Liu Xiuying, Qie Lujiang, Ma Zhiguang, Shan Jinhuan, Shen Shigang
(College of Chemistry & Environmental Science, Hebei University, Baoding 071002, China)

Received Apr. 8, 2002.

Abstract In this paper, N-benzylphthalimide was firstly synthesized under microwave irradiation. The influence of some factors to the product yield, such as the proportion of reaction reagents, the irradiation time and alkaline reagents K₂CO₃-Al₂O₃ was investigated and the optimum experimental conditions were found. The yield of the product could attain 98% in 420s, which not only increased the yield ,but also took much shorter time than that reaction time reported by the predecessors.
Keywords N-benzylphthalimide, Phthalimide, Benzyl Chloride, Microwave irradiation, Synthesis

1 INTRODUCTION   
The compounds of N-alkylating phthalimides are important intermediate of synthesizing the fatty primary amine and a -amino acid. The classical method is to make salt with the reaction of phthalimide and KOH, then the N-alkylating phthalimides is produced with the reaction of halogenated compound and the salt in the presence of N,N-dimethylformamide (DMF). At present some methods are reported as: the phase transfer catalytic effects of polyethylene glycols in N-alkylation^[1]; convenient synthesis of N-substituted phthalimide^[2]; the reaction of N-alkylation of phthalimide by the inorganic supported reagent^[3,4]. The above reported reaction time is longer. Take the synthesis of N-benzylphthalimide as an example: the reaction time usually takes about 10h, the fastest reaction time takes 4 h and the highest yield is 94%.
    In recent years, it is reported that the speed of chemical reaction by heating chemicals with microwave is times or even thousands times faster than that of general chemical reactions. Since Canadian chemist Richard Gedye and others reported to make chemical reactions with microwave oven in 1986, the studies on synthetic chemistry of using microwave technology become more and more common^[5].This paper is reported on the study of the conditions to synthesize N-benzylphthalimide with microwave irradiation to raise the yield and speed up the reaction. It has got the better results.

3 RESULTS AND DISCUSSION
3.1 Influence of the proportion of reaction reagents to yield
The relationship between the proportion of reaction reagents and yield was shown in Table 1. When phthalimide: benzyl chloride =1:1 (molar ratio), the yield was very low; when phthalimide: benzyl chloride =1:2 (molar ratio), the maximum yield (98%) could be obtained. But when benzyl chloride was increased further, the yield did not change.

    As could be seen in it, the yield was 80% when the continuous irradiation time was 330s,and that was 98% when it was 420s. It should be noted that the yield decreased as the products obtained changed color or became coked if the reaction time was longer once more.

3.3 Influence of the supported alkaline reagents K₂CO₃-Al₂O₃ to the yield                       
The reagent Al₂O₃ was selected from Huamao Plant of Shanghai (referred as A) and No.3 Chemical Reagent Plant of Tianjin (referred as B). The two kinds of supported alkaline reagents K₂CO₃-Al₂O₃ (A) and K₂CO₃-Al₂O₃ (B) were made by the same method described above and the experiments had been separately carried out with these two kinds of alkaline reagents. The results were shown in the Table 3.

    As it could be seen in Table3, the product yield could attain 98% when 5g K₂CO₃-Al₂O₃ (A) was used; When it was less than 5g , the yield was lower; and when it was more than 5g, the color of products would become yellow and the yield was much lower. When 5g K₂CO₃-Al₂O₃ (B) was added, the products would be coked. This would be caused by the larger surface of Al₂O₃ (B), which absorbed too much microwave energy. When K₂CO₃-Al₂O₃ (B) added was less than 5g, the yield was not high. The results described above showed that it was suitable to use 5g K₂CO₃-Al₂O₃ (A) as supported carrier in the experiment.

4 CONCLUSION   
In the paper, N-benzylphthalimide was successfully synthesized under microwave irradiation and the optimum conditions were obtained (Phthalimide: Benzyl Chloride: K₂CO₃-Al₂O₃=1:2:1.2 under continuous irradiation for 420s in 750W microwave oven with suitable amount DMF as solvent). The yield could attain 98%, it was the highest value, and the reaction time needed was only 420s, which was much shorter than the fastest time (4h) reported by others^[1]. In addition, the method was easy to operate, the inorganic supported reagent could be recovered, and the pollution could be reduced as well.