Epoxidations with dioxygen

Introduction of mesityl groups at the porphyrin ring can prevent the formation of the dimeric products and the reaction with dioxygen now leads to ruthenium(VI)-dioxo complexes of TMP (tetramesitylporphyrin) [35], The tram-Ru(VI)02-TM P species can catalyse the epoxidation of alkenes as well as whole range of other oxidation reactions. After transfer of one oxygen atom to an organic substrate Ru(IV)0-TMP is formed, which disproportionates to an equilibrium of Ru02 and llu ). 

Epoxidation of c/.v-P-mcthylstyrcnc and trara-P-methylstyrene produce the epoxides with retention of configuration [36], The reaction is first order in styrene and Ru02 and the reaction is faster when the styrene molecules contain electron donating substituents. This shows that the oxo attack at the alkene is an electrophilic attack [37], Epoxidation of 1-alkenes gives aldehyde as the 

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. 

23 Kokubo, T. Sugimoto, T. Uchida, T. Tanimoto, S. Okano, M. J. Client. Soc. Chem. Commun. 1983,769. 

34 Ezhova, M. B. James, B. R. in Advances in Catalytic Activation of Dioxygen by Metal Complexes , Ed. Simandi, L. S. Volume 26, Catalysis by Metal Complexes, Kluwer Academic Publishers, Dordrecht, 2003. 

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Epoxidation with dioxygen is also possible if it is carried out in the presence of added reducing agents to transform the second oxygen atom into H20.307 Since the... 

Initially, the products of these reactions suggested radical ions were involved138. In particular, when hexamethyl Dewar-benzene was epoxidized with MCPBA, the nature of the products depended on whether or not the iron(III) porphyrin hemin was added to the reaction mixture1381 . Furthermore, when Z-stilbene was epoxidized with dioxygen, catalyzed by (tetraphenylporphorinato)iron(III) chloride, -stilbene appeared in the reaction mixture139. 

The following reactions are observed when an olefin is epoxidized with dioxygen ... 

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

Oleic acid has been reported to undergo epoxidation with dioxygen around 30 °C in the presence of cobalt acetate catalyst and added benzaldehyde [114]. A free radical mechanism involving the C H CO ... 

In the presence of methyl, tert-butyl, or (-)-menthyl esters of 2-oxocyclopentanecarboxylic acids, the cobalt(II)-salen type complexes and Jacobsen-type manganese(ni) complexes shown in Figure 17 are active catalysts for alkene epoxidation with dioxygen . 

Mononuclear peroxo complexes, such as MoO(02)2(HMPT), show a more electrophilic character in their reactions. This particular complex may be used for the epoxidation of alkenes.176 Much interest has been shown in the reaction of small molecules with dioxygen complexes of this type with a view to catalytic oxidation. These reactions and their products are sumarized in Table 10. The... 

A dinuclear ruthenium complex with both ruthenium in 2+ oxidation states catalyzes the oxidation of adamantane to hydroxy adamantane and alkene to epoxide by dioxygen. Suggest a possible mechanism. 

An especially elegant use of superoxide anion, in combination with dioxygen, is as both an EGB and as an epoxidation reagent [52]. In this way the ROJ species, formed from the carbanion R , reacts in situ with an enone, which is converted into the epoxide (Scheme 15). Specifically, the carrier (PhCHoCN), which has been dubbed an auxiliary carbon acid, is deprotonated by the superoxide anion, and the resulting anion reacts with O2. The Ph2C(CN)OJ species thus formed reacts in situ with an enone, which is converted into the epoxide with elimination of cyanide anion from the carrier and concomitant formation of ketone (Scheme 15). Another possible auxiliary carbon acid is MeCH(C02Et)2 and, apart from the example in Scheme 15, 4,4-dimethyl-2-cyclohexen-l-one, 4-methylpent-3-en-2-one, and trans-cha -cone (PhCH=CHCOPh) may be converted [52] into the corresponding epoxides in >80% yield. 

Another product of the reaction of an olefin with the O2/ CO2 mixture in the presence of Rh is the cyclic carbonate (VII) [3]. Only few reports can be found in the literature on the direct synthesis of carbonates from olefins, dioxygen and carbon dioxide, despite the usefulness of this reaction that avoids, with respect to the reaction of epoxides with carbon dioxide, the preliminary synthesis of epoxides. We have found that the product distribution in the oxidative carboxylation of styrene depends on the O2/ CO2 ratio and on the temperature (Scheme 3). As the epoxide is one of the oxidation products of styrene, it could be the origin of styrene carbonate. We have evidence that the formation of the... 

T. Iwahama, G. Hatta, S. Sakaguchi, Y. Ishii, Epoxidation of alkenes using alkyl hydroperoxides generated in situ by catalytic autoxidation of hydrocarbons with dioxygen, Chem. Commun. (2000) 163. 

In continuation to our studies with molecular oxygen as the primary oxidant[8], we now report metal phthalocynanine catalyzed oxidation of sulphides to sulphones and olefins to epoxides by dioxygen-isobutyraldehyde system under ambient conditions (Scheme-1). 

The stoichiometric reaction with in situ produced Fe =0 produces only epoxide, with a larger yield under aerobic conditions, where dioxygen from air appears in the epoxide product... 

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