Epoxide iron-catalyzed

Abstract In this review, recent developments of iron-catalyzed oxidations of olefins (epoxidation), alkanes, arenes, and alcohols are summarized. Special focus is given on the ligand systems and the catalytic performance of the iron complexes. In addition, the mechanistic involvement of high-valent iron-oxo species is discussed. 

Scheme 1 BPMEN and TPA ligands for the iron-catalyzed epoxidation of olefins...

Scheme 4 Selected ligands for iron-catalyzed epoxidation of alkenes...

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

Other examples of oxidant-iron(III) adducts as intermediates in iron porphyrin-catalyzed reactions have been published as listed in references 54a-k. Competitive alkene epoxidation experiments catalyzed by iron porphyrins with peroxy acids, RC(0)00F1, or idosylarenes as oxidants have been proposed to have various intermediates such as [(porphyrin)Fe (0-0-C(0)R] or [(porphyrin)Fe (0-I-Ar)]. Alkane hydroxylation experiments catalyzed by iron porphyrins with oxidant 3-chloroperoxybenzoic acid, m-CPBA, have been proposed to operate through the [(porphyrin)Fe (0-0-C(0)R] intermediate. J. P. CoUman and co-workers postulated multiple oxidizing species, [(TPFPP )Fe =0] and/or [(TPFPP)Fe (0-I-Ar)] in alkane hydroxylations carried out with various iodosylarenes in the presence of Fe(TPFPP)Cl, where TPFPP is the dianion of me50-tetrakis(pentafluorophenyl)porphyrin. ... 

The iron-catalyzed addition of Grignard reagents to propargylic epoxides developed by Furstner and Mendez allows one to prepare a yw-allenol, which is an important intermediate for the synthesis of a precursor of the amphidinolide X (Scheme 67). 

Although the iron-catalyzed synthesis of allenes from propargylic halides was reported by Pasto and coworkers in 1978 [67], little progress was achieved in this field until recently [68]. In 2003, Fiirstner and coworkers discovered propargylic epoxides as valuable substrates for the reaction with Grignard reagents in presence of catalytic amounts of Fe(acac)3 to generate 2,3-allenol derivatives (Scheme 5.24) [69]. 

CpFe(CO)2(thf)]+, is generated by protonation of the respective iron-methyl complex with HBF4 in THF solution. Iron-catalyzed reactions of phenyldiazomethane with aldehydes resulted in mixtures of ketones and epoxides [29]. 

Scheme 9.15 Iron-catalyzed epoxidation and ring-opening reaction.

Scheme 9.45 Carbonylative iron-catalyzed ring expansion of epoxides.

The iron-catalyzed ring expansion reaction is a complementary alternative to Ti(III) chemistry for the ring expansion of epoxides [110]. However, so far the reaction is limited to styrene oxide derivatives, while the alkene can be broadly varied. 

An intermolecular iron-catalyzed ring expansion reaction involving epoxides and alkenes provided tetrahydrofurans via radical processes <07CEJ4312>. Cp2TiCl is able to promote cyclization of 2,3-epoxy alcohols containing a p-(alkoxy)acrylate moiety to form tetrahydrofurans <07TL6389>. As shown in the following example, an intramolecular addition of carbon radicals to aldehydes was reported to afford tetrahydrofuran-3-ols... 

M. Fujita, L. Que Jr., In situ formation of peracetic acid in iron-catalyzed epoxidations by hydrogen peroxide in the presence of acetic acid, Adv. Synth. Catal. 346 (2004) 190. 

Bio-Inspired Iron-Catalyzed Olefin Oxidations Epoxidation Versus c/s-Dihydroxylation... 

The discovery of iron complexes that can catalyze olefin czs-dihydroxylation led Que and coworkers to explore the possibility of developing asymmetric dihydroxylation catalysts. Toward this end, the optically active variants of complexes 11 [(1R,2R)-BPMCN] and 14 [(1S,2S)- and (lP-2P)-6-Me2BPMCN] were synthesized [35]. In the oxidation of frans-2-heptene under conditions of limiting oxidant, 1R,2R-11 was foimd to catalyze the formation of only a minimal amount of diol with a slight enantiomeric excess (ee) of 29%. However, 1P-2P-14 and 1S,2S-14 favored the formation of diol (epoxide/diol = 1 3.5) with ees of 80%. These first examples of iron-catalyzed asymmetric ds-dihydroxylation demonstrate the possibility of developing iron-based asymmetric catalysts that may be used as alternatives to currently used osmium-based chemistry [45]. 

Fe =0 species have also been implicated in one recent study [65] to explain the dramatic effect of acetic acid in enhancing the epoxide yield and selectivity of olefin oxidations mediated by 6 and 9 (see Section 3) [40]. NMR evidence was obtained by Talsi and coworkers for the formation of Fe =0 species from the reaction of 6 or 9 with H2O2 in the presence of acetic acid at 50 °C. The Fe =0 species may be formed as a consequence of the iron-catalyzed in situ formation of peracetic acid as proposed by Fujita et al. [42], which has been shovm to react with 9 efficiently to form [(TPA)Fe O] [66]. It remains to be established whether such species indeed participate in epoxidation catalysis at higher temperature. 

R. Mas-Balleste, M. Fujita, C. Hemmila, L. Que, Jr., Bio-inspired iron-catalyzed olefin oxidation. Additive effects on the cis-diol/epoxide ratio, /. Mol. Catal. A Chem. 251 (2006) 49. 

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. 

R. Mas-Balleste, L. Que, Jr., Iron catalyzed olefin epoxidation in the presence of acetic acid. Insights into the nature of metal-based oxidant, J. Am. Chem. Soc. 129 (2007) 15964. 

From 2, it was concluded that the ferryl complex is the catalytically active species. Observation 1 suggested that 80% of the epoxide product in the aerobic reaction is derived from a carbon-based radical, which is quenched by O2 (autoxidation), and this is known to produce epoxide in reactions with cyclooc-tene (325). Methanol (observation 3) is known to quench radicals. The fact that the diols formed are a mixture of cis and trans products (observation 1 this is very unusual in iron-catalyzed olefin oxidations) suggested that the diol results from the capture of OH radicals by the putative carbon-based radical. 

Kimura and co-workers reported the first iron-catalyzed olefin epoxidation with H2O2 as oxidant and Fe(acac)3 as the catalyst (89). The oxidation of cis- and troras-stilbene afforded troras-epoxide as the major product with a product yield of 96%. Similar stereochemical features were observed in the epoxidation of other substrates, such as methyl oleate and cis- or trons-9-octadecen-l-ol (89). 

J.T. Groves and T.E. Nemo, Epoxidation reactions catalyzed by iron porphyrins.. ... 

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