Design and Synthesis of Donor-Acceptor-Donor (D-A-D) Type Aromatic Ketones for Photocatalytic C(sp3)—H Coupling Reactions

doi:10.6023/cjoc202501013

Abstract

A series of donor-acceptor-donor (D-A-D) aromatic ketone photosensitizers (PC3~PC8) were successfully designed and synthesized. These compounds feature a diacyltetrafluorobenzene electron-acceptor core connected with multi- substituted aryl electron-donor groups on both sides. They demonstrated excellent performance at photocatalytic systems for C(sp³)—H coupling reactions between methylarenes and aryl bromides. The study found that PC8, which uses meta-diacyl- tetrafluorobenzene as the electron-acceptor core and mesitylene as donor groups, shows outstanding photocatalytic performance. Its activity is 460 times higher than traditional benzophenone. More notably, this work reported the activation of solid methylarene substrates, which overcame the key limitation of using methylarenes solely as reaction solvents and significantly broadened the substrate scope. Theoretical calculations revealed that the D-A-D structure of photosensitizer PC8 effectively separates the electron density distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). This reduces the singlet-triplet energy gap (ΔE_S-T). The ΔE_S-T value of PC8 (1.48 eV) is significantly lower than that of benzophenone (1.84 eV), which facilitates the intersystem crossing (ISC) process. The absorption spectra revealed that PC8 has a characteristic absorption peak at 310 nm. Steady-state fluorescence analysis further proved that PC8 has 5 times higher fluorescence quantum yield than benzophenone. These spectroscopic characterizations demonstrate that PC8 possesses superior photophysical properties, effectively driving the enhancement of quantum efficiency in the photocatalytic cycle.

Key words: aromatic ketone photosensitizer, photocatalysis, C(sp³)—H activation, coupling reaction, quantum efficiency

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Hui Zhu, Peng Wu, Chenming Zhong, Shuming Li, Gang Lin, Xuefen Liu, Shuping Luo. Design and Synthesis of Donor-Acceptor-Donor (D-A-D) Type Aromatic Ketones for Photocatalytic C(sp³)—H Coupling Reactions[J]. Chinese Journal of Organic Chemistry, 2025, 45(9): 3441-3449.

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Fig. & Tab. 10

Figure 1. (A) Fluorenone photocatalytic Diels-Alder reaction; (B) visible light-driven synthesis of highly substituted pyrroles; (C) isoquinoline-derived diaryl ketone-based photocatalysts for catalytic dehydrogenation coupling; (D) aryl coupling of p-bromobenzonitrile with toluene; (E) C—S functionalisation of toluene derivatives

Figure 2. Structures of compounds PC1~PC8

Entry	Photocatalyst (mol%)	Solvent	Base		Time/h	Yield^b/%
1	PC1 (20)	Toluene	NaHCO₃		18	30
2^c	—	Toluene	NaHCO₃		18	None
3	PC2 (20)	Toluene	NaHCO₃		18	92
4	PC3 (20)	Toluene	NaHCO₃		18	93
5	PC4 (20)	Toluene	NaHCO₃		18	33
6	PC5 (20)	Toluene	NaHCO₃		18	79
7	PC6 (20)	Toluene	NaHCO₃		18	57
8	PC7 (20)	Toluene	NaHCO₃		18	79
9	PC8 (20)	Toluene	NaHCO₃		18	99
10	PC8 (5)	Toluene	NaHCO₃		18	99
11^d	PC8 (5)	Toluene	NaHCO₃	18		None
12^e	PC8 (5)	Toluene	NaHCO₃		18	None
13^f	PC8 (5)	Toluene	NaHCO₃		18	None
14	PC8 (5)	Toluene	NaHCO₃		10	99
15	PC8 (5)	Toluene	K₂HPO₄		10	84
16	PC8 (5)	Toluene	CH₃COONa		10	23
17	PC8 (5)	Toluene	K₂CO₃		10	33
18	PC8 (5)	Toluene	Na₂CO₃		10	56
19	PC8 (5)	Toluene	NaHCO₃		0.5	96
20	PC8 (5)	DMSO (0.1 mol/L)	NaHCO₃		18	None
21	PC8 (5)	Hexane (0.1 mol/L)	NaHCO₃		18	Trace
22	PC8 (5)	EtOAc (0.1 mol/L)	NaHCO₃		18	7
23	PC8 (5)	MeCN (0.1 mol/L)	NaHCO₃		18	32
24	PC8 (5)	MeCN (0.2 mol/L)	NaHCO₃		18	55
25	PC8 (5)	MeCN (0.4 mol/L)	NaHCO₃		18	99
26	PC8 (5)	MeCN (0.4 mol/L)	NaHCO₃		0.5	87

Table 1. Reaction conditions for photocatalytic C(sp3)—H coupling reaction of toluene with bromobenzene

Figure 3. Substrate scope of methylarenes and aryl bromides a Reaction conditions: a mixture of 2a (0.4 mmol, 1.0 equiv.), NaHCO3 (0.8 mmol, 2.0 equiv.), NiCl2•6H2O (0.02 mmol, 5 mol%), PC8 (5 mol%), and dtbbpy (0.02 mmol, 5 mol%), in solvents (5 mL) was stirred at room temperature irradiated by 20 W blue LEDs under an Ar atmosphere for 0.5 h. b Product yield determined by HPLC using diethyl phthalate as the internal standard; c Acetonitrile/benzene (V:V=3:1) replaced pure acetonitrile. d Temperature was 50 ℃. e K3PO4 instead of NaHCO3.

Figure 4. UV absorption spectra of PC1~PC8

Figure 5. Fluorescence emission spectra of PC1~PC8

Figure 6. (a) Geometry of PC8; (b) HOMO orbital localization for PC8; (c) LUMO orbital localization for PC8

Entry	PC	E_LUMO/eV	E_HOMO/eV	ΔE/eV	ΔE_S-T/eV
1	PC1	-2.08	-6.86	4.78	1.84
2	PC2	-2.76	-7.76	5.00	2.06
3	PC3	-2.64	-7.57	4.94	2.00
4	PC8	-2.50	-6.92	4.42	1.48

Table 2. Density functional theory (DFT) calculations

Figure 7. Experiment of free benzyl radical capture

Figure 8. Possible mechanism for the photocatalytic C(sp3)—H coupling reaction of arylmethyl groups with aryl bromides

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供体-受体-供体(D-A-D)型芳香酮设计合成与光催化C(sp³)—H偶联反应性能研究

Design and Synthesis of Donor-Acceptor-Donor (D-A-D) Type Aromatic Ketones for Photocatalytic C(sp³)—H Coupling Reactions

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供体-受体-供体(D-A-D)型芳香酮设计合成与光催化C(sp3)—H偶联反应性能研究