A palladium–bisoxazoline supported catalyst for selective synthesis of aryl esters and aryl amides via carbonylative coupling reactions

Abstract

The catalytic synthesis of aryl esters and amides has been successfully achieved in the presence of the efficient palladium–bisoxazoline supported on Merrifield's resin (Pd–BOX-M). The palladium heterogeneous catalyst was prepared and characterized using various spectroscopic techniques. A bisoxazoline ligand having a suitable functional group (BOX-OH) was first synthesized, characterized, chemically supported on Merrifield's resin, and finally coordinated to palladium(II) chloride. The catalytic activity and the recycling ability of the new palladium supported catalyst have been investigated in the alkoxycarbonylation and aminocarbonylation of various aryl iodides using different alkyl and aromatic alcohols and amines as nucleophiles. The palladium heterogeneous catalyst demonstrated excellent catalytic activity and very high recycling ability in the above two carbonylation reactions. The palladium heterogeneous catalysts showed an excellent stability under carbon monoxide and under the experimental conditions.

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1. Introduction

Palladium catalyzed carbonylation reactions of aryl halides (carbonylative coupling) in the presence of alcohol or amine nucleophiles represent major industrial processes for the production of value-added bulk and fine chemicals. It is a versatile synthetic pathway which allows the obtaining of a wide range of linear, branched and cyclic carboxylic acids and their derivatives in one step and from easily accessible starting precursors.^1–8 The products of alkoxycarbonylation and aminocarbonylation reactions are extensively used as building blocks for various materials ranging from polymers,⁹ light sensitive and electrically conductive materials to detergents, flavors, fragrances and various pharmaceuticals.^10,11

A plethora of homogeneous palladium catalysts have been described to successfully catalyze carbonylative coupling reactions with high selectivity, activity and low catalyst loading.^12–15 However, the complete removal of the homogeneous catalyst from the cross coupling products is a tedious and costly process. This reduces the chances of industrial implementation of most homogeneous palladium catalysts since metal contamination in the final products is highly regulated by the industries.¹⁶

A suitable method for overcoming the separation problem is by immobilizing the homogeneous catalyst on a solid support.¹⁷ Other than easy removal from the coupling products, the immobilized catalyst can be also effectively recycled and re-used.¹⁸ The ability to separate and reuse the supported catalyst makes it more viable alternative especially from economical point of view. Taking into consideration the substantial advantages of supported catalysts over the homogeneous catalysts, the interest towards the immobilization of palladium catalysts has been increasing rapidly. Several supported palladium catalysts have been reported in literature for the alkoxycarbonylation and aminocarbonylation of aryl iodides, where palladium particles or complexes have been immobilized on organic polymers,^19,20 functionalized silica,^21,22 ZIF-8,²³ MOF-5,²⁴ porous organic polymer,^25,26 and Pd/C.^27–30 However, despite of their potential utility, key obstacles for practical applications of these heterogeneous catalysts include the use of expensive and moisture-sensitive phosphine ligands and non-recyclable metal precursors with low catalytic activity, harsh reaction conditions and short-term stability. Hence, it is necessary to develop highly active and stable heterogeneous catalysts for potential industrial applications. To the best of authors' knowledge, the application of supported palladium-bis(oxazolines) catalysts in carbonylative coupling reactions has not been explored.

As a continuation of our interest in exploring the chemistry and catalytic activities of palladium–bisoxazoline catalysts,^31–33 we wish to report in this study, the synthesis and characterization of a palladium–bisoxazoline complex supported on Merrifield's resin (Pd–BOX-M). The catalytic activity and recycling ability of the new supported catalyst in alkoxycarbonylation and aminocarbonylation reactions of aryl iodides have been studied.

2. Experimental

2.1. Materials

Materials for the synthesis of ligands and complexes were purchased from Sigma-Aldrich Company and were used as received. All solvents (reagent grade) used in the synthesis were distilled and dried before use. Merrifield's peptide resin (50–100 mesh, extent of labelling: 2.5–4.0 mmol g⁻¹ Cl-loading, 1% crosslinked with divinylbenzene) was also purchased from Sigma-Aldrich. The products were purified using flash column chromatography packed with silica gel 170–400 mesh, Fisher chemical (Fisher scientific, US). Merck 60 F₂₅₄ silica gel plates (250 μm layer thickness) were used for thin-layer chromatography (TLC) analyses.

2.2. Instrumentation

¹H and ¹³C NMR spectral data were obtained using 500 MHz NMR machine (Joel 1500 model). Chemical shifts were recorded in ppm using tetramethyl silane (TMS) as a reference and CDCl₃ was used as NMR solvent. Solid state NMR spectral data was recorded using CP-MAS on a Bruker Avans 400 MHz machine. IR spectra were recorded in wave numbers (cm⁻¹) using FT-IR spectrometer (Perkin-Elmer 16F model). A Varian Saturn 2000 GC-MS machine equipped with a 30 m capillary column was used to analyze the products. Agilent 6890 gas chromatograph (GC) was used to monitor the reactions and analyze the products. Elemental analyses were performed on Perkin Elmer Series 11 (CHNS/O) Analyzer 2400. Palladium loading was estimated using inductively coupled plasma-mass spectrometer, X-series 2 ICP-MS, thermos scientific. X-ray photoelectron spectroscopy (XPS) (Thermo Scientific, ESCALAB-250Xi) was employed to find the chemical composition of the supported palladium–bisoxazoline catalysts. The XPS spectra were recorded using Al K radiation (1486.6 eV) as excitation source. The take-off angle θ of the emitted photoelectrons was adjusted to 45° with respect to the normal surface.

2.3. Synthesis of bisoxazoline ligand (BOX-I)

The BOX ligand was prepared using an earlier published procedure.^31–33 A solution of 4-iodophthalonitrile (4.0 mmol) and zinc triflate (5.0 mol%, 0.2 mmol) in dried chlorobenzene (30 mL) was stirred at room temperature for 15 minutes. A solution of 2-amino-2-methyl-1-propanol (8.0 mmol) in dry chlorobenzene (5 mL) was slowly added. The temperature was raised to 135 °C and the reaction mixture was refluxed for 24 hours. The solvent was removed using rotary evaporator. The crude product was dissolved in 30 mL of dichloromethane and extracted twice with distilled water (2 × 20.0 mL). The aqueous layer was then separated and the combined organic layers were dried with anhydrous sodium sulfate. The dichloromethane was removed using a rotary evaporator to obtain the impure product, which was then purified using silica gel column chromatography with dichloromethane/ether (4/1) as eluent.

2,2′-(4-Iodobenzene-1,2-diyl)bis(4,4-dimethyl-4,5-dihydro-1,3-oxazole) (BOX-I). Yield 94%; waxy solid; ¹H NMR (500 MHz, CDCl₃) δ (ppm): 8.01 (s, 1H, C-3 arom), 7.73 (d, J = 10 Hz, 1H, C-5 arom), 7.38 (d, J = 10 Hz, 1H, C-6 arom), 3.99 (s, 2H, OCH₂), 3.98 (s, 2H, OCH₂), 1.30 (s, 6H, CH₃ × 2); 1.29 (s, 6H, CH₃ × 2); ¹³C NMR (125 MHz, CDCl₃) δ (ppm); 27.95 (CH₃ × 4), 67.92 (NCH × 2), 79.40 (OCH₂ × 2), 96.12 (C-4 arom), 127.89 (C-1 arom), 130.11 (C-2 arom), 130.97 (C-6 arom), 138.26 (C-5 arom), 139.07 (C-3 arom), 160.82 (C-4′), 161.47 (C-1′); IR (KBr) ν (cm⁻¹) 2963, 2884, 1642, 1458, 1398, 1352, 1303, 1194, 1089, 1039, 964, 824, 723; GC-MS m/z 398 (M⁺); anal. calc. for C₁₆H₁₉IN₂O₂ (398.24): C, 48.26; H, 4.81; N, 7.03. Found: C, 48.44; H, 4.88; N, 7.22.

2.4. Synthesis of hydroxyl functionalized bisoxazoline ligand (BOX-OH)

2,2′-(4-Iodobenzene-1,2-diyl)bis(4,4-dimethyl-4,5-dihydro-1,3-oxazole) (BOX-I) (0.50 mmol), PdCl₂ (0.025 mmol, 5.0 mol%), K₂CO₃ (1.0 mmol, 2.0 mol equivalent), DMF (2 mL), distilled water (2 mL) and the 4-hydroxy phenylboronic acid (0.6 mmol), were added in a 10 mL round bottom flask. The mixture was stirred at 70 °C for 6 h. After completion of the reaction, the mixture was cooled down and acidified with 1 M HCl. The acidified solution was extracted 3 times with EtOAc and the combined EtOAc extract was dried using anhydrous MgSO₄. The solvent was removed under reduced pressure and the product was purified by silica gel column chromatography using hexane–EtOAc (1 : 9) as an eluent.

3,4′-Bis(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)biphenyl-4-ol (BOX-OH). Yield 94%; light brown oil; ¹H NMR (500 MHz, CDCl₃) δ (ppm): 7.81 (s, 1H), 7.79 (d, J = 10 Hz, 1H), 7.57 (d, J = 10 Hz, 1H), 7.25 (s, 2H), 6.82 (d, J = 10 Hz, 2H), 4.18 (s, 2H, OCH₂), 4.14 (s, 2H, OCH₂),1.51 (s, 6H, NC(CH₃)₂), 1.44 (s, 6H, NC(CH₃)₂); ¹³C NMR (125 MHz, CDCl₃) δ (ppm); 28.0 (NC(CH₃)₂), 28.1 (NC(CH₃)₂), 67.7 (NC(CH₃)₂), 67.9 (NC(CH₃)₂), 79.6 (OCH₂), 79.9 (OCH₂), 116.1, 116.2, 125.7, 127.9, 128.2, 130.3, 130.4, 143.4, 157.3, 162.5, 164.1; IR (CH₂Cl₂) ν (cm⁻¹) 3189, 2968, 2929, 2893, 1650, 1605, 1522, 1460, 1358, 1276, 1177, 1098, 964, 829. GC-MS m/z 364 (M⁺); anal. calc. for C₂₂H₂₄N₂O₃ (364.44): C, 72.51; H, 6.64; N, 7.69. Found: C, 72.25; H, 6.38; I; N, 7.52.

2.5. Synthesis of supported bisoxazoline ligand on Merrifield's resin (BOX-M)

NaH (0.50 mmol) was added in one portion to a stirred solution of 3,4′-bis(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)biphenyl-4-ol (BOX-OH) (0.30 mmol) in dry DMF in a dry flask. The mixture was stirred for 2 h at room temperature and under argon atmosphere. Merrifield's resin (0.30 mmol) was added and the mixture was stirred at 90 °C for 12 h. The solid product was filtered and washed successively with methanol, water, acetone and dichloromethane. The product was dried at room temperature under vacuum.¹⁸

Supported bisoxazoline ligand on Merrifield's resin (BOX-M). Yellow solid; 91% yield; CP-MAS NMR: δ 28.2, 41.7, 67.9, 80.3, 91.4, 120–140 (several signals), 168.8; IR: v_max (cm⁻¹) 3081, 3024, 2965, 2923, 1652, 1604, 1517, 1491, 1452, 1351, 1311, 1244, 1086, 1036, 825, 759, 699.

2.6. Synthesis of palladium(II)–bisoxazoline supported on Merrifield's resin(Pd–BOX-M)

The Merrifield's resin supported bisoxazoline ligand (BOX-M) (0.30 mmol) was stirred in anhydrous ethanol for 30 min. An ethanolic solution of bis(benzonitrile) palladium(II) chloride (0.30 mmol) was added and the resulting mixture was stirred at 50 °C for 12 h. The solid product was filtered, washed thoroughly with ethanol and dried in vacuum.³⁴

Palladium–bisoxazoline supported on Merrifield's resin (Pd–BOX-M). Dark brown solid, 95% yield; CP-MAS NMR: δ 27.5, 41.4, 70.5, 81.7, 96.1, 120–140 (several signals), 182.3; IR: v_max (cm⁻¹) 3056, 3021, 2919, 2844, 1634, 1599, 1494, 1452, 1371, 1327, 1221, 1178, 1068, 1011, 950, 824, 757, 699.

Metal loading from ICP-MS: 6.7% corresponding to 0.6 mmol g⁻¹.

2.7. General procedure for the alkoxycarbonylation of aryl iodides

A basic stainless steel autoclave equipped with a glass liner, gas inlet valve and pressure gauge was used for the reaction. (Pd–BOX-M) catalyst (0.0050 mmol based on palladium), iodobenzene (1.0 mmol), KOH (2.0 mmol) and alcohol (5.0 mL) were added in the glass liner which was then placed in 45 mL autoclave. The autoclave was vented three times with CO and then pressurized to 100 psi CO. The mixture was heated to 100 °C and maintained at this temperature under stirring for the required time. After the reaction is complete, the mixture was cooled down to room temperature and the excess of CO was released under fume hood. The catalyst was carefully separated from the product. The product mixture was immediately analyzed with GC and GC-MS. The recovered catalyst was carefully washed and dried under vacuum in a desiccator before the next use.

2.8. General procedure for the aminocarbonylation of aryl iodides

A basic stainless steel autoclave equipped with a glass liner, gas inlet valve and pressure gauge was used for the reaction. (Pd–BOX-M) catalyst (0.0050 mmol based on palladium), iodobenzene (1.0 mmol), amine (2.0 mmol), triethylamine (3.0 mmol), and acetonitrile (5.0 mL) were added to the glass liner. The glass liner was then placed in 45 mL autoclave. The autoclave was vented three times with CO and then pressurized to 100 psi CO. The mixture was heated to 120 °C and maintained at this temperature under stirring for the required time. After the reaction is complete, the mixture was cooled down to room temperature and the excess of CO was released under fume hood. The catalyst was carefully separated from the product. The product mixture was immediately analyzed with GC and GC-MS. The recovered catalyst was carefully washed and dried under vacuum in a desiccator before the next use.

3. Results and discussion

3.1. Synthesis and characterization of the palladium-bisoxazoline supported on Merrifield's resin (Pd–BOX-M)

The iodo functionalized ligand (BOX-I) was prepared from the reaction of 4-iodophthalonitrile with 2 mol equivalent of 2-amino-2-methyl-1-propanol. The hydroxyl functionalized ligand (BOX-OH) was prepared from the Suzuki–Miyaura cross coupling reaction of BOX-1 with 4-hydroxyphenylboronic acid (Scheme 1).

Scheme 1 Synthesis of Merrifield's supported BOX ligand (BOX-M).

The Merrifield's resin was functionalized with the bisoxazoline ligand (BOX-OH) to form Merrifield's resin supported BOX ligand (BOX-M) followed by a subsequent complexation with palladium chloride to form the polymer supported palladium-bisoxazoline catalyst (Pd–BOX-M) (Scheme 2). The BOX ligands were characterized using analytical and spectroscopic techniques including ¹H and ¹³C NMR, FT-IR, GC-MS, and elemental analysis. The formation of the Pd–BOX-M catalyst was confirmed by FT-IR, CP-MAS NMR, ICP-MS. SEM, XPS and TGA techniques.

Scheme 2 Synthesis of supported Pd–BOX-M catalyst.

The formation of resin supported bis(oxazoline) ligand was confirmed by the appearance of a strong band at 1652 cm⁻¹ in the free ligand and at 1634 cm⁻¹ due to the its complexation with palladium. This peak was initially absent in the spectrum of the unmodified Merrifield's resin. This band is due to the stretching of imino (–C=N–) bond of the oxazoline ring. The shift in the position of the imino band (Δν = 18 cm⁻¹) confirms the coordination of palladium with the supported ligand.

The Merrifield' resin supported BOX-M ligand and its palladium catalyst (Pd–BOX-M) were further analyzed using solid state ¹³C NMR. The NMR data of the polymer bound ligand and its palladium catalyst showed that the resonance due to imino carbon (–C=N) was observed at δ 168.8 in the spectrum of the supported ligand. This band shifted to δ 182.3 after complexation.

The palladium loading on the polymer supported palladium catalyst which was analyzed using ICP-MS, was estimated as 6.7% (0.6 mmol g⁻¹). The thermal stability of both the resin supported ligand and its palladium catalyst was established from the TGA analyses. The Merrifield's resin supported BOX ligand and its palladium catalyst where found to possess high thermal stability with decomposition temperature greater than 350 °C.

XPS analysis was carried out on the supported catalyst in order to get insight on the nature of the palladium atom in the complex. The XPS spectrum of the fresh supported catalyst showed several peaks for palladium in the range of 335 to 341 eV. Two distinctive 3d peaks were identified. The first peak is with binding energy of 334.98 eV (Pd3d_5/2) and the second peak is with binding energy of 340.28 (Pd3d_3/2). These peaks correspond to palladium(II) forms, which confirm that palladium(II) is the main form of palladium in the supported catalyst.

3.2. Evaluation of the catalytic activity of Pd–BOX-M catalyst in the alkoxycarbonylation of aryl iodides

Palladium-catalyzed alkoxycarbonylation reaction of aryl halides is a versatile reaction for the synthesis of various aromatic carboxylic acids and their derivatives. The reaction is of synthetic value due to the exceptionally low cost of carbon monoxide and from the diversity of aromatic esters that can be achieved by selecting the proper alcohol. In our investigation, we have chosen the methoxycarbonylation of iodobenzene using Pd–BOX-M catalyst as a model reaction. We have studied the influence of various reaction parameters including temperature, base, solvent, and the type of palladium catalyst.

A preliminary alkoxycarbonylation reaction of iodobenzene (1a) with methanol (2a) using the catalytic system ([Pd–BOX-M]/KOH/CH₃OH/70 °C) (Table 1, entry 1) yielded 50% of methyl benzoate (3aa) after 3 h reaction. The catalytic activity was found to be highly temperature dependent. The yield of 3aa increased significantly from 50% to 85% when the temperature was raised from 70 °C (Table 1, entry 1) to 100 °C (Table 1, entry 2) and to 90% yield at 110 °C (Table 1, entry 3). Full conversion of iodobenzene and almost quantitative yield of the ester was obtained on raising the temperature to 120 °C (Table 1, entry 4). The selectivity was not affected by changing the reaction temperature.

Table 1 Palladium–bisoxazoline supported on Merrifield's resin (Pd–BOX-M) catalyzed methoxycarbonylation of iodobenzene (1a)^a

Entry	Pd-complex	Solvent (5 mL)	Base	T (°C)	Yield 3aa^b (%)
a Reaction conditions: [Pd] (0.0050 mmol), iodobenzene (1.0 mmol), solvent (5.0 mL), base (2.0 mmol), CO (100 psi), 3 h.b GC yield.
1	Pd–BOX	Neat CH3OH	KOH	70	50
2	Pd–BOX	Neat CH3OH	KOH	100	85
3	Pd–BOX	Neat CH3OH	KOH	110	90
4	Pd–BOX	Neat CH3OH	KOH	120	99
5	Pd–BOX	Neat CH3OH	NaOH	120	99
6	Pd–BOX	Neat CH3OH	K2CO3	120	96
7	Pd–BOX	Neat CH3OH	Et3N	120	93
8	Pd–BOX	CH3CN/CH3OH	KOH	120	99
9	—	Neat CH3OH	KOH	120	Traces
10	Pd(PPh3)2Cl2	Neat CH3OH	KOH	120	96
11	Pd(PhCN)2Cl2	Neat CH3OH	KOH	120	99
12	Pd(OAc)2	Neat CH3OH	KOH	120	95
13	Pd/C	Neat CH3OH	KOH	120	84

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We have further screened various at 120 °C. Potassium hydroxide gave excellent yield (99%) of the product 3aa (Table 1, entry 4). Similarly, full conversion and almost quantitative yield was obtained with NaOH (Table 1, entry 5). Potassium carbonate gave 96% of the methyl benzoate (Table 1, entry 6). The use of an organic base such as triethylamine resulted in an excellent yield of the required ester (93%) (Table 1, entry 7). We have then studied the optimized reaction using acetonitrile as a solvent and methanol as a nucleophile (Table 1, entry 8), where comparable excellent yield of the ester was achieved.

The effect of the type of palladium catalyst on the methoxycarbonylation of iodobenzene was investigated. No product was obtained in the absence of palladium catalyst (Table 1, entry 9). Pd–BOX-M (Table 1, entry 4) yielded a full conversion and 100% selectivity in favor of the methyl benzoate. We have further compared the catalytic activity of our newly prepared Pd–BOX-M with some commercially available palladium complexes and salts such as Pd(PPh₃)₂Cl₂ (Table 1, entry 10) (96%), Pd(PhCN)₂Cl₂ (Table 1, entry 11) (99%), Pd(OAc)₂ (Table 1, entry 12) (95%) and Pd/C (Table 1, entry 13) (84%). Interestingly, the activity of the palladium salts and complexes reported in this study and the supported complexes reported in literature^19–30 were similar in activity to our Pd–BOX-M catalyst. However, Pd–BOX-M has the potential to be recycled and reused for large number of runs for the same or similar reactions, and consequently possessing high turnover number (TON) and turnover frequency (TOF) values.

3.3. Methoxycarbonylation of iodobenzene catalyzed by Pd–BOX-M catalyst. Recycling ability

The recycling ability of Pd–BOX-M supported catalyst was investigated in the methoxycarbonylation reaction of iodobenzene at 100 psi CO pressure and a temperature of 100 °C for 6 h. The result of the recycling experiment is presented on Fig. 1. Remarkably, the supported catalyst could be recycled up to ten times devoid of substantial loss in its catalytic activity. The TON of the supported catalyst was estimated for the 10 cycles as 1884, while the TOF was estimated as 314/h. In order to confirm the effectiveness and the high activity realized with the supported catalyst, we have conducted experiment with the amount of iodobenzene equal to the total amount used in all the ten cycles (10.0 mmol) using the same quantity of supported palladium–bisoxazoline catalyst (0.005 mmol) (substrate to catalyst ratio equals to 2000). Excellent yield of methyl benzoate (95%) was recorded. Similarly, the TON of the supported palladium–bisoxazoline catalysts in the later experiment was estimated as 1860, while the TOF was estimated as 310/h.

Fig. 1 Methoxycarbonylation reaction of iodobenzene. Recycling ability of Pd–BOX-M. Reaction conditions: [Pd–BOX-M] (0.0050 mmol), iodobenzene (1.0 mmol), methanol (5.0 mL), KOH (2.0 mmol), CO (100 psi), 100 °C, 6 h.

3.4. Pd–BOX-M catalyzed alkoxycarbonylation of aryl iodides. Effect of various substrates

The excellent recycling ability realized with the new supported palladium–bisoxazoline catalyst (Pd–BOX-M) in the methoxycarbonylation of iodobenzene encouraged us to study the scope of the supported catalyst in the alkoxycarbonylation reaction of a broad range of substrates using a CO pressure of 100 psi and KOH as a base. Thus, the alkoxycarbonylation of iodobenzene with various alcohols including aliphatic (primary, secondary and tertiary aliphatic alcohols) as well as aromatic alcohols were studied (Table 2). All considered alcohols gave excellent conversions, and in some cases the corresponding esters were isolated in excellent yields. Primary aliphatic alcohols (Table 2, entries 1–3) reacted smoothly to yield the corresponding aromatic esters. The reactivity of primary alcohols was not affected by the length of the carbon chain, however, secondary and tertiary aliphatic alcohols were relatively less reactive compared to the primary alcohols and therefore longer reaction time was required to achieve full conversions of the alkoxycarbonylation reactions (Table 2, entries 4 and 5). In these reactions, the alcohol served both as a nucleophile and as a solvent. The alkoxycarbonylation reaction of iodobenzene with phenol was carried out using acetonitrile as a solvent (Table 2, entry 7).

Table 2 Alkoxycarbonylation of aryl iodide by the Pd–BOX-M catalyst^a

Entry	Aryl iodide 1a–d	Alcohol 2a–h	Product (ester) 3a–k	Time (h)	Yield^b^,^c (%)
a Reaction conditions: [Pd–BOX-M] (0.0050 mmol), aryl iodide (1.0 mmol), alcohol (5 mL), KOH (2.0 mmol), CO (100 psi), 100 °C.b GC yield.c Isolated yields are given in brackets.d Phenol (2.0 mmol), CH3CN (5.0 mL), 120 °C.
1		CH3OH 2a		6	99
2				6	99
3				6	96
4				12	95
5				12	99 [92]
6				12	99 [93]
7^d				6	99 [95]
8				6	99 [92]
9		CH3OH 2a		6	99 [94]
10		CH3OH 2a		6	96 [90]
11		CH3OH 2a		12	99 [92]

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The impact of electronic effect of various aryl iodides on their reactivity in the carbonylation reactions was also studied. Both activated and deactivated aryl iodides worked effectively to lead to the corresponding aromatic esters. The presence of a deactivating group on the aryl iodide enhanced its reactivity and the corresponding ester was isolated in excellent yield (94%) (Table 2, entry 9). Similarly, 4-iodoanisole reacted smoothly to give methyl 4-methoxybenzoate (Table 2, entry 10) in high yield. The methoxycarbonylation of 1,4-diiodobenzene was also successful and yields 92% of dimethylbenzene-1,4-dicarboxylate (Table 2, entry 11).

3.5. Aminocarbonylation of iodobenzene with diisobutylamine catalyzed Pd–BOX-M catalyst. Recycling ability

Palladium-catalyzed aminocarbonylation of aryl halides is a widely used methodology in the synthesis of carboxamides from easily accessible starting materials. Various amides including those with bulky N-substitutions can easily be produced by selecting the suitable aryl halide and amine. We have tested the catalytic activity of our palladium supported catalyst (Pd–BOX-M) in the aminocarbonylation of aryl iodides. Various amines were considered in this reaction.

The recycling ability of Pd–BOX-M was also investigated in the aminocarbonylation reaction of iodobenzene with diisobutylamine (DIBA) in the presence of trimethylamine as a base at 100 psi CO pressure and a temperature of 120 °C for 6 h. The results of the recycling experiments are presented on Table 3. The supported catalyst was recycled up to six times with no significant loss in its catalytic activity. However, a drop in the conversion of iodobenzene to 61% was observed with the supported catalyst during the seventh cycle, which reflects some deactivation of the catalyst due to the leaching of palladium in the presence of trimethylamine and amine as a nucleophile, particle aggregation or poisoning.

Table 3 Aminocarbonylation of iodobenzene with diisobutylamine (DIBA). Recycling ability of Pd–BOX-M catalyst^a

Cycle	Conversion^b%	Product distribution^c%
		5aa	6aa
a Reaction conditions: [Pd–BOX-M] (0.0050 mmol), iodobenzene (1.0 mmol), DIBA (2.0 mmol), Et3N (3.0 mmol), acetonitrile (5.0 mL), CO (200 psi), 120 °C, 6 h.b Determined by GC based on iodobenzene.c Determined by GC.
1	99	96	4
2	99	95	5
3	99	96	4
4	92	96	4
5	88	96	4
6	85	96	4
7	61	95	5

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3.6. Aminocarbonylation of various aryl iodides catalyzed by Pd–BOX-M catalyst

We have extended the scope of the aminocarbonylation reaction to different amines including primary and secondary amines as nucleophiles and various aryl iodides (Table 4, entries 1–7) as substrates. The aminocarbonylation reaction was found to be highly dependent on the type of the amine employed. For instance, secondary amines such as diisobutylamine (4a) and dicyclohexylamine (4b) reacted smoothly with iodobenzene (1a) (Table 4, entries 1 and 2, respectively). The reaction of iodobenzene with diisobutylamine yielded the corresponding carboxamide (5aa) in 96% selectivity and 4% of ketocarboxamide (6aa). However, the reaction of iodobenzene with dicyclohexylamine yielded the carboxamide (6ab) as the only product. The aminocarbonylation of iodobenzene with aniline (4c) as nucleophile was also successful and yielded N-phenylbenzamide (6ac) selectively as the sole product (Table 4, entry 3). The supported catalyst Pd–BOX-M also gives excellent conversions in the aminocarbonylation of iodobenzene with primary amines as nucleophiles (Table 4, entry 4). However, in contrast to secondary amines, primary amines show relatively poor selectivity and a mixture of mono carbonylation and double carbonylation products were obtained (Table 4, entries 4 and 5).

Table 4 Aminocarbonylation of aryl iodides using Pd–BOX supported on Merrifield's resin as catalyst^a

Entry	Aryl halide 1a–d	Amine 4a–e	Conversion^b%	Product distribution^c%
				5	6
a Reaction conditions: Pd–BOX-M (0.0050 mmol), aryliodide (1.0 mmol), amine (2.0 mmol), Et3N (3.0 mmol), CO (100 psi), 120 °C, 6 h.b Percent conversion determined by GC based on aryl iodide.c Determined by GC.
1			99	96 5aa	4 6aa
2			99	100 5ab	0
3			99	100 5ac	0
4			99	65 5ad	35 6ad
5			99	70 5ae	30 6ae
6			99	96 5ba	4 6ba
7			99	93 5ca	7 6ca
8			99	100 5db	0

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The effect of the type of the substituent on the aryl iodide was also investigated in our aminocarbonylation reaction (Table 4). Interestingly, Pd–BOX-M was highly active in the aminocarbonylation reactions of both activated and deactivated aryl iodides. For instance, the aminocarbonylation of 4-iodoanisole (1b) and methyl 4-iodobenzoate (1c) with diisobutylamine were achieved to give excellent conversions and very high selectivities toward the production of the corresponding expected carboxamides (Table 4, entries 6 and 7).

The aminocarbonylation of 1,4-diiodobenzene (1d) with dicyclohexylamine (4b) was also successful and yields 99% of diamide as the only product of the reaction (Table 4, entry 8).

3.7. Characterization of the recycled Pd–BOX-M catalyst

The ability to reuse the supported palladium–bisoxazoline complex several times and in various reactions without significant loss in their catalytic activities demonstrates their interesting stabilities. The interesting results realized with the supported catalysts urged us to carry out further investigations to asses any change in the physical and chemical structures of the used catalysts in comparison with the unused complexes. The recovered catalysts from all the two applications were analyzed with Fourier Transform Infrared Spectroscopy (FT-IR), X-ray Photoelectron Spectroscopy (XPS) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) techniques.

The recovered palladium–bisoxazoline catalyst was washed successively with distilled water, acetone and methanol. The catalyst was then dried in an oven at 100 °C prior to analysis. The dried catalyst was pressed into disc with KBr and analyzed using FT-IR. The FT-IR spectrum for the recovered Merrifield's resin supported palladium–bisoxazoline catalysts (Fig. 2a) was found to be similar with the spectrum of the unused catalyst (Fig. 2b).

Fig. 2 FT-IR spectra of the supported Pd–BOX catalyst: (a) fresh catalyst and (b) recovered catalyst from the alkoxycarbonylation reaction.

The percentage of palladium on the supported catalyst recovered after the tenth cycle of the alkoxycarbonylation reaction were determined using ICP-MS and were found to be 6.0%. Whereas, the amount of palladium on the supported catalyst recovered after the seventh cycle of the aminocarbonylation reaction was estimated as 5.1%.

These results indicate that the amount of palladium on the supported catalyst recovered from alkoxycarbonylation reactions are similar to the amount of palladium in the unused catalyst and could be the reason for the promising recycling ability observed in the alkoxycarbonylation reaction.

On the other hand, the amount of palladium on the supported catalyst recovered from aminocarbonylation reaction was much less than the amount of palladium on the unused supported complex (there is a 24% loss of palladium). This could be the reasons for the decrease in the catalytic activity observed in the later cycles of the aminocarbonylation experiments.

The XPS spectra of the supported palladium–bisoxazoline recovered after the tenth cycle of the alkoxycarbonylation reaction (Fig. 3a) show that the oxidation state of palladium remains unchanged after the catalytic application (Fig. 3b). Similar to the unused supported catalyst, the 3d spectrum resolved into 3d_5/2 and 3d_3/2 spin orbit pairs with binding energies of 334.88 eV and 339.98 eV, respectively.^34,35

Fig. 3 XPS spectrum of Pd–BOX-12 recovered from alkoxycarbonylation reaction showing the Pd3d, (right spectrum) spectrum of fresh catalyst is displayed on the left.

3.8. Study of the palladium leaching from the support during carbonylation reactions

The main objective of supporting a homogeneous catalyst is to enable its easy separation from the product and to minimize the level of contamination caused by the toxic metal. The possible palladium leaching into the product was analyzed using ICP-MS. After the tenth cycle of the alkoxycarbonylation and the seventh cycle of the aminocarbonylation, the products of each reaction were combined in separate containers. Samples were taken from each reaction for the analysis. The samples were digested using concentrated nitric acid. The solutions were then analyzed with ICP-MS technique.

The results of the ICP-MS analysis show that the concentration of palladium in the alkoxycarbonylation product is 2.0 ppb. This means that less than 0.1% of the total palladium on the supported complex was leached into the solution. On the other hand, the amount of palladium that leached into the solution during the aminocarbonylation reaction was 195.0 ppb. This amount was estimated as 3.4% of the total palladium on the supported catalyst. These results clearly show that the palladium leaching during aminocarbonylation reaction was significantly higher than the alkoxycarbonylation reaction, which explains the reason for the relatively lower recycling ability encountered during the aminocarbonylation reaction as compared to the alkoxycarbonylation reaction. The higher leaching of palladium observed with the aminocarbonylation reaction could be attributed to the coordination of palladium to the amine (either trimethylamine base or the amine nucleophile). This results in the formation of homogeneous complexes that are highly soluble in the liquid phase.³⁶

4. Conclusions

In summary, we reported successfully the application of an innovative palladium–bisoxazoline complex supported on Merrifield's resin as a highly active heterogeneous catalyst in the production of aryl esters and amides via alkoxycarbonylation and aminocarbonylation of various aryl iodides. The palladium heterogeneous catalysts showed an excellent stability and recycling ability. The process represents a simple and attractive method for the production of highly value-added esters and amides that should be of high interest to many chemical and petrochemicals companies.

Acknowledgements

This project was funded by the National Plan for Science, Technology and Innovation (MARIFAH) – King Abdulaziz City for Science and Technology – through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM), the Kingdom of Saudi Arabia, award number (14-PET2737-04).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra15506e

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