Steric and electron-withdrawing effects of substituents governing chemical stability and catalytic activity of Mn(III)-tetraarylporphyrins in HOCl/ClO− alkene expoxidations
✍ Scribed by S. Banfi; F. Montanari; S. Quici
- Publisher
- Elsevier Science
- Year
- 2010
- Tongue
- English
- Weight
- 604 KB
- Volume
- 109
- Category
- Article
- ISSN
- 0165-0513
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✦ Synopsis
Abstract
Mn(III)‐tetrakis(2,4,6‐trimethyl‐3,5‐dihalogenophenyl)porphyrins 5‐7 (halogen = Cl, Br, I) have been synthesized and their catalytic activity has been tested in HOCl/ClO^−^ alkene epoxidations. These porphyrins are characterized by structural separation of the two parameters governing chemical stability towards the oxidants: the steric protection of both the metal centre and of the meso positions of the porphyrin ring; and the electron‐withdrawing effect of substitutents (methyl groups and halogen atoms in the o,o′ and m,m′ positions, respectively, of the meso‐aromatic rings). The stability of 5–7 towards oxidative degradation is similar to that of Mn(III)‐tetrakis(2,6–dichlorophenyl)porphyrin (1). Catalytic activities of the porphyrins 5‐7 and 1 are optimized under quite different conditions. This behaviour can be explained in terms of mono‐ (K~1~) and bis‐coordination (β~2~) constants of porphyrins 1, 5 and 6 with the axial ligand, 1‐hexylimidazole, which account for the concentration of the mono‐ligated species Mn(P)L and of the free ligand at the equilibrium.