A Study of the Strength of a Template Molecule—A Functional Monomer Interaction That Affects the Performance of Molecularly Imprinted Polymers and Its Application to Chiral Amplification

Abstract

A novel type of molecularly imprinted polymer (MIP), N-benzoyl-(S)-valine anilide-imprinted polymer (IP-2), was prepared using hydrogen-bonding interactions as a main force in the pre-polymerization step. The performance of the IP-2 was evaluated via batch procedure and compared with a (S)-valine anilide-imprinted polymer (IP-1) that was prepared using an ionic interaction that is stronger than hydrogen bonding. Although both polymers showed a preferential adsorbability for (S)-amino acid derivatives, different performances were observed in terms of adsorbability and enantioselectivity. In addition, the IP-2 was able to recognize the enantiomer of a valine-derived chiral catalyst. This phenomenon was applied to a chiral amplification reaction, and a highly selective asymmetric Mannich-type amination was achieved using the combination of a racemic catalyst and a MIP.

Molecularly imprinted polymers (MIPs)^1–3) have been energetically pursued in the field of molecular recognition, and various types of applications such as a stationary phase of HPLC,^4,5) solid-state extraction,⁶⁾ and chemical sensors^7,8) have been reported due to qualities such as synthetic simplicity, stability, and a high level of recognition selectivity.

There are several methods for the preparation of imprinted polymers, including acid–base interaction, simple hydrogen bonding, dipole–dipole interaction, and hydrophobic interaction at the pre-polymerization step. Among these, the acid–base interaction^4,5,9) and/or hydrogen bonding^10,11) are mainly used due to the effectiveness of bonding strength in regards to the tailor-made construction of three-dimentional binding sites.

Therefore, MIPs prepared using an acid–base interaction appear to show a higher level of performance than those without such an ionic interaction. However, unexpectedly, only a few reports¹²⁾ described the comparison of the different performances of MIPs between the presence of acid–base interaction and the absence of such a strong interaction. Therefore, it would be desirable to establish a new methodology, which would enable a convenient and precise comparison of the performance of MIPs that could lead to the development of a new type of MIP.

We previously reported¹³⁾ the evaluation of basic (S)-amino acid anilide (such as (S)-1)-imprinted MIPs that were prepared using an acid–base interaction by batch procedure that allowed independent observation of adsorbability and selectivity, which were not precisely measured by a HPLC column system.

In this paper we describe the evaluation of the performance of neutral (S)-N-benzoyl amino acid anilide (such as (S)-5)-imprinted MIPs, which were prepared in the presence of a weaker hydrogen-bonding interaction in the pre-polymerization step, by batch procedure. We also report a MIP with a (S)-N-benzoyl valine anilide cavity and its application to chiral amplification (Fig. 1).

Fig. 1. Structures of (S)-1 and (S)-5

Experimental

General

Ethylene glycol dimethacrylate (EGDM), methacrylic acid (MAA) and chloroform were freshly distilled prior to use. (RS)-Alanine, (S)- and (RS)-valine, (RS)-phenylalanine, (RS)-tert-leucine, methacryloyl chloride, 2,2′-azobis(isobutyronitrile) (AIBN), 2-aminoethanol, p-toluenesulfonyl chloride, cyclohexane (c-hex), diethylcyanophosphonate (DEPC), o-anisidine, o-anisic acid, La(NO₃)₃·6H₂O and H-D-Val-O^tBu were purchased and used without further purification. n-Hexane (n-hex), 2-propanol, ethanol (EtOH), and acetonitrile were all HPLC grade and used without further purification. Tetrahydrofuran (THF; dehydrated) and dichloromethane (CH₂Cl₂; dehydrated) were purchased (Kanto Chemical, Co., Inc.), stored under an argon atmosphere and used without further purification. Diethyl ether (Et₂O), ethyl acetate (EtOAc), N,N′-dimethylformamide (DMF) and methanol (MeOH) were purified by distillation.

The preparation of (S)-valine anilide-imprinted polymers (IP-1), using an acid–base interaction, and the adsorbability of various types of amino acid anilides onto IP-1 have been reported.¹³⁾

Synthesis

(S)-N-Benzoyl-valine Anilide ((S)-5); General Procedure for the Preparation of N-Benzoyl Amino Acids

(S)-Valine anilide ((S)-1) (1.34 g, 7.00 mmol) and benzoic acid (0.900 g, 7.35 mmol) were dissolved in DMF (41.7 mL). To the mixed solution was added Et₃N (1.10 mL, 7.89 mmol) and DEPC (1.10 mL, 7.35 mmol), and the solution was stirred at 0°C for 45 min. The resultant mixture was diluted with EtOAc (300 mL), and the organic layer was washed with satd. NaHCO₃ aq. (50 mL×3) and satd. NaCl aq. (50 mL×3) and dried over anhyd. Na₂SO₄. After concentration of the organic layer in vacuo, the residue was purified by recrystallization (hexane–CH₂Cl₂) to give (S)-5 (1.60 g, 5.38 mmol) as a colorless solid: mp 216–218°C; [α]_D²²−46.9 (c 0.52, CHCl₃); ¹H-NMR (300 MHz/CDCl₃) δ: 9.54 (1H, br s), 7.82–7.80 (2H, m), 7.51–7.45 (4H, m), 7.36–7.33 (2H, m), 7.05–7.02 (1H, m), 4.98 (1H, t, J=8.4 Hz), 2.41–2.36 (1H, m), 1.16 (3H, d, J=7.0 Hz), 1.14 (3H, d, J=7.0 Hz); ¹³C-NMR (75 MHz/CDCl₃) δ: 170.5, 168.1, 137.8, 133.9, 131.7, 128.7, 128.5, 127.3, 124.2, 120.3, 60.0, 31.7, 19.3, 18.9; IR (KBr) 3278, 1668, 1633, 1603, 1531, 1446 cm⁻¹; high resolution (HR)-MS (FAB) m/z Calcd for C₁₈H₂₁N₂O₂ [M+H]⁺ 297.1603. Found 297.1586.

Racemic N-Benzoyl Amino Acids,

(RS)-6, 7 and 8 were synthesized by the same procedure as (S)-5 from corresponding amino acid derivatives. Then, (RS)-6 and (RS)-8 were purified by recrystallization from hexane–CH₂Cl₂, and (RS)-7 was purified by recrystallization from EtOAc.

(RS)-N-Benzoyl-alanine Anilide ((RS)-6)

Colorless solid; mp 169–171°C; ¹H-NMR (300 MHz/CDCl₃) δ: 9.42 (1H, br s), 7.87–7.84 (2H, m), 7.60 (2H, d, J=7.7 Hz), 7.52 (1H, dd, J=9.5, 7.7 Hz), 7.44–7.40 (3H, m), 7.31–7.25 (2H, m), 7.11–7.06 (1H, m), 5.15–5.05 (1H, m), 1.58 (3H, d, J=7.0 Hz); ¹³C-NMR (75 MHz/CDCl₃) δ: 170.9, 167.7, 138.0, 133.5, 132.0, 128.9, 128.6, 127.2, 124.3, 120.0, 50.1, 18.5; IR (KBr) 3297, 1686, 1633, 1606, 1548, 1489, 1442 cm⁻¹; HR-MS (FAB) m/z Calcd for C₁₆H₁₇N₂O₂ [M+H]⁺ 269.1290. Found 269.1284.

(RS)-N-Benzoyl-phenylalanine Anilide ((RS)-7)

Colorless solid; mp 236–239°C; ¹H-NMR (300 MHz/DMSO-d₆) δ: 10.20 (1H, br s), 8.72 (1H, d, J=8.1 Hz), 7.83 (2H, d, J=6.6 Hz), 7.62 (2H, d, J=8.1 Hz), 7.54–7.40 (5H, m), 7.34–7.25 (4H, m), 7.17 (1H, dd, J=7.7, 6.6 Hz), 7.08–7.03 (1H, m), 4.87–4.82 (1H, m), 3.17–3.11 (2H, m); ¹³C-NMR (75 MHz/DMSO-d₆) δ: 170.4, 166.5, 138.9, 138.1, 133.9, 131.3, 129.2, 128.7, 128.1, 128.1, 128.0, 127.4, 126.3, 123.3, 119.3, 55.8, 37.2; IR (KBr) 3292, 3268, 1676, 1635, 1551, 1535, 1446 cm⁻¹; HR-MS (FAB) m/z Calcd for C₂₂H₂₁N₂O₂ [M+H]⁺ 345.1603. Found 345.1583.

(RS)-N-Benzoyl-tert-leucine Anilide ((RS)-8)

Colorless solid; mp 226–228°C; ¹H-NMR (300 MHz/CDCl₃) δ: 9.50 (1H, br s), 7.87–7.84 (2H, m), 7.57–7.52 (3H, m), 7.48–7.43 (2H, m), 7.27–7.20 (3H, m), 7.09–7.04 (1H, m), 5.17 (1H, d, J=9.2 Hz), 1.14 (9H, s); ¹³C-NMR (75 MHz/CDCl₃) δ: 169.4, 167.5, 137.9, 134.1, 131.9, 128.8, 128.72, 128.71, 127.1, 124.2, 120.0, 61.2, 35.9, 26.7; IR (KBr) 3305, 3267, 2962, 1678, 1627, 1554, 1539, 1490, 1446 cm⁻¹; HR-MS (FAB) m/z Calcd for C₁₉H₂₃N₂O₂ [M+H]⁺ 311.1760. Found 311.1741.

(RS)-N-(2-Hydroxyphenyl)-2-(2-hydroxybenzoylamino)-3-methylbutylamide ((RS)-9)

(RS)-9 was synthesized by the same procedure as previously reported.¹⁴⁾ Colorless solid; mp 195–197°C; ¹H-NMR (300 MHz/DMSO-d₆) δ: 11.81 (1H, s), 9.76 (1H, s), 9.41 (1H, s), 8.87 (1H, d, J=8.2 Hz), 7.98 (1H, d, J=7.9 Hz), 7.75 (1H, d, J=7.9 Hz), 7.42–7.38 (1H, m), 6.95–6.85 (4H, m), 6.79–6.74 (1H, m), 4.69 (1H, dd, J=7.7, 7.3 Hz), 2.28–2.21 (1H, m), 1.00 (3H, d, J=5.7 Hz), 0.98 (3H, d, J=5.9 Hz).

(S)-N-(2-Methoxyphenyl)-2-(2-methoxybenzoylamino)-3-methylbutyramide ((S)-10)

To a mixture of (S)-N-Boc-Val-OH (2.50 g, 11.5 mmol) and Et₃N (1.60 mL, 11.5 mmol) in THF (30 mL) was added ethyl chloroformate (1.10 mL, 11.5 mmol) in THF (13 mL) dropwise over 15 min at −15°C. After stirring for 15 min, o-anisidine (1.36 mL, 12.1 mmol) was subsequently added and stirred at room temperature (r.t.) for 15 h. The resultant mixture was diluted with EtOAc (300 mL), then washed with 10% KHSO₄ aq. (50 mL×3), satd. NaHCO₃ aq. (50 mL×3), satd. NaCl aq. (50 mL×3) and dried over anhyd. Na₂SO₄. After concentration in vacuo, the residual solid was washed with a mixture of hexane–Et₂O. To a solution of the washed solid (2.38 g) in CH₂Cl₂ was added Amberlyst 15 (19.6 g) with stirring at r.t. for 15 h. The resultant mixture was filtered, and the resin was washed with hexane, MeOH and THF. The resin was poured into NH₃/ⁱPrOH (197 mL) and stirred at r.t. for 40 min. The resin was filtered off, and the filtrate was concentrated in vacuo. The residue was dissolved in DMF (35 mL) and o-anisic acid (0.930 g, 6.10 mmol); Et₃N (0.85 mL, 6.10 mmol) and DEPC (0.870 mL, 5.80 mmol) were sequentially added at 0°C followed by stirring at 0°C for 30 min. The resultant mixture was diluted with EtOAc (300 mL), washed (satd. NaHCO₃ aq. (50 mL×3) and satd. NaCl aq. (50 mL×3)) and dried over anhyd. Na₂SO₄. After concentration in vacuo, the residue was purified by silica gel column chromatography (CH₂Cl₂ to CH₂Cl₂–EtOAc (97.5: 7.5)) to give (S)-10 as a colorless solid (1.60 g, 4.49 mmol); mp 108–110°C; [α]_D²² +39.9 (c 0.70, CHCl₃); ¹H-NMR (300 MHz/CDCl₃) δ: 8.51 (1H, d, J=8.4 Hz), 8.38 (1H, br s), 8.35 (1H, dd, J=7.9, 1.6 Hz), 8.21 (1H, dd, J=7.7, 1.8 Hz), 7.50–7.44 (1H, m), 7.11–6.83 (5H, m), 4.71 (1H, dd, J=8.4, 5.9 Hz), 4.01 (3H, s), 3.80 (3H, s), 2.44–2.40 (1H, m), 1.09 (3H, d, J=6.8 Hz), 1.07 (3H, d, J=6.8 Hz); ¹³C-NMR (75 MHz/CDCl₃) δ: 169.4, 165.4, 157.6, 148.1, 133.0, 132.3, 127.3, 123.9, 121.2, 121.0, 120.9, 119.9, 111.4, 110.0, 59.9, 56.1, 55.7, 30.8, 19.4, 18.0; IR (KBr) 3305, 2953, 1668, 1630, 1535, 1493, 1461, 1432 cm⁻¹; HR-MS (FAB) m/z Calcd for C₂₀H₂₅N₂O₄ [M+H]⁺ 357.1814. Found 357.1812.

Synthesis of (S)-N-Phenyl-2-(2-methoxybenzoylamino)-3-methylbutyramide ((S)-11)

(S)-Valine anilide ((S)-11) (200 mg, 1.04 mmol) and o-anisic acid (166 mg, 1.09 mmol) were dissolved in DMF (6.20 mL). Et₃N (0.150 mL, 1.10 mmol) and DEPC (0.160 mL, 1.00 mmol) were added sequentially to the solution at 0°C, and the mixture was stirred at 0°C for 45 min. The resultant mixture was diluted with EtOAc (150 mL) and washed with satd.NaHCO₃ aq. (50 mL×3), satd. NaCl aq. (50 mL×3) and dried over anhyd. Na₂SO₄. After the organic layer was concentrated in vacuo, the residue was purified by silica gel column chromatography (CH₂Cl₂ to CH₂Cl₂–EtOAc (87.5 : 12.5)) to give a colorless solid; mp 103–105°C; [α]_D²²−30.6 (c 0.54, CHCl₃); ¹H-NMR (300 MHz/CDCl₃) δ: 8.82 (1H, br s), 8.54 (1H, d, J=8.2 Hz), 8.20 (1H, dd, J=7.9, 1.5 Hz), 7.54–7.46 (3H, m), 7.27–7.24 (2H, m), 7.08–7.03 (3H, m), 4.74 (1H, dd, J=8.2, 6.9 Hz), 4.02 (3H, s), 2.50–2.38 (1H, m), 1.12 (3H, d, J=6.8 Hz), 1.10 (3H, d, J=6.8 Hz); ¹³C-NMR (75 MHz/CDCl₃) δ: 170.0, 165.8, 138.0, 133.2, 132.2, 128.7, 124.0, 121.2, 121.0, 120.1, 111.5, 59.8, 56.1, 30.9, 19.5, 18.4; IR (KBr) 3284, 2958, 1635, 1601, 1535, 1489, 1444 cm⁻¹; HR-MS (FAB) m/z Calcd for C₁₉H₂₃N₂O₃ [M+H]⁺ 327.1709. Found 327.1719 (Fig. 2).

Fig. 2. Structures of the Compounds

General Procedure for the Preparation of Polymers

Preparation of a Molecular-Imprinted Polymer (MIP) (Table 1)

We prepared several types of polymers as follows. A N-benzoyl-(S)-valine anilide ((S)-5)-imprinted MIP (IP-2) was prepared by the polymerization of a mixture of (S)-5, ethylene glycol dimethacrylate (EGDM), methacrylic acid (MAA), and AIBN in chloroform under irradiation followed by an appropriate amount of grinding and washing. In a similar manner, IP-3 and IP-4 were prepared by following the IP-2 procedure. In a glass vial, (S)-5 (889 mg, 3.00 mmol) was dissolved in chloroform (12.0 mL). To this solution were added MAA (1.00 mL, 12.0 mmol), EGDM (9.00 mL, 48.0 mmol), and AIBN (99.6 mg, 0.610 mmol). After sufficient mixing, the solution was separated into four borosilicate screw-capped test tubes, and each test tube was flushed with argon gas followed by irradiation with a 100 W high-pressure mercury vapor lamp (5°C, 24 h) to form a polymer. Bulk polymers were ground with a mill (OSAKA Chemical, Wonder Blender, WB-1) and a ball mill (Nitto Kagaku, ANZ-50S). These were then sieved through a 500 µm mesh filter, and particles of <500 µm were collected. The polymer was washed (by soxhlet extractor (MeOH–AcOH (7 : 3)), 20 h), filtered (Kiriyama Funnel (filter paper: No. 5B)), immersed in THF, and refluxed for 5 h. The polymer was collected by filtration (Kiriyama Funnel (filter paper: No. 5B)), washed again (CH₂Cl₂), and dried in vacuo.

The preparation of a (S)-valine anilide ((S)-1)-imprinted MIP (IP-1) has been reported previously.¹³⁾

Table 1. Composition of the MIPs

Entry	Polymer	Template	Template (mmol)	MAA (mmol)	EGDM (mmol)	CHCl3 (mL)
1	IP-1a)	(S)-1	1	4	16	4
2	IP-2	(S)-5	3	12	48	12
3	IP-3	10	0.25	1	4	1
4	IP-4	11	0.25	1	4	1

a) See ref. 13.

General Procedure for Evaluating the Ability of a MIP Using the “Batch Procedure”

In a glass vial, (RS)-5 (2.60 µmol) was dissolved in a mixture of CH₂Cl₂ and n-hexane (10 mL; CH₂Cl₂–hexane (1 : 1)). This concentration was defined as [5]₀, which was an average of [5]_S0 (concentration value of (S)-form) and [5]_R0 (concentration value of (R)-form). To this solution was added a MIP (46.0 mg, 13.0 µmol as recognizing sites), followed by stirring at 24°C for 2 h. The MIP was filtered off, and the concentrate of this filtrate was defined as [5]_S and [5]_R. [5]₀ and [5] were determined by HPLC¹⁵⁾ using N-p-toluenesulfonyl-2-aminoethanol (Ts-C2AA; 12)¹⁶⁾ as an internal standard. The adsorbed amount and selectivity were calculated using the following equations:   

S_ad and R_ad were used as the indices of adsorbability, and e.e.p. was used as an index of selectivity.

The number of recognition sites in the MIP was calculated by dividing the number of template molecules (mmol) by the assumption of 100% of the yield of MIP (g), i.e.,   

In this paper, we calculated the equivalents of the MIPs based on this value.

Application: Asymmetric Amination

Preparation of cat. A and cat. B

(RS)-9 (65.7 mg, 0.200 mmol) was dissolved in a mixture of cyclohexane, CH₂Cl₂ and EtOAc (153 mL; c-hex–CH₂Cl₂–EtOAc=7 : 2 : 1). To this solution was added IP-2 (3.50 g), which was stirred at r.t. for 30 min. The IP-2 was filtered off (Kiriyama Funnel (filter paper: No. 5B)) and washed with cyclohexane (15 mL). The combined filtrates were concentrated in vacuo to obtain unadsorbed 9, with a slight incline to the (R)-isomer (7% ee) as a colorless solid. To the solid was added a mixed solvent of n-hexane and CH₂Cl₂ (4 mL; hexane–CH₂Cl₂ (6 : 4)), followed by filtration (minisalt SRP 15) to remove the racemate ((RS)-9) as a solid (95% recovered yield). The filtrate was concentrated in vacuo to afford (R)-9 (1.5 mg) with 94% ee as a colorless solid that was used as cat. A.

The filtered MIP was washed with MeOH (15 mL×3) and CH₂Cl₂ (15 mL×1). The eluent was concentrated in vacuo, followed by the same procedure as the filtered solution described above to give (S)-9 (1.5 mg) with 94% ee as a colorless solid that was used as cat. B.

The recovered MIP was reused after desiccation in vacuo, without further operation.

Asymmetric Amination^14,17)

The cat. A and La(NO₃)₃·6H₂O (4.40 mg, 10.0 mmol) were dissolved in EtOAc under an argon atmosphere. To the solution was added H-D-Val-O^tBu in EtOAc (60.0 mL, 30.0 mmol) and ethyl 3-amino-3-oxo-2-phenylpropanoate 13 at r.t., and the reaction mixture was cooled to 0°C. To the mixture was added di-tert-butyl azodicarboxylate 14 (55.3 mg, 0.240 mmol) in EtOAc (0.20 mL) at 0°C with stirring for 48 h. The reaction was quenched by the addition of 1 M HCl aq. (1.0 mL), and the resultant mixture was extracted with CH₂Cl₂ (5.0 mL×3). The organic layer was washed with satd. NaHCO₃ aq. (5.0 mL×3), satd. NaCl aq. (5.0 mL×3), and dried over anhyd. Na₂SO₄. After concentration in vacuo, the residue was purified by silica gel column chromatography (hexane–EtOAc=5 : 1 to 3 : 1) to give (R)-15 (80.1 mg, 0.183 mmol, 92, 97% ee) as a colorless foam: ¹H-NMR (300 MHz/CDCl₃) δ: 8.65–8.53 (1H, br m), 7.68–7.57 (2H, m), 7.47–7.28 (3H, m), 6.66 (1H, br s), 6.29–6.17 (1H, br m), 4.34–4.29 (1H, br m), 1.49–1.09 (21 H, m).

The same experiment using cat. B instead of cat. A gave (S)-15 (86.6 mg, 0.198 mmol, 99, 97% ee) as a colorless foam.

The enantiomeric excess of the obtained products was determined via HPLC; DAICEL CHIRALPAK AD-H, hexane–MeOH=95 : 5, 1.0 mL/min, 40°C, 254 nm, t_R=7.8 min (S), 10.4 min (R). Spectroscopic data was consistent with that previously reported.¹⁷⁾

Results and Discussion

Evaluation of IP-2 and the Solvent Effect

We first evaluated the adsorbability of the racemic N-benzoyl-valine anilide ((RS)-5) onto IP-2 in dichloromethane via batch procedure (Table 2). As described above, IP-2 was prepared using hydrogen-bonding interactions as a main force in the pre-polymerization step and IP-1 was similarly prepared using an acid–base interaction.¹³⁾ Despite the lack of a stronger acid–base interaction that was advantageous for molding the recognition sites, surprisingly, IP-2 showed a higher stereoselectivity than expected (entry 1). In CH₂Cl₂, N-benzoylvaline anilide exhibited lower adsorbability onto IP-2 than that of a valine anilide onto IP-1 (see Table 3, entry 2). Such a weak point for IP-2, however, was overcome by decreasing the polarity of the solvent, and adequate adsorbability of IP-2 was achieved. Comparing the results of adsorbability and enantioselectivity, we chose a mixed-solvent system that consisted of dichloromethane–n-hexane (1 : 1) for further research (entry 2).

Table 2. The Effects of Solvents for the Adsorbability of (RS)-5 onto IP-2^a)

Entry	Solvent	Sad (%)	Rad (%)	e.e.p. (%)	e.e.s. (%)
1	CH2Cl2	25	12	35	8
2	CH2Cl2–c-Hex=50 : 50	46	30	21	13
3	CH2Cl2–c-Hex=25 : 75	64	54	8	12

a) Conditions: (RS)-5, 0.26 mM, IP-2 (5 eq), 24°C, 2 h.

Comparison of Substrates

In our previous report, we found that IP-1 synthesized using (S)-valine anilide as a template molecule, with the aid of a stronger acid–base interaction than hydrogen bonding, could recognize not only the enantiomers of a template molecule but also those of similar amino acid derivatives (Ala, Phe and t-Leu anilides), which showed similar enantioselectivities for all substrates, but also variations in the amount of adsorbed substrates¹³⁾ (Table 3).

Table 3. Enantioselective Adsorption of Amino Acid Anilides 1–4 by IP-1^a,b)

Based on such curious results, we next evaluated the adsorbabilities and enantioselectivities of IP-2 toward various types of N-benzoyl amino acid anilides (Table 4).

Table 4. Enantioselective Adsorption of N-Benzoyl Amino Acid Anilides 5–8 by IP-2^a)

To our delight, as shown in Table 4, IP-2 could effectively differentiate not only the enantiomers of template molecules, N-benzoyl valine anilide 5, but also those of similar N-benzoyl amino acid derivatives 6, 7, and 8 to yield slightly (R)-enriched solutions, along with preferentially (S)-adsorbed IP-2. N-Benzoyl phenylalanine anilide 7 possessed benzyl moieties that were larger than those of methyl, isopropyl, or tert-butyl groups in the amino acid scaffold and, intriguingly, it was adsorbed by IP-2 in a ratio similar to other N-benzoyl amino acid anilides, but with an inferior level of enantioselectivity. The trend in these results concerning the adsorption of phenylalanine anilide onto IP-2 was apparently different than the results with IP-1, which showed lower adsorbability and a similar enantioselectivity in the presence of acid–base interactions (see Table 3).

According to observations from the present study, the different properties of IP-1 and IP-2 with regard to adsorbability and enantioselectivity can be summarized as follows. The adsorbability of IP-1 was strongly influenced by the basicity of amino acids. Phenylalanine has a lower basicity than other amino acids such as alanine, valine or tert-leucine,¹⁸⁾ which is suspected to be caused by sterical bulkiness, and, therefore, the weaker acid–base interaction between the acidic moiety in the mold of a MIP and phenylalanine showed adsorption that was inferior to those of other amino acid anilides. However, once the amino acid anilides were attracted to the recognition sites of a MIP, the stronger acid–base interaction dominated the steric repulsion between the mold and the substrates, which then resulted in similar enantioselectivities for all the amino acid anilides we tested.

On the other hand, weak hydrogen-bonding attracting interactions and steric repulsion existed between the mold of IP-2 and the N-benzoyl amino acid anilide substrates. Therefore, the steric repulsion between IP-2 and the side chains of amino acids, particularly the benzyl group of phenylalanine, would affect the enantioselectivities for the adsorbance of IP-2 in a more apparent fashion than that for IP-1, which introduces a slight decrease in the adsorbance of (S)-enantiomers to show a lower e.e.p. value. However, the strength of the hydrogen bonding between IP-2 and N-benzoyl amino acid anilides seemed comparable, which would lead to similar adsorbabilities for amino acid derivatives toward IP-2.

Application

From our delightful results showing that MIPs molded using hydrogen bonding as a main attracting force caused an incline in the enantiomeric ratio, we envisaged the application of such a MIP to chiral amplification.^19–21) Thus, treatment of a solution of racemic catalyst to a MIP would cause a slight lean in the ratio of enantiomers, and the use of such an “enantioenriched solution” for asymmetric synthesis via chiral amplification could produce enantiomerically pure compounds. It is noteworthy that a racemic catalyst would produce optically pure compounds with the help of a MIP that would be easily recovered and recyclable.

Kumagai, Shibasaki and colleagues, previously reported the non-linear effect of chiral catalyst 9¹⁷⁾ for an asymmetric Mannich-type reaction. In their report, the catalyst was prepared by intentional mixing of the pure enantiomers of (S)-9 and (R)-9. Considering the structural similarities of 9 and 5, we attempted to apply the above-mentioned methodology to the racemic solution of 9 for the generation of an enantioenriched solution of 9, followed by application to an asymmetric Mannich-type reaction.

We first tried to prepare several types of MIPs for the enantioselective recognition of 9, using various N-benzoyl-(S)-valine anilide derivatives (10, 11) as chiral templates. Except for the use of 9,²²⁾ two types of MIPs, IP-3 and IP-4, using 10 and 11 as template molecules, respectively, were newly synthesized (see Table 1).

The adsorbability and enantioselectivity of IP-3 and IP-4, along with IP-2, was evaluated in the presence of (RS)-9 as a substrate (Table 5, entries 1–3). As a result, contrary to our expectations, only IP-2 gave satisfactory levels of adsorbability and enantioselectivity to a racemic solution of 9, although its effect as a substrate was inferior to that of (RS)-5.

Table 5. The Effects of Concentration and Composition of the Solvents for the Enantioselective Adsorption of (RS)-9 by IP-2–IP-4^a)

Entry	Conc. (mM)	MIP (eq)	Solvents (%)			Sad (%)	Rad (%)	e.e.p. (%)	e.e.s. (%)
			c-Hex	CH2Cl2	EtOAcb)
1	0.26	IP-2 (5)	75	20	5	40	31	13	7
2	0.26	IP-3 (5)	75	20	5	20	20	—	—
3	0.26	IP-4 (5)	75	20	5	24	22	5	1
4	1.3	IP-2 (1)	75	20	5	23	20	7	2
5	1.3	IP-2 (3)	75	20	5	53	48	4	4
6	1.3	IP-2 (5)	75	20	5	69	64	4	8
7	1.3	IP-2 (5)	70	20	10	47	41	7	6
8	1.3	IP-2 (5)	75	15	10	52	47	5	5

a) Conditions: Substrate was used (RS)-9, 24°C, 2 h. b) See ref. 23.

We next optimized the adsorption conditions (Table 5, entries 4–8). A greater concentration of the substrate (RS)-5 led to a greater level of adsorption (entries 1 and 6). We also concluded that the addition of a 3–5 eq amount of IP-2 was optimal for the production of an adequate e.e.s. value (entries 4–6). The optimization of the composition of a solvent system was also determined, and an increase in the proportion of EtOAc decreased the amount of adsorption (entries 6–8).

Based on the results assembled in Table 5, the condition shown in entry 7 seemed to be the optimized choice for an appropriate adsorbability that would be compatible with enantioselectivity, and we applied this condition to the actual asymmetric Mannich-type amination system. Thus, the treatment of a racemic solution of 9 with 5 eq of IP-2 at r.t. for 2 h followed by separation gave the filtrate including a slightly (R)-inclined 9, which was transformed into (R)-enriched 9 with 94% ee by single reprecipitation of the racemate. Moreover, adsorbed 9, which was slightly inclined to an (S)-form, was also collected via the washing of IP-2 with methanol, followed by the same procedure as the filtered solution described above to give (S)-enriched 9 with a 94% ee Both of the gained enantiomers, (S)-9 and (R)-9, were applied to an asymmetric Mannich-type amination reaction, and, to our delight, both enantiomers gave the aminated products, (R)-15 and (S)-15, respectively, in high yields with high enantioselectivities (Chart 1).

Chart 1. Application to Chiral Amplification

Conclusion

We prepared a novel type of N-benzoyl-(S)-valine anilide-imprinted polymer, IP-2, using hydrogen-bonding interactions as a main force for molding and evaluated the details of the performance of IP-2 via batch procedure. It appeared that IP-2 could differentiate not only the enantiomer of a template molecule but also those of other types of N-benzoyl amino acid anilides, which led to the preferential adsorption of N-benzoyl-(S)-amino acid anilides in good order. Interestingly, the properties of IP-2 differed from those of IP-1, which was synthesized using an acid–base interaction for the molding of adsorbability and enantioselectivity.

Based on such curious findings, we also found that IP-2 could recognize the enantiomers of valine-derived bis(2-hydroxyphenyl)diamide 9, the analogue of N-benzoylvaline anilide 5, and that both (R)- and (S)-enriched 9 with 94% ee were easily afforded from the filtrate and filtered MIP, respectively, with the help of a single reprecipitation of the racemate. The (R)- and (S)-enriched 9 was applied to an asymmetric Mannich-type amination reaction, and the corresponding aminated products were obtained in high yields with high enantioselectivities.

These intriguing results clearly prove that the treatment of a racemic catalyst with MIP triggered a highly stereoselective asymmetric synthesis via chiral amplification. Furthermore, adsorbed MIP was reusable after an appropriate work-up (washing with polar solvent, substitution to a non-polar solvent and desiccation. Table 6).²⁴⁾

Table 6. Recyclability of IP-2^a)

Run	Sad (%)	Rad (%)	e.e.s. (%)	e.e.p. (%)
1	46	39	8	7
2	44	36	9	7
3	44	37	9	6

a) Conditions: (RS)-9 (0.20 mmol), IP-2 (5 eq), c-Hex–CH₂Cl₂–EtOAc=7 : 2 : 1, r.t., 30 min.

Thus, we proved the great potential of a MIP for application to asymmetric syntheses and to recyclability for environmental friendliness.

Further, and more precise, mechanistic studies and synthetic applications to various asymmetric reactions are currently in progress.

Conflict of Interest

The authors declare no conflict of interest.

References and Notes

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• 15) HPLC condition were as follows; Table 2, Table 4, entry 2: DAICEL CHIRALPAK IA, hexane–EtOH=85 : 15, 0.75 mL/min, 254 nm, 12 as internal standard, t_R=8.8 min (R), 18.4 min (S), 27.8 min (internal standard); Table 4, entry 1: DAICEL CHIRALPAK IA, hexane–EtOH=85 : 15, 1.0 mL/min, 254 nm, 12 as internal standard, t_R=9.2 min (R), 13.0 min (S), 19.6 min (internal standard); Entry 3: DAICEL CHIRALPAK IA, hexane–EtOH=85 : 15, 1.0 mL/min, 254 nm, 12 as internal standard, t_R=13.7 min (S), 16.1 min (R), 19.4 min (internal standard); Entry 4: DAICEL CHIRALPAK IA, hexane–EtOH=85 : 15, 0.75 mL/min, 254 nm, 12 as internal standard, t_R=6.7 min (R), 9.6 min (S), 25.7 min (internal standard); Table 5: DAICEL CHIRALPAK IC, hexane–EtOH=95 : 5, 0.75 mL/min, 254 nm, N-benzoyl-2-phenethylamine as internal standard, t_R=11.8 min (S), 13.4 min (R), 26.1 min (internal standard).
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• 22) Unfortunately, compound (RS)-9 did not tolerate radical polymerization condition and we could not prepare the corresponding MIP.
• 23) (RS)-9 could not dissolve in non-polar solvent without polar co-solvent. For this reason, ethyl acetate was added in the presence of (RS)-9 as a substrate.
• 24) The recyclability of IP-2 is shown in Table 6. These results were part of the adsorption experiments of (RS)-9 using IP-2, as shown in Chart 1.Through the recycling process, none of the turbidity by the pulverization of the polymer particle in the filtrate was found. Considering this observation and the results of Table 6, we concluded that IP-2 was reusable.