Synthesis of Novel 4,4-Dialkyl- and 4,4-Diarylindeno[1,2-b]Thiophenes and their 2-Bromo Derivatives*

A series of new 4,4-dialkyl- and 4,4-diaryl-4H-indeno[1,2-b]thiophenes and their 2-bromo derivatives were synthesized and characterized. An economical one-pot method for the preparation of the key starting material 2-(2-thienyl)benzoic acid and improved procedures for the synthesis of indeno[1,2-b]thiophen-4-one and 4,4-dialkyl-4H-indeno[1,2-b]thiophenes have been developed. The described methods were shown to be efficient for the preparation of the title compounds on a multigram scale.

4H-Indeno[1,2-b]thiophene is an attractive molecule with potentially interesting properties owing to its unique structure highly reminiscent to fluorene. Compounds possessing this heterocyclic moiety were found to display a wide range of functional material properties and are useful for the construction of sensitizers for dye-sensitized solar cells [1, 2], p-type organic semiconductors for organic thin film transistors [3–6], chromophores in luminescent materials [7], or as spacers in D-π-A [8] dyes for liquid or solid state solar cells. Continuing our program focused on the search of novel heterocyclic compounds with valuable properties [9–11], we present herein a synthesis of the title compounds and some improved procedures for the preparation of intermediates.

For the construction of 4H-indeno[1,2-b]thiophene ring system, 2-(2-thienyl)benzoic acid or the corresponding benzoates are the most often used starting materials [3, 12]. These compounds, as well as their precursor, 2-(2-thienyl)benzonitrile, can be obtained by Ullmann coupling [12], palladium-catalyzed desulfinylative [13, 14] or Stille [15, 16] reactions, or using manganese-catalyzed oxidative cross coupling of the corresponding Grignard reagents [17].

Taking into account that manganese-catalyzed reactions require large excess of one of the components and palladium prices are steadily rising, the purpose of the present investigation was to establish a more economical synthesis method, suitable for the preparation of the title compounds on a multigram scale. In this connection, for the synthesis of 2-(2-thienyl)benzoic acid (4) we decided to apply the Kumada-type Ni(0)-catalyzed cross-coupling reaction between 2-chlorobenzonitrile (1) and 2-thienylmagnesium bromide (2) (Scheme 1). Upon optimization of the reaction conditions, it was noticed that the success of the cross-coupling reaction depends on the amount of catalyst. When 2 mol% NiCl₂(PPh₃)₂ were used only 10% conversion of nitrile 1 to the desired product 3 was observed. Increase of catalyst loading up to 4 mol% raised the conversion to 30%, and 2-chlorobenzonitrile (1) was completely consumed in 8 h when 8 mol% NiCl₂(PPh₃)₂ was employed in the reaction. Formation of compound 3 was confirmed, and conversion of the starting material was estimated by GC/MS (m/z 185.1 [M]⁺). Distillation of the crude product under reduced pressure gave a mixture consisting of 2-(2-thienyl)-benzonitrile (3) and triphenylphosphine. This mixture was used in the next step without any further purification. Hydrolysis of compound 3 with KOH in ethylene glycol furnished 2-(2-thienyl)benzoic acid (4) in overall two-step yield 44%.

Scheme 1

Literature survey revealed that the synthesis of the key intermediate, 4H-indeno[1,2-b]thiophen-4-one (5) is explored insufficiently. To our knowledge, the only method of synthesis of compound 5 from benzoic acid 4 described in the literature includes the synthesis of the corresponding benzoyl chloride and its subsequent cyclization with SnCl₄ in dry benzene [3, 12]. Therefore, we decided to investigate other well-known cyclization reactions. First, we tried to apply the direct cyclization reactions using POCl₃ or PPA. However, the reaction of compound 4 with POCl₃ did not yield even traces of product. Heating compound 4 with PPA at 100°C afforded 4H-indeno[1,2-b]thiophen-4-one (5) in low yield (18%). Further, the corresponding chloro-anhydride was obtained by treating compound 4 with SOCl₂. Various Lewis acids were tested in intramolecular Friedel–Crafts acylation reaction. Using FeCl₃, AlCl₃, and TiCl₄ in dichloroethane afforded compound 5 in 36%, 46%, and 52% yields, respectively. The best result was achieved using in situ generated AlCl₃–MeNO₂ complex: indenothiophenone 5 was obtained in 70% yield over two steps (Scheme 1). Wolff–Kishner reduction of compound 5 afforded 4H-indeno[1,2-b]thiophene (6) [12] in 82% yield. Methylation of compound 6 with methyl iodide using phase transfer catalyst benzyltriethylammonium chloride in the two-phase solvent system after 96 h of stirring at room temperature afforded compound 7a in 82% yield. Alkylation of compound 6 under the same reaction conditions with alkyl halides, which had longer carbon chains, showed even longer reaction times, and 4,4-dialkyl-4H-indeno[1,2-b]thiophenes 7b-e could not be separated from the monoalkylated by-products. In order to reduce the reaction time and increase yields of dialkylated compounds 7a-e, the alkylation reaction using an excess of potassium tert-butoxide with alkyl halides was carried out (Scheme 1). This method produced 4,4-dialkyl-4H-indeno[1,2-b]thiophenes 7a-e in good yields without any traces of monoalkylated by-products. The total reaction time was 2 h. The method works well with both alkyl bromides and alkyl iodides.

The synthetic route towards 4,4-diarylindeno[1,2-b]thiophenes is outlined in Scheme 2. Ester 8, prepared from acid 4, reacted with an excess of aryl Grignard reagents to give the corresponding alcohols 9a,b as intermediates. Without further purification, these alcohols were subjected to acid-catalyzed intramolecular annelation to afford 4,4-diaryl-4H-indeno[1,2-b]thiophenes 10a,b [7].

Scheme 2

The obtained 4,4-dialkylindeno[1,2-b]thiophenes 7a-e and 4,4-diaryl-4H-indeno[1,2-b]thiophenes 10a,b were selectively brominated at position 2 by NBS in DMF under exclusion of light to give compounds 11a-g in high yields (Scheme 3).

Scheme 3

In summary, we have synthesized and characterized a series of new 4,4-dialkyl- and 4,4-diaryl-4H-indeno[1,2-b]thiophenes and their 2-bromo derivatives. The synthesized compounds can find application as precursors or substrates for the development of novel functional materials with valuable light-emitting and semiconductive properties. Improved procedures for the synthesis of 2-(2-thienyl)benzoic acid and 4,4-dialkyl-4H-indeno[1,2-b]thiophenes allow the preparation of the title compounds on a multigram scale.

Experimental

¹H and ¹³C NMR spectra were recorded on a Varian Inova spectrometer (300 and 75 MHz, respectively) in CDCl₃. The NMR chemical shifts were referenced to residual solvent peaks (7.29 and 77.3 ppm for ¹H and ¹³C nuclei, respectively). Mass spectra and conversion of compounds were acquired on an Agilent 5975 GC/MS instrument with EI ionization (70 eV). Elemental analysis was performed on a ThermoFischer Scientific Flash 2000 Elemental Analyzer. Melting points were determined in open capillaries with a digital melting point ThermoFischer Scientific IA900 series apparatus and are uncorrected. Thin-layer chromatography was performed using TLC-aluminum sheets with silica gel Merck 60 F254. Visualization was accomplished by UV light. Column chromatography was performed using Merck Silica gel 60 (0.040-0.063 mm). All reagents, catalysts, and solvents were purchased from Sigma-Aldrich and were used without further purification.

2-(2-Thienyl)benzoic Acid (4). A mixture of NiCl₂(PPh₃)₂ (144.5 g, 0.221 mol), PPh₃ (115.8 g, 0.442 mol), and Zn (14.3 g, 0.221 mol) was stirred at room temperature for 30 min under argon atmosphere till the reaction mixture became dark-red. Then a solution of 2-chlorobenzonitrile (1) (379.5 g, 2.760 mol) in THF (1000 ml) was added to the reaction mixture. Thiophene Grignard reagent (2), prepared by reacting 2-bromothiophene (500 g, 3.07 mol) with magnesium turnings (76.5 g, 3.15 mol) in THF (1000 ml), was added dropwise to the reaction mixture at such a speed that temperature would not rise above 30°C. The reaction mixture was stirred at room temperature for 2 h and left overnight, then poured into ice water, acidified with concentrated hydrochloric acid to pH 2-4, and extracted with CH₂Cl₂ (2×750 ml). The combined extracts were dried over Na₂SO₄ and filtered. After the removal of the solvent from the filtrate, the obtained viscous dark-brown residue was distilled under vacuum and the fraction with bp 145-155°C/1 mbar was collected to give 300.0 g of 2-(2-thienyl)benzonitrile (3) as a viscous yellow oil. The obtained crude 2-(2-thienyl)benzonitrile (3) was added to a mixture of ethylene glycol (1200 ml), H₂O (50 ml), and KOH (320 g, 5.7 mol). The reaction mixture was stirred at 130-140°C for 24 h. After cooling to room temperature, the mixture was poured into water, acidified with concentrated HCl to pH 1-2, and extracted with CH₂Cl₂ (3×500 ml). After removal of the solvent, the brown residue was recrystallized from hexane to give 250 g (44%) of compound 4 as a white solid, mp 91-93°C (hexane) (mp 92-94°C (Et₂O) [3]). ¹H NMR spectrum, δ, ppm (J, Hz): 7.08-7.14 (2H, m, H-3,4 thienyl); 7.39 (1H, dd, J = 4.8, J = 1.5, H-5 thienyl); 7.41-7.54 (2H, m, H-4,5 Ph); 7.57 (1H, dd, J = 7.5, J = 1.5, H-3 Ph); 7.94 (1H, dd, J = 7.5, J = 1.5, H-6 Ph); 10,09 (1H, s, COOH). ¹³C NMR spectrum, δ, ppm: 126.4; 127.1; 127.5; 128.1; 130.5; 130.8; 132.1; 132.2; 135.4; 141.9; 173.9. Found, %: C 64.32; H 3.96. C₁₁H₈O₂S. Calculated, %: C 64.69; H 3.95.

4 H -Indeno[1,2- b ]thiophen-4-one (5). A mixture of 2-(2-thienyl)benzoic acid (4) (150 g, 0.735 mol), SOCl₂ (88 g, 0.74 mol), and 1,2-dichloroethane (250 ml) was refluxed for 8 h. Then the reaction mixture was cooled to -18°C, and a solution of AlCl₃ (103 g, 0.75 mol) and MeNO₂ (122 g, 2 mol) in 1,2-dichloroethane (300 ml) was added dropwise within 1 h. After the addition was complete, the reaction mixture was left to warm to room temperature for 1 h. The mixture was poured into 5 M HCl (500 ml) and stirred for 30 min. The organic layer was separated, washed twice with water, dried over Na₂SO₄, and filtered. After removal of the solvent from the filtrate, the obtained dark-brown residue was distilled under vacuum at 122°C/0.5 mbar and recrystallized from hexane to give 96 g (70%) of compound 5 as orange crystals, mp 101-102°C (hexane) (mp 99-101°C (benzene) [12]). ¹H NMR spectrum, δ, ppm (J, Hz): 7.13-7.22 (4H, m, H-2,3,6,8); 7.35 (1H, td, J = 7.5, J = 0.9, H-7); 7.46 (1H, dd, J = 7.5, J = 0.9, H-5). ¹³C NMR spectrum, δ, ppm: 119.5; 121.7; 123.9; 128.7; 129.3; 134.1; 136.7; 139.0; 142.2; 159.2; 187.5. Found, %: C 70.69; H 3.24. C₁₁H₆OS. Calculated, %: C 70.94; H 3.25.

4 H -Indeno[1,2- b ]thiophene (6). A mixture of 4H-indeno[1,2-b]thiophen-4-one (5) (40 g, 0.215 mol), hydrazine hydrate (36 g of 50% solution in water, 0.36 mol), and diethylene glycol (150 ml) was heated at 110°C for 3 h, then cooled to 40°C, and KOH (36 g, 0.645 mol) was added. The reaction mixture was heated to 110°C and stirred for 4 h. After cooling to room temperature, the mixture was poured into water (400 ml) and extracted with CH₂Cl₂ (3×150 ml). The organic layer was separated and washed with water until neutral reaction (pH 7). After removal of the solvent, the solid was recrystallized from hexane to give 32 g (82%) of compound 6 as a yellowish solid, mp 68-70°C (hexane) (mp 68-69°C (Et₂O) [12]). ¹H NMR spectrum, δ, ppm (J, Hz): 3.74 (2H, s, CH₂); 7.16 (1H, d, J = 4.5, H-3); 7.24-7.34 (2H, m, H-5,8); 7.35 (1H, d, J = 4.5, H-2); 7.52-7.55 (2H, m, H-6,7). ¹³C NMR spectrum, δ, ppm: 34.2; 119.0; 122.9; 125.0; 125.3; 127.1; 127.3; 139.1; 143.4; 146.3; 147.4. Mass spectrum, m/z (I _rel, %): 172 [M]⁺ (100), 139 (10), 127 (20), 86 (14). Found, %: C 76.46; H 4.63. C₁₁H₈S. Calculated, %: C 76.70; H 4.68.

Synthesis of 4,4-Dimethyl-4 H -indeno[1,2- b ]thiophene (7a) Under Phase Transfer Catalysis. A mixture of 4H-indeno[1,2-b]tiophene (6) (2.00 g, 11.60 mmol), benzyltriethylammonium chloride (0.18 g, 0.79 mmol), MeI (2.5 ml, 40 mmol), DMSO (20 ml), and 50% aqueous solution of NaOH (20 ml) was stirred at room temperature for at least 96 h. The reaction mixture was quenched with water and extracted with CH₂Cl₂. After removal of the solvent, the product was purified by column chromatography on silica gel (eluent hexane) to give compound 7a. Yield 1.90 g (82%), white powder, mp 84-85°C (2-PrOH). ¹H NMR spectrum: δ, ppm (J, Hz): 1.52 (6H, s, 2CH₃); 7.08 (1H, d, J = 5.1, H-3); 7.24-7.32 (2H, m, H-5,8); 7.34 (1H, d, J = 5.1, H-2); 7.48-7.50 (2H, m, H-6,7). ¹³C NMR spectrum, δ, ppm: 26.3; 45.7; 119.1; 120.9; 122.6; 125.4; 127.2; 127.8; 136.8; 140.0; 156.5; 158.6. Found, %: C 77.45; H 6.04. C₁₃H₁₂S. Calculated, %: C 77.95; H 6.04.

4,4-Dialkyl-4 H -indeno[1,2- b ]thiophenes 7a-e (General Method). Potassium tert-butoxide (3.9 g, 34.8 mmol) was added to a solution of 4H-indeno[1,2-b]-thiophene (6) (2.0 g, 11.6 mmol) in THF (30 ml). The reaction mixture was stirred at room temperature for 1 h. Then the corresponding alkyl iodide (for compounds 7a,b,e) or bromide (for compounds 7c,d) (34.8 mmol) was added, and the reaction mixture was stirred for an additional hour. The reaction was quenched with water and extracted with CH₂Cl₂ (3×50 ml); the organic layer was separated and washed with water until neutral reaction (pH 7). After evaporation of the solvent, the product was recrystallized from 2-PrOH (compound 7a) or purified by column chromatography (eluent hexane) (compounds 7b-e).

4,4-Dimethyl-4 H -indeno[1,2- b ]thiophene (7a). Yield 2.0 g (86%).

4,4-Dibutyl-4 H -indeno[1,2- b ]thiophene (7b). Yield 2.5 g (76%), yellow oil. ¹H NMR spectrum, δ, ppm (J, Hz): 0.69-0.85 (4H, m, 2CH₂CH ₂CH₂CH₃); 0.75-0.87 (6H, t, J = 7.5, 2CH₃); 1.13-1.21 (4H, m, 2CH₂CH₂CH ₂CH₃ ); 1.89-1.99 (4H, m, 2CH ₂CH₂CH₂CH₃); 7.20 (1H, d, J = 4.8, H-3); 7.31-7.33 (3H, m, H-5,6,7); 7.38 (1H, d, J = 4.8, H-2); 7.45 (1H, d, J = 7.5, H-8). ¹³C NMR spectrum, δ, ppm: 14.1; 23.3; 26.6; 39.0; 54.0; 118.8; 121.8; 122.8; 125.2; 126.9; 127.3; 138.5; 141.4; 154.1; 155.6. Found, %: C 80.22; H 9.03. C₁₉H₂₄S. Calculated, %: C 80.22; H 8.59.

4,4-Dipentyl-4 H -indeno[1,2- b ]thiophene (7c). Yield 2.3 g (64%), yellow oil. ¹H NMR spectrum, δ, ppm (J, Hz): 0.71-0.89 (4H, m, 2CH₂CH ₂(CH₂)₂CH₃); 0.83 (6H, t, J = 7.2, 2CH₃); 1.14-1.23 (8H, m, 2(CH₂)₂(CH ₂)₂CH₃); 1.97-2.07 (4H, m, 2CH ₂(CH₂)₃CH₃); 7.04 (1H, d, J = 4.8, H-3); 7.30-7.36 (3H, m, H-5,6,7); 7.39 (1H, d, J = 4.8, H-2); 7.51 (1H, d, J = 7.2, H-8). ¹³C NMR spectrum, δ, ppm: 14.3; 22.6; 24.1; 32.6; 39.3; 54.1; 118.9; 121.8; 122.8; 125.3; 127.0; 127.4; 138.6; 141.5; 154.2; 155.6. Mass spectrum, m/z (I _rel, %): 312 [M]⁺ (70), 241 (90), 185 (100), 171 (35), 152 (14). Found, %: C 80.7; H 9.03. C₂₁H₂₈S. Calculated, %: C 80.71; H 9.03.

4,4-Dihexyl-4 H -indeno[1,2- b ]thiophene (7d). Yield 2.8 g (71%), yellow oil. ¹H NMR spectrum, δ, ppm (J, Hz): 0.72-0.89 (4H, m, 2CH₂CH ₂(CH₂)₃CH₃); 0.88 (6H, t, J = 7.5, 2CH₃); 1.12-1.21 (12H, m, 2(CH₂)₂(CH ₂)₃CH₃); 1.96-2.06 (4H, m, 2CH ₂(CH₂)₄CH₃); 7.07 (1H, d, J = 4.8, H-3); 7.27-7.38 (3H, m, H-5,6,7); 7.38 (1H, d, J = 4.8, H-2); 7.51 (1H, d, 7.2, H-8). ¹³C NMR spectrum, δ, ppm: 14.3; 22.9; 24.4; 30.0; 31.9; 39.3; 54.1; 118.9; 121.8; 122.8; 125.2; 127.0; 127.4; 138.6; 141.5; 154.1; 155.6. Mass spectrum, m/z (I _rel, %): 340 [M]⁺ (63), 255 (85), 185 (100), 171 (32), 152 (12). Found, %: C 80.92; H 9.44. C₂₃H₃₂S. Calculated, %: C 81.11; H 9.47.

4,4-Diheptyl-4 H -indeno[1,2- b ]thiophene (7e). Yield 3.6 g (84%), yellow oil. ¹H NMR spectrum, δ, ppm (J, Hz): 0.75-0.91 (4H, m, 2CH₂CH ₂(CH₂)₄CH₃); 0.91 (6H, t, J = 7.2, 2CH₃); 1.17-1.29 (16H, m, 2(CH₂)₂(CH ₂)₄CH₃); 1.91-2.07 (4H, m, 2CH ₂(CH₂)₅CH₃); 7.03 (1H, d, J = 5.1, H-3); 7.24-7.36 (3H, m, H-5,6,7); 7.36 (1H, d, J = 5.1, H-2); 7.47 (1H, d, J = 7.2, H-8). ¹³C NMR spectrum, δ, ppm: 14.3; 22.8; 24.4; 29.3; 30.3; 32.1; 39.3; 54.1; 118.9; 121.8; 122.8; 125.3; 127.0; 127.3; 138.5; 141.4; 154.1; 155.6. Found, %: C 80.96; H 9.87. C₂₅H₃₆S. Calculated, %: C 81.46; H 9.84.

Methyl 2-(2-Thienyl)benzoate (8). A mixture of 2-(2-thienyl)benzoic acid (4) (30 g, 0.147 mol), p-TsOH (5 g, 0.022 mol), and MeOH (100 ml) was refluxed for 14 h. Then the reaction mixture was cooled and poured into ice water (300 ml) and extracted with CH₂Cl₂ (2×100 ml). Removal of the solvent gave 29 g (91%) of yellowish oil (slightly-yellow oil [3]). ¹H NMR spectrum, δ, ppm (J, Hz): 3.77 (3H, s, COOCH₃); 7.06-7.10 (2H, m, H-3,4 thienyl); 7.37 (1H, dd, J = 5.0, J = 1.5, H-2 thienyl); 7.43-7.52 (3H, m, H-4,5,6 Ph); 7.75 (1H, td, J = 7.6, J = 1.5, H-3 Ph). ¹³C NMR spectrum, δ, ppm: 52.2; 125.9; 126.3; 127.2; 127.7; 129.4; 131.0; 131.1; 131.7; 134.1; 142.0; 169.1.

4,4-Diaryl-4 H -indeno[1,2- b ]thiophenes 10a,b (General Method). Grignard reagent prepared from 1-bromo-4-hexylbenzene or 1-bromo-3-hexylbenzene (13.3 g, 0.055 mol) and magnesium turnings (1.45 g, 0.06 mol) in THF (25 ml) was added to a stirred solution of methyl 2-(2-thienyl)benzoate (8) (5 g, 0.023 mol) in THF (50 ml). After the addition was complete, the reaction mixture was refluxed for 3 h, then cooled to room temperature and poured into 1 M NH₄Cl solution (200 ml), extracted with CH₂Cl₂, and washed with water twice. After removal of the solvent, the obtained yellow oil was dissolved in acetic acid (40 ml) and heated to 80°C. When the required temperature was reached, 2 drops of concentrated H₂SO₄ was added, and stirring at 80°C was continued for 2 h. The reaction mixture was poured to ice water, extracted with CH₂Cl₂, and washed with water 3 times. Removal of the solvent gave the crude product, which was recrystallized from 2-PrOH.

4,4-Bis(4-hexylphenyl)-4 H -indeno[1,2- b ]thiophene (10a). Yield 4.3 g (38%), white solid, mp 61-63°C (2-PrOH). ¹H NMR spectrum, δ, ppm (J, Hz): 0.94 (6H, t, J = 6.0, 2CH₃); 1.35-1.63 (16H, m, 2CH₂(CH ₂ ) ₄CH₃); 2.59 (4H, t, J = 7.5, 2CH ₂(CH₂)₄CH₃); 7.08-7.24 (10H, m, H-2,3, H-2,3,5,6 2Ar); 7.31-7.35 (2H, m, H-6,7); 7.43 (1H, d, J = 7.5, H-5); 7.51 (1H, d, J = 7.5, H-8). ¹³C NMR spectrum, δ, ppm: 14.3; 22.8; 29.3; 31.6; 31.9; 35.8; 63.1; 119.5; 123.4; 125.8; 126.6; 127.5; 128.0; 128.1; 128.5; 137.5; 141.2; 141.5; 142.2; 154.0; 156.2. Mass spectrum, m/z (I _rel, %): 492 [M]⁺ (100), 459 (9), 407 (18), 331 (16), 260 (14). Found, %: C 85.02; H 7.89. C₃₅H₄₀S. Calculated, %: C 85.31; H 8.18.

4,4-Bis(3-hexylphenyl)-4 H -indeno[1,2- b ]thiophene (10b). Yield 3.2 g (28%), white solid, mp 45-46°C (2-PrOH). ¹H NMR spectrum, δ, ppm (J, Hz): 0.91 (6H, t, J = 6.0, 2CH₃); 1.25-1.57 (16H, m, 2CH₂(CH ₂)₄CH₃); 2.53 (4H, t, J = 7.8, CH ₂(CH₂)₄CH₃); 6.96-7.08 (7H, m, H-2, H-2,4,6 2Ar); 7.16 (2H, t, J = 7.5, H-5 2Ar); 7.22 (1H, d, J = 7.5, H-5); 7.30-7.41 (3H, m, H-3,6,7); 7.51 (1H, d, J = 7.5, H-8). ¹³C NMR spectrum, δ, ppm: 14.3; 22.8; 29.1; 31.7; 31.9; 36.2; 63.6; 119.5; 123.3; 125.2; 125.8; 126.5; 127.0; 127.6; 128.1; 128.2; 128.5; 137.5; 141.3; 143.1; 144.9; 153.8; 156.0. Mass spectrum, m/z (I _rel, %): 492 [M]⁺ (100), 459 (10), 407 (20), 331 (16), 260 (14). Found, %: C 84.96; H 8.11. C₃₅H₄₀S. Calculated, %: C 85.31; H 8.18.

2-Bromo-4,4-dialkyl- and 4,4-Diaryl-4 H -indeno[1,2- b ]thiophenes 11a-g (General Method). NBS (1.47 g, 8.25 mmol) was added in portions to a solution of the corresponding 4,4-dialkyl- or 4,4-diaryl-4H-indeno[1,2-b]thiophene 7a-e, 10a,b (7.5 mmol) in DMF (25 ml), cooled to 0°C, and protected from the daylight. The reaction mixture was stirred at 0°C for 2 h and then allowed to reach room temperature. The reaction was quenched with water (100 ml) and extracted with CH₂Cl₂. After removal of the solvent, the products were purified by column chromatography on silica gel (eluent hexane).

2-Bromo-4,4-dimethyl-4 H -indeno[1,2- b ]thiophene (11a). Yield 1.9 g (90%), white solid, mp 54-55°C (hexane). ¹H NMR spectrum, δ, ppm: 1.48 (6H, s, 2CH₃); 7.08 (1H, s, H-3); 7.25-7.33 (2H, m, H-5,6); 7.38-7.40 (2H, m, H-7,8). ¹³C NMR spectrum, δ, ppm: 26.1; 46.7; 113.7; 119.0; 122.0; 124.2; 125.7; 127.3; 136.4; 140.2; 155.1; 157.4. Found, %: C 56.23; H, 3.98. C₁₃H₁₁BrS. Calculated, %: C 55.92; H 3.97.

2-Bromo-4,4-dibutyl-4 H -indeno[1,2- b ]thiophene (11b). Yield 2.2 g (82%), white solid, mp 61-63°C (hexane). ¹H NMR spectrum, δ (J, Hz): 0.73-0.84 (4H, m, 2CH₂CH ₂CH₂CH₃); 0.77 (6H, t, J = 7.2, 2CH₃); 1.12-1.17 (4H, m, 2CH₂CH₂CH ₂CH₃); 1.81-1.92 (4H, m, 2CH ₂CH₂CH₂CH₃); 7.02 (1H, s, H-3); 7.20-7.29 (2H, m, H-5,6); 7.31-7.39 (2H, m, H-7,8). ¹³C NMR spectrum, δ, ppm: 14.1; 23.2; 26.5; 38.9; 55.0; 113.3; 118.8; 122.8; 124.8; 125.5; 127.1; 138.1; 141.7; 152.7; 154.4. Mass spectrum, m/z (I _rel, %): 364 [M]⁺ (60), 307 (50), 251 (20), 226 (100), 198 (46), 184 (75), 139 (24). Found, %: C 63.13; H 6.55. C₁₉H₂₃BrS. Calculated, %: C 62.80; H 6.38.

2-Bromo-4,4-dipentyl-4 H -indeno[1,2- b ]thiophene (11c). Yield 2.7 g (92%), yellow oil. ¹H NMR spectrum, δ, ppm (J, Hz): 0.77-0.82 (4H, m, 2CH₂CH ₂(CH₂)₂CH₃); 0.81 (6H, t, J = 6.9, 2CH₃); 1.11-1.16 (8H, m, 2(CH₂)₂(CH ₂)₂CH₃); 1.72-1.98 (4H, m, 2CH ₂(CH₂)₃CH₃); 7.01 (1H, s, H-3); 7.20-7.30 (2H, m, H-5,6); 7.31-7.41 (2H, m, H-7,8). ¹³C NMR spectrum, δ, ppm: 14.2; 22.6; 24.0; 32.4; 39.1; 55.1; 113.3; 118.8; 122.8; 124.8; 125.5; 127.1; 138.1; 141.7; 152.7; 154.7. Found, %: C 64.92; H 7.06. C₂₁H₂₇BrS. Calculated, %: C 64.44; H 6.95.

2-Bromo-4,4-dihexyl-4 H -indeno[1,2- b ]thiophene (11d). Yield 2.8 g (89%), yellow oil. ¹H NMR spectrum, δ, ppm (J, Hz): 0.79-0.85 (4H, m, 2CH₂CH ₂(CH₂)₃CH₃); 0.82 (6H, t, J = 6.9, 2CH₃); 1.15-1.22 (12H, m, 2(CH₂)₂(CH ₂)₃CH₃); 1.81-198 (4H, m, 2CH ₂(CH₂)₄CH₃); 7.03 (1H, s, H-3); 7.21-7.32 (2H, m, H-5,6); 7.37-7.44 (2H, m, H-7,8). ¹³C NMR spectrum, δ, ppm: 14.2; 22.8; 24.3; 29.8; 31.8; 39.0; 55.1; 113.3; 118.8; 122.8; 124.9; 125.5; 127.1; 138.1; 141.7; 152.7; 154.6. Found, %: C 65.69; H 7.43. C₂₃H₃₁BrS. Calculated, %: C 65.86; H 7.45.

2-Bromo-4,4-diheptyl-4 H -indeno[1,2- b ]thiophene (11e). Yield 2.61 g (78%), yellow oil. ¹H NMR spectrum, δ, ppm (J, Hz): 0.78-0.84 (4H, m, 2CH₂CH ₂(CH₂)₄CH₃); 0.86 (6H, t, J = 7.0, 2CH₃); 1.12-126 (16H, m, 2(CH₂)₂(CH ₂)₄CH₃); 1.78-1.95 (4H, m, 2CH ₂(CH₂)₅CH₃); 7.01 (1H, s, H-3); 7.20-7.30 (2H, m, H-5,6); 7.32-7.39 (2H, m, H-7,8). ¹³C NMR spectrum, δ, ppm: 14.3; 22.8; 24.3; 29.2; 30.2; 32.0; 39.1; 55.1; 113.3; 118.8; 122.8; 124.9; 125.5; 127.1; 138.1; 141.7; 152.7; 154.5. Found, %: C 66.93; H 7.92. C₂₅H₃₅BrS. Calculated, %: C 67.10; H 7.88.

2-Bromo-4,4-bis(4-hexylphenyl)-4 H -indeno[1,2- b ]thiophene (11f). Yield 3.8 g (89%), white powder, mp 93-95°C (hexane). ¹H NMR spectrum, δ, ppm (J, Hz): 0.94 (6H, t, J = 6.6, 2CH₃); 1.36-1.63 (16H, m, 2CH₂(CH ₂)₄CH₃); 2.61 (4H, t, J = 7.8, 2CH ₂(CH₂)₄CH₃); 7.08-7.19 (8H, m, H-2,3,5,6 2Ar); 7.14 (1H, s, H-3); 7.23-7.45 (4H, m, H-5,6,7,8). ¹³C NMR spectrum, δ, ppm: 14.3; 22.8; 29.4; 31.6; 32.0; 35.8; 63.9; 114.2; 119.5; 120.6; 126.1; 126.5; 127.8; 127.9; 128.6; 137.1; 141.5; 141.6; 141.8; 152.8; 155.0. Found, %: C 73.52; H 6.96. C₃₅H₃₉BrS. Calculated, %: C 73.54; H 6.88.

2-Bromo-4,4-bis(3-hexylphenyl)-4 H -indeno[1,2- b ]thiophene (11g). Yield 3.0 g (70%), yellow viscous oil. ¹H NMR spectrum, δ, ppm (J, Hz): 0.92 (6H, t, J = 6.9, 2CH₃); 1.29-1.61 (16H, m, 2CH₂(CH ₂)₄CH₃); 2.56 (4H, t, J = 7.8, 2CH ₂(CH₂)₄CH₃); 6.97 (2H, dt, J = 7,5, J = 1.5, H-6 2Ar); 7.05 (2H, s, H-2 2Ar); 7.07-7.14 (3H, m, H-3, H-4 2Ar); 7.17 (2H, t, J = 7.5, H-5 2Ar); 7.23 (1H, dd, J = 7.5, J = 0.9, H-5); 7.33 (1H, td, J = 7.5, J = 0.9, H-6); 7.37-7.45 (2H, m, H-7,8). ¹³C NMR spectrum, δ, ppm: 14.3; 22.9; 29.1; 31.6; 31.9; 36.2; 64.5; 114.2; 119.5; 125.1; 126.1; 126.3; 126.5; 127.2; 127.7; 128.4; 128.4; 137.1; 141.5; 143.2; 144.3; 152.6; 154.8. Found: C 73.54; H 6.91. C₃₅H₃₉BrS. Calculated, %: C 73.54; H 6.88.

The research was funded by a grant (No. MIP-027/2013) from the Research Council of Lithuania.