Styrenes epoxidation

Scheme 5.3 Biocatalytic (a) and chemical catalytic (b) styrene epoxidation.

Bis(pyrazolyl)borate copper complex 47 has been employed as a catalyst in the homogeneous and heterogeneous styrene epoxidation reactions <00JCS(CC)1653>. Pyrazole palladacycles 48 have proven to be stable and efficient catalysts for Heck vinylations of aryl iodides <00JCS(CC)2053>. An asymmetric borane reduction of ketones catalyzed by N-hydroxyalkyl-/-menthopyrazoles has been reported <00JHC983>. 

Fig. 51. Correlation between the intensity of Ti-superoxo ([A + A] and [B + C]) signals and selectivity for styrene oxide and non-selective products in the styrene epoxidation reaction. The effects of titanosilicates, oxidants, and solvent on the correlation are depicted [from Srinivas et al. (52)].

Closely related substrates such as )5-methyl styrene epoxide and dihydro-naphthalene epoxide and derived trans diols were not substrates for this particular microorganism. 

Bromine-atom atomic resonance absorption spectrometry (ARAS) has been applied to measure the thermal decomposition rate constants of CF3Br in Kr over the temperature range 1222-1624 K. The results were found to be consistent with recently published theory. The formation of cyclopent[a]indene and acenaphthylene from alkyl esters of biphenyl-mono- and -di-carboxylic acids has been observed in flash vacuum pyrolyses at 1000-1100 °C. The kinetics and mechanisms of free-radical generation in the ternary system containing styrene epoxide, / -TsOH, and i-PrOH have been examined in both the presence and absence of O2. ... 

Figure 5.5 A Portion of the NMR Spectrum of Styrene Epoxide (100 MHz as a 5% solution in CCI4)...

Styrene epoxide on reaction with 3-buten-l-ol in the presence of a catalytic amount of BiCl3 gave two possible isomers, of which the m-isomer was found to be the major one [25] (Fig. 1). The scope and versatility of the method is depicted in... 

Epoxidation of propene and oct-l-ene was effected with tran5-Ru(0)j(TMP)/ O ll atm)/water-CH2Cl2. After some 40 turnovers in a day, the deactivated form of the complex, Ru(C0)(TMP).H20 was detected (vide infra). Use of (l-( C)-oct-l-ene suggest that, in part at least, the carbon atom of the Ru(CO)(TMP) formed derives from the first C atom of the octene [591]. For styrene epoxidation by trans-Ru(0)2(TMP)/(LN0)/CgH (LNO=N-oxides of 2,3,5,6-tetramethylpyrazine, acridine, 2-methylquinoline and 3,6-dichloropyridazine) the mono- and bis-A-oxides of tetramethylpyrazine were the most effective co-oxidants [586]. 

Gao and Friend showed that chlorine is an effective promoter for enhancing the selectivity of styrene epoxidation on Au(l 11) by inhibiting secondary oxidation [247]. 

Several extensions of these reactions are possible with respect to the electrophilic reagent as well as to the structure of the adduct. Thus functionalized alkyl groups, such as 2-hydroxyethyl and 2-hydroxy-2-phenylethyl, can be introduced into the pyridine ring at the position by treating l-lithio-2-phenyI-l,2-dihydropyridine with ethylene epoxide and styrene epoxide, respectively.144 When polyhalides such as CF3I are used, bis-(substituted-pyridyl)methanes and the dimeric substituted dipyridyls are obtained along with other products.144... 

In some cases, oxidation of double bonds does not stop at the epoxide, but proceeds further to oxidative cleavage of the double bond. It was reported that the reaction of a-methyl styrene with H2O2 in the presence of TS-1 or TS-2 produces a-methyl styrene epoxide (15%), a-methyl styrene diol (10-40%) and acetophenone (40-60%) (Reddy, J. S. et al., 1992). However, results similar to those obtained with titanium silicates were obtained for many other catalysts, such as HZSM-5, H-mordenite, HY, A1203, HGa-silicalite-2, and fumed Si02. These materials have much different properties and differ significantly from titanium silicates thus, the results cast some doubt on the role of the catalyst in this reaction. Furthermore, the oxidation of styrene is reported to proceed with C=C cleavage and formation of benzaldehyde, in contrast to previous reports of the formation of phenylacetaldehyde with 85% selectivity (Neri et al., 1986). 

The reinvestigation of this reaction has shown that styrene epoxide is formed as the initial product, but it rapidly undergoes isomerization to phenylacetaldehyde. In the presence of solvent methanol, however, addition of the solvent to the epoxide produces 32.8% of 2-methoxy-2-phenylethanol, decreasing the phenylacetaldehyde yield. In all cases the reaction is accompanied by further oxidation to benzaldehyde (Kumar et al., 1995). 

Sharpless epoxidation of the alkenylsilanol 214 gave, after protodesilylation of the silyl epoxide 215, styrene epoxide 216 in 95% e.e. the stereochemical course of the reaction follows that predicted by Sharpless for allylic alcohols341. 

The catalytic properties, in particular in acetone, are excellent and are, e.g. for cyclohexene and styrene epoxidation, comparable as well/1101 The use of a filtration experiment points to the heterogeneous nature of the catalyst. In such... 

IR and UV-vis spectroscopy show that the majority of the encapsulated complexes still have structural integrity. As observed for many other oxidation catalysts, entrapment shows significantly enhanced activity. This effect is more pronounced when the degree of ligand substitution with Cl is higher. The simultaneous occurrence of two mechanisms is confirmed again, the selectivity of the encapsulated Mn(salen) complexes for the formation styrene epoxide is only 30%, the dominant product being benzaldehyde. 

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