Fused quinoxaline-2,3-dicarbonitrile derivatives, their photochemical synthesis and use thereof

Abstract

Title: Fused quinoxaline-2,3-dicarbonitrile derivatives, their photochemical synthesis and use thereof The present invention relates to a new photochemical synthetic route towards fused quinoxaline-2,3-dicarbonitriles of general formula (I) wherein R refers to hydrogen, halogen, (C1-C6)alkyl, (C1-C6)alkoxy or (C1-C6)alkylthio, the method comprises photochemical cyclization of the corresponding disubstituted 5,6-di(thiophen-2-yl)pyrazine-2,3-dicarbonitrile, and is specific in the use of light of the wavelength ranging from 400 to 500 nm. The present invention further relates to quinoxaline-2,3-dicarbonitrile derivatives of general formula (I), and to the use thereof in catalysis of photoredox reactions.

Description

[0001]Fused quinoxaline-2,3-dicarbonitrile derivatives, their photochemical synthesis and use thereof Field of Art The invention relates to a facile photochemical synthetic pathway towards quinoxaline-2,3- dicarbonitrile scaffold fused with five-membered thiophene moieties. The invention further relates to said fused quinoxaline-2,3-dicarbonitrile derivatives and their use in photoredox catalysis. Background Art Cyano-disubstituted aromatic molecules represent an important class of electron-deficient systems with manifold applications in organic electronics as well as photoredox catalysis. In 2014, we have reported X-shaped push-pull chromophores based on pyrazine-2,3- dicarbonitrile [Y. Zhao, C. Zhang, K. F. Chin, O. Pytela, G. Wei, H. Liu, F. Bureš, Z. Jiang, RSC. Adv.2014, 4, 30062–30067.] that showed noticeable nonlinear optical and photoredox activity. Especially 5,6-bis(5-methoxythiophen-2-yl)pyrazine-2,3-dicarbonitrile (DPZ, Scheme 1) forms a long-lived triplet state and may act as one-electron oxidant in photoinduced electron-transfer (PET). On the contrary, the simultaneously formed radical anion (DPZ^–) may serve upon excitation as a very strong one-electron reductant [Z. Burešová, H. B. Gobeze, M. Grygarová, O. Pytela, M. Klikar, R. Obertík, R. Cibulka, T. Islam, K. S. Schanze, F. Bureš, J. Catal.2024, 430, 115348.]. The synthesis of DPZ can be carried out in a straightforward, two-step, one-pot procedure starting from commercially available precursors [EP3679033]. DPZ’s molecular structure reveals planar dicyanopyrazine ring and twisted 5-methoxythiophene moieties that hinder efficient intramolecular charge-transfer from the peripheral methoxy donors to the cyano acceptors. More significantly, DPZ shows photobleaching upon irradiation with RoyalBlue LED (^maxE= 450 nm) exceeding 12 hours and its low photostability represents a significant drawback in the photoredox catalysis. Scheme 1 Disclosure of the Invention The main purpose of the present invention is a facile photochemical construction of a new C-C bond towards planar quinoxaline-2,3-dicarbonitrile (DTQ) derivatives fused with two additional thiophene rings as potential efficient photoredox catalysts. The main advantages of the synthetic pathway are: i) the use of a low-energy light source for irradiation, ii) synthetic accessibility of the starting derivatives of general formula II, iii) no need of any further chemical reagents and additives, iv) facile purification of the products, v) possible multigram synthesis, and vi) high photoredox catalytic activity of target DTQ derivatives. One object of the present invention is synthetic method of DTQ compounds of general formula (I) wherein R is selected from the group comprising hydrogen, halogen, (C1-C6)alkyl, (C1- C6)alkoxy, and (C1-C6)alkylthio.Said method comprises the following steps: a) providing an intermediate of general formula (II) wherein R is as defined above; b) photochemical cyclization of the intermediate of general formula (II), by irradiating said intermediate with the light with the wavelength in the range of from 400 to 500 nm, resulting in formation of the compound of general formula (I). Starting from compounds of general formula (II), the synthetic pathway of the compound of general formula (I) according to the present invention is depicted in Scheme 2. Scheme 2 Said intermediates of general formula (II) are accessible e.g. via one-pot two-step reaction of 2-substituted thiophenes with oxalyl dichloride and diaminomaleonitrile, as disclosed in EP3679033, wherein the intermediate of general formula (II) in step a) is obtained by Friedel-Crafts reaction of precursor of general formula (III) , wherein R is as defined above, with oxalylchloride to form a compound of general formula (IV) , wherein R is as defined above; followed by Lewis acid catalysed condensation reaction of the compound of general formula (IV) with diaminomaleonitrile to form the intermediate of general formula (II). In one preferred embodiment of the present invention, the photochemical cyclization of step b) is carried out by irradiating the reaction with the light having the wavelength 450 nm that overlaps well with the absorption maxima of the compounds of general formula (II). In one embodiment of the present invention, the used light source is RoyalBlue LED (1 W), preferably 1030 mW at 700 mA with the photon flux of 2.4×10-7einstein·s-1. This represents a commercially available LED source widely utilized in photoredox catalysis. In one embodiment of the present invention, step b) of the reaction can be carried out in a solvent selected from the group comprising dichloromethane, 1,2-dichloroethane, ethanol, dioxane, dimethylsulfoxide, preferably in dioxane. The reaction carried out in dioxane affords target compounds of general formula (I) in a short reaction time, highest yield and allows facile isolation via filtration. In one embodiment of the present invention, the reaction in step b) is irradiated for at least 1 hour, preferably for several hours, most preferably for 3 to 24 h. The structure and electronic effects of the substituents R affect the reaction time, while the longest reaction time is required for the cyclization of methylsulfanyl-substituted derivative of general formula II (R = SCH3). In one preferred embodiment of the present invention, the photochemical cyclization is carried out at temperature within the range of 0 to 30 °C, more preferably at about 25 °C representing a common laboratory temperature. In one preferred embodiment of the present invention, the product of general formula (I) precipitates from the reaction mixture, and further purification is possible via recrystallization. The above-described synthetic pathway is easily scalable and it is possible to proceed with a multigram synthesis. In one preferred embodiment of the present invention, the reaction is carried out in a batch of from 0.25 to 1.5 mmol of the starting intermediate of general formula (II). Another object of the present invention is the compound of general formula (I) as defined above. The compounds of general formula (I) are efficient and stable photoredox catalysts, especially for use in photoredox reactions, such as annulation reaction between a tertiary amine and maleimide, photoreduction reactions (e.g. photoreduction of nitrobenzene), photooxidation reactions, addition to a double bond, deuteration of aldehydes etc. In one embodiment, R is selected from the group consisting of hydrogen, Br, methyl, methyloxy, methylthio, hexyloxy. Most preferably, R is OCH3 and / or OHex, A further object of the present invention is the use of the compound of general formula (I) as a photocatalyst for photoredox reactions. Preferably, the reactions are selected from those utilizing photoinduced electron transfer, including both oxidation and reduction. More preferably, the photoredox reactions are selected from chemodivergent reduction of nitroaromatics (e.g. photoreduction of nitrobenzene), the oxidation of amines, the addition to olefins, the addition and subsequent annulation to aryl olefins (annulation reaction between a tertiary amine and maleimide), and deuteration of aldehydes. Abbreviations and definitions: DTQ - quinoxaline-2,3-dicarbonitrile DPZ - 5,6-bis(5-methoxythiophen-2-yl)pyrazine-2,3-dicarbonitrile The term „halogen“ means F, Cl, Br, I. The term “(C1-C6)alkyl” means a saturated hydrocarbon chain with one to six carbon atoms, which may be straight, branched or cyclic or cycle-containing, and which is derived from an alkane by removal of one hydrogen atom. An example of alkyl is methyl, ethyl, isopropyl, hexyl. The term “(C1-C6)alkoxy” means a group of -O-alkyl with one to six carbon atoms. An example of an alkoxy group is methoxy (methyloxy), ethoxy, hexyloxy. The term “(C1-C6)alkylthio“ means a group of -S-alkyl with one to six carbon atoms. An example of an alkylthio group is methylthio. Examples The invention can be documented but is not limited to the following experiments. Example 1: General method for preparing compounds of general formula (II) Compounds of general formula II were prepared according to the preceding synthetic procedure [EP3679033 and Y., Zhao, C., Zhang, K. F., Chin, O., Pytela, G., Wei, H., Liu, F., Bureš, Z., Jiang, RSC Adv., 2014, 4, 30062-30067]. Generally, Friedel-Crafts reaction of a (substituted) thiophene of general formula (III), wherein R is hydrogen, halogen, (C1- C6)alkyl, (C1-C6)alkoxy, and (C1-C6)alkylthio, with oxalyldichloride is performed, followed by Lewis acid catalysed condensation reaction with diaminomaleonitrile (DAMN). The catalyst for the Friedel-Crafts reaction may be selected from AlCl3, SnCl4, FeCl3, BF3·Et2O, TiCl4. Typically, the solvent of said Friedel-Crafts reaction is tetrahydrofuran, ethyl acetate, toluene or l,2-dichloroethane. Characterization of three newly prepared compounds II is provided below. 5,6-Bis[5-(methylthio)thiophen-2-yl]-pyrazine-2,3-dicarbonitrile (R = SCH3) The title compound was synthesized from 2-(methylthio)thiophene (650 mg; 5 mmol), oxalyldichloride (318 mg; 2.5 mmol), TiCl4 (945 mg; 5 mmol), pyridine (440 mg; 5.5 mmol) and DAMN (810 mg; 7.5 mmol) following the general method. Yield: 458 mg (47 %); orange solid. Rf= 0.67 (SiO2; DCM / Hex 1:1). Mp = 168.8^169.3 °C.1H NMR (acetone-d6, 500 MHz, 25 °C): ^ = 2.69 (s, 6H, SCH3), 7.05 (d, J = 4, 2H, CHth), 7.69 (d, J = 4, 2H, CHth) ppm.13C NMR (acetone-d6, 125 MHz, 25 °C): ^ = 19.6, 114.6, 128.6, 128.8, 132.9, 138.3, 147.4, 149.3 ppm. HR-FT-MALDI-MS (DCTB): calcd for C16H10N4S4 ([M]+) 385.97828, found 385.97766. 5,6-Bis(5-methylthiophen-2-yl)pyrazine-2,3-dicarbonitrile (R = CH3) The title compound was synthesized from 2-methylthiophene (490 mg; 5 mmol), oxalyldichloride (318 mg; 2.5 mmol), AlCl3(665 mg; 5 mmol), pyridine (440 mg; 5.5 mmol) and DAMN (810 mg; 7.5 mmol) following the general method. Yield: 280 mg (35 %); orange solid. Rf= 0.35 (SiO2; DCM / Hex 1:1). Mp = 172.8^173.7 °C.1H NMR (acetone-d6, 500 MHz, 25 °C): ^ = 2.57 (s, 6H, CH3), 6.88 (d, J = 4, 2H, CHth), 7.56 (d, J = 4, 2H, CHth) ppm.13C NMR (acetone-d6, 125 MHz, 25 °C): ^ = 15.5, 114.6, 128.1, 128.8, 132.7, 136.9, 148.4, 149.2 ppm. HR-FT-MALDI-MS (DCTB): calcd for C16H10N4S2 ([M]+) 322.03414, found 322.03410. 5,6-Bis(5-bromothiophen-2-yl)pyrazine-2,3-dicarbonitrile (R = Br) The title compound was synthesized from 2-bromothiophene (815 mg; 5 mmol), oxalyldichloride (318 mg; 2.5 mmol), AlCl3(665 mg; 5 mmol), pyridine (440 mg; 5.5 mmol) and DAMN (810 mg; 7.5 mmol) following the general method. Yield: 90 mg (8 %); orange solid. Rf = 0.5 (SiO2; DCM / Hex 1:1). Mp = 207.0-208.8 °C.1H NMR (acetone-d6, 500 MHz, 25 °C): ^ = 7.28 (d, J = 4, 2H, CHth), 7.63 (d, J = 4, 2H, CHth) ppm.13C NMR (acetone-d6, 125 MHz, 25 °C): ^ = 114.4, 120.8, 129.9, 132.9, 133.4, 140.6, 147.6 ppm. HR-FT-MALDI- MS (DCTB): calcd for C14H4Br2N4S2 ([M]+) 449.82386, found 449.82396. Example 2: General method for preparing compounds of general formula (I) In a 5 ml open snap vial equipped with a magnetic stirring bar, a compound of general formula (II) (0.085 mmol) was dissolved in 1,4-dioxane (3 ml). The reaction mixture was irradiated with Royal Blue LED (1×1 W) from 5 cm distance for 3^24 h at 25 °C. The precipitated product of general formula (I) was filtered from the reaction mixture and was crystallized from 1,2-dichloroethane and hexane. 6,9-Dimethoxydithieno[2,3-f:3',2'-h]quinoxaline-2,3-dicarbonitrile (R = OCH3) The title compound was synthesized from 5,6-bis(5-methoxythiophen-2-yl)pyrazine-2,3- dicarbontrile (30 mg) following the general method (reaction time: 5 h). Yield: 15 mg (51 %); orange solid. Rf = 0.4 (SiO2; DCM / Hex 1:1). Mp > 300 °C (dec.).1H NMR (acetone-d6, 500 MHz, 25 °C): ^ = 4.23 (s, 6H, OCH3), 7.29 (s, 2H, CHth) ppm.13C NMR (acetone-d6, 125 MHz, 25 °C): ^ = 61.6, 101.2, 115.6, 174.5 ppm (four signals are missing). HR-FT- MALDI-MS (DCTB): calcd for C16H8N4S2O2 ([M]+) 352.00832, found 352.00803. Dithieno[2,3-f:3',2'-h]quinoxaline-2,3-dicarbonitrile (R = H) The title compound was synthesized from 5,6-di(thiophen-2-yl)pyrazine-2,3-dicarbontrile (25 mg) following the general method (reaction time: 12 h). Yield: 10 mg (40 %); orange solid. Rf = 0.6 (SiO2; DCM / Hex 1:1). Mp > 400 °C (dec.).1H NMR (acetone-d6, 500 MHz, 25 °C): ^ = 8.21 (d, J = 5, 2H, CHth), 8.42 (d, J = 5, 2H, CHth) ppm.13C NMR (acetone-d6, 125 MHz, 25 °C): ^ = 124.9, 136.5 ppm (five signals are missing). HR-FT-MALDI-MS (DCTB): calcd for C14H5N4S2 ([M+H]+) 292.99501, found 292.99476. 6,9-Dihexyloxydithieno[2,3-f:3',2'-h]quinoxaline-2,3-dicarbonitrile (R = OHex) The title compound was synthesized from 5,6-bis(5-hexyloxythiophen-2-yl)pyrazine-2,3- dicarbontrile (42 mg) following the general method (reaction time: 5 h). Yield: 18 mg (43 %); orange solid. Rf= 0.8 (SiO2; DCM / Hex 1:1). Mp = 167.9-169.1 °C.1H NMR (acetone- d6, 500 MHz, 25 °C): ^ = 0.93 (t, J = 7 Hz, 6H, CH3), 1.37^1.44 (m, 8H, CH2), 1.53^1.59 (m, 4H, CH2), 1.93^1.98 (m, 4H, CH2), 4.42 (t, J = 6.5 Hz, 4H, OCH2), 7.12 (s, 2H, CHth) ppm.13C NMR (acetone-d6, 125 MHz, 25 °C): ^ = 14.4, 23.3, 26.3, 29.7, 32.3, 75.5, 101.5, 115.6, 122.9, 127.7, 137.7, 139.9, 173.8 ppm. HR-FT-MALDI-MS (DCTB): calcd for C26H28N4S2O2 ([M]+) 492.16482, found 492.16453. 6,9-Bis(methylthio)dithieno[2,3-f:3',2'-h]quinoxaline-2,3-dicarbonitrile (R = SCH3) The title compound was synthesized from 5,6-bis(5-(methylthio)thiophen-2-yl)pyrazine- 2,3-dicarbonitrile (33 mg) following the general method (reaction time: 24 h). Yield: 10 mg (30 %); red solid. Rf= 0.1 (SiO2; DCM). Mp > 200 °C (dec.).1H NMR (acetone-d6, 500 MHz, 25 °C): ^ = 8.56 (s, 2H, CHth) ppm.13C NMR not measurable due to a low solubility. HR-FT-MALDI-MS (DCTB): calcd for C16H8N4S4([M]+) 383.96263, found 383.96267. 6,9-Dimethyldithieno[2,3-f:3',2'-h]quinoxaline-2,3-dicarbonitrile (R = CH3) The title compound was synthesized from 5,6-bis(5-methylthiophen-2-yl)pyrazine-2,3- dicarbonitrile (27 mg) following the general method (reaction time: 3 h). Yield: 16 mg (59 %); orange solid. Rf= 0.1 (SiO2; DCM). Mp > 410 °C (dec.).1H NMR (acetone-d6, 500 MHz, 25 °C): ^ = 7.85 (s, 2H, CHth) ppm.13C NMR not measurable due to a low solubility. HR-FT-MALDI-MS (DCTB): calcd for C16H9N4S2 ([M+H]+) 321.02631, found 321.02636. 6,9-Dibromodithieno[2,3-f:3',2'-h]quinoxaline-2,3-dicarbonitrile (R = Br) The title compound was synthesized from 5,6-bis(5-bromothiophen-2-yl)pyrazine-2,3- dicarbonitrile (38 mg) following the general method (reaction time: 4 h). Yield: 18 mg (47 %); dark yellow solid. Rf= 0.9 (SiO2; DCM / Hex 1:1). Mp > 405 °C (dec.).1H NMR (acetone-d6, 500 MHz, 25 °C): ^ = 8.34 (s, 2H, CHth) ppm.13C NMR not measurable due to a low solubility. HR-FT-MALDI-MS (DCTB): calcd for C14H3Br2N4S2([M+H]+) 448.81604, found 448.81619. Example 3: Photoredox catalysis – Annulation towards 7 The catalytic performance of DTQ derivatives of general formula I has been screened in a benchmark annulation reaction between N,N-dimethylaniline 5 and N-phenylmaleimide 6 according to Scheme 3. All the above-prepared DTQ derivatives I afforded product 7 in the yields ranging from 65 to 100 %, while the original photocatalysts of general formula II showed diminished yields within the range of 56–75 % (Table 1). DTQ derivatives I with R = OCH3or OHex proved to be the most catalytically efficient substances furnishing 7 quantitatively. Their catalytic loading as well as the reaction time can be reduced down to 0.2 mol% / 1.5 h without affecting the reaction outcome. Hence, further photoredox transformations were catalysed by DTQ derivatives I with R = OCH3. Scheme 3 Table 1 Photocatalyst R Yield of 7 [%] I H 98 II H 70 I OCH3100 II OCH3 75 I OHex 100 II OHex 74 I SCH3 65 II SCH3 56 I CH3 66 II CH3 58 I Br 68 II Br 67 In a 5 ml snap vial equipped with a magnetic stirring bar, N,N-dimethylaniline 5 (63 μl, 0.5 mmol) and N-phenylmaleimide 6 (43 mg, 0.25 mmol) were dissolved in acetone (3 ml) and 6,9-dimethoxydithieno[2,3-f:3',2'-h]quinoxaline-2,3-dicarbonitrile (0.5 mol%) was added. The reaction mixture was irradiated with Royal Blue LED (1×1 W) from 5 cm distance at 25 °C for 2.2 h. The solvent was removed in vacuo and the crude product was purified by column chromatography (Al2O3; PE / EA = 3:1, Rf = 0.45). Yield: 69 mg (95 %). Mp = 199– 202 °C.1H NMR (acetone-d6, 500 MHz, 25 °C): δ = 2.81 (s, 3H), 3.08–3.05 (dd,2J = 4.5 Hz,3J = 11.5 Hz, 1H), 3.55–3.52 (dd,2J = 2.7 Hz,3J = 11.5 Hz, 1H), 3.72–3.68 (m, 1H), 4.26 (d,3J = 9.6 Hz, 1H), 6.77 (d,3J = 8 Hz, 1H), 6.82 (t,3J = 7.5 Hz, 1H), 7.19–7.16 (m, 1H), 7.23–7.22 (m, 2H), 7.37–7.34 (m, 1H), 7.43 (t,3J = 7.5 Hz, 3H) ppm.13C NMR (CDCl3, 125 MHz, 25 °C): δ = 38.9, 42.2, 43.5, 50.8, 112.4, 119.1, 119.7, 126.7, 128.1, 128.2, 128.7, 130.3, 133.0, 148.9, 175.8, 177.8 ppm. MS-EI (70 eV): m / z = 292 (M+, 80%), 144 (100%). The spectral data are consistent with that published in the literature [S., Kumari Prasanna, B., Naveen, P., Kumar Suresh, S., Selva Ganesan, Chem. Pap., 2023, 77, 151-158.] Example 4: Photoredox catalysis – Photoreduction towards benzeneamine 9 – Method A The catalytic performance of photocatalysts of general formula I (R = OCH3) has been screened in the reduction of nitrobenzene 8 providing product 9, according to Scheme 4. Scheme 4 In a 5 ml open snap vial equipped with a magnetic stirring bar, nitrobenzene 8 (26 μl, 0.25 mmol) was dissolved in THF (4 ml). Subsequently triethanolamine (200 μl, 1.5 mmol; TEOA) and 6,9-dimethoxydithieno[2,3-f:3',2'-h]quinoxaline-2,3-dicarbonitrile (0.5 mol%) were added. The reaction mixture was irradiated with Royal Blue LED (1×1 W) from 5 cm distance at 25 °C for 3 h. The combined content of two vials was diluted with water (5 ml) and extracted with dichloromethane (3×10 ml). The combined organic extracts were dried over Na2SO4 and the solvents were removed in vacuo. The crude product 9 was purified by column chromatography (SiO2; DCM / Hex 4:1, Rf= 0.65). Yield: 22 mg (93 %).1H NMR (CDCl3, 500 MHz, 25 °C): δ = 3.64 (s, 2H), 6.69 (d, J = 7.6 Hz, 2H), 6.78–6.75 (m, 1H), 7.18–7.15 (m, 2H) ppm.13C NMR (CDCl3,125 MHz, 25 °C): δ = 115.2, 118.6, 129.4, 146.5 ppm. MS-EI (70 eV): m / z = 93.1 (M+, 100%), 66.1 (30%), 52.1 (10%). The spectral data are consistent with that published in the literature. [D. Panja, A. Sau, S. D. Thakur, S. Dey, R. Sahu, S. Kundu, Adv. Synth. Catal.2023, 365, 2959-2968.] Example 5: Photoredox catalysis – Photoreduction towards benzeneamine 9 – Method B The catalytic performance of photocatalysts of general formula I (R = OCH3) has been screened in the reduction of nitrobenzene 8 providing product 9, according to Scheme 5. Scheme 5 In a 5 ml open snap vial equipped with a magnetic stirring bar, nitrobenzene 8 (26 μl, 0.25 mmol) was dissolved in THF (4 ml). Subsequently Hantzsch ester (190 mg, 0.75 mmol; HE) and 6,9-dimethoxydithieno[2,3-f:3',2'-h]quinoxaline-2,3-dicarbonitrile (0.5 mol%) were added. The reaction mixture was irradiated with Royal Blue LED (1×1 W) from 5 cm distance at 25 °C for 1h. The combined content of two vials was diluted with water (5 ml) and was extracted with dichloromethane (3×10 ml). The combined organic extracts were dried over Na2SO4 and the solvents were removed in vacuo. The crude product 9 was purified by column chromatography (SiO2; PE / EA 3:1 Rf= 0.5). Yield: 17 mg (71 %). Example 6: Photoredox catalysis – Photoreduction towards nitrosobenzene 10 The catalytic performance of photocatalysts of general formula I (R = OCH3) has been screened in the reduction of nitrobenzene 8 providing product 10, according to Scheme 6. Scheme 6 In a Schlenk flask equipped with a magnetic stirring bar, nitrobenzene 8 (26 μl, 0.25 mmol) was dissolved in dry THF (4 ml) under argon atmosphere. HE (158 mg, 0.63 mmol) and 6,9- dimethoxydithieno[2,3-f:3',2'-h]quinoxaline-2,3-dicarbonitrile (1 mol%) were added. The reaction mixture was irradiated with Royal Blue LED (2×1 W) from 5 cm distance at ^30 °C for 1h. The combined content of four vials was evaporated under reduced pressure at 30 °C. The crude product 10 was purified by steam distillation. Yield: 17 mg (65 %). Mp = 63– 65 °C.1H NMR (CDCl3, 500 MHz, 25 °C): δ = 7.60 (t, J = 7.5 Hz, 2H), 7.69 (t, J = 7.5 Hz, 1H), 7.89–7.87 (m, 2H) ppm.13C NMR (CDCl3, 125 MHz, 25 °C): δ = 121.1, 129.5, 135.8, 165.9 ppm. MS-EI (70 eV): m / z = 107.1 (M+, 80%), 77.1 (100%), 51.1 (60%), 63.1 (10%). The spectral data are consistent with that published in the literature [Q. Niu, Q. Huang, T. Y. Yu, J. Liu, J. W. Shi, L. Z. Dong, S. L. Li, Y. Q. Lan, J. Am. Chem. Soc.2022, 144, 18586- 18594.] Example 7: Photoredox catalysis – Photoreduction towards N-Acetoxy-N-phenylacetamide 11 The catalytic performance of photocatalysts of general formula I (R = OCH3) has been screened in the reduction of nitrobenzene 8 providing product 11, according to Scheme 7. Scheme 7 In a Schlenk flask equipped with a magnetic stirring bar, nitrobenzene 8 (26 μl, 0.25 mmol) was dissolved in MeOH (2 ml) under argon atmosphere. HE (190 mg, 0.75 mmol), 6,9- dimethoxydithieno[2,3-f:3',2'-h]quinoxaline-2,3-dicarbonitrile (0.5 mol%) and acetic anhydride (2 ml) were added. The reaction mixture was irradiated with Royal Blue LED (1×1 W) from 5 cm distance at 25 °C for 1h. The combined content of two vials was diluted with water (5 ml) and was extracted with dichloromethane (3×10 ml). The combined organic extracts were dried over Na2SO4 and the solvents were removed in vacuo. The crude product 11 was purified by column chromatography (SiO2; PE / EA 1:1, Rf= 0.6). Yield: 47 mg (98 %). Mp = 38–41 °C.1H NMR (CDCl3, 500 MHz, 25 °C): δ = 2.02 (s, 3H), 2.16 (s, 3H), 7.44– 7.39 (m, 5H) ppm.13C NMR (CDCl3, 125 MHz, 25 °C): δ = 18.5, 21.7, 129.6, 139.5, 168.0 ppm. HR-FT-MALDI-MS (DCTB) m / z: calcd for C10H11NO3([M+Na]+): 216.06312; found 216.06316. The spectral data are consistent with that published in the literature [J. Ghorai, K. Ramachandran, P. Anbarasan, P. J. Org. Chem.2021, 86, 14812-14825.] Example 8: Photoredox catalysis – Photoreduction towards 1,2-diphenyldiazene 1-oxide 12 The catalytic performance of photocatalysts of general formula I (R = OCH3) has been screened in the reduction of nitrobenzene 8 providing product 12, according to Scheme 8. Scheme 8 In a 5 ml open snap vial equipped with a magnetic stirring bar, nitrobenzene 8 (26 μl, 0.25 mmol) was dissolved in acetonitrile, ACN (4 ml). HE (190 mg, 0.75 mmol), and 6,9- dimethoxydithieno[2,3-f:3',2'-h]quinoxaline-2,3-dicarbonitrile (0.5 mol%) were added. The reaction mixture was irradiated with Royal Blue LED (1×1 W) from 5 cm distance at 25 °C for 24 h. The combined content of two vials was diluted with water (5 ml) and extracted with dichloromethane (3×10 ml). The combined organic extracts were dried over Na2SO4and the solvents were removed in vacuo. The crude product 12 was purified by column chromatography (SiO2; PE / EA 5:1 Rf = 0.8). Yield: 24 mg (97 %). Mp = 39–42 °C.1H NMR (CDCl3, 500 MHz, 25 °C): δ = 7.39–7.36 (m, 1H), 7.57–7.45 (m, 5H), 8.16–8.13 (m, 2H), 8.31–8.28 (m, 2H), ppm.13C NMR (CDCl3, 500 MHz, 25 °C): δ = 122.6, 125.7, 128.9, 129.0, 129.8, 131.8, 144.2, 148.6 ppm. HR-FT-MALDI-MS (DCTB) m / z: calcd for C12H10N2O ([M]+): 198.07876; found 198.07871. The spectral data are consistent with that published in the literature [A. Yaghoubian, G. K. Hodgson, M. J. Adler, S. Impellizzeri, Org. Biomol. Chem.2022, 20, 7332-7337.] Example 9: Photoredox catalysis – Photoreduction towards 1,2-diphenyldiazene 13 The catalytic performance of photocatalysts of general formula I (R = OCH3) has been screened in the reduction of nitrobenzene 8 providing product 13, according to Scheme 9. Scheme 9 In a Schlenk flask equipped with a magnetic stirring bar, nitrobenzene 8 (26 μl, 0.25 mmol) was dissolved in dimethoxysulfoxide (4 ml). HE (127 mg, 0.5 mmol) and 6,9- dimethoxydithieno[2,3-f:3',2'-h]quinoxaline-2,3-dicarbonitrile (0.5 mol%) were added under argon. The reaction mixture was irradiated with Royal Blue LED (1×1 W) from 5 cm distance at 25 °C for 48 h. The combined content of two vials was diluted with water (5 ml) and was extracted with dichloromethane (3 × 10 ml). The combined organic extracts were dried over Na2SO4and the solvents were removed in vacuo. The crude product 13 was purified by column chromatography (SiO2; Hex / acetone 200:1 Rf = 0.45). Yield: 21 mg (94 %). Mp = 65–68 °C.1H NMR (CDCl3, 500 MHz, 25 °C): δ = 7.47–7.45 (m, 2H), 7.52–7.49 (m, 4H), 7.91 (d, J = 7 Hz, 4 H) ppm.13C NMR (CDCl3, 125 MHz, 25 °C): δ =123.1, 129.3, 131.2, 152.9 ppm. HR-FT-MALDI-MS (DCTB) m / z: calcd for C12H10N2([M]+): 182.08385; found 182.08382. The spectral data are consistent with that published in the literature [S. Dey, D. Panja, A. Sau, S. D. Thakur, S. J. Kundu, S. J. Org. Chem.2023, 88, 10048-10057.] Example 10: Photoredox catalysis – Photooxidation towards N-benzylidenebenzylamine 15 The catalytic performance of photocatalysts of general formula I (R = OCH3) has been screened in the oxidation of dibenzylamine 14 providing product 15, according to Scheme 10. Scheme 10 In a 5 ml snap vial equipped with a magnetic stirring bar, dibenzylamine 14 (39 mg, 0.2 mmol) was dissolved in acetonitrile (3.8 ml) and H2O (0.2 ml), whereupon 6,9- dimethoxydithieno[2,3-f:3',2'-h]quinoxaline-2,3-dicarbonitrile (1 mol%) was added. The reaction mixture was irradiated with Royal Blue LED (1×1 W) from 5 cm distance at 25 °C for 20 min and the solvent was removed in vacuo. The crude product 15 was purified by flash chromatography (SiO2; Hex / EA = 9:1, Rf = 0.7). Yield: 39 mg (69 %).1H NMR (CDCl3, 500 MHz, 25 °C): δ = 4.87 (s, 2 H), 7.32–7.28 (m, 1H), 7.47–7.38 (m, 7H), 7.84– 7.82 (s, 1H) ppm.13C NMR (CDCl3, 125 MHz, 25 °C): δ = 65.2, 127.1, 128.1, 128.4, 128.6, 128.7, 130.9, 136.3, 139.4, 162.1 ppm. HR-FT-MALDI-MS (DCTB) m / z: calcd for C14H14N ([M+H]+) 196.11208; found 196.11204. The spectral data are consistent with that published in the literature [X. Liu, R., Qi, S., Li, W., Liu, Y., Yu, J., Wang, S., Wu, K., Ding, Y., Yu, J. Am. Chem. Soc.2022, 144, 23396-23404.] Example 11: Photoredox catalysis – Addition towards 18 The catalytic performance of photocatalysts of general formula I (R = OCH3) has been screened in the addition between 4-iodopyridine 16 and 1,1-diphenylethylen 17 providing product 18, according to Scheme 11. Scheme 11 In a Schlenk flask equipped with a magnetic stirring bar, 1,1-diphenylethylen 17 (689 μl, 3.9 mmol) and 6,9-dimethoxydithieno[2,3-f:3',2'-h]quinoxaline-2,3-dicarbonitrile (10 mol%) were dissolved in dimethylsulfoxide (2 ml) under argon atmosphere. Subsequently, diisopropylethylamine (68 μl, 0.39 mmol) and 4-iodopyridine 16 were added. The reaction mixture was irradiated with Royal Blue LED (1×1 W) from 5 cm distance at 25 °C for 24 h. Water (10 ml) was added followed by extraction with dichloromethane (3 × 10 ml). The combined organic extracts were dried over Na2SO4 and the solvents were removed in vacuo. The crude product 18 was purified by column chromatography (SiO2; DCM / EA = 1:1, Rf = 0.45). Yield: 41 mg (80 %).1H NMR (CDCl3, 500 MHz, 25 °C): δ = 3.38 (d, J =8 Hz, 2 H), 4.27–4.24 (t, J = 8 Hz, 1H), 6.96 (d, J = 6 Hz, 2H), 7.22–7.19 (m, 6H), 7.30–7.27 (m, 4H), 8.41 (d, J = 6 Hz, 2H) ppm.13C NMR (CDCl3, 125 MHz, 25 °C): δ = 41.6, 52.3, 60.6, 124.7, 126.7, 128.1, 128.7, 143.8, 149.5, 149.6 ppm. HR-FT-MALDI-MS (DCTB) m / z: calcd for C19H18N ([M+H]+) 260.14338; found 260.14316. The spectral data are consistent with that published in the literature [J. Li, L. He, X. Liu, X. Cheng, G. Li, Angew. Chem. Int. Ed., 2019, 58, 1759-1763.] Example 12: Photoredox catalysis – Deuteration of aldehydes towards 20 The catalytic performance of photocatalysts of general formula I (R = OCH3) has been screened in deuteration of 4-phenylbenzaldehyde 19 providing product 20, according to the reaction Scheme 12. Scheme 12 In a Schlenk flask equipped with a magnetic stirring bar, 4-phenylbenzaldehyde 19 (36 mg, 0.2 mmol), 6,9-dimethoxydithieno[2,3-f:3',2'-h]quinoxaline-2,3-dicarbonitrile (1 mol%) and PhCO2Na (7 mg, 0.05 mmol) were dissolved in anhydrous ethyl acetate (1 ml) and D2O (1 ml) under argon atmosphere. Triisopropylsilanhiol (14 μl, 0.065 mmol) was added. The reaction mixture was irradiated with Royal Blue LED (1×1 W) from 5 cm distance at 25 °C for 8 h. NaHSO3 (5 ml, sat. aq. sol.) was added and the reaction mixture was extracted with dichloromethane (3 × 5 mL). The combined organic extracts were dried over Na2SO4and the solvents were removed in vacuo. The crude product 20 was purified by flash chromatography (SiO2; Hex / acetone = 18:1, Rf = 0.1). Yield: 26 mg (71 %). Mp = 55–57 °C.1H NMR (CDCl3, 500 MHz, 25 °C): δ = 7.44 (t, J = 7 Hz, 1H), 7.52 (t, J = 7 Hz, 2H), 7.75 (d, J = 8Hz, 2H), 7.89 (d, J = 8 Hz, 2H), 8.02 (d, J = 8 Hz, 2H), 10.09 (s, 0.001H) ppm.13C NMR (CDCl3, 125 MHz, 25 °C): δ = 127.5, 127,9, 128.7, 129.2, 130.5, 135.2 (t, J = 3.5 Hz), 139.8, 147.4, 191.9 (t, J = 26.5 Hz) ppm. MS-EI (70 eV): m / z = 183 (M+, 50%), 184 (10%), 182 (70%), 181 (100%), 152 (65%). The spectral data are consistent with that published in the literature [W. Wang, Synthesis of deuterated aldehydes. WO2021045879A1, 2021.]

Claims

CLAIMS 1. A compound of general formula (I)wherein R is the same and is selected from the group consisting of hydrogen, halogen, (C1-C6)alkyl, (C1-C6)alkoxy, and (C1-C6)alkylthio.

2. The compound of general formula (I) according to claim 1, wherein R is selected from the group consisting of hydrogen, Br, methyl, methyloxy, methylthio, hexyloxy.

3. A method of synthesis of compounds of general formula (I) according to claim 1, characterized in that it comprises the following steps: a) providing an intermediate of general formula (II)wherein R is as defined in claim 1; b) photochemical cyclization of the intermediate of general formula (II) by irradiating said intermediate with the light with the wavelength in the range of from 400 to 500 nm, resulting in formation of the compound of general formula (I).

4. The method according to claim 3, characterized in that the intermediate of general formula (II) in step a) is obtained by Friedel-Crafts reaction of precursor of general formula (III), wherein R is as defined in claim 1, with oxalylchloride to form a compound of general formula (IV), wherein R is as defined in claim 1; followed by Lewis acid catalyzed condensation reaction of the compound of general formula (IV) with diaminomaleonitrile to form the intermediate of general formula (II).

5. The method according to claim 3 or 4, characterized in that step b) is carried out in a solvent selected from the group comprising dichloromethane, 1,2-dichloroethane, ethanol, dioxane, and dimethylsulfoxide; preferably the solvent is dioxane.

6. The method according to claim 3, 4 or 5, characterized in that the time of irradiation in step b) is at least 1 hour, preferably the time of irradiation is in the range of from 3 to 24 hours.

7. The method according to any one of the preceding claims 3 to 6, characterized in that step b) is carried out at temperature in the range of from 0 to 30 °C, preferably the reaction temperature is 25 °C.

8. Use of the compound of general formula (I) according to claim 1 or 2 in photoredox catalysis.

9. Use according to claim 8, wherein the photoredox reactions are selected from reactions utilizing photoinduced electron transfer, including both reduction and oxidation;preferably the photoredox reactions are selected from the group comprising the chemodivergent reduction of nitroaromatics, the oxidation of amines, the addition to olefins, the addition and subsequent annulation to aryl olefins and the deuteration of aldehydes.

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