Homogeneous epoxidation porphyrin complexes

Synthetic iron porphyrin complexes such as Fe(TPP) (tetraphenylporphyrin = TPP), Fe(TMP) (Tetramesitylporphyrin = TMP), and Fe(TDCPP) (tetrakis (dichlorophenyl)porphyrin = TDCPP) (Fig. 9) have been used as models for P450 and peroxidase (9, 50-54). Early pioneering work showed that epoxida-tion catalyzed by Feln(TPP) was successfully carried out by the use of iodosylbenzene (Ph—1=0) as an oxidant (50). A very interesting feature of this model epoxidation is that the cis olefin is readily oxidized while the trans olefin is hardly oxidized (e.g., d.v-stylbene can be oxidized in 80% yield, but fraws-stylbene gave only a trace amount of the epoxide under the same conditions) (50, 55). Most of the model reactions are carried out in homogeneous organic solvents such as chloroform, dichloromethane, and acetonitrile, thus, the c/.v-epoxidation is expected to be a kinetically favorable process over the trans-epoxidation. 

Traylor et al. reached different conclusions in studies of alkene epoxidation. In homogeneous solutions in which iodosylbenzene was solubilized in dichloro-methane/alcohol/water, very rapid epoxidation (300 turnovers/sec) was found to be independent of substrates or concentration of alkane (124). No dependence on alkene concentrations was reported by Dicken et al. when they employed /7-cyanodimethylaniline 7V-oxide (725). Meanwhile, Traylor et al. found an accumulation of 7V-alkyliron porphyrin complexes (35) during the course of epoxidation of norbornene (126), and showed that it is the only intermediate accumulated in either the homogeneous system or the heterogeneous system 35 was found to form with a bimolecular rate constant of 450 M sec and decomposes at a rate of 0.07 sec. Epoxide formation occurs with a rate constant of at least 10 Af" sec in these conditions. Thus 35 is not an intermediate in the production of epoxide however, it could account for at least a small percentage of the products, since 35 was found to catalyze alkene epoxidation by utilizing PhIO (Scheme XIX). 

Application of Ru-porphyrin complexes for catalytic, homogeneous alkene epoxidation is limited because of small turnover numbers, only moderate enantioselectivities in the case of chiral systems, the cost of porphyrin and the 0-atom donor (unless O2), and often limited stability of the porphyrin ring under the oxidizing conditions. In attempts to improve possible plicability, Ru-porphyrins have been heterogenized on various supports °, following methodology developed previously for Ee- and Mn-... 

A tetracationic Mn porphyrin complex immobilized on monmorillonite was found to be efficient for alkene epoxidation and alkane hydroxylation by PhIO, with a higher ability than the corresponding homogeneous or silica-supported Mn porphyrin complexes... 

The most common route to cyclic carbonates is the reaction of epoxides with CO2, which is promoted by a variety of homogeneous, heterogeneous and supported catalysts either cyclic carbonates or polymers are obtained [89]. Main group metal halides [90a] and metal complexes [90b], ammonium salts [91] and supported bases [92], phosphines [93], transition metal systems [88, 94], metal oxides [95], and ionic liquids [96] have been shown to afford monomeric carbonates. A1 porphyrin complexes [97] and Zn salts [89, 94, 98] copolymerize olefins and CO2. 

Supercritical CO2 is a non-polar, aprotic solvent and promotes radical mechanisms in oxidation reactions, similar to liquid-phase oxidation. Thus, wall effects might occur as known, e.g. from olefin epoxidation with 02 or H202 which may decrease epoxide selectivities. The literature covers the synthesis of fine chemicals by oxidation either without catalysts (alkene epoxidation, cycloalkane oxidation, " Baeyer-Villiger oxidation of aldehydes and ketones to esters ), or with homogeneous metal complex catalysts (epoxidation with porphyrins, salenes or carbonyls ). Also, the homogeneously catalysed oxidation of typical bulk chemicals like cyclohexane (with acetaldehyde as the sacrificial agent ), toluene (with O2, Co +/NaBr ) or the Wacker oxidation of 1-octene or styrene has been demonstrated. 

Chiral porphyrin ligands have been used in asymmetric direct 02catalytic oxidations. The first catalytic asymmetric homogeneous aerobic epoxidation, not requiring an aldehyde coreductant, was achieved with a Ru(Vl)dioxo-(D4-porphyrinato) complex, 15 (Scheme 5.12) [43]. Although these chiral mthe-nium complexes have promoted good asymmetric induction in the product, the TON is still quite low [38]. Currently, a synthetically viable homogeneous aerobic epoxidation catalyst without coreductants has not been developed. 

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