g-C3N4 - Singlet Oxygen Made-Easy for Organic Synthesis; Scope

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g-C3N4 - Singlet Oxygen Made-Easy for Organic Synthesis; Scope and Limitations Irene Camussi, Barbara Mannucci, Andrea Speltini, Antonella Profumo, Chiara Milanese, Lorenzo Malavasi, and Paolo Quadrelli ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06164 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 7, 2019

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ACS Sustainable Chemistry & Engineering

g-C3N4 - Singlet Oxygen Made-Easy for Organic Synthesis; Scope and Limitations Irene Camussi,a Barbara Mannucci,a Andrea Speltini,b Antonella Profumo,a Chiara Milanese,a Lorenzo Malavasi,a* Paolo Quadrellia* a. Dipartimento di Chimica, Università degli Studi di Pavia, Viale Taramelli 12, 27100 – Pavia (Italy) b. Dipartimento di Scienze del Farmaco, Università degli Studi di Pavia, Viale Taramelli 14, 27100 – Pavia (Italy) E-mail: [email protected], [email protected] Abstract. Oxidized g-C3N4 displays very good catalytic performances in the generation of 1O2 under photochemical conditions. The hetero Diels-Alder and ene reactions were conducted on a variety of dienes and alkene showing a general ability of oxidized g-C3N4 to promote chemoselective and unselective oxidative processes in strong dependence from the substrates. The results offer a good panorama of the ability of this type of catalysts to be employed in organic reactions to prepare valuable synthons that can be used in several value-added preparations. A fine-tuning of the oxidative properties through further investigations on the synthesis of suitably modified g-C3N4 will be of great importance in determining a sustainable change in the approach to 1O2 generation methods, allowing for greener ways to perform organic reactions.

Keywords. g-C3N4, Singlet oxygen, Diels-Alder cycloadditions, Ene reactions, Sustainable oxidation reactions.

Introduction

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Singlet oxygen (1O2) is a highly reactive species photochemically produced by the use of organic molecules (sensitizers) in charge of absorbing ultraviolet or visible light and, after a singlet-to-triplet intersystem crossing, transferring its energy to the triplet ground state of molecular oxygen.1 Besides the classical hydrogen peroxide decomposition promoted by hypochlorite or hypobromite and ozonides decomposition (Scheme 1, path a), porphyrin (P), rose bengal (RB), and methylene blue (MB) are the most common sensitizers (Sens) in homogeneous solutions preparation methods (path b). Moreover, 1O2 can also be obtained by thermal decomposition of unstable molecules (path c), such as arene endoperoxides (path d), by photolysis of oxone (path e).2

Scheme 1. 1O2 generation methods. Sensitizers: P, Porphyrin; RB, Rose Bengal; MB, Methylene Blue.


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The synthetic applications of 1O2 are also well known and the investigations on this topic continue to attract the attention of many research groups. 1O2 mostly targets olefins, dienes and sulfides producing the corresponding dioxetane or allylic hydroperoxides, endoperoxides or sulfoxides.3 These processes have been thoroughly investigated from a mechanistic point of view, also expanding the field to 1O2 reaction with heterocyclic compounds. Indeed, among heterocycles, furans have a particular tendency to react with 1O2 to produce a variety of products that can be used as an easily accessible library of building blocks, for instance to exploit chemical diversity in drug discovery.4 Usually, butenolide formation from the reaction of furans with 1O2 is a key step in the synthesis of several natural products and has been also investigated in ionic liquids.5 Depending on the 1O2 demanding applications, new methods of producing singlet oxygen are required since classical sensitization is not always feasible in each medium (e.g. living cells or tissues, natural environment, etc.) or when access to specific wavelength irradiation is limited. Ideally, methods should aim at producing singlet oxygen in a reproducible fashion, without being affected by surrounding conditions. Photoproduction of 1O2 could be implemented in the absence of a sensitizer by irradiation with visible light in halogenated solvents,6 taking advantage of the heavy atom effect which makes the triplet-to-singlet transition “less forbidden”. A typical issue is represented by selfquenching of the photosensitizer at higher concentrations, limiting the possibility to increase the efficiency of 1O2 production. The latter could be affected also by the physical and chemical quenching of 1O2 by the sensitizer itself, an effect that could take place also at low sensitizer concentration and must be taken into account when reporting quantum yields of 1O2 production from new dyes.7 An interesting approach has been introduced with the design of aggregation-induced sensitizers that take advantage of conformational changes of biarylic systems that upon aggregation are able to sensitize the production of 1O2.8 Other systems are based on supported or encapsulated dyes but, in this case the problem of partial dye leakage must be considered. The recent silica-supported BODIPY sensitizers have represented a potential solution to this issue, also taking advantage of the introduction of iodine substituents, which increased the yields of 1O2 production.9


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Nevertheless, novel materials displayed unexpected potentialities and recent studies showed their ability to act as catalysts and photocatalysts. This is the case of graphitic carbon nitride (g-C3N4), a polymeric semiconductor consisting of carbon and nitrogen atoms, prepared by bulk and hard template pyrolysis of melamine (MLM), or alternatively from other carbon/nitrogen containing sources (e.g. urea). The broad possibilities for its functionalization, with the aim to control its reactivity, make g-C3N4 as one of the most promising materials for the development of new ecologically-friendly reactions. An interest in this material has been growing steadily in the recent years.10 Moreover, g-C3N4 is the most stable allotropic form of carbon nitride whose large interest is due to its unique electronic properties as well as catalytic and photocatalytic activity.11 In this context, g-C3N4 has been studied as visible-light-driven catalyst for CO2 reduction, selective organic reactions, abatement of pollutants, and H2 production from water.12 In the field of photocatalysis, its great potentiality mainly relies on its band gap (~ 2.7 eV) smaller compared to TiO2, the most famed inorganic photocatalyst. This indicates that light harvesting, the first fundamental step in a photocatalytic process that generates charge carriers, is possible also under visible light. The possibility to be exfoliated into 2D thin nanosheets provides an improvement of the overall photocatalytic performance because of the more efficient exposure of active sites (due to surface area increase), higher light absorption and charge carriers separation.13-15 Compared to TiO2, the more negative conduction band edge stands for a much stronger reduction capability of photo-induced electrons, useful for instance for H2 evolution from water.13 At the same time, the photo-generated holes of g-C3N4 with moderate oxidation ability can only achieve oxygen evolution from water oxidation, instead of the formation of the non-selective hydroxyl radicals. For this, g-C3N4 photocatalysts are suitable candidate for selective photooxidation and related transformations of organic compounds in aqueous media, avoiding direct mineralization to CO2.12 The easy preparation and versatile tuning of g-C3N4 structure, from the phase point of view, attracted the attention of several research groups in widening the application also in organic chemistry. Recently, the prepared N-doped g-C3N4 exhibited a reasonable catalytic activity in the Knoevenagel


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condensation between benzaldehyde and ethylcyanoacetate yielding ethyl-α-cyanocinnamate (conversion of benzaldehyde up to 100%, yield of the desired product up to 51%). In the studied reaction conditions, weak basic sites assigned as C-N=C species are responsible for selective formation of the desired product. An increase of tertiary nitrogen content leads to an increase of benzaldehyde conversion with a simultaneous decrease of selectivity towards ethyl-αcyanocinnamate due to a higher basicity. g-C3N4 obtained using silica as a hard template allows achieving the highest yield of the desired product (51%).16 Another example is the one represented by the preparation of amines, very important for both the chemical industry and renewable feedstock processing. Nevertheless, difficulties remain in finding a catalytic system that is sufficiently active and environmentally benign for producing amine compounds. It was reported that g-C3N4 nanosheets as support materials can significantly boost the efficiency of Pd nanoparticles for the reduction of nitro compounds to primary amines. Using formic acid as a hydrogen donor and water as a solvent, the optimized 5 wt% Pd/g-C3N4 catalyst exhibited an unprecedented performance in the conversion of nitrobenzene into aniline, yielding the best activity ever reported for heterogeneously catalyzing nitro compound reduction. Pd/g-C3N4 catalyst was also active for the one-pot reductive amination of carbonyl compounds with nitro compounds to obtain the corresponding secondary amines with excellent selectivity (>90%). Furthermore, Pd/gC3N4 was highly stable with a wide scope in the syntheses of various amine compounds. A new approach for the transfer hydrogenations of nitro compounds to produce primary or secondary amines in green chemistry is clearly opened by these preliminary results.17 Looking back to the 1O2 generation, it was recently demonstrated that, given their conjugated structures, g-C3N4 and its derivatives might involve energy transfer between excited triplet excitons and ground-state oxygen molecules to give 1O2. It was demonstrated that the triplet-exciton yield is significantly enhanced by incorporating the carbonyl groups into the g-C3N4 polymer matrix via a simple oxidization treatment. The specifically designed oxidized g-C3N4 was found to greatly enhance the efficiency of 1O2 generation and concomitantly suppress the production of other reactive


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oxygen species (ROS), enabling its excellent performance in 1O2-need organic synthesis. By taking the selective conversion of sulfide to sulfoxide as an example, the authors of this research paved the way to other important investigations on this topic.18 A recent minireview summarizes the progress in photoredox catalysis mediated by heterogeneous carbon nitride materials and the organic reactions that can be performed with these types of catalysts.19 Here, we wish to report preliminary results concerning the 1O2 generation through oxidized g-C3N4 catalysts and the scope of the method in Diels-Alder (DA) and ene reactions with representative dienes and alkenes, shedding some light on the dichotomy 1O2 promoted processes and simple sensitized oxidation reactions. Investigating the scope of 1O2 promoted reactions, we found interesting as well as valuable limitations occurring with some substrates. The obtained value-added compounds are commonly the synthons for a remarkably high range of synthetic transformations, widely applied in modern organic synthesis and here derive from a sustainable and greener 1O2 methodology in organic synthesis.

Results and discussion A few g-C3N4 catalysts were prepared to be tested in 1O2 generation and applied in DA, ene reactions and general oxidation processes. CAT-1 is an oxidized g-C3N4 catalyst obtained from MLM polymerization by heating up to 600 °C (N2 atmosphere) followed by an oxidation step in concentrated HNO3/H2SO4 (2:1, v/v) mixture, according to the reported procedure used for the sulfide oxidation to sulfoxide.18 For sake of comparison, X-ray diffraction (XRD) analysis was conducted and the pattern was found identical to that reported in literature. Similarly, the oxygen content was verified by X-ray photoelectron spectroscopy (XPS) and the data were perfectly superimposable with those from the literature. CAT-2 is an oxidized g-C3N4 catalyst obtained from dicyandiamide (DCD) polymerization by heating up to 500 °C (N2 atmosphere) that successively underwent 3 hours oxidation in HNO3/H2O (1:1, v/v) mixture, according to the reported procedure.20


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A third catalyst CAT-3 was prepared from DCD polymerization prolonging the oxidation time to 9 h, as previously reported.21 For comparison, XRD analyses on CAT-2 and CAT-3 were conducted and the pattern was found identical to that reported in literature. Similarly, the oxygen content was verified by XPS and data were perfectly superimposable with those from previous works.20,21 In order to verify the role of the oxidation step in the catalyst preparation procedure with respect to the performances in 1O2 generation, two non-oxidized catalysts were also prepared (control samples). MLM polymerization (at 600 °C) afforded cat-4, analogous to CAT-1 with respect to the general procedure.18 Similarly, from DCD polymerization at 500 °C (see procedure for CAT-2) a nonoxidized g-C3N4 catalyst cat-5 was also obtained.20,21 We tested the synthetized g-C3N4 catalysts, oxidized CAT-1-3 and non-oxidized cat-4,5, in a typical 1O

2

DA cycloaddition reaction with 1,3-cyclohexadiene using acetonitrile as solvent, at room

temperature (Scheme 2). Besides the solubility that was verified to be the best in acetonitrile for the reagents, this solvent also allows for the optimum performances of the catalyst under irradiation.10-15

Scheme 2. DA cycloaddition reaction of 1,3-cyclohexadiene and 1O2 used as benchmark reaction to test the efficiency of g-C3N4 catalysts. We chose 1,3-cyclohexadiene because of its reduced reactivity with respect to cyclopentadiene in order to limit the potentially expected polymerization of the diene as side reaction under these experimental conditions. Irradiation was conducted under simulated solar light for 24 hours saturating the acetonitrile phase with pure oxygen, and leaving the reaction under oxygen atmosphere (using a balloon as the reservoir connected to the photochemical vessel). Table 1 reports the data relative to the different catalysts used and the yields values for the reaction at hand. In all the reactions, we verified that the 1,3-cyclohexadiene conversions were quantitative by GC-MS analyses that served also for the detection of the reaction products, here and in the other reported experiments. ACS Paragon Plus Environment

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Intermediate 1 that represents the primary DA cycloadduct was never detected in the experiments performed. The 4-hydroxycyclohex-2-en-1-one (2) was the only product of the reactions in entries 13, a known compound derived from the reductive cleavage of the peroxide O-O bond and simultaneous oxidation of one of the hydroxy functionalities, simultaneously promoted by the catalyst; the structure was confirmed by comparison with authentic samples (standard) as well as NMR characterization.22 The best result was obtained with CAT-1 that provided 55% yield of the cycloadduct. Lower yields were obtained with catalyst oxidized by aqueous HNO3 for different reaction times. Both non-oxidized catalysts cat-4 and cat-5 were ineffective for the DA cycloaddition and only tar material was isolated from the reaction mixture (entries 4,5). We have also confirmed that the absence of catalyst did not afford the desired products (entry 7) and that the use of other photoactive catalysts with an higher band gap, such as the commercially available P25 TiO2, also did not promote 1O2 generation and subsequent trapping of the desired DA cycloadduct (entry 6). Table 1. Catalysts and chemical yields of the DA cycloaddition reaction between 1,3-cyclohexadiene and photocatalyzed generated 1O2. Entry

Catalyst§

Product 2 (%)

1

CAT-1

55

2

CAT-2

15

3

CAT-3

3

4

cat-4

-

5

cat-5

-

6

P25 TiO2

6

7

-

-

§.

The amount of catalyst is 13% w/w with respect to 1,3-cyclohexadiene.

Based on the above-illustrated results we have extended the investigations to other dienes and alkenes to probe the scope of the set-up protocol by using a 13% w/w of catalyst with respect to the reagents.


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Scheme 3. DA cycloaddition reaction of 1O2 with cyclopentadiene in the presence of oxidized gC3N4 as the catalyst. Scheme 3 shows as the reaction of 1O2, photogenerated with g-C3N4 CAT-1, with freshly distilled cyclopentadiene allowed for obtaining the products of oxidation of just the DA dimer of cyclopentadiene 5a-e. The reaction conversion is complete but affected by an important amount of cyclopentadiene polymerization material. The composition of the reaction mixture is reported in Scheme 3 as it was found by GC-MS analyses. The experimental conditions clearly activate a very fast cyclopentadiene dimerization that is responsible for the reaction outcome as shown in Scheme 3. The primary DA cycloadduct 3 was neither isolated nor detected in the final reaction mixtures from independent experiments, as well as compound 4 that derives from the peroxide bond reductive cleavage. All the reported products are known in literature and the structures are consistent with the reported data.23 The catalyst was simply filtered at the end of each reaction and disposed. Freshly prepared catalysts were employed for new reactions. Another remarkable reaction where 1O2 plays a pivotal role is the ene reaction, extremely useful for the synthesis of allylic alcohols and unsaturated carbonyl compounds. The investigation in this field was conducted on a variety of alkenes, ranging from the highly substituted ones, such as tetramethyl ethylene (TME) and trimethyl ethylene (tme), expanding also the studies on some aromatic alkenes, such as -methyl styrene, indene and dihydroanthracene. All the reactions were performed by using CAT-1 as the catalyst under the typical set-up experimental conditions above reported.


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Scheme 4 shows the results obtained with cyclopentene, cyclohexene and TME in the ene reactions. Besides the tme case that did not give any result and the adduct 9 that was not detected (ND), the results show that in all the other cases the primary ene adducts, the peroxides 6a, 7a and 8a were detected in the reaction mixtures as major products or even the only ones as in 8a. All reactions display a good chemoselectivity because, besides the primary peroxidic ene adducts, their composition includes the alcohol 6b derived from the reduction of the primary peroxide and the ketones 6c and 7c, derived from the oxidation of the previously cited alcohols. This could somewhat be a limit in the synthetic applications of ene reactions of alkenes with singlet oxygen under catalytic conditions if highly pure peroxides are required. At the moment it is difficult to rationalize the behavior of tme and a wider range of alkenes has to be investigated with respect to the ene reaction. All the detected products are known compounds reported in literature,24 and their structures were confirmed upon comparison with authentic samples. From the results obtained it is confirmed that the best catalyst is CAT-1, the same g-C3N4 material that allowed for the oxidation of sulfides to sulfoxides, obtained from MLM polymerization.18

Scheme 4. Ene reactions of 1O2 with cyclic and linear alkenes in the presence of oxidized g-C3N4 as the catalyst. In order to ascertain the applicability of this catalyst to wider range of substrates in DA or ene reaction we have investigated some different compounds, also containing aromatic rings.


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The 1-methoxycyclohexa-1,3-diene (10) was allowed to react with 1O2 in situ generated under the typical experimental conditions (Scheme 5) to give the HDA cycloadduct 11 that was neither isolated nor detected in the reaction mixture and afforded the hydroquinone (12) as single product in quantitative yield, with complete conversion of the diene 10, deriving from the peroxide O-O bond reductive/oxidative cleavage. Part of the diene was obviously involved in side polymerization. The hydroquinone (12) is a well-known compound and was found identical to authentic specimen as reported in literature.25

Scheme 5. Oxidized g-C3N4-promoted oxidation reactions of 1-methoxycyclohexa-1,3-diene (10) and -methyl styrene (13). More intriguing is the reaction with the -methyl-styrene (13). The primary ene adduct 14 was not detected even in the crude reaction mixture. However, acetophenone (15) was obtained in 100% with 92% of styrene conversion and derives from the [2+2] photochemical cycloaddition26 to the styrene double bond to give the not-isolable intermediate 16 that undergoes intramolecular cycloreversion with loss of formaldehyde to afford the final product 15. Quite interestingly, the reaction can be described as a replacement of a methylene with an oxygen atom.


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Furthermore, indene (17) is prone to be oxidized with our catalytic method although with poor chemoselectivity since the primary ene adduct 18a (Scheme 6) was detected in the crude reaction mixture in 21% yield along with many other compounds 18b-d belonging to the alcohol and ketone families derived from the oxidation of 18a or directly from the indene (17). The diol 18b was found in 37% as mixture of cis and trans isomers. Ketones 18c and 18d were found in different ratio and their formation presumably comes from a direct oxidation of the sp2 carbon atom out of the ene reaction pathway. Products 18a-d have been already reported in literature and their structures were confirmed by comparison with authentic samples.27

Scheme 6. Oxidized g-C3N4-promoted oxidation reactions of indene (17) and dihydroanthracene (19). On the other side a strong chemoselectivity was obtained in the reaction with the dihydroanthracene (19) that afforded the anthraquinone 20 in quantitative yields in just 4h. The formation of this known compound is reasonably due to the fact that the position 9 and 10 of the anthracene moiety are prone to be oxidized as typically happens for these types of aromatic compounds. The novelty is that this oxidation occurs quantitatively in a shorter reaction time and in a clean process. Finally, we also investigated the oxidation of allylbenzene (21) that bears a not conjugated double bond with aim to oxidize the end on the side chain shifting the unsaturation in the styrenic position.

Scheme 7. Oxidized g-C3N4-promoted oxidation reactions of allylbenzene (21). ACS Paragon Plus Environment

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Scheme 7 shows the outcome of the reaction: the primary ene adduct 22a is the 26% of the reaction mixture along with just 4% of the corresponding cinnamyl alcohol 22b. Again the simultaneous oxidation pathway leads to the cinnamaldehyde (22c), which is present in 70%. Products 22a-c are known compounds.28 In summary, the results in the DA cycloaddition reaction showed a remarkable catalytic activity of oxidized g-C3N4 CAT-1 thus leading to one-pot the product 2 deriving from the not-isolable primary cycloadduct of 1O2 to 1,3-cyclohexadiene. The peroxide cycloadduct 1 undergoes a simultaneous reduction/oxidation that produces the isolable compound 2, with desimetrization of the molecule. The DA with cyclopentadiene reaction is less clean due to the strong tendency of this diene to polymerize. Nevertheless, the products represent a valuable group of compounds that confirm the reaction mechanism as they derive from the primary DA cycloadduct 3 that undergoes reduction/oxidation reaction to afford the detected products. Quite interesting and remarkably useful from the synthetic point of view, the ene reactions surprise for the simplicity as the products are obtained in good yields. The most interesting case is that of methyl styrene that is converted into acetophenone in quantitative yields as well as that of the dihydroanthracene to give quantitatively the corresponding quinone. New efforts are actively conducted for a better tuning of the oxidative capacities of the catalyst with the aim to improve the chemoselectivity on a wider range of substrates or a fine optimization of the catalyst on a specific type of reaction (e.g. DA or ene reactions). Conclusions We have shown as the oxidized g-C3N4 displays good catalytic performances in the generation of 1O2 under photochemical conditions. Graphitic carbon nitride prepared by bulk and hard template pyrolysis of melamine is seen as one of the most promising materials for the development of new ecologically-friendly reactions.10 The oxidized g-C3N4 catalyst CAT-1, gave from good to very good performances in DA cycloaddition reaction of in situ generated 1O2 to 1,3-cyclohexadiene and with


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alkenes in the case of ene reactions. It also displayed good oxidative potentialities towards aromatic olefins. The HDA and ene reactions were conducted on a variety of dienes and alkenes showing a general ability of oxidized g-C3N4 to promote chemoselective and unselective oxidative processes in strong dependence on the substrates. These results offer a good panorama of the ability of the catalyst to be employed in organic reactions to prepare valuable synthons that can be used in several value-added preparations. The g-C3N4-based methodology is not free from drawbacks, mainly due to the low chemoselectivity in the presence of some substrates or even the absence of any reaction. These outcome lead to introduce the possibility to differentiate between photooxidation reactions of Type I and Type II as shown in Scheme 8.29

Scheme 8. Definition of Type I and Type II photooxidation reactions. In the first case, radicals or radical ions are photogenerated by suitable triplet state sensitizers and oxygen is captured to give the products (Type I); for example, the lack of 1O2 ene reactions in some cases in Scheme 4 and other non 1O2 products such as in indene are candidates for Type I reactions derived from oxygen radicals and oxygen radical ions.30 In the second case, the sensitizer promotes the 1O2 formation that undergoes oxidation reactions with variable substrates (Type II). To discern between these two types of mechanisms new studies are needed (chemical traps investigations) and can be reasonably planned. From the above-presented results it can be clearly seen that the products deriving from 1O2 formation pathway are the major ones although in the mixtures photooxidized products can be found in different ACS Paragon Plus Environment

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relative amounts. A fine-tuning of the oxidative properties through further investigations on the synthesis of suitably modified g-C3N4 will be of great importance in determining a change in the approach to singlet oxygen generation methods that may open other ways to perform organic reactions through greener and sustainable methodologies.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: Experimental details with selected NMR and GC-MS spectra. Irradiation was performed under simulated solar light using a Solar Box 1500e (Co.Fo.Me.Gra S.r.l., Milan, Italy) set at a power factor 500 W m-2, and equipped with UV outdoor filter of soda lime glass IR treated.

Author Information Corresponding Authors *L. Malavasi. E-mail: [email protected]. Phone: +39-0382-987921. Fax: +39 0382 528544. *P. Quadrelli. E-mail: [email protected]. Phone: +39-0382-987315. Fax: +39 0382 528544. ORCID L. Malavasi: 0000-0003-4724-2376 P. Quadrelli: 0000-0001-5369-9140 A. Profumo: 0000-0001-5697-9260 C. Milanese: 0000-0002-3763-6657 B. Mannucci: 0000-0002-7371-7243 A. Speltini: 0000-0002-6924-7170

Author Contributions


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Equal contribution.

Notes The authors declare no competing financial interest.

Acknowledgements Financial support by University of Pavia, MIUR (PRIN 2015, CUP: F12F16001350005) are gratefully acknowledged. We also thank “VIPCAT – Value Added Innovative Protocols for Catalytic Transformations” project (CUP: E46D17000110009) for valuable financial support. References 1. Klan, P.; Wirz, J. Photochemistry of Organic Compounds, Wiley, J. and Sons, Eds.; Chichester: 2009, pp. 404-451. 2. Bonnett, R. Photosensitizers of the porphyrin and phthalocyanine series for photodynamic therapy. Chem. Soc. Rev. 1995, 24, 19-33, DOI 10.1039/cs9952400019. 3. De Rosa, M. C.; Crutchley, R. J. Photosensitized singlet oxygen and its applications. Coord. Chem. Rev. 2002, 233-234, 351-371, DOI 10.1016/S0010-8545(02)00034-6. 4. Pibiri, I, Buscemi, S.; Piccionello, A. P.; Pace, A. Photochemically Produced Singlet Oxygen: Applications

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8. Garcia-Fresnadillo, D.; Georgiadou, Y.; Orellana, G.; Braun, A. M.; Oliveros, E. Singlet‐Oxygen (1Δg) Production by Ruthenium(II) complexes containing polyazaheterocyclic ligands in methanol and in water. Helv. Chim. Acta 1996, 79, 1222-1238, DOI 10.1002/hlca.19960790428. 9. Kim, S.; Zhou, Y.; Tohnai, N.; Nakatsuji, H.; Matsusaki, M.; Fujitsuka, M.; Miyata, M.; Majima, T.

Aggregation‐Induced

Singlet

Oxygen

Generation:

Functional

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TOC

Diels-Alder 1

O2

O2

Oxidized g-C3N4 Singlet Oxygen Made-Easy by g-C3N4 Photocatalyst and Easily Used in Organic Oxidation Reactions.


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g-C3N4 - Singlet Oxygen Made-Easy for Organic Synthesis; Scope

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