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Article

t-BuOOH/TiO2 Photocatalytic System as a Convenient Peroxyl Radical Source at Room Temperature under Visible Light and Its Application for the CH-Peroxidation of Barbituric Acids

by
Elena R. Lopat’eva
,
Igor B. Krylov
* and
Alexander O. Terent’ev
*
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky Prospekt, Moscow 119991, Russia
*
Authors to whom correspondence should be addressed.
Catalysts 2023, 13(9), 1306; https://doi.org/10.3390/catal13091306
Submission received: 31 July 2023 / Revised: 15 September 2023 / Accepted: 18 September 2023 / Published: 19 September 2023
(This article belongs to the Section Catalysis in Organic and Polymer Chemistry)
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Abstract

:
TiO2 is one of the most promising heterogeneous photoredox catalysts employed in oxidative pollutant destruction, CO2 reduction, water splitting, disinfection, solar cell design and organic synthesis. Due to the wide bandgap of TiO2, visible light energy is not sufficient for its activation, and electron/hole pairs generated upon UV irradiation demonstrate limited selectivity for application in organic synthesis. Thus, the development of TiO2-based catalytic systems activated by visible light is highly attractive. In the present work we demonstrate the generation of t-BuOO• radicals from tert-butylhydroperoxide catalyzed using commercially available unmodified TiO2 under visible light. This finding was used for the highly selective CH-peroxidation of barbituric acids, which contrasts with the behavior of the known TiO2/H2O2/UV photocatalytic system used for deep oxidation of organic pollutants.

Graphical Abstract

1. Introduction

Heterogeneous photocatalysis has recently gained a lot of attention as an ideal tool for green chemistry [1]. It utilizes the energy of light using non-toxic environmentally friendly photocatalysts that can be easily recycled and reused. Titanium dioxide (TiO2) is the most widely used heterogeneous photocatalyst due to its excellent chemical stability, low toxicity, moderate cost and availability [2]. The most profound applications of TiO2 are currently solar cell design [3,4], the oxidative destruction of pollutants [1,5,6,7], photodisinfection [8], hydrogen generation [9,10,11,12], CO2 reduction [2,13] and water splitting [14]. One of the most important challenges for TiO2 application in organic synthesis is to shift its activity form UV to visible light [15,16,17,18]. Visible light is an attractive energy source for chemical transformations, as it is responsible for the greatest part of sunlight’s irradiation power compared with UV light. UV-mediated photochemistry is frequently associated with additional safety precautions, expensive light sources (compared with visible light) and the need for UV-transparent quartz glassware. Moreover, most organic compounds, including solvents, absorb UV radiation, which leads to lower energy efficiency and unwanted photochemical side reactions. Visible light quanta have lower energy and, thus, are more promising for the development of selective and widely applicable photochemical synthetic methods based on suitable photocatalysts. Numerous attempts were directed to the photosensitization of titanium dioxide in the visible light region by another semiconductors [19,20,21,22], organic photocatalysts [23] or via element doping [24,25,26,27,28,29]. Another opportunity is to use organic compounds, especially bearing hydroxyl or carboxyl groups, such as phenols [30,31], salicylic acid [32,33], carboxylic acids [34,35,36] or N-hydroxyphthalimide [37,38], which can modify the TiO2 surface to absorb visible light. In our work, we discovered that peroxides can also lead to visible light sensitization: the mixing of TiO2 suspension in MeCN with tert-butylhydroperoxide resulted in a change in color from white to pale yellow (the appearance of the mixture can be found in the Supplementary Materials).
In our previous work, it was found that some hydroperoxides, in particular ethylbenzene hydroperoxide [37] and tert-butyl hydroperoxide (Scheme 1a) [38], can be decomposed on the TiO2 surface when irradiated with visible light (443 nm). Previously, studies of peroxide-TiO2 photochemistry were mainly focused on H2O2 and UV light usage [39,40,41,42,43,44,45]. Data on photoreactions involving unmodified TiO2 and organic peroxides [46], especially under visible light irradiation, are rare (Scheme 1a) [25,47].
Organic peroxides are widely used as green metal-free oxidants [48,49,50] and O-reagents for cross-dehydrogenative C–O coupling reactions [51,52,53,54,55,56,57,58]. As a rule, the generation of peroxyl radicals from organic peroxides requires high temperatures [48,49,56,57] or salts of transition metals, such as Mn [55], Fe [51,58,59], Co [52], Ni [60], Cu [50,53,54,59] or Ru [61]. Direct photolysis of organic peroxides requires UV [62,63,64,65] or high-power white light [66], so various organic photoredox catalysts such as Eosin Y [67,68], Rose Bengal [69], Rhodamine B [70] or metal complex dyes, including Ir and Ru complexes [71,72], were proposed for the visible-light-induced photodecomposition of peroxides. However, examples of heterogeneous photocatalysts used to generate peroxyl radicals are still rare [38,47,73,74,75].
In this work, we investigated the decomposition of different organic peroxides using commercially available unmodified TiO2 under visible light and demonstrated that the t-BuOOH/TiO2 system can be used for the generation of peroxyl radicals under visible light (Scheme 1b).
Medicinally relevant compounds, barbituric acids, were chosen as substrates. Barbituric acids represent an important class of biologically active compounds [76], the direct functionalization of which is currently a hot topic for research [77,78,79]. The proposed methods for the peroxidation of barbituric acids often deal with transition metal catalysis [53,80] or proceed at high temperatures [53], and therefore the development of mild metal-free conditions for the peroxidation of barbituric acids is an attractive task.

2. Results

2.1. Study of Organic Peroxide Decomposition on TiO2 under Visible Light

In the first step, we studied the decomposition of organic peroxides of various classes (hydroperoxides, dialkyl peroxides, diacyl peroxides and peroxyacids) under visible light in a suspension of commercially available nanosized titanium dioxide (Table 1). Based on our previous research [37,38], we used TiO2 Hombikat UV100 with a high specific surface area (300 m2·g−1) as a heterogeneous photocatalyst and a 443 nm blue LED with 10 W input power as a light source. In all cases, corresponding alcohols 2 (or benzoic acid, in the case of mCPBA) were detected as the main decomposition products.
Hydroperoxides (t-BuOOH and cumyl hydroperoxide) were found to be the most susceptible to photodecomposition (entries 1, 3). Tert-butyl hydroperoxide (TBHP) decomposes at a higher conversion than cumyl hydroperoxide (Table 1, entries 1, 3). This fact can be attributed to the higher lipophilicity of cumyl hydroperoxide compared with TBHP and, thus, the lower degree of adsorption of cumyl hydroperoxide on the TiO2 surface. It should be noted that TBHP did not decompose in the absence of light (entry 2). Another important note is that the fraction of the beta-decay product, acetone, for the tert-butoxyl radical is negligible (no acetone signals are observed in the 1H and 13C NMR spectrum), while for cumyl hydroperoxide, about a quarter of the alkoxyl radicals undergo β-scission with the formation of acetophenone and the release of the methyl radical. Dibenzoyl peroxide (BzOOBz, entry 4) is rather stable under reaction conditions. The traces of benzoic acid observed in the reaction mixture were present in the starting commercial BzOOBz. Meta-chloroperbenzoic acid (mCPBA, entry 5) decomposes mainly to meta-chlorobenzoic acid (mCBA). For the correct determination of mCPBA conversion and the yield of meta-chlorobenzoic acid, 1H NMR spectra were recorded both from the starting mCPBA and reaction mixture in MeCN with the internal standard C2H2Cl4. The initial mCPBA is of 75% purity (contains mCBA and water), and the molar ratio mCPBA/mCBA is 5.1/1, while after the reaction the molar ratio mCPBA/mCBA decreases to 1/1, which means that conversion of mCPBA is 40%. Di-tert-butyl peroxide turned out to be stable under the experimental conditions (entry 6), which gives hope that other dialkyl peroxides, in particular target peroxidation products, will not undergo decomposition on TiO2 under visible light.
Hydroperoxide decomposition on TiO2 can start from either single-electron oxidation (Scheme 2, A) leading to peroxyl radical, or single-electron reduction, (Scheme 2, B) leading to alkoxyl radical. Peroxyl radicals can undergo bimolecular decay (Scheme 2, C) with the formation of alkoxyl radicals [81,82,83,84]. In turn, alkoxyl radicals can abstract hydrogen atoms from the comparatively weak O–H bond of hydroperoxides with the formation of alcohols and peroxyl radicals (Scheme 2, D) [82]. The formation of ketone products (observed for cumyl hydroperoxide, Table 1, entry 3) is a sign of the β-scission process in the alkoxyl radical (Scheme 2, E) [83].
To prove that tert-butylperoxyl radicals are formed in TBHP/TiO2 systems under visible light, we set up radical trapping experiments with 1,1-diphenylethylene 3 and 2,6-di-tert-butyl-4-methylphenol (BHT, 6) (Scheme 3). The isolation of tert-butylperoxy products 4 and 7 unambiguously confirmed the presence of tert-butylperoxyl radicals in the system.
Summing up all the data obtained, tert-butyl hydroperoxide is the most suitable source of peroxyl radicals and can be used for the peroxidation of substances containing relatively weak CH bonds with the formation of unsymmetrical organic peroxides that are hoped to be stable under the reaction conditions.

2.2. Synthesis of Alkyl Peroxides from Barbituric Acids

In the next stage, the peroxidation of barbituric acids using a tert-butyl hydroperoxide/TiO2 system under visible light was studied using 1,3-dimethyl-5-benzylbarbituric acid 8a as a model substrate (Table 2).
Under general conditions, we obtained the desired product 9a in a 52% yield (entry 1). In the absence of light or photocatalyst, no conversion of 8a was observed (entries 2, 3), proving that the target reaction is indeed a photocatalytic process. Carrying out the reaction in an air atmosphere led to a decrease in 9a yield (entry 4), presumably due to the interaction of C-centered radicals formed from 8a with molecular oxygen. Reducing the reaction time to 1 or 2 h (entries 5 and 6) led to the incomplete conversion of 8a. A longer reaction time than required for complete conversion (8 h, entry 7) did not lead to a decrease in the yield of 9a, which indicates that the product 9a is stable under the reaction conditions. The use of a two-fold excess or equimolar amount of t-BuOOH resulted in the incomplete conversion of 8a and low yields of 9a (entries 8 and 9). Increasing the excess of t-BuOOH over four equivalents did not lead to a further increase in the yield of 9a (entry 10). The decrease in the TiO2 loading had a negative effect on the yield of the target product 9a (entry 11). The increase in TiO2 dosage from 10 mg (standard conditions) to 20 mg (entry 12) led to slight decrease in 9a yield. Switching the solvent from MeCN to other polar solvents (DMSO, DMF, AcOH) led to lower yields of the target product (entries 13–15). 1,2-Dichloroethane (DCE) was less effective due to its immiscibility with water from aqueous t-BuOOH, leading to the aggregation of TiO2 particles in the water phase (entry 16). Another widely used visible-light active photocatalyst g-C3N4 can also promote the photochemical peroxidation of barbituric acid 8a, but much less efficiently than TiO2 (entry 17). At last, we studied the scalability of the developed procedure (entry 18). It turns out that scaling up to 1 mmol of 8a led to a slight increase in 9a yield; therefore, the conditions of entry 18 were chosen as optimal for the peroxidation of other barbituric acids. As one can note, the yields of peroxide 9a in many cases were significantly lower than conversions of the starting barbituric acid 8a. The main side product observed with NMR analysis of the reaction mixture was 5-hydroxylated barbituric acid 10a (5-benzyl-5-hydroxy-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione). The isolated yields of the side product 10a are not reported because it was not isolated in an analytically pure form in all cases.
In the next step, we extend our method to the synthesis of other 5-tert-butylperoxybarbituric acids 9 (Scheme 4).
The peroxidation of N-substituted barbituric acids proceeds in moderate yields (27–62%), with the main by-product being the 5-hydroxy derivative. It should be noted separately that in the case of the substrate with the furanyl substituent 8l, no product is formed, and the conversion of the initial substrate is insignificant. In the absence of substituents on both nitrogen atoms, the target reaction is not observed; in this case, the main product is the 5-hydroxy product (10t, 10u). In the next step, we tested some other classes of CH-reagents in peroxidation using the developed TBHP/TiO2 photocatalytic system. Selective formation of the peroxide 11 from the tetrahydroisoquinoline is quite unusual, because TBHP is frequently used as an oxidant in reactions of tetrahydroisoquinolines with different nucleophiles [85,86], and the peroxide intermediate is easily intercepted by the nucleophile present in the system or oxidized to isoquinolin-1(2H)-one [87,88]. Other CH-acidic substrates, such as Meldrum’s acids, oxindoles and cyclic β-ketoethers, demonstrate high stability to oxidation under reaction conditions, showing that our method is tolerant to easily oxidizable CH-acidic or enol groups.
To illustrate the substrate selectivity of the TBHP/TiO2 peroxidation system, we conducted a series of experiments with competitive CH-acidic substrates (β-ketoester 12, β-diketone 13 and malononitrile 14, Scheme 5). In all cases, we obtained the desired peroxidation product 9a with good yields (44–70%). With β-diketone 13, we observed the incomplete conversion of barbituric acid 8a, which led to a decrease in the 9a yield.
Radical trapping EPR experiments employing DMPO (5,5-Dimethyl-1-pyrroline N-oxide) as a radical acceptor were conducted to study the formation of t-BuOOH-derived radicals in the t-BuOOH-TiO2 system (see SI). It interesting to note that the radical formation in this system was observed even under dark conditions. However, the peroxidation reaction did not proceed without blue LED irradiation (Table 1, entry 2) and an increase in radical production was observed with EPR upon blue LED irradiation of the t-BuOOH-TiO2 mixture in MeCN. The simulation of the observed DMPO radical adduct EPR spectrum employing the EasySpin 5.2.35 program [89] resulted in the following hyperfine splitting values of the main observed signal: aN = 1.31 mT, aHβ = 1.04 mT and aHγ = 0.13 mT. The obtained values are in agreement with those reported for the t-BuO• radical adduct to DMPO [90,91]; however, unambiguous assignment of the signal is not possible because very close values can be expected for t-BuO• and t-BuOO• adducts [92] (see the SI for details). Based on the experimental data and the existing literature, we proposed the following mechanism (Scheme 6).
Tert-butylhydroperoxide decomposes on TiO2 under visible light irradiation either by single-electron oxidation, leading to a tert-butylperoxyl radical (t-BuOO•), or single-electron reduction, leading to a tert-butoxyl radical (t-BuO•). The short-lived tert-butoxyl radical transforms to t-BuOH via hydrogen atom abstraction from TBHP with the additional formation of a tert-butylperoxyl radical. Thus, t-BuOO• radicals are main oxygen-centered radical species in the reaction mixture. Reactive tert-butylperoxyl and tert-butoxyl radicals abstract a H atom from barbituric acid 8a with the formation of C-centered radical 15, which is intercepted by t-BuOO• to give the desired product 9a, stable under reaction conditions (see Table 2, entry 7). The C-centered radical 15 can be intercepted by molecular oxygen (derived from air or t-BuOOH decomposition) with the formation of hydroperoxyl radical 16, which undergoes hydrogen atom transfer (HAT) from the medium, forming unstable hydroperoxide 17. Hydroperoxide 17 decomposition leads to 5-hydroxy by-product 10a.

3. Materials and Methods

3.1. General

Room temperature (rt) stands for 23–25 °C. 1H and 13C NMR spectra were recorded on Bruker (Bruker AXS Handheld Inc., Kennewick, WA, USA) AVANCE II 300 and Bruker Fourier 300HD (300.13 and 75.47 MHz, respectively) spectrometers in CDCl3 and DMSO-D6. Residual signals of CDCl3 (7.26 in 1H NMR, 77.16 in 13C NMR) were used as reference signals for precise chemical shift determination. FT-IR spectra were recorded on Bruker Alpha instrument. IR spectra were registered in KBr pellets for solid compounds, and liquid compounds were placed between two KBr windows to make a thin layer. High-resolution mass spectra (HR-MS) were measured on a Bruker maXis instrument using electrospray ionization (ESI). The measurements were performed in a positive ion mode (interface capillary voltage—4500 V); mass range from m/z 50 to m/z 3000 Da; external calibration with Electrospray Calibrant Solution (Fluka). A syringe injection was used for all acetonitrile solutions (flow rate 3 µL/min). Nitrogen was applied as a dry gas; interface temperature was set at 180 °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. Tert-butyl peroxide (TBHP, 70% aqueous), cumyl hydroperoxide (CHP, 80%), dibenzoyl peroxide (75%), meta-chloroperoxybenzoic acid (mCPBA, 75%), di-tert-butylperoxide (DTBP, 99%) and 1,1,2,2-tetrachloroethane (98.5%) were used as is from commercial sources. Barbituric acids 1a–1x were synthesized according to the literature procedures. Bulk g-C3N4 was prepared analogously to previously reported methods [93,94]: the urea was heated in a covered alumina crucible for 4 h at 550 °C (heating rate 5 °C·min−1). Dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), acetic acid (AcOH) and 1,2-dichloroethane (DCE) were used as is from commercial sources. MeCN was distilled over P2O5. The reaction mixtures were sonicated in an ultrasonic bath (HF-Frequency 35 kHz, ultrasonic nominal power 80 W) before the irradiation.

3.2. Experimental Procedures

Experimental details for Table 1
Peroxide (1 mmol), TiO2 (10 mg) and a solvent (MeCN, 2 mL) were placed in a 50 mL round-bottomed flask. The reaction mixture was sonicated for 2 min in an ultrasonic bath, then magnetically stirred in a thermostated water bath at 25 °C (±1 °C) under irradiation of 10 W blue (443 nm) LED for 3 h. Then, the C2H2Cl4 internal standard (40–60 mg) was added, and the reaction the mixture filtered through Celite. Yields and conversions were determined with 1H from the MeCN solutions using C2H2Cl4 as the internal standard.
Experimental details for Scheme 3
1,1-diphehylethylene 3 (1 mmol, 180 mg) or 2,6-di-tert-butyl-4-methylphenol 6 (BHT, 1 mmol, 220 mg), TiO2 (20 mg), t-BuOOH 70% aq. (4 mmol, 515 mg) and a solvent (MeCN, 2 mL) were placed in a round-bottomed flask. The reaction mixture was vacuumed and flushed with argon 3 times. The reaction mixture was sonicated for 2 min in an ultrasonic bath, then magnetically stirred in a thermostated water bath at 25 °C (±1 °C) under irradiation of 10 W blue LED for 8 h. After reaction, the mixture was diluted with CH2Cl2 (10 mL) and poured into the water (20 mL). The layers were shaken, and the organic layer was separated. Water layer was extracted with CH2Cl2 (2 × 10 mL). Combined organic extracts were washed with brine (20 mL), dried over MgSO4 and rotary-evaporated. Products 4 and 7 were isolated via column chromatography using Petroleum ether/EtOAc as eluent.
Experimental details for Table 2
Barbituric acid 8a (0.5 mmol, 124 mg), TiO2 (0–10 mg), t-BuOOH 70% aq. (0.5–2 mmol, 64–257 mg) and a solvent (1 mL) were placed in a round-bottomed flask. The reaction mixture was vacuumed and flushed with argon 3 times. The reaction mixture was sonicated for 2 min in an ultrasonic bath, then magnetically stirred in a thermostated water bath at 25 °C (±1 °C) under irradiation of 10 W blue LED for 1–8 h. Then, the mixture was diluted with CH2Cl2 (10 mL) and poured into the water (20 mL). The layers were shaken, and the organic layer was separated. Water layer was extracted with CH2Cl2 (2 × 10 mL). Combined organic extracts were washed with brine (20 mL), dried over MgSO4 and rotary-evaporated. Product 9a was isolated via column chromatography using Petroleum ether/EtOAc = 1/5 as eluent.
Experimental details for Scheme 4
Barbituric acid 8 (1 mmol), TiO2 (20 mg), t-BuOOH 70% aq. (4 mmol, 515 mg) and a solvent (MeCN, 2 mL) were placed in a round-bottomed flask. The reaction mixture was vacuumed and flushed with argon 3 times. The reaction mixture was sonicated for 2 min in an ultrasonic bath, then magnetically stirred in a thermostated water bath at 25 °C (±1 °C) under irradiation of 10 W blue LED for 5 h. After reaction, the mixture was diluted with CH2Cl2 (10 mL) and poured into the water (20 mL). The layers were shaken, and the organic layer was separated. Water layer was extracted with CH2Cl2 (2 × 10 mL). Combined organic extracts were washed with brine (20 mL), dried over MgSO4 and rotary-evaporated. Products 9a–10u were isolated via column chromatography using Petroleum ether/EtOAc as eluent.
Experimental details for Scheme 5
Barbituric acid 8a (0.5 mmol, 124 mg), competitive CH-substrate (ethyl 2-methyl-3-oxobutanoate, 12, 0.5 mmol, 72 mg or 3-benzylpentane-2,4-dione, 13, 0.5 mmol, 95 mg or 2-benzylmalononitrile 14, 0.5 mmol, 78 mg), TiO2 (10 mg), t-BuOOH 70% aq. (2 mmol, 257 mg) and MeCN (2 mL) were placed in a round-bottomed flask. The reaction mixture was vacuumed and flushed with argon 3 times. The reaction mixture was sonicated for 2 min in an ultrasonic bath, then magnetically stirred in a thermostated water bath at 25 °C (±1 °C) under irradiation of 10 W blue LED for 5 h. Then, the reaction the mixture was diluted with CH2Cl2 (10 mL) and poured into the water (20 mL). The layers were shaken, and the organic layer was separated. Water layer was extracted with CH2Cl2 (2 × 10 mL). Combined organic extracts were washed with brine (20 mL), dried over MgSO4 and rotary-evaporated. Product 9a was isolated via column chromatography using Petroleum ether/EtOAc = 1/5 as eluent.

3.3. Characterization Data of the Synthesized Products

(1,2-Bis(tert-butylperoxy)ethane-1,1-diyl)dibenzene 4 [95] was isolated via column chromatography (EtOAc/petroleum ether = 1/40, Rf = 0.4) as a colorless liquid (30 mg, 8%). 1H NMR (300 MHz, Chloroform-d) δ 7.43–7.37 (m, 4H), 7.36–7.27 (m, 6H), 4.84 (s, 2H), 1.24 (s, 9H), 1.12 (s, 9H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 142.2, 127.9, 127.7, 127.5, 85.8, 80.6, 79.8, 77.8, 26.8, 26.4.
2,6-Di-tert-butyl-4-(tert-butylperoxy)-4-methylcyclohexa-2,5-dien-1-one 7 was isolated via column chromatography (EtOAc/petroleum ether = 1/50, Rf = 0.9) as yellow powder (43 mg, 14%). Mp = 84–85 °C (lit. Mp = 88–90 °C [53]). 1H NMR (300 MHz, Chloroform-d) δ 6.55 (s, 2H), 1.31 (s, 3H), 1.21 (s, 18H), 1.17 (s, 9H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 186.8, 146.8, 142.0, 79.5, 77.6, 76.3, 34.8, 29.6, 26.6, 24.4. HR-MS (ESI): m/z = 331.2245, calcd. for C19H32O3 + Na+: 331.2244.
5-Benzyl-5-(tert-butylperoxy)-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione 9a was isolated via column chromatography (EtOAc/petroleum ether = 1/5, Rf = 0.6) as a white powder (187 mg, 56%). Mp = 93.0−94.0 °C (lit. Mp = 95 °C [53]). 1H NMR (300 MHz, Chloroform-d) δ 7.25–7.18 (m, 3H), 7.03–6.95 (m, 2H), 3.26 (s, 2H), 3.10 (s, 6H), 1.23 (s, 9H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 168.3, 150.2, 130.9, 129.6, 128.9, 128.4, 84.1, 82.4, 41.9, 28.6, 26.5. HR-MS (ESI): m/z = 357.1416, calcd. for C17H22N2O5 + Na+: 357.1421.
5-(Tert-butylperoxy)-5-(4-fluorobenzyl)-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione 9b was isolated via column chromatography (EtOAc/petroleum ether = 1/5, Rf = 0.5) as a white powder (200 mg, 57%). Mp = 102−103 °C (lit. Mp = 102−105 °C [53]. 1H NMR (300 MHz, Chloroform-d) δ 7.07–6.84 (m, 4H), 3.25 (s, 2H), 3.13 (s, 6H), 1.21 (s, 9H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 168.2, 162.7 (d, J = 247.9 Hz), 150.2, 131.4 (d, J = 8.2 Hz), 126.8 (d, J = 3.6 Hz), 115.9 (d, J = 21.4 Hz), 83.8, 82.4, 40.7, 28.6, 26.5. HR-MS (ESI): m/z = 370.1771, calcd. for C17H21FN2O5 + NH4+: 370.1773.
5-(Tert-butylperoxy)-5-(4-chlorobenzyl)-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione 9c was isolated via column chromatography (EtOAc/petroleum ether = 1/6, Rf = 0.5) as a white powder (215 mg, 58%). Mp = 115–116 °C. 1H NMR (300 MHz, Chloroform-d) δ 7.23–7.15 (m, 2H), 7.00–6.92 (m, 2H), 3.26 (s, 2H), 3.15 (s, 6H), 1.21 (s, 9H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 168.1, 150.2, 134.5, 131.2, 129.6, 129.1, 83.6, 82.5, 40.7, 28.7, 26.5. FTIR (KBr): νmax = 2980, 2937, 1705, 1686, 1491, 1445, 1429, 1380, 1367, 1305, 1294, 1231, 1198, 1178, 1116, 1092, 1068, 1031, 1016, 870, 845, 805, 752, 538, 501, 409 cm−1. HR-MS (ESI): m/z = 391.1035, calcd. for C17H21ClN2O5 + Na+: 391.1031.
5-(Tert-butylperoxy)-5-(4-methoxybenzyl)-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione 9d was isolated via column chromatography (EtOAc/petroleum ether = 1/10, Rf = 0.3) as a white powder (103 mg, 28%). Mp = 120−121 °C (lit. Mp = 122−124 °C) [53]. 1H NMR (300 MHz, Chloroform-d) δ 6.93–6.87 (m, 2H), 6.76–6.70 (m, 2H), 3.73 (s, 3H), 3.21 (s, 2H), 3.12 (s, 6H), 1.22 (s, 9H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 168.4, 159.6, 150.3, 130.8, 122.6, 114.2, 84.1, 82.3, 55.3, 41.0, 28.6, 26.5. HR-MS (ESI): m/z = 382.1974, calcd. for C18H24N2O6 + NH4+: 382.1973.
5-(Tert-butylperoxy)-5-(4-isopropylbenzyl)-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione 9e was isolated via column chromatography (EtOAc/petroleum ether = 1/10, Rf = 0.3) as a viscous liquid (206 mg, 55%) [53]. 1H NMR (300 MHz, Chloroform-d) δ 7.09–7.00 (m, 2H), 6.93–6.82 (m, 2H), 3.20 (s, 2H), 3.07 (s, 6H), 2.79 (hept, J = 6.9 Hz, 1H), 1.20 (s, 9H), 1.14 (d, J = 6.9 Hz, 6H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 168.3, 150.1, 149.1, 129.5, 128.0, 126.7, 84.2, 82.1, 41.6, 33.7, 28.4, 26.4, 23.9. HR-MS (ESI): m/z = 394.2328, calcd. for C20H28N2O5 + NH4+: 394.2336.
5-(Tert-butylperoxy)-1,3-dimethyl-5-(2-methylbenzyl)pyrimidine-2,4,6(1H,3H,5H)-trione 9f [53] was isolated via column chromatography (EtOAc/petroleum ether = 1/10, Rf = 0.4) as a colorless oil (162 mg, 47%). 1H NMR (300 MHz, Chloroform-d) δ 7.12–6.92 (m, 3H), 6.88–6.75 (m, 1H), 3.26 (s, 2H), 3.06 (s, 6H), 2.15 (s, 3H), 1.18 (s, 9H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 168.2, 150.2, 137.5, 131.1, 130.1, 129.2, 128.3, 125.9, 83.9, 82.2, 38.6, 28.7, 26.5, 19.4. HR-MS (ESI): m/z = 366.2020, calcd. for C18H24N2O5 + NH4+: 366.2023.
5-(Tert-butylperoxy)-1,3-dimethyl-5-(2-nitrobenzyl)pyrimidine-2,4,6(1H,3H,5H)-trione 9g was isolated via column chromatography (EtOAc/petroleum ether = 1/5, Rf = 0.3) as a white powder (116 mg, 30%). Mp = 139–141 °C (lit. Mp = 139–140 °C) [53]. 1H NMR (300 MHz, Chloroform-d) δ 7.98 (d, J = 8.1 Hz, 1H), 7.58 (t, J = 7.5 Hz, 1H), 7.51–7.38 (m, 2H), 3.66 (s, 2H), 3.28 (s, 6H), 1.17 (s, 9H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 167.3, 150.7, 149.5, 134.4, 132.9, 129.2, 128.2, 125.2, 82.6, 81.7, 38.6, 29.1, 26.5. HR-MS (ESI): m/z = 397.1716, calcd. for C17H21N3O7 + NH4+: 397.1718.
5-(Tert-butylperoxy)-1,3-dimethyl-5-(3-nitrobenzyl)pyrimidine-2,4,6(1H,3H,5H)-trione 9h was isolated via column chromatography (EtOAc/petroleum ether = 1/5, Rf = 0.7) as a colorless oil (100 mg, 26%). 1H NMR (300 MHz, Chloroform-d) δ 8.14–8.09 (m, 1H), 7.94–7.91 (m, 1H), 7.49–7.36 (m, 2H), 3.40 (s, 2H), 3.17 (s, 6H), 1.21 (s, 9H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 167.7, 150.1, 148.4, 136.1, 133.5, 129.9, 124.7, 123.4, 83.0, 82.8, 40.6, 28.8, 26.4. HR-MS (ESI): m/z = 402.1273, calcd. for C17H21N3O7 + Na+: 402.1272.
5-(Tert-butylperoxy)-5-(2,4-dichlorobenzyl)-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione 9i was isolated via column chromatography (EtOAc/petroleum ether = 1/10, Rf = 0.4) as a white powder (221 mg, 55%). Mp = 101–102 °C (lit. Mp = 101–103 °C) [53]. 1H NMR (300 MHz, Chloroform-d) δ 7.32 (d, J = 1.9 Hz, 1H), 7.16 (dd, J = 8.3, 2.0 Hz, 1H), 7.11 (d, J = 8.3 Hz, 1H), 3.37 (s, 2H), 3.21 (s, 7H), 1.21 (s, 9H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 167.2, 150.5, 135.6, 134.9, 133.4, 129.6, 128.6, 127.2, 82.5, 38.6, 59.0, 26.5. HR-MS (ESI): m/z = 420.1077, calcd. for C17H20Cl2N2O5 + NH4+: 420.1088.
5-(Tert-butylperoxy)-1,3-dimethyl-5-(naphthalen-1-ylmethyl)pyrimidine-2,4,6(1H,3H,5H)-trione 9j was isolated via column chromatography (EtOAc/petroleum ether = 1/5, Rf = 0.6) as slightly yellow crystals (105 mg, 27%). Mp = 92–93 °C (lit. Mp = 92–93 °C) [53]. 1H NMR (300 MHz, Chloroform-d) δ 7.96–7.88 (m, 1H), 7.84–7.77 (m, 1H), 7.77–7.72 (m, 1H), 7.55–7.41 (m, 2H), 7.37–7.28 (m, 1H), 7.23–7.17 (m, 1H), 3.75 (s, 2H), 2.81 (s, 6H), 1.29 (s, 9H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 168.2, 149.9, 133.8, 131.9, 129.3, 129.0, 128.7, 127.2, 126.5, 126.1, 124.9, 123.6, 84.2, 82.3, 38.5, 28.5, 26.6. HR-MS (ESI): m/z = 402.2016, calcd. for C21H24N2O5 + NH4+: 402.2023.
5-(Tert-butylperoxy)-1,3-dimethyl-5-(thiophen-2-ylmethyl)pyrimidine-2,4,6(1H,3H,5H)-trione 9k was isolated via column chromatography (EtOAc/petroleum ether = 1/5, Rf = 0.7) as a white powder (177 mg, 52%). Mp = 84–85 °C (lit. Mp = 86–88 °C) [53]. 1H NMR (300 MHz, Chloroform-d) δ 7.19–7.11 (m, 1H), 6.91–6.84 (m, 1H), 6.75 (d, J = 3.5 Hz, 1H), 3.53 (s, 2H), 3.20 (s, 6H), 1.21 (s, 9H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 168.2, 150.5, 132.0, 128.6, 127.4, 126.4, 83.7, 82.5, 35.5, 28.8, 26.5. HR-MS (ESI): m/z = 358.1429, calcd. for C15H20N2O5S + NH4+: 358.1431.
5-(Tert-butylperoxy)-1,3-dimethyl-5-phenylpyrimidine-2,4,6(1H,3H,5H)-trione 9m was isolated via column chromatography (EtOAc/petroleum ether = 1/4, Rf = 0.5) as a white powder (113 mg, 35%). Mp = 112–113 °C. 1H NMR (300 MHz, Chloroform-d) δ 7.37 (m, 5H), 3.40 (s, 6H), 1.32 (s, 9H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 167.7, 150.9, 130.4, 129.1, 126.7, 82.7, 29.3, 26.6. FTIR (KBr): νmax = 2983, 1695, 1441, 1423, 1375, 1291, 1194, 1129, 1064, 1029, 867, 756, 717, 691, 634 cm−1. HR-MS (ESI): m/z = 343.1271, calcd. for C16H20N2O5 + Na+: 343.1264.
5-(Tert-butylperoxy)-1,3,5-trimethylpyrimidine-2,4,6(1H,3H,5H)-trione 9n was isolated via column chromatography (EtOAc/petroleum ether = 1/5, Rf = 0.5) as a white powder (144 mg, 56%). Mp = 118–119 °C. 1H NMR (300 MHz, Chloroform-d) δ 3.34 (s, 6H), 1.60 (s, 3H), 1.18 (s, 9H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 168.9, 151.0, 82.2, 79.3, 29.0, 26.4, 21.4. FTIR (KBr): νmax = 2983, 1761, 1711, 1677, 1466, 1445, 1418, 1381, 1291, 1195, 1114, 1071, 870, 753 cm−1. HR-MS (ESI): m/z = 281.1111, calcd. for C11H18N2O5 + Na+: 281.1108.
5-Butyl-5-(tert-butylperoxy)-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione 9o [53] was isolated via column chromatography (EtOAc/petroleum ether = 1/5, Rf = 0.7) as a colorless liquid (162 mg, 59%). 1H NMR (300 MHz, Chloroform-d) δ 3.29 (s, 6H), 1.97–1.89 (m, 2H), 1.27–1.17 (m, 2H), 1.11 (s, 9H), 1.07–0.96 (m, 2H), 0.78 (t, J = 7.3 Hz, 3H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 168.6, 150.9, 82.8, 81.9, 35.1, 28.7, 26.3, 24.9, 22.5, 13.6. HR-MS (ESI): m/z = 232.1578, calcd. for C14H24N2O5 + Na+: 323.1577.
5-(Tert-butylperoxy)-5-hexyl-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione 9p [51] was isolated via column chromatography (EtOAc/petroleum ether = 1/7, Rf = 0.6) as a colorless liquid (152 mg, 44%). 1H NMR (300 MHz, Chloroform-d) δ 3.31 (s, 6H), 2.00–1.89 (m, 2H), 1.27–1.16 (m, 6H), 1.14 (s, 9H), 1.12–1.00 (m, 2H), 0.86–0.75 (m, 3H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 168.7, 151.0, 82.9, 82.0, 35.4, 31.3, 29.0, 28.8, 26.4, 22.9, 22.5, 14.0. HR-MS (ESI): m/z = 346.2332, calcd. for C16H28N2O5 + NH4+: 346.2336.
5-Benzyl-5-(tert-butylperoxy)-1-methylpyrimidine-2,4,6(1H,3H,5H)-trione 9r was isolated via column chromatography (EtOAc/petroleum ether = 1/5, Rf = 0.3) as a white powder (172 mg, 54%). Mp = 130–132 °C (lit. Mp = 132–133 °C, dec.) [53]. 1H NMR (300 MHz, Chloroform-d) δ 8.22 (s, 1H), 7.27–7.15 (m, 3H), 7.09–6.97 (m, 2H), 3.28 (s, 2H), 3.07 (s, 3H), 1.21 (s, 9H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 169.0, 167.6, 149.0, 130.6, 130.0, 129.0, 128.5, 84.1, 82.6, 41.1, 28.0, 26.5. HR-MS (ESI): m/z = 343.1266, calcd. for C16H20N2O5 + Na+: 343.1264.
5-(Tert-butylperoxy)-5-(4-methoxybenzyl)-1-methylpyrimidine-2,4,6(1H,3H,5H)-trione 9s was isolated via column chromatography (EtOAc/petroleum ether = 1/4, Rf = 0.3) as a white powder (230 mg, 62%). Mp = 137–138 °C. 1H NMR (300 MHz, Chloroform-d) δ 8.92 (s, 1H), 6.95 (d, J = 8.6 Hz, 2H), 6.72 (d, J = 8.7 Hz, 2H), 3.70 (s, 3H), 3.22 (s, 2H), 3.08 (s, 3H), 1.20 (s, 9H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 169.1, 168.0, 159.5, 149.3, 131.0, 122.2, 114.3, 84.0, 82.4, 55.2, 40.2, 27.9, 26.4. FTIR (KBr): νmax = 3473, 3415, 3244, 2980, 1763, 1724, 1697, 1613, 1514, 1447, 1386, 1368, 1305, 1253, 1183, 1073, 1031, 870, 844, 820, 793, 532 cm−1. HR-MS (ESI): m/z = 373.1380, calcd. for C17H22N2O6 + Na+: 373.1370.
5-Benzyl-5-hydroxy-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione 10a was isolated via column chromatography (EtOAc/petroleum ether = 1/5, Rf = 0.1) as a white powder. Mp = 112–114 °C (lit. Mp = 113–114 °C [96]). 1H NMR (300 MHz, Chloroform-d) δ 7.28–7.16 (m, 3H), 6.96–6.86 (m, 2H), 3.75 (bs, 1H), 3.21 (s, 2H), 3.06 (s, 6H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 169.9, 149.9, 132.0, 129.3, 128.8, 128.6, 77.1, 50.0, 28.6. HR-MS (ESI): m/z = 285.0844, calcd. for C13H14N2O4 + Na+: 285.0846.
5-butyl-5-hydroxypyrimidine-2,4,6(1H,3H,5H)-trione 10t was isolated via column chromatography (EtOAc/DCM = 1/1, Rf = 0.4) as a white powder (141 mg, 52%). Mp = 175–177 °C. 1H NMR (300 MHz, DMSO-d6) δ 11.26 (s, 2H), 6.00 (s, 1H), 1.88–1.64 (m, 1H), 1.40–1.08 (m, 2H), 0.81 (t, J = 6.8 Hz, 1H).13C{1H}NMR (75.48 MHz, CDCl3) δ 172.2, 150.0, 74.8, 39.0, 24.8, 22.0, 13.8. FTIR (KBr): νmax = 3396, 3301, 3219, 3101, 2961, 2873, 1759, 1708, 1436, 1419, 1405, 1375, 1320, 1266, 1246, 1221, 1167, 1124, 1098, 822, 762, 528, 500 cm−1. HR-MS (ESI): m/z = 223.0688, calcd. for C8H12N2O5 + Na+: 223.0689.
5-Hexyl-5-hydroxypyrimidine-2,4,6(1H,3H,5H)-trione 10u was isolated via column chromatography (EtOAc/DCM = 1/1, Rf = 0.5) as a white powder (84 mg, 35%). Mp = 128–129 °C. 1H NMR (300 MHz, DMSO-d6) δ 11.24 (s, 2H), 5.98 (s, 1H), 1.81–1.72 (m, 2H), 1.28–1.14 (m, 8H), 0.83 (t, J = 6.3 Hz, 3H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 172.0, 149.9, 74.7, 39.2, 30.9, 28.4, 22.5, 21.9, 13.8. FTIR (KBr): νmax = 3483, 3219, 3073, 2959, 2922, 2855, 1740, 1698, 1436, 1368, 1316, 1239, 1223, 1208, 1164, 1094, 796, 500 cm−1. HR-MS (ESI): m/z = 251.1003, calcd. for C10H14N2O5 + Na+: 251.1002.
1-(Tert-butylperoxy)-2-phenyl-1,2,3,4-tetrahydroisoquinoline 11 [97] was isolated via column chromatography (EtOAc/petroleum ether = 1/7, Rf = 0.8) as slightly yellow crystals (163 mg, 55%). Mp = 60–61 °C. 1H NMR (300 MHz, Chloroform-d) δ 7.42–7.36 (m, 1H), 7.34–7.18 (m, 5H), 7.18–7.13 (m, 2H), 6.90–6.82 (m, 1H), 6.21 (s, 1H), 3.81–3.67 (m, 1H), 3.64–3.50 (m, 1H), 3.17–2.90 (m, 2H), 1.15 (s, 9H). 13C{1H}NMR (75.48 MHz, CDCl3) δ 149.0, 136.7, 133.1, 129.2, 129.1, 128.7, 126.1, 119.0, 115.0, 90.8, 80.1, 42.7, 28.3, 26.7. HR-MS (ESI): m/z = 331.2245, calcd. for C19H32O3 + Na+: 331.2244.

4. Conclusions

It was found that commercial unmodified TiO2 can catalyze the decomposition of organic hydroperoxides (e.g., tert-butylhydroperoxide, cumyl hydroperoxide) under visible light irradiation (443 nm). The t-BuOOH/TiO2 photocatalytic system can be used for the generation of tert-butylperoxyl radicals under mild conditions. The synthetic application of the t-BuOOH/TiO2 system was demonstrated by the peroxidation of barbituric acids. The peroxidation is highly chemoselective and can proceed in the presence of weak benzylic CH-bonds or CH-acidic substrates such as Meldrum’s acids, β-ketoesters, β-diketones and malononitriles.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13091306/s1, Figure S1: EPR spectra of spin trapping experiments under air. Black lines—EPR spectra before irradiation, Figure S2: EPR spectra of spin trapping experiments under argon atmosphere, Figure S3: Experimental (black) and simulated (red) EPR spectra for mixtures in MeCN, Table S1: Simulation parameters for spectra presented in Figure S3 in comparison with literature data, Figure S4: Appearance of TiO2 suspension in MeCN in the presence (left) and absence (right) of t-BuOOH, Figure S5: UV-Vis spectrum of 10W Blue LED used for photochemical syntheses in the present study (λmax = 443 nm), Spectral data of the synthesized compounds.

Author Contributions

Conceptualization, I.B.K.; methodology, I.B.K. and E.R.L.; investigation, E.R.L.; 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, A.O.T. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Russian Science Foundation (Grant No. 21-43-04417).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 13 01306 sch001
Scheme 1. Organic peroxides in TiO2-photocatalyzed oxidative transformations. Previous works on benzylic alcohol oxidation [47], oxidative coupling of alcohols with N-hydroxyphthalimide [25], and cross-dehydrogenative coupling of ethers with electron-deficient heteroarenes [38] (a) and present work (b).
Scheme 1. Organic peroxides in TiO2-photocatalyzed oxidative transformations. Previous works on benzylic alcohol oxidation [47], oxidative coupling of alcohols with N-hydroxyphthalimide [25], and cross-dehydrogenative coupling of ethers with electron-deficient heteroarenes [38] (a) and present work (b).
Catalysts 13 01306 sch001
Catalysts 13 01306 sch002
Scheme 2. Plausible schematic mechanism of TiO2-photocatalyzed decay of organic hydroperoxides.
Scheme 2. Plausible schematic mechanism of TiO2-photocatalyzed decay of organic hydroperoxides.
Catalysts 13 01306 sch002
Catalysts 13 01306 sch003
Scheme 3. Tert-butylperoxyl radical trapping (isolated yields are given. NMR yields are given in parenthesis).
Scheme 3. Tert-butylperoxyl radical trapping (isolated yields are given. NMR yields are given in parenthesis).
Catalysts 13 01306 sch003
Catalysts 13 01306 sch004
Scheme 4. Scope of substrates suitable for the peroxidation in a TBHP/TiO2 system under visible light.
Scheme 4. Scope of substrates suitable for the peroxidation in a TBHP/TiO2 system under visible light.
Catalysts 13 01306 sch004
Catalysts 13 01306 sch005
Scheme 5. Experiments with competitive CH-acidic substrates.
Scheme 5. Experiments with competitive CH-acidic substrates.
Catalysts 13 01306 sch005
Catalysts 13 01306 sch006
Scheme 6. Plausible mechanism.
Scheme 6. Plausible mechanism.
Catalysts 13 01306 sch006
Table 1. Photodecomposition of organic peroxides on TiO2 under blue light irradiation.
Table 1. Photodecomposition of organic peroxides on TiO2 under blue light irradiation.
Catalysts 13 01306 i001
EntryPeroxideConversion, %Yield a 2, %
1t-BuOOH8158
2 bt-BuOOH0nd
3 cPhC(OOH)(CH3)2 (80%)6331
4BzOOBz (75%)<5trace
5mCPBA (75%)4040
6t-BuOOt-Bu<5nd
a Yields were determined with 1H NMR using C2H2Cl4 as internal standard, mixture composition was additionally confirmed with 13C NMR. b Dark conditions. “nd” stands for not detected by 1H NMR. c 11% of acetophenone was formed.
Table 2. Optimization of amount of t-BuOOH, solvent and time for the peroxidation of 8a.
Table 2. Optimization of amount of t-BuOOH, solvent and time for the peroxidation of 8a.
Catalysts 13 01306 i002
№Changes to the General ConditionsConversion, %Yield a 9a, %
1none10054
2no light0nd
3no TiO20nd
4air atmosphere10037
5reaction time 1 h7938
6reaction time 2 h9440
7reaction time 8 h10055
81 equiv. of t-BuOOH7024
92 equiv. of t-BuOOH8135
106 equiv. of t-BuOOH10050
115 mg of TiO28431
1220 mg of TiO210046
13DMSO as a solvent597
14DMF as a solvent10014
15AcOH as a solvent10032
16DCE as a solvent7829
17g-C3N4 instead of TiO25623
18scaled up to 1 mmol of 8a10056
a Isolated yields are given. “nd” stands for not detected by 1H NMR.
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Lopat’eva, E.R.; Krylov, I.B.; Terent’ev, A.O. t-BuOOH/TiO2 Photocatalytic System as a Convenient Peroxyl Radical Source at Room Temperature under Visible Light and Its Application for the CH-Peroxidation of Barbituric Acids. Catalysts 2023, 13, 1306. https://doi.org/10.3390/catal13091306

AMA Style

Lopat’eva ER, Krylov IB, Terent’ev AO. t-BuOOH/TiO2 Photocatalytic System as a Convenient Peroxyl Radical Source at Room Temperature under Visible Light and Its Application for the CH-Peroxidation of Barbituric Acids. Catalysts. 2023; 13(9):1306. https://doi.org/10.3390/catal13091306

Chicago/Turabian Style

Lopat’eva, Elena R., Igor B. Krylov, and Alexander O. Terent’ev. 2023. "t-BuOOH/TiO2 Photocatalytic System as a Convenient Peroxyl Radical Source at Room Temperature under Visible Light and Its Application for the CH-Peroxidation of Barbituric Acids" Catalysts 13, no. 9: 1306. https://doi.org/10.3390/catal13091306

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