Chromatography
The Purifi cation of Crown Ether Samples by a Quaternary Solvent Flash Chromatography System
Haifeng Wu, Wenjun Qiu and Bo Xu* Application R&D Center, Santai Technologies Inc, China *Corresponding Author:
BoXu@santaitech.com
Crown ethers are heterocyclic chemical compounds that consist of a ring containing several ether groups. The most common crown ethers are oligomers of ethylene oxide, the repeating unit being ethyleneoxy (-CH2
CH2 O-). Important members of this series are the tetramer (n = 4, or four ethyleneoxy units), the pentamer (n = 5),
and the hexamer (n = 6). The term ‘crown’ refers to the resemblance between the structure of a crown ether bound to a cation, and a jewelled regal crown. The fi rst number in the name of a crown ether refers to the number of atoms in the cycle, and the second number refers to the number of those atoms that are oxygen. Crown ethers are much broader than the oligomers of ethylene oxide; an important group is derived from catechol.
Experimental
The sample used in this application was a benzocrown ether derivative which was kindly provided by a university chemistry lab. The chemical structure of the sample molecule is shown in Figure 2. The sample has poor solubility in weak polar organic solvents since the components of the sample have diverse polarities. Therefore dichloromethane (DCM) was used as the solvent of choice for complete dissolution of the raw sample.
Figure 1. Jewelled crown (left) and Crown ether (right).
In 1967, Charles Pedersen, a chemist working at DuPont, discovered a simple method of synthesising crown ethers whilst trying to prepare a complexing agent for divalent cations [1]. Citing earlier work on the dissolution of potassium in 16-crown-4, he realised that the cyclic polyethers represented a new class of complexing agents that were capable of binding alkali metal cations. He proceeded to report systematic studies of the synthesis and binding properties of crown ethers in a seminal series of papers [2]. The fi elds of organic synthesis, phase transfer catalysts, and other emerging disciplines benefi ted from the discovery of crown ethers. Pedersen shared the 1987 Nobel Prize in Chemistry for the discovery of the synthetic routes to and binding properties of crown ethers.
The special properties of crown ethers are their strong coordination ability for alkali metal and alkaline earth metal cations. Depending on the size of the ether ring opening, the type of heteroatom and the number of atoms, this coordination effect shows distinct selectivity for specifi c metal ions. As important compounds in many fi elds such as coordination chemistry, extraction chemistry, phase transfer catalysis, ion-selective carriers, etc. crown ethers have attracted more and more attention [3-8]. The different properties of the substituent groups on the crown ether compound will affect its complexing strength with metal ions, selectivity and solubility. Compared with unsubstituted crown ethers, benzocrown ethers have attracted more attention in research due to their better lipophilicity [9,10]. However, benzocrown ethers exhibit poor solubility in solvents with low polarity, making them diffi cult to be used directly. In order to improve the properties of benzocrown ethers, in recent years many researchers have focused their work on the research and development of benzocrown ether derivatives.
In this application, the sample to be purifi ed is a benzocrown ether derivative obtained by organic synthesis. The sample is composed of multiple components with diverse polarities, making it impossible to separate each component by conventional binary solvent gradient. Considering the sample properties, a quaternary solvent system was utilised employing a SepaBean machine T combined with SepaFlash normal phase silica cartridge (Santai Technologies) for the purifi cation of the sample. By online switching between solvents of different polarity, the target products meeting the purity requirements were successfully obtained, suggesting a feasible solution for the purifi cation of complex sample composed of multiple components with diverse polarity.
Figure 2. The chemical structure of benzocrown ether derivative sample.
To prepare the sample solution, 4.9 g of the raw sample was dissolved in small amount of DCM and absorbed onto 3.5 g of silica gel 200-400 mesh. The DCM was removed by vacuum and the absorbed sample was then placed in a 4g SepaFlash iLOK empty cartridge (order number: SD-0000-004) for solid sample loading. The sample was then eluted automatically by a SepaBean T Flash chromatography system employing the parameters shown in Table 1.
Results and Discussion
Considering the sample property, conventional binary solvent system DCM/methanol was employed as the mobile phase for the elution of the sample. The Flash chromatogram of the sample in this condition was shown in Figure 3.
Figure 3. The Flash chromatogram of the sample by binary solvent gradient.
INTERNATIONAL LABMATE - FEBRUARY 2020
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