Solvoselective effect of Cu–B₂O₃/porous boron nitride on catalytic conversion of arylboric acids to phenols and biphenyls

Abstract

A novel Cu–B₂O₃/porous boron nitride (PBN) composite was developed as a highly efficient and reusable heterogeneous catalyst for the solvent-controlled conversion of arylboronic acids. In water, the catalyst promotes ipso-hydroxylation to phenols, while in methanol, it facilitates aerobic homocoupling to biphenyls, using air as a green oxidant. The synergistic interaction between Cu nanoparticles and B₂O₃ sites enhances both catalytic activity and stability. This work offers a sustainable and selective approach for synthesizing phenolic and biphenyl compounds under mild conditions.

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1. Introduction

Arylboronic acids are versatile reagents widely used in cross-coupling reactions for constructing C–C, C–N, and C-heteroatom bonds, with broad applications in pharmaceuticals, materials science, and fine chemicals.^1–5 Among their transformations, the synthesis of phenols and biphenyls is of particular interest due to the utility of these structures in drug intermediates, agrochemicals, and polymers.^6–10

Traditional methods for phenol synthesis, such as diazonium salt hydrolysis¹¹ and hydroxylation of aryl halides,^12–14 often suffer from harsh conditions or poor selectivity. In contrast, the ipso-hydroxylation of arylboronic acids represents a greener and more atom-economical route. Various catalytic systems have been explored,^15–20 including BN–OH,²¹ graphene oxide,²² N-oxide,²³ hypervalent iodine, and supported metal nanoparticles (Pd, Au, Ag, Cu, etc.).^24–33 However, many of these systems rely on H₂O₂ as an oxidant, which increases cost and limits sustainability. Molecular oxygen, especially from air, offers an ideal alternative, though efficient and stable catalysts for aerobic hydroxylation remain scarce.

Two-dimensional materials such as graphene and boron nitride have emerged as promising catalyst supports due to their high surface area and tunable surface chemistry. However, the weak interaction between metals and the basal plane of hexagonal boron nitride (h-BN) often leads to poor metal dispersion.³⁴ To address this, we developed a Cu–B₂O₃/PBN composite with enhanced metal–support interaction and catalytic performance.

In this work, we report the solvoselective behavior of Cu–B₂O₃/PBN: in water, it catalyzes the ipso-hydroxylation of arylboronic acids to phenols, while in methanol, it promotes their homocoupling to biphenyls, using air as the sole oxidant. The catalyst exhibits excellent stability and reusability, underscoring its potential for sustainable chemical synthesis.

2. Experimental section

2.1 Materials

Cu(NO₃)₂·3H₂O was purchased from Beijing InnoChem Science and Technology Co., Ltd. All substrates, including phenylboronic acid and its derivatives, were obtained from Shanghai Macklin Biochemical Co., Ltd. Melamine (C₃H₆N₆) and boric acid (H₃BO₃) were acquired from Shanghai Aladdin Biochemical Technology Co., LTD (Shanghai, China). All materials were commercially available and were used without further purification. PBN was prepared according to the procedure reported by our group.³⁵

2.2 Preparation of Cu–B₂O₃/PBN nanocatalyst

To prepare the Cu–B₂O₃/PBN nanocatalyst, Cu(NO₃)₂·3H₂O (0.046 g) was dissolved in 15 mL of deionized water. Subsequently, 0.2 g of the as-obtained PBN powder was dispersed into the above solution, and then the mixture was stirred at room temperature for 5 h. After that, the mixed solution was stirred at 60 °C until the solvent had evaporated, followed by drying at 60 °C for 8 h. The reddish-brown Cu–B₂O₃/PBN powder was obtained by a reduction process conducted at a temperature of 400 °C in Ar/H₂ (5%) for a duration of 2 h (Fig. S1).

2.3 Cu–B₂O₃/PBN catalyzed for ipso-hydroxylation of phenylboronic acids in water

In a typical reaction, 0.2 mmol of phenylboric acid and 5 mg of Cu–B₂O₃/PBN were dispersed in 5 mL of water and stirred continuously at 80 °C. HPLC was used to evaluate the reaction's performance, while the yield was determined by using the standard curve. Once the reaction was finished, the product was subjected to extraction using methylene chloride. The purification of all phenol synthesis products was achieved using column chromatography, resulting in the production of white acicular crystals. The acquired compounds were further characterized using ¹H-NMR spectra. After completion of the reaction, the reaction mixture was centrifuged to separate the catalyst and reused in further reactions. Following the completion of the reaction, the reaction mixture underwent centrifugation to isolate the catalyst, which was then used in subsequent processes.

2.4 Cu–B₂O₃/PBN catalyzed for homocoupling of phenylboronic acids in methanol

A solution was prepared by dissolving 0.2 mmol of phenylboric acid (1a) in 5 mL of methanol at room temperature. Following this, 5 mg of Cu–B₂O₃/PBN catalyst was introduced under continuous stirring. The reaction progress was tracked using high-performance liquid chromatography (HPLC), and the yield was quantified using a standard calibration curve (Fig. S2 and S3). For the substrate scope experiments, 0.3 mmol of the corresponding derivatives was used. After the reaction, the catalyst was removed by filtration, and methanol was evaporated under reduced pressure. The product, white crystals of 1(b–m), was obtained through separation and purification via column chromatography (silica gel, eluent: petroleum ether and ethyl acetate). After each cycle, the catalyst was recovered from the solution, washed three times with water, dried under vacuum to remove residual solvents, and then reused for subsequent reaction cycles.

2.5 Characterization

The morphology of the catalysts was observed by transmission electron microscopy (TEM, JEM-2100F). The X-ray diffraction (XRD) patterns of the synthesized samples were analyzed using an X-ray diffractometer (D/MAX-2500, Rigaku, Japan), set to operate at 40 kV and 150 mA with Cu Kα radiation (λ = 0.15406 nm) and a scanning range of 2θ from 5° to 100°. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific K-Alpha instrument with 72 W Al Kα X-ray as the excitation source. The binding energies were calibrated by contaminant C at 284.8 eV. Fourier transform infrared (FT-IR) spectra were performed on a Thermo-Fisher scientific Prestige-21 FT-IR instrument using KBr as dilute agent. UV-vis absorption spectra were measured at room temperature with a UV-vis spectrophotometer (TU-1901, Persee, China). The actual Cu content in the Cu–B₂O₃/PBN catalyst was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent 5110). The morphology and elemental composition of the catalysts were observed using field emission scanning electron microscopy (FESEM) (TESCAN MIRA LMS, Czech Republic) with energy dispersive X-ray spectrometer (EDX). Thermal stability was evaluated using thermogravimetric-differential scanning calorimetry (TG-DSC) (PerkinElmer STA 6000, USA). The N₂ adsorption–desorption isotherms were recorded at 77 K using a Micromeritics ASAP 2460 (USA), and the surface area was calculated using the BET method.

3. Results and discussion

3.1 Characteristics of the Cu–B₂O₃/PBN nanocatalysts

The structure of the Cu–B₂O₃/PBN sample was analyzed using XRD (Fig. 1a). The XRD pattern of Cu–B₂O₃/PBN displays two distinct diffraction peaks at 2θ = 26.7° and 43.6°, which are closely aligned with the characteristic diffraction peaks observed in PBN (BN, PDF #34-0421). The crystal structure of boron oxide on the Cu–B₂O₃/PBN sample was validated by comparing the two prominent and distinct diffraction peaks at 2θ = 14.6° and 27.8° with the standard pattern of B₂O₃ (PDF #06-0297). The distinct peaks seen at 2θ = 43.19°, 50.29°, 73.88°, and 89.61° correspond to the Cu crystal planes (111), (200), (220), and (311), respectively (PDF #70-3038). These peaks confirm the successful incorporation of Cu. The actual Cu loading determined by ICP was 6.21 wt%, which is in good agreement with the nominal value based on the precursor amount used during synthesis. In the FT-IR spectra (Fig. 1b), the prominent peaks at 1439 and 782 cm⁻¹ are associated with the in-plane stretching and out-of-plane bending vibrations of the B–N bond, similar to those observed in the PBN structure. The shift of the absorption band from 806 cm⁻¹ in PBN to 782 cm⁻¹ in Cu–B₂O₃/PBN is indicative of a strong interaction between Cu and the hydroxyl groups on the PBN support. The absorption peak at 1195 cm⁻¹ corresponds to the stretching vibration of the B–O bond, which is formed due to the oxidation of the N–B bond.^35,36 The peaks at 926 and 883 cm⁻¹ are attributed to the out-of-plane bending vibrations of N–B–O bonds, providing further evidence for the formation of B₂O₃ within the Cu–B₂O₃/PBN composite. The morphology of Cu–B₂O₃/PBN samples was examined using TEM (Fig. 1c). The TEM images reveal a homogeneous distribution of Cu nanoparticles, measuring around 7.9 nm in size, on the PBN matrix (Fig. S4). In Fig. 1d, the HRTEM picture was magnified to a resolution of 5 nm, and the lattice fringes of copper nanoparticles on the Cu–B₂O₃/PBN sample were identified. The measurement of the lattice spacing yielded a value of 0.21 nm, which matches the lattice spacing of the (111) crystal plane in the face-centered cubic phase copper crystal. Simultaneously, the presence of lattice fringes in B₂O₃ was observed in the vicinity of the Cu nanoparticles. The lattice spacing was determined to be 0.32 nm, indicating the lattice spacing is specifically associated with the face-centered cubic B₂O₃ (310) crystal plane. The XPS analysis (Fig. S5) was used to describe the composition and chemical condition of the material. A blue shift was seen on the B1s orbital (Fig. 1e) in comparison to the B–N bond of PBN at 190.6 eV. This shift may be attributed to the replacement of some nitrogen atoms with oxygen atoms.³⁷ The observed peak at 193.1 eV may be ascribed to the presence of the B–O bond, which aligns with the findings from XRD and FTIR analysis. This observation suggests that the introduction of B₂O₃ occurs during the synthesis of Cu–B₂O₃/PBN samples. A B–O bond is seen at 533.0 eV on the O 1s orbital (Fig. S6). The high-resolution X-ray photoelectron spectroscopy (XPS) analysis of Cu 2p (Fig. 1f) reveals a complex chemical state for the Cu component in Cu–B₂O₃/PBN. Deconvolution of the peaks at 933.9 and 952.7 eV suggests the presence of Cu⁺/Cu⁰ states. Due to the close binding energies, which make it difficult for XPS to distinguish between Cu⁺ and Cu⁰, Auger electron spectroscopy (AES) was employed for further analysis of Cu–B₂O₃/PBN. The Auger peak at 914.6 eV, as shown in Fig. 1g, is attributed to Cu⁺. After Ar ion etching, the Cu 2p XPS spectrum of the Cu–B₂O₃/PBN sample is shown in Fig. 1h. The Cu LMM spectrum reveals two distinct peaks corresponding to Cu⁰ at 918.8 eV and Cu⁺ at 914.6 eV. This study indicates that the copper nanoparticles within the Cu–B₂O₃/PBN structure possess a core–shell configuration (Fig. 1i). The porous structure of Cu–B₂O₃/PBN was characterized by N₂ adsorption–desorption measurements (Fig. S7). The material exhibits a type IV isotherm with a BET surface area of 20 m² g⁻¹, pore volume of 0.023 cm³ g⁻¹. This porous structure facilitates mass transfer and provides abundant active sites for catalytic reactions. The thermal stability of the Cu–B₂O₃/PBN catalyst, a crucial factor for its application, was assessed by TGA-DSC (Fig. S8). The minimal weight loss and absence of thermal events below 80 °C confirm the structural integrity of the material under the reaction conditions employed (≤80 °C).

Fig. 1 (a) XRD patterns of the samples. (b) FTIR spectra of Cu–B₂O₃/PBN. (c) and (d) HRTEM images of Cu–B₂O₃/PBN. The yellow circles in the inner image of (c) mark the enlarged Cu particles. XPS spectra of Cu–B₂O₃/PBN: (e) B 1s and (f) Cu 2p. Auger Cu LMM of Cu–B₂O₃/PBN: (g) no etching and (h) argon ion etching (30 s). (i) Core–shell structure of surface Cu particles.

3.2 The ipso-hydroxylation of arylboronic acid over Cu–B₂O₃/PBN

The reaction progress of the ipso-hydroxylation of phenylboronic acid in water was closely monitored using HPLC (Fig. 2a), with comprehensive optimization results presented in Table 1. Initially, we dissolved 0.2 mmol of phenylboronic acid in 5.0 mL of water within a 25 mL round-bottom flask, maintaining constant stirring in open air, which yielded a 37.6% phenol production (Table 1, entries 1). Recognizing that bases often enhance yields in various catalytic systems, we experimented with different bases. Our findings indicated that 10 mol% K₂CO₃ notably accelerated the reaction, achieving a 99.0% yield within 300 minutes (Fig. 2b). We further optimized the reaction conditions by adjusting variables such as temperature, catalyst concentration, and solvent type (Table 1). Notably, increasing the reaction temperature to 80 °C in a water solvent significantly enhanced the product yield to 99% (Fig. 2c). A temperature of 80 °C was thus identified as the optimal condition for this ipso-hydroxylation reaction in open air.

Fig. 2 (a) HPLC spectra of benzene-boric acid converted to phenol in water. (b) The effect of different bases on yield. (c) Catalyst temperature effect on yield. (d) Effect of different dosages on yield.

Table 1 Optimization of the reaction condition^a

Entry	Different catalysts	Catalyst amount (mg)	Reaction condition	Different base	Amount of base (%)	Reaction temperature (°C)	Solvent	Yield^b (%)
a Typical reaction conditions: 0.2 mmol of substrate 1a, 2.5–10 mg of catalyst, 5 mL of solvent, at room temperature for 5 h. b Based on HPLC yield.
1	Cu–B2O3/PBN	5	Open-air	—	—	30	H2O	37.6
2	Cu–B2O3/PBN	5	Open-air	K2CO3	10 mol	80	H2O	99
3	Cu–B2O3/PBN	5	Open-air	TEA	10 mol	80	H2O	90
4	Cu–B2O3/PBN	5	Open-air	NaOH	10 mol	80	H2O	36
5	Cu–B2O3/PBN	5	Open-air	Py	10 mol	80	H2O	35
6	Cu–B2O3/PBN	5	Open-air	Na2CO3	10 mol	80	H2O	35
7	Cu–B2O3/PBN	5	Open-air	K2CO3	5 mol	80	H2O	85
8	Cu–B2O3/PBN	5	Open-air	K2CO3	15 mol	80	H2O	92
9	Cu–B2O3/PBN	5	Open-air	K2CO3	20 mol	80	H2O	86
10	Cu–B2O3/PBN	5	Open-air	K2CO3	10 mol	30	H2O	20
11	Cu–B2O3/PBN	5	Open-air	K2CO3	10 mol	50	H2O	80
12	Cu–B2O3/PBN	2.5	Open-air	K2CO3	10 mol	80	H2O	23
13	Cu–B2O3/PBN	7.5	Open-air	K2CO3	10 mol	80	H2O	80
14	Cu–B2O3/PBN	10	Open-air	K2CO3	10 mol	80	H2O	76
15	Cu–B2O3/PBN	5	Open-air	K2CO3	10 mol	80	EtOH	4
16	Cu–B2O3/PBN	5	Open-air	K2CO3	10 mol	80	DMF	3
17	Cu–B2O3/PBN	5	Open-air	K2CO3	10 mol	80	MeCN	0
18	Cu–B2O3/PBN	5	Open-air	K2CO3	10 mol	80	EtOAc	0
19	Cu–B2O3/PBN	0	Open-air	K2CO3	10 mol	80	H2O	0
20	Cu–B2O3/PBN	5	Nitrogen	K2CO3	10 mol	80	H2O	17
21	Cu–B2O3/PBN	5	Open-air	—	—	80	H2O	59.6

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To determine the optimal conditions, we conducted the reaction using varying amounts of the Cu–B₂O₃/PBN catalyst, finding that 5 mg of catalyst was sufficient to complete the reaction in 300 minutes at 80 °C (Table 1, entries 2, 12–14; Fig. 2d). We also explored the solvent's influence on reaction reactivity, identifying water as the most suitable solvent for this system (Table 1, entry 2, 15–18). Notably, in the absence of the catalyst, no product formation occurred (Table 1, entry 19). Conducting the ipso-hydroxylation reaction under N₂ conditions yielded only a 17% product (Table 1, entry 20), suggesting the critical role of oxygen from air in phenol formation.

Furthermore, we extended our study to include the ipso-hydroxylation of phenylboronic acid using other copper-supported catalysts and commercially available copper compounds under optimized conditions. These variations produced phenol in moderate to low yields (Table 2, entry 1–8). The influence of substituents on Cu–B₂O₃/PBN's catalytic activity was investigated using various phenylboronic acid derivatives (Table 3). Conducted under optimized conditions, this study revealed that phenolic products with electron-donating groups exhibit a decreasing yield over time, likely due to increased susceptibility to oxidation. Conversely, the yield for phenolic products with electron-withdrawing groups remained relatively stable (Table S1). Notably, the catalyst also demonstrated excellent compatibility with strong electron-withdrawing groups. As shown in Table 3, 4-cyanophenylboronic acid and 4-nitrophenylboronic acid were smoothly converted to the corresponding phenols 2i and 2j in high yields of 85% and 88% within 3 h, respectively. This result highlights the remarkable functional group tolerance and high reactivity of our Cu–B₂O₃/PBN catalytic system. After 90 minutes, we detected a 10% yield of 1-naphthol (2g), but no product was observed after 3 h. Additionally, 2-hydroxythiophene (2h) was not detected (Table S1).

Table 2 Effect of other catalysts on the yield of phenol^a

Entry	Different catalysts	Catalyst amount (mg)	Reaction condition	Different base	Amount of base (%)	Reaction temperature (°C)	Solvent	Yield^b (%)
a Typical reaction conditions: 0.2 mmol of substrate 1a, 2.5–10 mg of catalyst, 5 mL of solvent, at room temperature for 5 h. b Based on HPLC yield.
1	PBN	5	Open-air	K2CO3	10 mol	80	H2O	14
2	B2O3–PBN	5	Open-air	K2CO3	10 mol	80	H2O	17.8
3	Cu/PBN	5	Open-air	K2CO3	10 mol	80	H2O	51.2
4	Cu(NO3)2·3H2O	5	Open-air	K2CO3	10 mol	80	H2O	90.4
5	CuCl	5	Open-air	K2CO3	10 mol	80	H2O	85.4
6	CuCl2	5	Open-air	K2CO3	10 mol	80	H2O	83.5
7	Cu2O	5	Open-air	K2CO3	10 mol	80	H2O	64
8	CuO	5	Open-air	K2CO3	10 mol	80	H2O	49

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Table 3 Substrate scope for 80 °C Cu–B₂O₃/PBN-catalyzed ipso-hydroxylation of arylboronic acid^a,b

a Reaction conditions: 1 mmol of substrate 1, 20 mg of catalyst, and 15 mL of H2O as the solvent. b Isolated yield.
2a 99% (5 h)	2b 74% (10 h)	2c 59% (18 h)
2d 37% (10 h)	2e 97% (10 h)	2f 41% (10 h)
2g 10% (1.5 h)	2h 0% (1 h)	2i 85% (3 h)
2j 88% (3 h)

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3.3 The C–C homocoupling of aryl boric acid over Cu–B₂O₃/PBN

The C–C homocoupling reactions of arylboronic acids were selected as the benchmark to further investigate the catalytic performance and selectivity of the synthesized Cu–B₂O₃/PBN catalyst. For initial experiments, 0.2 mmol of phenylboronic acid, serving as a model substrate, was mixed with 5 mg of the Cu–B₂O₃/PBN catalyst and 5 mL of methanol, and the reaction was conducted at room temperature in open air. The results demonstrated the high reactivity of the catalyst, achieving a biphenyl yield of 94% (Fig. 3a). It was observed that increasing the catalyst amount to 5 mg maximized the yield of biphenyl, with additional catalyst not significantly altering the outcome (Table 4, entries 1–5; Fig. 3b).

Fig. 3 (a) HPLC spectra of benzene-boric acid converted to biphenyls in methanol. (b) The effect of different dosages on yield. (c) The effect of different solvents on yield. (d) Yield efficiency for 6 successive reaction cycles.

Table 4 Optimization of the reaction condition^a

Entry	Different catalysts	Catalyst amount (mg)	Reaction condition	Different base	Reaction time	Solvent	Yield^b (%)	Yield^b (%)
a Typical reaction conditions: 0.2 mmol of substrate 1a, 2.5–10 mg of catalyst, 5 mL of solvent, at room temperature for 5 h. b Based on HPLC yield.
1	Cu–B2O3/PBN	5	Open-air	None	300 min	MeOH	94	3
2	Cu–B2O3/PBN	0	Open-air	None	300 min	MeOH	0	0
3	Cu–B2O3/PBN	2.5	Open-air	None	300 min	MeOH	79	2
4	Cu–B2O3/PBN	7.5	Open-air	None	300 min	MeOH	94	3
5	Cu–B2O3/PBN	10	Open-air	None	300 min	MeOH	85	2
6	Cu–B2O3/PBN	5	Open-air	None	300 min	EtOH	11	0.3
7	Cu–B2O3/PBN	5	Open-air	None	300 min	DMF	8	0.24
8	Cu–B2O3/PBN	5	Open-air	None	300 min	MeCN	0	0
9	Cu–B2O3/PBN	5	Open-air	None	300 min	EtOAc	0	0
10	Cu–B2O3/PBN	5	Open-air	None	300 min	H2O	0	99
11	Cu–B2O3/PBN	5	Open-air	K2CO3	120 min	MeOH	93	2.8
12	Cu–B2O3/PBN	5	Open-air	Py	120 min	MeOH	58	1.7
13	Cu–B2O3/PBN	5	Open-air	Na2CO3	120 min	MeOH	46	1.4
14	Cu–B2O3/PBN	5	Open-air	TEA	120 min	MeOH	19	0.58
15	Cu–B2O3/PBN	5	Open-air	NaOH	120 min	MeOH	0	0
16	Cu–B2O3/PBN	5	Open-air	KOH	120 min	MeOH	0	0

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To optimize the reaction parameters, we explored the effect of various solvents on the yield (Table 4, entries 6–10; Fig. 3c), testing both protic and aprotic solvents. Methanol emerged as the most effective solvent for the C–C homocoupling reaction. Notably, conducting the reaction in water did not yield any biphenyl; instead, a considerable amount of phenol was produced. Thus, the optimal conditions for the arylboronic acid homocoupling reaction were identified as using Cu–B₂O₃/PBN as a catalyst in methanol under open air at room temperature for 5 h.

The catalyst Cu–B₂O₃/PBN is essential to this reaction since it would not be able to continue without it. Cu–B₂O₃/PBN is significantly more active and selective than those copper supported catalysts and commercially available copper compounds. We further compared the catalytic activity of this catalyst with other copper supported catalysts and the commercially available CuO, Cu₂O, and copper salts like CuCl₂ and Cu(NO₃)₂ for the model reaction under the standard conditions (Table 5, entries 1–5).

Table 5 Effect of other catalysts on the yield of biphenyl^a

Entry	Different catalysts	Catalyst amount (mg)	Reaction condition	Different base	Reaction time	Solvent	Yield^b (%)	Yield^b (%)
a Typical reaction conditions: 0.2 mmol of substrate 1a, 2.5–10 mg of catalyst, 5 mL of solvent, at room temperature for 5 h. b Based on HPLC yield.
1	CuCl2	5	Open-air	None	360 min	MeOH	13	0.39
2	Cu(NO3)2·3H2O	5	Open-air	None	360 min	MeOH	23	0.69
3	Cu2O	5	Open-air	None	360 min	MeOH	41	1.23
4	CuO	5	Open-air	None	360 min	MeOH	60	1.8
5	PBN	5	Open-air	None	360 min	MeOH	0	0

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The Cu–B₂O₃/PBN catalyst also demonstrated broad applicability in the aerobic homocoupling of various arylboronic acids. As summarized in Table 6, uniformly high yields were obtained regardless of whether electron-donating or electron-withdrawing substituents were present. For sterically demanding substrates such as naphthalene-1-boronic acid, an 85% yield was achieved by extending the reaction time. In contrast, heteroarylboronic acids gave moderate yields, around 50%.

Table 6 Substrate scope for room-temperature Cu–B₂O₃/PBN-catalyzed C–C homocoupling of aryl boric acid^a,b

a Reaction conditions: 1 mmol of substrate 1, 20 mg of catalyst, and 15 mL of methanol as the solvent. b Isolated yield.
3a 94% (5 h)	3b 94% (5 h)	3c 91% (5 h)
3d 90% (5 h)	3e 91% (5 h)	3f 92% (5 h)
3g 80% (15 h)	3h 50% (20 h)	3i 60% (5 h)
3j 65% (5 h)

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Notably, electron-deficient substrates bearing strong electron-withdrawing groups (–CN and –NO₂) were also successfully transformed into the corresponding biphenyls 3i and 3j, with yields of 60% and 65%, respectively. The somewhat lower yields compared to electron-rich substrates can be attributed to the electron-withdrawing effect, which likely decelerates the transmetalation step in the catalytic cycle. Despite this, the results clearly demonstrate the versatility of the Cu–B₂O₃/PBN system in promoting C–C bond formation across substrates with diverse electronic properties.

3.4 Reusability of Cu–B₂O₃/PBN catalyst

The stability and reusability of a heterogeneous catalyst are critical factors for its commercial application. In evaluating these properties for the Cu–B₂O₃/PBN catalyst across both ipso-hydroxylation and C–C homocoupling reactions, we recovered the catalyst post-reaction via centrifugation and subjected it to three ethanol wash cycles. Subsequently, the catalyst was dried under vacuum at 60 °C overnight before being redeployed in new rounds of both reaction types. Remarkably, the Cu–B₂O₃/PBN catalyst demonstrated consistent performance, maintaining its catalytic efficiency through up to six cycles of reuse with negligible activity loss in both reactions, as illustrated in Fig. 3d. The excellent stability and reusability were further confirmed by characterization of the spent catalyst. FESEM images (Fig. S9–S11) show maintained morphology without significant aggregation after six cycles. EDAX analysis confirms minimal copper leaching (∼0.24–0.92 wt% loss). XRD patterns of the reused catalyst (Fig. S12) display identical diffraction peaks to the fresh catalyst, indicating preserved crystal structure of both Cu nanoparticles and B₂O₃ phases.

3.5 The proposed catalytic mechanism of Cu–B₂O₃/PBN

The reaction mechanism was presented through a set of experiments conducted step by step. When conducting the control experiment, a nitrogen atmosphere surrounding was available, and a significance of low yield of the desired product (17%) was attained (Table 1, entry 20). We furthered the reaction experiment without the Cu–B₂O₃/PBN catalyst in air, while no desired product could be detected (Table 1, entry 19). All these can indicate that both the catalyst and the air are indispensable for the transformation.

N,N-diethyl-p-phenylenediamine sulfate (DPD) and horseradish peroxidase (POD) solutions were used to monitor the generation of H₂O₂ in water. With no presence of the catalyst, H₂O₂ was not detected in the reaction solution, as shown in Fig. S13. Whenever the catalyst was added, the ultraviolet-visible spectrum of the DPD-POD mixed solution became pink from the past colorless. Hence, it can indicate that there existed the production of H₂O₂ during the reaction. It can be speculated that superoxide radicals are primarily responsible for the oxidative dehydrogenation of hydrogenated azobenzene. In order to investigate the chemical state of Cu nanoparticles, XPS and AES were adopted to analyze the used catalysts. The results signified no significant difference in the surface chemical state of the used catalyst, compared to the fresh catalyst (Fig. S14 and S15). Further control experiments were carried out to explore the reaction pathway and the role of the catalyst support. The findings demonstrated that B₂O₃–PBN can serve as an effective catalyst support compared to commercial PBN (Fig. S16–S18), and that copper ions played a crucial role in catalysis.

Based on our observations and previous studies,^38,39 we hypothesize that the mechanism for phenol formation involves the generation of radicals, leading to a proposed reaction pathway (see Scheme 1). Initially, oxygen in the air is activated by Cu nanoparticles, resulting in the generation of superoxide radicals and H₂O₂. Subsequently, the H₂O₂ molecules on the surface of Cu–B₂O₃/PBN react with phenylboronic acid to form an adduct. This adduct undergoes rearrangement and subsequent hydrolysis, ultimately yielding phenol. The proposed mechanism for biphenyl formation involves a Cu(I)/Cu(III) catalytic cycle.^40–42 Initially, Cu(I) undergoes double transmetallation with two molecules of phenylboronic acid to form a Ph–Cu(I)–Ph complex. This complex is then oxidized by air to generate a Cu(III) intermediate, which undergoes reductive elimination to release the homocoupled product Ph–Ph. Furthermore, a comparative analysis with other reported catalytic systems (Table S2) reveals that our Cu–B₂O₃/PBN catalyst not only achieves high yields in both ipso-hydroxylation and homocoupling reactions but also operates under milder and greener conditions (e.g., air as oxidant, low catalyst loading, and reusable heterogeneous nature). This positions our catalyst as a competitive and environmentally benign option for the synthesis of phenols and biphenyls.

Scheme 1 The possible mechanism of the solvent selectivity effect of Cu–B₂O₃/PBN on the catalytic conversion of arylboronic acid to phenol and biphenyl.

4. Conclusion

In this study, we successfully synthesized a Cu–B₂O₃/PBN composite material through a straightforward impregnation process. The integration of Cu not only promoted the crystallization of B₂O₃ but also ensured its stable attachment to the PBN framework. This composite material proved to be an efficient and selective heterogeneous catalyst for two distinct reactions: the ipso-hydroxylation of arylboronic acids to phenols in an aqueous medium and the aerobic homocoupling of arylboronic acids to biphenyls in methanol. Both reactions utilized environmentally benign air as the oxidant under mild conditions, highlighting the sustainability of this catalytic system. Moreover, the Cu–B₂O₃/PBN catalyst exhibited excellent stability and reusability, maintaining its catalytic performance over multiple reaction cycles without significant loss of activity. This work underscores the potential of Cu–B₂O₃/PBN as a versatile and robust catalyst for green chemistry applications.

Author contributions

Yumei Zhang: conceptualization, formal analysis, funding acquisition, methodology, resources, supervision, writing – original draft, writing – review & editing. Yue Wang: data curation, formal analysis, investigation, writing – original draft. Dongle Wang: investigation, data curation. Yating Liu: investigation, data curation. Tong Gao: investigation, data curation. Qingqiang Zhou: investigation, data curation. Qingzhi Luo: investigation, resources. Jing An: methodology. Xueyan Li: methodology. Yandong Duan: formal analysis, methodology, funding acquisition, writing – original draft. Desong Wang: conceptualization, formal analysis, resources, supervision.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI).

Supplementary information: supplementary data to this article can be found online. See DOI: https://doi.org/10.1039/d5cy00986c.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 22278349) and Natural Science Foundation of Hebei Province (No. B2025208009). The authors extend their gratitude to Mr. Li Cai (from Scientific Compass https://www.shiyanjia.com) for providing invaluable assistance with the XRD analysis.

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