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PCN Pincer Palladium(II) Complex Catalyzed Enantioselective Hydrophosphination of Enones: Synthesis of Pyridine-Functionalized Chiral Phosphine Oxides as NCsp3O Pincer Preligands

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posted on 17.10.2014 by Xin-Qi Hao, Juan-Juan Huang, Tao Wang, Jing Lv, Jun-Fang Gong, Mao-Ping Song
A series of chiral PCN pincer Pd­(II) complexes VIXIII with aryl-based aminophosphine–imidazoline or phosphinite–imidazoline ligands were synthesized and characterized. They were examined as enantioselective catalysts for the hydrophosphination of enones. Among them, complex IX, which features a Ph2PO donor as well as an imidazoline donor with (4S)-phenyl and N-Tol-p groups, was found to be the optimal catalyst. Thus, in the presence of 2–5 mol % of complex IX a wide variety of enones reacted smoothly with diarylphosphines to give the corresponding chiral phosphine derivatives in high yields with enantioselectivities of up to 98% ee. In particular, heteroaryl species such as 2-thienyl-, 2-furyl-, and 2-pyridinyl-containing enones that have a strong coordination ability to the Pd center were also appropriate substrates for the current catalytic system. For example, hydrophosphination of 2-alkenoylpyridines with diphenylphosphine followed by oxidation with H2O2 afforded the corresponding pyridine-functionalized chiral phosphine oxides in good yields with good to excellent enantioselectivities (10 examples, up to 95% ee). Furthermore, it had been demonstrated that the obtained pyridine-containing phosphine oxide acted as a tridentate ligand in the reaction with PdCl2 to form an intriguing NCsp3O pincer Pd­(II) complex via Csp3–H bond activation, which to our knowledge is the first example of a chiral DCsp3D′ Pd pincer (D ≠ D′; D and D′ denote donor atoms such as P, N, etc.).