Halogen replacement

Sulfur tetrafluoride [7783-60-0] SF, replaces halogen in haloalkanes, haloalkenes, and aryl chlorides, but is only effective (even at elevated temperatures) in the presence of a Lewis acid catalyst. The reagent is most often used in the replacement of carbonyl oxygen with fluorine (15,16). Aldehydes and ketones react readily, particularly if no alpha-hydrogen atoms are present (eg, benzal fluoride [455-31-2] from benzaldehyde), but acids, esters, acid chlorides, and anhydrides are very sluggish. However, these reactions can be catalyzed by Lewis acids (HP, BF, etc). 

The reactivity of the metal fluoride appears to be associated with the oxidation potential of the metal. For example, AgF replaces halogen in organic compounds, whereas Agp2 replaces hydrogen. 

For completeness, it should be mentioned that the reaction of trifluoromethyl radicals to replace halogens is extremely general, and not confined solely to metal species. Plasma-generated trifluoromethyl radicals will react with halocarbons according to the reaction 21)... 

Tri-rc-butylstannane is able to reductively replace halogen by hydrogen. Mechanistic studies indicate a free radical chain mechanism.199 The order of reactivity for the halides is RI > RBr > RC1 > RF, which reflects the relative ease of the halogen atom abstraction.200... 

Lithium derivatives of triphenylsilanes have also been used to replace halogens in halogenopentacarbonylmetallates of the chromium group . ... 

Organometallic anions react with halophosphazenes to replace halogen atoms by organometallic units. The first reactions of this type discovered are illustrated in Scheme 5. The metallophosphazenes are surprisingly stable. Moreover, as shown below, hexachlorocyclotrlphosphazene reacts with an organometallic dl-anlon to yield both a dlmetallo derivative (XXXI) and a trl-metallic cluster derivative (XXXII). The latter compound is... 

Divalent chromium reduces triple bonds to double bonds (trans where applicable) [195], enediones to diones [196], epoxides to alkenes [192] and aromatic nitroso, nitro and azoxy compounds to amines [190], deoxygenates amine oxides [191], and replaces halogens by hydrogen [197,198],... 

To eliminate arylating activity, replace halogen ortho/para to the nitro group(s) to the meta position in nitroaromatic compounds. 

Table 3 lists some of the exchange reactions carried out for replacing halogens with fluorine in halogenated ketones. 

An efficient reagent system to replace halogens with fluorine has been found in the semi-molten mixture of tetrabutylammonium bromide and cesium fluoride.168 A number of organo-fluorine compounds 13 have been prepared by this method by replacing the halogens in the starting organic halides 12. 

Further applications of cesium fluoride in replacing halogens and sulfonate groups with fluorine are listed in Table 10. 

Rubidium fluoride is a fairly expensive reagent and has been used rarely in replacing halogens and sulfonates with fluorine. l-Chloro-2,4-dinitrobenzene (19) has been reacted with rubidium chloride to give l-fluoro-2,4-dinitrobenzene (20).185... 

The reaction of aryl and hetaryl halides with the nitrile-stabilized carbanions (RC -CN) leads to derivatives of the type ArCH(R)CN. Sometimes, however, dimeric products of the type ArCH(R)CH(R)Ar are formed (Moon et al. 1983). As observed, 1-naphthyl, 2-pyridyl, and 2-quinolyl halides give the nitrile-substituted products, while phenyl halides, as a rule, form dimers. The reason consists of the manner of a surplus electron localization in the anion radical that arises upon replacing halogen with the nitrile-containing carban-ion. If the resultant anion radical contains an unpaired electron within LUMO covering mainly the aromatic ring, such an anion radical is stable, with no inclination to split up. It is oxidized by the initial substrate and gives the final product in the neutral form, Scheme 1-7 ... 

I. A. Rybakova, E. N. Prilezhaeva, V P. Litvinov, Methods of Replacing Halogen in Aromatic Compounds by RS-Funclions." Russ. Chem. Rev. (Engl. Transl.) 1991, 60, 1331. 

Toxicity and environmental concerns led to submission of a proposal to European Union to ban the use of PBDPE in 1989 (111-4301-89-EN Draft). The proposal was rejected on the basis of recommendations issued by a thorough debate between scientists, regulators, producers, and users of fire retardants, stating that banning would involve an unacceptable fire risk since alternatives were not available to replace halogen-based fire retardants with comparable effectiveness. Ever since, fire-retardant research has been mostly devoted to the development of nonhalogenated replacements for the halogen fire retardants. 

The ease of replacing halogens by fluorine increases from Mathieson Chemical Corporation, Niagara Falls, N.Y. 

Nucleophiles containing elements of the second row have also been used in photosubstitution reactions of aryl halides. The selenocyanate ion739, selenourea740 and thiourea741 are capable of replacing halogen atoms in various derivatives of benzene, naphthalene, pyridine and pyrimidine. Both chlorine atoms in 6>rj/zc>-dichlorobenzene are substituted upon irradiation in (MeO)3P at 60 °C for 5 days and a 50% yield of 1,2-bis(phosphino)-benzene is obtained742. 

Sulphite ion has been successfully used to replace halogen in 4-chloro-l-naphthol743,744,... 

Not only salts of nitrous acids but also nitrous acid itself can replace halogen by the nitro group. This has been discovered by Wuster and Scheibe [194] when they reacted sodium nitrite with bromodimethylaniniline in hydrochloric acid ... 

The tetrakis(trifluoromethylthio)pyrrolyl radical replaces halogen in PX3 (X = Cl, Br, I). Monosubstitution is observed when stoichiometric amounts of the pyrrolle or its salts are reacted with PX3. 

The Jackson-Meisenheimer reaction is of considerable theoretical and practical significance for analytical and preparatory work in the laboratory and in industry. The latter refers to replacing halogen or alkoxy groups in nitroaro-matics by acting with bases and it received full attention in Vol. I (pp. 197,453, 461,467,484, 545, 547, 549, 559, 563, 571) and Vol. Ill (pp. 47,61). 

Most primary and secondary amines replace halogen more rapidly than ammonia6. Condensation of amines with 2,4-dinitrophenyl halides was used to prepare derivatives for... 

Manufacture of many important amino intermediates used for dyes and other purposes is usually by conversion or replacement of a substituent. For example, as already mentioned, in substituted nitro compounds, the nitro groups may be reduced with iron/hydrochloric acid, hydrogen and catalyst, or zinc in aqueous alkali. Partial reductions can be brought about with sodium sulfide. Amino groups may be introduced by replacing halogens in the aromatic ring. Another approach to amination is by attack on a substituted aromatic compound with ammonia or amines. Thus, for example, direct amination of p-chloronitrobenzene (15a) in the presence of a copper catalyst affords p-nitroaniline (15b) in almost quantitative yield l,4-dichloro-2-nitrobenzene (16) is converted in a similar way to 4-chloro-2-nitroaniline (17). Reactions of ammonia with carboxylic acids or anhydrides are carried out on an industrial scale. 

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