Oxygen-harvesting carbon dot photocatalysts for ambient tandem oxidative synthesis of quinazolin-4(3H)-ones

Abstract

An efficient oxidative route to quinazolin-4(3H)-ones using common alcohol precursors is critical for the drug industry since the existing methodologies inevitably rely on transition metal assistance in the oxidative step and a high synthesis temperature to augment dehydration in the final step. Herein, we introduce acidic and oxygenphilic waste derived carbon dots (CDs) as an inexpensive metal-free photocatalyst cum oxidant for the one pot quantitative synthesis of quinazolin-4(3H)-ones with ultrahigh efficiencies, including five potent drug molecules and establish the reaction mechanism. The protocol eliminates the need for external oxidants, ligands, or additives and enables identical efficiencies in air and oxygenated atmospheres, paving the way for using air in industrial processes. With an Eco-Scale of >80% for greenness and sustainability, gram-scale production with uncompromised yields, and lesser purification needs, the CDs derived from waste plastics are a sustainable photocatalyst and a viable green alternative for such transformations.

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Green foundation

1. We converted waste plastics into carbon dots (CDs) that serve as eco-friendly catalysts cum oxidants, generating reactive oxygen species upon light irradiation. We also synthesized quinazolin-4(3H)-one based potent drug molecules photochemically at room temperature, achieving an Eco-Scale above 80 with no side products.

2. Our work meets seven principles of green chemistry: (a) Waste prevention: CDs from plastics act as sustainable catalysts. (b) Reduce derivatives: CDs replace additional oxidants and metals. (c) Less hazardous synthesis: Eco-friendly process to synthesize potent drugs. (d) Catalysis: Ensure selectivity without by-products. (e) Design for degradation: CDs self-degrade under light, minimizing harm. (f) Energy efficiency: Inexpensive precursors, CDs harness oxygen from air. (g) Safer chemistry: Focused on accident prevention and safety.

3. This work exemplifies green chemistry and can be made more sustainable by adjusting carbon dot properties, facilitating more oxidative transformations and new drug candidates.

Developing green and economic methodologies to access medicinally important scaffolds remains a vital research challenge due to the faster spread of infectious diseases from the ease of travelling and the expanding market size. Quinazolin-4(3H)-ones are one such compound that exhibits potent medicinal activities such as anticancer, anti-inflammatory, anti-hypertensive, anti-lipid accumulation, anti-depression activities, etc. (Fig. 1a).^1,2

Fig. 1 (a) A few quinazolin-4(3H)-one-based marketed drugs.² (b) Schematic representation of PE-CDs.

Numerous transition metal-assisted and metal-free organo-catalytic methodologies have been developed to synthesize quinazolin-4(3H)-ones derivatives.^3–8 Despite achieving good efficiencies and high yields, the transition-metal-assisted approaches are emerging as non-preferred due to the difficulties in removing trace metal impurities from the products.^4–6 On the other hand, the necessity of using extra additives in the metal-free approaches, often used in excess stoichiometric amounts to match the efficacy of the metal-based approaches, generates more waste and, therefore, is associated with high E-factors.⁷ Such transformations sometimes require the use of acids since protonation is a necessary facilitator for the polarization of the aldehyde functional group for feasible attack by the nucleophiles to form the Schiff bases, the key intermediate for the synthesis of quinazolin-4(3H)-ones. Besides, the use of higher reaction temperatures produces greenhouse gases. Therefore, a simple and affordable protocol to produce such drug candidates is necessary from the viewpoint of sustainability. In this regard, a room-temperature photocatalytic approach to drug production using renewable solar energy and ambient air as oxidants while choosing suitable catalysts from a plethora of recently developed photoactive materials could be a promising alternative but has barely been explored for this purpose.

Carbon dots (CDs) have emerged as an alternative to conventional reagents and catalysts due to their benign, abundant, and inexpensive nature.⁹ CDs are an emerging class of carbon nanomaterials with sizes below 10 nm with excellent physicochemical and optoelectronic properties as electron donors and acceptors.¹⁰ They have been increasingly used as catalysts in organic transformations due to their unique tunable photoluminescence and surface functional moieties that lead to different electronic properties and adjustable catalytic performances.^11–15 A limitation of carbon dots (CDs) in organic transformations, is their tendency to be unstable due to photo-bleaching when exposed to light.¹⁶

Recently developed CDs produced from waste polyethylene (PE) feedstock by chemical oxidative fragmentation following acid treatment present several opportunities for synthesizing quinazolin-4(3H)-ones. These PE-CDs exhibit arrested bleaching characteristics in the presence of other reactants. However, light exposure in the absence of additional reactants results in their self-inflicted photo-oxidation and elimination from the reaction medium forming CO₂, which is critical for the easy disposal of the spent CDs.¹⁷ The self-eliminating property of PE-CDs helps resolve the issue of separation of catalyst in a homogeneous catalysis without the need for additional techniques or instrumentation (details in ESI Note S1†). Besides, sulphuric acid treatment of PE leaves behind the sulfonic acid groups anchored on the CD surfaces to assist the conversion (Fig. 1b). More importantly, we have discovered two exciting novel properties of the PE-CDs: these can harvest molecular oxygen from ambient air and act as a reservoir and diffuser of O₂ in solvents. On average, over a hundred O₂ molecules are reversibly absorbed on each PE-CD surface and are in equilibrium with dissolved oxygen molecules in aqueous surrounding.^18,19 This property allows light-induced hypoxia, enabling tunable O₂ harvesting using light as a non-contact control.²⁰ Under light irradiation, PE-CDs convert absorbed O₂ molecules into various reactive oxygen species (ROS), which are expected to play a crucial role in the initial oxidation of benzyl alcohols to benzaldehydes. This process avoids the need for high pressure, toxic oxidizing agents, or expensive oxygen partial pressures, serving as an initial step in the tandem oxidative synthesis of quinazolin-4(3H)-ones.

In this paper, we introduce a room-temperature, one-step photocatalytic approach to produce several potent quinazolinone-based drug molecules using PE-CDs as a highly active photocatalyst. To add to the benefits, we show that the unique oxygen harvesting properties of the CDs facilitate the use of molecular O₂ present in ambient air as an inexpensive but highly facile oxidant for the oxidative step of synthesis. We further establish the gram-scale production prospects of this protocol. To the best of our knowledge, this is the first example of photoproduction of the widely used quinazolinone-based drug molecules where the utilization of waste plastics as feedstock in catalyst synthesis makes it even greener.

Fig. 2a is the transmission electron microscopic (TEM) and high-resolution (HR) TEM images of the PE-CDs showing their quasi-spherical morphologies with average particle sizes of 4–6 nm and a size distribution of ±2 nm. These CDs consist of graphitic regions with clear lattice fringes containing ∼0.23 nm spacings related to the (001) crystal plane of the graphite. The powder X-ray diffraction (XRD) pattern of these CDs exhibited a broad peak centering at 24–26° corresponding to the (002) crystal plane of graphite (Fig. S1†). The chemical transformations that originated during the conversion of PE to char to the CDs, leading to their acidic characteristics, were probed by FTIR and Raman measurements (Fig. 2c & Fig. S2†). Briefly, the C–H stretching and bending peaks disappeared in the FTIR spectrum of PE after acid treatment under reflux condition due to charring, accompanied by the appearance of the acidic sulfonic and C–O–C groups grafted on the surface. Oxidation of this char produces the CDs while retaining the acidic groups and introducing many surface –OH, giving rise to a broad peak at ∼3420 cm⁻¹. We further checked their acidic strength at a catalytically relevant concentration. As seen in Fig. 2b, 50 mg of PE-CDs highly dispersed in 2 ml of water change the aqueous pH from ∼7 to ∼2 due to the presence of the surface –SO₃H groups, beneficial for the proposed transformation. Fig. 2d illustrates the oxygen harvesting ability of the PE-CDs (details in ESI Note S2†).

Fig. 2 (a) TEM images of the PE-CDs: top inset is a histogram of particle size distribution, and the bottom one displays lattice spacings, (b) pH of 2 ml water with 50 mg PE-CDs, (c) FTIR spectra of PE-precursor, PE-char, and PE-CDs. (d) Temporal variation of O₂ content in an aq. PE-CD solution while purging N₂ and subsequent intake of O₂ from the ambient air by the same solution. (e) Contour plot showing PL emission spectra of PE-CDs at λ_ex (300–500 nm, 10 nm increment), (f) PL decay profile of CDs at λ_em of ∼520 nm using λ_ex ∼375 nm.

Using the PE-CDs, an initial study was conducted to determine the optimum conditions for the synthesis of quinazolin-4(3H)-ones, choosing anthranilamide (50 mg, 0.36 mmol) and 4-methoxybenzylalcohol (126 mg, 0.91 mmol) as specimen substrates (details in ESI Note S3†). During the optimization, it was confirmed that PE-CDs can afford quinazolin-4(3H)-ones within 6 hours of reaction time under the irradiation of 400 W of Xe lamp without any external oxidizing agents but using molecular oxygen extracted from the air as the sole oxidant (Scheme 1). As discussed later, the reaction yield consistently increases with time and completes in 6 h. Besides, the reaction does not proceed without light or the CDs alone, confirming its photo-catalytic nature.

Scheme 1 Optimized reaction condition while using ambient air.

To check the general applicability of the protocol, a series of benzyl alcohols were tested by reacting with anthranilamide under the optimized conditions (Table S1, entry 23†).

Benzyl alcohols with various electron-donating and withdrawing groups react efficiently, providing the respective products with appreciable yields (Table 1, entries 3aa–3al). Electron-donating groups such as methyl and methoxy substituents in the benzyl alcohol produced >95% of isolated yields, while electron-withdrawing groups such as –Cl and –Br onto the aromatic ring led to a slight decrease in the yield (73%–90%) due to the –I effect. A low-reactive heterocyclic alcohol, furfuryl alcohol, was also tested for the protocol. Its furan ring is more electron-deficient and may undergo ring-opening reactions, limiting its reactivity and product yields of 50–65% as found in earlier approaches.²¹ In our photocatalytic approach, >76% yield was obtained in 6 hours using furfuryl alcohol (Table 1, entry 3aj). Further, aliphatic alcohol (Table 1, entry 3ak) was also tested and showed potential tolerability with the photocatalytic protocol. Substituted anthranilamide with –Cl group was also tested (Table 1, entry 3al) and produced >78% yield despite the –I effect of the Cl group, confirming PE-CDs as a potent and the first photocatalyst cum oxidant for transforming a wide variety of anthranilamide under ambient air.

Table 1 Reaction scope for potent drug and other molecules

Reaction conditions: 1a (50 mg, 0.36 mmol), 2aa (110–120 mg, 0.91 mmol), CDs (51 mg), DMSO (2 ml) at 400 W full spectrum of Xe light for 6 hours (Scheme 1).

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One of the notable aspects of our methodology is that five of the synthesized molecules are potent drug molecules (Table 1, entries 3aa–3ae). One of them, (Table 1, entry 3ac) functions as a potent drug for antitumor, α-glucosidase inhibition, and antileishmanial activity.²² Three molecules (Table 1, entries 3aa, 3ab, 3ad) serve for both α-glucosidase inhibition and antileishmanial activity,²³ while another one (Table 1, entry 3ae) functions as an α-glucosidase inhibitor.²⁴ We believe that the high isolated yields (∼90%) for these potent drug molecules while using waste polyethylene-derived CDs as a sunlight-harvesting catalyst may inspire industrial production.

Quinazolin-4(3H)-one synthesis, under the conventional procedure, involves the oxidation of the benzyl alcohol to benzaldehyde through a radical process followed by Schiff base formation, cyclization, and dehydrogenation.²⁵ We have conducted several controlled experiments using the optimized conditions to compare and confirm the reaction pathway induced by the CDs’ photo-excitons. Considering benzyl alcohol to benzaldehyde transformation is a radical oxidation process,²⁶ radical scavengers were added to the reaction mixture before exposing to light, which lowered the yield to below 30% (Table S1, entry 18†), confirming the photooxidative alcohol to aldehyde transformation.

We further found that this oxidative step is the rate-determining step for the transformation. Taking, for instance, 1a and 2aa (Fig. 3a) as substrates, the quinazolin-4(3H)-ones yields were found to be ∼2–30% in 2 hours, which increased to 45–50% in 3 h, and to ∼80% in 5 hours. The optimum yield of >95% was obtained after 6 hours. We compared this kinetics with the corresponding alcohol (2aa) to aldehyde transformation kinetics, where the reaction was carried out without 1a.

Fig. 3 Comparison of oxidation kinetics of benzyl alcohol vs. overall synthesis of quinazolin-4(3H)-ones (i & ii) by PE-CDs in (a) open-air (b) O₂ environment.

As seen from Fig. 3a, one can easily notice that aldehyde and quinazolin-4(3H)-one formation kinetics are nearly identical. Therefore, as the benzaldehyde (2ab) starts forming, there is a simultaneous conversion to the corresponding quinazolin-4(3H)-ones (3aa) upon reacting with the anthranilamide (1a).

Using pressurized oxygen in industrial processes incurs additional costs and should preferably be avoided. Due to the oxygen harvesting capacity of the PE-CDs from ambient air and their ability to produce ROS under light illumination, we investigated the efficiency of these transformations under oxygen-bubbling conditions vs. in ambient air. First, a controlled reaction was done in the O₂ environment in the absence of the CDs to demonstrate its role in the oxidative synthesis of quinazolin-4(3H)-ones (Table S1, entry 17†). The transformation did not occur to conclude that the molecular O₂ alone is insufficient to lower the reaction's activation barrier for oxidative transformation without the CDs. Further, the reaction was carried out and analyzed hourly in ambient air and O₂ using two substrates and the optimized quantity of CDs (reactions (i) & (ii) in Fig. 3). As seen in Fig. 3a and b, the reactions progressed identically in time before completion at 6 hours, leading to similar isolated yields.

CDs are known to photodegrade a variety of organic pollutants by uncontrolled oxidation.²⁷ Therefore, we extended the reaction beyond completion to 8 hours. As seen in Fig. 3, there was no change in the product yield, confirming the stability of the substrates against unwanted photodegradation.

Motivated by the successful drug molecule production by facile use of ambient air, several controlled experiments were performed to confirm the reaction mechanism.

As seen from Scheme 2a, the presence of both light and PE-CDs is essential for the initial transformation step, confirming its photocatalytic and rate-determining nature as described in Fig. 3a. Besides, DMSO is essential to facilitate product selectivity since solvent-free conditions resulted in partial oxidation, even though DMSO could not conduct the reaction by itself.

Scheme 2 (a) Controlled and (b) quenching experiments to establish the reaction mechanism.

We carried out quenching experiments to identify the specific reactive oxygen species (ROS) that drives the PE-CDs mediated photocatalytic activation of molecular oxygen (Scheme 2b). The reaction's progression was significantly hindered by the addition of 1,4-Diazabicyclo[2.2.2]octane (DABCO), a quencher for singlet oxygen. Likewise, the individual introduction of benzoquinone (BQ) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO), both quenchers for superoxide anions, also considerably obstructed the reaction, suggesting that the reaction involves both energy transfer (ET) and single electron transfer (SET) pathways to molecular oxygen.²⁸ The formation of singlet oxygen and superoxide radicals was confirmed by electron paramagnetic resonance (EPR) and trapping experiments with TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), respectively (Fig. S5 and S6†). In situ spectroscopic detection of singlet and superoxide radicals was also carried out and quantified (Fig. S7†). The details are added in ESI Note S4.†

In the presence of TEMPO, we are able to detect the TEMPO-adduct (performing an oxidation reaction in the optimized condition taking 4-Cl benzylalcohol as specimen substrate) by mass spectrometry (Fig. S6†). Based on these observations and considering PE-CD band alignments with reaction intermediates,^17,29 the following reaction mechanism is proposed for the synthesis of quinazolin-4(3H)-ones (Scheme 3).

Scheme 3 Proposed mechanism for the synthesis of quinazolin-4(3H)-ones.

In the ET pathway, the excited PE-CDs transfer energy to O₂, resulting in the formation of singlet oxygen (¹O₂). In the SET pathway, the excited PE-CDs transfer an electron to O₂, generating a superoxide anion (˙O₂⁻). This singlet oxygen and superoxide anion radical are the key reactive species due to which the benzyl alcohol (1) is first selectively oxidized to corresponding benzaldehyde (2). In situ generated benzaldehyde further reacts with anthranilamide and undergoes a condensation reaction to form a Schiff base (4) in the presence of CDs as a catalyst and DMSO as a solvent. The delocalization of the lone pair on the nitrogen atom to the electrophilic carbon atom, followed by the abstraction of the proton (5) resulted in a cyclized product (6). In the final step, protonation occurs in the presence of water released during the previous step of Schiff base formation, followed by oxidation via CDs to form the desired quinazolin-4(3H)-one derivative (8). Mechanistic investigation of the proposed mechanism has been carried out using computational study (Fig. S8 & S9†) and details are presented in ESI Note S5.†

We calculated the Eco-Scale for our protocol to check the sustainability of the developed protocol over other methodologies for synthesizing quinazolin-4(3H)-ones.³⁰

As detailed in the ESI Note S6,† the high Eco-Scale values of >95% for at least three substrates, >90% for another four, and >80% for the remaining substrates justify the promising sustainability of the protocol (Fig. 4).

Fig. 4 Eco-Scale of the synthesized products.

Finally, we have expanded our protocol to check the industrial applicability by performing large-scale synthesis. We have taken the reaction (i) in Fig. 3 as a model reaction for scale-up syntheses. Three reactions at various scales were carried out using substrates 1a (3.67 mmol) with 2aa (9.17 mmol), 1a (7.34 mmol) with 2aa (18.35 mmol), and 1a (10.92 mmol) with 2aa (27.30 mmol) under the optimized reaction conditions. The corresponding product 3aa, the α-glucosidase inhibitor, was obtained in good yields (85%, 75%, and 70% isolated yields, respectively), indicating the industrial viability of the protocol (Fig. 5).

Fig. 5 Scale-up synthesis (0.5 g, 1 g, and 1.5 g) of α-glucosidase inhibitor.

In conclusion, we have established an efficient and sustainable protocol for the efficient use of waste polyethylene-derived nanosized carbon dots for the tandem oxidative synthesis of quinazolin-4(3H)-ones. The acidic and oxygen-philic properties of the CDs relegate the use of pressurized oxygen by providing identical yields in ambient air. The protocol demonstrates access to quinazolin-4(3H)-ones from anthranilamide and benzyl alcohols, eliminating the use of transition metals, toxic oxidants, additives, and ligands. Moreover, the methodology was successfully used to synthesize five potent drug molecules in a simple way, including gram-scale synthesis. To the best of our knowledge, this is the first report demonstrating the synthesis of medicinal drugs by utilizing waste upcycling, an essential perspective for sustainable industrial practices.

Author contributions

B. D.: conceptualization, data curation, formal analysis, methodology, software, writing manuscript. B. K.: data curation, methodology, software, writing manuscript. K. K.: data curation, methodology, formal analysis, software. A. K. G.: computational study. U. K. G.: conceptualization, supervision, writing & analysing final draft. D. S.: conceptualization, supervision, writing & analysing final draft.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

BD is thankful to IISER Mohali for providing a post-doctoral fellowship. The authors thank IISER Mohali for providing various central facilities. KK is thankful to UGC for the junior research fellowship. Support from the SERB, India, under Grant CRG/2021/001420, is acknowledged. The authors acknowledge CSIC, Dibrugarh University, and IISER Mohali for NMR measurements and Dibrugarh University and IISER Mohali for providing all infrastructural facilities. BD and DS thank the Department of Science and Technology (DST), India, for financial assistance under the DST-FIST programme (No. SR/FST/CS-I/2020/152) and PURSE programme [No. SR/PURSE/2022/143 (C)]. The authors acknowledge Gokaldas Exports Charitable Foundation for a CSR grant for carbon dot research.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5gc00962f

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