By Halogen Exchange

Diorgano tellurium dihalides, when refluxed with silver fluoride in toluene, tetrahydro-furan, or acetone, precipitate silver halides. The diorgano tellurium difluorides crystallize upon cooling the filtrate of the reaction mixture or evaporating the filtrate under vacuum. 

Diphenyl Tellurium Difluoridefi A mixture of 15 w/ of dry toluene and 2 g (16 mmol) of silver fluoride is heated to reflux, and 1.3 g (3 mmol) of diphenyl tellurium diiodide are added in portions to the refluxing silver fluoride suspension. Additional diiodide is added after the reddish color of the solution has disappeared. The hot solution is filtered under aspirator vacuum, The filtrate deposits the crystalline product on cooling. The difluoride is filtered off, washed with toluene, and recrystallized from benzene the pure product has m.p. 154°. 

The same product was obtained when diphenyl tellurium dibromide was used as reagent. 

Similarly prepared were the following tellurium difluorides  

Irgolic Organo Tellurium Compounds with 2 Te —C Bonds or 1 Te = C Bond 

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Various halogenating agents have been used to replace hydroxyl with chlorine or bromine. Phosphoms trihaUdes, especially in the presence of pyridine, are particularly suitable (17,18). Propargyl iodide is easily prepared from propargyl bromide by halogen exchange (19). 

Zirconium tetrabromide [13777-25-8] ZrBr, is prepared direcdy from the elements or by the reaction of bromine on a mixture of zirconium oxide and carbon. It may also be made by halogen exchange between the tetrachloride and aluminum bromide. The physical properties are given in Table 7. The chemical behavior is similar to that of the tetrachloride. 

Zirconium tetraiodide [13986-26-0], Zrl, is prepared directly from the elements, by the reaction of iodine on zirconium carbide, or by halogen exchange with aluminum triiodide. The reaction of iodine with zirconium oxide and carbon does not proceed. The physical properties are given in Table 7. 

Fluorinated and iodinated derivatives are usually prepared by halogen exchange reactions, although the Baltz-Schiemann reaction has been applied to the synthesis of 2-fluoroquin-oxaline (66JHC435>. 

This reaction, parallel with 10-77, is the standard method for the preparation of sulfonyl halides. Also used are PCI3 and SOCI2, and sulfonic acid salts can also serve as substrates. Sulfonyl bromides and iodides have been prepared from sulfonyl hydrazides (ArS02NHNH2, themselves prepared by 10-126) by treatment with bromine or iodine.Sulfonyl fluorides are generally prepared from the chlorides, by halogen exchange. 

Thiazyl fluoride (73) can be prepared from the corresponding chloride by halogen exchange the chloride itself is obtained by the thermal depoly-... 

Perdeuteriated isopropyl, t-butyl, and t-pentyl fluorides were prepared from the corresponding deuteriated alkyl chlorides (bromides) by halogen exchange. The perdeuteriated alkyl fluorides were then used in the formation of the deuteriated carbonium ions under similar conditions as those for the protium complexes. The resonance spectra were obtained at 9 2 Mc/s and the data are summarized in Table 7... 

More recently, 6-astatomethyl-19-norcholest-5(10)-en-3/S-ol (V) has been prepared rapidly, in high yield (60-70%), and at high specific activity by halogen exchange (At /I) in the presence of crown ethers. The crown ethers may fulfill a catalytic role by acting as specific cationic sinks, and thus facilitating rapid nucleophilic exchange. The product was identified by TLC with an Rp value very close to that of the iodinated derivative (95). 

Trifluoromethylthiocarbonyl fluoride can be converted to the chloride by halogen exchange with aluminum trichloride. By chlorine addition this compound furnishes another sulfenyl chloride (55) ... 

Ethyl a-(hydroxyraethyl)acrylate can be used for the synthesis of chloro and bromomethyl acrylates. The fluoro and iodo compounds have been prepared easily by halogen exchange from ethyl a-(bromomethyl)acrylate, ... 

Starr Finger Chem. Ind. (London) 1962, 1328 Shiley Dickerson Finger J. Fluorine Chem. 1972,2. 19 Kimura Suzuki Tetrahedron Lett. 1989, 30. 1271. For the use of phase transfer catalysis in this reaction, sec Yoshida Kimura Chem. Lett. 1988, 1355. For a review of the preparation of aryl fluorides by halogen exchange, sec Dolby-Glover Chem. Ind. (London) 1986, 518-523. 

The methods that are known for introducing trifluoromethoxy groups into arenes are generally not effective. An interesting onc-pot O-alkylation of phenols followed by halogen exchange to form trifluoromethyl ethers 6 has been described227 (see also Houben-Weyl, Vol. E4, pp 626-627). 

In cases where only difluorophosphanes are formed from dichlorophosphanes by halogen exchange with antimony(III) fluoride, the former can be converted into tetrafluoro-25-phosphanes by treatment with chlorine gas in the presence of antimony(III) fluoride.46,52... 

The orange-yellow NbBrs and yellow TaBr5 are prepared in the same way as the pentachlorides.1,3,19 NbBr5 has also been obtained by halogen exchange between NbCI5 and BBr3 at room temperature. 

This isomerization reaction allows the synthesis of fluorohaloalkanes containing tri-fiuoromethyl groups, which are often not directly accessible by halogen-exchange reactions. These reactions are commercially important in the production of replacements for chloro-fiuorocarbons (CFCs) (see Vol. ElOa, p 65). Much of the work in this area has been published only in the patent literature. 

Lithiation of the exocyclic vinyl position of methylenecyclopropane is achieved by halogen exchange reaction of /-BuLi with (bromomethylene)cyclopropane in ether. The vinyl... 

C-Terminal ketone derivatives of peptides have been used as effective inhibitors for a variety of proteases including serine, cysteine, and aspartyl proteases. 271 This class of peptides includes diazomethyl ketones (Section 15.1.2), halomethyl ketones (Section 15.1.3), and fluoromethyl ketones (Section 15.1.4). In general, the A -amino group and side chain must be protected. The diazomethyl ketones serve as good intermediates for conversion into chloromethyl and bromomethyl ketones. Fluoromethyl ketones, the most widely known class of peptide haloketones, can also be prepared from diazomethyl ketones or by halogen-exchange reactions. Other methods for the synthesis of fluoromethyl ketones are described in Section 15.1.4. 

Vinyl halides are so inert that none has been converted to a fluoride by halogen exchange. Vinyl fluorides have been synthesized from saturated polyhalides by dehalogenation with zinc and by dehydrohalo- genation with alcoholic alkali, and from acetylene by addition of one molecule of hydrogen fluoride.12 18... 

Fluorination of sulfuryl halides by halogen exchange is apparently a very difficult process to force to completion even at temperatures as high as 300° the usually effective Swarts reagent takes sulfuryl chloride only to the chloride... 

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