Five-membered ring 330 INDEX

Several aromaticity indices (bond lengths, bond orders, Jug and Francois s aromaticity index) indicate that, despite the nonplanarity of the five-membered ring in 2,5-diphenylthiophene-l-oxide (108), this compound is intermediate in aromaticity between the corresponding thiophene 107 and the nonplanar 1,1-dioxide 109 (Scheme 48).149 The theoretical calculations were supported by experimental electrochemical data.150... 

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>. 

The lability of thieno[3,4-6]thiophene (3) and other iso-annelated systems, such as benzo[c]thiophene and benzole] furan, may be due to the strain effect (Mills-Nixon effect see also Zwanenburg et alP and references therein) in the condensed five-membered ring. The stability of the iso-annelated dithienothiophenes 7—9 is noteworthy. Simple LCAO MO method calculations on benzo[c]thiophene indicate that its instability is due to low specific delocalization energy and high free valence index at position 1. 

Bird aromaticity index (I) - An index of aromatic character based on a statistical evaluation of the extent of variation of ring bond orders compared to those of the nondelocalized Kekule structure. Bond orders are determined from experimentally determined bond lengths, or from accurate calculated values. The index was introduced for five-membered ring (7S) in 1985 by Clive Bird and subsequently extended to six-membered rings If) and bicyclic systems (7S,6 and 766). A universal index 7a unifies the approach (7a = 76 = 1.235 7s = 1.840 76,6 = 2.085 I tf) <1992T335>. For examples see Sections 2.24.2.3, 2.34.2.3, and 244.2.3. 

For benzo[. ]thiophene, there is a it-MO delocalization between the two aromatic rings. For benzo[r]thiophene, the orbitals of the five-membered rings are localized on the heteroatom, C-1 and C-3, and there is no it-MO delocalization on the heterocyclic five-membered rings. These results are in agreement with the theoretical aromaticity of these molecules as are theoretical results from the reactivity indexes. 

Table 65 Aromaticity index (/ring) for heterooyoies with five-membered rings...

Table 66 Aromaticity indexes for some five-membered-rings and benzene...

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

Bird, C. W. 1985. A new aromaticity index and its application to five-membered ring heterocycles. Tetrahedron Al 1409-1414. 

Validation of the new Aromaticity Index to Six- and Five-Membered Rings... 

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