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(1,3-Dihalo-2-azulenyl)diphenylphosphines 2halogen Cl (a), Br (b) and homologous bis(1,3-dihalo-2-azulenyl)phenylphosphines 4have been synthesized, and the effect of the nonalternant azulenyl group on the reactivity and properties of these phosphines has been studied in comparison with that of the group 15 congeners. The electronic structure of the nonalternant conjugation is dramatically affected by the change in the valency at the phosphorus. Thus, oxidation of 2and 4with hydrogen peroxide proceeded with a dramatic color change from green to blue, giving the corresponding phosphine oxides 3and 5, respectively, in high yield. Similar color changes were observed when a solution of each phosphine, 2band 4b, was left standing under ambient conditions, while 2a, 4a, and (2-azulenyl)diphenylphosphine (7) showed no such changes. (1-Bromo-2-azulenyl)diphenylphosphine oxide (9) was isolated from the solution of 2b. This unexpected reaction was found to occur via electrophilic substitution at the five-membered ring through protonation. The bismuth and antimony congeners of 2bshowed no such reactivity, suggesting that the higher nucleophilicity of the phosphorus atom was the deciding factor. Such debromination of 2bmay be ascribed to the steric congestion around the bromine atom, which is inferred from the X-ray crystallographic study. The unexpected reaction was successfully applied to a convenient synthesis of 2-substituted azulene using triphenylphosphine as a reagent for debromination. The 13C NMR study shows the enhanced -polarization in the azulenyl group of the phosphine oxides compared to that of the parent phosphines, but the degree is not as marked as that observed in the bismuth and antimony systems. Comparison of the crystal structure of 3bwith that of 9with respect to the intermolecular interaction between the azulenyl groups showed that the slight structural change in the nonalternant azulenyl group brings about a dramatic change in the packing. Thus, 3bhas head-to-tail − stacking, but 9forms a mutual halogen−hydrogen interaction of the bromine atom with a hydrogen atom of its positively charged seven-membered ring. [ABSTRACT FROM AUTHOR]

In spite of having small π-conjugation systems, azulenes show large binding constants (104–105) to C60 and C70, which are larger than those of monoporphyrins and alternant aromatic hydrocarbons. [ABSTRACT FROM AUTHOR]