Styrenes oxidative rearrangement

Some related reactions are worth mentioning in this context. Addition of allylnickel bromide to styrene oxide to give an alcohol has been reported (example 7, Table IV). Tsutsumi has described the Darzens-type reaction of two molecules of a-bromoketones to give dimethylfurans (example 8, Table IV). This reaction consists of the addition of the ketomethylenic group to the carbonyl group of another molecule, followed by epoxide formation and bromide elimination. A subsequent rearrangement leads to a dialkylfuran. 

In contrast to the examples reported above, some other aryl- and vinyl-stabilized lithiooxiranes show a strong electrophilic behavior and undergo rearrangement reactions (Scheme 83, see also Section V.A.2.a for other examples). Lithiated styrene oxide has been engaged in 1,2-metallate rearrangement with zirconacycles . 

Varied oxidizing agents may be employed in the laboratory synthesis of carbonyl compounds. Styrenes undergo oxidative rearrangement when treated with Pb(OAc)4 in CF3COOH, but only aldehydes are produced in good yields 556... 

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. 

The aim of the present paper is to demonstrate the involvment of both internal and external site involvment in the rearrangement of styrene oxide and substituted derivatives over various modified zeolites. 

Styrene oxide is easily and quantitatively rearranged into phenylacetal-dehyde over the zeolites mentioned above in relatively mild conditions (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. 

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). 

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)]. 

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. 

Examples of intramolecular trapping of carbonyl ylide dipoles by alkenes have now been reported.These include, for example, the conversion of the oxirane (172) into the tetrahydrofuran (173). Carbonyl ylides have also been prepared by irradiation of 2,3-bis-(p-methoxyphenyl)oxirane in the presence of dicyanoanthracene as electron-transfer sensitizer direct or triplet-sensitized irradiation, however, leads mainly to rearrangement via carbon-oxygen bond cleavage. In contrast, cyclohexene oxide and styrene oxide, on naphthalene-sensitized irradiation in alcohols, undergo solvolysis via oxide anion-radical intermediates. ... 

When using conventional homogeneous Lewis or Br0nsted acidic catalysts only liquid-phase reactions are applicable. With heterogeneous catalysts gas-phase reactions, which are readily performed continuously, can also be realized. The product is readily separated from the catalyst and higher efficiency is usually achieved (space-time yield). The rearrangement of styrene oxides in the gas phase described later in this section [8,15,16] is an example of the improvement of yields by changing the reactor concept from liquid- to gas-phase. 

The rearrangement of styrene oxides to aldehydes (Figure 2) yields valuable compounds or intermediates for the produetion of fragranees, pharmaceuticals, and insecticides, fungicides, and herbicides, particularly when halogenated derivatives are needed. 

The gas-phase reactions of styrene oxide derivatives also gave very good results. 2-Methylstyrene oxide is, however, rearranged to a mixture of 2-phenylpro-panal and phenylacetone with selectivity of approx. 20% aldehyde and 60% ke-... 

Liquid-phase rearrangements of styrene oxides and their derivatives over zeolites in their H-form were deseribed by K. Smith et al. [24]. They obtained poor results with protic solvents such as methanol (30% conversion of styrene oxide). Use of chlorinated aprotic solvents sueh as dichloromethane led to higher yields of phenylacetaldehyde (74 % over H-ZSM-5 and 78 % over H-/8 at 20 °C,... 

The rearrangement of styrene oxide can also be performed in liquid-phase reactions with the well known catalyst TS-1 [11,25]. The framework-titanium gives the MFI-type zeolite Lewis acidic properties and 100 % conversion and 98 % phenylacetaldehyde selectivity was achieved after 1-2 h at 70 °C in a batch reaction with acetone (100 mL) as solvent, epoxide (50 g), and catalyst (3 g) as feed. 

Because TS-1 is also a catalyst for oxidation reactions it can be used for liquid-phase bifunctional catalysis of the oxidation of styrene to styrene oxides with H2O2, then in situ rearrangement of the styrene oxides to phenylacetaldehydes... 

By analogy with the rearrangement of styrene oxide compounds, more complicated substances, e. g. phenylpyruvic acid methyl ester derivatives, can be synthesized from readily available glycidic acid esters, as shown in Figure 3. These esters can be used as intermediates for herbicides (e. g. the triazinones) and for the synthesis of L-amino acids. 

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