Furan bromination

It is to be expected that furo[2,3-ft]pyrazines would show an appreciably lower reactivity in electrophilic substitution than benzo[fe]furans. Bromination of 2-phenylfuro[2,3-6]pyrazine requires high temperatures and long reaction times the substitution takes place in position 3. By contrast, nitration occurs mainly in the phenyl group. Reaction with peracids easily provides the N-7 monoxide. This is to be expected since quantum mechanical calculations (HMO, PPP, CNDO) all predict a higher charge density at N-7 compared with N-5. 

Also obtained by saponification of 4-acetoxy-2-benzoyloxy-3,5-dibromobenzo-phenone (SM) with potassium hydroxide in boiling ethanol for 1 h (73%). SM was prepared in three steps first, bromination of 6-hydroxy-2,3-diphenylbenzo-furan (bromine/carbon tetrachloride), followed by acetylation of the 5,7-dibromo-6-hydroxybenzofuran so obtained (acetic anhydride/sodium acetate) and oxidation of the resulting 6-acetoxy-5,7-dibromo-2,3-diphenylbenzofuran (chromium trioxide/acetic acid) [1197]. 

The reaction of 2-amino-4(2-furyl)thiazole in acetic acid with bromine yields product 198 brominated on the furan ring (Scheme 126). The... 

Furan can be catalyticaHy oxidized in the vapor phase with oxygen-containing gases to maleic anhydride (93). Oxidation with bromine or in an electrochemical process using bromide ion gives 2,5-dimethoxy-2,5-dihydrofuran [332-77-4] (19) which is a cycHc acetal of maleic dialdehyde (94—96). 

Heterocychc compounds range from those, such as furan which is readily halogenated and tends to give polyhalogenated products, to pyridine which forms a complex with aluminum chloride that can only be brominated to 50% reaction (23). 

The reactivity sequence furan > tellurophene > selenophene > thiophene is thus the same for all three reactions and is in the reverse order of the aromaticities of the ring systems assessed by a number of different criteria. The relative rate for the trifluoroacetylation of pyrrole is 5.3 x lo . It is interesting to note that AT-methylpyrrole is approximately twice as reactive to trifluoroacetylation as pyrrole itself. The enhanced reactivity of pyrrole compared with the other monocyclic systems is also demonstrated by the relative rates of bromination of the 2-methoxycarbonyl derivatives, which gave the reactivity sequence pyrrole>furan > selenophene > thiophene, and by the rate data on the reaction of the iron tricarbonyl-complexed carbocation [C6H7Fe(CO)3] (35) with a further selection of heteroaromatic substrates (Scheme 5). The comparative rates of reaction from this substitution were 2-methylindole == AT-methylindole>indole > pyrrole > furan > thiophene (73CC540). 

Furan reacts vigorously with chlorine and bromine at room temperature to give poly-halogenated products. Low temperature (-40 °C) reaction of furan with chlorine in dichloromethane yields mainly 2-chlorofuran and reaction of furan with dioxane dibromide at 0 °C affords 2-bromofuran in good yield. 2-Iodofuran is obtained by treatment of 2-furoic acid with iodine and potassium iodide in aqueous sodium hydroxide. 

Halogens react with benzo[6]furan by an addition-elimination mechanism to give 2- and 3-substituted products (76JCS(P2)266). Treatment of benzo[6]thiophene with chlorine or bromine in acetic acid gives predominantly 3-substituted products (71JCS(B)79). 2,2,3,3,4,5,6,7-Octachloro-2,3-dihydrobenzothiophene is obtained when benzo[6]thio-phene is treated with chlorine in the presence of 1 mole of iodine (80JOC2l5l). 

Benzo[b]furan, 2,3-dihydro-2-phenyl-synthesis, 4, 697 Benzo[b]furan, 2,3-dihydroxy-tautomerism, 4, 37 Benzo[6]furan, 4,6-dimethoxy-acylation, 4, 606 Benzo[6]furan, 2,3-dimethyl-acetylation, 4, 606 bromination, 4, 605 photooxygenation, 4, 642 Benzo[b]furan, 5,6-dimethyl-2,3-diphenyl-applications, 4, 709 Benzo[b]furan, 1,3-diphenyl-vertical ionization potential, 4, 587 Benzo[b]furan, 2-ethoxy-5-hydroxy-synthesis, 4, 127... 

Benzo[b]furan, 2-nitro-reduction, 4, 74, 647 Benzo[b]furan, 5-nitro-mass spectrometry, 4, 583 Benzo[b]furan, 2-(4-nitrophenyl)-properties, 4, 708 Benzo[b]furan, 2-phenyl-bromination, 4, 605 chloromethylation, 4, 607 nitration, 4, 604 photochemical reactions, 4, 636 properties, 4, 697 Benzo[b]furan, 3-phenyl-synthesis, 4, 697... 

Benzo[b]furan-2-carboxylic acids bromination, 4, 602 chloromethylation, 4, 602 from coumarins, 3, 686 IR spectra, 4, 590 methyl ester... 

Furan, 3-acetoxy-2,4,5-triphenyl-synthesis, 4, 659 Furan, 2-acetyl-isopropylation, 4, 607 rotamers, 4, 544 synthesis, 4, 665 toxicity, 1, 136 Furan, 3-acetyl-bromination, 4, 604 Furan, 3-acetyI-2-amino-reactions, 4, 74 Furan, 2-acetyl-3,5-dimethyl-synthesis, 4, 691 Furan, 2-acetyl-5-ethyl-synthesis, 4, 691 Furan, 2-acetyl-3-hydroxy-synthesis, 4, 649... 

Furan-2-carbonyl chloride, 5-alkyl-3,4-dichloro-synthesis, 4, 690 Furancarboxamides rotational isomerism, 4, 543 Furan-2-carboxylic acid, 5-acetylamino-ethyl ester reactions, 4, 647 Furan-2-carboxylic acid, amino-properties, 4, 708 Furan-2-carboxylic acid, 5-bromo-nitration, 4, 603, 711 Furan-2-carboxylic acid, 3-methyl-methyl ester bromination, 4, 604 Furan-2-carboxylic acid, 5-methyl-nitration, 4, 602... 

Furan-3-carboxylic acid, 2-methyl-methyl ester bromination, 4, 604 synthesis, 4, 686 Furancarboxylic acids acidity, 4, 71... 

The 2,5-dialkoxy-2,5-dihydrofurans can be obtained by electrolytic oxidation of furan in alcoholic ammonium bromide or by bromine oxidation of furan in the appropriate alcohol. ... 

Using these assumptions and conventions, Imoto and co-workers have correlated a number of series of reactions of thiophenes and furans. The reactions studied are the acid-base equilibria pK values) and the acid catalyzed methylations (thiophenes only) of thiophene-and furan-carboxylic acids and the alkaline hydrolyses of their ethyl esters the side-chain bromination of the a-acetylthiophenes, and the a-mercuration of thiophenes and the polarographic half-wave potentials of the methyl esters of thiophene- and furan-carboxylic acids and of nitrothiophenes. The pK values were determined and the ester hydrolyses studied for all three substitution orientations in the thiophene series. For the 4-R-2-Y and 5-R-2-Y series, the p-values do not appear significantly different and the data could probably be combined into a single series unfortunately, however, no limits of accuracy are reported for the p-values, and some of the raw data are not readily available so recalculation is not easily possible. For the 5-R-3-Y series the p-values deviate considerably from the other values however, whereas they are higher for the pK values, they are lower for the ester hydrolyses, and it is possible that the differences are neither systematic nor significant. 

Very little is known concerning the simple, monocyclic 3-hydroxy-furans (cf. reference 15). Both the oxo and hydroxy forms of the substituted 3-hydroxyfurans 26 and 27 (R = H, CcHn) have been isolated/ but the individual tautomers slowly undergo interconversion. The enol forms give a positive reaction with ferric chloride, react rapidly with bromine, and form a peroxide with oxygen. From chemical evidence, the benzo derivatives of 3-hydroxyfuran, 28 and 29, appear to exist predominantly in the oxo form, and this is further supported by ultraviolet spectral data. Stefanye and Howard- ... 

N. A-Diethyl-2.3-dihydro-8//-thieno[2,3-r/]azcpin-7-amine (20), which is obtained in 49 % yield by photolysis of 6-azido-2,3-dihydrothiophene in a mixture of diethylamine and tetrahydro-furan. on treatment with bromine yields the fully unsaturated derivative 21.1,4... 

The presence of methyl groups in the 2- and 3-position directs bromination to the adjacent carbon. When both positions are substituted by methyl there is an initial addition process, but the ultimate products are bromi-nated in a side chain. Even 3-methylbenzo[h]furan reacted with NBS to give 2-bromo-3-bromomethylbenzo[h]furan (48HCA78 84BAU445). The 2-phenyl derivative of 1 gave a mixture of 3-bromo and 3,6-dibromo... 

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