Formamides are important intermediates in organic synthesis [1, 3], and are also precursors for preparation of formamidine, isocyanides  and many pharmaceutically important heterocycles, such as flouroquinolines , oxazolidinones , 1, 2-dihydroquinolines , substituted arylimidazoles  and cancer chemotherapeutic compounds . In addition, formyl group has been widely used as amino protecting group through peptide synthesis . Furthermore, using as reagent in vilsemiere formylation  and as Lewis base catalyst , are other applications of formyl group. Numerous formylating methods have been reported in recent years using acetic formic anhydride , chloral , activated formic acid using DCC  or EDCI , activated formic esters [14-16], ammonium formate , solid supported reagents , 2, 2, 2-trifluoroethyl formate , aq. 85% formic acid with ZnO  and sulfonic acid supported hydroxyapatite encapsulated γ-Fe2O3 . However, some limitation factors such as expensiveness and toxicity of formylating reagents, formation of byproducts, high temperature and prolonged reaction times are besides the usefulness of these methods. Moreover, due to the environmental considerations, development of environmentally benign chemical reactions has been emerged as a demanding research area in modern organic chemical research . Recently, application of zeolites as a heterogeneous catalyst has gained remarkable interest, due to their distinctive chemical and physical properties such as their thermal stability, their acidic and basic nature or shape selectivity which affects the reaction efficiency, and easy separation from reaction mixture which facilitates work up and purification of products [23-25]. Furthermore, zeolites are known to catalyze different types of organic transformations much more effectively and selectively than the Lewis acid catalysts . Recently, natrolite zeolite has been reported as a natural catalyst for synthesis of formamides . Also N-formylation of amines using natural HEU zeolite has been reported . In the other hand, organic-inorganic hybrid materials are of great interest as serve by thermal and mechanical stability of their inorganic moieties . Therefore, exploiting these unique properties to address the drawbacks of previously reported procedures and in continuation of our interest toward environmentally benign procedures for organic transformations, herein we have designed NaY/SA/Cu(Ⅱ) as a recyclable organicinorganic hybrid catalyst and used in general procedure for N-formylation of amines under solvent-free conditions (Scheme 1).
|Scheme1. Chemoselective N-formylation of amines with formic acid and NaY/SA/ Cu(Ⅱ)|
2. Results and discussion 2.1. Characterization of NaY/SA/Cu(Ⅱ)
The FT-IR spectra of all hybrid materials (Fig. 1) indicate an intense band about 1049 cm-1 attributed to the asymmetric stretching of Al-O-Si chain of zeolite. The symmetric stretching and bending frequency bands of Al-O-Si framework of zeolite appear at 765 and 455 cm-1, respectively . The out of plane bending and stretching of O-H group appeared at 785 and 3400-3500 cm-1 respectively. The characteristic peak appears at 1150 assigned to S=O stretching vibrations which confirm the presence of SO3H group (Fig. 1, curves c, d). The symmetric and asymmetric stretching vibration mode of amine group was observed at 3200-3400 cm-1 (Fig. 1, curves b-d) The weak band at 2900 cm-1 can be attributed to C-H stretching vibration (Fig. 1, curve d).
|Fig. 1. The comparative FT-IR spectra of (a) NaY zeolite, (b) NaY zeolite-NH2, (c) NaY/SA and (d) NaY/SA/Cu(Ⅱ)|
The microstructure of the catalyst was investigated by scanning electron microscopy (SEM). The SEM image of NaY/SA/Cu(Ⅱ) in Fig. 2b shows particles with mean diameter of about 300 nm in a nearly spherical shape which demonstrate structural integrity in the terms of particle's size and shape in comparison to NaY/SA in Fig. 2a. The EDX analysis of NaY/SA/Cu(Ⅱ) that is available in supporting information file, clearly reveals the presence of Cu as well as other expected main elements including N, Al, Si, O, C, Na and S (Fig. 3b). The lower intensity of Na in compare to that for NaY/SA demonstrated the exchange of Na+ as a monovalent cation by divalent cation Cu2+ in zeolite's pores without any changes in zeolite's structure.
Thermal analysis was used to monitor the decomposition profile of NaY/SA/Cu(Ⅱ) (Fig. 3). In the first stage, it shows a weight loss of 12% up to 350 ℃, due to the adsorbed water in catalyst. The second stage weigh loss of 11% was observed at the temperature range about 350-430 ℃ due to the organic moiety. The weight loss above 450 ℃ is attributed to the zeolite decomposition.
The same peaks are observed in XRD patterns of both NaY/SA and NaY/SA/Cu(Ⅱ) (Fig. 4) that demonstrates no changes in the general structure and morphology of NaY zeolite during the immobilization procedures.
2.2. Catalytic studies
We initiated our investigation to optimize the formylation of aniline as a model reaction by varying reaction parameters including solvent, catalyst type, catalyst loading and amount of formic acid (Table 1). As the reaction was performed with prolonged reaction time in the absent of any catalyst, gave dramatic yield (Table 1, entry 19), among different catalyst, NaY/ SA/Cu(Ⅱ) (0.01 g) was found to provide the best result (Table 1, entry 6). Then, the feasibility of the reaction was probed in various solvent (Table 1, entries 1-5). An increase in reaction times and a decrease in the yields were observed with using any solvent and the best result was obtained in solvent-free conditions. Also lowering the amount of formic acid, increased the reaction time in lower yield. The best yield of formamide was obtained with 5 mmol formic acid and 1 mmol aniline in the presence of 0.01 g NaY/SA/Cu(Ⅱ), under solvent free conditions at room temperature without any byproduct (Table 1, entry 6). Then the feasibility of the reaction was probed in various solvent (Table 1, entries 1-5). An increase in reaction times and a decrease in the yields were observed with using any solvent and the best result was obtained in solvent-free conditions. Also lowering the amount of formic acid, increased the reaction time in lower yield. The best yield of formamide was obtained with 5 mmol formic acid and 1 mmol aniline in the presence of 0.01 g NaY/SA/Cu(Ⅱ), under solvent free conditions at room temperature without any byproduct (Table 1, entry 6).
Having established the optimized conditions for easy transformation of aniline to N-phenylformamide, formylation of variety of amines were investigated via this protocol. A series of aromatic, heterocyclic and aliphatic amines were subjected to reaction with formic acid using NaY/SA/Cu(Ⅱ) under solvent free conditions at room temperature to obtain the formamide in excellent yields (Table 2). This methodology showed a wide reaction scope and was found to be effective in formylation of both electron rich and electron poor anilines in high to excellent yields in short reaction times. This protocol is very valuable from generality point of view and can be applied successfully to both primary and secondary amines compared to the recent formylation method by use of polyethylene glycol which is only applicable for aromatic primary amines . However, anilines bearing electron withdrawing groups such as -NO2 carried out the reaction in longer reaction times than ones bearing electron donating groups such as -Methyl. So, electronic factors play an important role on yields and reaction times as we observed for 4, 6-dimethylpyrimidin-2-amine (Table 2, entry 14) which produced the product in lower yield and longer reaction time in compare to pyridin-2-amine (Table 2, entry 13). It seems that the presence of two electronegative SP2 nitrogen atom in heterocyclic ring of 4, 6-dimethylpyrimidin-2-amine caused the reaction proceed slightly slower. Primary amines (Table 2, entries 1-16) easily reacted to give N-formyl product, while secondary amines (Table 2, entries 17-22) smoothly proceeded to provide the corresponding formamide. We also observed that sterically hindered amines (Table 2, entries 14, 17, 21 and 22) took a longer reaction time. It seems that steric factors play an important role on yields and reaction times as well as electronic factors (Table 2, entry 22). Diphenylamine as a hindered secondary amine which previously has been reported unreactive  or provided lower yield , was reactive following this reaction protocol (Table 2, entry 21). Interestingly the N-formylation of 1, 2-phenylendiamine afforded di-formylated product (Table 2, entry 16) in high yield instead of formation benzimidazole which previously reported in literature . Unfortunately, acyclic aliphatic amines showed poor activity than cyclic aliphatic amines and gave a significant lower yield (Table 2, entry 27).
It is noteworthy to mention that the reaction was found to be selective only toward N-formylation. O-Formylation of phenols did not take place under this protocol, such as, phenol, p-nitrophenol, benzyl alcohol, and (Ph)2CHOH. So, any substrate bearing both -OH and -NH2 groups, will only be N-formylated which indicate the high chemoselectivity of our method (Table 2, entries 10 and 11). Also chemoselectivity was observed in reaction of formic acid with a mixture of primary and secondary amine when only primary amine was formylated (Scheme 2). Therefore, the primary amines can be formylated in the presence of secondary amines by controlling the reaction time.
|Scheme2. Chemoselectivity in formylation of primary amine in the presence of secondary amine|
A plausible mechanism is shown in Scheme 3. It probably seems that NaY/SA/Cu(Ⅱ) has an important role in activation of carbonyl of formic acid as similar mechanism that has been reported for Nformylation of amines using ZnCl2 . The catalyst as a Lewis and protic acid coordinates to carbonyl group of formic acid results in enhancement of its electrophilic character to nucleophilic attack by the amine nitrogen atom.
To evaluate the recyclability of NaY/SA/Cu(Ⅱ), the reaction of aniline and formic acid was studied under the optimal reaction conditions using recycled catalyst from the previous run. The catalyst was separated by simple filtration, adequately washed with CHCl3, dried at 60 ℃ and reused for further reactions. The recovered catalyst could be reused up to 6 cycles for N-formylation without any significant loss of activity (Fig. 5) as its FT-IR spectrum showed no distinct change in its structure in compare to FT-IR spectrum of fresh catalyst (Fig. 6).
|Fig. 6. FT-IR spectra of fresh catalyst and the six times reused catalyst|
Finally, to establish the efficiency of this methodology, it was compared with some reported methods. As Table 3 shows this method clearly improved yields and reaction times. Also, the reusability of the catalyst makes this method less contaminate and nearly green. In addition, since the catalyst is heterogeneous, the work-up procedure is easy and chromatography or other complex purification methods are not necessary. According to the obtained results, it seems that NaY zeolite also helps the catalytic effect of sulfonic acid group and accelerates the reaction.
We have developed an efficient method for chemoselective Nformylation of various amines by use of NaY/SA/Cu(Ⅱ) as a reusable heterogeneous organic-inorganic hybrid catalyst to give access formamides at room temperature under solvent-free conditions. Prominent advantages of this protocol include short reaction times, easy work up, high to excellent product yields and the recoverability of the catalyst. Also this rapid synthesis of formamides make this protocol, practical and energy efficient and it can be considered as "green procedure" as no volatile solvent is released into the environment.4. Experimental 4.1. Materials
All used chemicals were purchased from Merck and Fluka chemical companies. Melting points were determined on an electro thermal digital melting point apparatus. The catalyst was characterized by atomic absorption instrument (Varian SpectrAA 200), X-Ray diffract meter (Philips 8440) with radiation at room temperature Cu Ka, FT-IR (Galaxy series FT-IR 5000 spectrometer), and TGA-DSC (Rheometric scientific STA-1500 thermo gravimetric analyser). NMR spectra were recorded on a Bruker (300 MHz) spectrometer. Chemical shifts (ppm) were referenced to the internal standards tetramethylsilane (TMS). Microanalyses were performed by the Elemental Analyzer (Elemental, Vario EL Ⅲ) at Arak University. The microanalyses results were agreed favorably with the calculated values. Reactions were monitored by thin layer chromatography using silica gel F254 aluminium sheets (Merck).4.2. Procedure for synthesis of NaY/SA/Cu(Ⅱ)
To a mixture of NaY zeolite in toluene (20 mL), trimethoxysilyl propylamine (2 mL) was added and stirred for 24 h under reflux conditions. Then zeolite was separated by filtration and dried at 60 ℃. For sulfonation, chlorosulfonic acid (2 mL) and triethylamine (0.2 mL) was added to a mixture of functionalized zeolite (1 g) in toluene (20 mL) and stirred at reflux conditions for 24 h. Finally, the participated solid was filtrated, washed with water and dried over 60 ℃ to obtain the NaY-SA compound. A pH analysis of NaY-SA showed 1.5 mmol/g loading of SO3H. In the next step, NaY-SA (1 g) was reacted with 100 mL of a 0.001 mol/L solution of Cu(OAc)2 in toluene for 24 h at room temperature. After sonication (2 × 3 min) the NaY/SA/Cu(Ⅱ) product was filtered, and dried at 60 ℃ (Scheme 4). Then the loading of copper in NaY/SA/Cu(Ⅱ) was determined to be 2% by atomic absorption spectroscopy.
4.3. General procedure for formylation of amines
To a mixture of amine (1 mmol) and NaY/SA/Cu(Ⅱ) (0.01 g), formic acid (5 mmol, 0.19 mL) was added and stirred for appropriate time at room temperature. After completion of the reaction as monitored by TLC (n-hexane/EtOAc = 8/2), NaY/SA/Cu (Ⅱ) catalyst was filtered and organic layer evaporated under reduced pressure to give pure product. The structure of the products was established from their physical properties and spectral (1H NMR, 13C NMR and IR) analysis and were compared with the data reported in the literature and are available as the Supporting information.
Physical and spectroscopic characterization data of selected new compounds (9, 13, 14 and 16) are shown below, and others were given in Supporting information.
N-(2, 4-Dichlorophenyl) formamide (9): Mp: 154-157 ℃. IR (KBr pellet, cm-1): vmax 3244, 3090, 3034, 2986, 2899, 1695, 1664, 1585, 1529, 1398, 1298, 1103, 1049, 817, 752, 700, 381. 1H NMR (DMSO-d6, 300 MHz): δ 9.74 (s, 1H, CHO). 7.37-8.14 (m, 3H, Ar-H), 7.15 (s, 1H, NH), 13C NMR (DMSO-d6, 75 MHz): δ 161.03, 133.88, 129.29, 128.79, 128.14, 124.73, 124.43. Elemental analysis for C7H5Cl2NO: Calcd. C 44.25, H 2.65, N 7.37; Found: C 44.00, H 2.80, N 7.37.
N-(Pyridin-2-yl)formamide (13): Mp: 130-132 ℃. IR (KBr pellet, cm-1): vmax 3319, 3157, 2876, 1668, 1624, 1589, 1454, 1369, 1327, 1248, 1134, 765, 387. 1H NMR (DMSO-d6, 300 MHz): δ 7.67-7.94 (m, 1H, CHO), 6.01-7.15 (m, 4H, Ar-H), 5.78 (s, 1H, NH). 13C NMR (DMSO-d6, 75 MHz): δ 160.08, 155.91, 149.63, 136.91, 120.72, 105.79. Elemental analysis for C6H6N2O: Cacld. C 59.01, H 4.95, N 22.94; Found: C 59.20, H 5.15, N 23.00.
N-(4, 6-Dimethylpyrimidin-2-yl)formamide (14): Mp: 145-148 ℃. IR (KBr pellet, cm-1): vmax 3325, 3182, 3094, 2930, 2858, 2735, 1682, 1662, 1635, 1574, 1589, 1373, 1323, 1136, 935, 553, 387. 1H NMR (DMSO-d6, 300 MHz): δ 7.90 (s, 1H, CHO), 6.11 (m, 2H, NH and Ar-H), 1.92 (s, 6H, CH3). 13C NMR (DMSO-d6, 75 MHz): δ 164.06, 159.63, 156.81, 137.39, 18.21. Elemental analysis for C7H9N3O: calcd. C 55.62, H 6.00, N 27.80; Found: C 56.00, H 5.86, N 27.81.
N, N'-(1, 2-Phenylene)diformamide (16): Mp: 180-182 ℃. IR (KBr pellet, cm-1): vmax 3101, 2922, 2856, 2808, 2696, 1682, 1620, 1560, 1485, 1452, 1398, 1249, 1120, 1035, 802, 605, 624, 426. 1H NMR (DMSO-d6, 300 MHz): δ 8.22-8.34 (m, 2H, CHO), 7.62-7.65 (m, 2H, Ar-H), 7.20-7.23 (m, 2H, Ar-H), 6.89 (s, 2H, NH). 13C NMR (DMSO-d6, 75 MHz): δ 163.62, 130.12, 124.64, 122.58. Elemental analysis for C8H8N2O2: Calcd. C 58.53, H 4.91, N 17.06. Found: C 59.19, H 5.00, N 17.39.Acknowledgment
We thank Arak University for financial support for this work.
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