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In the 1980s, polysaccharide-based chiral stationary phases (CSPs) were identified as versatile and useful chiral sorbents for separation of enantiomers/stereoisomers in high performance liquid chromatography (HPLC). Chiral discrimination abilities of these CSPs can be derived from the highly organized structure of the left-handed 3/2 helical chain conformations [1]. Some chiral cavities with specific configuration can be formed on the CSPs, which provide the suitable site for a particular enantiomer and make it easier to interact with CSPs by hydrogen bonding and π-π interactions. This leads to enantioseparation of chiral compounds by different retention and elution on CSPs between their enantiomers [2]. Okamoto et al. reported that the introduction of various kinds of substituents on the hydroxyl group of polysaccharides can improve their stereoslectivity [3].
Cellulose is an important polysaccharide, it is also a highly crystalline polymer which occurs with various crystal structures. In the 1970s, Hesse and Hagel first synthesized microcrystalline cellulose triacetate (MCTA), and thought its chiral recognition ability might originate from secondary structures creating chiral cavities upon swelling, which can clamp stereoselectively compounds with aromatic residues [4]. In recent years, different cellulose derivatives have been synthesized, coated or covalently bonded on decorative silica gel, and broadly used as CSPs in enantiomeric separation of chiral compounds especially on pesticides and pharmaceuticals. These derivatives exhibit powerful chiral recognition ability towards a wide number of different racemic compounds. More and more commercial cellulose-based CSPs including cellulose acetate, benzoate and phenylcarbamates are being developed and applied in enantioseparation [2,3].
Chiral compounds account for 25% of all agrochemical compounds used commercially and for 26% of the total value of the world agrochemical market [5]. The enantiomers of chiral pesticides possess similar physicochemical properties in a non-chiral environment while they show different activities in biological systems due to enantioselective interactions with enzymes, receptors, and other enantiomeric biological entities. For example, triadimenol is a systemic fungicide and has four stereisomers due to the presence of two chiral centers in its molecule. Of the four, the (1S, 2R)-isomer shows the highest fungicidal activity (up to 1000-fold more active than the other three) [6]. However, most chiral pesticides are produced and formulated as racemic mixture even though the desired biological activity may be derived from only one enantiomer. It is therefore very important to be able to separate enantiomers of chiral pesticides in order to prepare single enantiomers, develop enantiomeric analysis methods and evaluate their bioactivity and environmental fates.
This work focuses mainly on a review of the development of cellulose derivatives for CSPs which are prepared as cellulose-based chiral columns by coating and bonding on supports, and their applications in stereoselective separations of chiral pesticides.
The cellulose-based CSPs generally are of two types: the coated and the bonded. The coated cellulose-based CSPs consisting of the low-molecular-weight cellulose benzoate or phenyl carbamate showed higher chiral recognition than the covalently bonded CSPs for most racemates. The major reason was considered to be an optimal secondary and supermolecular structure for the chiral recognition mechanism of polysaccharide derivatives under coated conditions [1,3]. However, the coated CSPs can only be used with a limited range of solvents as mobile phases such as alkanes, alcohols, acetonitrile, or aqueous solvents including alcohols or acetonitrile because CSPs may dissolve in ‘strong’ solvents such as tetrahydrofuran (THF) and chloroform (CHCl3). Such a dissolution would damage or destroy the CSPs. This limited the application range of the coated CSPs on separation and preparation of chiral compounds, because the solubility of the sample in the mobile phase is very important to increase the amount of racemates loaded on CSPs, especially on a preparative large-scale separation [7].
The bonded CSPs were prepared by covalently bonding cellulose derivates to silica gel. They can be applied to a wider range of resolving conditions than the coated type. The fixation can affect the conformation of cellulose derivates and make it difficult to obtain optimal supermolecular structure. This results in lower chiral recognition ability of the bonded-type CSPs. However, the fixation improves versatility in the solvent selection, and allows the use of some solvents that cannot usually be applied on the coated CSPs as mobile phases or sample dissolving reagents [8].
The commercial cellulose-based CSPs including the coated and the bonded CSPs currently in use are summarized in Table 1. As can be seen, there are only two columns (Chiralpak IB and Chiralpak IC) prepared from cellulose derivatives by bonding out of 13 commercial chiral columns. This means that the coated CSPs include more cellulose derivatives and are more frequently used for the resolution of chiral compounds than the bonded CSPs. Some of these chiral columns can be selectively used in normal-phase HPLC (NP-HPLC), like Chiralcel OD, Chiralcel OA, Chiralcel OB, Chiralcel OC, Chiralcel OF, Chiralcel OG and Chiralcel OJ etc.; some can be used in reversed-phase HPLC (RP-HPLC), like Chiralcel OD-R, Chiralcel OZ-R and Chiralcel OJ-R; and some can be used in both NP-HPLC and RP-HPLC, like Lux Cellulose-1, Lux Cellulose-2, Lux Cellulose-3, Lux Cellulose-4, Chiralpak IB and Chiralpak IC [9,10]. Some studies have been done to evaluate comparatively the enantioselective and chromatographic properties of Chiralcel OD and Chiralpak IB using a set of 48 compounds that differ in their physical and chemical properties [11]. The uses of these CSPs in different mobile phases mainly depend on their different preparation methods.
Table 1
The list of commercial cellulose-based CSPs in the present.
2.1. The development of coated cellulose-based CSPs
Various cellulose derivatives were reported as CSPs in recent years, especially on cellulose benzoates and phenylcarbamates because of their higher enantiomeric discrimination ability and wide applications. Okamoto et al, synthesized some cellulose triphenylcarbamate derivatives and absorbed them on silica gel as CSPs, and then compared optical resolution abilities with the characteristics of the substituents on the phenyl rings. The results showed that dimethylphenyl- and dichlorophenylcarbamates substituted at 3,4- or 3,5-positions exhibited better chiral recognition for most reacemates than monosubstituted derivaties. Of the these, cellulose tris-(3,5-dimethylpheyl-carbamate) (CDMPC) offered the highest enantiomeric separability [12]. In another investigation on chiral recognition ability of cellulose phenylcarbamate derivatives, cellulose-tris-(3-fluoro-5-methylphenylcarbamate) was reported to be better than 3,5-difluoro- and 3,5-dimethylphenylcarbamates of cellulose for enantioseparation of ten racemates [13].
The investigations of four regioselectively substituted cellulose derivatives having two different substituents at 2-, 3-, and 6-positions showed better enantioseparations were sometimes obtained on these CSPs, compared to the corresponding homogeneously tris-substituted cellulose derivatives-based CSPs. Cellulose 2,3-(3-chloro-4-methylphenylcarbamate)-6-(3,5- dimethylphenylcarbamate), and 2,3- (3,5-dimethylphenyl- carbamate)-6-(3-chloro-4-methylphenylcarbamate) exhibited the most efficient enantioseparations for tested racemates in four CSPs [14]. The cellulose derivative of benzoylcarbamate also showed a higher chiral discrimination ability compared to those of phenylcarbonate, p-toluenesulfonylcarbamate, and benzoylformate when they used as CSPs on HPLC. This discrimination could be achieved by hydrogen bonding of the racemates’ hydrogen atoms with the carbonyl group of the benzoylcarbamates [15].
Chiral recognition abilities of cellulose-methoxyphenylcarbamates were significantly influenced by the position, bulkiness, and number of alkoxy groups introduced on the phenyl group. The 3-position was found to be the best for introducing an alkoxy group, and cellulose-tris-(3-methoxyphenylcarbamates) exhibited much higher recognitions. Additionally, the recognition abilities also increased with the increases of the bulkiness of the 3-alkoxy group [16]. Cellulose-tris- (3-trifluoromethylphenylcarbamate) also exhibited characteristic enantioseparation and were better to resolve some chiral compounds than Chiralcel OD [17 ].
During the preparation of polymer cellulose-based CSPs by coating on silica gel, chiral additives such as (+)-L-Mandelic acid, (+)-1-phenyl-1,2-ethanediol and (-)-2-phenyl-1-propanol for CSPs of cellulose tribenzoate, and (-)-2-phenyl-1- propanol and (+)-phenylsuccinic for CSPs of cellulose trisphenylcarbamate have a substantial effect on the resolution and efficiency of the CSPs, and can improve chiral recognition ability compared to the original CSPs [18].
Some new supports other than decorative silica gel were also used to prepare the coated CSPs. For example, a new CSP of CDMPC was prepared by coating CDMPC on TiO2/SiO2 particles. Its good chiral separation ability and a comparably low column pressure proved that TiO2/SiO2 could be used as an alternative to silica gel, and could enlarge the range of base materials when preparing CSP [19].
2.2. The development of bonded cellulose-based CSPs
CDMPC and CDCPC were covalently bonded to decorative silica gel to obtain the bonded chiral columns of Chiralpak IB and Chiralpak IC respectively [9]. CTPC regioselectively bonded at the 6-position to silica gel exhibited a higher chiral recognition than either CTPC regioselectively bonded at the 2- or 3-position or non-regioselectively bonded at the 2-, 3-, and 6-positions [20]. When cellulose derivatives bearing pyridyl and bipyridyl residues were compared in chiral recognition abilities, the results showed that the regioselectively substituted derivatives exhibited higher recognition compared with cellulose derivatives bearing these residues at the 2-, 3- and 6-positions of a glucose ring. This ability was significantly influenced by the coordination of Cu(II) ion to the bipyridyl groups that resulted in the difference of the higher-order structures of cellulose derivatives [21].
CSP with poly[styrene-b-cellulose 2,3-bis-(3,5-diphenylcarbamate)] was prepared by the surface-initiated atom transfer radical polymerization (SI-ATRP) of cellulose 2,3-bis-(3,5-dimethylphenylcarbamate)-6-acrylate after the SI-ATRP of styrene on the surface of silicon dioxide supports in pyridine. This CSP showed considerably high column efficiency for the resolution of tested racemates [22].
Laureano Oliveros et al, prepared five mixed 10-undecenoate/benzoates of cellulose and linked them to allyl silica gel by means of a radical reaction. The investigation of chiral recognition ability showed that CSP5 (10-undecenoate/3,5-dichlorobenzoate) has the highest enantioselectivity for most of tested racemates, followed by CSP3 (10-undecenoate/4-methylbenzoate) and CSP4 (10-undecenoate/benzoate). These CSPs showed lower resolution than the coated CSPs although they have higher column efficiency. The reason may be the lack of polar amino groups on the surface of the CSPs. However, when being compared with the coated CSPs, these CSPs can tolerate the use of more polar solvents such as chloroform in the mobile phase [23].
Three cellulose-based CSPs were prepared by reticulation of the same cellulose derivative on three end-capped silica gels with different pore sizes (50Å, 100Å and 4000Å). The comparison of chiral recognition ability among them showed that CSPs with higher pore size exhibited higher selectivity factors, because it can accommodate a larger amount of accessible cellulose derivative on its surface [7].
Four mixed 10-undecenoyl-3,5-dimethylphenylaminocarbonyl derivatives of cellulose with increased proportion of alkenoyl groups were bonded on allylsilica gel. Their comparison showed that CSPB presents the best chiral recognition and can separate the widest range of the tested racemates. The reason may be the higher number of substitution of glucose units. The important decrease in the recognition ability of these CSPs could be attributed to their higher degree of reticulation. More heterogeneous reaction sites of allysilica gel with cellulose derivatives can result in lower degree of reticulation in CSPs and therefore improve their recognition ability [24].
Azido cellulose phenylcarbamate (AzCPC) was synthesized regioselectively and chemically immobilized onto amino-functionalized silica gel to obtain urea-bonded CSPs. Enantioseparation using CHCl3 on these CSPs showed better separation than traditional hexane/2-propanol in mobile phases for some tested racemates. The pre-coating of AzCPC onto silica gel prior to chemical immobilization could significantly improve immobilization efficiency, and obtained better enantioselectivity [25].
Generally, benzoate and phenylcarbamate derivatives of cellulose were prepared by reaction between cellulose and excess benzoyl chloride or phenyl isocyanate derivatives in dry pyridine (Figure 1). These derivatives are then coated onto macro-porous 3-aminopropylsilica (APS) from a solution by evaporation of the solvent to obtain coated CSPs. The APS was prepared beforehand by silanizing silica gel with a solution of 3-aminopropyltriethoxysilane. Finally, the CSPs were packed into HPLC columns by the slurry method, to obtain coated chiral columns [18, 26]. For example, CDMPC was synthesized by reaction of microcrystalline cellulose with 3,5-dimethylphenylcarbimide in pyridine; the product was filtered off, washed with methanol and dried at 60° C for 24h. CDMPC was then dissolved in THF and coated on the APS under vacuum to dryness. Finally, the coated CDMPC were packed into a stainless-steel column at 3.7×107Pa by the high-pressure slurry method to obtain the corresponding CSP [26].
Figure 1.
The synthesized routes of cellulose benzoates or phenylcarbamates.
Investigations on the influence of the pore size of silica gel, the coating amount, the coating solvent, and the column temperature on chiral discrimination of CDMPC showed that CSPs prepared with a large-pore silica gel having a small surface area exhibited higher recognition abilities. An increase in the amount of coating of CDMPC on the silica gel can improve the loading capacity of racemates, and a CSP coated with 45% CDMPC by weight can be used for both analytical scale and semi-preparative scale separations. CSPs coated with acetone showed higher enantioselectivity than those coated with THF or a mixture of CH2Cl2 and phenol [27].
3.2. The preparation method of covalently bonded CSPs
Generally, cellulose-derived CSPs covalently bonded on silica gel are prepared by using a benzoyl chloride or a phenyl isocyanate to react with cellulose in homogeneous conditions, to obtain the corresponding benzoates or carbamates. However, other methods to prepare this type of CSP have been reported. Ikai et al. summarized various immobilization methods of the polysaccharide derivatives mainly onto silica gel: immobilization using diisocyanate, vinyl groups by polymerization and copolymerization with a vinyl monomer etc. [28,29]. Several methods of synthesis are shown in Figures 2 to 4.
CDMPC can be efficiently immobilized on silica gel as CSPs by copolymerizing with vinyl monomers. The introduction of vinyl groups or the employment of vinyl monomers can readily tune the immobilization efficiency and the chiral recognition of cellulose derivatives [30]. The new method was applied to immobilize CDMPC onto bare silica gel via the intermolecular polycondensation of triethoxysilyl groups, which were introduced onto the glucose unit by the epoxide ring-opening reaction under acidic conditions. The CSPs thus obtained also exhibited high chiral recognition ability for 10 tested racemates and could be used with various eluents that are not compatible with the conventionally coated CSPs [31]. One-pot method was applied to synthesize CDCPC bearing a small amount of 3-(triethoxysilyl) propyl residues, and then immobilized onto silica gel through intermolecular polycondensation. The immobilized CSPs exhibited chiral recognition abilities similar to the corresponding coated CSP and slightly different from the commercial Chiralpak IC [32].
Figure 2.
The covalent bonding of 3,5-dichloro- and 3,5-dimethylphenylcarbamate of cellulose onto APS [33].
Figure 3.
Regioselective covalent bonding of CDMPC to positions 2 and 3 of the glucosidic rings.
Figure 4.
Regioselective covalent bonding of CDMPC to position 6 of the glucosidic rings [34].
Cellulose-(diphenymethyldicarbamate/phenylcarbamate) covalently bonded to APS showed some chiral recognition ability [35]. Cellulose-tris-phenylcarbamate was covalently bonded to silica gel with different spacers. The results showed CSPs prepared with spacer 1(4-(1-(3-(triethoxysilyl)-propyl)urea)-benzyl-4-isocyanatobenzene) exhibited higher resolution ability than spacer TEPI (3-(triethoxysilyl) propyl isocyanate) with the same preparation procedure. The amount of spacer in the synthesis influences the optical resolution ability of CSPs, and a lower amount can produce higher resolution ability [36].
Polar monodisperse amine terminated polymer (2-aminoethyl methacrylate-co- ethylenedimethacrylate) beads can be used as the replacement of silica gel, and are suitable as supports for the preparation of cellulose-based CSPs coated by simple adsorption and immobilized with a diisocyanate linker. However, the chiral recognition abilities of these CSPs shows no enhancement because the uses of cellulose-based selectors and preparation methods may completely cover the surface of polymer supports. Thus, the analytes have no access to the native surface of the support and non-specific interactions with the surface functionalities are not observed. [37].
4. The application of cellulose-based CSPs in enantioseparation of chiral pesticides
Chiral HPLC is a good method to separate enantiomers/stereoisomers of chiral pesticides because it facilitates the preparation of single enantiomers for study of enantiomeric bioactivity, toxicology and environmental fate. In recent years, cellulose-based CSPs prepared with different cellulose derivatives and methods resulted in their very broad application for chiral separation of pesticides such as organophosphates [38], organochlorine, triazole, synthetic pyrethroids, acylanilides, imidazolinones, phenoxypropanoic-acid herbicides and related compounds [39].Table 2 summarizes the resolution results of 79 chiral pesticides in current references.
As shown in Table 2, the stereoselective separations of most of chiral pesticides can be achieved on NP-HPLC and some on RP-HPLC using cellulose-based CSPs. The most efficient CSP with the highest chiral recognition ability is CDMPC, available under the commercial names of Chiralcel OD, Chiralcel OD-H, Chiralcel OD-R, Chiralcel OD-RH, Lux Cellulose-1 and Kromasil CelluCoatTM. The coated CDMPC on APS exhibited higher chiral discrimination for most of pesticides than the bonded type available under the commercial names of Chiralpak IB and Chiralpak IC. For example, the resolution factor (Rs) of systemic fungicide-metalaxyl on the coated CDMPC is 4.54 with hexane/IPA (80:20) as the mobile phase, which is significantly higher than that on the bonded CDMPC with an Rs of 0.632 using hexane/IPA (97/3) as the mobile phase.
The second most efficient CSP in terms of resolution is CTMB available under the commercial names of Chiralcel OJ, Chiralcel OJ-H, Chiralcel OJ-RH, Lux Cellulose-3. It exhibited higher chiral discrimination for some chiral pesticides than CDMPC. For example, the Rs of triazole fungicide-imazalil on Chiralcel OJ-H is 5.21, which is significantly higher than 1.51 obtained on Chiralcel OD-H using the same mobile phase of hexane/IPA (100/3) and the same flow rate of 0.8 mL/min on NP-HPLC. The combination of CDMPC and CTMB on NP-HPLC and RP-HPLC can separate most chiral pesticides listed in Table 2.
The separations on NP-HPLC were better than those on RP-HPLC for most chiral pesticides. The cellulose-based CSPs on NP-HPLC can generally give better resolution and yield a larger amount of a single enantiomer in one injection. However, its application is limited because some racemates are polar and difficult to dissolve in the weak polar solvents used as mobile phase on NP-HPLC. For this reason, the amount of racemates loaded on CSPs cannot be increased. The separation on RP-HPLC is sometimes less effective than on NP-HPLC, but it can use more methanol, acetonitrile or water in the mobile phase and can thus significantly improve the solubility of some racemates that will not readily dissolve in the hexane, heptane and isopropanol used in NP-HPLC. This is very helpful to prepare optically pure enantiomer of polar chiral compounds and obtain more enantiomer in a shorter time. Additionally, the use of HPLC in the reversed phase can easily be connected in tandem with mass spectrometry, which makes it possible to establish more sensitive and more efficient analytical methods for enantioselective studies of chiral pesticides [40-].
Table 2
Summary of resolution results of chiral pesticides on cellulose-based CSPs
Cellulose derivatives have high chiral recognition abilities for racemates and have already become a very popular and useful source material for CSPs. Cellulose-based CSPs can be prepared by coating or bonding cellulose derivatives on decorative silica gel or other supports with various preparation methods. The coated CSPs exhibite higher discrimination abilities for chiral pesticides and are more popular than the bonded CSPs. However, the bonded CSPs can tolerate broader solvent ranges, including THF and CHCl3, which cannot be used on coated CSPs as mobile phases because they have strong dissolution abilities that can dammage or destroy them. Coated CDMPC and CTMB had the broadest application in the stereoselective separations of chiral pesticides. For most pesticides, better separations were obtained on NP-HPLC than on RP-HPLC. However, RP-HPLC can improve the amount of racemates loaded on CSPs as it allows the use of more polar solvents to enhance the solubility of racemates in mobile phases. Additionally, it can be easily connected in tandem with MS, allowing for the development of more sensitive methods for analysis of enantiomers/stereoisomers. The cellulose-based CSPs on NP-HPLC and RP-HPLC provide very powerful tools to prepare individual enantiomers and study the activity, toxicity and environmental fates of chiral pesticides.
Acknowledgement
The financial support from National Natural Science Foundation of China (Project number 21177156 and 20907073) for this work is hereby gratefully acknowledged.
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Written By
Jing Qiu, Shouhui Dai, Tingting Chai, Wenwen Yang,
Shuming Yang and Hualin Zhao
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