Second-Generation Manganese(III) Porphyrins Bearing 3,5-Dichloropyridyl Units: Innovative Homogeneous and Heterogeneous Catalysts for the Epoxidation of Alkenes

Neves Neves,Faustino Faustino,Simões Simões,Rebelo Rebelo

doi:10.3390/catal9110967

Abstract

The synthesis, characterisation and homogeneous catalytic oxidation results of two manganese(III) porphyrins of the so-called second-generation of metalloporphyrin catalysts, containing one or four 3,5-dichloropyridyl substituents at the meso positions are reported for the first time. The catalytic efficiency of these novel manganese(III) porphyrins was evaluated in the oxidation of cyclooctene and styrene using aqueous hydrogen peroxide as the oxidant, under homogeneous conditions. High conversions were obtained in the presence of both catalysts, obtaining the corresponding epoxide as the major product. The asymmetric metalloporphyrin, chloro[5,10,15-tris(2,6-dichlorophenyl)-20-(3,5-dichloropyridin-4-yl)porphyrinate]manganese(III), CAT-4, evidences a similar activity to that obtained with the well-known and highly efficient second-generation metalloporphyrin catalyst, chloro[5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrinate]manganese(III), CAT-2. CAT-4 was covalently attached onto Merrifield resin and 3-bromopropylsilica supports. The solid materials obtained were characterized by several techniques including diffuse reflectance, UV—VIS spectrophotometry, SEM and XPS. The catalytic results for the oxidation of cyclooctene and styrene using the immobilized catalysts are also presented. The Merrifield-supported catalyst showed to be very efficient, leading to five catalytic cycles in the oxidation of cyclooctene, using tert-butyl hydroperoxide as the oxidant.

Highlights

Metalloporphyrins are effective and well-known biomimetic catalysts of the cytochrome P450(CYP450) enzymes in the oxidation of several organic compounds [1,2,3,4,5,6,7,8,9,10,11]
The novel free-base porphyrins and their manganese(III) complexes were synthesized according to the synthetic approach summarized in Scheme 1—Steps I and II, following well-established procedures [31,32]
The strong alterations observed in the Soret band (λ ≈ 417 nm) and Q bands (λ ≈ 500–650 nm) regions of the free-bases Porph-3 and Porph-4 spectra confirmed the success of manganese insertion and, the formation of CAT-3 and CAT-4

Summary

Introduction

Metalloporphyrins are effective and well-known biomimetic catalysts of the cytochrome P450(CYP450) enzymes in the oxidation of several organic compounds [1,2,3,4,5,6,7,8,9,10,11]. The so-called first-generation of metalloporphyrins as model systems of CYP450 was based on the metal complexes of the free-base 5,10,15,20-tetraphenylporphyrin (Porph-1). Significant progress has been achieved by developing more robust and efficient biomimetic oxidation metalloporphyrin [4,13,14,15]. Introduction electronSince significant progress hascatalysts been achieved by The developing more of robust and withdrawing or bulkyoxidation substituents at the meso-phenyl rings of[4,13,14,15]. The macrocycle, is the case of efficient biomimetic metalloporphyrin catalysts. 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin ledrings to theofso-called second-generation of electron-withdrawing or bulky substituents at the(Porph-2), meso-phenyl the macrocycle, as is the case metalloporphyrins (Figure 1), known as more resistant towards oxidative degradation [13,14]. 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin ledrings to theofso-called second-generation of electron-withdrawing or bulky substituents at the(Porph-2), meso-phenyl the macrocycle, as is the case metalloporphyrins (Figure 1), known as more resistant towards oxidative degradation [13,14]. of

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Conclusion