Citraconic anhydride

A large variety of bisimides and polymers containing maleimide and citraconimide end groups have also been reported (21—26). Thus polymers based on bisimidobenzoxazoles from the reaction of maleic anhydride and citraconic anhydride with 5-aniino-2-(p-aniinophenyl)benzoxazole and 5-aniino-2(y -aniinophenyl)benzoxazole are found to be thermally stable up to 500°C in nitrogen. 

Citraconic anhydride [616-02-4] M 112.1, m 8-9°, b 47°/0.03mm, 213°/760mm, d4 1.245, ng 1.472. Possible contamination is from the acid formed by hydrolysis. If the IR has OH bands then reflux with AC2O for 30 min, evaporate then distil the residue in a vacuum otherwise distil in a vacuum. Store in a dry atmosphere. [Biochem J 191 269 1980.]... 

The major by-products of this process are maleic anhydride, benzoic acid, and citraconic anhydride (methylmaleic anhydride). Maleic anhydride could be recovered economically. ... 

Whereas maleic anhydride can react with furan (139a) at ambient pressure, citraconic anhydride (140) reacts only at high pressures due to the strong deactivating effect of the methyl group (Schemes 5.21 and 5.22). The two-step synthesis [53] of the palasonin (141), in an overall yield of 96 %, is a good example of the acceleration of the Diels-Alder by high pressure (Scheme 5.21). Previous synthesis [54] based on the thermal Diels-Alder reaction of furan with methoxy carbonyl maleic anhydride required 12 steps. 

A convenient alternative to LP-DE is lithium trifluoromethanesulfonimide (LiNTf2) in acetone or diethyl ether (LT-AC, LT-DE). Representative examples are the Diels-Alder reactions of citraconic anhydride with cyclopen-tadiene and of dimethyl acetylenedicarboxylate with isoprene [47] (Scheme 6.26). 

Citraconic anhydride (Methyl maleic anhydride) was found to be produced from pyruvic acid by an oxidative decarboxy-condensation. The best catalyst is iron phosphate with a P/Fe atomic ratio of 1.2. The presence of oxygen is required to promote the reaction. The main side-reaction is formation of acetic acid and CO2 by oxidative C-C bond fission. The best results are obtained at a temperature of 200°C. The yield of citraconic anhydride reaches 71 mol% at a pyruvic acid conversion of 98%. 

In the reaction of lactic acid to form pyruvic acid over the iron phosphate catalysts, formation of a new compound was observed. As the extent of reaction increased, the amount of pyruvic acid increased to a maximum and then decreased, while that of the new compound increased steadily. It was therefore concluded that the new compound is formed from pyruvic acid in parallel with acetic acid and CO2. According to gas-mass analyses, the molecular weight was determined as 112. However, there are many compounds with molecular weigth of 112. After the NMR analyses and X-ray diffraction analyses for the single crystal, the new compound was determined to be citraconic anhydride, i.e., mono-methyl maleic anhydride. 

Citraconic anhydride formation from pyruvic acid by oxidative decarboxy-condensation has not been known prior to these studies. Therefore, in this paper, we attempted to get more insight into the new reaction. 

It is clear that the V-P oxide and Mo-P heteropoly compound catalysts are not effective for the production of citraconic anhydride. The acidic catalysts such as... 

Performance of oxide catalysts for production of citraconic anhydride... 

Si-P and Si-AI are not effective, either. The basic catalyst such as Si-K is very active for decomposition of pyruvic acid, but citraconic anhydride is not produced. The best results are obtained with W and W-based catalyst. The one-pass yield of citraconic anhydride reaches about 60 mol% at a pyruvic acid conversion of about 92%. The combination of P to W decreases the activity markedly. The combination of Mo or K decreases the selectivity to citraconic anhydride. The combination of Ti or Sn is scarcely effective, when the amount is less than 10 atomic %. When the amount of Sn or Ti is high, the selectivity falls. 

Since the best results were obtained with the W and W-based oxide catalysts, the reaction was studied in more detail using 20 g portions of these catalysts. The reaction was performed at 230°C, with feed rates of pyruvic acid, air, and water = 10.5, 350, and 480 mmol/h. The contact time defined as volume of catalyst (ml)/rate of gaseous feed (ml/s) was about 5.2 s. The main products were citraconic anhydride and CO2. The amount of acetic acid was very small. No other products were detected except for very small amounts of CO, acetone, and acetaldehyde. A relatively large discrepancy was observed between the amount of consumed pyruvic acid and that of the sum of produced citraconic anhydride and acetic acid. This discrepancy was defined as "loss". 

The product distributions are shown in Figure 1 as a function of the time-onstream. At the begining of reaction, that is in the first 1 h on stream, the conversion of pyruvic acid reached 92% and the yields of citraconic anhydride and acetic acid were 61 and 5 mol%, respectively. The amount of loss was about 26 mol%. 

The reaction was studied using the iron phosphate catalyst at 230°C with feed rates of pyruvic acid, air, and water = 10.5, 350, and 480 mmol/h. The main products were citraconic anhydride, acetic acid, and CO2. When the amount of catalyst used was lOg, that is, when the contact time is about 2.6 s, the conversion of pyruvic acid reached 95% and the yields of citraconic anhydride and acetic acid were 50 and 28 mol%, respectively the loss was about 17 mol%. The selectivity to citraconic anhydride is clearly lower and that to acetic acid is higher than in the case of the W-based oxide catalysts. However, the catalytic activity was very stable. No clear change in the yield of citraconic anhydride was observed during the reaction for 10 h. 

The reaction was then performed using different amounts of catalyst from 1 to 20g. The yields of citraconic anhydride and acetic acid and the loss are plotted as a... 

It should also be noted that the selectivities remain unchanged with a large variation in the extent of reaction. This indicates that the citraconic anhydride and acetic acid produced are stable enough under the reaction conditions used. 

The reaction was performed over the iron phosphate catalyst by changing the feed rate of oxygen from zero to 350 mmol/h, while fixing the sum of feed rates of oxygen and nitrogen at 350 mmol/h. The feed rate of pyruvic acid was fixed at 10.5 mmol/h. The yields of citraconic anhydride obtained at a temperature of 230°C and a short contact time of 0.52 s (amount of catalyst used = 2 g) are plotted as a function of the feed rate of oxygen in Figure 3. 

The formation of citraconic anhydride increases with an increase in the feed rate of oxygen up to about 70 mmol/h (air). However, with a further increase in oxygen feed rate, the formation of citraconic anhydride levels off. It is clear that the presence of oxygen is required to form citraconic anhydride from pyruvic acid. 

It is clear that the selectivity to citraconic anhydride increases and that to acetic acid decreases with a decrease in the oxygen concentration. 

The selectivity to citraconic anhydride decreases and that to acetic acid increases as the temperature is raised. The results indicate that the activation energy for the formation of citraconic anhydride is much lower than that for the formation of acetic acid. The selectivity to acetic acid decreases steadily with a lowering of the temperature. However, the highest selectivity to citraconic anhydride is obtained at 200°C. Possibly vaporization of pyruvic acid may become difficult at temperatures below 200°C. The yield of citraconic anhydride reached 71 mol% and that of acetic acid was 7 mol% at the pyruvic acid conversion of 98% the loss was about 20 mol%. 

Since formation of citraconic anhydride from pyruvic acid is one of "acid to acid type" transformations, such as reactions from isobutyric acid to methacrylic acid and from lactic acid to pyruvic acid, the required catalysts must be acidic [11). If the catalysts are basic, it may be impossible to obtained acidic products, because basic catalysts activate selectively acidic molecules and, as a result, they show a very high activity for the decomposition of acidic products [11]. 

The results shown in Figure 4 indicate that the oxygen dependency of the acetic acid formation is higher than that of the citraconic anhydride formation. Therefore, use of a low oxygen concentration is beneficial to the selectivity to citraconic anhydride, though it is disadvantageous to the reaction rate. 

As for the reaction path from pyruvic acid to citraconic anhydride, it is considered that a condensation reaction first takes place by a reaction between an oxygen atom of carbonyl group and two hydrogn atoms of methyl group in another molecule, followed by oxidative decarboxylation to form citraconic acid. The produced citraconic acid is dehydrated under the reaction conditions used. The proposed reaction path is shown in Figure 7. 

In order to exemplify the potential of micro-channel reactors for thermal control, consider the oxidation of citraconic anhydride, which, for a specific catalyst material, has a pseudo-homogeneous reaction rate of 1.62 s at a temperature of 300 °C, corresponding to a reaction time-scale of 0.61 s. In a micro channel of 300 pm diameter filled with a mixture composed of N2/02/anhydride (79.9 20 0.1), the characteristic time-scale for heat exchange is 1.4 lO" s. In spite of an adiabatic temperature rise of 60 K related to such a reaction, the temperature increases by less than 0.5 K in the micro channel. Examples such as this show that micro reactors allow one to define temperature conditions very precisely due to fast removal and, in the case of endothermic reactions, addition of heat. On the one hand, this results in an increase in process safety, as discussed above. On the other hand, it allows a better definition of reaction conditions than with macroscopic equipment, thus allowing for a higher selectivity in chemical processes. 

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