Bromination aryl ethers

When bromine is added to two beakers, one containing phenyl isopropyl ether and the other containing cyclohexene, the bromine color in both beakers disappears. What observation could you make while performing this test that would allow you to distinguish the alkene from the aryl ether ... 

Bromine adds to the alkene but substitutes on the aryl ether, evolving gaseous HBr. 17.10. Strong acid is used for nitration, and the amino group of aniline is protonated to a deactivating — NH3+ group. 17.12. (a) 2,4- and 2,6-dinitrotoluene (b) 3-chloro-4-nitrotoluene and 5-chloro-2-nitrotoluene (c) 3- and 5-nitro-2-bromobenzoic acid ... 

Problem 17.23 Because of their highly reactive benzene rings, aryl ethers may decolorize bromine in carbon tetrachloride. How could this behavior be distinguished from the usual unsaturation test Hint Sec Sec. 6.30.)... 

Allylphenols. A regioselective rearrangement of 2-alkenyl aryl ethers to the brominated o-position is realized via the lithio derivative (via reaction with r-BuLi) by treatment of the latter species with CuCN-LiBr. 

Bromination. Active arenes such as phenols, aryl ethers, and anilines are brominated by ZrBr4 in the presence of diisopropyl azodicarboxylate in CH2CI2. A free para position is the preferred site for bromination. 

Pyridine dibromide-HBr (pyridinium perbromide) can be used to mono-brominate polyhydric phenols such as pyrocatechol, pyrogallol, and also aryl ethers at or below room temperature.66-69,453 The solid reagent may be used (for preparation see page 112) alternatively glacial acetic acid containing a known amount of bromine is dropped into a mixture of the phenol or ether with pyridine hydrochloride (prepared by passing dry HC1 into an ethereal solution of pyridine) or with quinoline sulfate. 

Aryl ethers can also be mono- or di-brominated in dry benzene or CC14 by adding iWbromosuccinimide (1 or 2 moles) (cf. pages 148 and 153) portionwise with stirring and cooling in ice-water. 

Poly(aryl ether ketone)s with two quaternary ammonium groups on pendant phenyls can be synthesized by a bromination followed by a quatemization reaction with trimethylamine [72]. Such polymers play an important role in the preparation of direct methanol fuel cells. 

Bromine Water. Since the aromatic ring is electron rich, aromatic ethers can undergo electrophilic aromatic substitution with bromine to yield the corresponding aryl ether-halide (s). Therefore, if elemental tests indicate that an aromatic group is present in an ether, treatment with the bromine water reagent may substantiate the presence of an aryl ether. 

In contrast to the well-known alkylation paths, namely, S l and Sn2, the fluoroalkylation reaction was rationalized by an unusual ionic chain mechanism [26]. As shown in Scheme 14.3, the reaction was initiated with the direct attack of electron-positive bromine (5+) on the BrCF2CF2Br by phenoxide. Tetrafluoroethy-lene (CF2=CF2) was generated in situ after loss of bromide anions. The phenoxides added to the CF2=CF2 to give the reactive fluorocarbanions, which were quickly terminated by bromide to form 2-bromo-tetrafluoroethyl aryl ethers. In the elimination step, zinc inserted into the C—Br bond of 2-bromo-tetrafluorethyl aryl ethers in a similar way to the preparation of Grignard reagents. Finally, the aryl trifluorovinyl ethers were obtained by the elimination of ZnBrF salt at elevated temperature. 

SCHEME 22.2 Intramolecular direct arylation of brominated/chlorinated diaryl ethers and diarylamines. 

Recendy, Guiver et al. reported a number of derivatives of polysulfone and poly(aryl sulfone).172 188 Polysulfones were activated either on the ortho-sulfone sites or the ortho-ether sites by direct lithiation or bromination-lithiation. The lithiated intermediates were claimed to be quantitatively converted to azides by treatment with tosyl azides. Azides are thermally and photochemically labile groups capable of being transformed readily into a number of other useful derivatives. 

The reaction can be carried out efficiently using aryl diazonium tetrafluoroborates with crown ethers, polyethers, or phase transfer catalysts.103 In solvents that can act as halogen atom donors, the radicals react to give aryl halides. Bromotrichloromethane gives aryl bromides, whereas methyl iodide and diiodomethane give iodides.104 The diazonium ions can also be generated by in situ methods. Under these conditions bromoform and bromotrichloromethane have been used as bromine donors and carbon tetrachloride is the best chlorine donor.105 This method was used successfully for a challenging chlorodeamination in the vancomycin system. 

Halogenatlon. Poly(2,6-dimethyl- and 2,6-diphenyl-l,4-phenylene ether) can be aryl-brominated simply by exposure to a bromine solution no catalyst is required.6 In fact, the use of Lewis acid catalysts to promote the chlorination of poly(2,6-dimethy1-1,4-phenylene ether) leads to substantial degradation of the molecular weight of the chlorinated products.7 Membranes produced from ring brominated PPO (40% wt Br) exhibited enhanced permeability to CHi and CO2 and proved to be more selective in separating CH4/CO2 mixtures.8... 

Periodic acid dihydrate, with iodine and durene to give iododurene, 51, 94 Phenols, from aryl methyl ethers, 53, 93 Phenylacetaldehyde, from 2-lithio-1,3,5-trithiane and benzyl bromide, 51, 43 Phenylacetic acid, bromination, 50, 31... 

Bromination of the enol ether product with two equivalents of bromine followed by dehydrobromination afforded the Z-bromoenol ether (Eq. 79) which could be converted to the zinc reagent and cross-coupled with aryl halides [242]. Dehydrobromination in the presence of thiophenol followed by bromination/dehydrobromination affords an enol thioether [243]. Oxidation to the sulfone, followed by exposure to triethylamine in ether, resulted in dehydrobromination to the unstable alkynyl sulfone which could be trapped with dienes in situ. Alternatively, dehydrobromination of the sulfide in the presence of allylic alcohols results in the formation of allyl vinyl ethers which undergo Claisen rearrangements [244]. Further oxidation followed by sulfoxide elimination results in highly unsaturated trifluoromethyl ketonic products (Eq. 80). 

This method can be applied to introduce CF, and CF3 groups at oxygen and nitrogen atoms in various molecules. oe,a-Difluoro ethers are prepared from O-alkyl(aryl) carbothioales in moderate to good yields by reaction with tetrabutylammonium dihydrogen trifluoride and /V-bromosuccinimide or /V-iodosuccinimide,64 or with bromine trifluoride.65... 

Figure 4-5. Reaction mechanism for the derivatization followed by reductive cleavage based on Lu and Ralph (1997). Reaction of the lignin with acetylbromide (AcBr) results in the acetylation of the y-carbon, while the a-carbon is brominated. Zinc (Zn) catalyzes the cleavage of the ether bond between the P-carbon of one residue and the 0-4 position of the adjacent residue. The resulting monomer is acetylated with acetic anhydride (Ac20) and pyridine (Py). R can be a proton or an aryl group. In H-residues R3 and R5 are protons, in G-residues R3 is a methoxyl group and R5 is a proton, whereas in S-residues both R3 and R5 are methoxyl groups. The wavy bonds indicate that both the S- and R- (4.26, 4.27) or E- and Z-stereo-isomers (4.28, 4.29) are present.

After debenzylation (Pd-C, cyclohexene) of the hypercores (e.g., 25) by transfer hydrogenation, they were treated with aryl branched, benzyl ether dendrons (Scheme 5.6) that were prepared by similar iterative transformations,1271 i.e., benzylic bromination and phenolic O-alkylation (See Section 5.4.2). Thus, hexaphenol core 27 was reacted with six equivalents of the benzylic bromide building block 28 to give the benzyloxy terminated dendrimer 29. Key features of these dendrimers include cores with flexible alkyl spacers and a three-directional, quaternary carbon branching center. 

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