Epoxidation using dioxygen

The use of epoxides characterized by a different molecular structure may be of interest as polymers with different structural properties may be obtained. Therefore, an issue of great interest is the discovery of new routes to the production of epoxides using dioxygen instead of hydrogen peroxide in the epoxidation of olefins. 

It is realised that both ruthenium and the substituted porphyrins are expensive catalyst components for industrial applications. Both turnover frequencies and turnover numbers are modest. Nevertheless it remains an interesting option to use dioxygen directly in epoxidation reactions. 

The first evidence of the capacity of supported gold catalysts to epoxidate propene was described by Haruta et al. using dioxygen in the presence of H2 as reductant, which allows 02 activation at low temperatures [237]. Although initial selectivities by Au/Ti02 were low, promising improvements were achieved with different supports [230-232, 238-246]. One of them, TS-1, is known to be suitable for the selective epoxidation of propene with H202 [246]. For that reason, many early studies focused on its use. 

Over the past 25 years, biomimetic model systems have been extensively studied and a wide variety of interesting oxidation processes such as the epoxidation of olefins, the hydroxylation of aromatics and alkanes, the oxidation of alcohols to ketones, etc., have been accomplished some of these are also known in enantioselective versions with spectacular ee s. The vast majority of these transformations were obtained using monooxygen donors such as those mentioned above as primary oxidants. The complexity of the catalysts and the practical impossibility to use dioxygen as the terminal oxidant have so far prevented the use of such systems for large industrial applications, but some small applications in the synthesis of chiral intermediates for pharmaceuticals and agrochemicals, are finding their way to market. 

S. Ozaki, T. Hamaguci, K. Tsuchida, Y. Kimata, M. Masui, Epoxidation catalysed by Mn" TPPCI using dioxygen activated by a system containing N-hydroxyphthalimide and styrene, 2-norbor-nene or indene, J. Chem. Soc. Perkin Trans. II. (1989) 951. 

T. Iwahama, S. Sakaguchi, Y. Ishii, Epoxidation of alkenes using dioxygen in the presence of an alcohol catalyzed by N-hydroxyphthalimide and hexafluoroacetone without any metal catalyst, Chem. Commun. (1999) TS. 

Figure 1. Catalytic cycle for olefin epoxidation with dioxygen using mthenium porphyrins.

The direct vapor phase epoxidation of propene using dioxygen (O2) and dihydrogen (H2) can be achieved using supported gold catalysts and is expected to contribute to future industrial processes for propene oxide production. At present, the following conclusions can be drawn ... 

Horwitz, C.P, S.E. Creager, and RW. Murray (1990). Electrocatalytic olefin epoxidation using manganese Schiff-base complexes and dioxygen. Inorg. Chem. 29, 1006-1011. 

Aerobic oxidation of alkanes is also possible, using dioxygen as the terminal oxidant. In these cases, Ru-porphyrin and RuCla systems have been shown to oxidize cyclohexane to cyclohexanone in the presence of acetaldehyde, with a fairly high turnover number (TON = 14,100 moles/(mole catalyst-h)). The mechanism for alkane oxidation remains largely unexplored but is suspected to be similar to the oxo-transfer mechanism that governs epoxidation of alkenes (44). 

Incorporation of Cu(II) into the galleries of a-ZrP resulted in a catalyst that is active for the oxidation of carbon monoxide in the presence of oxygen, and the catalytic activity was comparable to that of a number of similar catalysts used for the oxidation of CO (129). Porphyrins and phthalocyanins intercalated into a-ZrP were used to oxidize olefins to epoxides by dioxygen, with considerable selectivity. While cyclohexene was oxidized to predominantly the epoxide and smaller amounts of allylic oxidation products, cis-stilbene gave rise to different... 

Epoxidation of Alkenes using Dioxygen as Terminal Oxidant... 

It has been shown that the cleavage of the olefin double bond can be avoided using a two-reactor system ((6.18) and (6.19)) in which a metal oxide transfers one lattice oxygen to an olefin with formation of the olefin oxide and is then re-oxidized using dioxygen in a separate reactor [116]. A single reactor can be used which is flushed with the olefin to get the epoxide and then with the correct amount of O2 to regenerate the oxide. 

Drennan and co-workers have identified the dioxygen binding site on iron in hydroxypropylphosphonic acid epoxidase (HppE), an unusual mononuclear iron enzyme that used dioxygen to catalyse the oxidative epoxida-tion of (S)-2-hydroxypropylphosphonic acid (715) to epoxide (716) and converted (i )-2-hydroxypropylphosphonic acid (717) to 2-oxo-propylphos-phonic acid (718) (Scheme 181). These structures illustrated how HppE was... 

Analysis This is the approach to 1,4-dioxygenated skeletons we used in frames 171-186. We need an illogical electrophile - in this case an epoxide ... 

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