Isomerization of isophorone oxide

Holderich et al. obtained high yields (up to 81 %) of keto aldehyde (13) over H-FER after 6 h in the liquid phase at 110 °C with toluene as solvent [37]. H-US-Y catalysts perform well in the formation of (13). The H-US-Y (Si/Al = 48)-HCl catalyst, modified by treatment with dilute HCl (pH 2) according to the method described in the preceding section [35,38] is particularly active, even at high weight hourly space velocities. The performance of this catalytic system is comparable with that of the system described by Sheldon et al. [36]. Reaction in benzene at 80 °C for 2 h over dealuminated H-mordenite gave 100% conversion and 85 % selectivity for (13). The ratio of the two main products (13) and (14) depends on the type and acidity of the zeolite used. For example, H-BEA with a more acidic outer surface was highly active but the selectivity for (13) was lower than for H-FER or H-US-Y catalysts [37]. 

Holderich et al. also reported vapor-phase rearrangement of (12) [37]. The conversions and selectivities found for different zeolites are shown in Table 2. 

Conversions were high for all the zeolites tested except Na-ZSM-5 and silica-lite. A shorter contact time seems to have a positive effect on aldehyde formation. 

1 bar pressure, numbers in parentheses are silicon to aluminum or boron ratios. 

3 Isomerization of Isophorone Oxide - Terpene epoxides are very reactive compounds. Some products formed by isomerization of such epoxides are 

They found that at room temperature using benzene as the solvent only 3% consisted of the diketone (3) and mainly the ring-contracted products were obtained 33% of keto aldehyde (2) and 28% of ketone (4). From an industrial point of view the desired compound is the keto aldehyde (2), which is an interesting intermediate for fragrance chemicals. The acid-catalysed reaction mechanism leading to the synthesis of keto aldehyde (2) has been discussed earlier.Therefore, it is of interest whether the product distribution changes in the presence of a heterogeneous catalyst system, and also whether the decarbony-lation of the compound (2) to compound (4) can be suppressed. 

In the presence of zeolites, the keto aldehyde (2), accompanied by the keto-enolic form of the a-diketone (3), is mainly formed as reported by Sheldon et al. and Holderich et al. 

The reports on catalytic isomerization using various zeolitic catalysts in comparison with the conventional catalysts previously used gives results of reactions carried out discontinuously in a batch reactor in the liquid phase, as well as for those carried out continuously in a fixed bed reactor in the vapor phase. The results in the liquid phase over heterogeneous catalysts are summarized in Table 1. 

The ratio between the two main products (2) and (3) depends on the type and acidity of zeolite used. For example, H-BEA with a higher acid outer surface, demonstrated a high activity but a low selectivity of (2) in comparison to H-FER or H-US-Y catalysts. 

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Terpene epoxides are very reactive compounds. Some products formed by isomerization of such epoxides are valuable raw materials for perfumes, synthetic flavourings and pharmaceuticals, and also provide useful intermediates in organic syntheses. The isomerization of isophorone oxide 23 (Eq. 15.2.9) was originally investigated by H.O. House and R.L. Wasson using boron trifluoride etherate as a homogeneous catalyst (24). 

Table 15.2 Conversion and selectivities obtained with used zeolites in the liquid phase isomerization of isophorone oxide 23...

In conclusion, the use of zeolites as catalysts in the isomerization of isophorone oxide 23, yields up to 86% keto aldehyde 24. The formation of 26 by... 

Table 1 Conversion and selectivities of used zeolites in liquid phase isomerization of isophorone oxide (1)...

In conclusion, the use of zeolites as catalysts in the isomerization of isophorone oxide (1), yields up to 86% keto aldehyde. The formation of (4) by decarbonyla-tion of (2) could be reduced by increasing the catalyst loading in a liquid phase batch reactor or by conducting the reaction under short contact time in the gas phase. The heterogeneously-catalysed isomerization of teipene epoxides over zeolites is a suitable, non-polluting method of preparing relevant and useful aldehydes for the synthesis of perfumes and synthetic flavours. 

Terpene epoxides are very reactive compounds. Some are prepared conventionally by isomerization using homogeneous catalysts such as Bp3.Et20 [35,36]. The rearrangement of isophorone oxide (Figure 6) yields the keto aldehyde (13), which is an intermediate for fragrance chemicals. If the reaction is performed in the presence of zeolites the keto aldehyde (13) and the keto-enol form of the a-diketone... 

Isomerization of jS-isophorone to a-isophorone has been represented as a model reaction for the characterization of solid bases 106,107). The reaction involves the loss of a hydrogen atom from the position a to the carbonyl group, giving an allylic carbanion stabilized by conjugation, which can isomerize to a species corresponding to the carbanion of a-isophorone (Scheme 9). In this reaction, zero-order kinetics has been observed at 308 K for many bases, and consequently the initial rate of the reaction is equal to the rate constant. The rate of isomerization has been used to measure the total number of active sites on a series of solid bases. Figueras et al. (106,107) showed that the number of basic sites determined by CO2 adsorption on various calcined double-layered hydroxides was proportional to the rate constants for S-isophorone isomerization (Fig. 3), confirming that the reaction can be used as a useful tool for the determination of acid-base characteristics of oxide catalysts. 

Preliminary experiments revealed that the selectivity of titania-silica aerogel in the epoxidation of P-isophorone was moderate. The selectivity related to the olefin converted was below 90 % at low temperature, and dropped rapidly at 80 °C or above. The most important side reactions were the formation of 3,5,5-trimethyl-2-cyclohexene-4-hydroxy-l-one (2) by ring opening of the epoxide (1), and the isomerization of P- to a-isophorone (3), as shown in Scheme 2. Epoxidation of 2 and 3, and the oxidation at the OH group of 2 to a dicarbonyl compound were slow and the amounts of these by-products were usually aroimd 1 % or less. 

The importance of the acid-catalyzed side reactions are illustrated in Table 3 by the product distribution obtained using either TBHP or cumene hydroperoxide (CHP) as oxidant. The epoxidation with TBHP is faster and considerably more selective. When using CHP, about 20 mol% of the coproduct 2-phenyl-2-propanol was dehydrated to a-methylstyrene. It is likely that the simultaneously formed water increases the (Brpnsted) acidity of the aerogel and thus accelerates the ring opening and - to a smaller extent - the isomerization reactions. No oxidation products were formed in the absence of peroxide, as expected. Slow isomerization from p- to a-isophorone catalyzed by titania-silica was the only reaction observed. The data in Table 3 indicate that the simultaneous presence of peroxide and catalyst in the reaction mixture markedly accelerates the acid-catalyzed isomerization reaction. 

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