Modern Chemistry
Volume 4, Issue 5, October 2016, Pages: 45-51

Convenient Synthesis of Benzo[b]thiophene-5,6-dicarboximide Derivatives and Their Photophysical Properties

Naoki Kobayashi1, Shinya Yamamoto1, Haruki Shimosasa1, Mitsunori Oda1, *, Ryuta Miyatake2

1Department of Chemistry, Faculty of Science, Shinshu University, Nagano, Japan

2Centre for Environmental Conservation and Research Safety, University of Toyama, Toyama, Japan

Email address:

(M. Oda)

*Corresponding author

To cite this article:

Naoki Kobayashi, Shinya Yamamoto, Haruki Shimosasa, Mitsunori Oda, Ryuta Miyatake. Convenient Synthesis of Benzo[b]thiophene-5,6-dicarboximide Derivatives and Their Photophysical Properties. Modern Chemistry. Vol. 4, No. 5, 2016, pp. 45-51. doi: 10.11648/j.mc.20160405.11

Received: October 5, 2016; Accepted: October 18, 2016; Published: November 10, 2016


Abstract: Phosphine-assisted annulation of thiophene-2,3-dicarbaldehyde with N-substituted maleimides provided the N-substituted benzo[b]thiophene-5,6-dicarboximides in good to high yields. Introduction of cyano and aryl groups to the thiophene moiety of the N-cyclohexyl product was achieved by metal-catalyzed coupling reactions via its bromo derivative. Photophysical properties of the products were also reported.

Keywords: Annulation, Phosphine, Copper-Mediated Cyanation, Mizoroki-Heck Reaction, Emission


1. Introduction

Arene-dicarboximides display unique photophysical and electrochemical properties and have been attractive to be used as organic electronic materials, such as light-emitting diodes, [1] semiconductors, [2] bio-sensors, [3-4] and electron acceptors in solar cells. [5-6] Thus, synthetic improvements to access to these imides have continuously made. [7-8]

Figure 1. Previously reported annulations by us.

We recently developed an efficient method of annulation between arene-1,2-dicarbaldehydes and maleimides [9-11] by improving the previously reported Haddadin’s result. [12] various naphthalene-, anthracene-, and isobenzofuran-dicarboximides 2–4 can be obtained in one-pot from the corresponding arene-1,2-dicarbaldehydes and maleimides in good to high yields (Fig. 1). These reactions proceed efficiently with the aid of trialkylphosphine in refluxing dioxane. It is worthy to note that the reaction procedure of this method is very simple, because the products were obtained directly from the reaction mixtures just by filtration.

In this paper, we present the synthesis of the benzo[b]thiophene-5,6-dicarboximides by applying this method to annulation of thiophene-2,3-dicarbaldehyde with N-substituted maleimides (Fig. 2), derivatization of the N-cyclohexyl product, and also photophysical properties of the products.

Figure 2. A synthetic route to N-substituted benzo[b]thiophene-5,6- dicarboximides described in this paper.

2. Results and Discussion

Commercially available thiophene-2,3-dicarbaldehyde (5) was subjected to the annulation reaction with various N-substituted maleimides in the presence of tri-n-octylphosphine in refluxing dioxane. Various N-substituted benzo[b]thiophene-5,6-dicarboximides (6a–i) were obtained in good to high yields. The results are shown in Table 1, indicating that both N-alkyl- and N-aryl-maleimides serve as a reactant for the annulation. Also, annulation of dialdehyde 5 with bismaleimides 7 and 8 provided bis(benzo[b]thiophene-5,6-dicarboximide)s 9 and 10 in good yields (Figure 3). It should be noted that most of all reactions completed for a short reaction time and the products were isolated directly from the reaction mixtures just by filtration. In literature there is only one reference, which described synthesis of benzo[b]thiophene-5,6-dicarboximides (Fig. 4). [13] Although this method is seems to be useful for preparing the 9-substituted compound, it requires an expensive samarium reagent and the yield of the two-pot procedures is moderate. Our method described in this paper is superior to the reported one in points of simplicity and efficiency of the reaction. Optical properties of some products are shown in Table 2. The N-4-methoxyphenyl derivative 6g shows clear dual emission in acetonitrile as seen in the spectra of N-arylnaphthalene-2,3-dicarboximides. [14] εmission quantum yields of 6 are smaller than those of the corresponding naphthalene-2,3-dicarboxyimides 2, [15] probably because of a heavy atom effect of the sulfur atom. [16]

Table 1. Reaction time and yields of N-substituted benzo[b]thiophene- 5,6-dicarboximides (6a–j).

entry R reaction time product yield (%)
1 Me 1 6a 86
2 Et 1 6b 86
3 c-hexyl 3 6c 78
4 Ph 1 6d 83
5 4-Br-C6H4- 3 6e 93
6 4-I-C6H4- 4 6f 71
7 4-MeO-C6H4- 2 6g 90
8 4-NO2-C6H4- 2 6h 89
9 4-Ph-C6H4- 2 6i 88
10 3-pyridyl 2 6j 75

Figure 3. Synthesis of bis(benzo[b]thiophene-5,6-dicarboximide)s 9 and 10.

Figure 4. A previously reported synthetic route to benzo[b]thiophene-5,6-dicarboximide derivatives.

Table 2. Optical properties of selected benzo[b]thiophene- 5,6-dicarboximides 6.

Imides solvent λabs / nm log ε λemi / nm Stokes shift / nm Φ / %
6a CH3OH 344 3.77 461 117 4
6b CHCl3 348 3.79 379 31 3
413sh 65
6c CHCl3 348 3.78 380 32 2
409sh 61
6d CH3CN 347 3.50 461 114 0.3
6d CHCl3 350 3.83 477 127 0.6
6e CH3CN 347 3.86 485 138 0.2
6f CH3CN 347 3.81 415 68 0.3
513 166
6g CH3CN 348 3.80 415 67 0.3
513 165
6g CHCl3 349 3.85 514 165 0.03

Next, we turned our attention to derivatization of the product in order to increase the quantum yield and to gain an insight to possibility for tuning absorption and emission wavelengths. Introduction of cyano and aryl groups to the thiophene moiety of 6c was examined via its bromo derivative. Since Lemaire et al. reported an efficient access to 2-aryl derivatives from benzo[b]thiophene itself, we applied their methodology to chemical modification in our work. [17] At first, 3-bromo compound 17 was synthesized from 6c. Bromination of 6c with N-bromosuccimide (NBS) in trifluoroacetic acid at 80°C for a short reaction time yielded 3-bromo derivative 7 in 96% yield, accompanied with a trace amount of 2,3-dibromo derivative 18. (Fig. 5) Although Lemaire et al. found that benzo[b]thiophene can be brominated by NBS in refluxing acetic acid, the reaction of 6c under the same conditions in refluxing acetic acid required a longer reaction time and the yield of 17 was found to be moderate. It is stressed that using trifluoroacetic acid as a solvent in the reaction of 6c improved both the reaction time and the yield. Introduction of a cyano group was achieved by heating 17 with CuCN in dimethylsulfoxide (DMSO). [18] While reaction of 17 with an excess of CuCN in DMSO at 125°C for 12 h resulted in formation of 19 in 17% yield, accompanied with 53% of recovery, the reaction of 17 at 160°C for a shorter reaction time of 6 h provided 19 in 78% yield.

Figure 5. Derivatization of N-cyclohexyl derivative 6c.

Figure 6. UV-vis absorption (solid line) and emission (broken line) spectra of 2-aryl-3-cyano-N-cyclohexylbenzo[b]thiophene-5,6-dicarboximides 20a (blue), 20b (green) and 20c (red) in chloroform.

Finally, the Mizoroki-Heck reactions of 19 with various aryliodides in the presence of Pd(OAc)2, potassium carbonate and tetra-n-butylammonium bromide produced 2-aryl derivatives 20a-c. Optical properties of 19 and 20a–c are summarized in Table 3. The UV-vis and emission spectra of 20a–c are shown in Fig. 6.

Although 19 shows weaker emission quantum yield (1%) than 6c does, 2-aryl derivatives 20a–c indicate the enhanced quantum yields up to 27% in spite of existence of a heavy atom of sulfur. The absorptions shift with a hyperchromic effect depending on the substituents at the para position of the phenyl group and similarly emission wavelength shifts, clearly indicating that it is possible to tune both absorption and emission wavelengths of the title compounds by introduction of aryl groups with an electron-donating substituent at the 2 position.

Table 3. Optical properties of some N-cyclohexylbenzo[b]thiophene-5,6- dicarboximides 19 and 20a–c in chloroform.

Imides λabs / nm log ε λemi / nm Stokes shift / nm Φ / %
19 338 3.67 368 30 1
20a 327sh 4.18 405 78 6
20b 350sh 4.37 436 86 17
20c 414 4.58 511 97 27

3. Experimental

3.1. General Remarks

Melting points were measured on a Yanaco MP-3 and are uncorrected. IR spectra were recorded on a JASCO FT/IR-4100 spectrometer and relative intensity is indicated with letters, vs, v, m, and w. as very strong, strong, medium, and weak, respectively. UV-vis spectra were recorded on a Shimadzu UV-2550 spectrometer. Emission spectra were recorded on a Shimadzu RF5300-PC spectrometer. Emission quantum yields were obtained by comparison with that of anthracene (F = 27% in ethanol). 1H- and 13C-NMR spectra were recorded on JEOL λ400 and ECA500 spectrometers. A chemical shift value of tetramethylsilane (δ = 0 ppm) for both 1H-NMR 13C-NMR spectra was used as internal standard. Mass spectra were measured on a JMS-700 mass spectrometer. Column chromatography was performed with Silica gel 60N from Kanto Chem. DMSO, ν,N-dimethylformamide (DMF), and 1,4-dioxane were purchased from Kanto Chem. and were distilled over CaH2. Dichloromethane, chloroform, trifluoroacetic acid, and acetonitrile were also purchased from Kanto Chem. Tri-n-octylphosphine, N-methylmaleimide, N- ethylmaleimide, N-cyclohexylmaleimide, N-phenylmaleimide, N,N’-(m-phenylene)bismaleimide, N-bromosuccimide (NBS), N-iodosuccimide (NIS), 4-iodoanisole, iodobenzene, and thiophene-2,3-dicarbaldehyde were purchased from Tokyo Chemical Industry, Inc. Copper(I) cyanide, palladium acetate, and tetra-n-butylammonium bromide were purchased from Wako Chem. N-(4-Methoxyphenyl)-, N-(4-bromophenyl)-, N-(4-iodophenyl)-, N-(p-biphenylyl)-, and N-3-pyridyl- maleimides were prepared according to a two-step procedure from maleic anhydride and corresponding amines reported by Cava et al. [19] N,N’-Hexamethylenedimaleimide was prepared by the method of Tona et al. [20] 4-Iodo-N,N-dimethylaniline was prepared by NIS iodination of N,N-dimethylaniline. [21]

3.2. General Procedure for Synthesis of Benzo[b]Thiophene-5,6-Dicarboximides 6

To a solution of the thiophene-2,3-dicarbaldehyde (1.0 mmol) and N-substituted maleimide (1.1 mmol) in 2 ml of dry dioxane was added tri-n-octylphosphine (1.2 mmol). The mixture was refluxed on a preheated oil bath under nitrogen atmosphere for 1–4 h, and was cooled to room temperature. To the reaction mixture was added 2 ml of hexane and the annulation product crystalized at ice-bath temperature. The crystals were collected by suction filtration and washed well with cold ether/hexane (1/1) to give a pure product.

6a: Colorless prisms, m.p. 224–226 ˚C. 1HNMR (CDCl3, 400 MHz) δ = 8.33 (d, J = 0.8 Hz, 1H), 8.26 (s, 1H), 7.76 (dd, J = 6.4, 0.8 Hz, 1H) 7.55 (d, J = 8.0 Hz, 1H), 3.22 (s, 3H) ppm; 13CNMR (CDCl3, 100 MHz) δ = 168.5, 168.3, 144.4, 143.1, 131.5, 128.4, 127.5, 125.0, 119.0, 118.4, 24.1ppm; IR (KBr) ν = 1764 (m), 1692 (vs) cm–1; UV-vis (CH3OH) λmax = 215 (logε = 4.11), 235 (4.35), 256 (4.62), 328 (3.64), 344 (3.77) nm; MS (70 eV) m/z (%) 218 (30), 217 (M+, 100), 216 (15), 189 (33), 188 (17), 173 (76), 161 (19), 160 (28), 133 (25), 132 (60), 94 (19), 66 (11). HRMS Calcd for C11H7NO2S (M+) 217.0198, found 217.0198.

6b: Colorless plates, m.p. 171–172 ˚C. 1HNMR (CDCl3, 400 MHz) δ = 8.33 (t, J = 0.7 Hz, 1H), 8.26 (d, J = 0.7 Hz, 1H), 7.75 (d, J = 5.5 Hz, 1H), 7.55 (dd, J = 5.5, 0.7 Hz, 1H), 3.79 (q, J = 7.3 Hz, 2H), 1.31 (t, J = 7.3 Hz, 3H) ppm; 13CNMR (CDCl3, 100 MHz) δ = 168.2, 168.1, 144.4, 143.1, 131.4, 128.4, 127.6, 125.0, 119.0, 118.4, 33.1, 14.0 ppm; IR (KBr) ν = 1761 (s), 1746 (s), 1695 (vs) cm–1; MS (70 eV) m/z (%) 231 (M+, 56), 216 (100), 203 (8), 189 (9), 161 (12), 132 (17). HRMS Calcd for C12H9NO2S (M+) 231.0354, found 231.0354.

6c: Colorless microcrystals, m.p. 232–233˚C. 1HNMR (CDCl3, 500 MHz) δ = 8.30 (t, J = 0.9 Hz, 1H), 8.23 (d, J = 0.9 Hz, 1H), 7.73 (d, J = 5.5 Hz, 1H), 7.54 (dd, J = 5.5, 0.9 Hz, 1H), 4.15 (tt, J = 12.8, 3.7 Hz, 1H), 2.25 (qd, J = 12.8, 3.7 Hz, 2H), 1.88 (dm, J = 12.8 Hz, 2H), 1.76 (dm, J = 12.8 Hz, 2H), 1.70 (dm, J = 12.8 Hz, 1H), 1.39 (qt, J = 12.8, 3.7 Hz, 2H), 1.29 (qt, J = 12.8, 3.7 Hz, 1H) ppm; 13CNMR (CDCl3, 126 MHz) δ = 168.4 168.3, 144.3, 143.1, 131.2, 128.3, 127.5, 125.0, 118.8, 118.2, 51.1, 29.9, 26.1, 25.1 ppm; IR (KBr) ν = 1715 (s), 1700 (vs), 1684 (s) cm–1; UV-vis (CHCl3) λmax = 260 (logε = 4.70), 333 (3.70), 348 (3.78) nm; MS (70 eV) m/z (%) 285 (M+, 61), 242 (38), 204 (100), 186 (25), 161 (10), 132 (14). HRMS Calcd for C16H15NO2S (M+) 285.08235, found 285.0828.

6d: Creamy white microcrystals, m.p. 250–251˚C. 1HNMR (CDCl3, 500 MHz) δ = 8.46 (t, J = 0.9 Hz, 1H), 8.38 (d, J = 0.9 Hz, 1H), 7.80 (d, J = 5.5 Hz, 1H), 7.60 (dd, J = 5.5, 0.6 Hz, 1H) 7.50 (m, 4H), 7.42 (tm, J = 6.5 Hz, 1H) ppm; 13CNMR (CDCl3, 126 MHz) δ = 167.4, 167.3, 145.0, 143.6, 132.02, 131.98, 129.2, 128.2, 128.0, 127.1, 126.7, 125.2, 119.7, 119.2 ppm; IR (KBr) n = 1774 (s), 1707 (vs) cm–1; UV-vis (CH3OH) λmax = 234 (logε = 4.26), 260 (4.61), 345 (3.66) nm; MS (70 eV) m/z (%) 280 (18), 279 (M+, 100), 236 (10), 235 (57), 132 (25). HRMS Calcd for C16H9NO2S (M+) 279.0354, found 279.0354.

6e: Colorless microcrystals, m.p. > 300 ˚C. 1HNMR (DMSO-d6 at 130˚C, 500 MHz) δ = 8.62 (s, 1H), 8.44 (s, 1H), 8.12 (d, J = 5.5 Hz, 1H), 7.75 (d, J = 5.5 Hz, 1H), 7.70 (d, J = 8.0 Hz, 1H), 7.46(d, J = 8.0 Hz, 1H) ppm; 13CNMR (DMSO-d6 at 130˚C, 126 MHz) δ = 165.9, 165.8, 144.1, 143.1, 132.7, 131.2, 131.2, 128.4, 127.1, 126.1, 124.6, 120.2, 118.8, 118.6 ppm; IR (KBr) ν = 1784 (m), 1717 (vs) cm–1; UV-vis (CH3CN) λmax = 228 (logε = 4.43), 262 (4.81), 334sh (3.78), 347 (3.86) nm; MS (70 eV) m/z (%) 359 (M+, 100), 357 (M+, 97), 315 (33), 160 (10), 139 (15), 132 (15). HRMS Calcd for C16H879BrNO2S (M+), 356.9459,found 356.9458.

6f: Creamy white microcrystals, m.p. > 300 ˚C. 1HNMR (DMSO-d6 at 130˚C, 400 MHz) δ = 8.28 (s, 1H), 8.04 (s, 1H), 7.74 (d, J = 5.6 Hz, 1H), 7.46 (dm, J = 8.4 Hz, 2H), 7.34 (d, J = 5.2 Hz, 1H), 6.86 (dm, J = 8.4 Hz, 2H) ppm; 13CNMR (DMSO-d6 at 130˚C, 126 MHz) δ = 165.8, 165.7, 144.1, 143.0, 137.1, 132.5, 131.7, 128.4, 127.1, 126.1, 124.6, 118.7, 118.5, 116.1 ppm; IR (KBr) ν = 1718 (vs), 1709 (vs), 1679 (s) cm–1; UV-vis (CH3CN) λmax = 231 (logε = 4.39), 263 (4.76), 334 (3.74), 347 (3.81) nm; MS (70 eV) m/z (%) 406 (19), 405 (M+, 100), 361 (23), 234 (23), 132 (45). HRMS Calcd for C16H15NIO2S (M+) 404.9320, found 404.9321.

6g: Yellowish microcrystals, m.p. 228–229 ˚C. 1HNMR (CDCl3, 500 MHz) δ = 8.45 (s, 1H), 8.36 (s, 1H), 7.79 (d, J = 5.2 Hz, 1H), 7.59 (d, J = 5.2 Hz, 1H), 7.38 (d, J = 9.0 Hz, 2H), 7.03 (d, J = 9.0 Hz, 2H), 3.86 (s, 3H) ppm; 13CNMR (CDCl3, 126 MHz) δ = 167.5, 167.4, 159.2, 144.7, 143.4, 131.7, 127.92, 127.89, 127.1, 125.0, 124.5, 119.5, 118.9, 114.4, 55.5 ppm; IR (KBr) ν = 1769 (s), 1714 (vs) cm–1; UV-vis (CH3CN) λmax = 228 (logε = 4.46), 260 (4.66), 334 (3.74), 345 (3.80) nm; MS (70 eV) m/z (%) 310 (20), 309 (M+, 100), 294 (40), 265 (9), 186 (9), 132 (11). HRMS Calcd for C17H11NO3S (M+) 309.0460, found 309.0461.

6h: Colorless microcrystals, m.p. >300 ˚C. 1HNMR (DMSO-d6 at 130˚C, 500 MHz) δ = 8.67 (s, 1H), 8.49 (s, 1H), 8.36 (d, J = 8.4 Hz, 2H), 8.15 (d, J = 5.2 Hz, 1H), 7.86 (d, J = 8.4 Hz, 2H), 7.78 (d, J = 5.2 Hz, 1H) ppm; 13CNMR (DMSO-d6 at 130˚C, 126 MHz) δ = 165.5, 165.4, 146.0, 144.3, 143.2, 137.5, 132.8, 126.9, 126.7, 125.9, 124.6, 123.3, 118.9, 118.7 ppm; IR (KBr) ν = 1765 (s), 1729 (vs) cm–1; MS (70 eV) m/z (%) 324 (M+, 100), 294 (25), 280 (24), 234 (22), 222 (10), 160 (14), 132 (45). HRMS Calcd for C16H8N2O4S (M+) 324.0205, found 324.0207.

6i: Yellowish leaflets, m.p. >300 ˚C. 1HNMR (DMSO-d6 at 130˚C, 500 MHz) δ = 8.63 (s, 1H), 8.46 (s, 1H), 8.12 (d, J = 5.6 Hz, 1H), 7.78 (m, 3H), 7.71 (d, J = 8.4 Hz, 2H), 7.58 (d, J = 8.4 Hz, 2H), 7.49 (t, J = 7.3 Hz, 2H), 7.39 (t, J = 7.3 Hz, 1H) ppm; 13CNMR (DMSO-d6 at 130˚C, 126 MHz) δ = 166.2, 166.1, 144.1, 143.1, 139.5, 139.1, 132.5, 131.1, 128.3, 127.2, 127.0, 126.7, 126.5, 126.3, 126.2, 124.6, 118.7, 118.4 ppm; IR (KBr) ν = 1774 (m), 1703 (vs) cm–1; MS (70 eV) m/z (%) 356 (25), 355 (M+, 100), 311 (20), 152 (13), 132 (22). HRMS Calcd for C22H13NO2S (M+) 355.0667, found 355.0670.

6j: Yellowish microcrystals, m.p. >300 ˚C. 1HNMR (DMSO-d6 at 130˚C, 400 MHz) δ = 8.76 (d, J = 0.7 Hz, 1H), 8.72 (d, J = 2.0 Hz, 1H), 8.63 (dm, J = 4.8 Hz, 1H), 8.52 d, J = 0.7 Hz, 1H), 8.20 (d, J = 5.5 Hz, 1H), 7.96 (ddm, J = 8.1, 2.0 Hz, 1H), 7.80 (d, J = 5.5 Hz, 1H), 7.60 (dd, J = 8.1, 4.8 Hz, 1H) ppm; 13CNMR (DMSO-d6 at 130˚C, 100 MHz) δ = 165.9, 165.8, 148.0, 147.0, 144.1, 143.1, 133.7, 132.7, 128.7, 127.1, 126.1, 124.6, 123.0, 118.8, 118.6 ppm; IR (KBr) ν = 1783 (m), 1708 (vs) cm–1; UV-vis (CH3CN) λmax = 230 (logε = 4.28), 261 (4.71), 334 (3.70), 347 (3.79) nm; MS (70 eV) m/z (%) 281 (18), 280 (M+, 100), 236 (53), 235 (44), 210 (11), 159 (11), 132 (61). HRMS Calcd for C15H8N2O2S (M+) 280.0307, found 280.0309.

3.3. General Procedure for Synthesis of Bis(Benzo[b]- Thiophene-5,6-Dicarboximide)s 9 and 10

To a solution of thiophene-2,3-dicarbaldehyde (1.0 mmol) and N-substituted maleimide (0.55 mmol) in 2 ml of dry dioxane was added tri-n-octylphosphine (1.2 mmol). The mixture was refluxed on a preheated oil bath under nitrogen atmosphere for 4 h, and was cooled to ice-bath temperature. The crystals formed were collected by suction filtration and washed well with cold ether/hexane (1/1) to give a product.

9: Brownish solids, m.p. 257–258 ˚C. 1HNMR (CDCl3, 500 MHz) δ = 8.31 (t, J = 0.8 Hz, 2H), 8.24 (d, J = 0.8 Hz, 2H), 7.74 (d, J = 5.5 Hz, 2H), 7.54 (dd, J = 5.5, 0.8 Hz, 2H), 3.71 (t, J = 7.3 Hz, 4H), 1.71 (quin, J = 7.3 Hz, 4H), 1.41 (quin, J = 7.3 Hz, 4H) ppm; 13CNMR (CDCl3, 126 MHz) δ = 168.3, 168.2, 144.3, 143.0, 131.3, 128.3, 127.5, 125.0, 119.0, 118.4, 38.0, 28.5, 26.5 ppm; IR (KBr) ν = 1758 (s), 1699 (vs) cm–1; MS m/z (%) = 488 (M+, 48), 272 (22), 230 (10), 216 (100), 204 (10), 186 (10), 161 (10). HRMS Calcd for C26H20N2O4S2 (M+) 488.0865, found 488.0860.

10: Brownish solids, m.p. >300 ˚C. 1HNMR (DMSO-d6 at 100˚C, 500 MHz) δ = 8.67 (s, 2H), 8.47 (s, 2H), 8.14 (d, J = 5.6 Hz, 2H), 7.77 (d, J = 5.6 Hz, 2H), 7.68 (m, 2H), 7.58 (m, 2H) ppm; 13CNMR (DMSO-d6 at 100˚C, 126 MHz) δ = 165.94, 165.85, 144.1, 143.1, 132.5, 132.3, 128.4, 127.1, 126.2, 125.8, 124.8, 124.6, 118.7, 118.5 ppm; IR (KBr) ν = 1770 (s), 1722 (vs) cm–1; MS m/z (%) = 481 (31), 480 (M+, 100), 240 (8), 132 (22). HRMS Calcd for C26H12N2O4S2 (M+) 480.0239, found 480.0242.

3.4. 3-Bromo-N-CyclohexylBenzo[b]Thiophene-5,6-Di- carboximide (17)

A mixture of 285 mg (1.00 mmol) of 6c and 890 mg (5.00 mmol) of NBS in 15 mL of trifluoroacetic acid was heated on an oil bath for 3 h. The reaction mixture was poured into water and was extracted with ether (30 ml x 3). The combined organic layer was washed with a saturated NaHCO3 aqueous solution and brine. After dryness over Na2SO4, the solvent was evaporated and the residue was purified by silica gel chromatography (CHCl3/hexane = 3/1) to give 252 mg of 17 (96%) as colorless microcrystals, followed by a trace amount of 18. Independently, 18 was obtained as colorless microcrystals by NBS bromination for a longer reaction time (14 h) from 17 in 84% yield.

17: M.p. 221–223˚C. 1HNMR (CDCl3, 500 MHz) δ = 8.28 (d, J = 0.7 Hz, 1H), 8.27 (d, J = 0.7 Hz. 1H), 7.71 (s, 1H), 4.17 (tt, J = 12.4, 3.7 Hz, 1H), 2.25 (qd, J = 12.4, 3.7 Hz, 2H), 1.89 (dm, J = 12.4 Hz, 2H), 1.77 (dm, J = 12.4 Hz, 2H), 1.71 (dm, J = 12.4 Hz, 1H), 1.25–1.44 (m, 3H) ppm; 13CNMR (CDCl3, 126 MHz) δ = 168.03, 167.97, 143.6, 141.5, 129.1, 128.6, 128.3, 118.8, 118.5, 109.4, 51.4, 30.0, 26.2, 25.3 ppm; IR (KBr) ν = 1766 (m), 1702 (vs) cm–1; MS (70 eV) m/z (%) 365 (M+, 57), 363 (M+, 55), 322 (32), 320 (30), 285 (14), 284 (100), 283 (27), 282 (98), 281 (15), 266 (23), 264 (22), 239 (12), 212 (13), 210 (13). HRMS Calcd for C16H1479BrNO2S (M+), 362.9927, found 362.9926.

18: M.p. 277–279 ˚C. 1HNMR (CDCl3, 400 MHz) δ = 8.17 (d, J = 0.4 Hz, 1H), 8.13 (d, J = 0.4 Hz, 1H), 4.13 (tt, J = 12.6, 3.4 Hz, 1H), 2.21(qd, J = 12.6, 3.4 Hz, 2H), 1.86 (dm, J = 12.6 Hz, 2H), 1.73 (dm, J = 12.6 Hz, 2H), 1.69 (dm, J = 12.6 Hz, 1H), 1.41–1.21 (m, 3H) ppm; 13CNMR (CDCl3, 100 MHz) δ = 167.6, 143.6, 141.3, 129.3, 128.6, 119.0, 118.6, 117.4, 113.3, 54.3, 51.3, 29.8, 26.0, 25.1 ppm; IR (KBr) ν = 1765 (s), 1705 (vs) cm–1; MS (70 eV) m/z (%) 445 (M+, 31), 443 (M+, 59), 441 (M+, 29), 402 (16), 400 (29), 398 (15), 364 (53), 363 (26), 362 (100), 344 (20), 342 (20), 319 (10), 290 (13). HRMS Calcd for C16H1379Br2NO2S (M+), 440.9034, found 440.9036.

3.5. 3-Cyano-N-CyclohexylBenzo[b]Thiophene-5, 6-Dicarboximide (19)

A mixture of 182 mg (0.500 mmol) of 17 and 134 mg (1.50 mmol, 3.0 eq.) of CuCN in 5 mL of dry DMSO was heated on an oil bath at 125˚C for 12 h. The reaction mixture was cooled to room temperature and was passed through a Celite® pad. The filtrate was poured into water and was extracted with ether (20 ml x 3). The combined organic layer was washed with a saturated NaHCO3 aqueous solution and brine. After dryness over Na2SO4, the solvent was evaporated and the residue was purified by silica gel chromatography (CHCl3) to give 96 mg of 17 (53% recovery), followed by 26.0 mg of 19 (32% based on consumed 17) as colorless microcrystals. The product 19 was obtained in 78% yield under similar reaction conditions at 160˚C for 6 h. M.p. 210–211˚C. 1HNMR (CDCl3, 500 MHz) δ = 8.44 (d, J = 0.8 Hz, 1H), 8.37 (s, 1H), 8.36 (d, J = 0.8 Hz, 1H), 4.18 (tt, J = 12.3, 3.7 Hz, 1H), 2.25 (qd, J = 12.3, 3.7 Hz, 2H), 1.89 (dm, J = 12.3 Hz, 2H,), 1.77 (dm, J = 12.3 Hz, 2H), 1.72 (dm, J = 12.3 Hz, 1H), 1.44–1.25 (m, 3H) ppm; 13CNMR (CDCl3, 126 MHz) δ = 149.4, 149.3, 129.9, 128.4, 128.3, 119.5, 119.1, 110.4, 109.9, 106.0, 102.5, 56.7, 39.4, 36.3, 35.6 ppm; IR (KBr) ν = 2230 (w), 1766 (s), 1702 (vs) cm–1; UV-vis (CHCl3) λmax = 252sh (logε = 4.70), 258 (4.77), 281sh (3.83), 324 (3.60), 338 (3.67) nm; MS (70 eV) m/z (%) 310 (M+, 40), 267 (31), 230 (15), 229 (100), 211 (21). HRMS Calcd for C17H14N2O2S (M+), 310.0776, found 310.0773.

3.6. General Procedure for Mizoroki-Heck Reactions of 19

A suspension of 19 (1.00 mmol), aryl iodide (1.25 mmol), tetra-n-butylammonium bromide (1.25 mmol), K2CO3 (2.50 mnol), and Pd(OAc)2 (0.010 mmol) in DMF (5.0 mL) was heated on an oil bath at 125˚C for 2 h under argon. After being cooled to room temperature, the reaction mixture was filtered through a Celite®pad, washed well with ether. The filtrate was poured into water and was extracted with ether (20 ml x 3). The combined organic layer was washed with a saturated NaHCO3 aqueous solution and brine. After dryness over Na2SO4, the solvent was evaporated and the residue was purified by silica gel chromatography with CHCl3 as eluant to give the pure product.

20a: Yellowish microcrystals, m.p. 245–246 ˚C. 1HNMR (CDCl3, 400 MHz) δ = 8.33 (s, 1H), 8.22 (s, 1H), 7.87–7.84 (m, 2H), 7.52–7.49 (m, 3H), 4.11 (tt, J = 12.4, 3.6 Hz, 1H), 2.18 (qd, J = 12.4, 3.6 Hz, 2H), 1.83 (dm, J = 12.4 Hz, 2H), 1.71 (dm, J = 12.4 Hz, 2H), 1.65 (dm, J = 12.4 Hz, 1H), 1.37–1.21 (m, J = 12.4, 3.6 Hz, 3H) ppm; 13CNMR (CDCl3, 100 MHz) δ = 167.6, 167.6, 159.1, 143.1, 141.8, 131.6, 130.7, 130.1, 129.8, 129.3, 128.5, 118.1, 117.9, 114.2, 103.3, 51.6, 30.0, 26.2, 25.2 ppm; IR (KBr) ν = 2224 (m), 1769 (m), 1701 (vs) cm–1; UV-vis (CHCl3) λmax = 259sh (logε = 4.41), 282 (4.52), 327sh (4.18) nm; MS (70 eV) m/z (%) 386 (M+, 72), 343 (29), 306 (23), 305 (100), 304 (44), 287 (20). HRMS Calcd for C23H18N2O2S (M+), 386.1089, found 386.1090.

20b: Yellowish microcrystals, m.p. 240–242 ˚C. 1HNMR (CDCl3, 500 MHz) δ = 8.33 (d, J = 0.8 Hz, 1H), 8.22 (d, J = 0.8 Hz, 1H), 7.89 (d, J = 8.8 Hz, 2H), 7.05 (d, J = 8.8 Hz, 2H), 4.15 (tt, J = 12.4, 3.6 Hz, 1H), 3.89 (s, 3H), 2.23 (qd, 2H, J = 12.4, 3.6 Hz, 1H), 1.87 (d, J = 12.4 Hz, 2H), 1.75 (d, J = 12.4 Hz, 2H), 1.69 (d, J = 12.4 Hz, 1H,), 1.42–1.26 (m, 3H) ppm; 13CNMR (CDCl3, 126 MHz) δ = 167.756, 167.750, 162.4, 159.1, 143.3, 141.3, 130.0, 129.0, 123.2, 117.9, 117.5, 115.2, 114.6, 101.8, 55.7, 51.7, 30.0, 26.2, 25.2 ppm; 13CNMR (CD2Cl2, 126 MHz) δ = 167.82, 167.79, 162.6, 159.3, 143.5, 141.7, 130.31, 130.05, 129.3, 123.5, 118.1, 117.4, 115.3, 114.8, 102.1, 56.0, 51.7, 30.2, 26.5, 25.6 ppm;[22] IR (KBr) ν = 2211 (w), 1765 (vs), 1706 (s) cm–1; UV-vis (CDCl3) λmax = 250 (logε = 4.45), 257 (4.49), 294 (4.64), 313 (4.31), 350sh (4.37) nm; MS (70 eV) m/z (%) 416 (M+, 71), 336 (14), 335 (100), 317 (10), 263 (10). HRMS Calcd for C24H20N2O3S (M+), 416.1195, found 416.1191.

20c: Orange microcrystals, m.p. 282–283 ˚C. 1HNMR (CDCl3, 500 MHz) δ = 8.28 (d, J = 0.8 Hz, 1H), 8.18 (d, J = 0.8 Hz, 1H), 7.89 (d, J = 5.5 Hz, 2H), 6.78 (d, J = 5.5 Hz, 2H), 4.16 (tt, J = 12.3, 3.7 Hz, 1H), 2.25(qd, J = 12.3, 3.7 Hz, 2H ), 1.89 (dm, J = 12.3 Hz, 2H), 1.77 (dm, J = 12.3 Hz, 2H), 1.71 (dm, J = 12.3 Hz, 1H), 1.43–1.28 (m, 3H) ppm; 13CNMR (CDCl3, 126 MHz) δ = 167.7, 167.7, 160.0, 152.2, 143.8, 140.5, 129.7, 129.4, 128.1, 117.8, 117.7, 116.7, 115.2, 111.9, 98.9, 51.2, 40.1, 29.9, 26.0, 25.1 ppm; IR (KBr) ν = 2212 (w), 1760 (s), 1704 (s), 1366 (vs) cm–1; UV-vis (CH3CN) λmax = 256sh (logε = 4.57), 266 (4.63), 310 (4.07), 328 (4.11), 414 (4.58) nm; MS (70 eV) m/z (%) 430 (29), 429 (M+, 100), 348 (10), 347 (33), 346 (20). HRMS Calcd for C25H23N23O2S (M+), 429.1511, found 429.1518.

4. Conclusion

We have demonstrated that the novel benzo[b]thiophene-5,6-dicarboximides 6, 9, and 10 can be synthesized easily by phosphine-assisted annulation between thiophene-2,3-dicarbaldehydes and various N-substituted maleimides. Introduction of a cyano group to the thiophene moiety of the N-cyclohexyl product was achieved by copper-mediated coupling reaction via its bromo derivatives and Mirozoroki-Heck reaction of thε 3-cyano product led to 2-aryl-3-cyano derivatives. Although the emission quantum yields of 6 are low, 2-aryl-3-cyano derivatives 20 indicate the enhanced emission quantum yield up to 27%. Also, we have demonstrated that the absorption and emission wavelengths are tunable by a substituent on the aryl moiety in this structure of 2-aryl-3-cyanobenzo[b]thiophene-5, 6-dicarboximide.


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