Electro-oxidative cyanidation of C—H bond by chloride ion in organic aqueous solution

doi:10.11949/0438-1157.20220023

Abstract

Abstract:

Most organic reaction substrates in organic electrochemical synthesis are insoluble in water, and it is often necessary to add an appropriate amount of organic solvent to form an organic-water mixed solvent. As a mediator, chloride ion (Cl^-) has attracted extensive attention in organic electrochemical synthesis, but it is often soluble in aqueous solution. Solvent is a very important factor in electrochemical system, therefore, the electrochemical properties of Cl^- in organic-H₂O mixed solvent should be studied systematically. In this paper, the kinetic parameters of Cl^- in organic aqueous solution were investigated by cyclic voltammetry (CV), linear scanning method (LSV) and chronocoulometry. The effects of anode material, scanning rate, water content, and organic solvent on the electrochemical behaviors of Cl^- were studied systematically. The apparent activation energy (E_a) of the electro-oxidation of Cl^- on Pt electrode was measured by constant overpotential method. At the same potential difference, the E_a decreased more rapidly at lower potential than that at high potential. Using Cl^- as a mediator, the indirect electrochemical cyanidation of p-methoxytoluene (p-MeOBT) under the condition of 12.5 mA·cm^-2 current density, 60℃ in CH₃CN-H₂O (volume ratio 7∶3) organic aqueous solution containing 0.15 mol·L^-1 hydroxylamine sulfate and 0.05 mol·L^-1 tetrabutyl ammonium perchlorate (TBAP) was studied. The yield of p-methoxybenzonitrile (p-MeOBCN) was 80%. A possible mechanism of the indirect electro-oxidative cyanidation of C—H bonds was proposed by monitoring the intermediate species in the electrochemical cyanidation process. Cl^- showed good catalytic performance in the indirect electro-oxidative cyanidation of C—H bonds, which provided the experimental and theoretical basis for the use of inorganic mediators in the indirect electrochemical conversion of organic substrate.

Key words: chloride ion, kinetics, electrolysis, indirect electro-oxidation, C—H activation, cyanidation

摘要：

有机电化学合成中大部分有机反应底物不溶于水，往往需要添加适量的有机溶剂形成有机-水混合溶剂。氯离子作为无机媒质在有机电化学合成中应用广泛，但氯离子往往溶解于水相中。而溶剂是电化学合成中的一个重要影响因素，因此很有必要对氯离子在有机-水混合溶剂中的电化学性能进行系统研究。采用循环伏安法(CV)、线性扫描法(LSV)、计时电量法研究了氯离子在有机-水混合溶剂中的动力学参数，考察了阳极材料、扫描速率、混合溶液中水的含量、有机溶剂的种类等对氯离子电化学氧化性能的影响。并以氯离子为媒质，在无隔膜电解槽中，通过间接电化学活化对甲氧基甲苯(p-MeOBT)甲基上的C—H键原位生成醛，并进一步转化为相应的腈类化合物。以p-MeOBT为反应底物，在乙腈-水混合溶液(体积比7∶3)中，恒电流电解(CCE) 12 h，60℃下，目标产物对甲氧基苯甲腈(p-MeOBCN)的收率达80%。通过对反应过程中各中间物种的监测，提出了可能的C—H键间接电腈化腈化反应机理。

关键词: 氯离子, 动力学, 电解, 间接电氧化, C—H活化, 腈化反应

CLC Number:

O 646

Youqun CHU, Zhanbang GE, Yufeng JIAO, Jianping ZHANG, Guanxuan GUO, Yinghong ZHU. Electro-oxidative cyanidation of C—H bond by chloride ion in organic aqueous solution[J]. CIESC Journal, 2022, 73(7): 3018-3025.

褚有群, 葛展榜, 焦玉峰, 张建平, 郭冠璇, 朱英红. 有机-水混合溶剂中氯离子对C—H键的电氧化腈化性能[J]. 化工学报, 2022, 73(7): 3018-3025.

Figures/Tables 15

Fig.1 The cyclic voltammograms of Cl- on different working electrodes (v = 50 mV·s-1)

Fig.2 The cyclic voltammograms of Cl- in different organic-aqueous solvents(v = 50 mV·s-1, Pt electrode)

Fig.3 The cyclic voltammograms of Cl- in different water contents of CH3CN-H2O solution (v = 50 mV·s-1)

Fig.4 The cyclic voltammograms of Cl- at different scanning rates (v = 50 mV·s-1)

Table 1 |Iox / Ire| of Cl- at different scanning rates

v/(mV·s^-1)	I_ox/mA	I_re/mA	$I o x ? / ? I r e$
25	2.42×10^-1	-9.23×10^-3	26.22
50	4.80×10^-1	-1.05×10^-2	4.57
100	5.37×10^-1	-1.61×10^-1	3.34
150	6.53×10^-1	-2.80×10^-1	2.33
300	8.20×10^-1	-3.61×10^-1	2.27

Table 1 |Iox / Ire| of Cl- at different scanning rates

v/(mV·s^-1)	I_ox/mA	I_re/mA	$I o x ? / ? I r e$
25	2.42×10^-1	-9.23×10^-3	26.22
50	4.80×10^-1	-1.05×10^-2	4.57
100	5.37×10^-1	-1.61×10^-1	3.34
150	6.53×10^-1	-2.80×10^-1	2.33
300	8.20×10^-1	-3.61×10^-1	2.27

Fig.5 The linear sweep of Cl- under different temperatures (v = 10 mV·s-1)

Fig.6 The relationship between lgJ and1/T

Table 2 The apparent activation energy of Cl- at different potentials

E/V	E_a/(kJ·mol^-1)
1.20	35.020
1.25	33.526
1.30	31.133
1.40	28.318

Fig.7 Thechronocoulometry of Cl- in different water contents of the CH3CN-H2O solution

Fig.8 The relationship between Q and t1/2

Fig.9 The diffusion coefficient of Cl- in different water contents of the CH3CN-H2O solution

Fig.10 The cyclic voltammograms of p-MeOBT in CH3CN-H2O mixed solution (v = 50 mV·s-1)

Table 3 Electrolysis results of p-MeOBT in different concentration of NaCl

Entry	NaCl /(mol·L^-1)	Yield/%					Conv./%
Entry	NaCl /(mol·L^-1)	p-MeOBOH	p-MeOBA	p-MeOBO	p-MeOBCN	p-MeOBCl	Conv./%
1	0.00	19	12	27	28	0	91
2	0.05	12	10	24	39	3	92
3	0.10	1	6	11	72	8	97
4	0.15	1	3	2	80	12	98
5	0.20	2	3	4	71	19	99

Fig.11 The contents change of each organics during electrolysis

Fig.12 Plausible reaction mechanism of the electro-oxidative cyanidation of p-MeOBT catalyzed by Cl-

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