Mechanism of the Synthesis of N-Phenylmaleimide and Improvement of Its Selectivityt

Abstract

Dehydration and ring closure of N-phenylmaleamic acid (PMA) to N-phenylmaleimide (PMI) in oxylene was investigated. A temperature-time plot showing a curvature was found to be divided into two parts, and the inflection point was observed at the boundary between these two parts. PMA crystals were dispersed in the organic solvent at the first half of the reaction time, and the organic phase became homogeneous at the latter half. The structure of ten types of compounds were separated from reaction mixture and identified by elemental analysis and IR, NMR, and GC/MS spectroscopies. Reactions yielding the identified compounds were proposed for the reaction stages. The following reactions are proposed for the first half of the reaction time.
1) PMI is produced by dehydration an d ring closure of PMA.
2) Hydrolysis of PMA yields maleic acid (MA) and anil ine (ANL).
3) PMI reacts with ANL to produce 2-anilino-N-phenylsuccinimide (APSI)
4) MA is dehydrated to be maleic anhydride (MAN).
5) Reaction of APSI with MAN leads to formation of N - (2, 5-dioxo-1-phenyl-3-pyrrolidinyl) -Nphenylmaleamic acid (PPMA).
6) MA isomerizes to fumaric acid (FA).
7) PMA also isomerizes to N-phenylfuma ramic acid (PFA)
During the latter half of the reaction, PPMA is mainly converted to PMI. The effects of the molar ratio of [MAN]/[ANL] and amount of catalyst on dehydration and ring-closure reactions of PMA were investigated. The reaction time of the first half increased with a decrease in the amount of catalyst, and the composition of the organic phase at the inflection point is almost constant. The amount of the catalyst influenced the degradation rate of PPMA at the latter half. APSI produced until the inflection point was converted to PPMA by adding MAN, and further reaction of PPMA to PMI after the inflection point improves the selectivity of PMI and increases the total yield of PMI.