Halides exchange reactions

All the halide exchange reactions mentioned above proceed more or less quantitatively, causing greater or lesser quantities of halide impurities in the final product. The choice of the best procedure to obtain complete exchange depends mainly on the nature of the ionic liquid that is being produced. Unfortunately, there is no general method to obtain a halide-free ionic liquid that can be used for all types of ionic liquid. This is explained in a little more detail for two defined examples the synthesis of [BMIM][(CF3S02)2N] and the synthesis of [EMIM][BF4]. 

The direct bromination of 2-hydroxypyridine is regioselective at C-3 whereas chlorination is not. Qu6guiner has developed a halide exchange reaction. Heating 3-bromo-2-hydroxypyridine in pyridine hydrochloride at reflux gives 3-chloro-2-hydroxypyridine in good yield <96TL(37)6695>. The directed aminomethylation of 3-hydroxy-2(lH)-pyridones and 3-hydroxy-4(lfl)-pyridones has been studied <96T(52)1835>. 

Heteronuclear /x-oxo complexes have been prepared by oxo/halide exchange reactions, then the reaction of metal halide complexes with the gold oxonium species gives complexes of the Jy [Rh2(dien)2 0(AuPPh3)2 (BF4)2 (476) (dien = COD, NBD) or [Pt(COD)2 OAu(PR3) 2] 2725 J "... 

Solutions of aluminium bromide in dichloromethane used as a catalyst in hydride-transfer equilibrium experiments should be kept cold, as a potentially dangerous exothermic halide exchange reaction occurs on warming. 

Quantitative assessment of the electrophilic character of various types of phosphenium ions has been attempted using computational studies on hydride and halide exchange reactions, and the results attribute to 1,3,2-diazaphospholenium ions a lower electrophilicity (and thus higher stability) than other types of phosphenium ions [20, 66], The gain in stability due to aromatic -delocalization is predicted to be somewhat larger than inductive stabilization resulting from exhaustive A-alkylation of the parent diaminophosphenium ion, [P(NH2)2]+. 

Scheme 6.145 Halide-exchange reactions in aryl halides.

The second aim of this work is to demonstrate the catalysis of the interlayer anions in organic reactions such as halide-exchange reactions between alkyl halides in toluene (eq. 1). 

For the halide-exchange reactions, into a SO-cm flask equipped with a condenser, 30 cm of toluene or DMF as a solvent and 1.0 g portion of the hydrotalcite-like material (3.3 mmol of interlayer Cl anions) were introduced. A 33 mmol of benzyl chloride was added to the mixture and the temperature was kept at 343 or 373 K with stirring, followed by the addition of a given amount of an alkyl bromide (or iodide) (33-195 mmol). Reactions were conducted under a nitrogen atmosphere. The liquid phase was withdrawn periodically and analyzed by a gas chromatograph equipped with a 2-m long SE-30 column and a... 

Because of the size of the alkyl halides, the halide exchange probably occurs at the external edge surfaces of the hydrotalcite-like material, not in the interlayer space. This is supported by the fact that no expansion of the interlayer space was observed after the material was used in the halide-exchange reaction. The diffusion of the halide ions in the interlayer space is not the rate-determining step, since the rate of the reaction greatly depends on the kind of alkyl bromide. 

The hydrotalcite-1ike material catalyzes organic reactions in which the interlayer Cl" anions play the role of catalyst. The material catalyzed the halide-exchange reactions between benzyl chloride with butyl bromide or butyl iodide in toluene. The hydrota1cite-1ike material also catalyzes a disproportionation of trimethoxysilane to give silane and tetramethoxysilane. 

BC13, BBr3, and BI3 undeigo exchange reactions to yield mixed boron halides. Exchange reactions also occur with trialkyl, triaryl, trialkoxy, or triaryloxy boranes and with diborane. Anhydrous metal bromides and iodides can be prepared by the exchange reaction of the metal chloride or oxide and BBr3 or BI3 (21). 

Metallation of alkynylcyclopropanes at the acetylenic end is accomplished either by deprotonation or via metal-halide exchange reaction with strong bases. Metallation of ethynylcyclopropane may be affected by KOH in DMF, ethereal EtMgBr or preferably BuLi in THF (equation 151)231. All three metal acetyl ides react with methyl ketones to give the corresponding alcohols. However, the instability of cyclopropyl ketones towards bases, especially at the reaction conditions required by KOH (20 °C, 6h), and the sensitivity of cyclopropenyl double bonds in cyclopropenyl ketone derivatives towards addition reactions of alkylmagnesium compounds, make the alkyllithium (-78 °C, instant reaction) superior to the other reagents. 

The best results have been obtained in the non-polar solvents heptane or 0-xylene. Polar aprotic solvents such as Me2CO, MeCN, DMSO or dioxane gave poor yields and byproduct formations. Water-free conditions to avoid hydrolysis of BC14- have been necessary. The best fluorination yields have been achieved in the 373-393 K temperature interval (in 0-xylene with TBAF, tetrabutylammonium fluoride). An increase in the yield was observed if an excess of the diazonium salt was used. The total labelling procedure took 50 min. The tetrachloroborate incorporates fluoride by a halide exchange reaction (equation 3)9 10. 

Alumina has been observed to function as a catalyst in halide exchange reactions. Thus chloride and iodide are exchanged with 1-bromooctane (equation l)79 ... 

Halide exchange reactions ( transhalogenation or Finkelstein reaction) have been, until recently, of particular synthetic importance only in the synthesis of alkyl (and strongly activated aryl) fluorides or iodides, where thermodynamic stability (for the fluorides) and solubility differences (for the iodides) shift the reversible exchange processes in the desired direction. With the advent of phase transfer catalysis (PTC) and transition metal catalysis (mainly homogeneous) this important family of reactions has been extended to practically all aromatic and aliphatic halides. 

Halide exchange reaction, 66, 87 2,5-Heptadien-4-ol, 3,4,5-trimethyl-, 65, 42 Heptanal (111-71-7), 65, 26 3-HEPTANONE, 4-METHYL-, (S)-, 65, 183 5-Heptynoic acid, 7-hydroxy-, methyl ester, 67, 193... 

Sommer et al. (136) found that optically active silyl halides readily undergo racemization induced by halide ions. The reaction is related to halide ion exchange discovered by Allen and Modena (137). The first tandem kinetic studies of the racemization and halide exchange reaction led to conclusion that the reaction proceeds via a silylenium ion pair [Eq. (28)],... 

The alkali halide—alkali halide exchange reaction CsCl + KI - Csl + KC1... 

An example of reaction type (d) in Table 5-4 is the Finkelstein halide exchange reaction between iodomethane and radioactive labeled iodide ion. The rate constant for this reaction decreases by about 10" on going from less polar acetone to water as solvent [66]. 

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