by reaction with LiCl or KCl (c) borylation with B(OnBu)3, followed by hydrolysis (d) CUCI2 (e) CuBr2 (f) compounds containing reactive bromo substituents, e.g., BrCX2 -CX2Br (X = F, Cl) or tosyl bromide (g) A-bromo succinimide (h) I2 (i) Br2.

Exhaustive mercuration of ferrocene with mercury(ii) trifluoroacetate, Hg(OCOCF3)2, is another possibility to synthesize polyhalogenated ferrocenes [37]. The reaction of deca(trifluoroacetato-mercuri)ferrocene, Fe[C5(Hg-OCOCF3)5]2, with the trihalides KBr3 and KI3 can be used to prepare Fe(C5Br5)2 and Fe(C5l5)2, respectively monosubstituted ferrocenes, Fc-R, react in an analogous manner (Scheme 5-5) ... 

The colorless zinc compound, Zn(CisH6)2, which sublimes at 160° under partial decomposition, is obtained in small yield from zinc chloride and cyclopentadienyl sodium in diethyl ether however, the less stable cadmium compound decomposes, with separation of cadmium, under these conditions (55). The mercury compound, Hg(CsH5)2, is produced in 20% yield by the action of the sodium derivative on mercuric chloride in tetrahydrofuran (215). The action of cyclopentadiene on the complex K2(HgI ) in aqueous alkaline solution results in the precipitation of a mixture of CsHsHgl and Hg(CsH6)2, from which the latter compound may be obtained in good yield by extraction with a mixture of tetrahydrofuran and petroleum ether (62). It forms pale yellow crystals which begin to decompose at about 60° and which melt at 83-85°. The compound is readily soluble in most solvents it decomposes slowly even when kept in the dark at room temperature it is insoluble in water and reacts with neither water nor bases. On the other hand, decomposition occurs in dilute hydrochloric acid. It converts ferric chloride to ferrocene quantitatively, and it yields an adduct with maleic anhydride (215). 

The usefulness of catalysts other than aluminum chloride, e.g., anhydrous hydrogen fluoride 202, 223), boron trifluoride 100), and phosphoric acid 94), has been examined. The mercuration reaction which is characteristic of aromatic systems takes place also for ferrocene, as has been described by Nesmeyanov and his co-workers 143,147) Through the mercury compounds these workers have also obtained the halogeno-ferrocenes. 

In Other variations, mercurated ferrocene was treated with olefins in the presence of Pd(II) salts to give vinyl ferrocene derivatives (9, 154). Aryl cobalt derivatives have also been used to produce the phenyl-palladium species 284-286). In another study it was found that even aryl phosphine ligands will transfer the aromatic to palladium 297). 

Transition-metal complexes containing cyclopentadiene and cyclobutadiene ligands are mercurated readily, e.g., ferrocene , (h -C5Hj)2Fe, reacts with Hg(02CCHj)2 in HO2CCH3, EtjO-EtOH, or PhH-EtOH to give mixtures of mono- and dimercurated ferrocenes in proportions dependent on the reagents ratio ... 

Other Carbon-Heteroatom Bonds. Carbon-mercury bonds are readily formed by treating the metalated species with mercuric chloride (Reaction 21) (27). The resulting chloromercury derivative in one case is a useful intermediate in the preparation of 2-iododimethylaminomethyl-ferrocene (27). 

MERCURIC NITRATE (10045-94-0) A powerful oxidizer. Violent reaction with reducing agents, combustibles, phosphinic acid, hypophosphoric acid, petroleum hydrocarbons. Forms heat- and/or shock-sensitive compounds with acetylene (forms mercury acetylide), ethanol (forms mercury fulminate), ferrocene, isobutene, phosphine, potassium cyanide, sulfur. Incompatible with strong acids, acetic anhydride, ammonia, ammonium hexacyanofer-rate(II), organic azides, citric acid, hydrazinium perchlorate, isopropyl chlorocarbonate, nitrosyl perchlorate, sodium thiosulfate, sulfamic acid, thiocyanates, hydrozoic acid, methyl isocyanoacetate, sodium peroxyborate, trinitrobenzoic acid, urea nitrate. Aqueous solution corrodes metals. 

The stereochemical aspect of electrophilic substitution of metallocenes is intriguing. In acylation, the attack comes from the outside (anti to the metal) and the order of reactivities is ferrocene > ruthenocene > osmocene (18). On the other hand, during mercuration the electrophile approaches from the inside (syn to the metal). It is clear that (/) hard electrophiles align themselves farthest from the essentially soft metal in the transition states, and the charge-controlled reactions (acylation) occur more readily with the harder metallocenes, and (ii) soft electrophiles (e.g., Hg ) approach the aromatic ring from the same side as the metal which permits soft-soft interaction. 

Electrophilic attack on coordinated cyclopentadienyl rings, particularly those in ferrocene, is well established. This process occurs in a similar fashion to electrophilic attack on arenes and was used to establish the binding mode of the Cp ligand in ferrocene (see Equation 3.88). These reactions of the Cp ligand with electrophiles are described in more detail in Chapter 12, which covers electrophilic attack on coordinated ligands. Friedel-Crafts acylation, formylation, aminomethylation, and mercuration are all known. - ... 

Multiple substitution of reactive aromatic compounds may occur. Thiophene gives a 2, 5-disubstituted and furan a 2,3,4, 5-tetrasubstituted derivative with mercury (II) acetate in boiling ethanol. The preferential mercuration of thiophene has been used for its removal from commercial benzene. Mercuration of ferrocene (p. 284) also occurs readily. 

Transmetallation reactions in order to obtain mercurated ferrocenes were carried out with a series of planar chiral cyclopalladated ferrocenylimines. ... 

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