2-氯-5-三氯甲基吡啶电化学氢化脱氯合成2-氯-5-甲基吡啶

Electrochemical hydrodechlorination of 2-chloro-5-trichloromethylpyridine to 2-chloro-5-methylpyridine

2-氯-5-三氯甲基吡啶(TCMP)选择性氢化脱氯制备2-氯-5-氯甲基吡啶(CCMP)或2-氯-5-甲基吡啶(CMP)在农药"吡虫啉"合成中具有重要应用价值。首先在弱酸性的甲醇/乙酸/水混合溶剂中研究了TCMP电化学脱氯合成CCMP或CMP的可行性;其次,研究了阴极材料和电解液组成对TCMP选择性脱氯反应的影响;最后,采用膜厚度极距的板框式电解槽分别研究了阴、阳极支持电解质对电解槽压和电流密度与底物浓度对脱氯反应效率的影响。实验结果表明,在弱酸性的甲醇/乙酸/水混合溶剂中,TCMP能在银网阴极上高选择性的氢化脱氯成CMP;CMP收率从高到低的阴极依次为:银>铜>锌>铅>钛>石墨>镍。阴阳极支持电解质从四丁基高氯酸铵分别换成乙酸锂和硫酸,电解槽压大幅度下降。降低电流密度和提高底物浓度有利于TCMP电化学氢化脱氯效率。在优化条件下(阴极液:含10%乙酸+5%水+0.2 mol·L^(-1)乙酸锂的甲醇溶液;阴极:银网;电流密度:333 A·m^(-2);温度:30℃),0.2 mol·L^(-1) TCMP能高效地转化为CMP(收率:91%),电流效率可达54%,电解槽压大约为3.0 V。

Electrochemical hydrodechlorination of 2-chloro-5-trichloromethylpyridine （TCMP） for the synthesis of 2-chloro-5-chloromethylpyridine （CCMP） or 2-chloro-5-methylpyridine （CMP） has important application in preparation of imidacloprid. Feasibility of electrochemical hydrodechlorination of TCMP to CCMP or CMP was first investigated in a weak acidic solvent mixture of methanol, acetic acid and water. Then, influence of cathode material and catholyte composition on electrochemical hydrodechlorination was studied. Finally, influence of supporting catholyte and anolyte on cell voltage and influence of current density and TCMP concentration on efficiency of electrochemical hydrodechlorination were investigated in a plate and frame cell. The experimental results show that： TCMP can be selectively hydrodechlorinated to CMP on a silver mesh cathode in the weak acidic solvent mixture, which cathode material with CMP production rate from high to low is silver, copper, zinc,lead, titanium, graphite, and nickel. Cell voltage is decreased substantially when lithium acetate （CH3COOLi） and sulfuric acid are used to replace tetrabutylammonium perchlorate （TBAP） as supporting cantholyte and anolyte. Efficiency of electrochemical hydrodechlorination is increased by reducing current density and raising initial concentration of TCME Under optimized condition （catholyte： 0.2 mol ·L^-1 CH3COOLi in 10%（vol） CH3COOH and 5%（vol） methanol aqueous solution. Cathode silver mesh, current density 333 A · m^-2, temperature 30℃）. 0.2 mol · L^-1 TCMP is hydrodechlorinated to CMP with yield of 91% at current efficiency of 54% and cell voltage of 3.0V.