Halide exchange

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]. 

A detailed examination of these systems leads us to conclude that these halide exchanges are most likely absent or negligible under our conditions, le., low tempera-... 

Halide exchange, sometimes call the Finkelstein reaction, is an equilibrium process, but it is often possible to shift the equilibrium." The reaction is most often applied to the preparation of iodides and fluorides. Iodides can be prepared from chlorides or bromides by taking advantage of the fact that sodium iodide, but not the bromide or chloride, is soluble in acetone. When an alkyl chloride or bromide is treated with a solution of sodium iodide in acetone, the equilibrium is shifted by the precipitation of sodium chloride or bromide. Since the mechanism is Sn2, the reaction is much more successful for primary halides than for secondary or tertiary halides sodium iodide in acetone can be used as a test for primary bromides or chlorides. Tertiary chlorides can be converted to iodides by treatment with excess Nal in CS2, with ZnCl2 as catalyst. " Vinylic bromides give vinylic iodides with retention of configuration when treated with KI and a nickel bromide-zinc catalyst," or with KI and Cul in hot HMPA." ... 

These reactions are most important for the preparation of acyl fluorides. " Acyl chlorides and anhydrides can be converted to acyl fluorides by treatment with polyhydrogen fluoride-pyridine solution" or with liquid HF at — 10°C. Formyl fluoride, which is a stable compound, was prepared by the latter procedure from the mixed anhydride of formic and acetic acids. Acyl fluorides can also be obtained by reaction of acyl chlorides with KF in acetic acid or with DAST. Carboxylic esters and anhydrides can be converted to acyl halides other than fluorides by the inorganic acid halides mentioned in 10-77, as well as with PhsPXa (X = Cl or but this is seldom done. Halide exchange can be carried out in a... 

Reaction of aryl organometallic compounds with halogens Aryl halide exchange halo-denitration halo-dehydroxylation Reaction between diazonium salts and iodide ion Heating of diazonium fluoroborates (Schiemann)... 

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>. 

In both oxa- and aza-alkyne Prins cyclization an unexpected halide exchange with halogenated solvents presumably caused by the vinyl cation intermediates was observed [37]. From a synthetic point of view, it is important to use the correct combination of FeXs and X-containing solvent in order to avoid the undesired halide scrambling (Scheme 28). 

Scheme 28 Halide exchange with halogenated solvents during the acetylenic Prins-type cyclization...

Trzcinskabancroft, B., Knachel, H., Dudis, D., Delord, T.J. and Marler, D.O. (1985) Experimental And Theoretical-Studies Of Dinudear Gold(I) And Gold(II) Phosphorus Ylide Complexes - Oxidative Addition, Halide Exchange, And Structural-Properties Including The Crystal And Molecular-Structures Of [Au (CH2)2PPh2]2 And [Au(CH2)2PPh2]2(CH3) Bri. Journal of the American Chemical Society, 107(24), 6908-6915. 

Chloroethyl)polystyrenes and (iodoethyl)polystyrenes are each prepared from the alcohol by common reagents in a single step without complications, but one-pot procedures fail to produce completely pure bromide, which must be prepared from the tosylate by assisted halide exchange (57). The preparation of (toluenesulfonyloxyethyl)polystyrene itself, if performed in ice-cold pyridine as for the free analogue (64, 65), required a week to complete (67) if quaternary ammonium and other side-products (68) are to be avoided. In contrast, with non-nucleophilic diisopropylamine (69) as acid acceptor instead of pyridine, (hydroxyethyl)polystyrene and toluene-sulfonyl chloride need only be refluxed in carbon tetrachloride for a few hours to give the desired tosic ester as sole product in quantitative yield (57). 

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]+. 

It is of interest to note that in the case of oxaazaphosphospholidines 302 derived from (-)-ephedrine (42), some inversion at the benzylic carbon occurred giving a mixture of the P-epimeric derivatives 303 (Scheme 71) [108], Halide exchange with inversion of configuration in the intermediate was suggested as the most likely explanation for the loss of stereospecificity. 

Scheme 6.145 Halide-exchange reactions in aryl halides.

Axelrad, G., Laosooksathit, S., and Engel, R., Halide exchange at vinylic position catalyzed by copper halide complexes of trivalent phosphorus, Synthetic Commun., 11, 405, 1981. 

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