Five-Membered Unsaturated Heterocycles

Pyrrole, the simplest five-membered unsaturated heterocyclic amine, is obtained commercially by treatment of furan with ammonia over an alumina catalyst at 400 °C. Furan, the oxygen-containing analog of pyrrole, is obtained by acid-catalyzed dehydration of the five-carbon sugars found in oat hulls and... 

Pyrrole, furan and thiophene five-membered unsaturated heterocycles... 

Thiophene is a sulphur-containing five-membered unsaturated heterocycle. The lone pair electrons of the sulphur are in the 35 orbital, and are less able to interact with the tt electrons of the double bonds. Therefore, thiophene is considered weakly aromatic. Acetylenic thiophene is found in some higher plant species. However, the thiophene ring is present in many important pharmaceutical products. 

Most heterocycles have the same chemistry as their open-chain counterparts. Lactones and acyclic esters behave similarly, lactams and acyclic amides behave similarly, and cyclic and acyclic ethers behave similarly. In certain cases, however, particularly when the ring is unsaturated, heterocycles have unique and interesting properties. Let s look first at the five-membered unsaturated heterocycles. 

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

Five-Membered Unsaturated Heterocycles 1151 Structures of Pyrrole, Furan, and Thiophene 1152 Electrophilic Substitution Reactions of Pyrrole, Furan, and Thiophene 1153 Pyridine, a Six-Membered Heterocycle Electrophilic Substitution of Pyridine Nucleophilic Substitution of Pyridine Fused-Ring Heterocycles 1158 Nucleic Acids and Nucleotides 1160 Structure of Nucleic Acids 1163 Base Pairing in DNA The Watson-Crick Model Nucleic Acids and Heredity 1166 Replication of DNA 1167... 

Five-Membered Unsaturated Heterocyclic Compounds The three well-known unsaturated five-membered heterocyclic compounds are furan, I, thiophene, II, and pyrrole, III, which may be considered as analogues of cyclopentadiene, TV. 

These compounds exhibit a very intense band below 220 mp due to the diene absorption and a low intensity band at longer wavelengths. The long wavelength band is most intense in the case of thiophene, possibly because of the interaction of the low-lying rf-orbitals. Some of the other unsaturated five-membered heterocycles for which the absorption spectra have been reported are imidazole, V, 1,2,3-triazole, VI, tetrazole, VII, and thiazole, VUI. The absorption data of the five-membered unsaturated heterocyclic compounds are summarized in Table 6.1. 

Furan (1), a volatile colourless liquid with an aromatic scent, is one of the major representatives of the five-member unsaturated heterocycle family, which also includes pyrrole (2) and thiophene (3), as well as several other less widespread homologues. 

The dehalogenation reaction has also been used for preparing unusual polymers that contain the Si-Si o-bonds in conjugation with an organic n-system. Thus, reaction of dichlorosiloles with lithium metal results in the formation of l,r-polysiloles (see Eq. 1.7) [20, 21]. Every silicon in these polymers is also a part of a five-membered unsaturated heterocyclic ring. 

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