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

(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.(1,3-二卤代-2-薁基（azulenyl）)二苯基膦类化合物2[卤素（halogen）为氯(a)、溴(b)]以及同系双(1,3-二卤代-2-薁基（azulenyl）)苯基膦4已被成功合成。本研究针对非交替薁基（azulenyl）对这类膦的反应活性及理化性质的影响展开了探究，并与第15族同系物的相关结果进行了对比分析。非交替共轭体系的电子结构会随磷原子价态的变化发生显著改变。因此，用过氧化氢氧化2和4时，体系发生了从绿色到蓝色的显著颜色变化，分别以高收率得到对应的氧化膦（phosphine oxides）3和5。当将膦类化合物2b和4b的溶液置于室温条件下静置时，观察到了类似的颜色变化；而2a、4a以及(2-薁基)二苯基膦（(2-azulenyl)diphenylphosphine）7则未出现此类变化。从2b的溶液中分离得到了(1-溴-2-薁基)二苯基氧化膦（(1-Bromo-2-azulenyl)diphenylphosphine oxide）9。研究发现该意外反应是通过质子化作用，在薁的五元环上发生亲电取代反应而进行的。2b的铋同系物与锑同系物未表现出此类反应活性，这表明磷原子更高的亲核性是决定该反应发生的关键因素。结合X射线晶体学研究（X-ray crystallographic study）结果推断，2b的此类脱溴反应可归因于溴原子周围的空间位阻效应。该意外反应被成功应用于以三苯基膦为脱溴试剂，便捷合成2-取代薁（azulene）的方法中。碳-13核磁共振（13C NMR）研究表明，相较于母体膦类化合物，氧化膦中薁基的π极化（π-polarization）程度有所提升，但提升幅度不及铋与锑体系中的观测结果。通过对比3b与9的晶体结构中薁基之间的分子间相互作用（intermolecular interaction），发现非交替薁基的微小结构变化会导致堆积方式发生显著改变。具体而言，3b呈现头对头π-π堆积（head-to-tail π−π stacking）结构，而9则通过溴原子与其带正电七元环上的氢原子形成相互的卤素-氢键相互作用（halogen−hydrogen interaction）。