Epoxidation of styrene and a-methylstyrene

Also in 2000, attachment of the Jacobsen catalyst to polymeric supports such as poly(ethylene glycol) and different polystyrene-based resins through a glutarate spacer was described [28]. Soluble as well as insoluble polymer-bound complexes were employed as catalysts in the epoxidation of styrene, cfs-2-methylstyrene, and dihydronaphthalene with wx-CPBA/NMO. Results were similar to those achieved with the nonsupported catalyst. Catalyst recycling was shown to be possible either by filtration or by precipitation and one catalyst system could be used for three cycles without significant loss of activity and enantioselectivity. 

Fruetel, J.A., J.R. Collins, D.L. Camper, G.H. Loew, and PR. Ortiz de Montellano (1992). Calculated and experimental absolute stereochemistry of the styrene and 3-methylstyrene epoxides formed by cytochrome P450cam. J. Am. Chem. Soc. 114, 6987-6993. 

A chiral [Mn (saZen)X] complex containing chloromethyl groups was anchored onto SBA-15, MCM-41, MCM 8 and amorphous silica previously modified with imidazole groups, Pigure 11.7. All the materials were veiy active in the epoxidation of styrene, a-methylstyrene and indene, with... 

Freire and coworkers described the intercalation of a chiral phosphonic acid derivatised [Mn (sa/e )Cl] complex within KlO-Montmorillonite layers by ion-exchange with the interlayer potassium cations. The catalytic performance of the heterogeneous catalyst (denoted as [Mn (sa/en)Cl] K10) was evaluated in the enantioselective epoxidation of styrene, a-methylstyrene and 6-cyano-2,2-dimethylchromene, at 0 °C in dichloromethane or acetonitrile, using three different oxidant systems m-CPBA/NMO, PhIO and NaOCl the results are summarised in Table 11.1. 

It is interesting to note that Uttle or no epoxide was obtained from either styrene or a-methylstyrene under conditions in which simple olefins gave good yields tert-butyl hydroperoxide, M0O3, 100 C) [372]. Instead, the products were a 4/1 mixture of a-phenylpropionaldehyde and acetophenone. Both products apparently resulted from the decomposition of the epoxide during reaction. At 60 C, however, epoxides are formed in good yield [378]. 

Such copolymers of oxygen have been prepared from styrene, a-methylstyrene, indene, ketenes, butadiene, isoprene, l,l-diphen5iethylene, methyl methacrjiate, methyl acrylate, acrylonitrile, and vinyl chloride (44,66,109). 1,3-Dienes, such as butadiene, yield randomly distributed 1,2- and 1,4-copolymers. Oxygen pressure and olefin stmcture are important factors in these reactions for example, other products, eg, carbonyl compounds, epoxides, etc, can form at low oxygen pressures. Polymers possessing dialkyl peroxide moieties in the polymer backbone have also been prepared by base-catalyzed condensations of di(hydroxy-/ f2 -alkyl) peroxides with dibasic acid chlorides or bis(chloroformates) (110). 

Ru(0)(biqn)(tmtacn)](C10 )2 and [Ru(0)(diopy)(tmtacn)](C10 )2 (biqn=C2 symmetric 1,T-biisoquinoline, diopy=(R,R)-3,3 -(l,2-dimethylethylenedioxy)-2,2 -bipyridine) aremadefrom [RuCl(L)(tmtacn)] + (L=biqn, diopy) and(NH )2[Ce(N03)J with Li(ClO ). Electronic and IR spectra were measured (v(Ru=(0) bands lie at 760 and 795 cm" respectively). The (diopy) complex is paramagnetic with 2.88 B.M. As stoich. [Ru(0)(biqn)(tmtacn)] + and [Ru(0)(diopy)(tmtacn)] VCH3CN they oxidised alkenes (styrene, cis and fran.y-P-methylstyrenes, fran -stilbene, nor-bomene, cyclohexene) to mixtures of aldehydes and epoxides. Conttary to expectation the (diopy) complex did not effect enantioselective epoxidations except with fran -stilbene, for which a moderate e.e. of 33% was observed [623]. 

RuCl(dmso)(bpy)2]Cl and [RuCl(SOMePh)(bpy) ]Cl, called respectively rac-Ru-1 and A-Ru-2 by the authors, are made by microwave irradiation of cis-RuCljtbpy) with the the sulfoxide ligands. Trani-stilbene, styrene and R-styrene (R=p-MeO, a and P-methyl) were epoxidised by [RuCl(dmso)(bpy)3]Vaq. Ph(IOAc)2/CH3Cl3/40°C, while with [RuCl(SOMePh)(bpy)3]V TBHP/water-CH Cl fran -stilbene, fran -P-methylstyrene gave the (R,R) epoxides [947]. 

In nonprotic solvents, alkenes are stoichiometrically oxidized by Vv-peroxo complexes to epoxides and consecutive oxidative cleavage products in a nonstereoselective fashion. For example, cis-2-butene gave an approximately 2 1 mixture of cis- and trans-epoxides (equation 37). The reactivity of alkenes increases with their nucleophilic nature. Alkenes containing phenyl substituents such as styrene, a- and jS-methylstyrene are also very reactive and mainly give oxidative cleavage products. 

Immobilization of a sulfonated chiral manganese-salen catalyst on a fimctio-nalized Merrifield resin yielded a remarkably active epoxidation catalyst [42]. Its activity and enantioselectivity was examined by epoxidation of 6-cyanochromene, indene, styrene, 4-methylstyrene, and trans-stilbene using m-CPBA/NMO and quantitative yields were obtained in less than 5 min. Enantioselectivities were between 33% (4-methylstyrene) and 96% ee (6-cyanochromene). The same complex was also supported on silica and a layered double hydroxide (LDEI) and the catalytic performances of the systems were compared. Recycling experiments were carried out and the silica-based system showed metal leaching combined with a significant decrease in yield and ee. The layered double hydroxide- and resin-catalysts exhibited a slight decrease in activity and constant ee values in five consecutive reactions. 

A series of polymer-anchored epoxidation catalysts was obtained by modifying Merrifield resin with imidazole [61], diphosphines [62], or piperazine [63] followed by treatment with UV-activated Mo(CO)6. High activities in the epoxidation of cyclic (cyclooctene, cyclohexene, indene, and a-pinene) as well as linear alkenes (styrene, a-methylstyrene, 1-heptene, 1-dodecene, cis- and frans-stilbene) were observed using TBHP as oxidant. The catalysts were recovered and reused up to 10 times in the epoxidation of cyclooctene without loss of activity. 

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