Furan aromaticity

Such nucleophilic displacements are likely to be addition-elimination reactions, whether or not radical anions are also interposed as intermediates. The addition of methoxide ion to 2-nitrofuran in methanol or dimethyl sulfoxide affords a deep red salt of the anion 69 PMR shows the 5-proton has the greatest upfield shift, the 3- and 4-protons remaining vinylic in type.18 7 The similar additions in the thiophene series are less complete, presumably because oxygen is relatively electronegative and the furan aromaticity relatively low. Additional electronegative substituents increase the rate of addition and a second nitro group makes it necessary to use stopped flow techniques of rate measurement.141 In contrast, one acyl group (benzoyl or carboxy) does not stabilize an addition product and seldom promotes nucleophilic substitution by weaker nucleophiles such as ammonia. Whereas... 

The most extensive research on furanic polyamides is recent (33) and deals essentially with furanic-aromatic structures, although an important effort was also devoted to all-fiiranic compositions. The reaction of the diacid 11a with various aromatic diamines leads to high-molecular weight polymers with good thermal stability and ciystallinity. Structure 23, obtained with p-phenylenediamine, exhibited features resembling closely those of polyaramides ... 

Polymer 24 was synthesized from the acid chloride 11b and the diamine 15. It had lower DPs than the furanic-aromatic counterparts and was less resistant to thermal degradation. This must stem fi om the relative instability of the diamine and the lability of the -Fu-CHt-NH- group. Polymer 25, obtained from the selfcondensation of aminoester b seems more promising, but more work is needed to improve its preparation and assess its properties. 

NaOH solution of the diol and an organic solution of the dihalide in the presence of a phase-transfer catalyst (31). The best results were obtained with dichloro-p-xylene as comonomer and gave polymers with the furanic-aromatic structure 26 ... 

It was shown <1998CCC681> that [l]benzothieno[3,2-3]furan 60, due to its low furan aromaticity, possesses dienophilic behavior and reacts with electron-rich dienes with the formation of a new heterocyclic system [l]ben-zothieno[3,2-3][l]benzofuran 93. Furthermore, it was reported <1999CCC389> that introduction of a vinyl moiety at C-2 of 57 created a reactive diene 94, which reacted with various dienophiles. Thus, both cycloaddition reactions led to new derivatives of heterocycle 93 substituted in the benzofuran part of the heterocyclic system (Scheme 10). 

Apart from direct photolysis that is highly substrate-dependent, indirect photoreactions sensitized by quinones, furans, aromatic carbonyls (e.g., benzaldehyde derivatives), all important constituents of atmospheric aerosols, can lead to the transformation of many organic compounds [33-35], In such processes the photosensitizer P absorbs radiation and is then able to cause transformation of a substrate S because of energy transfer, electron or atom abstraction. 

This intermediate dihydrofuran volves through an electronic movement beginning in the hydroxyl electron-pair, and ending by expulsion of bromide, leading to a furan aromatic ring. 

Anhydrides (R C0)20 + H2O 2R COOH Conductimetric, polarimetric, thermometric, or UV Alkenes, furans, aromatic, ketones, formic acid, aldehydes, amides, and oximes range 0.2-10% water depending upon the sample... 

Solvents. Most reactions proceed most readily under homogeneous conditions, and where fairly low temperatures are necessary, as in organometallic chemistry, this means that solution techniques are commonly used. Generally organic solvents such as ethers (diethyl ether, tetrahydro-furan), aromatic hydrocarbons and saturated hydrocarbons are most suitable. 

The golden, highly crystalline material is air stable. It is paramagnetic having MEFF 5.10 BAl. as measured by the nmr technique. It is soluble in tetrahydro-furan, aromatic solvents, and chlorinated solvents. 

Furanes and benzofuranes in which an oxygenated ring is condensed into one or more aromatic rings. 

The Pd—C cr-bond can be prepared from simple, unoxidized alkenes and aromatic compounds by the reaction of Pd(II) compounds. The following are typical examples. The first step of the reaction of a simple alkene with Pd(ll) and a nucleophile X or Y to form 19 is called palladation. Depending on the nucleophile, it is called oxypalladation, aminopalladation, carbopalladation, etc. The subsequent elimination of b-hydrogen produces the nucleophilic substitution product 20. The displacement of Pd with another nucleophile (X) affords the nucleophilic addition product 21 (see Chapter 3, Section 2). As an example, the oxypalladation of 4-pentenol with PdXi to afford furan 22 or 23 is shown. 

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

The cross-coupling of aromatic and heteroaromatic rings has been carried out extensively[555]. Tin compounds of heterocycles such as oxazo-lines[556,557], thiophene[558,559], furans[558], pyridines[558], and seleno-phenes [560] can be coupled with aryl halides. The syntheses of the phenylo.xazoline 691[552], dithiophenopyridine 692[56l] and 3-(2-pyridyl)qui-noline 693[562] are typical examples. 

Cyclic compounds that contain at least one atom other than carbon within their ring are called heterocyclic compounds, and those that possess aromatic stability are called het erocyclic aromatic compounds Some representative heterocyclic aromatic compounds are pyridine pyrrole furan and thiophene The structures and the lUPAC numbering system used m naming their derivatives are shown In their stability and chemical behav lor all these compounds resemble benzene more than they resemble alkenes... 

The oxygen m furan has two unshared electron pairs (Figure 11 16c) One pair is like the pair m pyrrole occupying a p orbital and contributing two electrons to complete the SIX TT electron requirement for aromatic stabilization The other electron pair m furan IS an extra pair not needed to satisfy the 4n + 2 rule for aromaticity and occupies an sp hybridized orbital like the unshared pair m pyridine The bonding m thiophene is similar to that of furan... 

Section 12 18 Heterocyclic aromatic compounds may be more reactive or less reactive than benzene Pyridine is much less reactive than benzene but pyrrole furan and thiophene are more reactive... 

The furan nucleus is a cycHc, dienic ether with some aromaticity (2). It is the least aromatic of the common 5-membered heterocycles. A comparison of the aromaticity (3) of several of these compounds is shown below. 

The balance between aromatic and aUphatic reactivity is affected by the type of substituents on the ring. Furan functions as a diene in the Diels-Alder reaction. With maleic anhydride, furan readily forms 7-oxabicyclo [2.2.1]hept-5-ene-2,3-dicarboxyhc anhydride in excellent yield [5426-09-5] (4). 

Lubricating Oil Extraction. Aromatics are removed from lubricating oils to improve viscosity and chemical stabihty (see Lubrication and lubricants). The solvents used are furfural, phenol, and Hquid sulfur dioxide. The latter two solvents are undesirable owing to concerns over toxicity and the environment and most newer plants are adopting furfural processes (see Furan derivatives). A useful comparison of the various processes is available (219). 

Aqueous mineral acids react with BF to yield the hydrates of BF or the hydroxyfluoroboric acids, fluoroboric acid, or boric acid. Solution in aqueous alkali gives the soluble salts of the hydroxyfluoroboric acids, fluoroboric acids, or boric acid. Boron trifluoride, slightly soluble in many organic solvents including saturated hydrocarbons (qv), halogenated hydrocarbons, and aromatic compounds, easily polymerizes unsaturated compounds such as butylenes (qv), styrene (qv), or vinyl esters, as well as easily cleaved cycHc molecules such as tetrahydrofuran (see Furan derivatives). Other molecules containing electron-donating atoms such as O, S, N, P, etc, eg, alcohols, acids, amines, phosphines, and ethers, may dissolve BF to produce soluble adducts. 

Pyrrole has a planar, pentagonal (C2 ) stmcture and is aromatic in that it has a sextet of electrons. It is isoelectronic with the cyclopentadienyl anion. The TT-electrons are delocalized throughout the ring system, thus pyrrole is best characterized as a resonance hybrid, with contributing stmctures (1 5). These stmctures explain its lack of basicity (which is less than that of pyridine), its unexpectedly high acidity, and its pronounced aromatic character. The resonance energy which has been estimated at about 100 kj/mol (23.9 kcal/mol) is intermediate between that of furan and thiophene, or about two-thirds that of benzene (5). 

With its sextet of 7T electrons, thiophene possesses the typical aromatic character of benzene and other similarly related heterocycles. Decreasing orders of aromaticity have been suggested to reflect the strength of this aromatic character benzene > thiophene > pyrrole > furan (9) and benzene > thiophene > selenophene > teUurophene > fuian (10). 

Manufacture of thiophene on the commercial scale involves reactions of the two component method type wherein a 4-carbon chain molecule reacts with a source of sulfur over a catalyst which also effects cyclization and aromatization. A range of suitable feedstocks has included butane, / -butanol, -butyraldehyde, crotonaldehyde, and furan the source of sulfur has included sulfur itself, hydrogen sulfide, and carbon disulfide (29—32). 

The reactive species that iaitiate free-radical oxidatioa are preseat ia trace amouats. Exteasive studies (11) of the autoxidatioa mechanism have clearly estabUshed that the most reactive materials are thiols and disulfides, heterocycHc nitrogen compounds, diolefins, furans, and certain aromatic-olefin compounds. Because free-radical formation is accelerated by metal ions of copper, cobalt, and even iron (12), the presence of metals further compHcates the control of oxidation. It is difficult to avoid some metals, particularly iron, ia fuel systems. 

In summary, all estimates of resonance energies indicate a decrease in aromaticity in the sequence benzene > thiophene > pyrrole > furan. Similar sequences are also found for the benzo[6] and dibenzo analogues. A somewhat different sequence is found for the benzo[c] fused heterocycles with isoindole > benzo[c]thiophene > benzo[c]furan. As would be anticipated, the resonance energies for the benzo[c] heterocycles are substantially lower than those for their benzo[6] isomers. 

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