Electron-Transfer Chain Catalysis of η²‑Arene, η²‑Alkene, and η²‑Ketone Exchange on Molybdenum

An oxidant-initiated, substitution process for dihapto-coordinated ligands is described for the {MoTp­(NO)­(DMAP)} system. Complexes of the form MoTp­(NO)­(DMAP)­(η2-alkene), MoTp­(NO)­(DMAP)­(η2-ketone), and MoTp­(NO)­(DMAP)­(η2-arene) (where Tp = hydridotris­(pyrazolyl)­borate and DMAP = 4-(dimethylamino)­pyridine) undergo an alkene-to-ketone exchange that is catalyzed by the addition of <0.1 equiv of a metallocene oxidant (ferrocenium, permethylferrocenium, or cobaltocenium). A similar acceleration was observed in the presence of the H-bond donor hexafluoroisopropanol (HFIP). From experimental observations, a radical chain propagation mechanism is proposed that is dependent on the equilibrium between dihapto-coordinated (C, O-η2) and monocoordinated (κ-O) isomers and the differing redox characteristics of these two isomeric forms. This concept was then applied to the search of sodium-free reduction conditions for the conversion of MoTp­(NO)­(DMAP)­(I) to various molybdenum(0) complexes of unsaturated ligands, including MoTp­(NO)­(DMAP)­(η2-naphthalene) and MoTp­(NO)­(DMAP)­(α-pinene).本文针对{MoTp(NO)(DMAP)}体系，报道了一类由氧化剂引发的双哈配位（dihapto-coordinated）配体取代反应过程。通式为MoTp(NO)(DMAP)(η²-烯烃)、MoTp(NO)(DMAP)(η²-酮)及MoTp(NO)(DMAP)(η²-芳烃)的配合物（其中Tp为氢化三(吡唑基)硼酸盐（hydridotris(pyrazolyl)borate），DMAP为4-(二甲氨基)吡啶（4-(dimethylamino)pyridine））可发生烯烃向酮的配体交换反应，该反应可通过添加少于0.1当量的茂金属氧化剂（二茂铁鎓、五甲基二茂铁鎓或钴茂鎓）实现催化。当体系中加入氢键供体六氟异丙醇（hexafluoroisopropanol，HFIP）时，反应也可得到类似的加速效果。基于实验观测结果，本文提出了一种自由基链传递反应机理：该机理依赖于碳氧双哈配位（C,O-η²）与单齿氧配位（κ-O）两种异构体间的平衡，以及这两种异构体迥异的氧化还原特性。随后，该机理概念被用于探索无钠还原条件，以实现将MoTp(NO)(DMAP)(I)转化为各类含不饱和配体的钼(0)配合物，包括MoTp(NO)(DMAP)(η²-萘)与MoTp(NO)(DMAP)(α-蒎烯)。