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Article

Heterogeneous Photocatalysis as a Potent Tool for Organic Synthesis: Cross-Dehydrogenative C–C Coupling of N-Heterocycles with Ethers Employing TiO2/N-Hydroxyphthalimide System under Visible Light

by
Elena R. Lopat’eva
,
Igor B. Krylov
*,
Oleg O. Segida
,
Valentina M. Merkulova
,
Alexey I. Ilovaisky
and
Alexander O. Terent’ev
*
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky Prospekt, 119991 Moscow, Russia
*
Authors to whom correspondence should be addressed.
Molecules 2023, 28(3), 934; https://doi.org/10.3390/molecules28030934
Submission received: 25 December 2022 / Revised: 11 January 2023 / Accepted: 13 January 2023 / Published: 17 January 2023
(This article belongs to the Special Issue Recent Developments in the Synthesis and Functionalization of Nitrogen Heterocycles)
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Abstract

:
Despite the obvious advantages of heterogeneous photocatalysts (availability, stability, recyclability, the ease of separation from products and safety) their application in organic synthesis faces serious challenges: generally low efficiency and selectivity compared to homogeneous photocatalytic systems. The development of strategies for improving the catalytic properties of semiconductor materials is the key to their introduction into organic synthesis. In the present work, a hybrid photocatalytic system involving both heterogeneous catalyst (TiO2) and homogeneous organocatalyst (N-hydroxyphthalimide, NHPI) was proposed for the cross-dehydrogenative C–C coupling of electron-deficient N-heterocycles with ethers employing t-BuOOH as the terminal oxidant. It should be noted that each of the catalysts is completely ineffective when used separately under visible light in this transformation. The occurrence of visible light absorption upon the interaction of NHPI with the TiO2 surface and the generation of reactive phthalimide-N-oxyl (PINO) radicals upon irradiation with visible light are considered to be the main factors determining the high catalytic efficiency. The proposed method is suitable for the coupling of π-deficient pyridine, quinoline, pyrazine, and quinoxaline heteroarenes with various non-activated ethers.

Graphical Abstract

1. Introduction

Heterogeneous photocatalysis in organic synthesis is a young and fast-growing area [1,2,3,4,5]. The semiconductor materials used in photocatalysis are inexpensive and widely available; their advantages include the ease of separation from organic products, stability and recyclability [1,5]. However, the development of this area is still hindered by several formidable obstacles, such as low catalytic efficiency due to the low degree of charge separation in photoexcited states and the fast recombination of electron–hole pairs [6,7], low visible light absorption and low selectivity due to the strong oxidation power of photogenerated valence-band (VB) holes in popular semiconductors (TiO2, ZnO, Bi2O3, WO3, etc.) [1,8]. This situation is reflected in the comparatively low number of synthetic methods in fine organic synthesis based on heterogeneous photocatalytic systems compared to the mainstream applications of heterogeneous photocatalysis: oxidative destruction of pollutants [9,10,11], hydrogen generation [12,13], CO2 reduction [14,15,16] and water splitting [17].
Currently, the scope of synthetic transformations enabled by heterogeneous photocatalysis is much less diverse compared to the scope of homogeneous photoredox-catalyzed reactions. Heterogeneous catalysis is mainly used in comparatively simple reactions; for example, alkylarene benzylic oxidation [18,19,20], the oxidation of benzylamines [4,5,21,22], alcohols [4,5] and sulfides [4,23], oxidative esterification [4], nitro-group reduction [4], tiol-ene reaction [24], alkene amination with aqueous ammonia [25] and the decarboxylation of carboxylic acids [26,27,28]. Cross-coupling reactions are much less developed and usually demand transition metal co-catalysts, such as palladium or nickel complexes [29,30,31,32,33].
UV irradiation, which is used frequently for the excitation of heterogeneous photocatalysts, is inconvenient due to safety issues, the comparatively high cost of UV light sources, incompatibility with common laboratory glassware (UV-transparent quartz is necessary) and possible side reactions due to the high energy of the light. The modification of heterogeneous photocatalysts, such as TiO2, in order to shift their photoactivity spectrum from UV to visible light [10,34,35,36,37] is the key task for expanding the scope of their applications in organic synthesis, increasing selectivity and making the of use cheap and available light sources for catalyst activation possible. At present, the following modification approaches have been proposed: the immobilization of dyes (organic compounds or metal complexes) on the photocatalyst surface [34,38,39,40,41], doping with metal ions or non-metal elements [42,43], semiconductor coupling [7,44,45,46,47,48,49] and modification with organic molecules bearing hydroxyl or carboxyl groups [34,50,51,52,53,54,55,56], which demonstrate the occurrence of visible light absorption when adsorbed on the surface of a semiconductor.
NHPI/TiO2 is one of the efficient catalytic systems activated by visible light based on industrially available substances (Scheme 1). The interaction of NHPI with the TiO2 surface leads to the occurrence of visible light absorption, resulting in the photogeneration of phthalimide-N-oxyl radicals (PINO) [20,22]. In our previous work [20], we demonstrated that the NHPI/TiO2 system could be successfully applied to the aerobic oxidation of alkylarenes under visible light irradiation (Scheme 1A). The conceptual novelty of this system arises from the conjunction of heterogeneous photocatalysis with homogeneous radical chain organocatalysis. A distinguishing feature of this system is the migration of PINO into the volume of solution, where the PINO/NHPI catalyzed radical chain process, once initiated on the TiO2 surface, produces the target product without the need for additional light absorption [20]. Thus, the energy efficiency of photocatalysis is fundamentally improved by combining heterogeneous photocatalysis with homogeneous organocatalysis. In the presence of additional organocatalyst (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) the effective oxidative homocoupling of benzylamines [22] was achieved previously (Scheme 1B).
In the present study, we demonstrate the successful application of the NHPI/TiO2 system to a more challenging cross-dehydrogenative C–C coupling process (Scheme 1C). In this case, previously reported CH-oxygenation processes [20] should be suppressed, which is a difficult task. In addition, the process of C–O coupling between NHPI-derived PINO radicals and CH-reagents [57,58,59] must be avoided. The oxidative coupling of ethers with π-deficient N-heteroaromatic compounds (a Minisci-type reaction) was chosen as a model reaction due to the practical importance for the functionalization of N-containing heterocycles with C–C bond formation. Minisci-type reactions [60,61,62,63,64,65,66,67,68] are based on the addition of nucleophilic C-centered radicals to electron-deficient arenes and represent one of the most important methods for the functionalization of such arenes, along with the nucleophilic aromatic substitution of hydrogen [69,70,71], and functionalization via transition-metal-catalyzed C(sp2)–H bond activation [72,73,74,75,76]. The products of the Minisci reaction are of great value for medicinal chemistry [61,64]. Thus, the development of new, milder, more efficient methods tolerant to a large number of functional groups based on Minisci chemistry remains a hot research topic.
To date, many photochemical protocols have been developed for the Minisci reaction, both with the use of metal complex photocatalysts [60,77,78,79,80] and organic photocatalysts [81,82,83]. In some specific cases, the Minisci reaction proceeds without a photocatalyst [84,85,86,87]. At the same time, examples of the application of heterogeneous photocatalysis for the Minisci reaction that are attractive from the practical point of view remain rare [88,89,90,91]. In this work, we demonstrate the use of the developed hetero-/homogeneous NHPI/TiO2 photocatalytic system for the Minisci reaction between π-deficient heteroarenes (pyridines, quinolines, isoquinolines, pyrazines, and quinoxaline) and non-activated ethers.

2. Results and Discussion

2.1. Optimization of Photocatalytic System Composition

Based on our previous work [20], TiO2 with high specific surface area (anatase nanopowder, Hombikat UV100) and industrially available N-hydroxyphthalimide were chosen as the components of the photochemical system. Blue LEDs (455 nm) with an input power of 10 W were used as light sources. In the first step, we optimized the conditions of the photochemical cross-dehydrogenative Minisci reaction between 4-methylquinoline 1a and tetrahydrofuran 2a (Table 1). Tert-butyl hydroperoxide (TBHP) was used as an inexpensive, easily available and metal-free oxidant.
The starting conditions (10 mg of TiO2, 20 mol.% of NHPI, 4 mmol of TBHP, 5 h, run 1) yielded 45% of the product 3aa. The absence of either TiO2 or NHPI resulted in the zero conversion of 1a (runs 2, 3), proving that both components of the catalytic system are essential. Without t-BuOOH, the reaction proceeded with low efficiency: only trace amounts of the product were formed (run 4). As a rule, the addition of a strong Brønsted acid, such as HCl [85] or TFA [77,79,82,84,86], increases the efficiency of the Minisci reaction. Acids protonate π-deficient N-containing heterocycles, making them more susceptible to attack by nucleophilic C-centered radicals [67]. However, in our case, the addition of trifluoroacetic acid (TFA, run 5) had no significant effect on the yield and conversion. The addition of 0.5 mL of water resulted in a drop in 3aa yield (run 6). Water breaks down the stable suspension of TiO2 in THF, causing the catalyst particles to aggregate in the water droplets. Both an increase and a decrease in the amount of THF lead to a decrease in the yield of 3aa (runs 7, 8). The dilution of the reaction mixture with such co-solvents as hexafluoroisopropanol (HFIP, run 9) and acetonitrile (MeCN, run 10) slowed down the reaction, and dilution with dichloroethane (DCE, run 11) led to the complete suppression of the target process. It is known that hydrogen peroxide can be used as the oxidant for the photocatalytic Minisci reaction [85]. However, the change of the oxidant from TBHP to aqueous H2O2 led to a dramatic drop in the yield (run 12). The lower efficiency of H2O2 compared to TBHP can be explained by the fact that H2O2 can not only initiate free-radical reactions but can also be an inhibitor via the formation of HOO• radicals [92,93,94]. The use of other organic peroxides, such as meta-chloroperoxybenzoic acid (m-CPBA, run 13), cumene hydroperoxide (run 14) and dicumyl peroxide (run 15) led to low yields or did not provide the product at all. Dibenzoylperoxide (BzOOBz, run 16) showed a yield comparable to TBHP, but the formation of a large amount of benzoic acid, which is poorly soluble in the system, complicates the isolation of the products and limits the scalability of the procedure. Therefore, TBHP was chosen as the optimal oxidant. The standard version of the Minisci reaction often uses inorganic persulfates as oxidants. In our system, the use of persulfates was less efficient than TBHP, and led to a significant drop in yield with increasing reaction time, presumably due to the overoxidation of the product (runs 17–20). An inert atmosphere did not increase the selectivity of the process (run 21), so we decided to carry out the reaction under air.
In the next step, we optimized the NHPI/TiO2/TBHP ratio and irradiation time to achieve the maximum yield of the coupling product 3aa (Table 2).
Increasing the amount of TiO2 increases the yield of 3aa (runs 1–4). However, when switching from the TiO2 loading of 20 mg to 40 mg, the efficiency increased only slightly. Therefore, the TiO2 loading of 20 mg was chosen as the optimal amount. Similarly, large loadings of NHPI resulted in an increase in the 3aa yield (runs 5–8), but the step from 20 to 40 mol.% of NHPI increased the yield of 3aa slightly, and a slight drop in selectivity was observed. The optimum excess of THBP was 4 mmol per 1 mmol of 1a (runs 9–11). The reaction proceeded with almost complete conversion in 8 h (run 15). It should be noted that visible-light-active heterogeneous photocatalyst g-C3N4 was ineffective for the model coupling reaction under the same conditions (run 16). The conditions of experiment 15 were chosen as optimal for further studies of the substrate scope for the developed method.

2.2. Application of the Designed Photocatalytic NHPI/TiO2 System to the Minisci Reaction

With the optimal conditions in hand (Table 2, run 15), we have synthesized a wide range of coupling products between N-heterocycles and ethers. The scope of ethers was explored first (Scheme 2). For substrates demonstrating lower conversions compared to 1a, the reaction time increased in some cases up to 48 h (the reaction times and conversions are given in Scheme 2).
Among the tested ethers, we obtained the best result with THF: after 8 h of reaction, the almost complete conversion of 4-methylquinoline 1a and a high yield of product 3aa (89%) were observed. As a rule, the reaction proceeds more slowly and with lower selectivity for other ethers. In the reaction of 4-methylquinoline with 2-methyltetrahydrofuran 2b, a mixture of products 3ab (as a diastereomeric mixture, major) and 3ab’ (minor) was observed. The observed regioselectivity can be explained by the fact that although the hydrogen atom abstraction is most favored from the weakest tertiary CH-bond (position 2 of 2-methyltetrahydrofuran) [95], the resulting C-centered radical is more stable and sterically hindered than the secondary radical and reacts less efficiently with 4-methylquinoline. For 1,3-dioxolane 2c, two isomeric products 3ac and 3ac’ were formed, and the major product 3ac corresponds to the breaking of the weakest C2-H bond in 1,3-dioxolane. With dioxane and tetrahydropyran, the reaction proceeded more slowly, but with a longer reaction time, its selectivity decreased simultaneously with an increase in conversion. With glyme, the dehydrogenative coupling product was not observed even after 24 h of reaction.
In the case of diethyl ether as a substrate, the reaction under the standard conditions was not effective due to the immiscibility of Et2O and H2O contained in TBHP (70% aq.), which led to the aggregation of TiO2 particles in water droplets and the low conversion of 1a. The solution to the problem was the use of anhydrous TBHP, prepared before the reaction (See experimental details for Scheme 2). The same problem limited the reaction time for the coupling of 1a with Et2O since the water generated during TBHP reduction accumulated in the reaction mixture and made the TiO2 suspension unstable.
In the next step, the scope of the electron-deficient N-heterocycles was tested (Scheme 3).
N-heterocycles with electron-donor groups reacted slower compared to substrates with electron-withdrawing groups, but at the same time, higher selectivity was observed (products 3ba, 3ea in comparison with 3ca). The reaction is sensitive to steric hindrance: 2-chloro-5-bromoquinoline 2d did not yield the target product of 3da, presumably due to the presence of a bulky Br substituent near the 4th position of the quinoline. Our photochemical system is also applicable to quinoxalines and pyrazines. It is worth noting that the products of 3ga and 3ha have not been previously reported (See Supplementary Materials for additional information). In general, the reaction is inefficient for pyridines with no substituents or with electron-donor substituents (pyridine, picolines, lutidine), but good yields have been obtained for pyridines with electron-acceptor substituents, such as pyridine-3-carboxylic acid methyl ester (product 3ia). 4-Methylquinoline-N-oxide reacted with the preservation of the N-oxide function (product 3ja). Good yields have also been obtained in the reaction with isoquinoline (product 3ka). In the reaction with imidazo [1,2-a]pyridine 2l, it was only possible to isolate the product of deep oxidation with the destruction of the ring—3la’. It should also be noted that the addition of acid (TFA) afforded increased yields in some cases (products 3ba, 3ca, 3ea, 3ga, 3ha,3ja and 3ka).
It turned out that carrying out the reaction to complete the conversion of π-deficient arenes in the NHPI/TiO2 photochemical system leads to a sharp drop in selectivity for target product 3. We assumed that product 3 could undergo further oxidation under the reaction conditions. To find out what role the individual components of the system play in oxidation, we performed control experiments in which the pure reaction product 3aa was placed under standard reaction conditions or irradiated in an inert atmosphere in the absence of NHPI or TBHP (Scheme 4).
Under the standard conditions, an 86% conversion of 3aa was observed in 8 h (Scheme 4, A). In the absence of TBHP under an air atmosphere, the product is also oxidized (88% conversion, Scheme 4, B), which suggests that a significant role in the decomposition of the product is played by air as an oxidant. The primary oxidation product was hydroperoxide 3aa’, which was detected in a mixture of oxidation products by 13C NMR and was confirmed by HRMS (See Supplementary Materials). The 13C signal with chemical shift typical for geminal alkoxyhydroperoxide fragment was observed [96]. However, carrying out the reaction under an argon atmosphere (Scheme 4, C) does not completely suppress the oxidation of product 3aa since TBHP or residual amounts of oxygen can serve as oxidants. The lowest conversion of the product was observed when the reaction was carried out in an argon atmosphere without the addition of NHPI (Scheme 4, D), implying that NHPI-derived PINO radicals play an important role in 3aa oxidation.
Based on the collected data, we proposed the following mechanism (Scheme 5). Upon irradiation with visible light, PINO radicals are generated from NHPI on the TiO2 surface. Simultaneously, the tert-butyl hydroperoxide decomposes on the TiO2 surface with the formation of tert-butoxyl radicals. Tert-butoxyl radicals can regenerate PINO by abstracting a hydrogen atom from the NHPI in solution [59]. Tert-butoxyl radicals can also generate tert-butylperoxy radicals from t-BuOOH [97,98]. Either tert-butoxy, tert-butylperoxy [99,100,101], or PINO radicals [59,95] can abstract a hydrogen atom from the α-CH bond in ether to form C-centered radical A. However, considering the fact that no cross-dehydrogenative coupling was observed without the addition of NHPI, the main role in H-atom abstraction is assumed to be played by the PINO radicals. Then, radical A undergoes addition to a heteroarene with the formation of the intermediate radical B, which is further subjected to HAT with the retrieval of aromaticity.

3. Materials and Methods

3.1. General

Room temperature (rt) stands for 23–25 °C.
Commercial TiO2 Hombikat UV 100 (anatase, specific surface area, BET: 300 m2·g−1, primary crystal size according to Scherrer <10 nm) was used as is. N-hydroxyphthalimide (NHPI, 98%, Acros Organics), 4-methylquinoline (99%, Acros Organics), 2-methylquinoline (97%, Acros Organics), 2-chloroquinoline (99%, Acros Organics), isoquinoline (97%, Acros Organics), quinoxaline (99%, Acros Organics), pyrazine (99+%, Acros Organics), 2-methylpyrazine (99+%, Acros Organics), Methyl nicotinate (99%, Acros Organics), 2-methoxyquinoline, 5-bromo-2-chloroquinoline were used as is from commercial sources. 4-methylquinoline 1-oxide was synthesized according to the literature procedure [102], 2-(4-bromophenyl)imidazo [1,2-a]pyridine was synthesized according to the procedure in the literature [103]. Bulk g-C3N4 was prepared analogously to previously reported methods [104,105], and the urea was heated in a covered alumina crucible for 4 h at 550 °C (heating rate 5 °C·min−1). MeCN was distilled over P2O5, and Ethers (THF, 2-Methyltetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, tetrahydropyran and diethyl ether, dimethoxyethane, bis(2-methoxyethyl) ether) were distilled over LiAlH4. The reaction mixtures were sonicated in an ultrasonic bath (HF-Frequency 35 kHz, ultrasonic nominal power 80 W) before the irradiation.
General reaction conditions: 4-methylquinoline 1a (1 mmol, 143.2 mg), TiO2 (10 mg), NHPI (0.2 mmol, 32.6 mg), t-BuOOH (70% aq., 4 mmol, 515 mg), THF 2a (25 mmol, 2 mL) and a magnetic stir bar (6 × 10 mm) were placed in a 50 mL round-bottom flask. The obtained mixture was sonicated for 5 min in an ultrasonic bath, then magnetically stirred (500 rpm) in a thermostated water bath at 25 °C (±1 °C) under irradiation of 10 W blue LED for 5 h under an air atmosphere (closed flask). Then, the solvent was rotary evaporated, and C2H2Cl4 (40–60 mg, 0.4–0.61 mmol) was added as a standard for NMR yield determination. The reaction mixture was centrifuged, and the NMR spectrum was recorded.
4-methylquinoline 1a (1 mmol, 143.2 mg), TiO2 Hombikat UV 100 (2.5–40 mg), NHPI (0.05–0.4 mmol, 8.2–65.2 mg), t-BuOOH 70% aq. (1–6 mmol, 129–772 mg) and THF 2a (25 mmol, 2 mL) and a magnetic stir bar (6 × 10 mm) were placed in a 50 mL round-bottom flask. The obtained mixture was sonicated for 5 min in an ultrasonic bath, then magnetically stirred (500 rpm) in a thermostated water bath at 25 °C (±1 °C) under irradiation of 10 W blue LED for 1–16 h under an air atmosphere (closed flask). Then, the solvent was rotary evaporated, C2H2Cl4 (40–60 mg, 0.4–0.61 mmol) was added as a standard for NMR yield determination. The reaction mixture was filtrated through a Celite layer, and the NMR spectrum was recorded.
Heterocycle 1 (1 mmol), TiO2 (20 mg), NHPI (0.2 mmol, 32.6 mg), t-BuOOH 70% aq. (4 mmol, 515 mg), CH-reagent 2 (25 mmol) and a magnetic stir bar (6 × 10 mm) were placed in a 50 mL round-bottom flask. The obtained mixture was sonicated for 5 min in an ultrasonic bath, then magnetically stirred (500 rpm) in a thermostated water bath at 25 °C (±1 °C) under irradiation of 10 W blue LED for 8 h under an air atmosphere (closed flask). If needed, another 4 mmol of the reaction t-BuOOH was added, and the reaction mixture was irradiated for another 8 h. At the end of the required time, the reaction mixture was poured into 20 mL of water and extracted with 3×15 mL of CH2Cl2. The combined organic extracts were washed with 2×20 mL of NaHCO3 saturated solution. The extracts were dried over MgSO4, and the solvent was evaporated in a vacuum membrane pump. The residue was purified using column chromatography to afford products 3aa–3ka. For the reaction of 1a with Et2O, anhydrous t-BuOOH was prepared. t-BuOOH 70% aq. (12 mmol, 1545 mg) was extracted with CH2Cl2 (10 mL). The organic layer was dried over MgSO4, and the solvent was rotary evaporated. The obtained anhydrous t-BuOOH was used instead of t-BuOOH 70% aq. For the longer reaction times, the new portion of anhydrous t-BuOOH (4 mmol, 360 mg) was added each 8 h.
4-methyl-2-(tetrahydrofuran-2-yl)quinoline 3aa (0.5 mmol), TiO2 (10 mg), NHPI (0.1 mmol, 16.3 mg), t-BuOOH 70% aq. (2 mmol, 257 mg) and a magnetic stir bar (6 × 10 mm) were placed in a 50 mL round-bottom flask. The obtained mixture was sonicated for 5 min in an ultrasonic bath. For the entries of C and D, the flask was vacuumed and then filled with Ar three times. The mixture was magnetically stirred (500 rpm) in a thermostated water bath at 25 °C (±1 °C) under irradiation of 10 W blue LED for 8 h. The conversion of 3aa was determined by 1H NMR in MeCN using C2H2Cl4 as the internal standard.

3.2. Characterization Data of the Cross-Dehydrogenative C–C Coupling Products

4-Methyl-2-(tetrahydrofuran-2-yl)quinoline 3aa [91] was isolated using column chromatography (Petroleum ether/EtOAc = 2/1) as a colorless viscous liquid (190 mg, 89%). 1H NMR (300.13 MHz, CDCl3) δ 8.07–7.99 (m, 1H), 7.92–7.85 (m, 1H), 7.66–7.58 (m, 1H), 7.48–7.41 (m, 1H), 7.40 (s, 1H), 5.10 (t, J = 6.9 Hz, 1H), 4.16–4.08 (m, 1H), 4.02–3.94 (m, 1H), 2.63 (s, 3H), 2.53–2.38 (m, 1H), 2.11–1.90 (m, 3H).13C{1H}NMR (75.48 MHz, CDCl3) δ 163.0, 147.3, 144.8, 129.5, 129.0, 127.4, 125.7, 123.6, 118.6, 82.0, 69.1, 33.2, 25.9, 18.8.
2-(2-hydroperoxytetrahydrofuran-2-yl)-4-methylquinoline 3aa’. 13C{1H}NMR (75.48 MHz, CDCl3) δ 159.4, 146.0, 145.7, 128.8, 128.7, 127.2, 126.1, 123.4, 119.7, 113.3, 69.5, 36.8, 24.8, 19.0. HR-MS (ESI): m/z = 246.1125, calcd. for C14H15NO3+H+: 246.1123.
Anti-4-methyl-2-(5-methyltetrahydrofuran-2-yl)quinoline 3ab was isolated using column chromatography (Petroleum ether/EtOAc = 2/1) as a colorless liquid (66 mg, 29%). 1H NMR (300 MHz, Chloroform-d) δ 8.07–8.02 (m, 1H), 7.98–7.93 (m, 1H), 7.66 (ddd, J = 8.4, 6.8, 1.5 Hz, 1H), 7.50 (ddd, J = 8.2, 6.8, 1.3 Hz, 1H), 7.46 (s, 1H), 5.26 (t, J = 7.1 Hz, 1H), 4.51–4.33 (m, 1H), 2.70 (s, 3H), 2.63–2.49 (m, 1H), 2.24–2.02 (m, 2H), 1.75–1.59 (m, 1H), 1.36 (d, J = 6.1 Hz, 3H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 163.6, 147.3, 145.1, 129.6, 129.2, 127.5, 125.9, 123.8, 118.6, 81.8, 76.7, 34.1, 34.0, 21.5, 19.0; FTIR (KBr): νmax = 2968, 2928, 2869, 1602, 1509, 1447, 1379, 1311, 1225, 1181, 1074, 910, 883, 760 cm−1. HR-MS (ESI): m/z = 228.1389, calcd. for C15H17NO+H+: 228.1383.
Syn-4-methyl-2-(5-methyltetrahydrofuran-2-yl)quinoline 3ab’ was isolated using column chromatography (Petroleum ether/EtOAc = 2/1) as a colorless liquid (59 mg, 26%). 1H NMR (300 MHz, Chloroform-d) δ 8.08–8.03 (m, 1H), 7.97 (dd, J = 8.4, 1.5 Hz, 1H), 7.68 (ddd, J = 8.4, 6.9, 1.5 Hz, 1H), 7.58–7.46 (m, 2H), 5.13 (dd, J = 7.6, 6.5 Hz, 1H), 4.33–4.21 (m, 1H), 2.72 (d, J = 0.7 Hz, 3H), 2.60–2.42 (m, 1H), 2.21–1.99 (m, 2H), 1.69–1.50 (m, 1H), 1.44 (d, J = 6.1 Hz, 3H).13C{1H}NMR (75.48 MHz, CDCl3) δ 163.3, 147.3, 145.2, 129.6, 129.3, 127.6, 126.0, 123.8, 118.8, 82.5, 76.9, 33.5, 33.2, 21.4, 19.1; FTIR (KBr): νmax = 2970, 2928, 2870, 1736, 1602, 1563, 1509, 1447, 1380, 1090, 1032, 913, 882, 760 cm−1. HR-MS (ESI): m/z = 228.1388, calcd. for C15H17NO+H+: 228.1383.
4-methyl-2-(2-methyltetrahydrofuran-2-yl)quinoline 3ab’ was isolated using column chromatography (Petroleum ether/EtOAc = 2/1) as a colorless liquid (28 mg, 12%). 1H NMR (300 MHz, Chloroform-d) δ 8.07 (d, J = 8.4, 1H), 7.99–7.94 (m, 1H), 7.67 (ddd, J = 8.4, 6.8, 1.5 Hz, 1H), 7.62–7.60 (m, 1H), 7.51 (ddd, J = 8.3, 6.9, 1.3 Hz, 1H), 4.13–4.02 (m, 1H), 3.95–3.83 (m, 1H), 2.88–2.75 (m, 1H), 2.71 (d, J = 1.0 Hz, 3H), 2.14–1.95 (m, 2H), 1.89–1.74 (m, 1H), 1.65 (s, 3H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 166.6, 147.5, 144.6, 129.9, 129.0, 127.2, 125.8, 123.7, 118.5, 86.2, 68.1, 37.7, 28.3, 26.1, 19.1; FTIR (KBr): νmax = 2977, 2931, 1600, 1447, 1383, 1363, 1196, 1101, 1033, 761 cm−1. HR-MS (ESI): m/z = 228.1380, calcd. for C15H17NO+H+: 228.1283.
2-(1,3-dioxolan-2-yl)-4-methylquinoline 3ac [65] was isolated using column chromatography (Petroleum ether/EtOAc = 2/1) as a colorless liquid (54 mg, 25%). 1H NMR (500 MHz, Chloroform-d) δ 8.16 (d, J = 8.4 Hz, 1H), 7.98 (d, J = 8.3 Hz, 1H), 7.74–7.67 (m, 1H), 7.59–7.53 (m, 1H), 7.49 (s, 1H), 5.95 (s, 1H), 4.27–4.19 (m, 2H), 4.16–4.08 (m, 2H), 2.71 (s, 3H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 156.7, 147.2, 145.7, 130.2, 129.5, 128.4, 126.9, 123.8, 118.7, 104.3, 65.8, 19.0.
2-(1,3-dioxolan-4-yl)-4-methylquinoline 3ac’ [65] was isolated using column chromatography (Petroleum ether/EtOAc = 2/1) as a colorless liquid (18 mg, 8%). 1H NMR (300 MHz, Chloroform-d) δ 8.05 (d, J = 8.4 Hz, 1H), 8.00 (dd, J = 8.4, 1.4 Hz, 1H), 7.71 (ddd, J = 8.4, 6.8, 1.4 Hz, 1H), 7.60–7.52 (m, 1H), 7.48 (s, 1H), 5.34 (s, 1H), 5.33–5.26 (m, 1H), 5.15 (s, 1H), 4.47–4.36 (m, 1H), 4.08 (dd, J = 8.3, 5.6 Hz, 1H), 2.73 (s, 3H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 160.0, 147.1, 146.0, 129.7, 129.5, 127.8, 126.5, 123.9, 118.8, 96.4, 78.3, 71.1, 19.1. FTIR (KBr): νmax = 2925, 2855, 16001, 1509, 1449, 1157, 1088, 1029, 936, 760 cm−1. HR-MS (ESI): m/z = 238.0841, calcd. for C13H13NO2+Na+: 238.0838.
2-(1,4-dioxan-2-yl)-4-methylquinoline 3ad [85] was isolated using column chromatography (Petroleum ether/EtOAc = 2/1) as white crystals (45 mg, 20%). Mp = 81–82 °C (lit. Mp = 82–83 °C [10.1039/C9OB02653C]). 1H NMR (300 MHz, Chloroform-d) δ 8.10 (d, J = 8.5 Hz, 1H), 7.98 (d, J = 8.7 Hz, 1H), 7.76–7.64 (m, 1H), 7.59–7.51 (m, 1H), 7.47 (s, 1H), 4.92 (dd, J = 10.3, 2.9 Hz, 1H), 4.25 (dd, J = 11.7, 2.9 Hz, 1H), 4.06–3.94 (m, 2H), 3.88–3.74 (m, 2H), 3.70–3.57 (m, 1H), 2.73 (s, 3H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 157.9, 147.4, 145.3, 129.9, 129.4, 127.7, 126.3, 123.8, 119.2, 78.9, 71.2, 67.2, 66.5, 18.9.
4-methyl-2-(tetrahydro-2H-pyran-2-yl)quinoline 3ae [85] was isolated using column chromatography (Petroleum ether/EtOAc = 2/1) as a colorless liquid (73 mg, 32%). 1H NMR (300 MHz, Chloroform-d) δ 8.06 (d, J = 8.4 Hz, 1H), 7.98–7.89 (m, 1H), 7.70–7.60 (m, 1H), 7.52–7.46 (m, 1H), 7.45 (s, 1H), 4.60 (dd, J = 11.0, 2.3 Hz, 1H), 4.25–4.15 (m, 1H), 3.75–3.60 (m, 1H), 2.68 (s, 3H), 2.16–2.04 (m, 1H), 2.03–1.88 (m, 1H), 1.83–1.66 (m, 2H), 1.66–1.51 (m, 2H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 162.2, 147.2, 145.1, 129.7, 129.1, 127.6, 125.9, 123.7, 118.9, 81.6, 68.9, 32.8, 25.9, 23.8, 18.9.
2-(1-ethoxyethyl)-4-methylquinoline 3af [85] was isolated using column chromatography (CH2Cl2/EtOAc = 20/1) as a colorless liquid (77 mg, 36%). 1H NMR (300 MHz, Chloroform-d) δ 8.07 (d, J = 8.4 Hz, 1H), 7.97 (d, J = 8.3 Hz, 1H), 7.68 (t, J = 8.4 Hz, 1H), 7.57–7.48 (m, 1H), 7.44 (s, 1H), 4.69 (q, J = 6.6 Hz, 1H), 3.57– 3.45 (m, 1H), 3.47–3.34 (m, 1H), 2.72 (s, 3H), 1.53 (d, J = 6.6 Hz, 3H), 1.22 (t, J = 7.1 Hz, 3H).13C{1H}NMR (75.48 MHz, CDCl3) δ 164.1, 147.2, 145.5, 129.6, 129.3, 127.8, 126.1, 123.8, 118.4, 79.7, 64.8, 22.7, 19.1, 15.6
2-methoxy-4-(tetrahydrofuran-2-yl)quinoline 3ba was isolated using column chromatography (EtOAc/petroleum ether 1/2) as a colorless liquid (122 mg, 53%). 1H NMR (300 MHz, Chloroform-d) δ 7.89 (d, J = 8.4 Hz, 1H), 7.81–7.72 (m, 1H), 7.68–7.54 (m, 1H), 7.44–7.31 (m, 1H), 7.07 (s, 1H), 5.52 (t, J = 6.9 Hz, 1H), 4.27–4.13 (m, 1H), 4.08 (s, 3H), 4.08–3.95 (m, 1H), 2.65–2.47 (m, 1H), 2.14–1.92 (m, 2H), 1.92–1.78 (m, 1H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 162.8, 152.3, 147.1, 129.2, 128.1, 123.8, 123.3, 122.9, 108.5, 76.8, 69.1, 53.4, 33.7, 26.0. FTIR (KBr): νmax = 2979, 2949, 1612, 1575, 1473, 1438, 1387, 1366, 1340, 1238, 1195, 1080, 1055, 1024, 761 cm−1. HR-MS (ESI): m/z = 230.1181, calcd. for C14H15NO2+H+: 230.1176
2-chloro-4-(tetrahydrofuran-2-yl)quinoline 3ca was isolated using column chromatography (Petroleum ether/EtOAc = 2/1) as a slightly yellow liquid (87 mg, 37%). 1H NMR (300.13 MHz, CDCl3) δ 8.04 (d, J = 8.5 Hz, 1H), 7.85 (dd, J = 8.4, 1.4 Hz, 1H), 7.71 (ddd, J = 8.4, 6.9, 1.4 Hz, 1H), 7.60–7.49 (m, 1H), 7.54 (s, 1H), 5.55 (t, J = 7.1 Hz, 1H), 4.22 (m, 1H), 4.02 (m, 1H), 2.70–2.55 (m, 1H), 2.13–1.95 (m, 2H), 1.94–1.76 (m, 1H).13C{1H}NMR (75.48 MHz, CDCl3) δ 153.1, 151.4, 148.1, 130.2, 129.5, 126.8, 124.5, 123.4, 117.9, 76.7, 69.2, 34.0, 26.1. FTIR (KBr): νmax = 2965, 2928, 2871, 1586, 1560, 1506, 1292, 1264, 1145, 1099, 1081, 1041, 1021, 878, 855, 792, 763 cm−1. HR-MS (ESI): m/z = 234.0688, calcd. for C13H12ClNO+H+: 234.0680.
2-methyl-4-(tetrahydrofuran-2-yl)quinoline 3ea [91] was isolated using column chromatography (Petroleum ether/EtOAc = 2/1) as a colorless liquid (161 mg, 75%). 1H NMR (300 MHz, Chloroform-d) δ 8.03 (d, J = 8.3 Hz, 1H), 7.81 (d, J = 8.4 Hz, 1H), 7.69–7.57 (m, 1H), 7.50–7.41 (m, 1H), 7.42 (s, 1H), 5.53 (t, J = 7.1 Hz, 1H), 4.24–4.14 (m, 1H), 4.00 (q, J = 7.1 Hz, 1H), 2.65–2.47 (m, 1H), 2.12–1.89 (m, 2H), 1.86–1.72 (m, 1H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 159.1, 149.4, 147.9, 129.4, 129.0, 125.5, 123.9, 123.0, 117.2, 76.8, 69.0, 33.9, 26.0, 25.6.
2-(tetrahydrofuran-2-yl)quinoxaline 3fa [66] was isolated using column chromatography (Petroleum ether/EtOAc = 2/1) as a colorless liquid (113 mg, 56%). 1H NMR (300.13 MHz, CDCl3) δ 9.02 (s, 1H), 8.12–8.07 (m, 1H), 8.07–8.01 (m, 1H), 7.76–7.69 (m, 2H), 5.21 (t, J = 7.0 Hz, 1H), 4.17 (q, J = 7.0 Hz, 1H), 4.05 (dd, J = 7.2 Hz, 1H), 2.57–2.46 (m, 1H), 2.21–2.11 (m, 1H), 2.11–2.00 (m, 2H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 157.7, 143.6, 142.0, 141.7, 130.2, 129.6, 129.3, 129.2, 80.6, 69.5, 33.0, 26.1.
2-(tetrahydrofuran-2-yl)pyrazine 3ga was isolated using column chromatography (CH2Cl2/MeOH = 50/1) as a colorless liquid (59 mg, 40%).1H NMR (300 MHz, Chloroform-d) δ 8.68 (s, 1H), 8.52–8.36 (m, 2H), 5.01 (t, J = 6.4 Hz, 1H), 4.14–4.02 (m, 1H), 4.00–3.87 (m, 1H), 2.49–2.29 (m, 1H), 2.11–1.86 (m, 3H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 158.2, 143.8, 143.5, 142.7, 79.8, 69.3, 32.9, 25.9. FTIR (KBr): νmax = 3389, 2959, 2882, 1724, 1701, 1406, 1304, 1140, 1052, 1020 cm−1. HR-MS (ESI): m/z = 151.0873, calcd. for C8H10N2O+H+: 151.0866.
2-methyl-3-(tetrahydrofuran-2-yl)pyrazine 3ha was isolated using column chromatography (CH2Cl2/MeOH = 50/1) as a slightly yellow liquid (76 mg, 46%).1H NMR (300 MHz, Chloroform-d) δ 8.37 (d, J = 2.6 Hz, 1H), 8.34 (d, J = 2.6 Hz, 1H), 5.15 (t, J = 7.0 Hz, 1H), 4.13–4.04 (m, 1H), 3.99–3.89 (m, 1H), 2.63 (s, 3H), 2.31–2.19 (m, 2H), 2.17–1.96 (m, 2H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 154.5, 152.5, 142.6, 141.5, 78.3, 69.2, 30.4, 26.3, 21.6. FTIR (KBr): νmax = 3240, 3051, 2959, 2878, 1774, 1726, 1701, 1405, 1299, 1169, 1130, 1105, 1055, 988, 923, 857, 732 cm−1. HR-MS (ESI): m/z = 165.1023, calcd. for C9H10N2O+H+: 165.1022.
Methyl 6-(tetrahydrofuran-2-yl)nicotinate 3ia [91] was isolated using column chromatography (EtOAc/DCM = 1/20→1/5) as an orange liquid (119 mg, 57%). 1H NMR (300.13 MHz, CDCl3) δ 9.11 (d, J = 2.2 Hz, 1H), 8.25 (dd, J = 8.2, 2.2 Hz, 1H), 7.52 (d, J = 8.2 Hz, 1H), 5.11–4.95 (m, 1H), 4.32–3.74 (m, 5H), 2.57–2.33 (m, 1H), 2.09–1.81 (m, 3H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 167.8, 165.9, 150.4, 137.9, 124.6, 119.4, 81.2, 69.3, 52.4, 33.2, 25.8.
4-methyl-2-(tetrahydrofuran-2-yl)quinoline 1-oxide 3ja [76] was isolated using column chromatography (Petroleum ether/EtOAc = 2/1) as a colorless liquid (78 mg, 34%). 1H NMR (300 MHz, Chloroform-d) δ 8.81–8.74 (m, 1H), 7.99–7.92 (m, 1H), 7.80–7.71 (m, 1H), 7.67–7.59 (m, 1H), 7.44 (s, 1H), 5.58 (t, J = 6.7 Hz, 1H), 4.17 (q, J = 6.9 Hz, 1H), 4.02 (q, J = 7.1 Hz, 1H), 2.90–2.76 (m, 1H), 2.68 (s, 3H), 2.13–1.98 (m, 1H), 1.99–1.82 (m, 2H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 150.8, 141.1, 135.4, 130.3, 128.8, 128.0, 124.8, 119.9, 118.9, 76.1, 69.5, 31.2, 26.0, 18.6.
1-(tetrahydrofuran-2-yl)isoquinoline 3ka [91] was isolated using column chromatography (CH2Cl2/EtOAc from 5/1 to 5/2) as a colorless liquid (130 mg, 65%). 1H NMR (300.13 MHz, CDCl3) δ 8.50 (d, J = 5.8 Hz, 1H), 8.34 (d, J = 8.3 Hz, 1H), 7.82 (d, J = 8.1 Hz, 1H), 7.75–7.52 (m, 3H), 5.72 (t, J = 7.1 Hz, 1H), 4.20 (q, J = 7.3 Hz, 1H), 4.03 (q, J = 7.5 Hz, 1H), 2.60–2.32 (m, 2H), 2.27–2.01 (m, 2H).13C{1H}NMR (75.48 MHz, CDCl3) δ 159.7, 141.4, 136.7, 130.1, 127.5, 127.3, 126.7, 125.5, 120.7, 79.2, 69.1, 30.9, 26.3.
2-(4-bromophenyl)imidazo [1,2-a]pyridine-3-carboxylic acid 3la’ [106] was isolated using column chromatography (CH2Cl2/EtOAc = 5/2) as slightly yellow crystals (82 mg, 30%). 1H NMR (300 MHz, Chloroform-d) δ 9.27 (bs, 1H, NH), 8.41 (d, J = 8.4 Hz, 1H), 8.28–8.19 (m, 1H), 7.84 (d, J = 8.5 Hz, 2H), 7.81–7.74 (m, 1H), 7.62 (d, J = 8.5 Hz, 2H), 7.09 (dd, J = 7.3, 4.9 Hz, 1H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 165.1, 151.5, 147.1, 139.3, 133.1, 132.2, 129.2, 127.4, 120.2, 114.8.

4. Conclusions

In this work, a new visible-light active heterogeneous photocatalyst system based on industrially available and non-toxic TiO2 and NHPI was proposed for the cross-dehydrogenative C–C coupling of electron-deficient N-heterocycles with ethers. In this photocatalytic system, phthalimide-N-oxyl radicals photogenerated on the surface of titanium oxide become active mediators of the reaction, which leads to 1) an increase in efficiency due to the homogeneous organocatalytic process in solution and 2) allows the selective cleavage of the weak CH bonds. We have proposed a new mild method for the generation of C-centered radicals from non-activated esters for the Minisci reaction. Despite the fact that acidic additives are frequently used in Minisci-type reactions, the addition of acid was not necessary in our procedure in the case of several substrates. Optimal conditions were chosen for the Minisci reaction between π-deficient pyridine, quinoline, pyrazine, and quinoxaline heteroarenes with non-activated ethers.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules28030934/s1, copies of NMR spectra of the synthesized products, the comparison of the developed method with the literature procedure, the determination of the side products of the studied reaction.

Author Contributions

Conceptualization, I.B.K.; methodology, I.B.K. and E.R.L.; investigation, E.R.L., O.O.S., V.M.M. and A.I.I.; writing—original draft preparation, E.R.L.; writing—review and editing, I.B.K. and A.O.T.; supervision, I.B.K. and A.O.T.; project administration, I.B.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We are grateful to the Department of Structural Studies, Zelinsky Institute of Organic Chemistry for the HRMS analysis.

Conflicts of Interest

The authors declare no conflict of interest.

Sample Availability

Not applicable.

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Molecules 28 00934 sch001 550
Scheme 1. Applications of NHPI/TiO2 photocatalytic system in organic synthesis: CH-oxygenation (A) [20], oxidative homocoupling of benzylamines (B) [22], and Minisi-type corss-dehydrogenative C–C coupling reported in the present work (C).
Scheme 1. Applications of NHPI/TiO2 photocatalytic system in organic synthesis: CH-oxygenation (A) [20], oxidative homocoupling of benzylamines (B) [22], and Minisi-type corss-dehydrogenative C–C coupling reported in the present work (C).
Molecules 28 00934 sch001
Molecules 28 00934 sch002 550
Scheme 2. Scope of ethers for the photocatalytic Minisci reaction with 4-methylquinoline 1a.
Scheme 2. Scope of ethers for the photocatalytic Minisci reaction with 4-methylquinoline 1a.
Molecules 28 00934 sch002
Molecules 28 00934 sch003 550
Scheme 3. Scope of π-deficient arenes for the photocatalytic Minisci reaction with tetrahydrofuran.
Scheme 3. Scope of π-deficient arenes for the photocatalytic Minisci reaction with tetrahydrofuran.
Molecules 28 00934 sch003
Molecules 28 00934 sch004 550
Scheme 4. Control experiments disclosing side processes of the photocatalytic Minisci reaction.
Scheme 4. Control experiments disclosing side processes of the photocatalytic Minisci reaction.
Molecules 28 00934 sch004
Molecules 28 00934 sch005 550
Scheme 5. Plausible mechanism of the photocatalytic Minisci reaction.
Scheme 5. Plausible mechanism of the photocatalytic Minisci reaction.
Molecules 28 00934 sch005
Table
Table 1. Influence of photocatalytic system composition, irradiation power, and nature of oxidant on the conversion of 4-methylquinoline 1a and yield of 3aa in photocatalytic Minisci reaction.
Table 1. Influence of photocatalytic system composition, irradiation power, and nature of oxidant on the conversion of 4-methylquinoline 1a and yield of 3aa in photocatalytic Minisci reaction.
Molecules 28 00934 i001
RunChanges to the General ConditionsConversion a 1a, %Yield a 3aa, %
1none5345
2no TiO200
3no NHPI00
4no TBHP64
5TFA (1.5 mmol) added5245
6H2O (0.5 mL) added239
7THF (12.5 mmol)3836
8THF (50 mmol)3227
9HFIP (1 mL) added1816
10MeCN (1 mL) added1716
11DCE (1 mL) added00
12H2O2 34% aq. b93
13m-CPBA 75% aq. b280
14PhCH(CH3)2OOH 80%1515
15PhCH(CH3)2OOPhCH(CH3)2 98% b160
16BzOOBz 75% aq. (1 mmol) b5944
17(NH4)2S2O8 b,c3933
18Na2S2O8 b,c3622
19K2S2O8 b,c4439
20K2S2O8 b,c, 16 h, Argon atmosphere9027
21Argon atmosphere4439
a The conversion of 1a and the yield of 3aa were determined by 1H NMR using C2H2Cl4 as an internal standard. b instead of TBHP. c 1 mL of water was used as co-solvent to dissolve the persulfate.
Table
Table 2. Optimization of NHPI/TiO2/TBHP ratio and reaction time for the synthesis of 3aa.
Table 2. Optimization of NHPI/TiO2/TBHP ratio and reaction time for the synthesis of 3aa.
Molecules 28 00934 i002
RunTiO2, mgNHPI, mmolTBHP, mmolTime, hConversion a 1a, %Yield a 3aa, %
12.50.12555
250.1251211
3200.1254440
4400.1255546
5100.052500
6100.1253936
7100.2254439
8100.4255241
9100.1153127
10100.1454942
11100.1653838
12100.12154
13100.1222015
14100.1253428
15200.2489689
16- b0.24897
a The conversion of 1a and the yield of 3aa were determined by 1H NMR using C2H2Cl4 as an internal standard. b Bulk g-C3N4 (20 mg) was used instead of TiO2 as heterogeneous photocatalyst.
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Lopat’eva, E.R.; Krylov, I.B.; Segida, O.O.; Merkulova, V.M.; Ilovaisky, A.I.; Terent’ev, A.O. Heterogeneous Photocatalysis as a Potent Tool for Organic Synthesis: Cross-Dehydrogenative C–C Coupling of N-Heterocycles with Ethers Employing TiO2/N-Hydroxyphthalimide System under Visible Light. Molecules 2023, 28, 934. https://doi.org/10.3390/molecules28030934

AMA Style

Lopat’eva ER, Krylov IB, Segida OO, Merkulova VM, Ilovaisky AI, Terent’ev AO. Heterogeneous Photocatalysis as a Potent Tool for Organic Synthesis: Cross-Dehydrogenative C–C Coupling of N-Heterocycles with Ethers Employing TiO2/N-Hydroxyphthalimide System under Visible Light. Molecules. 2023; 28(3):934. https://doi.org/10.3390/molecules28030934

Chicago/Turabian Style

Lopat’eva, Elena R., Igor B. Krylov, Oleg O. Segida, Valentina M. Merkulova, Alexey I. Ilovaisky, and Alexander O. Terent’ev. 2023. "Heterogeneous Photocatalysis as a Potent Tool for Organic Synthesis: Cross-Dehydrogenative C–C Coupling of N-Heterocycles with Ethers Employing TiO2/N-Hydroxyphthalimide System under Visible Light" Molecules 28, no. 3: 934. https://doi.org/10.3390/molecules28030934

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