Salen-Mn (Ⅲ),styrene,(R)/(S)-styrene oxide with a purity of 99% were supplied by J&K Scientific Ltd. Dichloromethane,acetone,sodium hypochlorite (the chlorine content≥5% ) were purchased from Kelong Chemical Co.,Ltd. Ethylbenzene was obtained from Aladdin. Methanol was supplied by Xilong Chemical Co.,Ltd.

The stock standard solution of styrene was prepared in acetone at a mass concentration of 3.11×10³ mg/L. The stock standard solutions of (R)/(S)-styrene oxides were also prepared in acetone at a mass concentration of 3.27×10³ mg/L and 3.22×10³ mg/L,respectively. The solution of 1.1×10⁴ mg/L ethylbenzene prepared in acetone was used as the internal standard.

All analyses were performed by using the Thermo Fisher Scientific GC-MS instrument. The chromatographic conditions were as follows: column,CP-Chirasl-Dex CB (30 m×0.25 mm×0.25 μ m,Varian,USA); carrier gas,helium at 1.0 mL/min; split ratio,50:1; injection port temperature,200 ℃; column temperature,initially being set at 80 ℃ for 1 min and programmed to rise at 2 ℃/min to 110 ℃,then hold for 1 min at 110 ℃; ion source temperature,220 ℃; ion mode,electron-impact ionization at 70 eV; full scan mode,m/z 50-350; SIM,target ions (m/z 91.00,118.96) for (R)/(S)-styrene oxide,target ions (m/z 78.01,104.00) for styrene,and target ions (m/z 91.01,106.02) for ethylbenzene (internal standard). The total run time for GC was within 17 min.

Reaction scheme for the asymmetric epoxidation of styrene is shown in Fig. 1. To simulate styrene epoxidation mixture,three stock standard solutions were mixed and diluted by acetone into a series of concentration gradient solutions,which were mixed standard solutions. After the addition of internal standard solution (ethylbenzene),these mixtures were analyzed by GC-MS.

Fig. 1

Fig. 1

Fig.1 Reaction scheme for the asymmetric epoxidation of styrene

The solution with approximately 0.55 mol/L NaClO and 0.05 mol/L Na₂HPO₄ was prepared,and the pH of the solution was adjusted by the addition of 1 mol/L NaOH solution. The mixed solution was added to a 25 mL round-bottom flask equipped with Salen-Mn (Ⅲ) and styrene in 0.5 mL CH₂Cl₂. The two-phase mixture was stirred and the reaction progress was monitored by thin-layer chromatography (TLC). After the completion of the reaction,CH₂Cl₂ was added to the mixture and the brown organic phase was separated,then the organic phase was washed twice with saturated NaCl solution and dried with anhydrous Na₂SO₄. After solvent removal,the residue was dissolved in a certain amount of acetone and determined by GC-MS after filtrating through a 0.45 μ m pore membrane.

Based on single factor experiment,the following main factors including the amount of the oxidant (NaClO),pH value,reaction time,reaction temperature,solvent and the amount of styrene,were selected for orthogonal experimental design. All samples prepared by orthogonal experiments of styrene epoxidation were analyzed by GC-MS and the optimal reaction conditions was determined according to the yield and ee values of samples (Table 1).

Table 1

Table 1

Table 1

Table 1 Factors and levels of orthogonal experiments for styrene epoxidation reaction Code Factor Levels 1 2 3 A amount of NaClO/mmol 0.20 1.00 3.00 B pH 7.00 9.00 11.30 C reaction time/h 4.0 6.0 10.0 D reaction temperature/℃ 0.0-4.0 25.0 37.0 E solvent CH2Cl2 CH2Cl2-MeOH MeOH (1∶2,v/v) F amount of styrene/mmol 0.3 0.5 0.7

Table 1 Factors and levels of orthogonal experiments for styrene epoxidation reaction

The total ion chromatograms of the mixed standard solution and the mixed styrene epoxidation solution are shown in Fig. 2 and Fig. 3. The mass spectra of ethylbenzene,styrene,styrene oxide in the mixture are shown in Fig. 4. The results indicate that each component of the mixture could be baseline separated and has strong characteristic ions,m/z 91.01,106.02 for ethylbenzene (internal standard),77.02,78.01,103.01,104.00 for styrene,89.02,91.00,92.06,118.96,119.98 for styrene oxide.

Fig. 2

Fig. 2

Fig.2 Total ion chromatogram of mixed standard solution

Fig. 3

Fig. 3

Fig.3 Total ion chromatogram of mixed solution of styrene epoxidation

Fig. 4

Fig. 4

Fig.4 Mass spectra of ethylbenzene,styrene,styrene oxide in the mixture

Analytical data of the mixed standard solutions at five mass concentrations were used to construct the standard curves,and good linear relationships were obtained for styrene and (R)/(S)-styrene oxide (Table 2). Three groups of mixed standard solutions with different mass concentrations were respectively added into three replicates of a certain sample (No.11 in Table 3),and then analyzed by GC-MS. The recoveries of the three substances ranged from 98.2% to 108.2% with RSDs of 1.2% -5.2% (n=5). The LOQs (S/N=10) of styrene and (R)/(S)-styrene oxide were 1.3,1.1 and 0.7 mg/L,respectively. These results demonstrate that the (GC-MS/SIM) method developed in this study is reliable in the determination of the asymmetric epoxidation mixture of styrene.

Table 2

Table 2

Table 2

Table 2 Linear equations,correlation coefficients (r),LOQs,recoveries and RSDs of styrene and (R)/(S)-styrene oxide Analyte Regression equation r LOQ/ (mg/L) Spiked/ (mg/L) Recovery/ % RSD/% (n=5) Styrene y=-0.359076+0.0698033x 0.9997 1.3 591.0 100.7 2.7 656.6 100.1 4.1 721.9 103.0 1.2 (R)-Styrene oxide y=-1.102280+0.0209811x 0.9932 1.1 116.1 99.1 3.3 128.9 101.3 2.8 141.8 105.5 3.2 (S)-Styrene oxide y=-0.399172+0.0383123x 0.9963 0.7 331.7 98.2 1.8 368.6 99.9 1.4 405.4 108.2 5.2 y: peak area of the component; x: mass concentration of the component,mg/L.

Table 2 Linear equations,correlation coefficients (r),LOQs,recoveries and RSDs of styrene and (R)/(S)-styrene oxide

The developed method was applied to screening for asymmetric epoxidation of styrene to be evaluated. From Table 3,it can be observed that there was minor difference between various R′ values,indicating ee not suitable for the core indicator. Therefore,the yield should be taken as the core indicator for screening various factors which affect the reaction. Comparing results of R values indicate that many factors would affect asymmetric epoxidation of styrene. The orderance in descending order was the solvent,the amount of NaClO,the amount of styrene,pH,reaction time and reaction temperature. Taking together with the results of each level,it can easily be concluded that the optimum reaction conditions were E₂A₃F₂B₃C₂D₁,which were 0.01 mmol Salen-Mn (Ⅲ),0.5 mmol styrene,5 mL CH₂Cl₂-MeOH (1:2,v/v),and 3 mmol NaClO (pH 11.3).

Table 3

Table 3

Table 3

Table 3 Results of orthogonal experiments No. Level Yield/% ee/% A B C D E F 1 1 1 1 1 3 2 34.6 81.6 2 2 1 3 3 1 3 4.5 44.6 3 3 2 2 1 2 3 41.7 66.2 4 1 2 3 2 3 1 11.1 78.0 5 2 3 1 3 2 1 22.7 71.4 6 3 3 2 2 1 2 10.2 69.2 7 1 1 2 2 3 3 26.7 74.5 8 2 1 3 1 2 1 32.4 55.4 9 3 2 1 2 2 2 61.8 58.9 10 1 2 2 3 1 1 26.3 69.1 11 2 3 1 1 1 3 17.6 85.8 12 3 3 3 3 3 2 65.9 61.5 13 1 1 1 2 1 1 10.0 73.3 14 2 1 2 3 2 2 32.7 79.6 15 3 2 1 3 3 3 9.3 66.3 16 1 2 3 1 1 2 5.7 80.9 17 2 3 2 1 2 3 42.8 78.9 18 3 3 3 2 2 3 37.1 69.1 K1 19.1 23.5 26.0 29.1 12.4 24.2 K2 25.4 26.0 30.0 26.2 38.1 35.2 K3 37.7 32.7 26.1 26.9 31.7 22.8 R 18.6 9.2 4.0 2.9 25.7 12.4 K1′ 76.2 68.2 72.8 74.8 70.5 71.0 K2′ 69.3 69.9 72.9 72.9 66.8 72.0 K3′ 65.2 72.7 64.9 65.4 73.5 67.8 R′ 11.0 4.5 8.0 9.4 6.7 4.2 A,B,C,D,E,F: code of each factor; K1,K2,K3: respective average yield of the three levels,K1′,K2′,K3′: respective average ee of three levels; R,R′: respective range of yield and ee.

Table 3 Results of orthogonal experiments