2
The Peculiar Chemical Features of the Furan Heterocycle and the Synthesis of Furfural and Hydroxymethylfurfural

Following the classic treatise where Dunlop and Peters covered systematically in 1953 the domain of furans in a comprehensive approach [1], several monographs discuss advances in the organic chemistry of furans [2], their naturally occurring compounds [3], and the pharmacological features of some derivatives [4]. This book does not deal with these issues, concentrating instead on furan polymers, but a brief outlook is provided here about the furan-specific chemical properties and reactivity, which are closely related to the polymerization behavior of furan monomers.

The furan heterocycle belongs to the family of five-member ring unsaturated compounds, and its structure can be displayed by the resonant forms shown in Figure 2.1.

Figure 2.1 The resonance-contributing structures associated with the furan heterocycle.

Substitution reactions of furans such as alkylation, halogenation, nitration, and sulfonation take place regioselectively at the C2 and/or C5 positions, which indicate that C and D are the dominant structures.

Compared with its sulfur and nitrogen homologues, furan possesses a reduced aromatic character and thus the most dienic, as illustrated in Figure 2.2, where benzene is the reference aromatic compound and cyclopentadiene the reference diene. The Dewar resonance energies decrease from 95 kJ mol-1 for benzene to 27 for thiophene, 22 for pyrrole, 18 for furan, and close to zero for cyclopentadiene.

Figure 2.2 The relative dienic and aromatic character of thiophene, pyrrole, and furan in relation to the fully aromatic benzene and the fully dienic cyclopentadiene.

This feature has an important bearing on much of the chemical behavior of furan, as briefly outlined below.

2.1 Free Radical Reactions

The addition of a free radical to a C2-substituted furan heterocycle takes place predominantly at the C2 or C5 position, as shown in Figure 2.3, depending on the chemical nature and steric hindrance of the substituent. The ensuing furyl radical unpaired electron is shared by the remaining three carbon atoms of the ring according to its dienic character, which is expected to confer a certain stability to the structure. The reactions of these radicals derived from furan monomers will be discussed in detail when dealing with their behavior in polymerization systems, but their sluggish reactivity must already be mentioned here in the sense that they do not display any tendency to oligomerize with the excess precursor, which emphasize their considerable degree of stabilization. Their termination in a context where the furan derivative is only confronted with primary radicals are illustrated in Figure 2.4, with the formation of a 2,5-dihydrofuran, or the corresponding C5-C5 dimer, depending on the terminating radical. These results were obtained in a systematic study of the bulk reaction of AIBN with furan and a wide selection of furan 2-substituted derivatives [5].

Figure 2.3 The addition modes of a primary radical to furan and 2-monosubstituted furan compounds.

Figure 2.4 Two alternative termination reactions of 2-furyl radicals.

2.2 Electrophilic Reactions

The addition of an electrophile to furan and 2-monosubstituted furans occurs predominantly at the C5 position, as shown in Figure 2.5. This very pronounced regiospecificity, estimated to more than 100:1 with respect to reactions at C3 or C4, is attributed again to the dienic character of the heterocycle, and bears strong consequences to the behavior of the cationic polymerization of furan monomers discussed in the appropriate section. Suffice to note here that the carbocation intermediate in these reactions is stabilized by the dienic property of the furan ring.

Figure 2.5 Electrophilic substitution reaction to a 2-substituted furan derivative.

2.3 Photochemistry

The electronic spectra of furan and alkyl furans display maxima at 205-215 nm associated with the conjugated diene pp* transition. Their mercury-sensitized photolysis produces both molecular fragmentation into CO, cyclopropenes, alkynes and allenes after ring contraction, and isomerization from the excited intermediates, as shown in Figure 2.6 for the case of 2-mehyl furan [6].

Figure 2.6 The mercury-sensitized photolysis of 2-methyl furan.

When a carbonyl function is attached to the heterocycle, the ring diene is now conjugated to it giving rise, in the case of furfural, to an electronic spectrum with a strong pp* transition peak at 260 (gas)-280 (solution) nm and a much weaker np* transition at 330 nm in the gas phase, which merges into the red-end of the pp* transition in the spectrum of liquid furfural. Furoic acid, HMF and 2-methylfuryl ketone exhibit similar spectral features, which have been exploited for the quantitative analysis of these compounds in processes involving their formation or presence as impurities.

The photolysis of furfural in the gas phase has been studied both by direct irradiation of both bands and by mercury sensitization [7]. In all instances, the singlet and triplet excited states rapidly revert to a ground-state vibrationally excited molecule, which either loses a CO molecule to yield furan, or suffers ring contraction, as in the case of furan, to give a cyclopropene dialdehyde intermediate that decomposes into two molecules of CO, propyne, allene, and cyclopropene. At higher pressures, some oligomerization is also observed. On the whole, therefore, this photochemical behavior is similar to that of furan, illustrated in Figure 2.6.

On the other hand, when liquid furfural sealed within a quartz tube is exposed to the full spectrum of a medium-pressure mercury arc for 10-90 h [8], the major product is an oligomeric material generated by two alternative reactions involving furfural excited state and a ground-state molecule, as shown in Figure 2.7. The ensuing vibrationally excited dimer reacts with further furfural molecules to give oligomers with Mn = 500±50, whose structure is close to that shown in Figure 2.8.

Figure 2.7 The interactions of electronically excited furfural molecules with ground-state counterparts.

Figure 2.8 The structure of the repeat unit of furfural oligomers produced by liquid-phase photolysis.

Moving to furan molecules with more extended conjugated moieties, 2-furanacrylic acid has been reported to cyclodimerize in the solid state to give cyclobutane isomers [9]. A water slurry of crystals of the corresponding 2,5-diacid (FDA) was irradiated with a 100 W tungsten bulb in a water-cooled reactor giving a near-quantitative conversion into the cyclodimer arising from the [p2+p2] coupling of the excited alkene moiety of a diacid molecule with the ground-state of the second one [10]. The structure of this product, shown in Figure 2.9, was confirmed by a thorough spectroscopic characterization. The use of a simple light bulb (and indeed also sunlight) was sufficient to promote this solid-state reaction stems from the fact that FDA displays a strong peak at 350 nm tailing off towards the visible domain, and photolysis with mercury lamps induced its oligomerization.

Figure 2.9 The photochemical dimerization of 2,5-furandiacrylic acid.

It is important to note that the dimer reverted to FDA when heated at 220 °C, a typical behavior related to the breaking down of the strained cyclobutane ring, which will be evoked in other similar systems.

In the same vein, several furan compounds bearing conjugated trans C=C moieties conjugated to carbonyl groups, appended at the C2 position of the heterocycle, were synthesized and submitted to UV irradiation in bulk and solution using a 500W medium-pressure mercury arc provided with a Pyrex filter to limit the excitation to wavelengths higher than 270 nm [11a]. Reactions were followed by FTIR spectroscopy, monitoring the decrease in the C=C band around 1640 cm-1, as well as by UV spectroscopy, monitoring the decrease of the peak around 300 nm. In all instances, cyclodimerization took place via the [p2+p2] coupling of the excited alkene moiety of a molecule with the ground-state of the other, as shown in Figure 2.10, which also illustrates the formation of the two isomeric dimers, identified spectroscopically.

Figure 2.10 Photoinduced cyclodimerization of C2-furan compounds bearing alkene-carbonyl groups.

In a systematic study on the photodimerization of furylene-vinylene and thienylene-vinylene compounds, the dimer arising from the self-condensation of 5-methyl furfural (5MF), which gave a UV maximum at 393 nm in methylene chloride, was irradiated in the solid state with both high- and medium-pressure mercury arcs using a filter to cut out any wavelength lower than 300 nm [11b]. The compound readily yielded the symmetrical dimer via the [p2+p2] coupling of the excited alkene moiety of a molecule with the ground-state of the other, as shown in Figure 2.11, which was fully characterized. This reaction was...