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 ¹H NMR, ¹³C NMR and mass spectrometry analysis. This Ni^II porphyrin complex 1 quantitatively catalyzed the epoxidation reaction of a wide range of olefins with meta-chloroperoxybenzoic acid (m-CPBA) under mild conditions. Reactivity and Hammett studies, H₂ ¹⁸O-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.