From α-pinene feedstock to value-added products: scalable and recyclable copper(II) catalysts for allylic oxidation

Abstract

α-Pinene is one of the most abundant and low-cost terpenes that can be used as an attractive and renewable feedstock for generating added-value products via allylic oxidation. Aiming to develop more sustainable catalytic materials for the functionalization of terpenes, in this work two new 2D coordination polymers (CPs), {[Cu₂(μ-pdc)(μ₃-pdc)(H₂mdea)(H₂O)₂]·2H₂O}_n (Cu-mdea) and {[Cu₂(μ-pdc)(μ₃-pdc)(H₃tipa)(H₂O)₂]·4H₂O}_n (Cu-tipa) {H₂pdc = 3,4-pyridinedicarboxylic acid, H₂mdea = N-methyldiethanolamine and H₃tipa = triisopropanolamine} were synthesized via a self-assembly method under green conditions. These compounds were obtained from low-cost and environmentally tolerable reagents, and their scale-up process was also optimized in a ‘crystalline reactor’ from the milligram to gram scale. The obtained CPs were applied as efficient and recyclable heterogeneous catalysts for the mild oxidation of α-pinene. The effects of various reaction parameters, temperature-dependent and mechanistic features, and catalyst stability were investigated in detail, leading to up to 93% of α-pinene conversion with good yields of 4-tert-butylperoxy-2-pinene (42%) and verbenone (25%) as main oxidation products. This study extends the application of α-pinene as a renewable feedstock for the synthesis of value-added oxidation products using recyclable heterogeneous catalysts and mild reaction conditions.

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Sustainability spotlight

The sustainability approach of the present work is addressed through the use of cost-efficient and recyclable heterogeneous catalysts, the exploration of α-pinene as a highly abundant and renewable substrate for added-value oxidation products, and the reaction process under mild conditions. This research aligns with several of the United Nations Sustainable Development Goals (SDGs), particularly those focused on responsible consumption and production (SDG 12), industry innovation and infrastructure (SDG 9), and climate action (SDG 13). This study showcases how sustainable, noble metal-free catalytic systems can deliver high performance, while addressing challenges in chemical transformations of renewable feedstocks.

Introduction

Transition-metal-catalyzed allylic oxidation is recognized as a powerful strategy for a variety of precursors that are important target molecules in the pharmaceutical and fragrance industries.^1,2 Allylic oxidation of alkenes is an excellent example of C–H bond activation with substantial synthetic importance.³ The most common catalysts and/or cocatalysts employed in such transformations are based on copper,⁴ selenium,^5,6 rhodium,⁷ and palladium^8–11 systems. In particular, the latter was extensively employed in the industry to achieve different depths of alkene oxidation, including the incorporation of oxygenated functional groups at the allylic position.^12,13 Due to elevated cost, low turnover numbers and toxicity of noble metals,³ there is a constant need for new catalytic materials and methodologies based on the use of more affordable, abundant and environmentally tolerable first-row transition metals.^14–16

Copper-based catalysis has emerged as a sustainable alternative for many reactions, including oxidative transformations.^17–22 Copper catalysts may provide significant benefits, such as low cost and toxicity, in addition to interesting selectivity and attractive substrate scope.¹⁷ Besides, there is a plethora of copper-containing enzymes with a recognized oxidation function (e.g., particulate methane monooxygenase, multicopper oxidases), which can activate molecular oxygen to generate a selective high-valence oxidizing species, thus motivating the design of biomimetic copper-based coordination compounds as catalysts for the oxidation of various substrates.^23–31 Copper(II) coordination polymers (CPs) may also act as heterogeneous catalysts with potential for recyclability.^{20,29,31–33} The most common approach to generate CPs employs carboxylic acids as building blocks, which ensure charge balance and versatile bridging modes within the coordination polymer structure.^34–37

With regard to substrates for allylic oxidation, nature provides a fascinating set of cheap, abundant, and renewable raw materials known as terpenes.^38–40 These molecules serve as versatile substrates for organic synthesis, owing to their wide-range applications as intermediates in the pharmaceutical, perfume, and fragrance industries.¹⁵ Specifically, α-pinene is one of the most abundant terpenes available from many coniferous trees and essential oils.⁴¹ It possesses a relatively simple structure and well-understood reactivity, thus being an attractive model substrate for investigating novel catalytic systems in oxidation reactions, including the allylic oxidation of terpenes.⁴¹

Considering the above discussion and our interest in developing copper(II) catalytic systems for the mild oxidation of hydrocarbons, the primary research objectives of the present work were: (1) to design and assemble new copper(II) CPs from a multicomponent reaction system composed of Cu²⁺ ions, amino alcohol chelators,^29,42–44 and 3,4-pyridinedicarboxylic acid (H₂pdc) linkers, and (2) to evaluate the catalytic behavior of the obtained CPs as heterogeneous catalysts in the allylic oxidation of α-pinene. As amino alcohol chelators, N-methyldiethanolamine (H₂mdea) and triisopropanolamine (H₃tipa) were explored on account of their low cost, aqueous solubility, and recognized chelating and structure-stabilizing behavior toward copper(II) ions.^45–48 The selection of H₂pdc was governed by the presence of two types of donor atoms (N,O) and its relatively underexplored use for the design of CPs.^49,50 In addition, such N-heterocyclic carboxylic acids and derived ligands may also promote the oxidation of hydrocarbons.⁵¹

Thus, we describe herein the syntheses, scale-up process, full characterization, and crystal structures, as well as a detailed catalytic investigation of two new copper(II) 2D coordination polymers {[Cu₂(μ-pdc)(μ₃-pdc)(H₂mdea)(H₂O)₂]·2H₂O}_n (Cu-mdea) and {[Cu₂(μ-pdc)(μ₃-pdc)(H₃tipa)(H₂O)₂]·4H₂O}_n (Cu-tipa). These CPs were easily assembled in a water/ethanol medium and applied as effective heterogeneous catalysts for the oxidation of α-pinene into value-added products, which include 4-tert-butylperoxy-2-pinene and verbenone.

Experimental

General synthetic procedure for Cu-mdea and Cu-tipa

3,4-Pyridinedicarboxylic acid (3,4-H₂pdc, 0.5 mmol, 83 mg) was dissolved in ethanol (7 mL), followed by the addition of aqueous NH₄OH (4 mmol, 1 mL, 4 M) to produce the reaction solution A (Fig. 1). In parallel, an aqueous solution of Cu(NO₃)₂·3H₂O (1.2 mmol, 1.2 mL, 1 M) was mixed with N-methyldiethanolamine (H₂mdea, 1 mmol, 119 mg) or triisopropanolamine (H₃tipa, 1 mmol, 191 mg) under stirring in air at 80 °C, producing the reaction solution B. Then, the reaction solution B was slowly added into A under constant stirring at 80 °C to give rise to mixture C. This was stirred for an additional 30 min, then filtered and refluxed for 1 h. Finally, the reaction mixture was transferred to an open glass vial that was kept for slow evaporation in the air. Blue crystals were formed in a week after partial evaporation of the filtrate, collected manually, and dried in air at room temperature to give Cu-mdea and Cu-tipa in 65 and 60% yields based on copper(II) nitrate, respectively. Analytical data: {[Cu₂(μ-pdc)(μ₃-pdc)(H₂mdea)(H₂O)₂]·2H₂O}_n (Cu-mdea), calculated for Cu₂C₁₉H₂₇N₃O₁₄ + H₂O (MW 666.5): C, 34.24%; H, 4.39%; N, 6.30%; found: C, 34.16%; H, 4.12%; N, 7.06%. FTIR-ATR (cm⁻¹): 3268 (s br) ν(OH/H₂O), 2108 (w) ν(CH), 1937 (w), 1617 (s) ν as(COO), 1598 (m), 1483 (w) ν s(COO), 1365 (s), 1193 (w), 1166 (w), 1119 (m), 1076 (m), 1060 (w), 1019 (w), 994 (w), 957 (w), 879 (w), 840 (w), 781 (s), 727 (s) and 683 (s). {[Cu₂(μ-pdc)(μ₃-pdc)(H₃tipa)(H₂O)₂]·4H₂O}_n (Cu-tipa), calculated for Cu₂C₂₃H₃₉N₃O₁₇ + H₂O (MW 774.7): C, 35.65%; H, 5.33%; N, 5.42%; found: C, 35.61%; H, 4.66%; N, 6.07%. FTIR-ATR (cm⁻¹): 3396 (s br) ν(OH/H₂O), 2975 (w) ν(CH), 2975 (w), 1627 (s) ν as(COO), 1585 (m), 1553 (m), 1483 (w) ν s(COO), 1382 (s), 1122 (w), 970 (m), 837 (w), 781 (w), 729 (w) and 683 (w). Additional characterization details are given in ESI Fig. S1–S7.†

Fig. 1 Synthesis of Cu-mdea and Cu-tipa in an aqueous ethanol medium.

Scale-up procedure for Cu-mdea and Cu-tipa

In a round-bottom flask, H₂pdc (4.1 g, 25 mmol), water (100 mL), and aqueous NH₄OH (100 mmol, 25 mL, 4 M) were combined to produce the reaction solution A, which was then stirred for 0.5 h at room temperature. In another flask, Cu(NO₃)₂·3H₂O (11.3 g, 47 mmol) was mixed with H₂mdea (47 mmol, 5.6 g) or H₃tipa (47 mmol, 9.0 g) in ethanol (100 mL) for 0.5 h at 80 °C to give the reaction solution B. Afterwards, the solutions A and B were mixed in a 300 mL Lab Reactor FlexyCUBE from SYSTAG, for 3 h at 80 °C. The real-time monitoring of the reactor ensured that the parameters were stable over the reaction time (pH 8–9; pressure 0.98–1.23 bar; reflux cooler: 5 °C; glass anchor stirrer: 300 rpm). Finally, the reaction mixture was filtered and split into twelve Falcon tubes. They were left to rest in a reflux system at 80 °C for 3 h. As a result, blue crystalline samples of Cu-mdea and Cu-tipa were instantly formed. To achieve higher yields, the Falcon tubes were opened and left to slowly evaporate at room temperature for one week. For details, see the diagram in Fig. S11 (ESI).† The above-mentioned optimized conditions were initially established by self-assembly synthesis, then confirmed through iterative experimentation using 10 mL vessels in a ‘crystalline reactor’ (Crystalline – Technobis Crystallization Systems) and finally tested in a 300 mL Lab Reactor. These scale-up conditions were successfully reapplied using a 500 mL round-bottom flask instead of Lab Reactor, demonstrating that the procedure is easily accessible and may be performed without using specialized equipment or precise parameters. The pivotal step in the procedure is to split the filtered reaction mixture and keep it without stirring under near reflux conditions (80 °C, 3 h).

Single-crystal X-ray diffraction

For Cu-mdea, a blue block-shaped crystal with the dimensions of 0.12 × 0.10 × 0.08 mm³ was selected for collecting the X-ray data on a Bruker APEX-II CCD diffractometer at 298 K. Data were measured using Mo Kα radiation (λ = 0.71073 Å). For Cu-tipa, a dark blue block-shaped crystal with the dimensions of 0.11 × 0.08 × 0.05 mm³ was mounted and data were collected using a Bruker D8 Venture, dual, Photon II diffractometer at 150 K and a Mo Kα radiation (λ = 0.71073 Å). Data reduction, scaling and absorption corrections were performed using SAINT V8.40B,⁵² for both Cu-mdea and Cu-tipa. The structure of Cu-mdea was solved with the ShelXS 2018/2 (ref. 53) program using iterative methods and Olex2 as the graphical interface.⁵⁴ For Cu-tipa, a multi-scan absorption correction was performed using TWINABS-2012/1,⁵⁵ followed by structure solution by the ShelXS⁵⁶ using direct methods. Both crystal structures were refined with ShelXL 2018/3 (ref. 57) using full matrix least squares minimisation on F². All non-hydrogen atoms were refined anisotropically. Most hydrogen atom positions were calculated geometrically (C–H = 0.87–0.98 Å) and refined using the riding model with U_iso(H) = 1.2U_eq(C) or 1.5U_eq(C) for H atoms bonded to oxygen atoms. The structure of Cu-tipa was refined as a 2-component twin [BASF 0.143(2)]. CCDC codes 2381054 and 2381055.

Powder X-ray diffraction

Powder X-ray diffraction data for the compounds were obtained using a D8 Advance diffractometer with Cu Kα (λ = 1.54056 Å), operating at 40 kV and 30 mA, with a Ni filter and a LynxEye linear detector. The diffractograms were collected in the 2θ angular range from 5 to 50°, with a typical step size of 0.02°, and counting time ranging from 0.5 to 1 s per step with a divergence slit of 0.6 mm and primary/secondary Soller slits of 2.5°. A comparison of simulated and experimental diffractograms confirmed the purity of the as-prepared microcrystalline products (Fig. S4 and S7, ESI†).

General procedure for oxidation of α-pinene

A Cu-based catalyst (0.5 to 3 mol% of solid catalyst vs. substrate) and a solution of α-pinene (96 μL, 0.6 mmol) in CH₃CN (1 mL) were combined in a 2 mL catalytic flask, equipped with an 11 mm stirring bar. Subsequently, TBHP (70% in H₂O, 167 μL, 1.2 mmol) was introduced. The catalytic flask was sealed, and the reaction medium was stirred at 1200 rpm for 5–12 h at 60 °C. Nitromethane (71 μL, 1.32 mmol) was used as an internal gas chromatography standard and introduced into the initial reaction mixture to monitor the reaction progress. Small aliquots were withdrawn and analyzed via gas chromatography (GC). Before collecting aliquots, the reaction mixture was allowed to rest for 3–5 min to ensure that the catalyst settled to the bottom of the vial. At the end of the reaction, the resulting solution was centrifuged at 7000 rpm for 2 min to separate the catalyst. Finally, the catalyst was washed with ethanol three times, left to slowly dry at room temperature for 24 h, and reused in the next run. For further details on the catalytic experiments, see the ESI.†

Results and discussion

Synthesis, scale-up, and characterization of Cu-mdea and Cu-tipa

The 2D coordination polymers {[Cu₂(μ-pdc)(μ₃-pdc)(H₂mdea)(H₂O)₂]·2H₂O}_n (Cu-mdea) and {[Cu₂(μ-pdc)(μ₃-pdc)(H₃tipa)(H₂O)₂]·4H₂O}_n (Cu-tipa) were assembled by reacting copper(II) nitrate with the corresponding amino alcohol chelators (H₂mdea or H₃tipa) and H₂pdc as a linker. The syntheses were performed in water–ethanol medium at 80 °C. To control the pH (up to 9) of the reaction mixture, an aqueous NH₄OH solution was used. The crystal structures of the obtained compounds were determined by single-crystal X-ray diffraction and further confirmed by standard methods, including FTIR spectroscopy, elemental and thermal analyses, and powder X-ray diffraction.

With an objective to scale-up the synthesis process from a milligram to gram scale, the reactions were carried out in a ‘crystalline reactor’. The quantities of the starting materials were increased proportionally, and the reaction parameters remained similar to those employed in the small-scale model reaction. The success of the scale-up process depended on splitting the reaction medium into different Falcon tubes and subjecting them to a reflux system at 80 °C without stirring. Over the next several hours, a blue crystalline powder formed on the walls of the Falcon tubes. This process yielded a significant amount of products (∼8 g scale, Fig. S11, ESI†). The main advantages of these findings include the consistent reproducibility of the syntheses of these two copper CPs on a larger scale. This scale-up process is robust and can be easily adapted for the synthesis of similar CPs. In a separate attempt, the reaction solution was left to reflux in a large reaction vessel. However, no crystals were obtained, emphasizing the importance of splitting the solution into smaller flasks for reflux to achieve good yields and synthetic reproducibility.

In fact, the optimized scale-up process resulted in higher yields of Cu-mdea (69%) and Cu-tipa (74%) when compared to a mg-scale self-assembly reaction (60%). Additionally, the similarity in PXRD patterns of the samples obtained by different methods confirmed that both compounds retain their pure crystalline phases (Fig. S12 and S13, ESI†).

FTIR spectroscopic data of Cu-mdea and Cu-tipa show broad absorption bands with maxima at 3268 and 3396 cm⁻¹ (Fig. S2 and S5, ESI†) that correspond to ν(OH/H₂O) vibrations of hydroxyl groups in amino alcohol ligands and water moieties. The ν(C=N)_py bands are observed at 1598 and 1594 cm⁻¹ for Cu-mdea and Cu-tipa, respectively. When compared to a free H₂pdc ligand (1641 cm⁻¹), these bands are shifted to lower wavenumbers, which is indicative of N_py coordination to the copper(II) center. Furthermore, strong absorptions with maxima at 1617 and 1365 cm⁻¹ for Cu-mdea and 1627 and 1385 cm⁻¹ for Cu-tipa are attributed to ν_as(COO⁻) and ν_s(COO⁻) bands, respectively.⁵⁸ These bands also support a deprotonated nature of pdc²⁻ ligands, which agrees with the X-ray crystallographic data.

Thermogravimetric analyses of both compounds (Fig. S3 and S6, ESI†) indicate three major mass loss events. The first one at 100–130 °C corresponds to a release of all water molecules, with the mass loss of 12.1% (calcd 12.6%) for Cu-mdea and 13.2% (calcd 13.4%) for Cu-tipa. Further thermal effects refer to the gradual thermal-decomposition of the samples that starts at ∼190–200 °C and is essentially complete at ∼380–400 °C, resulting in the formation of 2CuO (sample remaining weight: exp. 22.5%, calcd 22.2% for Cu-mdea; exp. 21.0%, calcd 21.3% for Cu-tipa).

Structural description

The compounds Cu-mdea and Cu-tipa are 2D coordination polymers, which crystallize in the centrosymmetric monoclinic system with the P2₁/c and C2/c space groups, respectively. X-ray crystallographic details are summarized in Table 1, while the selected geometric parameters are listed in Tables S1–S4 (ESI).† Both CPs possess two types of copper(II) centers, two distinct μ-pdc²⁻ and μ₃-pdc²⁻ linkers and one neutral amino alcohol ligand (Fig. 2). In Cu-mdea, the ‘central’ 5-coordinated Cu(II) atom adopts an almost ideal square pyramidal {CuO₃N₂} geometry (τ₅ = 0.04).⁵⁹ In equatorial positions, there are two oxygen and two nitrogen atoms from the bridging pdc²⁻ ligands, whereas the water ligand occupies an axial position. The second Cu(II) atom adopts a distorted square pyramidal {CuO₄N} environment (τ₅ = 0.27),⁵⁹ which is taken by two O and one N donors of H₂mdea, along with a carboxylate oxygen from pdc²⁻ and a water ligand. Similar to what is observed in Cu-mdea, the ‘central’ 5-coordinated Cu(II) atom in Cu-tipa shows almost an ideal square pyramidal {CuO₃N₂} environment (τ₅ = 0.07 for Cu1 and τ₅ = 0.01 for Cu3). The coordination distances between Cu–O (1.97–2.41 Å) and Cu–N (∼2.00 Å) are within the range expected for related structures.⁴⁹ Besides, the Cu2 center with a {CuO₄N} environment in the crystal structure of Cu-tipa is also 5-coordinated by H₃tipa and a water ligand (equatorial position), and displays an almost ideal square pyramidal geometry (τ₅ = 0.05 for Cu2). In both structures, each of the “central” copper units, {CuO₃N₂}, is cross-linked to four adjacent units via the pdc²⁻ moieties, thus leading to 2D coordination polymer structures with square-like voids of 8.9 × 8.9 Å dimensions (Fig. 3). From a topological perspective, the crystal structures can be defined as an uninodal 4-connected net with a (4⁴.6²) point symbol, which corresponds to an sql (Shubnikov tetragonal plane net) topology.^60,61 The potential open windows are blocked due to the intercalated arrangement of the crystalline packing in the 2D layers and the positioning of the [Cu(aminoalcohol)]²⁺ units at the center of the square windows (Fig. S9 and S10, ESI†). Despite this, both compounds may endow materials with bulk mesoporous properties.^62,63 Although neither H₂mdea nor H₃tipa contributes to the network topology, their incorporation plays a structure-stabilizing role.

Table 1 Crystal data and structure refinement details for Cu-mdea and Cu-tipa

Compound	Cu-mdea	Cu-tipa
Formula	{[Cu2(μ-pdc)(μ3-pdc)(H2mdea)(H2O)2]·2H2O}n	{[Cu2(μ-pdc)(μ3-pdc)(H3tipa)(H2O)2]·4H2O}n
D calc./g cm^−3	1.705	1.611
μ/mm^−1	1.758	1.446
Formula weight	648.51	755.64
Size/mm^3	0.12 × 0.10 × 0.08	0.11 × 0.08 × 0.05
Crystal system	Monoclinic	Monoclinic
Space group	P21/c	C2/c
a/Å	13.9122(6)	17.6112(11)
b/Å	10.8868(4)	20.3103(11)
c/Å	16.8263(7)	17.6199(11)
α/°	90	90
β/°	97.444(2)	98.672(3)
γ/°	90	90
V/Å^3	2527.02(18)	6230.4(6)
Z	4	8
Radiation type	Mo Kα	Mo Kα
Measured Refl's	49 574	172 915
Indep't Refl's	4615	5795
Refl's I ≥ 2 s(I)	4087	5052
Rint	0.0367	0.0879
Parameters	328	435
Restraints	6	5
GooF	1.044	1.043
wR2	0.1678	0.1884
R1	0.0589	0.0632

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Fig. 2 Crystal structure fragments of Cu-mdea (left) and Cu-tipa (right). Color codes: Cu green balls, O red, N blue, C gray. H atoms and solvent molecules were excluded for clarity.

Fig. 3 (a) Illustration of ‘central’ {CuO₃N₂} blocks interconnected by four adjacent square pyramidal units through pdc²⁻ linkers in Cu-tipa (similar interconnections are seen in Cu-mdea, top). Color codes: Cu green balls, O red, N blue, C gray. (b) 2D metal–organic layers formed in Cu-mdea (top) and Cu-tipa (bottom) directed by the {CuO₃N₂} units. Topological representation of the simplified uninodal 4-connected underlying net with the sql topology; green balls are the centroids of the {CuO₃N₂} units.

Catalytic oxidation of α-pinene

Oxidation of α-pinene (Scheme 1) is a good model to investigate allylic oxidation reactions catalyzed by copper(II) coordination polymers, owing to the abundance, low cost, and, most importantly, structure of this substrate.⁶⁴

Scheme 1 Cu-catalyzed oxidation of α-pinene ((A) α-pinene, (B) 4-tert-butylperoxy-2-pinene, (C) verbenone, (D) pinene oxide).

To determine the optimal conditions for α-pinene oxidation, a screening of many variables was investigated in this catalytic study, including temperature, solvent, substrate, oxidant, and catalyst loading. Initially, several commercial oxidants [tert-butyl hydroperoxide (TBHP), 70% in H₂O; H₂O₂, 50% in H₂O, tert-butyl peroxybenzoate (TBPB); m-chloroperoxybenzoic acid (m-CPBA); and peracetic acid (CH₃COOOH), 50% in H₂O] were screened alongside Cu-mdea as a catalyst to identify the most effective oxidant for this type of catalytic system. The reactions were run in an aqueous acetonitrile medium, and a summary of the results is presented in Table 2. A typical chromatogram of the reaction mixture in the α-pinene oxidation is given in Fig. S15 (ESI).†

Table 2 Oxidation of α-pinene with different oxidants catalyzed by Cu-mdea

Entry	Oxidant	Oxidant-to-substrate molar ratio^a	Conversion (%)^c		Main identified products^b
			1 h	9 h
a Conditions: α-pinene (0.6 mmol), Cu-mdea (1 mol%), oxidant (1.2–1.8 mmol), CH3CN (1 mL), 1–9 h, 1200 rpm, 60 °C, catalytic flask (2 mL). b Traces of these products were identified by GC-MS. c Conversion: [moles of initial α-pinene – moles of final α-pinene/(moles of initial α-pinene) × 100%].
1	TBHP, 70% in H2O	2	59.0	89.0	4-tert-Butylperoxy-2-pinene and verbenone
2	H2O2, 50% in H2O	3	94.0	97.0	Catalyst decomposition^b
3	TBPB	3	56.0	64.0	Partial oxidation of solvent and catalyst decomposition^b
4	m-CPBA	2	—	97.0	Epoxide derivatives (diols, verbenone, α-campholenal)^b
5	CH3COOOH, 50% in H2O	3	30.5	41.0	Epoxide derivatives (diols, α-campholenal)^b

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As a result, a noticeable increase in the conversion of α-pinene was achieved using TBHP (89% conversion), H₂O₂ (97%), and m-CPBA (97%) as oxidants after 9 h (Table 2, entries 1, 2, and 4). In contrast, a moderate conversion was observed when the same reaction is carried out with TBPB and peracetic acid (64 and 41% conversion, respectively; Table 2, entries 3 and 5). Although the reactions with H₂O₂ (1 h) and m-CPBA (9 h) led to the highest conversion values in this study, low product selectivity and catalyst degradation pose challenges for the application of these systems in the allylic oxidation reactions. These non-selective reactions can be explained either by the reactivity order at an α-position (CH > CH₂ > CH₃), in which the abstraction of an H atom favors the formation of diverse resonance-stabilized species, resulting in different products, such as 4-tert-butylperoxy-2-pinene, verbenol and verbenone; or via a transition-metal-catalyzed epoxidation of a double-bond, giving rise to pinanediol and α-campholenal (Scheme S1, ESI†).^24,65

Based on these preliminary studies, TBHP was selected as an oxidant due to its reactivity and tolerance toward the catalyst, thus facilitating its recovery. This assumption is consistent with prior studies on the Cu-catalyzed allylic oxidation with TBHP.⁶⁶ Moreover, the optimal molar ratio of oxidant-to-pinene in the oxidation reaction using CH₃CN as the solvent was determined to be 2 : 1, with no significant variations compared to the molar ratio of 3 : 1 (Table S5, ESI†). The gradual addition of TBHP also did not imply any influence on the oxidation reaction (Fig. S16, ESI†).

Since Cu-mdea and Cu-tipa possess isoreticular coordination networks, it was expected that their catalytic activities might be similar. Indeed, both 2D CPs exhibited comparable catalytic behavior in the allylic oxidation of α-pinene at 60 °C (Fig. 4), resulting in the maximum substrate conversion of 90–92%, with the formation of 4-tert-butylperoxy-2-pinene (31%, Fig. 4B) and verbenone (13.5%, Fig. 4C) as main products after 12 h. The formation of alkyl tert-butyl peroxide as the primary reaction product is particularly interesting, given its status as a highly reactive intermediate with potential application in organic synthesis.⁶⁷ In contrast, in the absence of catalyst (blank test), there is no significant conversion of α-pinene (25.5% conversion after 12 h), reaching a total 6.5% yield of the main products (Table S6, ESI†).

Fig. 4 Cu-catalyzed oxidation of α-pinene with TBHP over Cu-mdea (▼) and Cu-tipa (▲) (1 mol%) compared to a blank test without Cu²⁺ (●). Conversion of α-pinene (A), accumulation of 4-tert-butylperoxy-2-pinene (B) and verbenone (C). Conditions: α-pinene (0.6 mmol), catalyst (6 μmol, Cu-mdea and Cu-tipa), TBHP (70% in H₂O, 1.2 mmol), CH₃CN (1 mL), 12 h, 1200 rpm, 60 °C, catalytic flask (2 mL).

Furthermore, to enhance our comprehension of the α-pinene oxidation reaction, an investigation was conducted involving variations of protic and aprotic solvents, using TBHP as the oxidant. Hence, acetonitrile, ethyl acetate, acetone, DMSO, propan-2-ol and THF were screened with Cu-mdea as catalyst (Table S7, ESI†). In polar aprotic solvents, the highest conversion (89%) and total product yield (44%) were achieved in CH₃CN, while in DMSO the respective values are 83% and 33%. However, when using propan-2-ol as a solvent, the conversion of α-pinene decreased significantly to 10.5%, possibly due to competing oxidation of the solvent and/or its binding to the catalyst.⁶⁸ These experimental data offer evidence that the reaction is more efficient in aprotic solvents with a relatively high dielectric constant (Table S7, ESI†).

Considering the essential role of catalyst loading in influencing the conversion rate, the catalytic activity of Cu-mdea was investigated by varying its concentration from 0.5 to 3 mol%. Fig. 5 and Table S8 (ESI)† illustrate the conversion of α-pinene (84% after 5 h) in a catalytic system comprising 1.5 mol% Cu-mdea. In contrast, a 3.0 mol% loading of the catalyst led to the highest substrate conversion (93%, Fig. 5A). Regarding the product yield, at 3.0 mol% Cu-mdea, an 18.5% yield of verbenone can be obtained after 5 h, which is higher than that for the reactions carried out with 0.5 mol% and 1.5 mol% catalyst loading (11.5 and 14.5% yields, respectively; Fig. 5C). For 4-tert-butylperoxy-2-pinene, the initial rate increased proportionally to catalyst quantity, leading to the most expressive yield of 42% at 3.0 mol% of Cu-mdea (Fig. 5B).

Fig. 5 Cu-catalyzed oxidation of α-pinene with TBHP over Cu-mdea (ranging from 0.5 mol% to 3.0 mol%) and a blank test without Cu²⁺. Conversion of α-pinene (A). Accumulation of 4-tert-butylperoxy-2-pinene (B) and verbenone (C). Conditions: α-pinene (0.6 mmol), Cu-mdea (0.5–3.0 mol%), TBHP 70% in H₂O (1.2 mmol), CH₃CN (1 mL), 9 h, 1200 rpm, 60 °C, catalytic flask (2 mL).

Based on these experimental data, the correlation observed between the quantity of catalyst and the enhanced conversion rates can be related to the enhanced availability of active sites. This assumption suggests that a higher number of active sites does not interfere with the diffusion of the reaction products. However, any increase in the conversion beyond the optimal catalyst quantity would yield only modest benefits within a 9 h timeframe. In addition, during the catalytic tests, the results were well reproducible, with the conversion values of α-pinene and product yields following the expected trends. Once the products are formed, they need to diffuse away from the catalyst surface to make room for the adsorption of new reactants. Consequently, the rate of product diffusion can impact the overall reaction rate and efficiency of the catalyst.^69,70 The conversions remain unaffected by the reaction conditions, indicating that the bulk of the 2D CPs is possibly a mesoporous material. Therefore, the product diffusion process is unlikely to be a limiting factor for the conversion values in this catalytic system.

The proposed mechanistic pathway begins with the Cu(II)-mediated conversion of TBHP into a t-butoxy radical (t-BuO˙) and t-butyl peroxy radical (t-BuOO˙).^71,72 The t-BuOO˙ radicals can facilitate not only the generation of alkyl radicals through hydrogen atom abstraction,⁷³ but also their coordination to a copper(II) center, leading to the formation of active copper-based peroxyl species.⁷⁴ These species may participate in the allylic peroxidation of α-pinene by introducing the peroxyl group at 4-position.⁷⁵ Subsequently, the homolytic cleavage of the peroxyl group can result in the formation of verbenone, as illustrated in Scheme 2.⁷³

Scheme 2 Mechanistic proposal for the allylic oxidation of α-pinene over a Cu-based catalyst with TBHP as oxidant.

The proposed mechanism can be experimentally supported by employing TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl) in a Cu-catalyzed oxidation of α-pinene. TEMPO reacts with t-BuOO˙ to catalyze the formation of dioxygen and t-BuO˙ (Scheme S2, ESI†).⁷⁶ Hence, in the absence of t-BuOO˙, there is no allylic peroxidation of α-pinene, as attested by a low yield of 4-tert-butylperoxy-2-pinene (2% after 6 h). In contrast, verbenone is obtained in a 19% yield (Fig. S18, ESI†), which is slightly higher than that under standard conditions (15%). Besides the homolytic cleavage of the peroxyl group, verbenone may also be formed in a radical system in the presence of O₂,⁷⁷ as promoted by the addition of TEMPO to the system (Scheme S2, ESI†).

The allylic peroxidation of the α-pinene was also validated by isolating 4-tert-butylperoxy-2-pinene from the reaction over Cu-mdea (1 mol%). This elucidation was explored by NMR spectroscopy (¹H, ¹³C, 2D HSQC and ¹H-NOESY) in accordance with the literature data (Fig. S19–S22, ESI†).⁷³

Temperature-dependent catalytic studies

In order to optimize the reaction conditions, a temperature-dependent study was conducted to investigate the oxidation of α-pinene using 1 mol% of Cu-dmea and Cu-tipa, with the reaction temperatures ranging from 40 to 80 °C. Fig. S28 (ESI)† presents an overview of this study, highlighting a similar catalytic behavior of both compounds at higher temperatures. From 60 to 80 °C, there is a 92% conversion of α-pinene into 4-tert-butylperoxy-2-pinene (32% yield) and verbenone (17% yield) after 12 h. Under these conditions, there is a noticeable increase in the initial reaction rate, as indicated by the upward trend in substrate conversion and the formation of the allylic peroxidation product. Another intriguing correlation is the variation in the ratio of 4-tert-butylperoxy-2-pinene (B) to verbenone (C) at different temperatures. At 80 °C, the yields of products B and C were 17 and 32%, respectively, resulting in a B : C molar ratio of 1.9. Conversely, when the reaction is performed at 40 °C, B and C are formed in 5% and 16% yields, respectively (B : C molar ratio of 1 : 3.2). This finding suggests that higher temperatures promote the conversion of reaction intermediates into verbenone. Furthermore, at lower temperatures (40, 50, and 60 °C), the accumulation of pinene epoxides barely surpasses 2%. However, at 70 and 80 °C, pinene epoxides (5.0%) were obtained in the first hour of the reactions. Then, these gradually decompose into products that could not be identified by GC or GC-MS analyses. Fig. S26–S28 and Tables S9–S13 (ESI)† portray the complete temperature-dependent investigation conducted with Cu-mdea and Cu-tipa.

Hot filtration test and catalyst reusability

To assess the influence of potential leaching of homogeneous species from Cu-tipa and Cu-mdea in the catalytic system, hot filtration tests were carried out. Their findings were compared to the standard reaction yields and conversions (Fig. 6). The complete hot filtration procedure is described in the ESI.†

Fig. 6 Hot filtration tests with Cu-mdea (■) and Cu-tipa (▲) compared to the standard oxidation reaction over Cu-mdea (▼). Conversion of α-pinene (A). Accumulation of 4-tert-butylperoxy-2-pinene (B) and verbenone (C). Conditions: α-pinene (0.6 mmol), catalyst (1 mol%), TBHP (70% H₂O, 1.2 mmol), CH₃CN (1 mL), 6 h, 1200 rpm, 60 °C, catalytic flask (2 mL).

The hot filtration test led to a decreased conversion of α-pinene from 92% to 36% after 6 h. This trend was slightly higher than that observed in the blank reaction (Fig. 6A). A similar pattern can be seen in the accumulation of verbenone, reaching the maximum yield of 3.5% (Fig. 6C).

However, an upward trend in the formation of tert-butylperoxy-2-pinene was noticeable, reaching 20% after 6 h (Fig. 6). As shown in Fig. S17 (ESI)† and despite the presence of a minor amount of soluble Cu species in the filtrate after the catalyst removal from the reaction medium, the Cu CPs essentially act as heterogeneous catalysts. Additionally, copper content in the reaction solution was analysed by ICP-OES. In fact, only a minor amount of copper (12.6 ppm) leached from Cu-tipa, while even a lower leaching (5 ppm) was observed in the system containing Cu-mdea. Therefore, both these heterogeneous catalysts are relatively stable during the catalytic process with less than 4% of the copper migrating from the catalyst bulk to the reaction medium.

The efficiency of catalyst recyclability was examined through the successive oxidation of α-pinene using 1 mol% of Cu CPs. The catalysts were reused in consecutive runs without undergoing any pre-treatment, such as thermal or chemical treatment. At the end of each cycle, Cu-mdea and Cu-tipa were recovered at 96% and 92%, respectively. As indicated in Fig. 7, the catalytic behavior of the recovered compounds closely resembles that of the initially used catalyst.

Fig. 7 Recycling test for (a) Cu-mdea and (b) Cu-tipa. Conditions: α-pinene (0.6 mmol), catalyst (1 mol%), TBHP (70% H₂O, 1.2 mmol), CH₃CN (1 mL), 6 h, 1200 rpm, 60 °C, catalytic flask (2 mL).

Cu-mdea maintained its structural integrity throughout the cycles, as confirmed by the powder X-ray diffraction (PXRD) patterns of both the fresh and recovered catalysts (Fig. S30, ESI†). Although Cu-tipa also retained its structure during the first three cycles, it exhibited a decline in activity by the fourth run, with the yield of α-pinene oxidation dropping from 41% to 25.5% (Table S14, entry 6, ESI†). This loss of catalytic activity is likely due to partial leaching of metal ions (Fig. S17, ESI†) and structural transformation, as indicated by additional crystallographic phases and the broadening of PXRD bands (Fig. S30, ESI†). In addition, the activity in terms of the TOF values was also sustained over the catalyst recycling, ranging from 13.6 to 12.6 h⁻¹ (Fig. 7 and Table S14, entries 1, 2, and 3, ESI†).

Several catalytic systems were explored for verbenone synthesis, particularly through the aerobic oxidation of α-pinene. Among these, a supported Co-polyoxometalate catalyst demonstrated notable performance. For example, Maksimchuk et al. achieved a 48% conversion and a maximum verbenone yield of 26% in acetonitrile (50 °C, 1 h).⁷⁸ Selvaraj et al. reported the high selectivity toward verbenone using a 2D mesoporous chromium silicate catalyst, achieving ∼75% conversion with 65% selectivity in chlorobenzene after 9 h.⁷⁹ Other notable catalytic systems include a bimetallic AuCu/TiO₂ (1/1), which achieved a 94% α-pinene conversion and 65% verbenone selectivity in acetonitrile,⁸⁰ and an iron hexadeca chlorinated phthalocyanine on a modified silica, which led to 84% conversion and 23% verbenone selectivity in acetone.⁸¹ In comparison, our study demonstrates superior α-pinene conversion (93%) and good yields of verbenone (25%) along with 4-tert-butylperoxy-2-pinene (42%) after 9 h, in addition to using recyclable copper CP catalysts. The present work thus demonstrates distinct advantages over these catalytic systems, not only in terms of verbenone yields but also in using scalable, recoverable, and easily obtainable 2D copper CPs as catalysts.

Regarding the production of 4-tert-butylperoxy-2-pinene, the most selective product in this study, only one comparable work was found. This previous study employed a Cu(I) complex in an acetonitrile: acetone system, which led to 85% of α-pinene conversion with 39% yield of 4-tert-butylperoxy-2-pinene after 10 days at 0 °C.⁷⁵ In contrast, our work demonstrated superior efficiency, resulting in higher α-pinene conversions and product yields after 9 h at 60 °C, under optimized conditions.

Conclusions

In conclusion, this study presents the successful synthesis, scale-up procedure, and structural characterization of two new 2D coordination polymers, Cu-mdea and Cu-tipa. These were obtained in good yields and under green conditions, using water–ethanol as a reaction medium. The obtained coordination polymers were fully characterized by standard analytical techniques, including FTIR spectroscopy, single and powder X-ray diffraction, and elemental and thermogravimetric analyses. Their structural and topological features were also discussed in detail. Overall, the scale-up process successfully reproduced the Cu-mdea and Cu-tipa materials on a larger gram scale with improved efficiency and cost-effectiveness. This robust process might be suitable for scaling up the synthesis of similar CPs.

Both coordination polymers act as promising heterogeneous catalysts for the allylic oxidation of α-pinene into value-added oxidation products. The catalysts also revealed reasonable structural stability even after several consecutive runs. Cu-mdea and Cu-tipa exhibit a relatively similar catalytic behavior under varying temperature-dependent conditions. Under the optimized conditions, the catalytic system comprising Cu-mdea can lead to a 93% conversion of α-pinene with the formation of up to 42% of 4-tert-butylperoxy-2-pinene and 25% of verbenone. These oxidation products, derived from a low-cost biobased feedstock, can be used as reactive intermediates in organic synthesis.

These findings highlight the potential of the obtained copper(II) compounds as recyclable catalysts for the selective oxidation of α-pinene, making them promising candidates for the development of greener and more sustainable chemical processes. Further research on the design of related catalytic materials and their exploration in the mild oxidative functionalization of terpenes is currently in progress in our laboratories.

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

This project has received funding from the EU's Horizon 2020 research and innovation programme under Grant Agreement No. 860762 (CHAIR project). We also thank the Foundation for Science and Technology, Portugal (FCT, PTDC/QUI-QIN/3898/2020, UIDB/00100/2020, UIDP/00100/2020, LA/P/0056/2020, and UID/00100/2023). C. H. J. Franco acknowledges the research contracts within the PTDC/QUI-QIN/29697/2017 and PTDC/QUI-QIN/3898/2020 projects (2021–2024). This work was partially carried out by G. Correia at Sorbonne Université (France) and Novartis Pharma (Switzerland) during his secondments within the CHAIR project. The authors also thank Tech. Daniel Naehrig for operational support in the process chemistry laboratory at Novartis Pharma and Dr Tiago Fernandes for assistance with some analyses.

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Footnote

† Electronic supplementary information (ESI) available: Additional data on the synthesis, characterization, and catalytic application of Cu-mdea and Cu-tipa (Schemes S1 and S2, Fig. S1–S30, and Tables S1–S14). CCDC 2381054 and 2381055. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5su00009b

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