Download PDF
Synlett 2005(2): 217-222  
DOI: 10.1055/s-2004-836055
LETTER
© Georg Thieme Verlag Stuttgart · New York

Reduction versus Rearrangement of 6,13-Dihydro-6,13-diarylpentacene-6,13-diols Affording 6,13- and 13,13′-Substituted Pentacene Derivatives

Nathalie Vets, Mario Smet, Wim Dehaen*
Department of Chemistry, K.U. Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium
Fax: +32(16)327990; e-Mail: Wim.Dehaen@chem.kuleuven.ac.be;
Further Information

Publication History

Received 14 June 2004
Publication Date:
02 December 2004 (online)

    Permissions and Reprints All articles of this category

Abstract

Pentacene has excellent semi-conducting properties but its practical use in organic thin film transistors (OTFTs) gives rise to a lot of problems caused by its sensitivity to oxygen and its very low solubility. In order to solve the problems involved in the use of pentacene, different aryl substituents were introduced on the 6- and 13-positions. Formation of the pentacene ring system may be accompanied by the rearrangement of the starting dihydropentacenediol to a 13,13′-disubstituted pentacen-6-one for electron-rich aryl substituents. 6-Monosubstituted pentacenes were also prepared.

Key words

substituted pentacenes - OTFTs - thiophene substituents - rearrangement - pentacenones

6

Synthesis of 6,13-Dihydro-6,13-dithien-2-ylpentacene-6,13-diol(2c):
n-BuLi (4.0 mL, 9.8 mmol) was added to a solution of thiophene (0.78 mL, 9.8 mmol) in THF (75 mL) at -78 °C under argon atmosphere. After 10 min pentacenequinone (1.0 g, 3.4 mmol) was added and the mixture was allowed to warm up to r.t. and stirred overnight. The reaction was worked up with 1 M HCl (50 mL), extracted with CH2Cl2 (150 mL) and washed with H2O (3 × 50 mL). After drying over MgSO4 and evaporation in vacuo, the crude product was purified by column chomatography over silica gel using petroleum ether-CH2Cl2 (15:85). The product was isolated in 58% yield as a mixture of the cis- and trans-isomers (4:6); mp 134 °C. 1H NMR (300 MHz, CDCl3): δ trans-isomer = 3.3 (s, 2 H), 6.8 (d, 2 H), 7.0 (t, 2 H), 7.3 (d, 2 H), 7.5 (m, 4 H), 7.9 (m, 4 H), 8.0 (m, 4 H), 8.2 (s, 4 H), 8.6 (s, 4 H); δ cis-isomer = 3.1 (s, 2 H), 5.9 (d, 2 H), 6.3 (t, 2 H), 6.9 (d, 2 H), 7.6 (m 4 H). 13C NMR (75 MHz, CDCl3): δ = 72.6, 124.2, 125.5, 126.2, 126.4, 127.2, 128.3, 132.8, 138.4, 149.0. MS (CI) [MH+]: m/z = 477 (and 459: rearranged product).
Synthesis of 6,13-Dihydro-6,13-bis(4-methoxy-phenyl)pentacene-6,13-diol (2a):
The procedure is the same as for 2c except purification was done by washing the crude product with MeOH (30 mL). The pure product was collected by filtration. Yield 34%, mp 232 °C. 1H NMR (300 MHz, CDCl3): δ = 3.1 (s, 2 H), 3.5 (s, 6 H), 6.1 (d, 4 H), 6.5 (d, 4 H), 7.5 (m, 4 H), 7.9 (m, 4 H), 8.4 (s, 4 H). 13C NMR (75 MHz, CDCl3): δ = 55.0, 76.4, 112.6, 124.8, 126.3, 128.1, 129.0, 132.7, 134.7, 139.6, 158.2. MS (CI) [MH+]: m/z = 525.
Synthesis of 6,13-Dihydroxy-6,13-(4-acetyl)phenyl-pentacene-6,13-diol (2b):
The procedure is the same as for 2c, the bromoacetophenone was protected with 1,2-ethanediol, except column chromatography over silica gel was carried out with EtOAc-petroleum ether-CH2Cl2 (2:4:4). Yield 11%; mp 229-232 °C. 1H NMR (300 MHz, CDCl3): δ = 2.4 (s, 6 H), 2.5 (s, 2 H), 6.9 (d, 4 H), 7.3 (d, 4 H), 7.6 (m, 4 H), 8.0 (m, 4 H), 8.4 (s, 4 H). 13C NMR (75 MHz, CDCl3): δ = 26.6, 76.4, 125.7, 127.1, 127.4, 128.3, 132.9, 135.7, 139.0, 147.4, 197.8. MS (CI) [MH+]: m/z = 549 (and 531: rearranged product).
Synthesis of 6,13-Dihydro-6′,13′-bis(benzothien-2-yl)pentacene-6,13-diol (2d):
The procedure is the same as for 2c except column chromatography over silica gel was carried out with petroleum ether-CH2Cl2 (1:9). Yield 87%; mp trans: 258-260 °C, cis: 296-300 °C. 1H NMR (300 MHz, CDCl3): δ trans-isomer = 3.0 (s, 2 H), 7.1 (s, 2 H), 7.3 (m, 4 H), 7.5 (m, 4 H), 7.6-7.7 (m, 2 H), 7.7-7.8 (m, 2 H), 7.9 (m, 4 H), 8.3 (s, 4 H); δ cis-isomer = 3.2 (s, 2 H), 5.9 (s, 1 H), 6.6 (d, 2 H), 6.7 (t, 2 H), 6.9 (t, 2 H), 7.3 (d, 2 H), 7.6 (m, 4 H), 8.0-8.1 (m, 4 H), 8.6 (s, 4 H). 13C NMR (75 MHz, CDCl3): δ trans-isomer = 75.4, 122.9, 124.5, 125.0, 125.1, 127.5, 127.6, 128.8, 133.7, 138.9, 139.8, 141.1, 153.0; δ cis-isomer = 53.7, 121.2, 123.6, 124.5, 126.9, 128.4, 133.1, 138.5, 139.9. MS (CI) [MH+]: m/z 559 (rearranged product).
Synthesis of 6,13-Dihydro-13,13′-bis(5-methoxythien-2-yl)pentacene-6-one (4):
The procedure is the same as for product 2c except column chromatography over silica gel was carried out with petroleum ether-CH2Cl2 (1:9). Yield 55%; the yield rose to 77% when the work up was done with NH4Cl. Mp 117-120 °C. 1H NMR (300 MHz, CDCl3): δ = 3.8 (s, 6 H), 5.9 (d, 2 H), 6.2 (d, 2 H), 7.5-7.6 (m, 4 H), 7.9 (d + s, 4 H), 8.0-8.1 (d, 2 H), 8.9 (s, 2 H). 13C NMR (75 MHz, CDCl3): δ = 53.2, 60.1, 102.4, 126.7, 127.2, 128.5, 128.8, 129.8, 130.2, 132.3, 135.3, 136.8, 143.0, 167.2. MS (CI) [MH+]: m/z = 519.
Synthesis of 6,13-Dihydro-6′,13′-bis-(5-methoxythien-2-yl)pentacene-6,13-diol (2e):
The procedure is the same as for product 2c except work up with NH4Cl and column chromatography over silica gel was carried out with petroleum ether-CH2Cl2 (1:9) and 0.5% Et3N. Yield 80%; mp 85-95 °C. 1H NMR (300 MHz, CDCl3): δ = 2.9 (s, 2 H), 6.0 (d, 2 H), 6.4 (d, 2 H), 7.5 (m, 4 H), 7.9 (m, 4 H), 8.2 (s, 4 H). 13C NMR (75 MHz, CDCl3): δ = 60.1, 74.5, 103.1, 125.0, 126.4, 126.7, 128.2, 133.0, 136.7, 139.0, 167.3. MS (CI) [MH+]: m/z = 519.2 (rearranged product), 559.2 (Na adduct).

7

Synthesis of 6,13-Dithien-2-ylpentacene (3c):
A suspension of diol 2c (2.0 g, 4.2 mmol), NaI (4.3 g, 29 mmol) and NaH2PO2 (4.8 g, 40 mmol) in HOAc (100 mL) was refluxed for 1 h. The product 3c was isolated by filtration and washed with H2O (3 × 50 mL) and MeOH (2 × 25 mL). After drying in vacuo, 6,13-dithien-2-ylpentacene was obtained in 87% yield; mp 300-305 °C. 1H NMR (300 MHz, CDCl3): δ = 7.3 (m, 4 H), 7.4 (d, 2 H), 7.5 (t, 2 H), 7.8 (m, 6 H), 8.5 (s, 4 H). 13C NMR (75 MHz, CDCl3): δ = 125.4, 125.5, 127.1, 127.4, 128.6, 130.0, 131.4, 139.5. MS (CI) [MH+]: m/z = 443.
Synthesis of 6,13-Bis(4-methoxyphenyl)pentacene (3a):
The procedure is the same as for 3c. Yield 91%; mp 303 °C. 1H NMR (300 MHz, CDCl3): δ = 4.1 (s, 6 H), 7.2 (m, 4 H), 7.3 (d, 4 H), 7.6 (d, 4 H), 7.8 (m, 4 H), 8.4 (s, 4 H). MS (MH+): m/z = 491.
Synthesis of 6,13-Bis(4-acetylphenyl)pentacene (3b):
The procedure is the same as for 3c. Yield 58%; mp >300 °C. 1H NMR (300 MHz, CDCl3): δ = 2.8 (s, 6 H), 7.3 (m, 4 H), 7.7 (m, 4 H), 7.8 (d, 4 H), 8.2 (s, 4 H), 8.3 (d, 4 H). MS [MH+]: m/z = 515.
Synthesis of 6,13-Bis(benzothien-2-yl)pentacene (3d):
The procedure is the same as for 3c. Yield 59%; mp >300 °C. 1H NMR (300 MHz, CDCl3): δ = 7.5 (m, 2 H), 7.6 (m, 6 H), 7.7 (m, 2 H), 7.9 (m, 4 H), 8.0 (m, 4 H), 8.6 (m, 4 H). MS [MH+]: m/z = 542.

9

Synthesis of 6,13-Dihydro-6,6′-bis(5-methoxythien-2-yl)-13,13′-dihydropentacene (5):
A solution of diol 2e (0.50 g, 0.90 mmol), ZnI2 (0.86 g, 2.7 mmol) and NaCNBH3 (0.57 g, 9.0 mmol) in 1,2-dichloroethane (50 mL) was refluxed during the night. After filtration of the salts, the filtrate was acidified, extracted with CH2Cl2 (50 mL) and washed with H2O (2 × 30 mL). After drying over MgSO4 and evaporation in vacuo. The crude product was purified by two successive column chromatographies over silica gel using CH2Cl2 and CH2Cl2-petroleum ether (5:5), respectively. The product was characterized to be 6,13-dihydro-6,6′-bis(5-methoxythien-2-yl)-13,13′-dihydropentacene; mp >300 °C. 1H NMR (300 MHz, CDCl3): δ = 3.8 (s, 6 H), 4.1 (s, 2 H), 6.0 (d, 2 H), 6.2 (d, 2 H), 7.4-7.5 (m, 6 H), 7.8 (m, 6 H). 13C NMR (75 MHz, CDCl3): δ = 36.9, 55.2, 60.1, 102.5, 125.4, 125.5, 126.3, 126.6, 127.0, 127.4, 128.5, 132.1, 132.8, 135.2, 135.3, 142.1, 166.6. MS (CI) [MH+]: m/z = 505.

10

Synthesis of 13,13′-dithien-2-ylpentacene-6-one (6):
A solution of diol 2c (0.26 g, 0.55 mmol) and BF3·OEt (0.10 mL, 8.2 mmol) in CH2Cl2 (20 mL) was stirred at r.t. during the night. After adding NaOH (1 M, 20 mL), the reaction mixture was extracted with CH2Cl2. The organic layer was washed with H2O (3 × 100 mL), dried with MgSO4 and evaporated in vacuo. The product 6 could be obtained after column chromatography over silica gel using CH2Cl2, with a yield of 44.5%; mp >300 °C. 1H NMR (300 MHz, CDCl3): δ = 6.6 (d, 2 H), 6.9 (t, 2 H), 7.3 (d, 2 H), 7.6 (m, 4 H), 7.8 (s + d, 4 H), 8.1 (d, 2 H), 8.9 (s, 2 H). 13C NMR (75 MHz, CDCl3): δ = 52.8, 125.2, 126.2, 126.4, 127.1, 128.3, 128.4, 128.8, 128.9, 129.7, 130.0, 132.2, 135.2, 143.6, 152.0, 184.8. MS (CI) [MH+]: m/z = 459.
Synthesis of 13,13′-Bis(4-methoxyphenyl)pentacene-6-one (7):
The procedure is the same as for 6 except column chromatography over silica gel was carried out with using CH2Cl2-petroleum ether (5:5); yield 1%. 1H NMR (300 MHz, CDCl3): δ = 3.7 (s, 6 H), 6.7, 6.8 (d, d, 8 H), 7.5 (s + m, 6 H), 7.7 (d, 2 H), 8.0 (d, 2 H), 8.8 (s, 2 H). 13C NMR (75 MHz, CDCl3): δ = 55.2, 113.2, 126.7, 128.1, 128.2, 128.5, 129.0, 129.1, 129.3, 129.5, 131.5, 131.6, 131.7, 134.9, 138.6, 144.9, 158.1, 186.0. MS (CI) [MH+]: m/z = 507.
Synthesis of 6,13-Dihydro-13,13′-bis(benzothien-2-yl)pentacene-6-one (8):
The procedure is the same as for 6 except column chromatography over silica gel was carried out with CH2Cl2-petroleum ether (5:5). Yield 18%, mp >300 °C. 1H NMR (300 MHz, CDCl3): δ = 6.9 (s, 2 H), 7.3 (m, 4 H), 7.6 (m, 6 H), 7.7 (d, 2 H), 7.8 (d, 2 H), 8.0 (s, 2 H), 8.1 (d, 2 H), 8.9 (s, 2 H). 13C NMR (75 MHz, CDCl3): δ = 56.9, 116.9, 118.4, 118.8, 119.1, 120.5, 121.4, 122.3, 122.8, 123.5, 123.6, 125.8, 128.3, 131.4, 133.2, 134.3, 142.7, 171.7. MS (CI) [MH+]: m/z = 559.

12

Synthesis of 13-Hydroxy-13′-thien-2-ylpentacen-6-one (9a):
n-BuLi (2.0 mL, 4.9 mmol) was added to a solution of thiophene (0.39 mL, 4.9 mmol) in THF (20 mL) under argon atmosphere at -78 °C. After 10 min, this solution was added dropwise to a suspension of pentacenequinone (2 g, 6.5 mmol) in THF (120 mL), which was cooled to -78 °C. The mixture was allowed to warm up to r.t. and stirred during the night. The reaction mixture was worked up with HCl (1 M, 25 mL). The precipitate was filtered, washed with H2O (3 × 30 mL), MeOH (2 × 20 mL), and dried in vacuo. The filtrate was extracted with CH2Cl2 (75 mL), washed with H2O (3 × 50 mL), dried over MgSO4 and evaporated in vacuo. After column chromatography over silica gel using petroleum ether-CH2Cl2 (3:7) on both fractions separately, the total yield was 39%; mp 248 °C. 1H NMR (300 MHz, CDCl3): δ = 3.3 (s, 1 H), 6.3-6.4 (d, 1 H), 6.7 (t, 1 H), 7.1 (d, 1 H), 7.5-7.6 (m, 4 H), 7.9 (d, 2 H), 8.0 (d, 2 H), 8.5 (s, 2 H), 8.8 (s, 2 H). MS [MH+]: m/z = 393.
Synthesis of 6,13-Dihydro-6-thien-2-yl-13-(4-methoxy-phenyl)pentacene-6,13-diol (10):
n-BuLi (0.60 mL, 1.4 mmol) was added to a solution of 4-bromoanisole (0.27 g, 1.5 mmol) in THF (30 mL) under argon atmosphere at -78 °C. After 10 min, ketone 9a (0.38 g, 0.97 mmol) was added. The mixture was allowed to warm up to r.t. and stirred during the night. The reaction mixture was worked up with HCl (1 M, 10 mL), was extracted with CH2Cl2 (50 mL), washed with H2O (3 × 50 mL), dried over MgSO4 and evaporated in vacuo. After purification by column chromatography on silica gel using petroleum ether-CH2Cl2 (2:8), diol 10 was used in the next reaction. Yield after column chromatography was 58%. MS [MH+]: m/z = 501.
Synthesis of 6-Thien-2-yl-13-(4-methoxy-phenyl)penta-cene (11):
A suspension of diol 10 (0.09 g, 0.2 mmol), NaI (0.38 g, 2.5 mmol) and NaH2PO2 (0.38 g, 4.3 mmol) was refluxed in HOAc (20 mL) during 1 h. The mixture was allowed to cool down and the product was filtered. The residue was washed with H2O (3 × 20 mL) and MeOH (2 × 10 mL). After drying in vacuo, 6-thien-2-yl-13-(4-methoxyphenyl)pentacene was obtained with a yield of 36%; mp 272-274 °C. 1H NMR (300 MHz, CDCl3): δ = 4.0 (s, 3 H), 7.2, 7.3 (m, 6 H), 7.4 (d, 1 H), 7.5 (t, 1 H), 7.55 (d, 2 H), 7.7, 7.8 (m, 5 H), 8.3 (s, 2 H), 8.5 (s, 2 H). 13C NMR (75 MHz, CDCl3): δ = 55.5, 114.1, 125.2, 125.4, 125.8, 127.0, 127.4, 128.2, 128.5, 128.9, 130.0, 130.2, 132.7, 138.3, 159.3. MS [MH+]: m/z = 467.

13

Smet, M. unpublished results

14

Synthesis of 6,13-Dihydro-6′-thien-2-yl-13,13′-dihydro-pentacene (12a):
A suspension of ketone 9a (0.1 g, 0.25 mmol), ZnI2 (0.24 g, 0.75 mmol) and NaBH3CN (0.16 g, 2.5 mmol) in 1,2-dichloroethane (40 mL) was refluxed overnight. The salts were filtered off and the filtrate was acidified with HCl (3 M, 20 mL). After extraction with CH2Cl2 (20 mL) and washing with H2O (3 × 50 mL), the organic layer was dried over MgSO4 and evaporated in vacuo. The product was purified by column chromatography over silica gel using CH2Cl2-petroleum ether (5:5). The title compound was obtained as a white solid in 92% yield; mp 228-230 °C. 1H NMR (300 MHz, CDCl3): δ = 4.2 (s, 2 H), 5.8 (s, 1 H), 6.5 (d, 1 H), 6.8 (t, 1 H), 7.1 (d, 1 H), 7.5 (m, 4 H), 7.8-7.9 (m, 4 H), 8.0 (s, 2 H). 13C NMR (75 MHz, CDCl3): δ = 48.4, 63.1, 124.8, 125.1, 125.5, 125.9, 126.0, 126.2, 126.6, 127.3, 127.6, 129.5, 132.8, 135.5, 137.8. MS (CI) [MH+]: m/z = 363. The same reaction in toluene gives a lower yield (44%) and 6-hydroxy-13-thien-2-ylpentacene (12%) as a side-product.
Synthesis of 6,13-Dihydro-6′-(4-methoxyphenyl)-13,13′-dihydropentacene (12b):
The procedure is the same as for 12a. Yield 51%; mp 155 °C. 1H NMR (300 MHz, CDCl3): δ = 3.7 (s, 3 H), 4.1 (s, 2 H), 5.5 (s, 1 H), 6.7 (d, 2 H), 7.0 (d, 2 H), 7.4 (m, 4 H), 7.8 (m, 8 H). 13C NMR (75 MHz, CDCl3): δ = 51.9, 55.6, 114.1, 125.9, 126.1, 127.0, 127.6, 129.7, 132.9, 133.0, 134.1, 136.3, 139.1, 158.5. MS (CI) [MH+]: m/z = 387.
Synthesis of 6,13-Dihydro-6′-(4-chlorophenyl)-13,13′-dihydropentacene (12b):
The procedure is the same as for 12a. Yield 50%; mp 174 °C. 1H NMR (300 MHz, CDCl3): δ = 4.0 (s, 1 H), 4.1 (s, 1 H), 5.5 (s, 1 H), 7.0 (d, 2 H), 7.1 (d, 2 H), 7.4 (m, 4 H), 7.8 (m, 8 H). 13C NMR (75 MHz, CDCl3): δ = 36.8, 51.7, 125.2, 125.5, 125.7, 126.0, 126.8, 127.4, 127.7, 128.5, 129.7, 132.5, 132.8, 135.8, 136.0, 138.0, 140.4. MS (CI) [MH+]: m/z = 391.

15

Synthesis of 6,13-Dihydro-13-hydroxy-13-(4-methoxy)phenylpentacene-6-on (9b):
Bromoanisole (2.8 mL, 22 mmol) in dry THF (10 mL) was added dropwise over a period of 30 min to a suspension of Mg (0.56 g, 23 mmol) in dry THF (10 mL) in the presence of a catalytic amount of iodine and bromoanisole. This mixture was than added dropwise to a suspension of pentacenequinone (6.0 g, 20 mmol) in THF (120 mL). The reaction mixture was stirred during the night and acidified with HCl (1 M). The mixture was extracted with CH2Cl2, washed with H2O and dried over MgSO4. After evaporation in vacuo, the crude product was purified by column chromatography on silica gel using petroleum ether-CH2Cl2 (4:6). The desired pentacene derivative 9b was obtained in 8% yield; mp 289-291 °C. 1H NMR (300 MHz, CDCl3): δ = 3.0 (s, 1 H), 3.7 (s, 3 H), 6.7 (d, 2 H), 7.2 (d, 2 H), 7.5-7.6 (m, 4 H), 7.9 (d, 2 H), 8.0 (d, 2 H), 8.3 (s, 2 H), 8.9 (s, 2 H). 13C NMR (75 MHz, CDCl3): δ = 55.2, 113.7, 126.5, 126.9, 128.1, 128.8, 129.0, 129.2, 129.8, 132.3, 136.0, 151.8, 155.6. MS (CI) [MH+]: m/z = 417.
6,13-Dihydro-13-hydroxy-13-(4-chloro)phenyl-pentacene-6-one (9c):
The procedure is the same as for 9b except column chromatography over silica gel using petroleum ether-CH2Cl2 (2:8); yield 8%. 1H NMR (300 MHz, CDCl3): δ = 3.2 (s, 1 H), 7.2 (s, 3 H), 7.3 (s, 3 H), 7.5 (d, 5 H), 7.8 (d, 2 H), 8.0 (m, 2 H), 8.2 (2 H), 8.9 (m, 2 H). MS (CI) [MH+]: m/z = 421. Melting point and 13C NMR data were not obtained, due to unremovable contaminating pentacenequinone.

16

Synthesis of 6-Thien-2-ylpentacene (13a):
Dihydropentacene 12a (90 mg, 0.25 mmol) was refluxed for 3 h in the presence of DDQ (64 mg, 0.28 mmol) in dioxane (20 mL). After cooling and evaporating in vacuo, the product was purified by column chromatography over silica gel using CH2Cl2. The yield of the impure product was 31%. 1H NMR (300 MHz, CDCl3): δ = 6.3 (m, 1 H), 6.8 (m, 1 H), 7.3 (m, 1 H), 7.5 (m, 4 H), 7.9 (d + s, 2 H + 2 H), 8.0 (d, 2 H), 8.2 (1 H), 8.4 (2 H). MS (CI) [MH+]: m/z = 361.

17

Spectral data: 1H NMR (300 MHz, CDCl3): δ = 6.0 (s, 1 H), 6.7 (d, 1 H), 6.8 (t, 1 H), 7.0 (d, 1 H), 7.4-7.5 (m, 4 H), 7.8 (d, 2 H), 7.9 (s, 2 H), 8.0 (d, 2 H), 8.8 (s, 2 H). MS [MH+]: m/z = 377.