Isomerization of styrene oxides

1 Isomerization of Styrene Oxides The conversion of styrene oxides to phenylacetaldehydes yields a substance which can be used as fragrances directly or as valuable intermediates for the production of fragrances. In addition, such compounds can also be used for manufacturing pharmaceuticals and insecticides, fungicides and herbicides in particular when halogenated derivatives are needed. The reaction is carried out in a fixed bed reactor, e.g. under gas phase conditions. 

The high regioselectivity to phenylacetaldehydes may be related to the stabilization of a developing a-cation. A possible side product is the trimeric 

B2O3- and P205-modified silica as well as bentonite show similar selectivities as pentasil zeolites, but they deactivate much faster. The reason for that is the formation of trimers by aldol condensation and aromatisation in the case of the non-shape-selective materials. In contrast, the use of MFI-zeolites avoid the trimer formation due to the steric constraints and consecutive reactions resulting in coke. 

The aim to produce a fine chemical with 100% yield is achieved by the use of a Cs-doped boron-pentasil zeolite having an extremely weak acidity. 

If the epoxide rearrangement of styrene oxide is carried out in the presence of hydrogen and by use of a bi-functional boron-pentasil zeolite catalyst having a hydrogenation component such as Cu, then 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. 

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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]. 

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. 

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 isomerization of styrene oxide to phenylacetaldehyde yields 100% using modified ZSM-5 zeolites, thereby the highest target achieved by catalysis has been fulfilled. A new process as well has been found for the heterogeneously-catalysed production of campholenic aldehyde from a-pinene oxide. By using low reaction temperatures of 0 °C and below in combination with HCl-treated H-US-Y zeolites, up to 85% yield is achieved. This process is competitive with the homogeneous ZnBr2 system. 

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). 

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. 

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. 

The thermal or photochemical homolysis of tertiary hydroperoxides leads to the formation of alkoxy macroradicals 3-scission of alkoxy macroradicals may occur. This leads to a, 3-unsaturated ketones on the butadiene component and induces the scission of the butadiene-SAN grafts. The macroradical so formed on the SAN macrophase is the precursor, after isomerization, of the oxidation of the styrenic component according to... 

Sulphated zirconia catalysts can be acidic or superacidic depending on the method of treatment. A variety of acid-catalysed reactions, referred to earlier in this section, can be carried out with sulphated zirconia. Yadav and Nair (1999) have given a state-of-the art review on this subject. Examples of benzylation of benzene with benzyl chloride / benzyl alcohol, alkylation of o-xylene with. styrene, alkylation of diphenyl oxide with 1-dodecene, isomerization of epoxides to aldehydes, acylation of benzene / chlorobenzene with p-chloro benzoylchloride, etc. are covered in the review. 

A similar mechanism of chain oxidation of olefinic hydrocarbons was observed experimentally by Bolland and Gee [53] in 1946 after a detailed study of the kinetics of the oxidation of nonsaturated compounds. Miller and Mayo [54] studied the oxidation of styrene and found that this reaction is in essence the chain copolymerization of styrene and dioxygen with production of polymeric peroxide. Rust [55] observed dihydroperoxide formation in his study of the oxidation of branched aliphatic hydrocarbons and treated this fact as the result of intramolecular isomerization of peroxyl radicals. 

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-... 

As seen from Scheme 7.2, the epoxy-ring cleavage and nickel oxidation proceed simultaneously. The nickel-oxygen bond is formed. This results in the formation of the carbon-nickel biradical in which Ph-CH fragment can rotate freely. The cleavage of the (NiO)-C bond leads to the formation of a mixture of styrenes. At early reaction stages (30 min), cis and trans olefins are formed in 50 50 ratio. After a prolonged contact (30 h), when all possible transformations should be completed, the trans isomer becomes the main product and cis trans ratio becomes 5 95. Such enrichment of the mixture with the trans isomer follows from the formation of the di-P-(trimethylsilyl)styrene anion-radical and its isomerization. The styrene formed interacts with an excess of the nickel complex. 

Hosokawa, Murahashi, and coworkers demonstrated the ability of Pd" to catalyze the oxidative conjugate addition of amide and carbamate nucleophiles to electron-deficient alkenes (Eq. 42) [177]. Approximately 10 years later, Stahl and coworkers discovered that Pd-catalyzed oxidative amination of styrene proceeds with either Markovnikov or anti-Markovnikov regioselectivity. The preferred isomer is dictated by the presence or absence of a Bronsted base (e.g., triethylamine or acetate), respectively (Scheme 12) [178,179]. Both of these reaction classes employ O2 as the stoichiometric oxidant, but optimal conditions include a copper cocatalyst. More recently, Stahl and coworkers found that the oxidative amination of unactivated alkyl olefins proceeds most effectively in the absence of a copper cocatalyst (Eq. 43) [180]. In the presence of 5mol% CUCI2, significant alkene amination is observed, but the product consists of a complicated isomeric mixture arising from migration of the double bond into thermodynamically more stable internal positions. 

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