Mercurated derivatives, halogenation

When treated with dilute hydrochloric acid, the mercuric derivatives regenerate the starting oxadiazoles. They also react with halogen to yield the 5-halogeno-oxadiazoles. 

There are a wide variety of methods for introduction of substituents at C3. Since this is the preferred site for electrophilic substitution, direct alkylation and acylation procedures are often effective. Even mild electrophiles such as alkenes with EW substituents can react at the 3-position of the indole ring. Techniques for preparation of 3-lithioindoles, usually by halogen-metal exchange, have been developed and this provides access not only to the lithium reagents but also to other organometallic reagents derived from them. The 3-position is also reactive toward electrophilic mercuration. 

Iodination of moderately reactive aromatics can be effected by mixtures of iodine and silver or mercuric salts.26 Hypoiodites are presumably the active iodinating species. Bis(pyridine)iodonium salts can iodinate benzene and activated derivatives in the presence of strong acids such as HBF4 or CF3S03H.27 Scheme 11.2 gives some specific examples of aromatic halogenation reactions. 

Methyloxadiazole and HgClg yield a complex XLIV. S-Phenyl-oxadiazole and HgCl gives directly the substituted derivative XLV. These mercurated compounds, on treatment with a halogen, yield the desired 5-halogeno-oxadiazoles 25a). 

It has been found on examination of the ammino-derivatives of mercuric halides that the nature of the halogen present has a marked effect on the stability ol the compound. Thus, diammino-mercuric chloride, [Hg(NH3)2]Cl2, is stable and does not readily lose ammonia, whereas the corresponding iodide, diammino-mercuric iodide, [ITg (NH3) JI2, readily loses ammonia at ordinary temperature. The stability decreases in the order chloride, bromide, iodide. 

Among the oxidative procedures for preparing azo compounds are oxidation of aromatic amines with activated manganese dioxide oxidation of fluorinated aromatic amines with sodium hypochlorite oxidation of aromatic amines with peracids in the presence of cupric ions oxidation of hindered aliphatic amines with iodine pentafluoride oxidation of both aromatic and aliphatic hydrazine derivatives with a variety of reagents such as hydrogen peroxide, halogens or hypochlorites, mercuric oxide, A-bromosuccinimide, nitric acid, and oxides of nitrogen. 

Chlorination or bromination of isothiochroman yields the corresponding 1-halo derivatives,241,252 253 which are intermediates for numerous isothiochromans since the halogen atoms are benzylic and, therefore, very susceptible to nucleophilic substitution. Thus, 1-chloroisothio-chroman wth mercuric cyanide gives the 1-cyano derivative, which provides the 1-carboxylic acid, 1-ester, and the 1-amide.239,252 The remaining 1-H of the 1-cyano derivative is acidic and, with base,... 

A new, more accurate electron diffraction study of gaseous mercuric chloride has been reported.159 The interatomic distances (Hg—Cl = 2.25 A, Cl—Cl = 4.48 A) are shorter than previously reported values by 0.02 to 0.09 A. A complete normal-co-ordinate analysis of bis(methylthio)mercury has also been reported.160 The Raman spectra of gaseous mercuric chloride, bromide, and iodide have been reported.161 The bond polarizability derivatives calculated from the data increase in the order Cl < Br < I, suggesting an increased degree of covalence in the mercury-halogen bond with increasing size of the halogen atom. 

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. 

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. 

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