English

• 1.   Introduction

  Chalcogen elements (i.e. S, Se and Te) possess unique features such as nucleophilic properties, the strong coordination with metals as well as the relatively weak bond energy facilitating the free radical generation via homo bond cleavages.^[1] In the area, the organoselenium compounds attract much attention for their good chemical- and bio- activities endowing the wide applications, such as the applications in medicinal chemistry, ^[2] organic synthesis^[3] and materials science.^[4] The unfolding investigations on organoselenium catalysis bring out a variety of new synthetic methods for the production of high valued-added chemicals^[5] as well as the pharmaceutical intermediates, ^[6] including the synthesis of chiral compounds.^[7] They are also employed in the oxidative polymerizations^[8] and received the interests of chemists in the field of environment- protection for the reversible coordination of Se with heavy metals^[9] and the strong catalytic activities to decompose the organic pollutants under mild and green conditions.^[10] The bio-metabolizable features of Se element make it relatively eco-friendly in comparison with the transition metals and organoselenium compounds are less toxic (some are even non-toxic) than inorganic selenium compounds. Moreover, tolerance of selenium residues is better than that of transition metals according to the latest revision of theUnited States Pharmacopoeia's New Standard and Method for Elemental Impurities Control in 2017.^[11] Investigations on organoselenium chemistry are booming during the past decade.

  Yet, transition metal catalysis is one of the most important key techniques in both academy and industry.^[12] In the field, copper is an especially practical metal because of the abundant resources. Despite of the developing trend of non-transition metal catalysis in recent years, ^[13] copper is still a significant metal in both academia and industry for its cheap price and the relatively low toxicity in comparison with many noble metals.^[14] However, although copper is a necessary element for human beings, over-taking of the metal may lead to poisoning and the permitting limit of copper residue in medicine is very strict. Copper pollution brings out tremendous threatens in many regions in the present. Therefore, developing techniques to eliminate copper pollutions are meaningful from the practical application viewpoint.^[15] Recently, it was found that 1, 2-bis(4- (benzyloxy)phenyl)diselane possessed very strong coordination with copper and could eliminate the metal pollutant at very high ratio over simple diselenides. This interesting finding may lead to unique protocols for developing copper scavengers.^[16]

  1.   Results and discussion

  The material could be synthesized from commercially available reagents and its synthetic route was illustrated in Scheme 1. First, the S_N2 reaction of 4-bromophenol (1) with benzyl bromide (2) under alkaline conditions led to the ether 3 in good yield (Scheme 1a). The aryl bromide moiety of 3 reacted with magnesium powder could produce the related Grignard reagent, which was then converted into the 4- (benzyloxy)benzeneselenol (4) via the subsequent Se-inser- tion and quenching with aqueous HCl (Scheme 1b). The generated 4 was oxidized by oxygen overnight to produce the designed molecule 5 in 72% yield (Scheme 1c).^[17]

  Scheme 1

  

  Scheme 1.  Synthetic route of 1, 2-bis(4-(benzyloxy)phenyl)- diselane.

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  Compound 5 was then used to remove the Cu²⁺ ion in its aqueous solution and ultraviolet-visible (UV-vis) spectra were used to determine the Cu²⁺ content. In the procedure, compound 5 was added into the Cu²⁺ solution for 3 h. After removing the precipitations by silica, the filtrate was extracted by ester while the aqueous layer was sent to UV analysis after the addition of DDTC-Na. As shown in Figure 1, the silica could absorb a few of Cu²⁺ (curves 2 vs. 1), while most of the metal was absorbed by compound 5 (curves 3 vs. 4). The standard curve of Cu²⁺ concentration versus the adsorption was then made, attesting that 99.3% Cu²⁺ was removed from its aqueous solution by using the unique substituted diaryl diselenide 5.

  Figure 1

  

  Figure 1.  UV-vis spectra of the samples in Cu-adsorption experiments

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  The effect of conditions on the adsorption progress was carefully investigated (Figure 2). In the progress, 41.5% Cu was removed within 1 h. The adsorption ratio rose along with the immersing time, and reached its peak at 4 h, at which 99.9% Cu was removed (Figure 2a). The adsorption was retarded under low temperature (e.g. 0 ℃, cooled by ice-water), while elevated temperature over 40 ℃ also led to the poor adsorption ratio due to the unstable Cu-Se coordination under heating (Figure 2b).^[9] The room temperature (20 ℃), as we initially employed, should be the preferable reaction temperature (Figure 2b). The effect of pH value of the condition was also examined and the result showed that the process was generally stable in the solution with pH>3 (Figure 2c). The adsorption ratio decreased sharply under the acidic conditions with pH < 2 (Figure 2c), and this was probably due to the competitive interaction of the environmental proton with the lone pair electron of the coordination elements such as Se and O. In practical applications, the system might involve a variety of ions disturbing the process. Thus, investigation on the effect of ion strength was then performed using KNO₃ as the simulative impurity. The process was found to be stable against the ion interferences, leading to the almost unaffected adsorption ratio of Cu with different KNO₃ concentration (Figure 2d).

  Figure 2

  

  Figure 2.  Cu-adsorption with compound 5 under different conditions

  (a) Test of the adsorption time; (b) Test of the temperature; (c) Test of the solution pH value; (d) Test of the condition ion strength

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  Moreover, a series of diselenides were employed as the Cu scavengers and the results were summarized for comparison (Figure 3a). Using (PhSe)₂ as the Cu cleaner, only 42.3% Cu was removed. Introducing methyl, methoxyl or phenoxyl as the electron-donating substituent did not improve efficiency, but using (4-CF₃C₆H₄Se)₂ and (PhCH₂-Se)₂ could remove over 80% Cu in solution. (n-C₄H₉Se)₂, as a representative of dialkyl diselenide, showed poor activity for the progress and only 67.7% Cu was removed. The structure of the diselenide might exert higher effect than the electrical property, and phenyl-containing substituent on to the aryl ring could obviously enhance the adsorption effect of the compound. The excellent performances of 5 probably attributed to its unique structure, which allowed the sufficient contact of Se with Cu for possible coordination.^[18] In the X-ray photoelectron spectroscopy spectra of the di- selenide 5 with/without Cu, an obvious shift was observed on the Se peaks, further attesting the coordination effect between Se and Cu (Figure 3b).

  Figure 3

  

  Figure 3.  Comparison of the adsorptions with 5 and the other typical diselenides

  (a) Parallel experimental results; (b) XPS spectra of the diselenide with/without Cu

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  The compound 5 has been successfully employed in our project to synthesize the Imatinib base, which is an efficient inhibitor of tyrosine kinases and is a "star medicine" for the treatment of chronic myeloid leukemia (CML) and gastro-intestinal stromal tumors (GIST). In our method, the last step of the process involved a Cu-catalyzed coupling reaction of 4-(pyridin-3-yl)pyrimidin-2-amine (6) with the aryl- C-Br of the major molecular skeleton (7), which led to ca 12.6×10^-6 of Cu residue in the final product (Scheme 2).^[16] Eliminating the Cu residue in product is a key for the whole project and compound 5 was employed to achieve this objective. After stirring 1 mol% compound 5 with the Cu-containing crude Imatinib base in 1, 4-dioxane for 5 h at room temperature, the product was then separated by concentration and filtration and was washed by petroleum ether. After the process, ca 85% purified Imatinib base was obtained. Inductively coupled plasma mass spectrometry (ICP-MS) analysis indicated that the Cu residue in the refined product was successfully depressed to be less than 10^-7, while the Se residue was not detected, showing good application potential of the compound 5 for copper adsorption (no matter the valence of the metal) in pharmaceutical industry.

  Scheme 2

  

  Scheme 2.  Last step of Imatinib base synthesis via a Cu- catalyzed coupling reaction

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  2.   Conclusion

  In conclusion, we discovered that 1, 2-bis(4-(benzyloxy)- phenyl) diselane was an excellent scavenger for copper pollutant elimination. The unique chemical structure allowing sufficient coordination of Se and O with Cu might be the reason for the excellent performances of the compound versus simple diselenides. The material has been successfully applied in removing the Cu residue from the Imatinib base in our drug development project, showing very good application potential in pharmaceutical industry. Further investigations on designing the Se-containing compounds with novel molecular structures are ongoing in our laboratory.

  3.   Experimental

  3.1   General methods

  Chemicals were purchased from reagent merchant with their purities over 98%. Solvents were analytically pure (AR) and were treated by standard method if necessary. NMR spectra were recorded on a Bruker Avance 400 instrument (400 MHz for ¹H NMR). Chemical shifts for ¹H NMR were referred to the internal Me₄Si (δ 0). Ultraviolet-visible (UV-vis) analysis was conducted using a Shimadzu UV2550 instrument. The Inductively coupled plasma mass spectrometry (ICP-MS) analyses were performed on a PerkinElmer Optima 7300 DV inductively coupled plasma spectrometer.

  3.2   Procedures for the synthesis of 1, 2-bis(4-(ben- zyloxy)phenyl)diselane

  3.2.1   Synthesis of 1-(benzyloxy)-4-bromobenzene (3)

  0.105 mol of 4-bromophenol (1), 0.066 mol of benzyl bromide (2), 0.197 mol of K₂CO₃ and 80 mL of N, N- dimethylformamide (DMF) were added into a 250 mL round bottom flask equipped with a condenser, which was then charged with N₂ and stirred for 24 h at reflux. The mixture was then filtrated and the solid was washed by EtOAc for 3 times. The solvent of the filtrate was removed by distillation under the reduced pressure, and the residue was dissolved by petroleum ether and washed by aqueous NaOH (0.5 mol/L) to neutralize the unreacted phenol. The organic layer was dried by anhydrous Na₂SO₄ and the product 1-(benzyloxy)-4-bromobenzene (3) was obtained in 84% yield after distilling the petroleum ether under reduced pressure.

  3.2.2   Synthesis of 1, 2-bis(4-(benzyloxy)phenyl)dise- lane (5)

  10 mmol of Mg powder and a piece of I₂ were added into a double necked 125 mL round bottom flask, which was charged with N₂ for protection. 10 mmol of 1-(benzyloxy)- 4-bromobenzene (3) in 50 mL of anhydrous ether was then dropped into the flask and the mixture was kept at room temperature for 3 h. 10 mmol of Se powder was slowly added into the mixture, which was stirred at reflux for 1 h. Then, the reaction liquid was poured into 100 mL of aqueous HCl (0.5 mol/L, cooled by ice-water) to quench the reaction. O₂ was introduced into the liquid to oxidize the generated 4-(benzyloxy)benzeneselenol (4) and led to the final product 1, 2-bis(4-(benzyloxy)phenyl)- diselane (5). The mixture was extracted by ether (50 mL×3) and the combined organic layer was dried by anhydrous NaSO₄. After removing the organic solvent via distillation, the product 5 could be obtained in 72% yield. The characterization data was given in section 3.5, while NMR spectra of the compound were given in supplementary material.

  3.3   Typical procedure for the Cu adsorption progress

  1.09 mmol of 1, 2-bis(4-(benzyloxy)phenyl)diselane (5) was dissolved in 10 mL of EtOH in a 50 mL volumetric flask. 26.3 mL of aqueous Cu(NO₃)₂ (0.4139×10^-4 mol/L) was then added. After constant volume process with water, the mixture was kept at room temperature for 3 h and 3.0 mL of the solution was moved and filtrated with 0.5 g of silica to remove the insoluble impurities. The silica was washed with distilled water (2 mL×3) and the combined filtrate was extracted by EtOAc (5 mL×3). After removing the EtOAc residue by distillation under reduced pressure, the aqueous layer was transfered into a 25 mL volumetric flask. 5 mL of 0.24×10^-3 mol/L of sodium diethyldithio- carbamate trihydrate (DDTC-Na) was then added into the volumetric flask and the pH value was adjusted to 9.0 by NH₃•H₂O (0.5 mol/L). After constant volume process with water, the sample was sent to UV-vis analysis and the concentration of the Cu²⁺ was calculated by internal standard curve (for details, please see supplementary data).

  3.4   Removing Cu residue with 1, 2-bis(4-(benzyl- oxy)phenyl)diselane from the crude Imatinib base

  10 mmol of the Imatinib base synthesized via our previous method^[16] and 0.1 mmol of 1, 2-bis(4-(benzyloxy)- phenyl)diselane were initially dissolved in 50 mL of 1, 4- dioxane. The solution was stirred for 5 h at room tem- perature. After concentration, the Imatinib base preci- pitated, and it was collected by filtration and washed by petroleum ether. The obtained sample was then sent to ICP-MS analysis to determine the Cu residue content (20 mg of sample was dissolved in 10 mL of 1 mol/L HNO₃ for a test).

  3.5   Characterization data of compounds 3 and 5

  1-(Benzyloxy)-4-bromobenzene (3): White solid, m.p. 62.3~63.1 ℃ (lit.^[19] 62~63 ℃); ¹H NMR (400 MHz, CDCl₃) δ: 7.44~7.35 (m, 7H), 6.89 (d, J=8.4 Hz, 2H), 5.04 (s, 2H); IR (KBr) v: 3056, 1850, 1585, 1487, 1288, 1245, 1168, 1068, 1040, 1025 cm^-1; MS (EI, 70 eV) m/z (%): 263 (11) [M⁺], 91 (100).

  1, 2-Bis(4-(benzyloxy)phenyl)diselane (5): Yellow solid, m.p. 65.1~65.5 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.45~7.32 (m, 14H), 6.89~6.84 (m, 4H), 5.05 (s, 4H); ¹³C NMR (100 MHz, CDCl₃) δ: 157.77, 136.46, 132.22, 128.58, 128.05, 127.38, 116.62, 113.05, 70.12; IR (KBr) v: 3688, 3524, 3447, 3283, 3165, 1710, 1575, 1486, 1445, 1285, 1242, 1032, 818, 732, 561 cm^-1; MS (EI, 70 eV) m/z (%): 263 (8), 91 (100). Anal. calcd for C₂₆H₂₂O₂Se₂ C 59.55, H 4.23; found C 59.62, H 4.30.

  Supporting Information    The analysis details and NMR spectra of the compounds. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn.

  
  
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• 

  Scheme 1  Synthetic route of 1, 2-bis(4-(benzyloxy)phenyl)- diselane.

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  Figure 1  UV-vis spectra of the samples in Cu-adsorption experiments

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  Figure 2  Cu-adsorption with compound 5 under different conditions

  (a) Test of the adsorption time; (b) Test of the temperature; (c) Test of the solution pH value; (d) Test of the condition ion strength

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  Figure 3  Comparison of the adsorptions with 5 and the other typical diselenides

  (a) Parallel experimental results; (b) XPS spectra of the diselenide with/without Cu

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  Scheme 2  Last step of Imatinib base synthesis via a Cu- catalyzed coupling reaction

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