Competitive Reaction of Axial Ligands during Biomimetic Oxygenations

Clean and Highly Selective Oxidation of Alcohols by the PhI(OAc)2/Mn(TPP)CN/Im Catalytic System

An efficient method for the oxidation of alcohols is presented. Using catalytic amounts of manganese porphyrin [Mn(TPP)CN] in combination with (diacetoxyiodo)benzene (PhI(OAc)2) allows the conversion of benzylic and primary aliphatic and aromatic alcohols into the corresponding aldehydes, and secondary alcohols to ketones as the sole products. This method provides a cost-effective and environmentally friendly oxidation procedure due to the utilisation of less toxic PhI(OAc)2 and biologically relevant manganese porphyrins. The amounts of the products (%) and the selectivities are very dependent upon the nature of the metalloporphyrin catalysts and also upon the electronic and steric properties of the starting alcohols.

Catalytic epoxidation of cis-stilbene and naphthalene by tetra-n-butylammonium hydrogen monopersulfate in the presence of manganese(III) tetraarylporphyrins and various anionic co-catalysts

The epoxidation of cis-stilbene by n-Bu4NHSO5 was studied in the presence of two different manganese porphyrin complexes, [MnTPFPP(Cl) and MnTPP(Cl)], and various n-Bu4NX (X = OAc, F, Cl, Br, OCN, NO3, BF4). In general direct correlation was found between stereoselectivity of the epoxidation reaction and the nucleophilic properties of these anionic co-catalysts. Also epoxidation of naphthalene was carried out in high yield and good selectivity by n-Bu4NHSO5 in the presence of MnTPFPP(Cl) in association with n-Bu4NOAc or n-Bu4NF co-catalysts.

Synthesis and redox properties of a [meso-tris(4-nitrophenyl) corrolato]Mn(III) complex

The Mn complexated corrole [ T (4- NO 2 P ) Corr ] Mn ( py ) (3) has been prepared ( py = pyridine ) where [ T (4- NO 2 P ) Corr ] is the trianion of the electron-poor 5,10,15-tris(4-nitrophenyl)corrole (1). The preparation of 3 includes a new synthetic method to form the corrole ligand 1 in a two-step synthesis. The Mn(III) complex gives a parallel mode EPR signal centered around g = 8.2 with six distinct hyperfine lines ( A || = 139 × 10-4 cm -1). Electrochemically 3 undergoes one reversible oxidation ( E 1/2 = 0.12 V vs Fc) and two reversible reductions ( E 1/2 = -1.45, -1.61 V). The oxidation is metal-centered and the product has been characterized by EPR spectroscopy as an S = 3/2 Mn(IV) species with no indication for oxidation of the macrocycle. The reductions of complex 3 are ligand based, and at the potential of the second reduction step a stable nitrogen centered radical with g = 2.0055 is generated. Chemical oxidation of 3 by iodosobenzene yields a Mn(IV) complex and epoxidation of cis-stilbene is not catalyzed by the Mn complex.

Effect of hydrogen bonding on catalytic activity of some manganese porphyrins in epoxidation reactions

Mn (III)-tetra phenyl porphyrin-acetate (MnTPPOAc) and some kinds of meso-phenyl substituted porphyrins by hydroxyl groups and their Mn (III) complexes were synthesized. These Mn -porphyrins were used as catalyst in the epoxidation of various alkenes with tetra-n-butylammonium hydrogen monopersulfate (n- Bu4NHSO5 ) as oxidant and tetra-n-butylammonium acetate (n- Bu4NOAc ) as the axial ligand. The following order of catalytic activity was observed for cyclooctene: T(2,3-OHP)PMnOAc ≫ T(2,4,6-OHP)PMnOAc ≥ T(4-OHP)PMnOAc ≥ T(2,6-OHP)PMnOAc ≥ TPPMnOAc and T(2,3-OHP)PMnOAc ≫ TPPMnOAc > T(4-OHP)PMnOAc > T(2,4,6-OHP)PMnOAc > T(2,6-OHP)PMnOAc for other alkenes. Different activity and stability of the catalysts were interpreted based on the hydrogen bonding between hydroxyl groups with appropriate orientation on the meso-position of the phenyl groups and axial bases or oxidant. T(2,3-OHP)PMnOAc catalyst has shown optimal condition for effective hydrogen bonding. In the case of other catalysts, electronic and steric factors overcome the hydrogen bonding effect.

Parallel mechanistic studies on the counterion effect of manganese salen and porphyrin complexes on olefin epoxidation by iodosylarenes

Counterions of manganese(III) porphyrin complexes influence diastereoselectivity in cis-stilbene epoxidation and product distribution in cyclohexene epoxidation markedly. In the epoxidation of cis-stilbene by iodosylbenzene carried out in a solvent mixture of CH(3)CN and CH(2)Cl(2), trans-stilbene oxide is the major product in the reaction of manganese complexes bearing a ligating anion (i.e., Cl(-)), whereas cis-stilbene oxide is the dominant product in the reactions of manganese complexes bearing a poorly-ligating anion (i.e., CF(3)SO(4)(-)). In cyclohexene epoxidation, the yields of allylic oxidation products such as cyclohexenol and cyclohexenone are higher when the counterion of the manganese catalysts is Cl(-) than when the counterion is CF(3)SO(4)(-). The product selectivities are also dependent on the nature of iodosylarenes and the axial and porphyrin ligands of the manganese porphyrin catalysts. The observation that product selectivities are different depending on the iodosylarenes may indicate the involvement of multiple oxidants in oxygen atom transfer reactions. These results are compared with those observed in manganese salen-catalyzed epoxidation of olefins by iodosylarenes.