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

Abstract

A new nickel(II) porphyrin complex, [Ni (porp)] (1), has been synthesized and characterized by H NMR, C NMR and mass spectrometry analysis. This Ni 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, H O-exchange experiments, and the use of PPAA (peroxyphenylacetic acid) as a mechanistic probe suggested that participation of multiple active oxidants Ni -OOC(O)R 2, Ni -Oxo 3, and Ni -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, CH Cl , and CH CN, multiple oxidants, Ni -(O)R 2, Ni -Oxo 3, and Ni -Oxo 4, operate simultaneously as the key active intermediates responsible for epoxidation reactions of easy-to-oxidize substrate cyclohexene, whereas Ni -Oxo 3 and Ni -Oxo 4 species become the common reactive oxidant for the difficult-to-oxidize substrate 1-octene. In a protic solvent system, a mixture of CH CN and H O (95:5), the Ni -OOC(O)R 2 undergoes heterolytic or homolytic O-O bond cleavage to afford Ni -Oxo 3 and Ni -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 -Oxo 3 and Ni -Oxo 4 species were the common reactive oxidant responsible for olefin epoxidation reactions.

Proton Switch in the Secondary Coordination Sphere to Control Catalytic Events at the Metal Center: Biomimetic Oxo Transfer Chemistry of Nickel Amidate Complex

“…To gain insights into the electronic nature of the reactive species involved in the olefin epoxidation reaction, we carried out competitive epoxidation reactions of various para ‐substituted styrenes (4‐X‐C 6 H 4 CHCH 2 , X = OMe; Me; H; Cl; CN). The Hammett analysis afforded a ρ value of −0.75 (Figure S15), which indicates the electrophilic nature of the reactive species similar to reported nickel complexes [12c,g,j, 25] …”

“…The selectivity showed a gradual decrease with increasing chain length. No nickel‐based catalysts are currently known that effect catalytic epoxidation of terminal olefins with such high efficiency and selectivity [12c,j] …”

Instant and quantitative epoxidation of styrene under ambient conditions over a nickel(ii)dibenzotetramethyltetraaza[14]annulene complex immobilized on amino-functionalized SBA-15

“…Fe, Cr, Ru, Mn, Ni, with facilely accessible high oxidation states, include porphyrin transition metal complexes, and using peroxycarboxylic acids such as m-CPBA as oxygen donor, this reaction probably operates by redox mechanism involving the formation of Ni II -OOAr and Ni-Oxo activated species, which transfer the oxygen to the olen either directly or by the intermediacy of a metallacycle, a radical, cation species, or via concerted transition state. 10,49,53,54 For porphyrins complexes, this mechanism was named by Groves as oxygen-rebound mechanism. 55 In this mechanism, an oxygen atom is transferred from the oxidant to the metal ion to form metal-oxo species (M-Oxo) with high oxidation state, which in turn transfers this oxygen atom to the olenic substrate.…”

“…Based on the literature, in aprotic solvents, tree pathways are possible for the cleavage of O-O bond: heterolysis, homolysis or direct epoxidation. 10,53 It is well known that O-O bond in peracids cleaves heterolytically. [59][60][61][62][63] Groves and Traylor have proposed that the M-Oxo forms by heterolysis.…”

“…[71][72][73][74] It is reported that Ni II -OOAr in aprotic solvents can undergo a direct epoxidation reaction. 53 In addition, Ni II active sites in Nitmtaa@N-SBA-15 catalyst are electron-poor metal centers, and would increase the proximal oxygen electrophilicity in N II -OOAr peroxide, making this complex an efficient oxygen-transfer agent. The nucleophilic attack of C]C double bond of the styrene on the proximal oxygen atom of the peroxy complex N II -OOAr leads to the formation of the concerted transition state (E).…”