Bromides, preparation from fluorides

An ethereal soln. of p-toluenesulfonyl fluoride added dropwise to ethylmagne-sium bromide prepared from ethyl bromide and Mg in ether, refluxed 13-15 hrs. 

Phenacyl esters can be prepared from the phenacyl bromide, a carboxylic acid, and potassium fluoride as base. These phenacyl esters can be cleaved by irradiation (313 nm, dioxane or EtOH, 20°, 6 h, 80-95 % yield, R = amino acids > 300 nm, 30°, 8 h, R = a gibberellic acid, 36-62% yield ). Another phenacyl derivative, RC02CH(C0C6H5)C6H3 3,5-(0CH3)2, cleaved by irradiation, has also been reported. ... 

Mesitaldehyde may be prepared from mesitylmagnesium bromide by the reaction with orthoformate esters3 or ethoxy-methyleneaniline 3 from acetylmesitylene by oxidation with potassium permanganate,4 from mesitoyl chloride by reduction,5 from mesityllithium by the reaction with iron pentacarbonyl and from mesitylene by treatment with formyl fluoride and boron trifluoride,7 by treatment with carbon monoxide, hydrogen chloride, and aluminum chloride,8 or by various applications of the Gatterman synthesis.9-11... 

In the Koenigs-Knorr method and in the Helferich or Zemplen modifications thereof, a glycosyl halide (bromide or chloride iodides can be produced in situ by the addition of tetraalkylammonium iodide) is allowed to react with a hydrox-ylic compound in the presence of a heavy-metal promoter such as silver oxide, carbonate, perchlorate, or mercuric bromide and/or oxide,19-21 or by silver triflu-oromethanesulfonate22 (AgOTf). Related to this is the use of glycosyl fluoride donors,23 which normally are prepared from thioglycosides.24... 

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." ... 

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. 

The order of reactivity of halides with magnesium is RI > RBr > RC1 (very few organomagnesium fluorides have been prepared), i) Aryl Grignard reagents are more easily prepared from aryl bromides and aryl iodides than from aryl chlorides, which react very sluggishly. 

The aryl bromide 40, prepared from cross-coupling between 1,2-dibromobenzene and (trimethylsilyl)acetylene, was converted to the corresponding arylzinc halide 41a and arylboronic acid 41b for subsequent coupling with the haloallenes 42 to produce the benzannulated enyne-allene 43 in -40% yield (Scheme 20.10) [38]. Desilylation with tetrabutylammonium fluoride (TBAF) then afforded 44 in 67% yield. 

Method F /V-Benzylcinchonium hydroxide, prepared from the bromide salt (0.46 g, I mmol) using 12.2.2, in MeOH (20 ml) is neutralized to pH 7 with aqueous HF (1M). The solution is evaporated and the residue is taken up in PhH MeCN (1 1, 10 ml). The solvent is removed and the procedure is repeated to yield the fluoride salt, which is dried overP2Os at 40 °C. 

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... 

A mixture of Re(CO)5Br dissolved in 8 mL of fluorobenzene and finely divided and freshly prepared silver fluoride (30 mg, 0.24 mmol, two-fold excess) is stirred in a glass vessel at room temperature for 2 h. The solution is filtered to remove the precipitated silver bromide and excess silver fluoride. The filtrate is heated to reflux at 90 to 100°C for 5 h, cooled to room temperature, and the solvent is removed under vacuum to give the yellow product. Yield 44 mg, 0.036 mmol (75% yield). This can be recrystallized from a diethyl ether solution into which fluorobenzene is allowed to diffuse and dried in vacuo to give needle or prismatic crystals. 

The procedure used is essentially that described by Meerwein and co-workers. 2, s The salt also has been prepared from ethyl fluoride and boron fluoride etherate, and from silver fluoborate, ethyl bromide, and ether.4... 

There was particular interest in the analogous reactions of 1-Oacetyl-2,3,5-tri-0-benzoyl-4-bromo-/ -D-ribose (37), because of the potential to prepare from it compounds related to nucleocidin, which is a 4 -fluoroaden-osine derivative having antitrypanosomal activity. With silver fluoride in acetonitrile, this bromide also reacted mainly with inversion of configuration, and gave the l-lyxo fluoride 140 (53% isolated yield), whereas, with silver tetrafluoroborate, much more of the more interesting 1-0-acetyl-2,3,4-tri-0-benzoyl-4-fluoro-/ -D-ribose (141) was obtained.38... 

Preparation from Other Precursors. Alkyl cations can be formed not only from halide precursors (the earlier investigation of generation from alkyl fluorides was later extended to alkyl chlorides, bromides, and even iodides) but also from alkenes in superacids like HF-SbF5 [Eq. (3.10)]. 

The nitroaldol reaction of silyl nitronates with aldehydes promoted by ammonium fluorides, which was originally introduced by Seebach and Colvin in 1978 [24], is a useful method for the preparation of 1,2-functionalized nitroalkanols. Recently, the present authors have succeeded in developing an asymmetric version of high efficiency and stereoselectivity by using a designer chiral quaternary ammonium bifluoride of type 6 as catalyst, which was readily prepared from the corresponding bromide by the modified method C in Scheme 9.5 [25]. 

Halogeno saccharides can be prepared by the displacement of a sulfonate ester with a halogen ion. Primary fluorides, bromides, chlorides and iodides can be prepared from mesylates (R0S02CH3) and tosylates (R0S02PhMe) (Scheme 3.3a).12 The latter derivatives can be obtained cheaply by treatment of alcohols with a sulfonyl chloride in pyridine or triethylamine. 

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