Alkyl(quinolin-8-yl)phosphine Oxides as Hemilabile Preligands for Palladium-Catalyzed Reactions

Preligands of quinolyl-substituted
secondary phosphine oxides (SPOs, 2a–d) were prepared and characterized.
The unique palladium complex 3, having a distorted-square-pyramidal
structure, was obtained from the reaction of 2 equiv of 2c with Pd­(COD)­Cl2 or [Pd­(μ2-Cl)­(η3-allyl)]2. In the crystal structure of 3, an apical chloride ligand and a supramolecular tetradentate ligand
composed of a deprotonated 2c′ and a neutral 2c′ were resolved (2c′: PA form
of 2c). Intriguingly, the gas-phase optimized geometry
of 3 converged to a distorted-square-planar structure,
which was predicted by density functional calculations. The solid-state
distorted-square-pyramidal structure of 3 can only be
explained with the consideration of environmental effects (i.e., the
electrostatic interactions between the surrounding molecules). As
also evidenced by 31P NMR experiments performed in different
deuterated solvents, the crystal structure of 3 is retained
in solution. In the crystal structure of 3, a long Pd–Cl
bond was analyzed by energy decomposition analysis, showing that the
bond is dominated by electrostatic character. Furthermore, application
of these SPOs using the Heck reaction shows good reactivity toward
common aryl bromides. The hemilabile preligand 2c also
tautomerizes to the competent ligand 2c′ for palladium-catalyzed
three-component reactions.

喹啉取代的二级氧化膦（secondary phosphine oxides, SPOs, 2a–d）类预配体已被制备并表征。将2当量的2c分别与双(1,5-环辛二烯)二氯化钯[Pd(COD)Cl₂]或二(μ₂-氯-η³-烯丙基)二钯{[Pd(μ₂-Cl)(η³-allyl)]₂}反应，得到了具有畸变四方锥结构的独特钯配合物3。在配合物3的晶体结构中，可解析出一个轴向氯配体，以及由一个去质子化的2c′与一个中性2c′构成的超分子四齿配体（2c′为2c的质子互变异构体）。值得注意的是，经密度泛函计算预测，3的气相优化几何构型收敛为畸变平面正方形结构。配合物3在固态下的畸变四方锥结构，仅能通过考虑环境效应（即周围分子间的静电相互作用）加以解释。此外，在不同氘代溶剂中开展的磷-31核磁共振（³¹P NMR）实验也证实，3的晶体结构在溶液中得以保留。在配合物3的晶体结构中，一条较长的Pd–Cl键通过能量分解分析进行了表征，结果显示该键以静电相互作用为主。进一步研究表明，这类SPOs应用于赫克反应（Heck reaction）时，对常见芳基溴化物表现出良好的反应活性。半稳定型预配体2c还可互变异构化为适用于钯催化三组分反应的活性配体2c′。