Palladium Bisphosphine Monoxide Complexes: Synthesis, Scope, Mechanism, and Catalytic Relevance

Recent studies in transition metal catalysis employing chelating phosphines have suggested a role for partial ligand oxidation in formation of the catalytically active species, with potentially widespread relevance in a number of catalytic systems. We examine the internal redox reaction of PdII(bisphosphine)­X2 (X = Cl, OAc, etc.) complexes to reveal previously underexplored aspects of bisphosphine monoxides (BPMOs), including evaluation of ligand structure and development of general reaction conditions to access a collection of structurally diverse BPMO precatalysts based on organopalladium oxidative addition complexes. In particular, a series of PdII(BPMO)­(R)­(X) (R = aryl, alkyl; X = I, Br) oxidative addition complexes bearing 24 different BPMO ligands were characterized by NMR and X-ray crystallography. Comparison of the catalytic performance of the oxidative addition complexes of bisphosphine versus bisphosphine monoxides as precatalysts is demonstrated to be an enabling diagnostic tool in Pd catalytic reaction development. Finally, the differences in catalytic behavior between bisphosphine and bisphosphine monoxide complexes were rationalized through solid-state parametrization and stoichiometric experiments.近年来，采用螯合膦配体（chelating phosphines）的过渡金属催化领域研究显示，配体部分氧化在催化活性物种的形成过程中具有关键作用，该现象在诸多催化体系中或具备广泛普适性。本研究针对二价钯(双膦配体（bisphosphine）)X₂（X=Cl、OAc等）配合物的分子内氧化还原反应展开探究，揭示了双膦单氧化物（bisphosphine monoxides, BPMOs）此前未被充分研究的诸多特性，包括配体结构评估，以及开发通用反应条件以获取一系列基于有机钯氧化加成配合物、结构多样的BPMO预催化剂。具体而言，本研究通过核磁共振波谱法（Nuclear Magnetic Resonance, NMR）与X射线晶体学（X-ray crystallography），对24种不同BPMO配体修饰的Pd(II)(BPMO)(R)(X)（R=芳基、烷基；X=I、Br）氧化加成配合物进行了表征。对比双膦配体与双膦单氧化物类预催化剂的氧化加成配合物的催化性能，被证实是钯催化反应开发中一项极具实用性的诊断工具。最后，本研究通过固态参数化与化学计量实验，阐明了双膦配体与双膦单氧化物配合物在催化行为上的差异成因。