Styrene oxides, isomerization

In the case of zeolites, if it can be assumed that the weak external sites are active enough to catalyze the easy styrene oxide isomerization, the participation of intracrystal1ine acidic sites cannot be excluded (ref. 17). 

The styrene oxide isomerization is known to be an easy reaction due to the carbonium stabilization by the aromatic nucleus. In the case of H-ZSM-5, taking into account the respective size of this medium-pore zeolite (5.5A) and the kinetic diameter of the styrene oxide molecule (5.9A), it was assumed that the weak external acidic sites are active enough to catalyze the reaction (ref. 16). If this were the case for all zeolites, no shape-selectivity could be obtained for any epoxide rearrangement. Nevertheless, for large-pore zeolites, the contribution of all the acidic sites cannot be excluded. 

In order to check the possibility of intracrystalline catalysis in the styrene oxide isomerization, we first studied this reaction over Y large pore zeolites, external sites of which were made inactive. 

In the case of the H-offretite, a similar decrease in the rate of styrene oxide isomerization over the modified zeolite compared to the untreated one is also observed, as shown in Table 4, but the reaction is not totally inhibited even with highly silanated catalyst. 

On the another hand, over the modified zeolite, the rearrangement of the substituted epoxide is slightly slower than the styrene oxide isomerization (vQ = 0.2.10- versus 0.3.10 3 mol.mn. g"1). 

Amidocarbonylation methodology can also be applied to the synthesis of N-acclyl- (D,L)-phenylalanine, a key intermediate for aspartame (1-aspartyl-l-phenylalanine methylester), from styrene oxide (via isomerization to phenac-etaldehyde) [24] or benzyl chloride [25] in good yields. 

Such stability is only relative, however, given the possibility of the acid-catalyzed 1,2-shift of a proton observed in some olefin epoxides of general structure 10.10 (Fig. 10.3) [12], Such a reaction occurs in the in vivo metabolism of styrene to phenylacetic acid the first metabolite formed is styrene oxide (10.10, R = Ph, Fig. 10.3, also 10.6), whose isomerization to phenyl-acetaldehyde (10.11, R = Ph, Fig. 10.3) and further dehydrogenation to phenylacetic acid has been demonstrated by deuterium-labeling studies. A com-... 

Phenylacetaldehyde can be obtained in high yield by vapor-phase isomerization of styrene oxide, for example, with alkali-treated silica-alumina [147]. Another process starts from phenylethane-l,2-diol, which can be converted into phenylacetaldehyde in high yield. The reaction is performed in the vapor phase in the presence of an acidic silica alumina catalyst [148]. 

Treatment of tt-methylstyrene oxide with f ri-butylmagnemuii chloride or phenylmagneaium bromide has been reported 24 171 p. yield 4,4-dimethyl 2phemyl-S-pentanol and 1.2-diphenyl-1-propanol respectively (Eq. 843). Both products presumably arise from methyl-phenylsoetsldehyde, formed by preliminary isomerization of a-mcthyl-styrene oxide. 

Styrene oxide and a-methylstyrene oxido undergo exclusive isomerization to phenylauetaldehyde and mcthylphenylacetaldehyde respectively (Eq. 447) under the action of various Lewis acids 2-ttl. jen. ini Hydride shifts are dearly dominant here. 

When the ethylene oxide contains an aromatic substituent, as in styrene oxide, there is a significant tendency for preliminary isomerization to oocur. Thus, treatment of styrene oxide with methyl-magnesium bromide or ethylmagnesium bromide yields 1 -phenyl-2-propauol and l-phemyJ-2-butanol respectively1 83 (Eq. 841). 

Styrene oxide and certain di- and trisubstituted epoxides cannot be assayed satisfactorily by titration with arid on account of their tendency to undergo isomerization to carbonyl compounds. 

The rearrangement of styrene oxide into phenyl acetaldehyde was studied over various zeolites (H-ZSM-5, HY, H-offretite). It was first shown that both external and internal acidic sites are involved in that easy isomerization. Moreover, a comparative study of the rearrangement of this epoxide and of its hindered analog, 1-phenyl-1,2-epoxycyclohexene, on silanated offretite, allowed a discrimination between the activities of these sites. 

In all the isomerization reactions carried out in heterogeneous conditions, the nature of the products and product ratio depended largely on the type of catalyst employed, and, moreover, in most of the cases no selectivity was found. Papers have recently appeared concerning the transformation of styrene oxide into phenyl acetaldehyde catalyzed by a series of natural silicates and amorphous silica-alumina (ref. 15) and by pentasil type zeolites (ref. 16). It is said that, in both cases, isomerization occurs on the acidic sites (si lands) of the external surface, which act as active centers even under mild experimental conditions. 

Isomerization of substituted styrene oxides allows the synthesis of aldehydes in high yields726 [Eq. (5.275)]. Cycloalkene oxides do not react under these conditions, whereas 2,2,3-trimethyloxirane gives isopropyl methyl ketone (85% yield). Isomerization of oxiranes to carbonyl compounds is mechanistically similar to the pinacol rearrangement involving either the formation of an intermediate carbocation or a concerted mechanism may also be operative. Glycidic esters are transformed to a-hydroxy-/3,y-unsaturated esters in the presence of Nafion-H727 [Eq. (5.276)]. 

In the isomerization of styrene oxides in a fixed bed reactor under gas phase conditions, the catalytic performance of various catalysts on the activity, selectivity and service time was screened at 300°C and WHSV = 2-3h" . As shown in Fig. 15.1, zeolites with MFI-structure are superior to other zeolite types and non zeolitic molecular sieves, as well as greatly superior to amorphous metal oxides. 

If the epoxide rearrangement (see chapter 15.2.1) of styrene oxide is carried out in the presence of hydrogen and by use of a bifunctional boron-pentasil zeolite catalyst having a hydrogenation component such as Cu, then 2-phenylethanol is obtained in one step. This hydro-isomerization renders high yields (> 85%) at 250 °C under the gas phase conditions. It is an example for multifunctional catalysis in a one pot-reaction, that means simultaneous rearrangement and hydrogenation. 

When an unsymmetrical secondary alcohol is formed, depending on which carbon-oxygen bond is cleaved. With propylene oxide, for example, a base-catalyzed reaction favors the formation of the secondary alcohol almost exclusively, whereas, a non-catalytic or acid-catalyzed alcoholysis yields a mixture of the isomeric ethers. However, the reactions of other a-epoxides, such as 3,4-epoxy-l-butene, 3,4-epoxy-l-chloropropane (epichlorohydrin), 3,4-epoxy-l-propanol (glycidol), and styrene oxide, are more complicated with respect to which isomer is favored. ... 

However, we have demonstrated the formation of a metallacycle [(dipy)(Cl)Rh-(0-CH2-CHPh) or (dipy)(Cl)Rh-(0-CHPh-CH2)] from styrene and dioxygen. These intermediates could give rise to both styrene oxide and the carbonate. The higher reaction rate when starting from styrene and dioxygen with respect to the epoxide can be, thus, justified. High temperature (> 353 K) often cause decomposition of the catalyst. Two mutually free cis positions are necessary for the formation of the metallacycle, that interacts with carbon dioxide and yields the carbonate so, in the presence of Rh(diphos)2Cl and Rh(dipy)2Cl, no conversion at all into the carbonate has been observed, either starting from styrene or from styrene oxide. In the latter case, only a minor isomerization into acetophenone and phenylacetaldehyde has been observed. 

Epoxides can be isomerized to carbonyl compounds. Industrial examples include the isomerization of styrene oxide to phenylacctaldehyde and that of a-pinene oxide to campholenic aldehyde. As Ti-containing zeolites as TS-1 [77J and the Ti-Beta [78] are good catalysts for these isomerizations it is tempting to combine the epoxidation and the isomerization step. Several substituted styrenes have been subjected [79[ to such a two-step one-pot procedure. 

The reaction of the lithium phosphide 7 with styrene oxide eliminates siloxide to give an isomeric mixture of phosphiranes dominated by the E-isomer (equation 16)24. 

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