Epoxidation of styrenes

Prepared by epoxidation of styrene with per-oxyelhanoic acid. Reactions are similar to those of aliphatic epoxides (s e, e.g. ethylene oxide). Reacts with alcohols to give mono-ethers, e g. PhCH(0Me)CH20H. Phenols give resins. 

The commonly held view of the uniqueness of Ag for ethylene epoxidation may soon change in view both of the propene epoxidation work of Haruta and coworkers on Au/Ti02 catalysts upon cofeeding H2 123 and also in view of the recent demonstration by Lambert and coworkers124 126 that Cu(lll) and Cu(110) surfaces are both extremely efficient in the epoxidation of styrene and butadiene to the corresponding epoxides. In fact Cu was found to be more selective than Ag under UHV conditions with selectivities approaching 100%.124-126 The epoxidation mechanism appears to be rather similar with that on Ag as both systems involve O-assisted alkene adsorption and it remains to be seen if appropriately promoted Cu124 126 can maintain its spectacular selectivity under process conditions. 

The same type of porphyrin-Ru complex was immobilized by coordina-tive adsorption on aminopropylsilicas (Fig. 26) as either amorphous or crystalline supports [79]. Mesoporous crystalline MCM-48 was the best support, as shown by the improved results obtained in the epoxidation of styrene with 2,6-dichloropyridine N-oxide (TON > 13 000 and 74% ee). The versatility of this catalyst was demonstrated in the intramolecular cyclopropanation of frans-cinnamyl diazoacetate. TON was ten times higher than that obtained in solution and 85% ee was observed. The solid was recycled and reused, although partial loss of selectivity occurred. 

Li, X. and Kutal, C. (2002) Photocatalytic selective epoxidation of styrene by molecular oxygen over highly dispersed titanium dioxide species on silica. Journal of Materials Science Letters, 21 (19), 1525-1527. 

Kureshy developed a polymer-based chiral Mn-salen complex (Figure 21). Copolymerization of styrene, divinylbenzene, and 4-vinylpyridine generated highly cross-linked (50%) porous beads loaded with pyridine ligands at 3.8 mmol g-1. Once the polymer was charged with the metal complex catalyst, enantioselective epoxidation of styrene derivatives was achieved with ee values in the range 16 46%. 79... 

C. Estavillo, Z. Lu, I. Jansson, J.B. Schenkman, and J.F. Rusling, Epoxidation of styrene by human cyt P-450 1A2 by thin film electrolysis and peroxide activation compared to solution reactions. Biophys. Chem. 104,291-296 (2003). 

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. 

The influence of pH on epoxidation of styrene with aqueous H202 catalyzed by TS-1 was also investigated. Conversion of styrene decreased, and styrene oxide selectivity increased marginally at high pH values (Table XXI). 

Effect of different oxidants on epoxidation of styrene and allylbenzene catalyzed by TS-1 and TS-2... 

A similar conclusion was also reached by Sankar et al. (46), who used EXAFS/ DFT techniques. From the selective decrease in the EPR intensity of the A type superoxo species during the epoxidation of styrene and allyl alcohol (Fig. 52), Srinivas et al. (52) concluded that these types of oxo species are preferentially consumed during the reaction. 

Figure 12.7 Enantioselective epoxidation of styrene to (S)-styrene oxide utilizing recombinant E. coli JMW1 (pSPZI0) as biocataiyst...

Scheme 4.12 Polymer-bound salen-complexes in asymmetric epoxidations of styrenes.

The unconsumed reactants and products were both easily removed from the reaction mixture by extraction with K-hexane, which is not miscible with [BMIMJPFg. The brown-red ionic liquid phase containing the catalyst was reused five times with PhI(OAc)2 as the oxidant. The recovered catalyst gave catalytic activity comparable to that of the original. In epoxidation of styrene and of cyclohexene, both catalytic activity and selectivity fell slightly after five reuses. In the conversion of hept-l-ene, the reused catalyst showed the same activity and selectivity as the fresh catalyst, and the catalyst was shown to be unchanged after the reaction. 

Most of the work reported with these complexes has been concerned with kinetic measurements and suggestions of possible mechanisms. The [Ru(HjO)(EDTA)] / aq. HjOj/ascorbate/dioxane system was used for the oxidation of cyclohexanol to cw-l,3-cyclohexanediol and regarded as a model for peroxidase systems kinetic data and rate laws were derived [773], Kinetic data were recorded for the following systems [Ru(Hj0)(EDTA)]702/aq. ascorbate/dioxane/30°C (an analogue of the Udenfriend system cyclohexanol oxidation) [731] [Ru(H20)(EDTA)]70j/water (alkanes and epoxidation of cyclic alkenes - [Ru (0)(EDTA)] may be involved) [774] [Ru(HjO)(EDTA)]702/water-dioxane (epoxidation of styrenes - a metallo-oxetane intermediate was postulated) [775] [Ru(HjO)(EDTA)]7aq. H O /dioxane (ascorbic acid to dehydroascorbic acid and of cyclohexanol to cyclohexanone)... 

The earliest work, by Kureshy et al. used complexes of chiral Schiff bases, e.g. Ru(PPh3)(Hp) (SB= 0 (Fig. 1.41) Ru(PPh3)(H30) (SB = )/Phl0/CH3Cy4°C catalysed the asymmetric epoxidation of styrene in the dark under an inert atmosphere. Yields of epoxide were however relatively low [95], but later systems of this... 

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