Iron asymmetric epoxidation

Until recently only few examples on asymmetric epoxidation using iron-based catalysts were reported in the literature (Scheme 6) [42-44]. With [Fe(BPMCN) (CF3S03)2] 10, 58% of the epoxide with 12% ee was obtained in the oxidation of frans-2-heptene [42]. 

A breakthrough in iron-catalyzed asymmetric epoxidation of aromatic alkenes using hydrogen peroxide has been reported by our group in 2008. Good to excellent isolated yields of aromatic epoxides are obtained with ee-values up to 97% for stilbene derivatives using diphenylethylenediamines 9 as ligands (Scheme 5) [45, 46]. 

Scheme 6 Chiral iron complexes for the asymmetric epoxidation of olefins...

Scheme 7 Recently reported ligands for iron-catalyzed asymmetric epoxidations...

Asymmetric epoxidation systems using iron porphyrin heme-mimics are also known, however the labor-intensive and expensive syntheses is hmiting their applications [49]. 

Scheme 10 Asymmetric epoxidation of tra/ts-2-heptene with the chiral iron complex and H2O2...

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... 

Better results for the porphyrin complex-catalyzed asymmetric epoxidation of prochiral olefins were achieved by Naruta et al.98 using iron complexes of chiral binaphthalene or bitetralin-linked porphyrin 128 as chiral catalysts. As shown in Scheme 4-45, asymmetric epoxidation of styrene or its analogs provided the product with good ee. Even better results were obtained with substrates bearing electron-withdrawing substituents. 

Collman et al.99 reported the asymmetric epoxidation of terminal olefins catalyzed by iron porphyrin complex 129. The catalyst was synthesized by connecting binaphthyl moieties to a readily available aa/ / -tetrakis(aminophenyl)-porphyrin (TAPP). Epoxidation of unfunctinalized olefins was carried out using iodosylbenzene as the oxidant. As shown in Scheme 4-46, excellent results were... 

The asymmetric epoxidation of /i-alkenes and terminal alkenes proved to be more difficult, though a recent finding, describing the use of a modified salen complex to epoxidize ( )-0-methylstyrene to form the corresponding epoxide in 83% ee, represents another important step forward. Alternatively, chiral (D2-symmetric) porphyrins have been used, in conjunction with ruthenium or iron, for efficient asymmetric oxidation of trans- and terminal alkenes[92]. 

Chiral porphyrins are also effective in the asymmetric epoxidation of alkenes. For example, a Cj-symmetiic iron porphyrin (29) <99JA460> catalyzes the efficient epoxidation of terminal alkenes, such as 30, with very good ee s and up to 550io turnovers. Similarly, trons-disubstituted... 

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. ... 

Apart from the commonly used NaOCl, urea—H2O2 has been used/ With this reaction, simple alkenes can be epoxi-dized with high enantioselectivity. The mechanism of this reaction has been examined.Radical intermediates have been suggested for this reaction, polymer-bound Mn -salen complex, in conjunction with NaOCl, has been used for asymmetric epoxidation. Chromium-salen complexes and ruthenium-salen complexes have been used for epoxidation. Manganese porphyrin complexes have also been used. Cobalt complexes give similar results. A related epoxidation reaction used an iron complex with molecular oxygen and isopropanal. Nonracemic epoxides can be prepared from racemic epoxides with salen-cobalt(II) catalysts following a modified procedure for kinetic resolution. 

F. G. Gelalcha, B. Bitterlich, A. Anilkumar, M. K. Tse, M. Beller, Iron-catalyzed asymmetric epoxidation of aromatic alkenes using hydrogen peroxide, Angew. Chem. Int. Ed. 46 (2007) 7293. 

Manganese and iron porphyrins substituted by chiral atropisomeric groups are models for cytochrome P-450 and several of such catalysts have been recommended by Groves, Kodadek, Mansuy and their coworkers in asymmetric epoxi-dations of olefins with hypochlorites or ArlO [957, 970], Recently, asymmetric epoxidation of 2-mtrostyrene has been accomplished with 89% enantiomeric excess, but this result lacks generality. A preliminary report on the use of threitol-strapped manganese poiphyrins in enantioselective epoxidations of unfunctionalized aryl substituted olefins appeared in 1993 [971] enantiomeric excesses in the range of 80% were observed. 

Groves, J.T. and R.S. Myers (1983). Catalytic asymmetric epoxidation with chiral iron porphyrins. J. Am. Chem. Soc. 105, 5791-5796. 

The oxidation of saturated hydrocarbons in the presence of iron- or manganese-containing catalysts can be achieved by using a variety of oxidants including alkyl hydroperoxides, peroxycarboxylic acids, iodosyl-benzene, dihydrogen peroxide, and dioxygen (9-11). It has been shown that chiral iron- and manganese-porphyrin complexes catalyze the asymmetric epoxidation of unfunctionalized alkenes (75). Except for a number of experiments in which up to 96 % enantiomeric excess (ee) has been reported (16,17), in most epoxidation reactions with chiral porphyrins only a low to moderate enantiomeric excess of the product is obtained (18,19). In association with these catalysts, alkyl hydroperoxides and iodosylbenzene are often used as primary oxidants (18,19). 

The asymmetric epoxidation of acyclic )S,)3-disubstituted o, )3-enones in acetonitrile, by peracetic acid and catalysed by an iron complex in which Fe(OTf)2 was coordinated by two 2-[l-(l-naphthyl)-2-naphthyl]-l,10-phenanthroline ligands (35) (R = m-xylyl), to the corresponding Q ,j8-epoxyketones with yield up to 88% and up to 92% ee was achieved. The epoxy ketone was further converted to functionalized )8-keto-aldehydes with an all-carbon quaternary centre." The transfer hydrogenation of acetophenone to 1-phenylethanol in isopropanol in the absence of added base was catalysed by a five-coordinated Fe(II) complex (36) and certain analogues. ... 

Table 1.6 Asymmetric epoxidation of trans-2-heptene with chiral iron complexes and H2O2. 

Gadissa Gelalcha, R, Bitterlich, B., AnUkumar, G., et al. (2007). Iron-Catalyzed Asymmetric Epoxidation of Aromatic Alkenes Using Hydrogen Peroxide, Angew. Chem. Int. Ed., 46, pp. 7293-7296. 

Gadissa Gelalcha, F., AniUcumar, G., Tse, M., et al. (2008). Biomimetic Iron-Catalyzed Asymmetric Epoxidation of Aromatic Alkenes by Using Hydrogen Peroxide, Chem. Eur. J., 14, pp. 7687-7698. 

Not only the porphyrins having binaphthyl groups, strapped type metalloporphyrins are also able to catalyze asymmetric epoxidation. For example, Mansuy et al. synthesized a "Basket-Handle" iron porphyrin bearing L-phenylalanine residues (Fig. 11(d)) [297]. Collman et al. employed threitol-strapped Mn porphyrins (Fig. 11(e)) [298]. Metalloporphyrin complexes conjugated with glycosylated groups [299], p-cyclodextrin [300], and camphanoyl groups [301] were also prepared. These chiral metalloporphyrin complexes were shown to catalyze asymmetric oxidations. 

Transition-metal catalysed asymmetric synthesis has again seen further developments over the past year. Of particular note are stereoselective aldol condensations, and asymmetric epoxidation reactions. Hydrogenation methods have seen few significant advances, and are considered only briefly this year. The important area of carbon-carbon bond-forming reactions continues to attract considerable attention although much work still uses palladium, iron, or copper, nickel and titanium chemistry is also being increasingly used, a trend that will no doubt continue. 

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