Furan, and aromaticity

Baumann, P.C. and D.M. Whittle. 1988. The status of selected organics in the Laurentian Great Lakes an overview of DDT, PCBs, dioxins, furans, and aromatic hydrocarbons. Aquat. Toxicol. 11 241-257. 

This section deals with the preparation of such structures, including all-furanic compositions and mixed ones calling upon furanic and aromatic or aliphatic monomers. 

The ortho-metallated product is an orange powder which is mildly air sensitive and should be stored and used in an inert atmosphere. It is generally more soluble than its precursor with increasing solubility in diethyl ether, tetrahydro-furan, and aromatic solvents. 

Conjugated polymers (particularly, polythiophene, polypyrrole, and poly-furan) and aromaticity 05CRV3448. 

Another clever example involving the hydroboration of cyclic systems was reported by Schreiber in a study of Paterno-Biichi cycloadducts produced from substituted furans and aromatic aldehydes (Scheme 7.15, see also... 

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. 

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... 

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... 

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). 

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. 

Examples of the remaining potential 3,4-dihydroxy heterocycles are presently restricted to furan and thiophene. Although the parent 3,4-dihydroxyfuran apparently exists as the dioxo tautomer (86), derivatives bearing 2-alkyl or 2,5-dialkyl substituents prefer the keto-enol structure (87) (71T3839, 73HCA1882). The thiophene analogues also prefer the tautomeric structure (87), except in the case of the 2,5-diethoxycarbonyl derivative which has the fully aromatic structure (88) (71T3839). 

The five-membered ring heterocycles possess Diels-Alder reactivity of varying degree. This is most pronounced in the case of furan and benzo[c] fused heterocycles such as isoindole. In this capacity they are functioning as heterocyclic analogues of cyclopentadiene, and high Diels-Alder reactivity can be correlated with low aromaticity. 

In the chemical shift range for alkenes and aromatic and heteroaromatic compounds enol ether fragments (furan, pyrone, isoflavone, 195-200 Hz) ... 

Pyrrole, furan, and thiophene, on the other hand, have electron-rich aromatic rings and are extremely reactive toward electrophilic aromatic substitution— rnore like phenol and aniline than benzene. Like benzene they have six tt electrons, but these tt electrons are delocalized over five atoms, not six, and ar e not held as strongly as those of benzene. Even when the ring atom is as electronegative as oxygen, substitution takes place readily. 

With respect to aromatic substrates, the Vilsmeier formylation reaction works well with electron-rich derivatives like phenols, aromatic amines and aromatic heterocycles like furans, pyrroles and indoles. However various alkenes are also formylated under Vilsmeier conditions. For example the substituted hexatriene 6 is converted to the terminal hexatrienyl aldehyde 7 in 70% yield ... 

Some of the typical conditions of polycondensations used for aliphatic and aromatic monomers are not suitable for furan derivatives, e.g., the melt polycondensation of 2,5-furan dicarboxylic acid chloride with 2,5-b/s(hydroxymethyl) furan at about 80 °C only yields a black insoluble product5. The hydrochloric acid liberated in the reaction is clearly responsible for the charring of the furanic diol which like its simpler homologue furfuryl alcohol, resinifies rapidly in acidic media (see below). 

Furan derivatives with an aromatic system fused on one of the ring s double bonds, such as benzofuran, naphthofuran etc., can be polymerized cationically through the other ring s double bond. In these polymerizations the complications encountered with furan and alkylfurans [see Section III-A-l-c] are absent because only one unsaturation is available for propagation, the other being tied up in the benzene system... 

All the derivatives examined above owe their instability to the presence of the furan ring similar aldehydes, carbinols, chlorides, etc. bearing aliphatic and aromatic substituents are not prone to resinify. An analogous singularity of behaviour is encountered in furan polymers, as underlined at the beginning of this chapter. In a polymer prepared from a furan derivative three different situations must be considered ... 

The diazotization of heteroaromatic amines is basically analogous to that of aromatic amines. Among the five-membered systems the amino-azoles (pyrroles, diazoles, triazoles, tetrazoles, oxazoles, isooxazoles, thia-, selena-, and dithiazoles) have all been diazotized. In general, diazotization in dilute mineral acid is possible, but diazotization in concentrated sulfuric acid (nitrosylsulfuric acid, see Sec. 2.2) or in organic solvents using an ester of nitrous acid (ethyl or isopentyl nitrite) is often preferable. Amino derivatives of aromatic heterocycles without ring nitrogen (furan and thiophene) can also be diazotized. 

Diels-Alder reactions of furans are markedly reversible because of the aromatic character of the furan nucleus [la]. The lability of the cycloadducts, even at relatively low temperatures, as well as the sensitivity to acidic conditions of both furans and cycloadducts, preclude the use of strong Lewis acids and have therefore given importance to the high pressure technique. 

The cycloaddition between furan and maleic anhydride was the first uncatalyzed aqueous Diels-Alder reaction reported in the literature and was studied by Diels and Alder themselves [11]. This cycloaddition was successfully revised by Woodward and Baer [12] and some years later by De Koning and coworkers [13]. The aqueous medium was also used in the cycloaddition of aromatic diazonium salts with methylsubstituted 1,3-butadienes [14]. 

Aromatic bromine compounds can be formed and transformed during various thermal processes, like aromatic chlorine compounds (ref. 22). Brominated dibenzodioxins and -furans and mixed brominated/chlorinated compounds have been detected in trace levels in the fly ash of a municipal waste incinerator (ref. 23).Chlorine is generally abundant compared to the bromine of typical municipal waste the chlorine vs. bromine ratio is in the range of 250 1. 

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