Iron-catalyzed Epoxide Formation

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. 

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. 

The reaction of olefin epoxidation by peracids was discovered by Prilezhaev [235]. The first observation concerning catalytic olefin epoxidation was made in 1950 by Hawkins [236]. He discovered oxide formation from cyclohexene and 1-octane during the decomposition of cumyl hydroperoxide in the medium of these hydrocarbons in the presence of vanadium pentaoxide. From 1963 to 1965, the Halcon Co. developed and patented the process of preparation of propylene oxide and styrene from propylene and ethylbenzene in which the key stage is the catalytic epoxidation of propylene by ethylbenzene hydroperoxide [237,238]. In 1965, Indictor and Brill [239] published studies on the epoxidation of several olefins by 1,1-dimethylethyl hydroperoxide catalyzed by acetylacetonates of several metals. They observed the high yield of oxide (close to 100% with respect to hydroperoxide) for catalysis by molybdenum, vanadium, and chromium acetylacetonates. The low yield of oxide (15-28%) was observed in the case of catalysis by manganese, cobalt, iron, and copper acetylacetonates. The further studies showed that molybdenum, vanadium, and... 

A carbene or nitrene transfer reaction to a carbon-carbon or carbon-heteroatom double bond system leads to the formation of three-membered rings, such as a cyclopropane, an aziridine or an epoxide. These processes can be catalyzed by applying iron catalysts and the different cyclic systems are discussed here. 

Two alternative mechanisms have been proposed. A direct substrate attack at the oxo ligand with concerted or sequential C-O bond formation is possible. Alternatively, the substrate may attack at both the metal and oxo centers to generate an oxametallocyclic intermediate. These two alternatives are shown in Fig. 9.9. Finally, note that in CytP450-catalyzed hydroxylation and epoxidation, an iron porphyrin intermediate of the type of 9.39 is involved. 

Biological systems overcome the inherent unreactive character of 02 by means of metalloproteins (enzymes) that activate dioxygen for selective reaction with organic substrates. For example, the cytochrome P-450 proteins (thiolated protoporphyrin IX catalytic centers) facihtate the epoxidation of alkenes, the demethylation of Al-methylamines (via formation of formaldehyde), the oxidative cleavage of a-diols to aldehydes and ketones, and the monooxygenation of aliphatic and aromatic hydrocarbons (RH) (equation 104). The methane monooxygenase proteins (MMO, dinuclear nonheme iron centers) catalyze similar oxygenation of saturated hydrocarbons (equation 105). ... 

Nakano, T, T.G. Traylor, and D. Dolphin (1990). The formation of N-alkyIporphyrins during epoxidation of ethylene catalyzed by iron(lll) meso-tetrakis(2,6-dichlorophenyl)porphyrin. Can J Chem. 10, 1859-1866. 

---