Topological charge stabilization

Whereas unsubstituted pentalene has not been obtained so far, its -n-isoelectronic inorganic analog (72) where there occurs the topological charge stabilization has, on the contrary, been synthesized [73AG(E)576]. 

Topological charge stabilization explains the greater stability of indole (88), benzo[b]furan (89), and benzo[6]thiophene (90) compared to their positional isomers (91)-(93) when the largest 7r-charge densities at the 1,3-positions of the five-membered ring in the 10 7r-electron indenyl anion (94) are taken into account (77JA1692). Such an order of the relative stability is consistent with the RE values (Table X). 

Kurihara etal. (90BCJ2531) discussed the stability and resonance energy of troponoids by application of the topological charge stabilization (TCS) rule. Many synthetic and natural troponoids prove to be energetically very stable molecules. The TSC rule can be applied to the formation mechanism of heterocyclotropones. Many benzazinotropone products are completely consistent with this rule, for example, tri- and higher-cyclic quinoxalotro-pones 27 and 166. 

Gimarc (83JA1979,86JA4303) proposed a rule of topological charge stabilization which states that heteroatoms prefer to be located at sites that conform to the pattern of relative electron densities determined by connectivity or topology in an isoelectronic, isostructural, homoatomic system that is called the uniform reference frame <83JA1979>. For the series of thienothiophene positional isomers, the pentalene dianion (5) serves as the uniform reference frame ((55) represents pentalene). 

Figure 5 Application of the topological charge stabilization rule to bicyclic heterocycles.

---