Benzo-fused five-membered heterocycles aromaticity

Figure 1.40 Several other five-membered and benzo-fused five-membered nitrogen-containing aromatic heterocycles.

The principal component analysis model has also been employed for the comparison of aromaticity for benzo-fused five- and six-membered heterocycles <90JPR(332)870>. The loadings for the bicyclic compounds show a close correlation to those of the corresponding monocyclic heterocycles. The data for the characteristics and the scores for thiophene and for benzo[6]thiophene (along with benzene and naphthalene) are listed in Table 20. 

SYNTHESIS OF AROMATIC BENZO-FUSED FIVE- AND SIX-MEMBERED HETEROCYCLES VIA PALLADIUM- AND COPPER-CATALYZED C-X BOND-FORMING REACTIONS... 

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. 

The C-H coupling of aromatic heterocycles with bis(pinacolato)di-borane(4) was carried out in octane at 80-100 °C in the presence of a half equivalent [IrCl(COD)]2-(4,4 -di-tert-butyl-2,2 -bipyridine) catalyst (3 mol%). The reactions of five-membered substrates such as thiophene, furan, pyrrole, and their benzo-fused derivatives exclusively produced 2-borylated... 

---