Porphyrin metal complex catalysts, alkene epoxidation

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. 

Simple alkenes do not normally react with IOB, unless there is catalysis by metal porphyrins or related metal complexes, in which case epoxidation occurs [1,2]. A great deal of work has been done in this field, especially with the relatively simple catalyst Fe(TPP)Cl (TPP is tetraphenylporphyrin) in some instances this approach can be used advantageously in comparison with other well-known methods of epoxidation. The Fe(TPP)Cl catalysed IOB epoxidation of alkenes is stereospecific, with cis substrates being considerably more reactive than trcrns. Several alkenes underwent efficient epoxidation with this system, e.g. cyclooctene (84% of epoxide), norbomene (67% exo-epoxide, accompanied by 3% of the enc/o-isomer) and... 

N-substituted iron porphyrins form upon treatment of heme enzymes with many xenobiotics. The formation of these modified hemes is directly related to the mechanism of their enzymatic reactivity. N-alkyl porphyrins may be formed from organometallic iron porphyrin complexes, PFe-R (a-alkyl, o-aryl) or PFe = CR2 (carbene). They are also formed via a branching in the reaction path used in the epoxidation of alkenes. Biomimetic N-alkyl porphyrins are competent catalysts for the epoxidation of olefins, and it has been shown that iron N-alkylporphyrins can form highly oxidized species such as an iron(IV) ferryl, (N-R P)Fe v=0, and porphyrin ir-radicals at the iron(III) or iron(IV) level of metal oxidation. The N-alkylation reaction has been used as a low resolution probe of heme protein active site structure. Modified porphyrins may be used as synthetic catalysts and as models for nonheme and noniron metalloenzymes. 

The more widely accepted mechanism for oxo-transfer involves direct substrate attack at the oxo ligand with concerted or sequential C-0 bond formation. In 1985, Groves proposed a transition state geometry for epoxidation by porphyrin complexes involving a side-on, perpendicular approach of the olefin to the metal-oxygen bond [17]. This trajectory accounted for the enhanced reactivity of cis- over frans-alkenes (4a vs. 4b) in porphyrin and other metal oxo catalyst systems (Fig. 2). This model has also helped explain the observed enantioselec-tivities in AE reactions with successful chiral catalysts [18,19]. 

Metal complexes of fluorous tetraarylporphyrins (1-5) have been used as catalysts in the epoxidation of alkenes under FB [9] or more traditional conditions [10], depending on their affinity for perfluorocarbons. Free base porphyrins 1-5 were readily metaUated with transition metal cations under standard conditions normally employed for their nonffuorous coimterparts. In particular, porphyrins 1-4 were metalated with Mn(OAc)2 4 H2O in boiling DMF to give their respective Mn(III) complexes Mn-l-Mn-4 [10], whereas the perffuorocarbon-soluble porphyrin 5 was similarly converted into the cobalt(II) complex Co-5 by treatment with Co(OAc)2 -4H20 [9],... 

Due to their importance for research but also for industrial chemistry, transition metal based catalysts are intensively investigated. Ananikov et al. [684] reviewed various appUcatimis of hybrid ONIOM methods within this field. This review involves reaction mechanisms and enantioselective reactions of transition metal complexes, e.g. Ti-catalyzed cyanation of benzaldehyde [685], Cu-catalyzed cyclopropanation [686], Mn-porphyrin catalyzed epoxidation of alkene [687], and Mo-catalyzed nitrogen activation [688]. These approaches involve QM/QM as well as QM/MM approaches. 

The epoxidation of alkenes by sodium hypochlorite in the presence of manganese porphyrins under phase-transfer conditions has been thoroughly studied. Kinetic studies of this reaction revealed a Michaelis-Menten rate equation. As in Scheme 12, the active oxidant is thought to be a high-valent manganese( V)-oxo-porphyrin complex which reversibly interacts with the alkene to form a metal oxo-alkene intermediate which decomposes in the rate determining step to the epoxide and the reduced Mn porphyrin. Shape selective epoxidation is achieved when the sterically hindered complex Mn(TMP)Cl is used as the catalyst in the hypochlorite oxidation. ... 

Aqueous sodium hypochlorite is another low-priced oxidant. Very efficient oxidative systems were developed which contain a meso-tetraarylporphyrinato-Mn(III) complex salt as the metal catalyst and a QX as the carrier of hypochlorite from the water phase to the organic environment. These reactions are of interest also as cytochrome P-450 models. Early experiments were concerned with epoxidations of alkenes, oxidations of benzyl alcohol and benzyl ether to benzaldehyde, and chlorination of cyclohexane at room temperature or 0°C. A certain difficulty arose from the fact that the porphyrins were not really stable under the reaction conditions. Several research groups published extensively on optimization, factors governing catalytic efficiency, and stability of the catalysts. Most importantly, axial ligands on the Mn porphyrin (e.g., substituted imidazoles, 4-substituted pyridines and their N-oxides), 2 increases rates and selectivities. This can be demonstrated most impressively with pyridine ligands directly tethered to the porphyrin [72]. Secondly, 2,4- and 2,4,6-trihalo- or 3,5-di-tert-butyl-substituted tetraarylporphyrins are more... 

Epoxidation. NaOCl provides the active oxygen for delivery to alkenes through interaction with metal-porphyrin complexes. Typically, 5,10,15,20-tetrakisaryl-porphyrins and their 2,3,7,8,12,13,17,18-octahalo derivatives form active catalysts with metal ions such as Mn. Another heterocyclic ligand (e.g., 4-methylpyridine) and a phase-transfer agent are also added to the reaction medium. 

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