Thiophene derivatives five-membered ring structure

Thiophene is an aromatic compound. Its structure can be assumed to be derived from benzene by replacement of two annular CH groups with sulfur. The sulfur atom in this five-membered ring acts as an electron-donating heteroatom by contributing two electrons to the aromatic sextet and thiophene is thus considered to be an electron-rich heterocycle. 

The NICS of each ring, as a criterion of aromaticity, has been used to explain the stability order of benzo[/)]thio-phene and its isomer. The results indicate that the benzene ring is aromatic in all the systems. The five-membered ring of benzo[. ]thiophene is also aromatic, whereas in benzo[r]thiophene it is nonaromatic. This could be an explanation of the stability of the former molecule. The MOS and the condensed Fukui functions derived from the electronic-structure calculations explain the expected electrophilic substitution of these compounds. The theoretical structure, ionization energies, order of aromaticity, stability, and reactivity are in good agreement with the experimental results <2003T6415>. 

It can be concluded that the B3LYP/6-31G will produce high-quality structural parameters for the five-membered rings and their benzo derivatives. Based on the structural uniformity principle and magnetic susceptibility anisotropies, the predicted relative aromaticity of these systems is found to be reliable. From the computed values, the relative stability of thiophene, benzo[ ]thiophene, and benzo[f]thiophene is accurately predicted. 

In the discussion of structural and spectroscopic aspects of thiophenes to follow, the reader does well to bear in mind that in fact aromaticity is a hallmark of thiophenes that dominates their properties and governs their reactions. Moreover, thiophene stands as uniquely aromatic among the five-membered heterocycles. In concrete terms this means that not only does thiophene undergo the reactions and have the physical and spectral properties associated with the concept of aromaticity, but that this aromatic character is steadfastly maintained through all manners of transformations including fusion with other aromatic rings. This attachment of substituents or fusion with other rings spawns thousands of aromatic derivatives. 

As expected from simple MO considerations, the radical cations of five-membered heterocycles, e.g. the blue species formed from furan, pyrrole, thiophene, and their alkyl derivatives, are n ions. The semi-occupied orbital is the n orbital with the heteroatom in the nodal plane (ai), see Scheme 2, structure 1. In radical cations of a,ca-bis-(l-pyrrolyl) alkanes the charge remains localized on a single ring, rather than being delocalized over both units [5, 10, 11]. 

Metal chelates are preser t in the complexes of Schiff bases of amino-(X = NR2), hydroxy- (X = O), and mercapto- (X = S) derivatives of monoheteroaromatic five-membered systems, azoles, and azines. A few publications on the complexes of azomethines of the monoheteroaromatic five-membered systems have appeared only recently. The X-ray structural study of copper(II) bis(2-N-n-octyliminomethyl)benzo[ 7]thio-phene-3-olate) 199 (04ZNK1696) is interesting in the sense that its square-planar structure is complemented by an extended octahedral one due to the intermolecular contacts of the thiophene sulfur with the copper site. Among the azole complexes, azomethine derivatives of p3Tazole (Equation (32)) prevail (05RCR193). There are several types of coordination units (N,N-, N,0-, N,S-, N,Se-) created by variation of the donor sites X in position 5 of the pyrazole ring. When X = O, S, Se, tautomer b is realized, whereas when X = NR, tautomer a predominates (Equation (32)). However, irrespective of the type of tautomer, in the chelates the coordination units have practically equalized bonds. 

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