One Heteroatom. Pyrrole, Furan and Thiophene

1439 cm . The third band lies between 1405—1336 cm (furans), 1255—1210 cm (thiophenes) and 1418—1384 cm (pyrroles). The in plane CH bends lie between 1300 and 1000 cm and are best characterised in the Raman spectra. 

The out of plane CH bends and ring modes are responsible for a number of strong bands between 1000 and 700 cm . These vary widely in position with substitution changes in the pyrroles and thiophenes, but much less so with the furans. Katritzky [42, 43] lists the positions of each of the bands in this region and gives the frequency ranges for various substitution patterns. 

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Five-membered Heterocycles containing One Heteroatom Pyrrole, Furan and Thiophene... 

The replacement of two CH groups in benzene by a neutral NR, O or S introduces into the new ring an electron-donating heteroatom. This electron-donor character is accentuated in the pyrrole anion where N is introduced. Thus the five-membered rings with one heteroatom are electron rich (rr-excessive), and the chemistry of pyrrole, furan and thiophene is dominated by this effect and is again considered together as a whole in Part 3. 

The first group consists of monocyclic heteroaromatic compounds with one heteroatom and without strongly electron-donating substitutents (OH, NH2). Pyrrole, furan, and thiophene are better electron donors than benzene. The order of their reactivities in azo coupling is thiophene > pyrrole > furan > benzene. 

We have looked at the five-membered aromatic heterocycles pyrrole, furan and thiophene in Section 11.5. Introduction of a second heteroatom creates azoles. This name immediately suggests that nitrogen is one of the heteroatoms. As soon as we consider valencies, we discover that in order to draw a five-membered aromatic heterocycle with two heteroatoms, it must contain nitrogen A neutral oxygen or sulfm atom can have only two bonds, and we cannot, therefore, have more than one of these atoms in any aromatic heterocycle. On the other hand, there is potential for having as many nitrogens as we like in an aromatic ring. 

The relation of the heteroatomic lone pair to the common double bond in matters relating to -electron mobility has long been accepted as a theoretically sound one.1 Because of a general lack of synthetic methodology, however, representation in the area of -excessive heterocycles was, until recently (early part of the past decade), limited to the long-known five-membered systems pyrrole, furan, and thiophene. In fact, it was not until the late 1960 s that well-developed synthetic procedures, such as (i) condensation with properly constructed Wittig reagents and (ii) the use of synthetic pericyclization, were successfully applied to the synthesis of the various substances now known. 

Pyrrole, furan, and thiophene are five-membered aromatic heterocycles with one heteroatom. In pyrrole, the nitrogen is sp2-hybridized and contributes two electrons to the 6n aromatic ring. Furan and thiophene are isoelectronic with pyrrole, the [—(N )=] unit being replaced by —( 0 )— and -( S )- units, respectively. Pyrrole, furan, and thiophene are electron-rich (there are six n electrons distributed over five atoms) and undergo electrophilic... 

Electrophilic substitution is an important type of reactions for five-membered heterocycles with one heteroatom and enables compounds with various substituents to be obtained. The present work is devoted to certain features of substrate and positional selectivities in electrophilic substitution reactions of derivatives of pyrrole, furan, thiophene and selenophene, and also the corresponding benzannulated systems, which had not been explained until recently. In a recent review (05RKZ(6)59), these problems were mainly discussed for thiophenes, while in a previous review (94H(37)2029) only monocyclic pyrrole, furan and thiophene derivatives were considered. 

Pyrrole, furan, and thiophene are the most common five-membered unsaturated heterocycles. Each has two double bonds and one heteroatom (N, O, orS). ... 

An annulene is a monocyclic hydrocarbon with alternating single and double bonds. A heterocyclic compound is a cyclic compound in which one or more of the ring atoms is a heteroatom—an atom other than carbon. Pyridine, pyrrole, furan, and thiophene are aromatic heterocyclic compounds. 

Pyrrole, furan, and thiophene are five-membered-ring heterocycles. Each has three pairs of delocalized rr electrons Two of the pairs are shown as rr bonds, and one pair is shown as a lone pair on the heteroatom. Furan and thiophene have a second lone pair that is not part of the rr cloud. These electrons are in an sp orbital perpendicular to the p orbitals. Pyrrole, furan, and thiophene are aromatic because they are cyclic and planar, every carbon in the ring has a p orbital, and the tt cloud contains three pairs of tt electrons (Sections 15.1 and 15.3). 

The TT-electron excess of the five-membered rings is accompanied by a high rr-donor character. The best measure of rr-donation is the value of first ionization potential, IP, which for all aromatic heterocycles with one heteroatom of pyrrole type reflects the energy of highest occupied rr-orbital. IP, values decrease in the sequence pyrrole > indole > carbazole furan > benzo[/ ]furan > dibenzofuran thiophene > benzo[/ ]thiophene (Section 2.3.3.9, Tables 21 and 23). Thus, the more extensive the rr-system, the stronger is its electron donor ability. Furan and thiophene possess almost equal rr-donation, which is considerably lower than that of pyrrole. 

The reactivity of five-membered rings with one heteroatom to electrophilic reagents has been quantitatively compared. Table 1 shows that the rates of substitution for (a) formylation by phosgene and V,iV-dimethylformamide, (b) acetylation by acetic anhydride and tin(IV) chloride, and (c) trifluoroacetylation with trifluoroacetic anhydride (71AHC(13)235) are all in the sequence furan > tellurophene > selenophene > thiophene. Pyrrole is still more reactive as shown by the rate for trifluoroacetylation, by the relative rates of bromination of the 2-methoxycarbonyl derivatives (pyrrole > furan > selenophene > thiophene), and by the rate data on the reaction of the iron tricarbonyl-complexed carbocation [C6H7Fe(CO)3]+ (Scheme 5) (2-methylindole ss V-methylindole > indole > pyrrole > furan > thiophene (73CC540)). 

The ionization potential was found to be 8.23 eV. The electron is derived from the HOMO, The dipole moment is 1.58 D. In contrast to furan and thiophene, the heteroatom represents the positive end of the dipole. This could be due to the fact that the heteroatom in pyrrole possesses only one nonbonding electron pair, whereas in furan and thiophene, there are two. As for furan and thiophene, the chemical shifts in the NMR spectrum are to be found in the region typical for aromatic compounds ... 

In these planar compounds, each heteroatom is sp hybridized, and its unhybridized 2p orbital is part of a closed loop of five 2p orbitals. In furan and thiophene, one unshared pair of electrons of the heteroatom lies in the unhybridized 2p orbital and is a part of the tt system (Figure 21.11). The other unshared pair of electrons lies in an sp hybrid orbital perpendicular to the 2p orbitals and is not part of the tt system. In pyrrole, the unshared pair of electrons on nitrogen is part of the tt system. In imidazole, the unshared pair on one nitrogen is part of the aromatic sextet the unshared pair on the other nitrogen is not. 

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