Effect of alkyl groups on emission properties of aggregation induced emission active N-alkyl arylaminomaleimide dyes

Abstract

Aggregation induced emission (AIE) active N-alkyl aminomaleimide dyes with various kinds of N-alkyl groups were synthesized in a one pot process. The synthesized dyes exhibited different emission behaviors depending on the chemical structure of the N-alkyl group, although they had no direct contribution to the π-conjugated system. Chain length, hydroxyl group and branching structure were important in the emission properties such as quantum yield and emission wavelength. Furthermore, it was found that one of the dyes showed mechanochromism; the green emission of crystal samples turned to blue emission after grinding. Single crystal X-ray diffraction analysis revealed that the surrounding around the luminophore was dominant in the emission behaviors.

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Introduction

Solid state emission is a challenging matter for the development of luminescent materials,¹ and a large number of studies have been carried out for intense emission in the solid state.² Among them, the aggregation induce emission (AIE) system has contributed a great deal to advancing this field.³ Since the first report on AIE in 2001,⁴ various kinds of AIE active dyes have been developed; tetraphenyl silole,^4,5 tetraphenyl ethylene (TPE),⁶ and so on.⁷ While no emission is observed in solutions, the AIE active dyes show emission in the solid state. Excitons of these AIE active dyes were deactivated without radiation in solutions due to molecular motions, whereas aggregation formation restricts these deactivation pathways. The state of the samples (e.g. solution, aggregation and solid) drastically changes the emission properties, and the environmentally-responsive property has been widely applied to sensors and bioprobes.^8,9 An important factor for solid state emission lies in the fashion of molecular packing, which are highly influenced by the substituents around the luminophores. This is why chemical structure which is not included in the π-conjugated system of the luminescent centers should be considered for molecular design of the AIE active dyes.¹⁰ Actually, side chains influence emission properties such as emission color and quantum yield. For example, the emission behavior of TPE was changed by chain length of the alkyl chain around the TPE luminescent center.^10c

Recently, we have reported AIE active N-aryl arylaminomaleimides (Chart 1a)¹¹ and N-alkyl arylaminomaleimides (Chart 1b).¹² The dye skeleton can be constructed in a one-pot process, and the obtained luminescent dyes are attractive because of their simple structures and versatile molecular design. The emission properties of N-aryl aminomaleimides were controlled by substituents on the 2- and 3-positions of the maleimide ring because the π-conjugated system is expanded to these substituents.^13,14 In addition, we found that the N-aryl group of the maleimides also affected the emission properties despite the little contribution to the π-conjugated system.¹³ This is because the molecular packing was changed by the N-aryl groups; the local surroundings around the maleimide ring highly influenced their emission properties. A maleimide ring has two carbonyl groups, which easily form interactions such as hydrogen bond in the solid states. Thus, short contacts to the luminescent center can be potentially controlled in a diverse way, leading to various emission behaviors of the AIE active aminomaleimide dyes.

Chart 1 Chemical structure of maleimide dyes. (a) N-Aryl maleimides, and (b) N-alkyl anilinomaleimides.

In a previous paper, we reported that N-alkyl aminomaleimides (Chart 1b) have advantages in high quantum yields, easy synthesis and good solubility, in comparison with N-aryl aminomaleimides.¹² N-Alkyl groups may work as steric hinderance around the maleimide ring to avoid co-facial π–π stacking toward the intense emission in the solid states. Therefore, it can be expected that the screening of the N-alkyl group realizes a variety of emission properties despite no direct electronic interaction to the π-conjugated system. Herein, we investigated effect of the N-alkyl groups of N-alkyl anilinomaleimides in a systematic way. Alkyl chain length and hydroxyl group and branching structure drastically changed emission properties such as emission wavelength and quantum yield. Furthermore, one of the synthesized dyes showed mechanochromism,^15,16 differently from the others. X-ray diffraction study revealed that molecular packing was changed in dependent on the N-alkyl group. This is, to the best of our knowledge, the first study on the relationship between the N-alkyl group and solid state emission of maleimide dyes, though various kinds of luminescent maleimide dyes have been reported to date.^11–14

Results and discussion

Synthesis

Synthesis of N-alkyl anilinomaleimides is shown in Scheme 1.¹⁷ Dimethyl acetylenedicarboxylate (1) and one equivalent of aniline were reacted at ambient temperature to give an adduct product, dimethyl anilinofumarate (2) nearly quantitatively. Subsequently, unreacted compounds were removed in vacuo. Without further purification, excess amount of a primary alkyl amine in methanol was added to 2 at ambient temperature. After stirring overnight, the reaction mixture was cooled to −20 °C to give N-alkyl anilinomaleimides (3–5) as yellow solids. The isolated yields of some products were relatively low because the obtained N-alkyl anilinomaleimides were slightly soluble in methanol. Notably, the synthetic procedure was one-pot, and the purification process was only recrystallization from a reaction solution.

Scheme 1 Synthesis of N-alkyl anilinomaleimides.

Alkyl chain length was examined from C1 to C12 in 3a–h, and cyclohexyl group was employed in 3i as a relatively rigid substituent. Hydroxyl group was introduced in 4a–c to provide a hydrogen bond donor. For incorporation of branching structure, chiral primary amines were used for (S)-5 and (R)-5, and the racemic mixture rac-5 was also synthesized. The chemical structures of the newly synthesized compounds were determined with ¹H and ¹³C NMR and high resolution mass spectroscopy.

Optical properties in general

The optical properties are summarized in Table 1. The absorption maxima were 380–383 nm in solutions (c = 1.0 × 10⁻⁴ mol L⁻¹ in chloroform), suggesting that the N-alkyl groups had no significant contribution to the π-conjugated systems in dilute solutions. Solid samples for photophysical measurement were prepared by reprecipitation in methanol. All of the obtained samples showed luminescence in the solid state, despite no emission in good solvents such as chloroform, dichloromethane, THF and so on. It implies that these dyes are AIE active. Emission wavelength and quantum yield of the dyes were dependent on the N-alkyl groups, suggesting that the surroundings of the aggregated molecules are different from each other.

Table 1 Optical properties of 3–5

Dye	λabs^a [nm]	λem^b [nm]	Φ^c
a Absorption maxima in CHCl3.b Emission maxima.c Absolute quantum yield in the solid state.
3a	380	491	<0.01
3b	380	498	0.32
3c	380	497	0.56
3d	380	495	0.59
3e	380	492	0.57
3f	380	492	0.48
3g	380	495	0.41
3h	380	494	0.44
3i	380	492	0.45
4a	382	516	0.18
4b	383	494	0.17
4c	381	496	0.11
rac-5	382	508	0.02
(R)- and (S)-5	382	507	0.02

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Effect of chain length

Methyl (3a), ethyl (3b), propyl (3c), butyl (3d), pentyl (3e), hexyl (3f), octyl (3g), dodecyl (3h) and cyclohexyl (3i) groups were examined to investigate chain length effect (Fig. 1). The emission wavelengths of 3a–i were approximately the same (491–497 nm), while quantum yields were completely different. Although the emissions of 3b–h were intense (Φ = 0.32–0.59), that of 3a was too weak to observe by naked eyes. This is probably because sterically small alkyl groups such as methyl group cannot prevent a luminescent center from concentration self-quenching. For the same reason, 3b (Φ = 0.32) showed weaker emission than 3c–e (Φ > 0.5). Therefore, a certain level of steric hinderance of the N-alkyl chain is necessary for intense solid state emission of the N-alkyl aminomaleimide dyes.

Fig. 1 (a) Photographs of 3a–h under UV irradiation at 365 nm. (b) Absolute quantum yields of 3a–3h in the solid state.

On the other hand, 3f–i had relatively low quantum yields in comparison with 3c–e, suggesting that the elongation of the alkyl chain offers flexibility and space which enable molecular motions. As a result, butyl group possessed moderate balance between steric hindrance and molecular rigidity. Judging from the results of 3f and 3i, there was no significant difference between hexyl and cyclohexyl groups, and topological effect was not observed.

The lifetimes of 3a–h were measured to examine the components of their emission in the solid state. The dyes had two distinct lifetimes (Fig. S23 and Table S3†). The ratio of shorter and longer lifetime components of 3a (τ₁ = 1.11 ns (84%), τ₂ = 4.57 ns (16%)) was different from those of 3b–h; elongation of the N-alkyl chain from methyl group raised the ratio of the longer lifetime component (approximately 30%), which is relatively easily deactivated. This means that small substituents such as methyl group cannot sufficiently prevent the excitons from non-radiative deactivation. Despite the minor difference, the emissions of 3b–h were derived from the similar fluorophore, suggesting that the N-alkyl chain did not directly influence the luminescent center.

Effect of hydroxyl group

In the case of 4 and 5, the hydroxyl group was incorporated into the N-alkyl chain. Although the UV-vis absorption spectra of all dyes 3–5 were approximately same, the introduction of the hydroxyl group changed emission properties in the solid state. The quantum yields of 5 were low (Φ = 0.02) in comparison with those of 4 (Φ = 0.11–0.18). Branching structure could change the packing structure to cause the significant decrease of quantum yield, though the detailed structural difference was unclear. The circular dichromism (CD) and circularly polarized luminescence (CPL) spectra of (S)- and (R)-5 were silent because the small methyl group of N-alkyl chain has no structural influence on the π-conjugated system in solutions.¹⁸

Notably, the emission maxima of 4a (λ_max = 516 nm) and 5 (λ_max = 507–508 nm) were red-shifted in comparison with those of 3 (λ_max = 491–498 nm). As representative examples, the PL spectra of 3c, 4a and rac-5 are shown in Fig. 2. It is possible that intermolecular and/or intramolecular interaction, e.g. hydrogen bond, between the hydroxyl group and the maleimide moiety caused lowering the LUMO level in the solid states. On the other hand, no significant red-shift of emission was observed in the case of 4b and 4c, which possessed longer alkyl chain than 4a.

Fig. 2 PL spectra of 3c, 4a and rac-5 (excited at 350 nm).

For structural analysis in the solid state, the IR spectra of 3c, 4a–c and rac-5 were measured with KBr method (Fig. 3). The signals of the carbonyl group of 4a (1650 cm⁻¹) and rac-5 (1644 cm⁻¹) were shifted from those of the others (1632–1637 cm⁻¹), suggesting the formation of interactions such as hydrogen bonds. In addition, the signals of the hydroxyl groups were defined in the case of 4a (3393 cm⁻¹) and rac-5 (3500 cm⁻¹). This is because 4a and rac-5 formed rigid structure based on the defined interaction, while 4b and 4c adopted various conformation in the solid state due to their longer alkyl chains. These results support the phenomenon in which only 4a and rac-5 exhibited different emission color. In the previous paper, we reported that the LUMO level is stabilized as the number of hydrogen bonds increase, resulting in red-shift of the emission.¹³ The hydroxyl groups probably directly or indirectly influenced the electronic surroundings of the maleimide luminescent center to cause the emission color change.

Fig. 3 FT-IR spectra of 3c, 4a–c and rac-5 (KBr method).

Mechanochromism

In the course of the investigation, we found that propyl-substitued dye 3c displayed mechanochromism (Fig. 4). To demonstrate the mechanochromic behavior, crystal samples of 3c (3c_cry) were prepared by recrystallization from dichloromethane and methanol, and 3c_cry was ground in a mortar to obtain 3c_grd. The emission color of 3c_cry was green (502 nm),¹⁹ while that of 3c_grd was greenish blue (489 nm).

Fig. 4 (a) Photographs (excited at 365 nm) and (b) PL spectra of 3c_cry and 3c_grd (excited at 350 nm).

For understanding on this phenomenon, single crystal X-ray diffraction (XRD) was carried out. As a representative dye, which did not show mechanochromism, 3i was selected. Suitable single crystals of 3c and 3i were prepared for the measurement. The molecules of 3c formed double hydrogen bonds (2.2 Å) between the secondary amine and carbonyl group (Fig. 5). In addition, short contacts between the protons of the benzene and maleimide rings and the carbonyl group were also observed (2.5–2.7 Å), and the weak hydrogen bonds constructed one-dimensional structure. A mechanical stimulus broke a part of these hydrogen bonds, and the surroundings of the luminophore were altered, resulting in the emission color change. The powder XRD pattern of 3c was simulated based on single crystal XRD. After grinding crystals of 3c, the broad signals were observed (Fig. S25†), suggesting the crystal structure was partial collapsed by grinding.

Fig. 5 Results of single crystal X-ray diffraction analysis on 3c. (a) Packing structure, (b) hydrogen bonds.

On the other hand, the result of 3i showed the molecules formed hydrogen bonds (2.1 Å) between the secondary amine and carbonyl group in a one-dimensional polymer fashion (Fig. 6). Three-dimensional structure was supported by the short contacts (2.5 Å and 2.7 Å) via the hydrogen bonds between the protons of the cyclohexyl groups and the carbonyl groups. The robust network structure could not be collapsed by a mechanical stimulus. It is possible that the dyes other than 3c adopted packing structures similar to that of 3i, resulting in the absence of mechanochromic property.

Fig. 6 Results of single crystal X-ray diffraction analysis on 3i. (a) Packing structure, (b) hydrogen bonds between the cyclohexyl group and the carbonyl group, and (c) hydrogen bonds between the secondary amino group and the carbonyl groups. Hydrogen atoms were omitted for clarity in (a).

Conclusions

We have systematically investigated effect of N-alkyl groups on the emission properties of AIE active aminomaleimide dyes. The UV-vis absorption and CD spectra suggested that the N-alkyl groups had no significant direct contribution to the π-conjugated system of the luminophore. Nevertheless, the chain length, hydroxyl group and branching structure were very important in the solid state emission. This is because the N-alkyl groups determined the packing structure, which was dominant in the surroundings of the luminophore. Interestingly, only 3c showed mechanochromism. Single crystal X-ray diffraction analysis revealed that the packing structure of 3c was completely different from that of 3i. A mechanical stimulus may cleave the hydrogen bonds to cause blue shift. This is the first study on the effect of the N-alkyl group of maleimide dyes on the emission properties, to the best of our knowledge. Easy synthesis and tunable emission properties are advantageous in practical use of the AIE active dyes. Further modification and functionalization of maleimide dyes are under way.

Experimental

Material

Methanol and dichloromethane were purchased from Nacalai Tesque, Inc. Dimethyl acetylenedicarboxylate was purchased from Tokyo Chemical Industry Co., Ltd. Aniline was purchased from Wako Pure Chemical Industry, Ltd. 3c, 3f and 3g were prepared by following the literature.¹²

Material

¹H (400 MHz) and ¹³C (100 MHz) NMR spectra were recorded on a Bruker DPX-400 spectrometer, and samples were analyzed in CDCl₃ using Me₄Si as an internal standard. The following abbreviations are used; s: singlet, d: doublet, m: multiplet. High-resolution mass spectra (HRMS) were obtained on a JEOL JMS-SX102A spectrometer. The UV-vis spectra were recorded on a Jasco spectrophotometer V-670 KKN. Emission spectra were obtained on a JASCO fluorescence spectrophotometer FP-8500, and absolute PL quantum yields (Φ) were determined using a JASCO ILFC-847S. The CD and UV-vis spectra were recorded on a JASCO J-820 spectropolarimeter with THF as a solvent at room temperature.

X-ray crystallographic data for single crystalline products

The single crystal was mounted on glass fibers with epoxy resin. Intensity data were collected at room temperature on a Rigaku RAXIS RAPID II imaging plate area detector with graphite monochromated Mo Kα radiation. The crystal-to-detector distance was 127.40 mm. Readout was performed in the 0.100 mm pixel mode. The data were collected at room temperature to a maximum 2θ value of 55.0°. Data were processed by the PROCESS-AUTO²⁰ program package. An empirical or numerical absorption correction²¹ was applied. The data were corrected for Lorentz and polarization effects. A correction for secondary extinction²² was applied. The structure was solved by heavy atom Patterson methods²³ and expanded using Fourier techniques.²⁴ Some non-hydrogen atoms were refined anisotropically, while the rest were refined isotropically. Hydrogen atoms were refined using the riding model. The final cycle of full-matrix least-squares refinement on F2 was based on observed reflections and variable parameters. In the case of the crystalline product recrystallized from acetone, the final cycle of full-matrix least-squares refinement on F was based on observed reflections and variable parameters. All calculations were performed using the crystal structure^25,26 crystallographic software package. Crystal data and more information on X-ray data collection are summarized in Table S1 and S2.†

General synthetic procedure

A dichloromethane solution of 1 (1.51 g, 10.7 mmol) was cooled to 0 °C with ice-water bath, and aniline (1.00 g, 10.7 mmol) was added dropwise. The reaction mixture was stirred at ambient temperature, and turned yellow gradually. After stirring overnight, the volatiles were removed in vacuo to obtain 2 as yellow oil product. Without further purification, a methanol solution (5 mL) of 2 was prepared. To the solution was added primary amine (32.1 mmol, 3 equivalent). After stirring overnight at ambient temperature, the reaction mixture was cooled to −20 °C to obtain N-alkyl aminomaleimide as yellow solid. Unless solid products were obtained, the volatiles were removed in vacuo, and small amounts of methanol (<5 mL) were added. Subsequently, the solution was cooled to −20 °C to isolate N-alkyl aminomaleimide.

2-Anilino-N-methylmaleimide (3a). Methylamine was employed as a primary amine. The isolated yield was 40%; mp 199.7–201.4 °C. ¹H NMR (CDCl₃, 400 MHz) δ 7.40 (t, J = 8.0 Hz, 2H), 7.35 (s, 1H), 7.15 (m, 3H), 5.52 (s, 1H), 3.05 (s, 3H) ppm. ¹³C NMR (CDCl₃, 100 MHz) δ 172.8, 168.4, 142.6, 138.4, 129.8, 124.5, 118.8, 89.0, 23.8 ppm. HRMS (FAB) calcd for C₁₁H₁₀O₂N₂ [M]⁺: 202.0742; found 202.0732. Anal. calcd for C₁₁H₁₀N₂O₂: C 65.34; H 4.98; N 13.85, found: C 65.20; H 4.93; N 13.83.

2-Anilino-N-ethylmaleimide (3b). Ethylamine was employed as a primary amine. The isolated yield was 67%; mp138.8–140.1 °C. ¹H NMR (CDCl₃, 400 MHz) δ 7.45 (s, 1H), 7.40 (t, J = 8.8, 7.2 Hz, 2H), 7.16 (m, 3H), 5.50 (s, 1H), 3.60 (q, J = 7.2 Hz, 2H), 1.22 (t, J = 7.2 Hz, 3H) ppm. ¹³C NMR (CDCl₃, 100 MHz) δ 172.6, 168.2, 142.5, 138.6, 129.7, 124.4, 118.8, 89.0, 32.8, 14.1 ppm. HRMS (FAB) calcd for C₁₂H₁₂O₂N₂ [M + H]⁺: calcd 216.0898; found 216.0892. Anal. calcd for C₁₂H₁₂N₂O₂: C 66.65; H 5.59; N 12.96, found: C 66.56; H 5.55; N 12.96.

2-Anilino-N-butylmaleimide (3d). Butylamine was employed as a primary amine. The isolated yield was 64%; mp 130.7–132.1 °C. ¹H NMR (CDCl₃, 400 MHz) δ 7.50 (s, 1H), 7.39 (t, J = 8.4 Hz, 2H), 7.16 (m, 3H), 5.50 (s, 1H), 3.53 (t, J = 7.2 Hz, 2H), 1.60 (m, 2H), 1.33 (m, 2H), 0.93 (t, J = 7.2 Hz, 3H) ppm. ¹³C NMR (CDCl₃, 100 MHz) δ 172.9, 168.4, 142.4, 138.6, 129.7, 124.4, 118.8, 88.9, 37.7, 30.8, 20.0, 13.7 ppm. HRMS (FAB) calcd for C₁₄H₁₆O₂N₂ [M]⁺: calcd 244.1211; found 244.1225. Anal. calcd for C₁₄H₁₆N₂O₂: C 68.83; H 6.60; N 11.47, found: C 68.81; H 6.55; N 11.51.

2-Anilino-N-pentylmaleimide (3e). Pentyl amine was employed as a primary amine. The isolated yield was 61%; mp 134.9–135.6 °C. ¹H NMR (CDCl₃, 400 MHz) δ 7.40 (t, J = 8.0 Hz, 3H), 7.16 (m, 3H), 5.50 (s, 1H), 3.53 (t, J = 7.4 Hz, 2H), 1.62 (m, 2H), 1.32 (m, 4H), 0.89 (t, J = 7.0 Hz, 3H) ppm. ¹³C NMR (CDCl₃, 100 MHz) δ 172.9, 168.4, 142.4, 138.5, 129.7, 124.4, 118.8, 88.9, 38.0, 28.9, 28.4, 22.3, 14.0 ppm. HRMS (FAB) calcd for C₁₅H₁₈O₂N₂ [M]⁺: calcd 258.1368; found 258.1375. Anal. calcd for C₁₅H₁₈N₂O₂: C 69.74; H 7.02; N 10.84, found: C 69.75; H 6.97; N 10.86.

2-Anilino-N-dodecylmaleimide (3h). Dodecylamine was employed as a primary amine. The isolated yield was 65%; mp 107.9–108.3 °C. ¹H NMR (CDCl₃, 400 MHz) δ 7.40 (t, J = 7.8 Hz, 2H), 7.25 (s, 1H), 7.15 (m, 3H), 5.50 (s, 1H), 3.53 (t, J = 7.2 Hz, 2H), 1.61 (m, 2H), 1.25 (m, 18H), 0.88 (t, J = 6.8 Hz, 3H) ppm. ¹³C NMR (CDCl₃, 100 MHz) δ 172.8, 168.3, 142.4, 138.5, 129.7, 124.4, 118.7, 89.0, 38.0, 31.9, 29.6, 29.4, 29.2, 28.7, 26.8, 22.7, 14.1 ppm. HRMS (FAB) calcd for C₂₂H₃₂O₂N₂ [M]⁺: calcd 356.2464; found 356.2463. Anal. calcd for C₂₂H₃₂N₂O₂: C 74.12; H 9.05; N 7.86, found: C 74.03; H 9.15; N 7.83.

2-Anilino-N-cyclohexylmaleimide (3i). Cyclohexylamine was employed as a primary amine. The isolated yield was 28%; mp 155.3–156.8 °C. ¹H NMR (CDCl₃, 400 MHz) δ 7.39 (t, J = 8.4 Hz, 2H), 7.34 (s, 1H), 7.14 (m, 3H), 5.47 (s, 1H), 3.93 (m, 1H), 2.06 (m, 2H), 1.83 (m, 2H), 1.69 (m, 3H), 1.25 (m, 3H) ppm. ¹³C NMR (CDCl₃, 100 MHz) δ 172.8, 168.1, 142.1, 138.6, 129.7, 124.3, 118.7, 89.0, 50.5, 30.1, 26.0, 25.1 ppm. HRMS (FAB) calcd for C₁₆H₁₈O₂N₂ [M]⁺: calcd 270.1368; found 270.1360. Anal. calcd for C₁₆H₁₈N₂O₂: C 71.09; H 6.71; N 10.36, found: C 70.82; H 6.72; N 10.31.

2-Anilino-N-(2-ethanol)maleimide (4a). 2-Aminoethanol was employed as a primary amine. The isolated yield was 39%; mp 145.3–146.8 °C. ¹H NMR (CDCl₃, 400 MHz) δ 7.42 (t, J = 7.6 Hz, 2H), 7.16 (t, J = 4.8, 3H), 5.54 (s, 1H), 3.82 (t, J = 5.6 Hz, 2H), 3.77 (t, J = 4.8 Hz, 2H), 2.32 (t, J = 5.6 Hz, 1H) ppm. ¹³C NMR (CDCl₃, 100 MHz) δ 173.2, 168.6, 142.6, 138.2, 129.8, 124.8, 118.9, 88.9, 61.3, 41.0 ppm. HRMS (FAB) calcd for C₁₂H₁₂O₃N₂ [M + H]⁺: calcd 233.0926; found 233.0926. Anal. calcd for C₁₂H₁₂N₂O₃: C 62.06; H 5.21; N 12.06, found: C 61.84; H 5.23; N 12.06.

2-Anilino-N-(3-propanol)maleimide (4b). 3-Aminopropanol was employed as a primary amine. The isolated yield was 53%; mp 149.3–149.9 °C. ¹H NMR (CDCl₃, 400 MHz) δ 7.42 (t, J = 7.6 Hz, 2H), 7.29 (br, 1H), 7.17–7.14 (m, 3H), 5.52 (s, 1H), 3.72 (t, J = 6.0 Hz, 2H), 3.63–3.59 (m, 2H), 2.54 (t, J = 6.4 Hz, 1H), 1.85–1.79 (m, 2H) ppm. ¹³C NMR (CDCl₃, 100 MHz) δ 173.4, 168.7, 142.8, 138.4, 129.9, 124.8, 119.0, 88.9, 59.1, 34.2, 31.5 ppm. HRMS (FAB) calcd for C₁₃H₁₄O₃N₂ [M + H]⁺: calcd 247.1083; found 274.1088. Anal. calcd for C₁₃H₁₄N₂O₃: C 63.40; H 5.73; N 11.38, found: C 63.16; H 5.67; N 11.40.

2-Anilino-N-(4-butanol)maleimide (4c). 4-Aminobutanol was employed as a primary amine. The isolated yield was 14%; mp 116.1–116.9 °C. ¹H NMR (CDCl₃, 400 MHz) δ 7.41 (t, J = 8.0 Hz, 2H), 7.25 (br, 1H), 7.18–7.14 (m, 3H), 5.50 (s, 1H), 3.69 (br, 1H), 3.67–3.54 (m, 2H), 2.46 (d, J = 5.2 Hz, 1H), 1.23 (d, J = 6.4 Hz, 3H) ppm. ¹³C NMR (CDCl₃, 100 MHz) δ 172.8, 168.3, 142.3, 138.4, 129.8, 124.6, 118.7, 89.0, 62.4, 37.6, 29.7, 25.2 ppm. HRMS (FAB) calcd for C₁₄H₁₇O₃N₂ [M + H]⁺: calcd 261.1239; found 261.1229. Anal. calcd for C₁₄H₁₆N₂O₃: C 64.60; H 6.20; N 10.76, found: C 64.15; H 6.11; N 10.69.

2-Anilino-N-(racemi-2-propanol)methylmaleimide (rac-5). racemi-2-Aminopropanol was employed as a primary amine. The isolated yield was 14%; mp 178.9–179.5 °C. ¹H NMR (CDCl₃, 400 MHz) δ 7.41 (t, J = 8.4 Hz, 2H), 7.30 (br, 1H), 7.19–7.15 (m, 3H), 5.54 (s, 1H), 4.05 (br, 2H), 3.60 (t, J = 6.8 Hz, 2H), 1.75–1.69 (m, 2H), 1.61 (t, J = 6.0 Hz, 2H), 1.43 (br, 1H) ppm. ¹³C NMR (CDCl₃, 100 MHz) δ 173.3, 168.7, 142.6, 138.2, 129.8, 124.8, 118.9, 88.9, 66.8, 45.6, 20.9 ppm. HRMS (FAB) calcd for C₁₃H₁₄O₃N₂ [M + H]⁺: calcd 247.1083; found 274.1078. Anal. calcd for C₁₃H₁₄N₂O₃: C 63.40; H 5.73; N 11.38, found: C 63.21; H 5.63; N 11.38.

2-Anilino-N-((S)-2-propanol)methylmaleimide ((S)-5). (S)-2-Aminopropanol was employed as a primary amine. The isolated yield was 22%.

2-Anilino-N-((R)-2-propanol)methylmaleimide ((R)-5). (R)-2-Aminopropanol was employed as a primary amine. The isolated yield was 23%.

Acknowledgements

We thank Prof. Y. Chujo and Prof. Y. Morisaki of Kyoto University for measurement of CD spectra. This study is a part of a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (No. 2401)” (No. 24102003) of The Ministry of Education, Culture, Sports, Science and Technology, Japan.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and spectrum data. CCDC 1423627 and 1423628. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra18690k

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