Silica supported palladium-phosphine as a reusable catalyst for alkoxycarbonylation and aminocarbonylation of aryl and heteroaryl iodides

Abstract

Silica-supported palladium phosphine complexes were prepared for alkoxycarbonylation and aminocarbonylation of aryl iodides. These catalysts were highly efficient for the carbonylation of unprotected hydroxy-aryl, amino-aryl, iodoindole and iodopyrazole. The carbonylation of unprotected iodopyrazole is challenging and their carbonylation was achieved for the first and obtained corresponding carbonylative products are biologically active. The applicability of developed protocols tolerates wide range of functional groups with excellent yields. The catalyst was easily recovered and shows significant recyclability up to five consecutive cycles without loss in its catalytic activity and selectivity. The prepared catalysts were characterized by different techniques such as FEG-SEM, EDS, FT-IR, XPS and ICP-AES spectroscopy.

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Introduction

Transition metal-catalyzed carbonylation reactions have been a versatile and most convenient methodology for the synthesis of aromatic carbonyl compounds such as ketones, aldehydes, acids, acid chlorides, esters, amides and anhydrides.¹ Moreover, they are used as essential building blocks for the fine and bulk chemical industry. Hence, these structural motifs are found in various natural products, agrochemicals, fragrances, photosensitizers, pharmaceuticals as well as biologically active compounds.² Typically, the most common approach for the synthesis of esters and amides by reaction of acyl halides or carboxylic acids and its derivatives with alcohols as well as amines.³ Subsequently, different synthetic methodologies such as transamidation, hydroamination of alkynes or by oxidation of benzyl alcohols, benzyl amines and benzaldehydes with amines as well as alcohols have been widely studied.⁴ Alternatively, aromatic esters and amides can be synthesized by palladium catalyzed carbonylation of aryl halides using alcohols and amines.⁵ These methods demonstrated their broad applicability and tolerated the wide variety of functional groups into good yields. In 2004, Beller et al. explored the carbonylation of haloindoles using Pd(PhCN)₂Cl₂/dppf as a catalyst.⁶ Carbonylation of heteroaryl halides have been also reported by employing (BINAP)PdCl₂ as homogeneous catalyst.⁷

Nowadays, efforts have been made by researchers to develop cost effective, simple, green and sustainable protocols. In this contexts, several groups have been disclosed the carbonylation of aryl halides by using immobilized palladium complexes on various supports such as activated carbon,⁸ silica,⁹ MCM-41,¹⁰ organic polymer,¹¹ SBA-15,¹² ZIF-8,¹³ Fe₃O₄,¹⁴ and MOF-5.¹⁵ Alper et al. demonstrated PAMAM dendrimer-palladium supported on silica for the carbonylation of aryl halides.¹⁶ Recently, Lei et al. reported the palladium supported on triphenylphosphine functionalized porous organic polymer for the alkoxycarbonylation of aryl iodides.¹⁷ However, these reported methods have limits in their substrate availability and intricate to carbonylation of unprotected nitrogen containing heterocycles. In addition, some reported methods required phosphine ligands, longer reaction time and non-recyclable Pd-precursors with quite severe reaction conditions. The use of homogeneous palladium precursors along with moisture sensitive phosphine ligands is a major problem associated with recovery and recycling of the catalyst. In addition, the use of heterogeneous catalyst would be valuable with respect to catalyst recycling, economically as well as environmentally benign.

In continuation of our previous work on heterogeneous catalyzed carbonylation reactions,¹⁸ herein we demonstrate an efficient, heterogeneous and recyclable silica supported palladium phosphine complexes I & II for alkoxycarbonylation as well as aminocarbonylation of aryl and heteroaryl iodides. The carbonylation of unprotected iodopyrazole is challenging and for the first time carbonylation achieved, which important subunit is in biologically as well as pharmaceuticals active molecules shown in Fig. 1. The 1H-pyrazole-4-carboxylate unit plays important role in the synthesis of highly biological active molecules.¹⁹ We can synthesize these important molecules as shown in Fig. 1 by N-arylation of 1H-pyrazole-4-carboxylate.²⁰ The present protocol tolerated a wide general applicability, such as aryl, heteroaryl and unprotected indole as well as pyrazole providing good to excellent yields (Scheme 1).

Fig. 1 Some highly biological active compounds containing 1H-pyrazole-4-carboxylate unit.

Scheme 1 Alkoxycarbonylation and aminocarbonylation of aryl iodides.

Experimental section

General

All the reactions were performed in inert conditions under the nitrogen atmosphere by using 100 mL stainless steel reactor. All the chemicals and reagents were purchased form Sigma Aldrich, S. D. Fine and WAKO commercial suppliers. The solvents were purchased with high purity from commercial suppliers and used without purification. Prepared catalyst was characterized by using FEG-SEM, EDS, FT-IR, XPS and ICP-AES (see the ESI†). The loading of catalyst was calculated by XRF measurements (SEA-2010, Seiko Electronic Industrial Co.). The XPS of PdCl₂_PPh₂Et@SiO₂ and Pd(OAc)₂_PPh₂Et@SiO₂ was measured using a PHI5000 Versa Probe with a monochromatic focused (100 μm × 100 μm) Al Kα X-ray radiation (15 kV, 30 mA) and dual beam neutralization using a combination of argon ion gun and electron irradiation. Reaction monitor by using Perkin Elmer Clarus-400 gas chromatography equipped with flame ionization detector with a capillary column (Elite-1, 30 m × 0.32 mm × 0.25 μm). GC-MS-QP 2010 instrument (Rtx-17, 30 m × 25 mm ID), film thickness (df) = 0.25 μm, was used for the mass analysis of the products. Products were purified by column chromatography on silica (100–200 mesh). The ¹H NMR spectrum was recorded on Bruker (400 MHz spectrometer) in CDCl₃ and DMSO-d6 using tetramethylsilane (TMS) as an internal standard. The ¹³C NMR spectrum was recorded on Bruker (100 MHz spectrometer) in CDCl₃ and DMSO-d6. The chemical shifts values are reported in parts per million (δ) relative to tetramethylsilane as an internal standard. The J (coupling constant) values were described in Hz. Splitting patterns of proton are depicted as s (singlet), d (doublet), t (triplet) and m (multiplet). The products were confirmed by the comparison of their GC, GC-MS, ¹H and ¹³C NMR spectra with those of authentic data.

Preparation of PdCl₂_PPh₂Et@SiO₂ (I) catalyst

The silica supported PdCl₂_PPh₂Et was prepared in a 100 mL round bottom flask 0.372 g of PdCl₂ and 3.0 g of PPh₂Et@SiO₂ (Aldrich 538019-5G, 2-diphenylphosphinoethyl functionalized silica, 0.7 mmol g⁻¹) were mixed, stirred and refluxed with acetonitrile under nitrogen atmosphere for 48 hours. The material showing yellow was filtered with 0.45 μm PTFE filter, washed with acetonitrile several time and dried at room temperature. The metal loading of PdCl₂_PPh₂Et@SiO₂ was 6.6 wt% as determined by XRF measurements (SEA-2010, Seiko Electronic Industrial Co.). The as prepared PdCl₂/PPh₂Et catalyst was characterized with FEG-SEM, EDS, FT-IR and XPS techniques. The FEG-SEM images of fresh and spent PdCl₂_PPh₂Et@SiO₂ catalyst were shown in Fig. 2a–c which shows rough surface morphology of catalyst. Fig. 2d shows the elemental analysis of catalyst by EDS measurement, displays the presence of Si, P, Cl and Pd for fresh catalyst.

Fig. 2 FEG-SEM images of (a) fresh, (b) 1^st recycled (c) 4^th recycled PdCl₂/PPh₂Et@SiO₂ catalyst and (d) EDS spectrum of fresh catalyst.

Preparation of Pd(OAc)₂_PPh₂Et@SiO₂ (II) catalyst

The silica supported Pd(OAc)₂_PPh₂Et was prepared as, in 100 mL round bottom flask 0.472 g of Pd(OAC)₂ and 3.0 g of PPh₂Et@SiO₂ (Aldrich 538019-5G, 2-diphenylphosphinoethyl functionalized silica, 0.7 mmol g⁻¹) were mixed, stirred and refluxed with acetonitrile under nitrogen atmosphere for 48 hours. The material showing yellow was filtered with 0.45 μm PTFE filter, washed with acetonitrile several time and dried at room temperature. The metal loading of Pd(OAc)₂_PPh₂Et@SiO₂ was 6.4 wt% as determined by XRF measurements (SEA-2010, Seiko Electronic Industrial Co.). The FEG-SEM and EDS analysis of fresh Pd(OAc)₂_PPh₂Et@SiO₂ catalyst are shown in ESI (see Fig. S4†).

General experimental procedure for alkoxycarbonylation reaction

To a 100 mL reactor, 1a (1 mmol), alcohol (5 mL), Pd catalyst (I) (0.6 mol%), and Et₃N (3 mmol) were added. The reactor was closed, purged three times with nitrogen, pressurized with 0.5 MPa of CO, and heated at 100 °C for 1–2 h. After the completion of the reaction, the reactor was cooled to room temperature, and the remaining CO gas was vented carefully, and the reactor was opened. The reactor vessel was thoroughly washed with ethyl acetate (2 × 10 mL) to remove any traces of product and catalyst. The catalyst was filtered, and the reaction mixture was evaporated under vacuum. The obtained residue was purified by column chromatography (silica gel, 100–200 mesh; petroleum ether/ethyl acetate) to afford the desired purified product.

General experimental procedure for recycling of PdCl₂_PPh₂Et@SiO₂ catalyst

At the end of the reaction, the reactor was cooled to room temperature, and the catalyst was recovered by simple filtration. The filtered catalyst was washed methanol (3 × 5 mL) to remove all traces of product or reactant present. The filtered catalyst was then dried under reduced pressure for 5 h. The dried catalyst was then used for the next run for the alkoxycarbonylation reaction.

General experimental procedure for aminocarbonylation reaction

To a 100 mL reactor, aryl iodide 1a (1 mmol), amine (2 mmol), Pd catalyst (I) (0.5–1.0 mol%), in toluene (8 mL), and Et₃N (3 mmol) were added. The reactor was closed, purged three times with nitrogen, and then pressurized with 0.5 MPa of CO, and heated at 100 °C for 8 h (the subsequent procedure is the same as that discussed above for the alkoxycarbonylation reaction).

Results and discussion

In order to evaluate the catalytic activity of prepared catalyst I and II were studied for the alkoxycarbonylation and aminocarbonylation of aryl iodides. The optimization study, iodobenzene 1a (1 mmol), Et₃N (3 mmol) in a methanol (5 mL) under 0.5 MPa of CO pressure was chosen as model reaction. For the progress of reaction yield, we examined various reaction parameters such as, effect of catalyst, catalyst loading, effect of solvents, effect of bases, effect of CO pressure and time. The results are summarised in Table 1. At first, the prepared silica supported palladium phosphine complexes I and II were used for the alkoxycarbonylation reaction (Table 1, entry 1 and 2). The catalyst loading was successfully reduced to 0.6 mol% of palladium under the optimized reaction conditions, and the 99% yield of 2a was obtained, while decreasing the catalyst loading results in decreased the yield of desired product 2a (Table 1, entry 3 and 4). Next, the effect of various catalysts were studied such as Pd/C, Pd(OAc)₂/PPh₃ and PdCl₂/PPh₃; among them complex I gave an excellent yield of 2a (99%) (Table 1, entries 1, 2 and 5–7). Then, we examined the effect of solvents using 3 mmol of MeOH with 1a under optimized condition, the obtained yield of corresponding esters were poor (Table 1, entries 8–10). However, methanol as a solvent as well as nucleophile was considered for the further optimization study. Next, we studied base effect and it was observed that the base played a crucial role in this transformation (Table 1). Various bases such as Et₃N, DBU, DABCO, K₂CO₃ and Na₂CO₃ were tested and among them Et₃N was found to be an excellent base to providing the desired product 2a in 99% yield (Table 1, entries 1 and 11–14). Next, we studied the effect of time on reaction condition and we observed that time 1.5 h provides 99% yield of the product 2a (Table 1, entry 16). Further, decreasing the reaction time to 1 h decreases the yield of the desired product to 87%. When the reaction temperatures were decreased to 70 °C, the yield of 2a was dramatically decreased to 73%. Then, we increased the reaction temperature up to 110 °C had no significant effect on reaction yield and it was observed that the 100 °C temperature was adequate for this complete consumption of starting material. Next, we checked the effect of CO pressure on the yield, the reaction were carried out with 0.2, 0.5 and 1 MPa (Table 1, entry 21 and 22). Hence, we concluded that iodobenzene 1a (1 mmol), Et₃N (3 mmol), Pd catalyst (0.6 mol%) in 5 mL MeOH under 0.5 MPa CO pressure at 100 °C for 1.5 h was adequate for the excellent yield of 2a (Table 1, entry 16).

Table 1 Optimization reaction parameters for the alkoxycarbonylation of iodobenzene^a

Entry	Pd catalyst	Base	Solvent	T (°C)	Time (h)	Conversion 1a/yield^b 2a (%)
a Reaction conditions: 1a (1 mmol), base (3 mmol), solvent (5 mL), catalyst (1 mol%), CO (0.5 MPa).b Conversion and yield were determined by GC.c 0.6 mol%.d 0.4 mol% catalyst loading.e MeOH (3 mmol).f 1 MPa.g 0.2 MPa CO pressure.
Effect of catalyst and catalyst loading
1	I	Et3N	MeOH	100	3	100/99
2	II	Et3N	MeOH	100	3	100/92
3^c	I	Et3N	MeOH	100	3	100/99
4^d	I	Et3N	MeOH	100	3	93/85
5	Pd/C	Et3N	MeOH	100	3	83/73
6	Pd(OAc)2/PPh3	Et3N	MeOH	100	3	94/81
7	PdCl2/PPh3	Et3N	MeOH	100	3	89/85
Effect of solvent
8^e	I	Et3N	DMF	100	3	74/40
9^e	I	Et3N	THF	100	3	59/55
10^e	I	Et3N	Toluene	100	3	65/48
Effect of base
11	I	DBU	MeOH	100	3	58/0
12	I	DABCO	MeOH	100	3	63/56
13	I	K2CO3	MeOH	100	3	42/40
14	I	Na2CO3	MeOH	100	3	50/46
Effect of time, temperature and CO pressure
15	I	Et3N	MeOH	100	2	100/99
16	I	Et3N	MeOH	100	1.5	100/99
17	I	Et3N	MeOH	100	1	93/87
18	I	Et3N	MeOH	80	1.5	86/82
19	I	Et3N	MeOH	70	1.5	75/73
20	I	Et3N	MeOH	110	1.5	100/95
21^f	I	Et3N	MeOH	100	1.5	100/99
22^g	I	Et3N	MeOH	100	1.5	91/85

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To explore the applicability of developed catalytic system were applied to a variety of aryl iodides with diverse alcohols under the optimized reaction condition to promote the corresponding esters shown in Table 2. The results of alkoxycarbonylation of iodobenzene demonstrate that changing the alcohol substrate from methanol had no significant effect on product yield (Table 2, entries 2, 4–6). But with a tert-butanol, a lower yield (43%) of carbonylation product was obtained (Table 2, entry 3). Both the electron-rich and electron-deficient substituents on the aryl iodides afforded in good to excellent yield of the corresponding products. At different position of substituents on the phenyl ring of aryl iodides had not affected on the reaction yield. For instance, substrate with electron-donating groups, such as 2-Me, 4-Me, 2-OMe and 4-OMe groups proceeded well towards the corresponding products (Table 2, entries 7–10). It was worth to note that the 4-OH and 4-NH₂ aryl iodides also worked very well under this transformation to achieve an excellent yield (Table 2, entries 11 and 12). Aryl iodides bearing electron deficient substituents such as 3-NO₂, 4-NO₂, 4-COCH₃, 4-COOH and 4-CN afforded an excellent yield of the corresponding esters (Table 2, entries 13–17). Unfortunately, the bromobenzene and chlorobenzene were failed to give the desired product under optimized conditions. Remarkably, the heteroaryl iodides such as 3-iodopyridine, 3-iodopyridine and 2-iodothiophene were also proven to be compatible reaction with aryl iodides and showed an excellent yield of corresponding esters (Table 3). To our delight, the carbonylation of unprotected 5-iodoindoles and 4-iodopyrazoles were also offered an excellent yield (Table 3, entries 4–6). The alkoxycarbonylation of 4-iodo-1H-pyrazole challenging and that was the first example afforded the corresponding product 2t in 70% yield under the optimal reaction condition. While, increasing the reaction time up to 4 h, the 87% and 90% yield of desired products 2t, 2u were obtained. The derivatives of 1H-pyrazole-carboxylate showed their potential application in pharmaceuticals and biologically active molecules.

Table 2 Scope of aryl iodides for the alkoxycarbonylation reaction^a

Entry	R	R1OH	Yield^b (%)
a Reaction conditions: 1 (1 mmol), R1OH (5 mL), Et3N (3 mmol), I (0.6 mol%), CO (0.5 MPa), 100 °C, 1.5 h.b Isolated yield.c R1OH (3 mmol) in THF (5 mL).
1	1a (H)	MeOH	2a, 99
2	1a (H)	EtOH	2b, 99
3	1a (H)	tert-BuOH	2c, 43, <10^c
4	1a (H)	Cyclohexanol	2d, 69, <10^c
5	1a (H)	PhOH	2e, 83^c
6	1a (H)	BnOH	2f, 97, 28^c
7	1b (4-Me)	MeOH	2g, 99
8	1c (2-Me)	MeOH	2h, 98
9	1d (4-OMe)	MeOH	2i, 98
10	1e (2-OMe)	MeOH	2j, 96
11	1f (4-OH)	MeOH	2k, 99
12	1g (4-NH2)	MeOH	2l, 99
13	1h (3-NO2)	MeOH	2m, 99
14	1i (4-NO2)	MeOH	2n, 95
15	1j (4-COCH3)	EtOH	2o, 96
16	1k (4-COOH)	MeOH	2p, 92
17	1l (4-CN)	MeOH	2q, 99
18	1m (1-naphthyl)	MeOH	2r, 98

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Table 3 Scope of heteroaryl iodides for the alkoxycarbonylation reaction^a

Entry	R1OH	Products	Yield^b (%)
a Reaction conditions: heteroaryl iodide (1 mmol), R1OH (5 mL), Et3N (3 mmol), I (0.6 mol%), CO (0.5 MPa), 100 °C, 1.5 h.b Isolated yield.c 4 h.
1	MeOH		99
2	MeOH		98
3	MeOH		96
4	MeOH		99
5	MeOH		70, 87^c
6	EtOH		90^c

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To extend the scope of the developed catalytic system was tested for the aminocarbonylation reaction. With the standard reaction conditions in hand, the scopes of amines were further explored for the aminocarbonylation reaction. Thus, the standard reaction condition consists of 1a (1 mmol), 2 (2 mmol), Pd catalyst (I) (0.5–1 mol%), Et₃N (3 mmol) in 8 mL of toluene under 0.5 MPa at 100 °C for 8 h. Initially, we examined the wide range of amines such as aliphatic as well as aromatic amines with iodobenzene afforded corresponding amides in good to excellent yield shown in Table 4. All the aliphatic amines were reacted smoothly under standard conditions and providing the desired products in excellent yields (Table 4, entries 1–4). The reaction with tert-butyl amine, gives 77% yield of 3e was obtained, because of the sterically hindered tert-butyl group. In addition, the cyclic amines were also well employed in this transformation under such standard reaction condition gave an excellent yield of corresponding amides 3e–3g (Table 4, entries 5–7). It was noteworthy that the allyl and diallyl amines also successfully provided 98% and 99% yields of respective amides (Table 4, entries 8–9). The benzyl amines and aniline were also examined and found to be good to excellent yield of the corresponding amides (Table 4, entries 10–12).

Table 4 Aminocarbonylation of aryl iodides with various amines^a

Entry	R2	R3	Yield^b (%)
a Reaction conditions: 1a (1 mmol), 2 (2 mmol), Et3N (3 mmol), I (0.6 mol%), toluene (8 mL), CO (0.5 MPa), 100 °C, 8 h.b Isolated yield.
1	Et	Et	3a, 98
2	n-Bu	n-Bu	3b, 96
3	n-Pr	n-Pr	3c, 97
4	H	ter-Bu	3d, 77
5	Pyrrolidinyl		3e, 94
6	Piperidinyl		3f, 96
7	Morpholino		3g, 93
8	Allyl	Allyl	3h, 99
9	H	Allyl	3i, 98
10	H	Ph	3j, 99
11	Bn	Bn	3k, 95
12	H	Bn	3l, 98

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Recyclability study

The reusability of catalyst reduces the cost of process chemistry and makes the protocol economically as well as environmentally valuable. The recyclability study was further explored for synthesis of esters by carbonylation of 1a with methanol under the optimal reaction condition as shown in Fig. 3. At the end of the reaction, the reactor was cooled to room temperature and remaining CO gas was carefully vented out. The catalyst was recovered from the reaction mixtures by simple filtration and resulting catalyst was found be effectively recycled up to five consecutive recycles without loss of its catalytic activity and selectivity. The 1^st and 4^th recycled sample was prepared for the palladium leaching study by ICP-AES analysis. The leaching of palladium was obtained below the detectable level. We also examined the leaching of palladium by hot filtration test, after 0.5 h the catalyst was filtered off from the reaction mixture and the filtrate was used as a further reaction up to 2 h and it was observed that the yield of 2a remains constant.

Fig. 3 Recycle study of PdCl₂_PPh₂Et@SiO₂ for alkoxycarbonylation of aryl iodide.

XPS analysis of the fresh and spent catalyst

The X-ray photoelectron spectroscopy analysis was performed for the fresh as well as spent catalyst shown in Fig. 4. The Fig. 4a indicates the wide scan of the fresh and spent PdCl₂_PPh₂Et@SiO₂ catalyst which shows the presence of Pd, P, O, C, I, Cl and Si elements on the surface of the catalyst. The high resolution XPS spectra for fresh, 1^st and 4^th recycled catalyst shown in Fig. 4b. This reveals that two bands of Pd 3d for the fresh catalyst appeared at 336.6 eV and 342.1 eV are assigned as Pd 3d_5/2 and Pd 3d_3/2 for the Pd(II) species. Furthermore, in XPS data of 1^st recycle the peaks shows at 336.4 eV to 342 eV, which assigned as Pd 3d_5/2 and Pd 3d_3/2 for Pd(II) species. But in case of 4^th recycle the reduction of Pd(II) to Pd(0) species takes place during the catalytic reaction and the new peaks were appeared at 334.5 eV and 339.9 eV assigned as Pd 3d_5/2 and Pd 3d_3/2 for the Pd(0) species. The peak at 132 eV corresponds to the P 2p recognized to the presence of P in the catalyst shown in Fig. 4c. These results indicate that the coordination bond between Pd and P are formed and they are still remains on silica support after 4^th recycle. In 4^th recycle, the region of Pd 3d shifted to lower binding energy, which indicates that Pd(0) is formed during catalytic reaction. Fig. 4d, suggests that the chlorine is replaced by iodine after first run because HI was formed during the course of reaction.

Fig. 4 (a) XPS survey spectrum; (b) Pd 3d; (c) P 2p; (d) Cl 2p and (e) I 3d for PdCl₂_Ph₂PEt@SiO₂ catalyst.

Conclusions

In summary, we prepared a novel silica supported palladium phosphine complex for the alkoxycarbonylation and aminocarbonylation of aryl iodides. The reactions were well proceeded under the optimized condition and allowing high substrate tolerance towards the corresponding products with good to excellent yields by using a well-defined heterogeneous PdCl₂_PPh₂Et@SiO₂ as a versatile catalyst. It is worth to mentioning that the alkoxycarbonylation of unprotected haloindoles and iodopyrazole was achieved in excellent yield which showed the potential application of corresponding esters in pharmaceuticals and biologically active molecules. The catalyst was found to be effectively reused up to five cycles without loss of its catalytic activity and selectivity. The catalysts were well characterized by FEG-SEM, EDS, FT-IR, XPS and ICP-AES techniques.

Characterisation data of products

Methyl benzoate (2a)¹⁴

Colourless liquid; isolated yield: 99%; ¹H NMR (500 MHz, CDCl₃) δ 8.00–7.98 (m, 2H), 7.46 (tt, J = 6.9, 1.4 Hz, 1H), 7.37–7.33 (m, 2H), 3.83 (d, J = 1.8 Hz, 3H); ¹³C NMR (126 MHz, CDCl₃) δ 166.92, 166.91, 132.82, 130.09, 129.48, 128.27, 51.92 ppm; GC-MS (EI, 70 eV): m/z (%) = 136 [M⁺] (36), 105 (100), 92 (4), 77 (61), 51 (19).

Ethyl benzoate (2b)^3b

Colourless liquid; isolated yield: 99%; ¹H NMR (500 MHz, CDCl₃) δ 7.99–7.97 (m, 2H), 7.43–7.31 (m, 3H), 4.28 (qd, J = 7.1, 2.6 Hz, 2H), 1.29 (td, J = 7.1, 2.8 Hz, 3H); ¹³C NMR (126 MHz, CDCl₃) δ 166.36, 132.67, 130.42, 129.41, 128.19, 60.76, 14.18 ppm; GC-MS (EI, 70 eV): m/z (%) = 150 [M⁺] (23.19), 122 (34), 105 (100), 77 (50), 51 (22).

tert-Butyl benzoate (2c)^5k

White solid; isolated yield: 43%; ¹H NMR (500 MHz, CDCl₃) δ 8.01 (d, J = 8.1 Hz, 2H), 7.55–7.49 (m, 1H), 7.38 (t, J = 7.7 Hz, 2H), 1.59 (s, 9H); ¹³C NMR (126 MHz, CDCl₃) δ 165.82, 133.71, 130.16, 128.79, 128.47, 28.20 ppm; GC-MS (EI, 70 eV): m/z (%) = 178 [M⁺] (1), 123 (100), 105 (93), 77 (42), 56 (64), 55 (33), 51 (14), 41 (27).

Cyclohexyl benzoate (2d)¹⁷

Colourless oil; isolated yield: 69%; ¹H NMR (500 MHz, CDCl₃) δ 8.07–8.05 (m, 1H), 7.49–7.39 (dd, J = 15.1, 11.3 Hz, 3H), 5.04 (ddd, J = 12.7, 8.5, 3.8 Hz, 1H), 1.95 (dd, J = 12.2, 5.3 Hz, 1H), 1.81–1.79 (m, 1H), 1.62–1.56 (m, 2H), 1.49–1.27 (m, 6H); ¹³C NMR (126 MHz, CDCl₃) δ 166.05, 133.69, 132.70, 130.96, 129.52, 128.27, 73.07, 31.62, 25.47, 23.65 ppm; GC-MS (EI, 70 eV): m/z (%) = 204 [M⁺] (1), 177 (4), 123 (86.6), 105 (100), 82 (21), 77 (48), 67 (21), 55 (9), 41 (11).

Phenyl benzoate (2e)^18a

White solid; isolated yield: 83%; ¹H NMR (500 MHz, CDCl₃) δ 8.25 (d, J = 8.3 Hz, 2H), 7.69–7.64 (m, 1H), 7.54 (t, J = 7.6 Hz, 2H), 7.46 (t, J = 7.7 Hz, 2H), 7.31 (td, J = 7.7, 0.9 Hz, 1H), 7.25 (d, J = 8.5 Hz, 2H); ¹³C NMR (126 MHz, CDCl₃) δ 165.22, 150.97, 133.62, 130.19, 129.53, 128.60, 125.92, 121.75 ppm; GC-MS (EI, 70 eV): m/z (%) = 198 [M⁺] (4), 105 (100), 77 (40.5), 65 (4), 50 (11).

Benzyl benzoate (2f)^18a

Colourless oil; isolated yield: 97%; ¹H NMR (400 MHz, CDCl₃): δ 8.02 (d, 2H, J = 8 Hz), 7.55–7.35 (m, J = 8H), 5.36 (s, 2H); ¹³C NMR (101 MHz, CDCl₃): δ 166.42, 136.01, 133.02, 130.08, 129.68, 128.58, 128.36, 128.23, 128.15, 66.68 ppm; GC-MS (EI, 70 eV): m/z (%) = 212 [M⁺] (16.90), 105 (100), 91 (49.35), 77 (32.70), 65 (12), 51 (11.83).

Methyl 4-methylbenzoate (2g)^8a

Colourless oil; isolated yield: 99%; ¹H NMR (500 MHz, CDCl₃) δ 7.93 (d, J = 8.1 Hz, 2H), 7.22 (dd, J = 7.9, 0.4 Hz, 2H), 3.89 (d, J = 0.7 Hz, 3H), 2.39 (s, 3H); ¹³C NMR (126 MHz, CDCl₃) δ 167.16, 143.53, 129.57, 129.04, 127.38, 77.35, 77.09, 76.84, 51.91, 21.60 ppm; GC-MS (EI, 70 eV): m/z (%) = 150 [M⁺] (49), 119 (100), 91 (53.96), 90 (24), 65 (21.86), 51 (4).

Methyl 2-methylbenzoate (2h)^8a

Colourless oil; isolated yield: 98%; ¹H NMR (500 MHz, CDCl₃) δ 7.91 (d, J = 7.5 Hz, 1H), 7.39 (d, J = 7.5 Hz, 1H), 7.27 (dd, J = 5.6, 4.9 Hz, 2H), 3.90 (d, 3H), 2.61 (s, 3H); ¹³C NMR (126 MHz, CDCl₃) δ 168.10, 140.16, 131.95, 131.84, 131.66, 130.54, 125.67, 51.82, 29.70, 21.72 ppm; GC-MS (EI, 70 eV): m/z (%) = 150 [M⁺] (46), 135 (7), 119 (100), 118 (74), 91 (77.4), 90 (24), 89 (14), 65 (25), 63 (10), 51 (5).

Methyl 4-methoxybenzoate (2i)^8a

White solid; isolated yield: 98%; ¹H NMR (400 MHz, CDCl₃): δ 7.97–7.95 (d, 2H, J = 8 Hz), 6.89–6.87 (d, 2H, J = 8 Hz), 3.85 (s, 3H), 3.81 (s, 3H); ¹³C NMR (101 MHz, CDCl₃): δ 166.82, 163.26, 131.53, 122.50, 113.54, 55.35, 51.83 ppm; GC-MS (EI, 70 eV): m/z (%) = 166 [M⁺] (35.63), 135 (100), 107 (17.56), 92 (15.25), 77 (23.87), 63 (9).

Methyl 2-methoxybenzoate (2j)^8a

Colourless oil; isolated yield: 96%; ¹H NMR (500 MHz, CDCl₃) δ 7.80–7.78 (m, 1H), 7.46–7.44 (m, 1H), 6.99 (dd, J = 7.4, 2.2 Hz, 2H), 3.89 (dd, J = 6.6, 1.2 Hz, 6H); ¹³C NMR (126 MHz, CDCl₃) δ 166.72, 159.07, 133.54, 131.64, 120.10, 111.97, 111.61, 55.95, 52.02 ppm; GC-MS (EI, 70 eV): m/z (%) = 166 [M⁺] (24.3), 135 (100), 133 (44), 105 (16), 92 (20), 77 (47.5), 63 (10), 51 (7).

Methyl 4-hydroxybenzoate (2k)^18a

White solid; isolated yield: 99%; ¹H NMR (400 MHz, CDCl₃): δ 7.90–7.87 (d, 2H, J = 8 Hz), 6.84–6.82 (d, 2H, J = 8 Hz), 4.34 (bs, 1H), 3.86 (s, 3H); ¹³C NMR (101 MHz, CDCl₃): δ 171.60, 165.57, 135.60, 135.60, 124.87, 118.87, 55.60 ppm; GC-MS (EI, 70 eV): m/z (%) = 152 [M⁺] (37.6), 121 (100), 93 (27.3), 65 (23.4).

Methyl 4-aminobenzoate (2l)^18a

White solid; isolated yield: 99%; ¹H NMR (400 MHz, CDCl₃): δ 7.82–7.80 (d, 2H, J = 8 Hz), 6.60–6.58 (d, 2H, J = 8 Hz), 4.02 (bs, 2H), 3.81 (s, 3H); ¹³C NMR (101 MHz, CDCl₃): δ 167.26, 151.02, 131.55, 138.14, 113.74, 51.62 ppm; GC-MS (EI, 70 eV): m/z (%) = 151 [M⁺] (45), 120 (100), 92 (37), 65 (34), 39 (13).

Methyl 3-nitrobenzoate (2m)^18a

White solid; isolated yield: 99%; ¹H NMR (400 MHz, CDCl₃): δ 8.83 (s, 1H), 8.38–8.33 (dd, 3H, 8.0 Hz), 3.96 (s, 3H); ¹³C NMR (101 MHz, CDCl₃): δ 164.92, 148.20, 135.25, 131.79, 129.62, 127.37, 124.55, 52.79 ppm; GC-MS (EI, 70 eV): m/z (%) = 181 [M⁺] (28), 150 (100), 135 (9), 120 (10), 104 (37.8), 76 (38), 50 (27), 39 (4).

Methyl 4-nitrobenzoate (2n)^4d

Colourless solid; isolated yield: 95%; ¹H NMR (500 MHz, CDCl₃) δ 8.29–8.24 (m, 2H), 8.21–8.15 (m, 2H), 3.96 (d, J = 0.8 Hz, 3H); ¹³C NMR (126 MHz, CDCl₃): δ 165.15, 150.48, 135.44, 130.68, 123.51, 113.73, 77.31, 77.06, 76.80, 52.83 ppm; GC-MS (EI, 70 eV): m/z (%) = 181 [M⁺] (3), 180 (29), 164 (23.84), 150 (100), 120 (22.56), 104 (28.55), 92 (19.6), 76 (25.75), 50 (17).

4-(Methoxycarbonyl)benzoic acid (2p)^5j

Colourless solid; isolated yield: 92%; ¹H NMR (400 MHz, CDCl₃): δ 8.12–8.11 (d, 4H), 3.96 (s, 3H); ¹³C NMR (101 MHz, CDCl₃): δ 171.77, 170.62, 139.47, 137.47, 138.62, 133.30, 56.25 ppm; GC-MS (EI, 70 eV): m/z (%) = 180 [M⁺] (28.60), 149 (100), 121 (26.90), 76 (8.7), 65 (27.4), 50 (10.60), 40 (10.10).

Methyl 1-naphthoate (2r)^8a

Colourless oil; isolated yield: 98%; ¹H NMR (500 MHz, CDCl₃) δ 8.77 (d, J = 7.8 Hz, 1H), 8.16 (d, J = 7.8 Hz, 1H), 7.87 (dd, J = 7.7, 1.6 Hz, 1H), 7.74–7.70 (m, 1H), 7.54–7.51 (m, 2H), 7.50 (dd, J = 7.6, 4.7 Hz, 1H), 4.02 (s, 3H); ¹³C NMR (126 MHz, CDCl₃) δ 167.66, 132.20, 130.94, 130.06, 128.83, 128.26, 127.01, 125.16, 124.45, 123.97, 52.93 ppm; GC-MS (EI, 70 eV): m/z (%) = 186 [M⁺] (69), 155 (100), 127 (98.63), 101 (6), 77 (11), 63 (10), 51 (4).

Methyl picolinate (Table 3, entry 1)^7a

Colourless oil; isolated yield: 99%; ¹H NMR (500 MHz, CDCl₃): δ 8.77 (d, J = 4.7 Hz, 1H), 8.16 (d, J = 7.8 Hz, 1H), 7.74–7.71 (m, 1H), 7.51–7.49 (m, 1H), 4.02 (s, 3H); ¹³C NMR (126 MHz, CDCl₃): δ 165.69, 149.79, 147.86, 137.11, 127.01, 125.16, 52.93 ppm; GC-MS (EI, 70 eV): m/z (%) = 137 [M⁺] (4), 107 (30), 106 (11), 79 (100), 78 (69), 51 (22).

Methyl nicotinate (Table 3, entry 2)^7a

Yellow oil; isolated yield: 98%; ¹H NMR (400 MHz, CDCl₃) δ 9.23 (s, 1H), 8.77 (s, 1H), 8.27–8.25 (d, J = 7.6 Hz, 1H), 7.37–8.35 (dd, J = 6.4, 3.1 Hz, 1H), 3.89 (s, 3H) ppm; GC-MS (EI, 70 eV): m/z (%) = 137 [M⁺] (52.5), 136 (27.6), 106 (100), 78 (79), 51 (33), 50 (15).

Methyl thiophene-2-carboxylate (Table 3, entry 3)^4d

Yellow oil; isolated yield: 96%; ¹H NMR (500 MHz, CDCl₃): δ 87.87–7.71 (m, 1H), 7.63–7.48 (m, 1H), 7.14–7.02 (m, 1H), 3.88 (d, J = 0.7 Hz, 3H); ¹³C NMR (126 MHz, CDCl₃): δ 162.69, 133.53, 133.46, 132.35, 127.73, 77.32, 77.06, 76.81, 52.14 ppm; GC-MS (EI, 70 eV): m/z (%) = 142 [M⁺] (46.67), 112 (8), 111 (100), 83 (18.62), 44 (6).

Methyl 1H-indole-5-carboxylate (Table 3, entry 4)^6b

Colourless solid; isolated yield: 99%; ¹H NMR (400 MHz, CDCl₃): δ 8.72 (bs, 1H), 8.43 (s, 1H), 7.91–7.89 (dd, 1H, 8 Hz), 7.39–7.37 (dd, 1H, 8 Hz), 7.24 (s, 1H), 6.83 (s, 1H), 3.93 (s, 3H); ¹³C NMR (101 MHz, CDCl₃): δ 168.50, 138.46, 127.44, 125.71, 123.71, 123.24, 121.68, 110.85, 103.85, 51.94 ppm; GC-MS (EI, 70 eV): m/z (%) = 175 [M⁺] (63.90), 144 (100), 116 (63.40), 89 (25.70), 63 (12.30), 58 (11.70).

Methyl 1H-pyrazole-4-carboxylate (Table 3, entry 5)^19b

Yellow semi-solid; isolated yield: 87%; ¹H NMR (400 MHz, DMSO-d6): δ 13.40 (bs, 1H, NH), 8.27 (s, 1H), 7.86 (s, 1H), 3.70 (s, 3H); ¹³C NMR (101 MHz, DMSO-d6): δ 163.98, 136.26, 114.13, 51.41 ppm; GC-MS (EI, 70 eV): m/z (%) = 126 [M⁺] (31), 96 (6.3), 95 (100), 68 (7), 39 (14.83), 31 (1).

Ethyl 1H-pyrazole-4-carboxylate (Table 3, entry 6)^19c

Yellow semi-solid; isolated yield: 90%; ¹H NMR (400 MHz, DMSO-d6): δ 13.38 (bs, 1H, NH), 8.28 (s, 1H), 7.84 (s, 1H), 4.17 (q, 2H) 1.21 (t, 3H); ¹³C NMR (101 MHz, DMSO-d6): δ 163.11, 140.50, 132.57, 113.96, 59.92, 14.73 ppm; GC-MS (EI, 70 eV): m/z (%) = 140 [M⁺] (17), 112 (41), 95 (100), 68 (9), 40 (8).

N,N-Diethylbenzamide (3a)^4b

Colourless oil; isolated yield: 98%; ¹H NMR (400 MHz, CDCl₃): δ 7.29 (m, 5H), 3.48 (s, 2H), 3.19 (s, 2H), 1.18 (s, 3H), 1.04 (s, 3H); ¹³C NMR (101 MHz, CDCl₃): δ 171.28, 137.13, 129.05, 128.33, 126.17, 77.43, 77.11, 76.79, 43.24, 39.19, 14.15, 12.84.; GC-MS (EI, 70 eV): m/z (%) = 177 [M⁺] (11), 176 (33), 105 (100), 77 (40), 51 (9).

N,N-Dibutylbenzamide (3b)^11b

Colourless oil; isolated yield: 96%; ¹H NMR (400 MHz, CDCl₃) δ 7.39–7.28 (m, 5H), 3.49 (s, 2H), 3.18 (s, 2H), 1.65 (s, 2H), 1.43 (dd, J = 20.4, 12.6 Hz, 4H), 1.13 (d, J = 6.7 Hz, 2H), 0.98 (s, 3H), 0.79 (d, J = 6.5 Hz, 3H) ppm; ¹³C NMR (101 MHz, CDCl₃) δ 171.65, 137.19, 129.85, 128.95, 128.25, 128.14, 126.35, 48.71, 44.40, 30.72, 29.56, 20.23, 19.66, 13.87, 13.54, 13.54 ppm; GC-MS (EI, 70 eV): m/z (%) = 233 (9) [M]⁺, 232 (11), 190 (8), 148 (5), 134 (4), 105 (100), 77 (26), 51 (4), 40 (13).

N,N-Dipropylbenzamide (3c)^5l

Colourless oil; isolated yield: 97%; ¹H NMR (400 MHz, CDCl₃) δ 7.35–7.32 (m, 3H), 7.30 (dd, J = 7.3, 4.6, 2.3 Hz, 2H), 3.41 (s, 2H), 3.11 (s, 2H), 1.65 (d, J = 6.8 Hz, 2H), 1.48 (d, J = 6.8 Hz, 2H), 0.93 (t, J = 6.4 Hz, 3H), 0.70 (d, J = 6.4 Hz, 3H); ¹³C NMR (101 MHz, CDCl₃): δ 171.76, 137.31, 128.96, 126.37, 50.63, 46.22, 21.85, 20.65, 11.40, 10.97 ppm; GC-MS (EI, 70 eV): m/z (%) = 204 [M⁺] (11.64), 176 (5.71), 134 (6.88), 105 (100), 77 (34.64), 51 (6.69), 39 (36.08).

N-(tert-Butyl)benzamide (3d)^4b

Colourless solid; isolated yield: 77%; ¹H NMR (400 MHz, CDCl₃): δ 7.70–7.68 (d, 2H, J = 8 Hz), 7.44–7.36 (m, 3H), 5.95 (s, 1H), 1.45 (s, 9H); ¹³C NMR (101 MHz, CDCl₃): δ 166.90, 135.85, 131.05, 128.43, 126.66, 51.57, 28.83 ppm; GC-MS (EI, 70 eV): m/z (%) = 177 [M⁺] (11), 162 (12), 122 (14.9), 105 (100), 77 (44), 51 (13), 41 (8).

Phenyl(pyrrolidin-1-yl)methanone (3e)^5l

Colourless oil; isolated yield: 94%; ¹H NMR (400 MHz, CDCl₃) δ 7.47–7.44 (m, 2H), 7.36–7.32 (m, 3H), 3.59 (t, J = 7.0 Hz, 2H), 3.37 (t, J = 6.6 Hz, 2H), 1.90 (dd, J = 13.7, 6.5 Hz, 2H), 1.82 (dd, J = 13.1, 6.6 Hz, 2H); ¹³C NMR (101 MHz, CDCl₃): δ 169.70, 137.11, 129.74, 128.19, 127.00, 49.58, 46.14, 26.33, 24.41 ppm; GC-MS (EI, 70 eV): m/z (%) = 175 [M⁺] (3), 174 (17), 156 (9), 146 (10), 105 (100), 95 (8), 77 (68), 56 (13), 51 (27), 41 (13).

Phenyl(piperidin-1-yl)methanone (3f)^5l

Colourless oil; isolated yield: 96%; ¹H NMR (400 MHz, CDCl₃): δ 7.34 (m, 5H), 3.66 (s, 2H), 3.29 (s, 2H), 1.62 (s, 4H), 1.46 (s, 2H); ¹³C NMR (101 MHz, CDCl₃): δ 170.28, 136.40, 129.32, 128.36, 126.71, 48.73, 43.07, 26.50, 25.58, 24.53 ppm; GC-MS (EI, 70 eV): m/z (%) = 188 [M⁺] (73.80), 160 (2.19), 105 (100), 84 (10), 77 (67.40), 51 (19.09).

Morpholino(phenyl)methanone (3g)^4b

Colourless oil; isolated yield: 93%; ¹H NMR (400 MHz, CDCl₃) δ 7.42–7.39 (m, 5H), 3.83–3.38 (m, 8H); ¹³C NMR (101 MHz, CDCl₃) δ 170.43, 135.15, 129.85, 128.51, 127.01, 66.80, 48.17, 42.50, 20.65; GC-MS (EI, 70 eV): m/z (%) = 191 [M⁺] (8), 190 (25.58), 176 (7), 105 (100), 91 (5), 86 (12), 77 (48), 56 (14), 51 (14).

N,N-Diallylbenzamide (3h)^4c

Yellowish oil; isolated yield: 99%; ¹H NMR (400 MHz, CDCl₃) δ 7.41–7.39 (m, 2H), 7.37–7.33 (m, 3H), 5.85 (s, 1H), 5.70 (s, 1H), 5.26–5.22 (d, J = 20.0 Hz, 1H), 5.19 (d, J = 1.0 Hz, 2H), 5.14 (s, 1H), 4.10 (s, 2H), 3.80 (s, 2H); ¹³C NMR (101 MHz, CDCl₃): δ 171.76, 136.15, 133.13, 132.69, 129.31, 128.33, 126.52, 117.61, 50.71, 46.92 ppm; GC-MS (EI, 70 eV): m/z (%) = 201 [M⁺] (3), 160 (9), 105 (100), 96 (6), 51 (11.25), 41 (10), 39 (8).

N-Allylbenzamide (3i)^4b

Yellowish oil; isolated yield: 98%; ¹H NMR (400 MHz, CDCl₃) δ 7.82–7.70 (m, 2H), 7.50–7.41 (m, 1H), 7.37 (td, J = 7.0, 1.0 Hz, 2H), 6.63 (s, 1H), 5.96–5.80 (m, J = 16 Hz, 1H), 5.24–5.09 (m, J = 16 Hz 2H), 4.02 (tt, J = 10 Hz, 2H); ¹³C NMR (101 MHz, CDCl₃): δ 167.56, 134.36, 134.12, 131.44, 128.49, 126.95, 116.49, 42.40 ppm; GC-MS (EI, 70 eV): m/z (%) = 161 [M⁺] (9), 146 (7), 105 (100), 77 (51), 56 (7), 51 (16), 39 (4).

N-Phenylbenzamide (3j)^18a

White solid; isolated yield: 99%; ¹H NMR (500 MHz, CDCl₃) δ 7.95 (s, 1H), 7.87 (d, J = 8.1 Hz, 2H), 7.65 (d, J = 8.2 Hz, 2H), 7.58–7.53 (m, 1H), 7.47 (dd, J = 11.1, 4.0 Hz, 2H), 7.37 (t, J = 7.4 Hz, 2H), 7.18–7.14 (m, 1H); ¹³C NMR (126 MHz, CDCl₃) δ 165.83, 137.91, 134.97, 131.84, 129.09, 128.77, 127.03, 124.58, 120.24; GC-MS (EI, 70 eV): m/z (%) = 197 [M⁺] (43.6), 105 (100), 91 (6), 77 (48), 65 (9), 51 (13.83).

N,N-Dibenzylbenzamide (3k)^4b

White solid; isolated yield: 95%; ¹H NMR (400 MHz, CDCl₃) δ 7.50–7.48 (m, 2H), 7.38–7.31 (m, 8H), 7.29 (t, J = 7.0 Hz, 4H), 7.13 (d, J = 5.7 Hz, 2H), 4.70 (s, 2H), 4.39 (s, 2H); ¹³C NMR (101 MHz, CDCl₃): δ 172.26, 136.09, 129.65, 128.84, 128.71, 128.55, 128.39, 127.65, 127.51, 127.01, 126.68, 51.50, 46.81 ppm; GC-MS (EI, 70 eV): m/z (%) = 301 [M⁺] (8.1), 210 (31.20), 105 (100), 91 (16.40), 77 (30.32), 65 (5.96), 51 (4).

N-Benzylbenzamide (3l)^18a

White solid; isolated yield: 98%; ¹H NMR (500 MHz, CDCl₃) δ 7.80 (d, J = 8.0 Hz, 2H), 7.49–7.23 (m, 8H), 6.67 (bs, 1H), 4.62 (d, J = 5.7 Hz, 2H); ¹³C NMR (126 MHz, CDCl₃) δ 167.44, 138.21, 134.34, 131.54, 128.76, 128.57, 127.88, 127.57, 126.99, 44.09, 29.71; GC-MS (EI, 70 eV): m/z (%) = 211 [M⁺] (56), 210 (21), 106 (28), 105 (100), 91 (10), 77 (59), 51 (14).

Acknowledgements

The author R. S. Mane gratefully acknowledged the Council of Scientific and Industrial Research (CSIR), India for providing Senior Research Fellowship (SRF).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Materials and methods, experimental procedure, ¹H, ¹³C NMR and GC-MS data of products. See DOI: 10.1039/c5ra18692g

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