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Original Article
2025
:18;
4742025
doi:
10.25259/AJC_474_2025

Visible light-mediated transition metal photocatalyst-free acylation/alkylation of para-quinone methides with unstrained ketone

, , , , , , ,
aCollege of Pharmacy, Hebei University of Chinese Medicine, Shijiazhuang 050200, P.R. China
bSchool of Pharmacy, China Pharmaceutical University, Nanjing, 211198, P.R. China
cInstitute of Integrative Medicine, College of Integrative Medicine, Hebei University of Chinese Medicine, Shijiazhuang, 050200, China
dHebei Higher Education Applied Technology Research Center of TCM Development and Industrialization, Hebei University of Chinese Medicine, Shijiazhuang, 050200, P.R. China
eApplication Technology Research and Development Center of Traditional Chinese Medicine in Hebei Universities, Hebei University of Chinese Medicine, Shijiazhuang, 050200, P.R. China

* Corresponding author: E-mail address: zyj1043011988@163.com (D. Ma); mdl_hebei@aliyun.con (Y. Zhang)

Received: , Accepted: ,
© 2025 Arabian Journal of Chemistry - Published by Scientific Scholar
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Abstract

α,α′-Diaryl-substituted carbonyl compounds are important organic intermediates. Current methods for their synthesis mainly focus on aryl aldehydes, carboxylic acids, or aromatic α-keto acids. These three are employed as benzoyl radical precursors and reacted with para-quinone methides (p-QMs) through 1,6-conjugate addition/aromatization to yield α,α′-diaryl-substituted carbonyl compounds. However, the application of this method is limited due to its harsh reaction conditions and the use of precious metals and expensive ligands. Herein, we present a transition metal photocatalyst and additive-free radical cross-coupling of unstrained ketone with p-QMs to explore alternative synthetic methods with mild reaction conditions and new substrate types. A benzoyl group could be introduced at the benzyl position using dihydroquinazolinone derived from unstrained ketone as a benzoyl radical precursor and reacting it with p-QMs under photocatalysis. We utilized cheap and readily available unstrained ketone as acyl radical sources for the 1,6-conjugate addition of p-QMs to obtain α,α′-diarylated carbonyl compounds. The transformation shows a broad substrate scope. Various substituted aryl ketones can be obtained. To our surprise, when utilizing trifluoroacetophenone and 2,2-difluoroacetophenone as raw materials for reaction with p-QMs, fluorine-containing compounds were acquired instead of α,α′-diaryl-substituted carbonyl compounds. Notably, dihydroquinazolinones could participate in the next reaction without complicated posttreatment, and the reaction can be applied on a large scale of 9 mmol.

Keywords

A transition metal photocatalyst- and additive-free radical cross-coupling
A radical precursor
p-Quinone methides
Unstrained ketone

1. Introduction

Ketones are important skeletons in drugs and natural products [1-4]. Their functionality has a remarkable influence on the pharmacodynamic and pharmacokinetic properties of drug molecules [5-7]. As important ketones, α,α′-diaryl-substituted carbonyl compounds have attracted extensive attention from chemists owing to their outstanding pharmacological activities [8-10]. In the last few decades, the synthesis of α,α′-diaryl-substituted carbonyl compounds has made considerable progress in the field of synthetic organic chemistry. For example, Anand and his colleague [11] reported a method for the synthesis of α,α’-diaryl ketones with aryl aldehydes and para-quinone methides (p-QMs) through 1,6-conjugate addition. p-QMs are structurally characterized by relatively high reactivity and a single reaction site due to the unique assembly of carbonyl and olefinic moieties [12-15]. In recent years, more methods for synthesizing α,α′-diaryl-substituted carbonyl compounds using p-QMs have been developed. Cai, Yang, Wu, and Yu [16-20] have reported the efficient synthesis of α,α′-diaryl-substituted carbonyl compounds via photoredox catalysis using p-QMs and aryl α-keto acids or carboxylic acids. Mechanistic experiments confirmed that α-keto acids or carboxylic acids, as benzoyl radical precursors, undergo an intermolecular 1,6-conjugate addition reaction with p-QMs to generate the target compounds. However, the above reaction has a substrate scope that is limited to carboxylic acids or aromatic a-keto acids and requires precious metals or expensive ligands. In 2022, Bica-Schröde and colleagues [21] developed a photocatalyst- and additive-free radical hydroacylation reaction of 4-acyl-substituted Hantzsch esters and p-QMs (Scheme 1a and b). Liao and group [22] reported a metallaphotoredox catalyzed benzoylation/benzylation of (hetero)arenes. Aromaticity serves as the driving force for the reaction, enabling the reaction to proceed in the forward direction. Many reports have been conducted on the aromatization-driven carbon–carbon bond cleavage of ketones [23-27]. However, the synthesis of α,α′-diaryl-substituted carbonyl compounds by the reaction of ketones as radical precursors with p-QMs has not been achieved (Scheme 1c).

(a-d) Acylation aromatization of p-QMs.
Scheme 1.
(a-d) Acylation aromatization of p-QMs.

Herein, ketones were used as radical sources and extended to the intermolecular 1,6-conjugate addition of p-QMs. We reported the visible-light-induced acylation aromatization of p-QMs to access α,α′-diarylated ketones using ketone-derived dihydroquinazolinones as radical precursors. This reaction features easily accessible starting materials, a broad substrate scope, and mild reaction conditions. Dihydroquinazolinones and p-QMs containing various substituents could yield the target compounds in good to excellent yields (Scheme 1d).

2. Materials and Methods

2.1. Material

All materials and reagents are commercially available. No special instructions are required.

2.2. Methods

2.2.1. Synthesis of dihydroquinazolinones I

According to relevant literature [28], to a 100 mL round bottomed flask was charged with anthranilamide (20.0 mmol, 1.0 equiv.), ketone (22.0 mmol, 1.1 equiv.), I2 (1.0 mmol, 0.05 equiv.) and 20 mL of dry dimethylformamide (DMF). The mixture was stirred for 72 h at 80°C. Upon its completion, the reaction was quenched with H2O (200 mL). The aqueous layer was extracted with ethyl acetate, and the combined organic layers were washed with saturated Na2S2O3 aqueous solution and saturated brine. It was then dried over Na2SO4, filtered, and concentrated under reduced pressure to afford the crude material. The crude material was directly used without further purification (Scheme 2).

Synthesis of dihydroquinazolinones I.
Scheme 2.
Synthesis of dihydroquinazolinones I.

2.2.2. Synthesis of dihydroquinazolinones II

According to relevant literature [29], anthranilamide (20.0 mmol, 1.0 equiv.), ketone (22.0 mmol, 1.1 equiv.), p-TSA (2.0 mmol, 0.1 equiv.), and 30 mL of dry EtOH were charged into a 100 mL round-bottomed flask. The mixture was stirred for 48 h at 80°C. Upon its completion, the reaction was quenched with H2O (200 mL). The aqueous layer was extracted with ethyl acetate, and the combined organic layers were washed with saturated brine. It was then dried over Na2SO4, filtered, and concentrated under reduced pressure to afford the crude material. The crude material was directly used without further purification (Scheme 3).

Synthesis of dihydroquinazolinones II.
Scheme 3.
Synthesis of dihydroquinazolinones II.

2.2.3. Synthesis of p-QMs

A 100 mL round-bottomed flask was charged with 2,6-di-tert-butylphenol (10 mmol, 1.0 equiv) and added with aldehyde (10 mmol, 1.0 equiv) in toluene (40 mL) and refluxed under N2. Piperidine (20 mmol, 2.0 equiv) was added dropwise within 1 h, and the resultant mixture was stirred at reflux temperature for 14 h. After being cooled to 100°C, acetic anhydride (20 mmol, 2.0 equiv) was added to the reaction system, and the resulting solution was stirred for 15 min at 100°C. Upon its completion, the reaction was quenched with H2O (50 mL). The aqueous layer was extracted with ethyl acetate, and the combined organic layers were washed with saturated brine, dried over Na2SO4, filtered, and concentrated under reduced pressure to afford the crude material [20]. The residue was purified through silica gel column chromatography to obtain the desired products (Scheme 4).

Synthesis of p-QMs.
Scheme 4.
Synthesis of p-QMs.

2.2.4. Synthesis of the desired products

(A procedure) Anthranilamide (0.3 mmol), ketone (0.3 mmol), I2 (10 mol%), and DMF were added to a round-bottomed flask with buffer balloons and stirred at 80°C for 24 h. After the reaction was completed, the mixture was extracted with water and ethyl acetate, and the organic layer was washed with saturated brine and dried over anhydrous Na2SO4. After filtration, the solvent was evaporated in vacuo. The crude dihydroquinazolinones I was used without purification [28].

Dihydroquinazolinones II (0.3 mmol, 1.5 equiv), p-QMs (0.2 mmol, 1.0 equiv), 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) (2 mol%), and dry DMF (2 mL) were added in a 10 mL tube equipped with a magnetic bar. It was then charged with nitrogen more than thrice. The resulting yellow solution was irradiated by blue LEDs (λ = 456 nm), under stirring at room temperature for 24 h. After the reaction, the mixture was extracted with water and ethyl acetate, and the organic layer was washed with saturated brine and dried over anhydrous Na2SO4. After filtration, the solvent was evaporated in vacuo and purified with a chromatography column.

(B procedure) A 25 mL round bottomed flask was charged with anthranilamide (0.3 mmol, 1.0 equiv.), ketone (0.3 mmol, 1.0 equiv.), p-TSA (0.03mmol, 0.1 equiv.), and 8 mL of dry EtOH. The mixture was stirred for 48 h at 80°C. Upon its completion, the reaction was quenched with H2O. The aqueous layer was extracted with ethyl acetate, and the combined organic layers were washed with saturated brine, dried over Na2SO4, filtered, and concentrated under reduced pressure to afford the crude dihydroquinazolinones II. The crude dihydroquinazolinones II was directly used without further purification. The operation process for the next step was consistent with 2.2.4 part (A procedure) [29].

3. Results and Discussion

At 80°C, the desired dihydroquinazolinone derivatives were obtained in a yield 83% by using 2-aminobenzamide and benzil as starting materials, p-toluenesulphonic acid as a catalyst, and ethanol as a solvent. Initially, the acylation aromatization of p-QMs was investigated using 2-benzoyl-2-phenyl-2,3-dihydroquinazolin-4(1H)-one (2a) and 4-benzylidene-2,6-di-tert-butylcyclohexa-2,5-dien-1-one (3a) as model substrates under visible-light photoredox catalysis (Table 1). After the extensive optimization of the reaction parameters, the optimal conditions were identified to be a combination of 4CzIPN (2.0 mol%) and Cs2CO3 (2.0 equiv.) in N,N-dimethylformamide (DMF) under 10W blue light emitting diode (LED) irradiation at 25°C. These conditions provided the desired ketone in 91% isolated yield (Table 1, entry 1). No conversion was observed in the absence of 4CzIPN, Cs2CO3, or blue LEDs (Table 1, entry 2). The substitution of 4CzIPN with the metal photosensitizer fac-Ir(ppy)3 or Ir[dF(CF3)ppy]2(dtbbpy)PF6 provided the desired product in 29% and 37% yields only (Table 1, entry 3). Low yields were observed with the organic photosensitizers Eosin Y, Rhodamine B, or Mes-Acr+ (Table 1, entry 4). In addition, various solvents were tested, revealing that polar non-protic dimethyl sulfoxide (DMSO), acetone, and dimethyl sulfoxide (MeCN) offered the target product in 49%, 30%, and 52% yields, respectively (Table 1, entry 5). When the bases were changed to K₂CO₃ or Na₂CO₃, the yield dramatically decreased. A notable detail is that potassium tert-butoxide (KOtBu) could cause the decomposition of 4CzIPN. After the reaction was completed, no 4CzIPN was detected by liquid chromatography mass spectrometry (LC-MS). The desired product did not form when the reaction was performed under air atmosphere. These results indicate that oxygen must be excluded from the reaction vessel. The control experiments confirmed that the photocatalyst, base, and solvent were essential for this transformation. The condensation of ketones and amines (2-aminobenzamide) gave dihydroisoquinolinones (2a) without necessitating complicated post-treatment for the next step and provided product 4a in 84% yield (Table 1, entry 1).

Table 1. Optimization of reaction conditionsa.
Entry Variations from the standard condition Yield (%) b
1 None 91% (84%c)
2 No 4CzIPN, or Cs2CO3, or blue LEDs NR
3 Ir[dF(CF3)ppy]2(dtbbpy)PF6 or Irppy3 instead of 4CzIPN 29%/37%
4 Eosin Y, Rhodamine B or Mes-Acr+ instead of 4CzIPN Trace/17%/21%
5 DMSO/acetone/MeCN 49%/30%/52%
6 K2CO3 instead of Cs2CO3 NR
7 Under air NR
a Reaction conditions: dihydroquinazolinones (0.3 mmol, 1.5 equiv), p-QMs (0.2 mmol, 1.0 equiv), 4CzIPN (2 mol%) and dry DMF (2 mL), room temperature, 24h, blue LEDs; bIsolated yield; cone-pot reaction, Step 1: ketones (0.3 mmol), I2 (0.06 mmol), 2-aminobenzamide (0.3 mmol), DMF 2 mL, 90°C, 48 hrs. Step 2: p-QMs (0.2 mmol), 4CzIPN (10 mol%), CS2CO3 (0.3 mmol), λmax = 456 nm Kessil. N2, 24 hrs, isolated yield NR= no reaction.

With the optimized reaction conditions in hand, various benzil-derived dihydroquinazolinones were explored, as illustrated in Schemes 5-7. Electron-donating groups (such as methyl, isopropyl, tert-, butyl-, and methoxy) or electron-withdrawing groups (such as chlorine and bromine) on the benzene ring could participate in the process to deliver the corresponding ketones in moderate to excellent yields (4a-4k). The change in the position of the substituents had no significant effect on the reaction yield. The substituents at the ortho (4j), meta (4g-4i), or para (4b-4f) positions on the phenyl ring of benzil-derived dihydroquinazolinones had minimal effects on the reaction efficiency, delivering the products in good yields. The polysubstituted benzil-derived dihydroquinazolinone underwent the reaction, resulting in 92% yield (4k). Afterwards, the substrate scope of substituted p-QMs was evaluated. Many p-QMs bearing the electron-donating or electron-withdrawing substitution, such as alkyl (4l, 4m), alkoxy (4n), halides (4p), and phenyl (4o) at the para-position of the phenyl ring, were observed and isolated in 73%-88% yield. The OPh (4q), Cl (4r), and Br (4s) substitutions at the meta-position of the phenyl ring were tolerated well, providing the corresponding phenols in moderate yields. A gram-scale reaction (1.92 g) of 2a with 3a was conducted under standard conditions to deliver 4a in 80% yield (See Supporting Information). The yield of the large-scale reaction (0.9 mmol) was lower than that of the small-scale reaction (0.3 mmol). The reaction time required by the scale-up transformation was prolonged. The findings suggested that the proposed transformation has remarkable potential in practical applications (Scheme 7).

Synthesis of α,α′-diaryl-substituted carbonyl compounds [28,29].
Scheme 5.
Synthesis of α,α′-diaryl-substituted carbonyl compounds [28,29].
Synthesis of fluoroalkane-substituted phenols [29].
Scheme 6.
Synthesis of fluoroalkane-substituted phenols [29].
Acylation of p-QMs.
Scheme 7.
Acylation of p-QMs.

The reaction scope could be expanded to trifluoroacetophenone, 2, 2-difluoroacetophen, and 2-phenylacetophenone, providing the products 4t-4z and 4aa-4ac in moderate yields (Scheme 8). Using dihydroquinazolinones derived from alkyl ketones as the radical precursor resulted in a poor reaction effect. For example, the target products could not be obtained when pentan-2-one, 3,3-dimethylbutan-2-one, and 4-phenylbutan-2-one were employed as the substrates and reacted with p-QMs. Although 3-methylbutan-2-one, 1-cyclobutylethan-1-one, and 1-cyclopentylethan-1-one could be utilized as reaction substrates, the reaction was complicated and could not yield a pure product. p-QMs bearing methyls at the ortho and meta-positions of the aromatic ring could not provide the target products. Additional information can be seen in the Supporting Information. (Unsuccessful examples part).

Alkylation of p-QMs.
Scheme 8.
Alkylation of p-QMs.

Control experiments were performed to understand the reaction mechanism further [30-32]. Firstly, the reaction could not proceed when the reaction atmosphere was changed from nitrogen to air. Next, a radical-trapping experiment was conducted. The radical-trapping reagent 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) was added to the reactions under standard conditions (Scheme 9). Although neither acylation nor alkylation products (4a or 4x) were found, TEMPO adducts were detected by gas chromatography-mass spectrometry (GC-MS). The results revealed that the addition of TEMPO inhibited the radical process [30-32]. Finally, a light-on-off experiment was performed. The results showed that continuous light irradiation was necessary. The reaction proceeded normally under the light-on conditions, whereas no significant progress was observed when the light-off conditions were applied, indicating that the reaction may not proceed through a free-radical chain mechanism [32].

Preliminary mechanistic studies.
Scheme 9.
Preliminary mechanistic studies.

A plausible reaction mechanism for the acylation/alkylation of p-QM was proposed based on the above experiments and relevant literature [30,33-36] and has been illustrated in Scheme 10. Under irradiation with 456 nm blue LED, 4CzIPN was photoexcited to 4CzIPN*. The excited 4CzIPN* underwent single electron transfer (SET) with 2a to deliver the radical cation 2aI. Meanwhile, 4CzIPN* gained an electron and was reduced into 4CzIPN˙‾. Driven by aromatization, 2aI underwent carbon–carbon bond cleavage and simultaneously lost a proton, generating the benzoyl radical intermediate 2aII. Subsequently, 2aII underwent intermolecular Giese radical addition with 3a to yield the intermediate 2aIII. Finally, the 4CzIPN˙‾ underwent a SET and regenerated to the ground state photocatalyst 4CzIPN, completing the photocatalyst cycle. The intermediate 2aIII gained an electron and a proton to give the final product 4a. The ability of the organic photocatalyst 4CzIPN to outperform metal-based alternatives, such as Ir[dF(CF3)ppy]2(dtbbpy)PF6, can be explained as follows: Relevant reports [29,37-38] have shown that the redox potential of 4CzIPN is superior to Ir[dF(CF3)ppy]2(dtbbpy)PF6 {Ir[dF(CF3)ppy]2(dtbbpy)PF6 (E1/2(P*/P•−) = +1.21 V vs SCE) and 4CzIPN (E1/2(P*/P•−) = +1.35 V vs SCE)}. The excited-state 4CzIPN is preferably quenched by dihydroquinazolinones (Eox = +1.21 V vs SCE).

Proposed catalytic cycle.
Scheme 10.
Proposed catalytic cycle.

4. Conclusions

This study reported a visible-light-driven acylation/alkylation of p-QMs with unstrained ketone. The reaction does not require any additives, and it has a broad substrate scope and excellent functional group tolerance under mild conditions. The method for synthesizing α,α′-diaryl-substituted carbonyl compounds has been extended to ketone compounds. Aromatic ketones exhibit good reaction efficiencies. However, this protocol is unsuitable for alkyl ketones, which provide alkyl radicals instead of acyl radicals. The use of radical scavengers confirms that the reaction follows a radical pathway. The light-on/off experiment demonstrates that the reaction does not involve a radical chain-growth mechanism. Instead, it undergoes single electron transfer, intermolecular proton transfer, and intermolecular Giese radical addition. This reaction provides a new method for the synthesis of benzyl benzoyl/fluoroalkane-substituted phenols. We aim to expand the scope of the acyl radical precursors that can be employed in this transformation.

Acknowledgment

We are grateful to the Project of Hebei Provincial Health Commission (20231566), Funded by Science and Technology Project of Hebei Education Department (BJK2023098) for the financial support.

CRediT authorship contribution statement

Zhihui Wei: Writing– original draft, Data curation. Bitao Zhang: Writing – original draft. Lina Li and Xuhong Duan: Writing–review & editing, Supervision. Yan Zhong, and Zichang Wang: Investigation. Donglai Ma: Funding acquisition. Yajing Zhang: Data curation, Methodology, Project, Administration, Nvestigation, and Funding acquisition.

Declaration of competing interest

The authors declare no conflict of interest.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of AI-assisted technology for assisting in the writing of the manuscript and no images were manipulated using AI.

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