Epoxidations Catalyzed by 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 

Epoxidation with PhIO in the presence of manganese(III) tetraphenylporphyrine chloride [Mn(TPP)Cl] has been extensively studied.244 245,307 The main characteristic of these reactions is the loss of stereochemistry at the double bond with product distributions depending on the porphyrin structure 134 

Epoxidation with NaOCl is of synthetic interest although this system is not stereoselective, either.244,245 Reactivity, chemo- and stereoselectivity, however, are 

Dramatic shape selectivities in competitive olefin epoxidation was observed with picnic basket metalloporphyrins312 313 designed to exclude bulky axial ligands on one sterically protected porphyrin face. When oxidized with PhIO in acetonitrile in the presence of the rigid p-xylyl-strapped porphyrin, cis-2-octene reacted selectively versus ds-cyclooctene or 2-methyl-2-pentene, giving 1000 reactivity ratios.313,314 Some immobilized manganese(III) porphyrins proved to be as efficient as their homogeneous equivalents in epoxidation with PhIO.151,315 

Certain robust manganese porphyrins [e.g., manganese(IH) tetra(2, 6 -dichloro-pheny])porphyrin chloride] are able to catalyze stereoselective epoxidations, when applied in the presence of imidazole324-327 or other heterocyclic nitrogen bases.326 NaI04324 or even H202325-327 can be used as oxidant. 

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Collman JP, Zhang X, Lee VJ et al (1993) Regioselective and enantioselective epoxidation catalyzed by metalloporphyrins. Science 261 1404-1411... 

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. 

Early model studies concerning the P-450 type oxygen activation, i.e., the reductive oxygen activation catalyzed by metalloporphyrins, were carried out by Tabushi et al. In 1979, Tabushi and Koga examined oxidation of cyclohexene by Mn(TPP)/02/NaBH4 and found the exclusive formation of cyclohexanol and cyclohexenol in a 4 1 ratio [175]. The production of cyclohexanol was explained by the reduction of cyclohexene oxide and cyclohexenone, respectively (Scheme 5) [175]. To avoid possible reduction of primary products, Tabushi and Yazaki replaced the reduction system with H2/colloidal Pt [176]. Under the optimal conditions, the Mn(TPP)/l-methylimidazole/02/H2-colloidal Pt system oxidized cyclohexene to cyclohexene oxide and cyclohexenone with a turnover number of 7,000. These systems were further improved to show stereospecific and regioselective mono epoxidation [177]. 

Bai, D. Duan, S. Hai, L. Jing, H. Carbon Dioxide Fixation by Cycloaddition with Epoxides, Catalyzed by Biomimetic Metalloporphyrins. Chem Cat Chem. 2012, 4, 1752-1758. 

Metalloporphyrin complexes have been shown to catalyze epoxidation of alkenes (Fig. 12), and in this case metal oxo intermediates have been implicated [19-21]. In most cases strong oxidants are required however, it has recently been reported that a ruthenium trans-dioxo complex catalyses direct epoxidation of an alkene with molecular oxygen as the oxidant [22]. It is believed that porphyrinato metal complex-catalyzed epoxidations are related to alkene epoxidations catalyzed by cytochrome P-A50 or other active enzymatic systems. It has recently been shown that creating site-isolation by anchoring (tetra-phenylporphinato)manganese(lll) acetate to a rigid polymer support considerably enhances the rate of epoxidation of cyclohexene by sodium hypochlorite [23]. 

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... 

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 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. 

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. 

C. L. Hill, R. B. Brown Jr., Sustained epoxidation of olefins by oxygen donors catalyzed by transition metal-substituted polyoxometalates, oxidatively resistant inorganic analogs of metalloporphyrins, J. Am. Chem. Soc. 108 (1986) 536. 

Two kinds of systems based on Fe(III) or Mn(III) porphyrins are available now for the oxidation of hydrocarbons. The first ones involve such a metalloporphyrin catalyst and an oxygen atom donor like PhIO, H2O2 or O2 and a reducing agent 2-10 They reproduce quite well the reactions catalyzed by cytochrome P450-dependent monooxygenases and involve a high-valent metal-oxo active species which is able to epoxidize alkenes, hydroxylate alkanes and aromatic compounds and perform N- or S- oxidations. 

Epoxidation of alkenes with iodosylbenzene can be effectively catalyzed by the analogous salen or chiral Schiff base complexes of manganese(in), ruthenium(II), or ruthenium(III). For example, the oxidation of indene with iodosylbenzene in the presence of (/ ,5)-Mn-salen complexes as catalysts affords the respective (15,2/ )-epoxyindane in good yield with 91-96% ee [704]. Additional examples include epoxidation of alkenes with iodosylbenzene catalyzed by various metalloporphyrins [705-709], corrole metal complexes, ruthenium-pyridinedicarboxylate complexes of terpyridine and chiral bis(oxazoUnyl)pyridine [710,711]. 

There has also been a renewal of interest in reactions catalyzed by ru-thenium(II) porphyrin complexes, simultaneously with the development of new chiral ruthenium porphyrins [175-178]. Although these reactions focus mainly on asymmetric epoxidation of olefins [179,180], in some cases asymmetric cyclopropanations were very successful As a recent example, the intermolecular cyclopropanation of styrene and its derivatives with ethyl diazoacetate afforded the corresponding cyclopropyl esters in up to 98% ee with high trans/cis ratios of up to 36 and extremely high catalyst turnovers of up to 1.1 X 10 [140]. The structure of the metalloporphyrin is given in Fig. 2. Asymmetric intramolecular cyclopropanations were also reported with the same catalyst [140]. hi this case, the decomposition of a series of aUyhc diazoacetates afforded the cyclopropyl lactones in up to 85% ee. Both the inter-and intramolecular cyclopropanation were proposed to proceed via a reactive chiral ruthenium carbene intermediate. The enantioselectivities in these processes were rationahzed on the basis of the X-ray crystal structures of closely related stable chiral carbene complexes obtained from the reaction of the chiral complex with N2CPh2 and N2C(Ph)C02CH2CH = CH2. 

Oxidations catalyzed by iron porphyrin complexes are not limited to hydroxylation of alkanes and epoxidation of alkenes. Many types of reactions catalyzed by heme enzymes such as P-450, peroxidases, and catalases have been modeled by synthetic metalloporphyrin complexes. 

Ion exchange resins are also useful to bind metalloporphyrins having ionic substituents on the porphyrin rings. For instance, Turk and Ford demonstrated the epoxidation of styrene in aqueous sodium hypochlorite solution catalyzed by tetra(2,6-... 

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. 

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. ... 

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... 

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