Anionic Ligand Effect on the Nature of Epoxidizing Intermediates in Iron Porphyrin Complex-Catalyzed Epoxidation Reactions

Abstract

We have studied an anionic ligand effect in iron porphyrin complex-catalyzed competitive epoxidations of cis- and trans-stilbenes by various terminal oxidants and found that the ratios of cis- to trans-stilbene oxide products formed in competitive epoxidations were markedly dependent on the ligating nature of the anionic ligands. The ratios of cis- to trans-stilbene oxides obtained in the reactions of Fe(TPP)X (TPP = meso-tetraphenylporphinato dianion and X(-) = anionic ligand) and iodosylbenzene (PhIO) were 14 and 0.9 when the X(-) of Fe(TPP)X was Cl(-) and CF(3)SO(3)(-), respectively. An anionic ligand effect was also observed in the reactions of an electron-deficient iron(III) porphyrin complex containing a number of different anionic ligands, Fe(TPFPP)X [TPFPP = meso-tetrakis(pentafluorophenyl)porphinato dianion and X(-) = anionic ligand], and various terminal oxidants such as PhIO, m-chloroperoxybenzoic acid (m-CPBA), tetrabutylammonium oxone (TBAO), and H(2)O(2). While high ratios of cis- to trans-stilbene oxides were obtained in the reactions of iron porphyrin catalysts containing ligating anionic ligands such as Cl(-) and OAc(-), the ratios of cis- to trans-stilbene oxide were low in the reactions of iron porphyrin complexes containing nonligating or weakly ligating anionic ligands such as SbF(6)(-), CF(3)SO(3)(-), and ClO(4)(-). When the anionic ligand was NO(3)(-), the product ratios were found to depend on terminal oxidants and olefin concentrations. We suggest that the dependence of the product ratios on the anionic ligands of iron(III) porphyrin catalysts is due to the involvement of different reactive species in olefin epoxidation reactions. That is, high-valent iron(IV) oxo porphyrin cation radicals are generated as a reactive species in the reactions of iron porphyrin catalysts containing nonligating or weakly ligating anionic ligands such as SbF(6)(-), CF(3)SO(3)(-), and ClO(4)(-), whereas oxidant-iron(III) porphyrin complexes are the reactive intermediates in the reactions of iron porphyrin catalysts containing ligating anionic ligands such as Cl(-) and OAc(-).

The bioinorganic chemistry of iron in oxygenases and supramolecular assemblies

“…Indeed, in a recent report by Jin et al (34), mutation of the conserved active site threonine-252 to alanine in P450 CAM was shown to disable camphor hydroxylation while maintaining some reactivity for more reactive olefinic substrates. Similarly, two reactive intermediates, as suggested by Volz et al (35) for the reactions of a thioether substrate, and also for model porphyrin systems described by Nam et al (36), could reasonably derive from a mechanistic spectrum of this type. An important precedent for this behavior is seen in the reactions of peroxyacids with model Fe(III) porphyrins.…”

Porphyrins as Catalysts in Scalable Organic Reactions

“…Besides the hydrogen peroxide decomposition [45] and other factors like the structural features of the porphyrin catalyst [46], the substrate concentration and the solvent [47] affect strongly the activity and the selectivity of these catalysts.…”

Iron porphyrins-catalysed oxidation of α-alkyl substituted mono and dimethoxylated benzyl alcohols

“…Mesotetrakis(4-N-methylpyridyl)porphyrin iron (III) chloride (FeTMPyPCl) and meso-tetrakis(4-sulfonatophenyl) porphyrin iron (III) chloride (FeTSPPCl) have been used as catalysts in aqueous solution in combination with KHSO 5 as the oxygen atom donor. For the reactions in organic solvent (CH 2 Cl 2 ) the efficient catalyst Mesotetrakis(pentafluorophenyl)porphyrin iron (III) chloride (FeTPFPPCl) has been used [22][23][24][25][26] [27,28]. …”