Side Reactions During Epoxidation

Scheme 10 Side-reaction during the alternating co-polymerization of epoxides with CO2.

The addition of activated molecular sieves (zeolites) to the asymmetric epoxidation milieu has the beneficial effect of permitting virtually all reactions to be earned out with only 5-10 mol % of the Ti-tartrate catalyst [3,4]. Without molecular sieves, only a few of the more reactive allylic alcohols are epoxidized efficiently with less than an equivalent of the catalyst. The role of the molecular sieves is thought to be protection of the catalyst from (a) adventitious water and (b) water that may be generated in small amounts by side reactions during the epoxidation process. 

Epoxide formation may be a side reaction occurring during initiation by t-butylperoxy radicals. The mechanism proposed for this process is as follows (Scheme 3,831, 1... 

The oxidative cyclization of vinylallenes need not be directed by a pendant hydroxyl group in order to succeed. The higher reactivity of the allene compared with the exocyclic methylene group in 73 (Eq. 13.23) with monoperphthalic acid leads primarily to the allene oxide which rearranges to cydopentenone 74 [27]. Inevitably some epoxidation of the alkene also takes place during the reaction. When m-CPBA is used as the oxidant, another side reaction is associated with m-chlorobenzoic add-mediated decomposition of the intermediate epoxide. It is possible to overcome this problem by performing the epoxidation in dichloromethane in a two-phase system with aqueous bicarbonate so as to buffer the add [28]. 

The curing rates are somewhat faster. The aziridines do not react with the secondary amines formed during the cure as the epoxides react with the hydroxyl groups. Hence, side reactions are avoided. The difficulties in producing di- and trifunctional materials of high purity have hindered the wide acceptance of aziridines except in speciality formulations. 

The behaviour in Fig. 20 was analyzed on the basis of chemical reaction during cure and the physical state of the system as a function of curing. During the early stages of curing, products may form from the epoxide groups reacting with H20, alcohol or HX to form products other than the oxymethylene units. Side reactions may also be caused by impurities. The by-products are felt to be less than 3 % and should not be responsible for the deviation from ideal behavior. 

During the opening of an epoxide, an alkoxide is liberated which may then be the cause of undesired side reactions. Ammonium chloride is often added to the reaction mixture to neutralise the alkoxide produced. 

It is well known 6 8, Ul that different side reactions may occur during the thermosetting of epoxides. This may lead to many differences in the chemical and physical structure of a final polymer. The main side reaction proceeding in a polymer under Tcure > 130 -140 °C22) is the reaction between secondary OH groups and unreacted epoxy rings ... 

Using route II [1], the desired silanes are accessible in a two-step synthesis The ft)-epoxyalkenes are obtained by partial epoxidation of the corresponding a,dienes with w-chloroperbenzoic acid. The lower product yields (47-50 %) compared to the direct epoxidation of side-reaction. The subsequent hydrosilylation requires the ethoxysilane HSi(OEt)3 as educt in order to exclude ring opening during the otherwise nescessary alcoholysis step. The lower reactivity of HSi(OEt)3 compared to chlorosilanes significantly reduces the formation of isomers but, on the other hand, considerably decreases the product yields (31-70 %). 

One interpretation of the ciurve in Figure 3 is that during the early stages of curing, the epoxide units react to form products in addition to the new oxymethylene units. The reaction of epoxy with water, alcohol or HX may consume epoxy groups without the formation of oxymethylene units (13). Another source of these side reactions may be the presence of impurities. The main crosslinking reaction may compete with the reaction of the impurities until they are consumed. However, the NMR spectra do not reflect the resonance of these new products. Although it is certain the by-products exist, they must be less than 2-3% and therefore would not account for the very substantial deviation from ideal behavior. 

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