Study On The Synthesis Of Piperonal

Piperonal is a synthetic perfume with the fragrance of jasmine,and also the synthetic intermediate of many perfumes,medicines and pesticides.At present,the industrial synthesis of piperonal mainly suffers from high production input and large industrial waste.Therefore,it is important to develop a green and energy saving method for the synthesis of piperonal.In this work,the major component safrole of Cinnamomum subavenium leaf essential oil was used to synthesize the isosafrole in presence of NaOH-KOH/PEG,and then isosafrole was converted into piperonal by electrochemical oxidation(indirect oxidation or direct oxidation).The results of the study are as follows:1.Synthesis of isosafroleThe Cinnamomum subavenium leaf essential oil was purified by column chromatography to obtain 99.5 wt%safrole.Isosafrole catalytized by NaOH:KOH(wNaoH:wKOH=1:2)was synthesized by isomerization in presence of PEG,and the structure of isosafrole was characterized using FT-IR and NMR.The effect of PEG type,reaction temperature,reaction time and base dosage on the yield of isosafrole was investigated by a single-factor test and orthogonal test.The optimum process conditions were obtained:the dosage of PEG800 was 9 wt%,the reaction temperature was 160℃,the reaction time was 3 h and the base dosage was 6 wt%.Under these conditions,the yield of isosafrole was 93.1%.2.Synthesis of piperonal by indirect electrooxidationPiperonal was synthesized from isosafrole by indirect electrochemical oxidation.The products were quantitatively and qualitatively analysed,and characterized by GC,TLC,FTIR and NMR.The optimum reaction conditions were obtained by single-factor test,and the electrochemical behaviour of the electrocatalyst and organic base additives in this reaction system was analysed by cyclic voltammetry.The platinum mesh electrode used as the auxiliary electrode,the optimum reaction conditions for synthesis of piperonal in an oxygen atmosphere were obtained as follows:N-hydroxyphthalimide(NHPI)as the electrocatalyst,conductive carbon paper as the working electrode,acetonitrile as the solvent,0.75 mmol 2,6-lutidine as the organic base additive,the reaction temperature of 45℃,the current strength of 5 mA and n-Bu4NClO4 as the supporting electrolyte.Under these reaction conditions,the yield of piperonal was 55.2%.To investigate the conversion process of this reaction,a density flooding theory(DFT)study was carried out to calculate the full reaction pathway,and the reaction mechanism was proposed according to the calculation results.The results show that there are three reaction pathways in this reaction system,among which the main product and the by-product compete,resulting in poor selectivity and low yield in the reaction system.3.Synthesis of piperonal by direct electrooxidationPiperonal was synthesized from isosafrole by direct electrochemical oxidation.The products were quantitatively and qualitatively analysed by GC,TLC.The optimum reaction conditions were obtained by single-factor test,oxidation potential of substrate and the electrochemical behavior of the electrode,slovent and organic base additive in this reaction system was analysed by cyclic voltammetry.The platinum mesh electrode used as the auxiliary electrode,the optimum reaction conditions for synthesis of piperonal in an oxygen atmosphere were obtained as follows:conductive carbon paper as the working electrode,acetonitrile as the solvent,0.6 mmol pyridine as the additive,the current strength of 10 mA and n-Bu4NC1O4 as the supporting electrolyte.Under these reaction conditions,the yield of piperonal was 85.9%.To investigate the electrocatalytic active site,a density flooding theory(DFT)study was carried out to calculate the spin density,and the reaction mechanism was proposed accoring to the calculation results.The results show that the highest spin density of the substrate positive ion is at β Carbon atom.Compared with the existing research reports,the electrochemical oxidation method used in this work has the advantages of low cost and low pollution,which provides a certain reference for the industrial synthesis of piperonal.