Epoxidation metalloporphyrin catalyzed

These reports sparked off an extensive study of metalloporphyrin-catalyzed asymmetric epoxidation, and various optically active porphyrin ligands have been synthesized. Although porphyrin ligands can make complexes with many metal ions, mainly iron, manganese, and ruthenium complexes have been examined as the epoxidation catalysts. These chiral metallopor-phyrins are classified into four groups, on the basis of the shape and the location of the chiral auxiliary. Class 1 are C2-symmetric metalloporphyrins bearing the chiral auxiliary at the... 

Table 1 Some examples of chiral metalloporphyrin-catalyzed epoxidation.

Epoxidation.1 This combination is known to oxidatively cleave double bonds but to effect epoxidation when catalyzed by a metalloporphyrin. Epoxidation of alkenes can also be effected by catalysis with a simple amine. The choice of the amine depends on the olefin. N,N-Dimethylethylenediamine is the most efficient ligand for epoxidation of a 1-alkene (68% yield). Pyridine is the best ligand for epoxidation of stilbene (93%), and imidazole is preferred for epoxidation of QH5CH=CHCH, (71% yield). 

In 1994, Mansuy and coworkers found that a simple ammonium salt, like ammonium acetate alone, is a very efficient cocatalyst for the metalloporphyrin-catalyzed epoxidation of simple alkenes by hydrogen peroxide ". Bases like sodium carbonate, sodium acetate or tetrabutylammonium hydroxide turned out to promote the porphyrin-catalyzed epoxidation without any other additive. Adducts of hydrogen peroxide (with Na2C03, urea, MesNO, PhsPO), which turned out to be particularly useful for reactions in which the concentration of H2O2 in solution needs to be controlled at a fixed level, have been employed by Johnstone and coworkers. 

In the wake of this report, many chiral iron(III)- and Mn(III)-porphyrin complexes have been synthesized and applied to the epoxidation of styrene derivatives [20]. Because these asymmetric epoxidations are discussed in the first edition of this book [21], the discussion on metalloporphyrin-catalyzed epoxidation here is limited to some recent examples. Most chiral metallopor-phyrins bear chiral auxi Maries such as the one derived from a-amino acid or binapthol. Differing from these complexes is complex 6, which has no chiral auxiliary but is endowed with facial chirality by introducing a strap and has been reported by Inoue et al. [20f]. Epoxidation of styrene by using only 6 as the catalyst shows low enantioselectivity, but the selectivity is remarkably enhanced when the reaction is performed in the presence of imidazole (Scheme 6B.11). This result can be explained by assuming that imidazole coordinates to the unhindered face of the complex and the reaction occur on the strapped face [20f. ... 

Metalloporphyrins catalyze the autoxidation of olefins, and with cyclohexene at least, the reaction to ketone, alcohol, and epoxide products goes via a hydroperoxide intermediate (129,130). Porphyrins of Fe(II) and Co(II), the known 02 carriers, can be used, but those of Co(III) seem most effective and no induction periods are observed then (130). ESR data suggest an intermediate cation radical of cyclohexene formed via interaction of the olefin with the Co(III) porphyrin this then implies possible catalysis via olefin activation rather than 02 activation. A Mn(II) porphyrin has been shown to complex with tetracyanoethylene with charge transfer to the substrate (131), and we have shown that a Ru(II) porphyrin complexes with ethylene (8). Metalloporphyrins remain as attractive catalysts via such substrate activation, and epoxidation of squalene with no concomitant allylic oxidation has been noted and is thought to proceed via such a mechanism (130). Phthalocyanine complexes also have been used to catalyze autoxidation reactions (69). 

J. Bernadou, A. Fabiano, A. Robert, B. Meunier, Redox tautomerism in high-valent metal-oxo-aquo complexes.Origin of the oxygen atom in epoxidation reactions catalyzed by water-soluble metalloporphyrins,/. Am. Chem. Soc. 116 (1994) 9375. 

De Poorter, B. and B. Meunier (1985). Metalloporphyrin-catalyzed epoxidation of terminal olefins with hypochlorite salts or potassium hydrogen persulfate. Perkin Trans. U,J. Chem. Soc. 1735-1740. 

Groves, J.T. and K.V. Shalyaev (1998). Paramagnetic H-NMR relaxation probes of stereoselectivity in metalloporphyrin catalyzed olefin epoxidation. Chirality 10, 106-119. 

Oxone has also been used in the metalloporphyrin-catalyzed diastereoselective epoxidation of allyl-substituted alkenes (eq 109) and in the manganese, salen-polystyrene-bound imidazole catalyzed deoxidation of alkenes (eq 110). ... 

Metalloporphyrin catalyzed epoxidations using 0-atom transfer reagents... 

Metalloporphyrin catalyzed olefin epoxidations using single oxygen donors... 

There have been several mechanistic proposals for interpreting the behavior of metalloporphyrin catalyzed epoxidation with single O-atom donors. 

The different mechanistic proposals for epoxidation reflect a complex oxidation pattern dependent on a variety of factors. At this time it does not seem either possible or necessary to accommodate all the observed features of metalloporphyrin catalyzed epoxidation within a single mechanism. The crucial importance of oxoiron(IV) porphyrin cation radicals has been established beyond doubt but the net 0-atom transfer to the olefin may perhaps occur via more than one kind of intermediate. 

Metalloporphyrin catalyzed epoxidations with 0 under reducing conditions... 

Another kind of metalloporphyrin catalyzed olefin epoxidation utilizes aldehydes, which generate peroxy radicals and subsequently peroxy acids [95,98,258]. The latter are complexed by the metalloporphyrin and serve as sources of 0-atoms for epoxidation either as the peroxy acid complex or as the oxidized ir-cation radical (oxometal species) [96,97]. The catalysis of propene epoxidation by... 

In the second oxidation method, a metalloporphyrin was used to catalyze the carotenoid oxidation by molecular oxygen. Our focus was on the experimental modeling of the eccentric cleavage of carotenoids. We used ruthenium porphyrins as models of cytochrome P450 enzymes for the oxidation studies on lycopene and P-carotene. Ruthenium tetraphenylporphyrin catalyzed lycopene oxidation by molecular oxygen, producing (Z)-isomers, epoxides, apo-lycopenals, and apo-lycopenones. 

Oxidizing enzymes use molecular oxygen as the oxidant, but epoxidation with synthetic metalloporphyrins needs a chemical oxidant, except for one example Groves and Quinn have reported that dioxo-ruthenium porphyrin (19) catalyzes epoxidation using molecular oxygen.69 An asymmetric version of this aerobic epoxidation has been achieved by using complex (7) as the catalyst, albeit with moderate enantioselectivity (Scheme 9).53... 

A new trend in the field of oxidations catalyzed by metalloporphyrin complexes is the use of these biomimetic catalysts on various supports ion-exchange resins, silica, alumina, zeolites or clays. Efficient supported metalloporphyrin catalysts have been developed for the oxidation of peroxidase-substrates, the epoxidation of olefins or the hydroxylation of alkanes. 

Cyclooctene epoxidation catalyzed by supported sulfonated metalloporphyrins. 

Epoxidation Catalyzed by Metalloporphyrins. Metalloporphyrins, which have thoroughly studied as catalysts in alkane oxygenations, have also been tested as epoxidation catalysts.119,122,244,245,307 Iodosylbenzene (PhIO), sodium hypochlorite, alkyl hydroperoxides, potassium hydrogen persulfate, and molecular oxygen are the oxygen sources used most frequently in these oxidations.119... 

Metallosalen complex [salen = N, A-ethylenebis(salicyldeneaminato)] has a structure similar to metalloporphyrin, and these two complexes catalyze the epoxidation of olefins. For example, Kochi et al. have found that metallosalen complexes such as (salen )manganese(III) [25] and (salen)chromium(IIl) complexes [26] (hereafter referred to as Mn- and Cr-salen complexes, respectively) serve as catalysts for the epoxidation of unfunctionalized olefins by using iodosylbenzene [25] or sodium hypochlorite [27], In particular, cationic Mn-salen complex is a good catalyst for epoxidation of unfunctionalized olefins, which proceeds through an oxo(salen)manganese(V) species (Scheme 6B.14) [25,28], The presence of oxo-Mn(V)-salen... 

In the last decade, transition metal complexes (e.g. metalloporphyrins) have been used to catalyze epoxidation. These entities can reproduce and mimic all reactions catalyzed by heme-enzymes (cytochromes P-450)54. Synthetic metalloporphyrins are analogous to the prosthetic group of heme-containing enzymes which selectively catalyze various oxidation reactions. The metallo complexes of Fe, Co, Cr, Mn, Al, Zn, Ru, etc. possessing porphyrin ligands have been mostly studied55 -57. Porphyrin ligands (4) are planar and can possess several redox states of the central metallic ions and hence they can exist as oxo metals. 

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