Novel Aromatic Cations and Anions: Aromaticity-Structure Relationships

In aromatic polycyclic systems both the local contribution of the components and the peripheral π-delocalization influence the net aromatic nature of the system. Polycyclic conjugated systems are classified according to the number of π-electrons involved in each ring. A (4n. + 2)π + (4n₂ + 2)π fusion results in a (4n + 2)π periphery. Indeed, these systems have aromatic character. However, steric effects (e.g. peri H-H interactions and biphenyl backbone) influence the gradual formation of the aromatic ions and direct their stereochemistry. A (4n_l + 2)π + 4n₂π fusion results in an antiaromatic 4nπ periphery. The fusion of a cyclo-pentadienyl anion to a planar COT component in a polycycle shows paratropic character. Oxidation of a polycyclic molecule which includes a planar COT component yields an aromatic system which includes a COT⁺⁺’ moiety. The fusion of 4n_lπ + 4n₂π systems represents an aromatic (4n + 2)π periphery, while the components are antiarcmatic. The preparation of benzannelated pentalene dications shows that they exhibit diatropicity and that the peripheral (4n + 2)π delocalization dominates. The two electron oxidation and reduction of dibenzopentalenes to the respective aromatic dications and dianions show that 4nπ conjugated polycycles behave similarly to the corresponding monocycles in attaining aromatic character.