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Characterization of Azulenylphosphine Derivatives. Unexpected Debromination and Its Synthetic Utility in the Preparation of 2-Substituted Azulene

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posted on 03.04.2020, 16:59 by A. F. M. Mustafizur Rahman, Toshihiro Murafuji, Toshihisa Shibasaki, Kouichi Suetake, Kei Kurotobi, Yoshikazu Sugihara, Nagao Azuma, Yuji Mikata
(1,3-Dihalo-2-azulenyl)diphenylphosphines 2 [halogen = Cl (a), Br (b)] and homologous bis(1,3-dihalo-2-azulenyl)phenylphosphines 4 have 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 2 and 4 with hydrogen peroxide proceeded with a dramatic color change from green to blue, giving the corresponding phosphine oxides 3 and 5, respectively, in high yield. Similar color changes were observed when a solution of each phosphine, 2b and 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 2b showed no such reactivity, suggesting that the higher nucleophilicity of the phosphorus atom was the deciding factor. Such debromination of 2b may 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 π-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 3b with that of 9 with 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, 3b has head-to-tail π−π stacking, but 9 forms a mutual halogen−hydrogen interaction of the bromine atom with a hydrogen atom of its positively charged seven-membered ring.