Aromatic Five-Membered Ring Heterocyclics

Section 21.9 Aromatic Five-Membered-Ring Heterocycles 897... 

In a metathesis-based approach to aromatic five-membered ring heterocycles [24], a series of tri- and disubstituted furans (74/79) has been synthesized [25] ... 

Imidazole is another aromatic five-membered ring heterocycle that has two nitrogens. These nitrogens, however, have different properties. The one with its lone pair in a p-orbital uses those electrons to complete the aromatic sextet and this nitrogen, as in pyrrole, is neutral. The other nitrogen has its lone pair contained within an sp orbital and those electrons are therefore in the plane of the ring, do not interact with the Jt-system, and can act as a base (Figure 1.39). 

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. 

An index of aromatic character based upon statistical evaluation of the deviations in peripheral bond orders has been devised and applied to five-membered ring heterocycles and their mesoionic derivatives including 1,2,3,4-thiatriazole <85TI409> and 5-phenyl-l,2,3,4-thiatriazol-3-oxide <93T8441>. See also <93QSAR146>. 

Oxazole and Isoxazole. These are five-membered ring heterocycles containing oxygen and a nitrogen. Two common forms are oxazole (1,3-oxazole) and isoxazole (1,2-oxazole), both of which undergo aromatic hydroxylation. 

Pyrazole and Imidazole. These are five-membered ring heterocycles containing two nitrogens, one basic and one neutral. Two common forms are pyrazole (1,2-diazole) and imidazole (1,3-diazole) both are prone to aromatic hydroxylation. 

Furan is the least aromatic of the five-membered ring heterocyclics and acts as a diene toward strong dienophiles. 

Four chapters in the present volume bring older reviews up to date. Those on the quinoxalines (Cheeseman and Werstiuk) and on hetero-aromatic quaternization (Zoltewicz and Deady) carry forward contributions on the same subjects in Volumes 2 (1963) and 3 (1964), respectively, of this Series, while those on the phenanthrolines (L. A. Summers) and the isatogens and indolones (Hiremath and Hooper) deal with topics previously covered elsewhere. The cyclazines, a relatively new field of chemistry, are reviewed by Flitsch and Kramer, and the azapentalenes— the wide variety of nitrogen-containing heterocycles formed by the fusion of two aromatic five-membered rings—are collected into one chapter by Elguero, Claramunt, and A. J. H. Summers. 

The fact remains that selenophene and its derivatives have aromatic character. This statement is, however, not easy to substantiate with quantitative data mainly because there are too many ambiguities for parent five-membered ring heterocycles. Some aromaticity indices for five-membered rings with one heteroatom, among them selenophene, were summarized and estimated values for thiophene and selenophene were similar [26], The data obtained from photoelectron ionization energies show that the aromatic stabilization in thiophene is slightly larger than for selenophene [27],... 

Bird, C.W. (1985). A New Aromaticity Index and its Application to Five-Membered Ring Heterocycles. Tetrahedron, 41,1409-1414. 

Orthometallated complexes are intermediates in numerous cyclic carbonylation reactions involving aromatics with nitrogen donor groups in their side chains. For example, aromatic ketoximes and azobenzene are carbonylated at elevated temperatures and pressures with a cobalt carbonyl catalyst to form five-membered ring heterocycles in which a carbonyl group has replaced the metal in the presumed orthometallated intermediate ... 

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). 

Relative Aromaticity of Five-Membered Ring Heterocycles ... 

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