Synthesis and properties of optically active helical polymers from (S)-3-functional-3′-vinyl-BINOL derivatives

Abstract

A family of optically active helical vinyl polymers bearing N-heterocycle substituted BINOL derivatives was synthesized by free radical polymerization. Their structures were characterized by ¹H NMR and ¹³C NMR spectra. The specific optical rotation of the obtained polymers changed tremendously compared with the corresponding monomers. Circular dichroism spectroscopic data showed that the obtained polymers can maintain the prevailing helicity of the backbone in solution.

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Introduction

Helical polymers are of great interest due to their broad potential applications in molecular recognition, asymmetric synthesis of enantiomers as well as chiral building blocks for self-assembled nanomaterial and devices.^1–7 Great efforts have been made to comprehend the properties of helical structures and to construct functional helical polymers^1,8 and oligomers.^9–12 One of the most basic methods for synthetic helical polymers is the polymerization of chiral monomers.^13–16 BINOL and its derivatives represent a class of important chiral molecules that possess extensive applications for their modifiable versatile backbones.^17–28 The BINOL moiety can be conveniently functionalized in various positions. The most frequent ones are the 4,4′ and 6,6′ positions, 3,3′ positions are also well documented.^25,29–38 With appropriate choices of substituents, BINOL derivatives can be transformed into various chiral monomers. Various modified BINOL derivatives have been synthesized and successfully applied to asymmetric catalytic reactions or molecular recognition.^39–42 Of particular importance in this respect is the 3-N-heterocycles substituted BINOL derivatives because of the coordination ability of nitrogen atoms.^43–45

In continuation of our ongoing interest in the development of helical polymers and the achievement that our group has made,⁴⁶ we present herein the synthesis of three 3-functional-3′-vinyl BINOL derivative polymers from BINOL and imidazole/indole/carbazole. The specific optical rotation, circular dichroism (CD) and UV-vis spectroscopic data show that the obtained polymers can keep a prevailing helicity of backbone in solution.

Experimental section

General

All reagents were used as supplied commercially. Tetrahydrofuran (THF) and toluene were distilled from sodium under N₂ before use. ¹H and ¹³C NMR spectra were recorded using a Bruker ARX400 MHz spectrometer using tetramethylsilane (TMS) as internal standard and CDCl₃ as solvent. Optical rotation data were measured on a Perkin Elmer Model 341 LC Polarimeter at 365 nm. Circular dichroism (CD) spectra were performed on Jasco J-810 CD instrument using CHCl₃ as solvent. Elemental analyses were carried out on Elementar Vario EL instrument. GPC analysis was performed with a JASCO-GPC system consisting of DG-1580-53 degasser, PU-980 HPLC pump, UV-970 UV/Vis detector, RI-930 RI detector, and CO-2065-plus column oven (at 38 °C) using two connected Shodex GPC-KF-804L columns in THF (sample concentration = 1 wt%; flow rate = 1.0 mL min⁻¹). Molecular weight and polydispersity data are reported relative to polystyrene standards.

Synthesis

(S)-3′-Hydroxymethyl-2,2′-bis(methoxymethoxy)-[1,1′-binaphthalene]-3-carbaldehyde, ((S)-2). NaBH₄ (0.11 g, 2.91 mmol) was added to a solution of the (S)-2,2′-bis(methoxymethoxy)-[1,1′-binaphthalene]-3,3′-dicarbaldehyde ((S)-1) (5.00 g, 11.62 mmol) in methanol (150 mL) at 0 °C. After stirred for 30 min, solvent was removed and the residue was purified by flash chromatography on silica (petroleum ether/ethyl acetate = 3/1) to give (S)-2 in 75% (3.77 g) as yellow oil: ¹H NMR δ 10.56 (s, 1H), 8.60 (s, 1H), 8.08 (s, 1H), 8.05 (s, 1H), 7.93 (d, J = 8.1 Hz, 1H), 7.51 (t, J = 7.4 Hz, 1H), 7.47–7.39 (m, 2H), 7.30 (d, J = 7.8 Hz, 1H), 7.23 (d, J = 8.5 Hz, 1H), 7.14 (d, J = 8.4 Hz, 1H), 5.01–4.96 (m, 1H), 4.90–4.85 (m, 1H), 4.65 (d, J = 2.6 Hz, 2H), 4.55 (d, J = 6.2 Hz, 1H), 4.49 (d, J = 6.2 Hz, 1H), 3.34 (t, J = 6.8 Hz, 1H), 3.21 (s, 3H), 2.91 (s, 3H). ¹³C NMR δ 190.67, 153.55, 152.73, 136.72, 134.48, 133.16, 131.73, 130.76, 130.04, 129.76, 129.31, 129.16, 128.77, 127.97, 126.54, 126.46, 125.96, 125.91, 125.44, 125.20, 124.11, 100.06, 99.13, 61.00, 56.71, 56.57. Elem. anal. calcd for C₂₆H₂₄O₆: C, 72.21; H, 5.59. Found: C, 72.25; H, 5.56.

(S)-3′-(Chloromethyl)-2,2′-bis(methoxymethoxy)-[1,1′-binaphthalene]-3-carbaldehyde, ((S)-3). A toluene (100 mL) solution of (S)-2 (3.00 g, 6.94 mmol) was cooled to 0 °C. Methanesulfonyl chloride (MsCl) (1.34 mL, 17.35 mmol) and Et₃N (3.38 mL, 24.29 mmol) were added to this solution and stirred for 1 h. Then the mixture was treated with LiCl (0.75 g, 17.35 mmol) and dimethylformamide (DMF) (25 mL) and stirred at room temperature using TLC track the reaction until the starting material disappeared. The mixture was washed with brine (50 mL × 2) and the separated aqueous layer was extracted with ethyl acetate (30 mL × 2). The combined organic layer was dried over Na₂SO₄. Solvent was evaporated under reduced pressure, and the residue was chromatographed (petroleum ether/ethyl acetate = 3/1) to give (S)-3 in 90% (2.82 g) as yellow oil: ¹H NMR δ 10.56 (s, 1H), 8.60 (s, 1H), 8.14 (s, 1H), 8.06 (d, J = 8.0 Hz, 1H), 7.93 (d, J = 8.1 Hz, 1H), 7.52 (d, J = 7.0 Hz, 1H), 7.43 (dd, J = 16.8, 7.8 Hz, 2H), 7.31 (t, J = 7.6 Hz, 1H), 7.25 (s, 1H), 7.15 (d, J = 8.4 Hz, 1H), 4.97 (d, J = 13.4 Hz, 2H), 4.66 (s, 2H), 4.59 (d, J = 5.7 Hz, 2H), 2.95 (s, 3H), 2.90 (s, 3H). ¹³C NMR δ 190.89, 154.01, 152.55, 137.01, 133.95, 131.93, 131.26, 130.75, 130.32, 130.06, 129.60, 129.01, 128.27, 127.39, 126.65, 126.25, 125.80, 124.95, 100.47, 99.81, 57.08, 56.86, 42.40. Elem. anal. calcd for C₂₆H₂₃ClO₅: C, 69.25; H, 5.14. Found: C, 69.27; H, 5.16.

(S)-3′-((1H-Imidazol-1-yl)methyl)-2,2′-bis(methoxymethoxy)-[1,1′-binaphthalene]-3-carbaldehyde, ((S)-4a). To a solution of (S)-3 (1.50 g, 3.33 mmol) in THF (100 mL) was added potassium hydroxide (KOH) (0.37 g, 6.66 mmol) and tetrabutylammonium bromide (TBAB) (0.11 g, 0.34 mmol) at room temperature. After stirred for 30 min, imidazole (0.23 g, 3.35 mmol) in THF (15 mL) was added and the mixture was stirred at the same temperature using TLC track the reaction until the starting material disappeared. Solvent was evaporated under reduced pressure. The residue was diluted in ethyl acetate (100 mL) and washed with brine (50 mL × 2). The separated aqueous layer was extracted with ethyl acetate (30 mL × 2). The combined organic layer was dried over Na₂SO₄. Solvent was evaporated under reduced pressure, and the residue was chromatographed (CH₂Cl₂/MeOH = 30/1) to give (S)-4a in 81% (1.30 g) as yellow oil: ¹H NMR δ 10.55 (s, 1H), 8.60 (s, 1H), 8.07 (d, J = 8.1 Hz, 1H), 7.80 (d, J = 8.1 Hz, 1H), 7.73 (s, 1H), 7.50 (s, 1H), 7.43 (s, 2H), 7.30 (d, J = 7.9 Hz, 2H), 7.23–7.19 (m, 2H), 7.16–7.10 (m, 2H), 5.51 (d, J = 21.3 Hz, 2H), 4.62 (s, 2H), 4.48 (s, 2H), 3.09 (s, 3H), 2.89 (s, 3H). ¹³C NMR δ 190.51, 153.75, 152.19, 137.66, 136.67, 133.52, 132.19, 130.60, 130.50, 130.25, 129.88, 129.58, 129.46, 128.89, 128.65, 128.07, 127.07, 126.29, 126.13, 125.93, 125.64, 124.66, 119.61, 100.28, 99.53, 56.84, 56.76, 46.70. Elem. anal. calcd for C₂₉H₂₆N₂O₅: C, 72.18; H, 5.43. Found: C, 72.17; H, 5.46.

(S)-3′-((1H-Indol-1-yl)methyl)-2,2′-bis(methoxymethoxy)-[1,1′-binaphthalene]-3-carbaldehyde ((S)-4b) and (S)-3′-((9H-carbazol-9-yl)methyl)-2,2′-bis(methoxymethoxy)-[1,1′-binaphthalene]-3-carbaldehyde ((S)-4c) were prepared in a similar manner (see ESI†).

(S)-1-((2,2′-Bis(methoxymethoxy)-3′-vinyl-[1,1′-binaphthalen]-3-yl)methyl)-1H-imidazole, ((S)-5a). A Schlenk flask charged with Ph₃PMeBr (2.44 g, 6.84 mmol) was added anhydrous THF (30 mL). The mixture was cooled to 0 °C, then potassium tert-butoxide (0.77 g, 6.84 mmol), the solution of (S)-4a (1.10 g, 2.28 mmol) in dry THF (10 mL) was added subsequently. The reaction mixture was stirred for 60 min and quenched with water, extracted with ethyl acetate. After removal of the solvent, the residue was purified by flash column chromatography (petroleum ether/ethyl acetate = 10/1) to give (S)-5a in 96% (1.05 g) as a white solid: ¹H NMR δ 8.15 (s, 1H), 7.91 (d, J = 8.1 Hz, 1H), 7.76 (d, J = 8.1 Hz, 1H), 7.71 (s, 1H), 7.44 (s, 1H), 7.43–7.38 (m, 2H), 7.29 (s, 1H), 7.25 (s, 1H), 7.23–7.17 (m, 3H), 7.14 (d, J = 8.4 Hz, 1H), 7.10 (s, 1H), 6.00 (d, J = 17.6 Hz, 1H), 5.56 (d, J = 16.0 Hz, 1H), 5.50–5.44 (m, 2H), 4.65 (d, J = 5.9 Hz, 1H), 4.56 (d, J = 5.8 Hz, 1H), 4.46 (dd, J = 12.2, 5.9 Hz, 2H), 3.12 (s, 3H), 2.57 (s, 3H). ¹³C NMR δ 151.59, 151.14, 137.34, 133.54, 133.04, 132.05, 131.30, 130.42, 130.18,130.06, 129.20, 127.84, 127.67, 127.43, 126.31, 125.76, 125.30, 125.06, 124.95, 124.74, 119.21, 116.00, 98.94, 98.61, 56.46, 55.82, 46.32. Elem. anal. calcd for C₃₀H₂₈N₂O₄: C, 74.98; H, 5.87. Found: C, 74.94; H, 5.88.

(S)-1-((2,2′-Bis(methoxymethoxy)-3′-vinyl-[1,1′-binaphthalen]-3-yl)methyl)-1H-indole ((S)-5b) and (S)-9-((2,2′-bis(methoxymethoxy)-3′-vinyl-[1,1′-binaphthalen]-3-yl)methyl)-9H-carbazole ((S)-5c) were prepared in a similar manner (see ESI†).

(S)-1-((3′-Ethyl-2,2′-bis(methoxymethoxy)-[1,1′-binaphthalen]-3-yl)methyl)-1H-imidazole, ((S)-6a). (S)-5a (1.00 g, 2.08 mmol), ammonium formate (0.31 g, 6.24 mmol), 10% Pd–C catalyst (0.05 g) and THF (25 mL) were added into a dry Schlenk flask. The mixture was stirred and refluxed for 20 h. After filtrating and removing of the solvent, the residue was purified by flash column chromatography (petroleum ether/ethyl acetate = 3/1) to afford (S)-6a in 80% (0.80 g) as yellow oil: ¹H NMR δ 7.86 (d, J = 11.0 Hz, 2H), 7.76 (d, J = 8.2 Hz, 1H), 7.71 (s, 1H), 7.43–7.38 (m, 3H), 7.28 (d, J = 1.2 Hz, 1H), 7.24 (dd, J = 8.5, 1.2 Hz, 1H), 7.20 (d, J = 9.0 Hz, 2H), 7.11 (d, J = 8.0 Hz, 2H), 5.51 (dd, J = 40.6, 16.1 Hz, 2H), 4.56 (d, J = 5.8 Hz, 1H), 4.41 (s, 2H), 4.37 (dd, J = 15.0, 8.0 Hz, 1H), 3.12 (s, 3H), 3.03 (dq, J = 14.8, 7.2 Hz, 1H), 2.90 (td, J = 15.2, 7.9 Hz, 1H), 2.83 (s, 3H), 1.41 (t, J = 7.5 Hz, 3H). ¹³C NMR δ 153.14, 151.82, 137.81, 137.58, 134.04, 132.53, 131.04, 130.56, 130.46, 129.59, 128.46, 127.93, 127.75, 127.60, 126.90, 126.13, 125.97, 125.60, 125.52, 125.05, 124.31, 119.73, 99.36, 99.01, 57.07, 56.48, 46.84, 23.76, 14.48. Elem. anal. calcd for C₃₀H₃₀N₂O₄: C, 74.67; H, 6.27. Found: C, 74.64; H, 6.28.

(S)-1-((3′-Ethyl-2,2′-bis(methoxymethoxy)-[1,1′-binaphthalen]-3-yl)methyl)-1H-indole, ((S)-6b) and (S)-9-((3′-ethyl-2,2′-bis(methoxymethoxy)-[1,1′-binaphthalen]-3-yl)methyl)-9H-carbazole, ((S)-6c) were prepared in a similar manner (see ESI†).

Polymerization (typical procedure)

(S)-5 (1.00 mmol) and AIBN (0.05 mmol) were added into a dry Schlenk flask. After three freeze–pump–thaw cycles, degassed toluene (1 mL) was added via degassed syringe. The polymerization was conducted in an oil bath at 75 °C for 24 h. The reaction mixture was then cooled to room temperature. Next, THF (10 mL) was added to give a homogenous solution. The THF solution was poured into methanol (100 mL) to precipitate the white polymer. The resulting polymer was filtrated and washed with methanol several times. The above mentioned process was repeated, and then the obtained precipitation was dried in vacuo at 50 °C to obtain the polymer (white powder, yield 72–78%).

Results and discussion

Synthesis

(S)-1 was prepared according to the literature⁴⁷ and the structure was confirmed by ¹H NMR. The synthetic route to (S)-2, (S)-3, (S)-4, (S)-5 and the polymer is outlined in Scheme 1. (S)-2 was synthesized by reduction one formyl-groups of (S)-1 to hydroxymethyl-group by NaBH₄. To explore the optimal amounts of NaBH₄, reactions of (S)-1 with different amounts of NaBH₄ were investigated (Table 1). 0.25 eq. NaBH₄ gave (S)-2 in 76% yield (Table 1, entry 2). With increasing amounts of NaBH₄, the ratio of (S)-2′ to (S)-2 increases (Table 1). (S)-3 was prepared in high yields (90%) by sulfonylation and chlorination of (S)-2. (S)-4 was obtained by reaction of (S)-3 and corresponding N-heterocycles catalyzed by KOH and TBAB. The monomer (S)-5 was prepared by Wittig reaction of (S)-4. Compound (S)-6 was synthesized via catalytic hydrogenation of (S)-5 using 10% Pd/C as a catalyst (Scheme 2). Synthesis and purification procedures and the spectroscopic data are described in Experimental part (Scheme 3).

Scheme 1 The synthetic route for the monomers and corresponding polymers.

Table 1 Reduction of (S)-1 by NaBH₄

Entry	NaBH4/(eq.)	Yield
		(S)-1/%	(S)-2/%	(S)-2′/%
1	0.20	27	70	3
2	0.25	20	75	5
3	0.35	11	65	24
4	0.45	5	41	54
5	0.50	3	22	75

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Scheme 2 The synthetic route for the model compounds.

Scheme 3 Reduction of (S)-1 by NaBH₄.

Polymerization

Fig. 1 shows the ¹H NMR spectra of the monomer (S)-5c and the corresponding polymer which was obtained by radical polymerization for 24 h at 75 °C in toluene using AIBN (0.05 eq.) as the initiator. The chemical shifts of the vinyl substituent of (S)-5c at 7.09 (s, 1H), 6.02 (d, J = 17.7 Hz, 1H), 5.49 (d, J = 10.9 Hz, 1H) (Fig. 1(a)), disappeared completely after polymerization (Fig. 1(b)).

Fig. 1 ¹H NMR spectra of (S)-5c (a) and poly-(S)-5c (b).

The tacticity of polymer chain, which could influence the chiroptical properties of polymer, was estimated by comparing ¹³C NMR spectra of the obtained polymers. As can be seen in Fig. 2, each broad signal around 57 ppm (a) and 98 ppm (b) corresponds to carbons in two different chemical environments. All of the other peaks in these spectra are narrow singlet without any shoulder peak, indicating the polymers a good stereoregularity.

Fig. 2 ¹³C NMR spectra of the polymers.

Chiroptical properties and the secondary structure of the polymers

The polymerization results and the chiroptical properties of the polymers are summarized in Table 2. The M_n of polymers is 5300–6500, which indicates it contains about 11 repeat units. All the polymers have narrow molecular weight distribution, the PDI varies from 1.25 to 1.31.

Table 2 Polymerization of the monomers

Entry	Polymers	Yield/%	Mn^a	PDI (Mw/Mn)^a
a Number-average molar mass (Mn), weight-average molar mass (Mw), and polydispersity (Mw/Mn) were determined by GPC in THF on the basis of standard polystyrene calibration.
1	Poly-(S)-5a	72	5360	1.28
2	Poly-(S)-5b	76	5905	1.25
3	Poly-(S)-5c	78	6500	1.31

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The absolute value of specific rotation for the polymers changed tremendously comparing to the corresponding monomers under the same condition (Table 3). The specific optical rotation [a]²⁵₃₆₅ of monomer (S)-5c is +188 and that of poly-(S)-5c is −346. The contribution of chiral pendant groups in the monomers and polymers should be similar, as can be seen nearly the same specific optical rotation of monomers (S)-5 and their model compounds (S)-6. Those large changes of specific optical rotation between the monomers and corresponding polymers may be attributed to the helical polymer backbone.

Table 3 The specific optical rotation of the compounds^a

Entry	[a]^25365
	Compounds	Monomers	Polymers	Model compounds
a c = 1.0 mg mL^−1 in CHCl3.
1	(S)-5a	+703	+1	+692
2	(S)-5b	+506	+82	+521
3	(S)-5c	+188	−346	+204

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The CD and UV-vis spectroscopy was used to further characterize the chiroptical properties of the polymers. As presented in Fig. 3(a)–(c), CD spectra show that all the compounds absorbed intensively at 244 nm, which meets well with the position in the UV-vis absorption spectra (Fig. 3(d)). These results indicate the electronic transitions of BINOL group. Another four opposite peaks in CD spectra are at 218 nm, 235 nm, 288 nm and 319 nm. The Cotton effect signals of the monomers are similar to their model compound as almost the same CD spectra are obtained, which was caused by the modified BINOL chiral groups. The Cotton effect signals of the polymers are 2–3 times stronger than corresponding monomers and model compounds, which means another chiral element, the preferred-handed helical structure of the polymers formed during the polymerization.

Fig. 3 CD absorption of monomers and polymers in CHCl₃ at 25 °C (a) (b) (c) and UV-vis absorption spectra of the polymers recorded in CHCl₃ at 25 °C (d).

Conclusions

In summary, we synthesized a family of optically active helical vinyl polymers bearing N-heterocycles substituted BINOL derivatives. The specific optical rotation and CD spectroscopic data indicate that the obtained polymers can keep a prevailing helicity of backbone in solution.

Acknowledgements

This work was supported by the National Science Foundation of China (21172186), the Higher Education Doctoral Science Foundation of China (20134301110004) and the Program for Innovative Research Cultivation Team in University of Ministry of Education of China (1337304).

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Footnote

† Electronic supplementary information (ESI) available: Charts of ¹H NMR, ¹³C NMR spectra for all the products are included. See DOI: 10.1039/c6ra08146k

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