Electronic structure, aromatic character, and chemical reactivity of o-quinonoidal heterocycles

Abstract

104, 893 (1967)]; however, it is likely that at least the 2,4-dinitrophenolate leaving group leaves without protonation.

Insights into the Mechanism of the Benzoannelated Thieno[3,2-b]furan Halogenation. Importance of HOMO–HOMO Interaction

“…It acquires the most negative charge on the C(2) carbon atom as shown in Table 4. Moreover, the angle α in π-CO is close to 90°(94.3°) and the bond length BL Br(1)−Br(2) (2.38 Å) is longer than that of the bromine molecule, BL Br(1)−Br(2) (2.33 Å); the bond orders BI C(2)−Br (1) and BI C(3)−Br(1) are 0.09 and 0.03, respectively. These facts indicate a weak interaction between the Br(1) atom and the C(2) and C(3) atoms of the thiophene ring and suggest the existence of a weak nonsymmetric three-membered π complex in π-CO between the bromine and the C(2) and C(3) atoms with the bromine positioned over the rim of thiophene ring, and oriented perpendicular to the heterocycle plane.…”

“…Aromatic electrophilic substitution (Ar-ES) reactions of benzofused electron-rich five-membered heterocycles are among the most thoroughly studied reactions in organic chemistry owing to their wide range of applications in material sciences and medicine. − Materials containing substituted electron-rich five-membered heterocycles are successfully applied to the design of novel liquid crystals. , Many experimental works dealing with an attempt to obtain halogenated electron-rich five-membered heterocycles have been extensively developed in this field. , Moreover, the experimental studies have been carried out to clarify the nature of the electrophilic substitution reaction in these heterocycles. − It was suggested that electrophilic substitution in the simple five-membered heterocycles furan and thiophene occurs with positional selectivity and via the aromatic σ-complex. − More recently Domingo et al rationalized the relative reactivities of five-membered heterocycles in electrophilic aromatic substitution reaction by means of the global nucleophilicity index. − Later Gromri et al extended this investigation and has shown that the experimental trends of the relative reactivities and regioselectivities of these reactions can be correctly predicted using calculated global nucleophilicity reactivity indices . Mineva reported and discussed the reactivity descriptors of thiophene, furan, and pyrrole using Fukui indices .…”

Heterocycles by cycloaddition. Part 9. Bridged heteroannulenes by cycloaddition–extrusion–ring-expansion reactions of mesoionic compounds with benzocyclopropene. A methanothiazonine, a methanothionine, and a methanothiecinone

“…Ph 1 660 cm-'; tjH 2.81, d, J 6.1 Hz, and 2.88, d, J 6.3 Hz). However, no fraction suggestive of the presence of either the methanooxonine (28) or the methano-oxecinone (19; S = NPh) could be found.…”

Computer-assisted mechanistic evaluation of organic reactions. 7. Six electron cycloadditions

“…19 It should be noted that although thioph-enes20 and pyrroles21 are quite inert due to a high degree of aromaticity, their incorporation into polycyclic aromatic systems increases their reactivity markedly. 22 A variety of heterocyclic dienes is illustrated in Chart I.…”