Synthesis, Characterization, and Efficient Catalytic Activities of a Nickel(II) Porphyrin: Remarkable Solvent and Substrate Effects on Participation of Multiple Active Oxidants

A new nickel(II) porphyrin complex, [Ni-II(porp)] (1), has been synthesized and characterized by (HNMR)-H-1, (CNMR)-C-13 and mass spectrometry analysis. This Ni-II porphyrin complex1 quantitatively catalyzed the epoxidation reaction of a wide range of olefins with meta-chloroperoxybenzoic acid (m-CPBA) under mild conditions. Reactivity and Hammett studies, (H2O)-O-18-exchange experiments, and the use of PPAA (peroxyphenylacetic acid) as a mechanistic probe suggested that participation of multiple active oxidants Ni-II-OOC(O)R 2, Ni-IV-Oxo 3, and Ni-III-Oxo 4 within olefin epoxidation reactions by the nickel porphyrin complex is markedly affected by solvent polarity, concentration, and type of substrate. In aprotic solvent systems, such as toluene, CH2Cl2, and CH3CN, multiple oxidants, Ni-II-(O)R 2, Ni-IV-Oxo 3, and Ni-III-Oxo 4, operate simultaneously as the key active intermediates responsible for epoxidation reactions of easy-to-oxidize substrate cyclohexene, whereas Ni-IV-Oxo 3 and Ni-III-Oxo 4 species become the common reactive oxidant for the difficult-to-oxidize substrate 1-octene. In a protic solvent system, a mixture of CH3CN and H2O (95:5), the Ni-II-OOC(O)R 2 undergoes heterolytic or homolytic O-O bond cleavage to afford Ni-IV-Oxo 3 and Ni-III-Oxo 4 species by general acid catalysis prior to direct interaction between 2 and olefin, regardless of the type of substrate. In this case, only Ni-IV-Oxo 3 and Ni-III-Oxo 4 species were the common reactive oxidant responsible for olefin epoxidation reactions.