Five-membered aromatic heterocycles

Oxidation of Aromatic Heterocycles Five-Membered Rings... 

The fully aromatic heterocyclic five-membered ring imidoyl halides have only a limited degree of imidoyl reactivity. However, their benzo derivatives are often considerably more reactive. For example, 2-chloro-benzthiazole (XXXI) reacts considerably faster than 2-chlorothiazole (XXXII) in nucleophilic substitution reactions ( ). 

In the design of NLO chromophores with large /ajS, the influence of the choice of aromatic end group can be significant (see. e.g., 4 vs. 5). Aromatic heterocyclic five-membered rings have six tt electrons spread over... 

Fig. 1 The aromaticity of five-membered heterocycles characterized by the Bird-Index [32]...

While it is clear that 1,2,5-thiadiazoles are clearly aromatic in nature, efforts have been made to quantify the degree of aromaticity. Three detailed comparative studies of relative aromaticity in five membered heterocyclic rings have been carried out by Bird <85T1409>, Katritzky <90JPR885>, and... 

Halogenation of heterocycles five-membered rings, 57, 291 fused to other aromatic and heteroaromatic rings, 59, 245 six- and seven-membered rings, 58, 271 of heterocyclic compounds, 7, 1 Hammett equation, applications to... 

Nucleus-independent chemical shift (NICS) values for a number of five-membered heterocycles have been reported and are used as a quantitative magnetic measure of aromaticity. They are a measure of diamagnetic ring current and are not a measure of thermodynamic stability. NICS values are theoretical parameters and the values depend on the computational method used and the position above the ring. Table 2 shows NICS values for a selection of heterocyclic systems where NICS is the value at the centre of the ring. The aromaticity of five-membered heterocycles is discussed in Sections 2.3.4.2 and 2.4.4.2. 

The aromaticity of a heterocycle depends on how effectively the lone-pair of the heteroatom contributes to the aromatic sextet. The aromaticity of five-membered heterocyclic compounds may be estimated from their reactivity in the Diels-Alder reaction.94 Spectrophotometry shows that furan, thiophene, and selenophene resemble benzene in that with maleic anhydride 1 1 complexes are formed which are stable up to 150°C in the case of thiophene, decompose at 150°C with selenophene (whereby selenium is formed together with a diene which gives a further adduct with another molecule of maleic anhydride), and produce the usual adduct at 20°C with furan. Thus, only furan is a normal diene as regards the Diels-Alder reaction. 

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. 

To confirm this finding based on the degree of the benzene ring C-C distortion, the magnetic susceptibility anisotropies and the relative energies for the benzene-fused five-membered heterocycles have been computed (Table 68). A similar approach was used by Schleyer et al. <1995AGE337> to evaluate the aromaticity of five-membered heterocycles using the ab initio methods. 

Formation of heterocycles. Five membered heterocycles Phenacylmalononitrile underwent cyclization in acidic media to yield substituted furans 136164 and 137166. Cyclization with anilines afforded pyrroles 138166. Benzylmalononitriles have been cyclized with hydrazine or hydroxylamine to yield 3,5-diaminopyrazoles or isoxazoles, respectively 6. Aminomalononitrile tosylate can be reacted with aliphatic or aromatic isothiocyanates to yield 2,5-diaminothiazole-4-carbonitrilesi6. ... 

Relative Aromaticity of Five-Membered Ring Heterocycles ... 

The potential application of C—C coupling constants in assessing the aromaticity of five-membered heterocycles has been discussed. ... 

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. 

A Hiickel model used for calculating aromaticity indicated that the isoxazole nucleus is considerably less aromatic than other five-membered heterocycles, including oxazole and furan. SCF calculations predicted that isoxazole is similar to oxazole. Experimental findings are somewhat difficult to correlate with calculations (79AHC(25)147). PRDDO calculations were compared with ab initio values and good agreement for the MO values was reported... 

---