Catal Lett (2009) 132:480–486 DOI 10.1007/s10562-009-0116-7
Novel Combination of Sodium Borohydride and Reusable Polyaniline Salt Catalyst for Rapid and Efficient Reductive Amination of Carbonyl Compounds Chebrolu Lavanya Devi Æ Oladoye Sunday Olusegun Æ Chebolu Naga Sesha Sai Pavan Kumar Æ Vaidya Jayathirtha Rao Æ Srinivasan Palaniappan
Received: 27 May 2009 / Accepted: 26 July 2009 / Published online: 8 August 2009 Ó Springer Science+Business Media, LLC 2009
Abstract A simple, fast and efficient method is developed for reductive amination of anthraldehydes and naphthaldehydes in a single step. In this method, sodium borohydride is used in combination with reusable polyaniline salt (PANI) catalyst for the first time. The reductive amination of anthraldehydes with amines was carried out in DCM-MeOH (3:1) solvent mixture at room temperature using NaBH4/PANI system. This method afforded the amines in excellent yields immediately without any side product. This method was successfully extended for the reductive amination of benzaldehyes, heterocyclic aldehydes, a,b-unsaturated aldehydes and ketones. Polyaniline salt catalyst is recoverable and reusable without losing its potency. In this method, we have established the use of various polyaniline salts containing mineral, solid/liquid organic protonic acid prepared by aqueous/emulsion polymerization pathway as polymer based solid acid catalyst in the reduction amination reaction. Keywords Reductive amination Anthraldehyde Naphthaldehyde Polyaniline Reusable catalyst
C. L. Devi O. S. Olusegun C. N. S. S. P. Kumar V. J. Rao Organic Chemistry Division-II, Indian Institute of Chemical Technology, Hyderabad 500007, India V. J. Rao National Institute of Pharmaceutical and Educational Research, Hyderabad 5000007, India S. Palaniappan (&) Organic Coatings & Polymers Division, Indian Institute of Chemical Technology, Hyderabad 500007, India e-mail:
[email protected]
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1 Introduction Reductive amination of aldehydes and ketones is one of the most useful methods to prepare amines, which are useful in biological and chemical systems as solvents, in the synthesis of intermediates for pharmaceuticals and agrochemicals, in polymers, dyes and raw materials for resins, textile additives, disinfectants, rubber stabilizers, corrosion inhibitors, and in the manufacturing of detergents and plastics [1, 2]. The methylamine derivatives of anthracene and naphthalene have various applications, including anticancer inhibitory effects [3–6], Fluorescent sensors [7–12] Fluorescent pH indicators [13, 14], and pH-controllable supramolecular switches [15]. Reductive amination of carbonyl compounds with the use of NaBH4 was carried out by microwave method [16], in micellar media [17], ionic liquids [18], titanium (IV) isopropoxide [19], NaBH4 along with catalysts such as various acids [20] boric acid/p-toulenesulfonic acid/benzoic acid [21], boric acid [22], acetic acid [23], trifluoroacetic acid [24], silica-phosphoric acid [25] iodine [26], indium chloride [27], guanidine-hydrochloride [28], 12-Tungstophosphoricacid [29] etc. However, the reductive amination of anthraldehyde and naphthaldehyde was carried out by two steps, i.e. condensation of anthraldehyde/naphthaldehyde with appropriate amines to generate imines, which are then reduced to the corresponding anthryl/naphthyl methylamines by NaBH4 [12, 30]. Also in most cases, reaction proceeded for a longer time. Despite several methods present in the literature for reductive amination, simple, efficient, high yielding and environmentally benign approaches for reductive amination is still needed. Very recently, we are establishing conducting polymer salt as polymer based acid catalyst in organic transformations [31–33] and in some reactions this catalyst does not
Novel Combination of Sodium Borohydride and Reusable PANI catalyst
show catalytic activity. In order to find the suitability of this catalyst in academic work and viability in industry, it is essential to carry out particular organic reaction with this catalyst. The choice of PANI catalyst is due to its number of intrinsic redox states, stability, ease of synthesis, cheaper, handling, versatility, simple workup procedure, mildness and recyclability. The structure of polyaniline is known as a Para-linked phenylene amineimine. The base form of polyaniline can, in principle, be described by the following general formula (Fig. 1): In the generalized base form (1 - x) measures the function of oxidized units. When (1 - x) = 0, the polymer has no such oxidized groups and is commonly known as a leucoemeraldine base. The fully oxidized form, (1 - x) = 1 is referred to as a pernigraniline base. The half-oxidized polymer, where the number of reduced units and oxidized units are equal, i.e. (1 - x) = 0.5, is of special importance and is termed as the emeraldine oxidation state or the emeraldine base. Partially oxidized emeraldine base is shown to be an alternating copolymer of reduced and oxidized repeat units. The value of x varies from 0 to 1, but the percentage of carbon, hydrogen and nitrogen will be almost the same. Taking these points into consideration, the following formula of polyaniline base (Fig. 2) and polyaniline salt (Fig. 3) is considered for simplicity. In this work, a simple, fast and efficient method is successfully developed for reductive amination of anthraldehydes and naphthaldehydes to amines in single step for the first time by the use of novel combination of NaBH4 and
H N
NH
N x
1-x
n
Fig. 1 Structure of polyaniline base
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various PANI as a polymer-based solid acid catalyst. In order to establish the versatility of the polymer-based catalyst, we also extended the reductive amination reaction for benzaldehyde, a,b-unsaturated aldehydes, heterocyclic aldehydes and ketones. We obtained the products immediately after mixing the reactants in excellent yield with NaBH4/PANI-HBF4 when compared to the earlier reports.
2 Experimental 2.1 General All chemicals were of research grade and were used as obtained from Aldrich and Fluka. The reactions were carried out in a round-bottomed flask of 25 mL capacity at room temperature in an efficient fume hood. Analytical thin layer chromatography was performed with E. Merck silica gel 60F glass plates and flash chromatography by use of E. Merck silica gel (60–120 mesh). Melting points were determined on a MEL-TEMP II melting point apparatus and were uncorrected. NMR spectra were recorded of Gemini 200 MHz Varian instrument and Avance 300 MHz Bruker UX 300 FT NMR. All NMR data were obtained in CDCl3 solution and chemical shifts (d) were given in ppm relative to TMS and are compared with the reported literature values. Mass spectra were recorded using ESI ion trap Mass Spectrometer (Thermofinnigan, Sanzox, CA, USA) and HRMS (high resolution mass spectrometer) were recorded using Applied Biosystems QSTAR XL high resolution mass spectrometer. IR spectra were recorded on a SHIMADZU 435 Infrared Spectrophotometer. Commercially available organic compounds were used without further purification except for the solvent, which was distilled by known methods before use. 10-Methyl 9-anthraldehyde was synthesized by using known method [34, 35]. 2.2 Synthesis of Polyaniline Salt Catalyst 2.2.1 Synthesis of Polyaniline Salt by Emulsion Polymerization Pathway
N H n ?
-
Fig. 2 Simplified structure of polyaniline base where H A is the acid group and y is the number of acid group per aniline unit
N H HA
n y
Fig. 3 Simplified structure of polyaniline salt
Polyaniline salt was synthesized by emulsion polymerization pathway by a procedure similar to our earlier report [32]. In a typical experiment, 3.0 g benzoyl peroxide was dissolved in 30 mL chloroform in 250 mL round bottomed flask. To this solution, 20 mL distilled water containing 1 g sodium. lauryl sulfate was added and the reaction mixture was stirred at 40 °C. Aqueous solution (50 mL) containing 5.5 mL fluoroboric acid and 1.0 mL aniline was prepared and added dropwise into the above initiator surfactant mixture for duration of 15 min. After
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addition, the reaction was continued for 8 h. Chloroform layer containing polyaniline salt was separated from aqueous layer and washed thrice with distilled water. Polyaniline salt was precipitated from the chloroform containing polyaniline salt layer with 300 mL acetone. Precipitate of polyaniline salt was separated by filtration, washed with water followed by acetone and dried under vacuum till a constant mass.
2.2.2 Synthesis of Polyaniline Salts by Aqueous Polymerization Pathway Polyaniline salts were synthesized by chemical oxidative method by a procedure similar to our earlier report [36]. In a typical reaction, 3.0 mL of aniline, 28.8 mL of hydrochloric acid were dissolved in 180 mL of distilled water taken in 250 mL round bottomed flask. The solution was kept under constant stirring at ambient temperature. To the above reaction mass, solution of 9.82 g of sodium persulfate in 120 mL water was added drop wise for 15–20 min. The reaction mass was maintained under constant stirring at ambient temperature for 4 h. The precipitated polyaniline salt was recovered from the polymerization vessel by filtration and then washed with distilled water until the washing liquid was colorless. In order to remove oligomers and other organic by products, the precipitate was washed with acetone until the acetone was colorless and subsequently dried at 60 °C till a constant mass. Polyaniline salt was dedoped to polyaniline base by aqueous sodium hydroxide solution. Polyaniline-hydrochloride salt (1.0 g) was stirred in 100 mL of 1.0 N sodium hydroxide solution for 4 h at ambient temperature. The solution was filtered, washed with sodium hydroxide (1.0 N) solution followed by distilled water and finally with acetone. Polyaniline base was dried at 60 °C till a constant weight was reached. Polyaniline base (0.4 g) was stirred with 50 mL of aqueous acid solution (4.3 mL of hydrochloric acid (1 N)/ 1.4 mL of sulphuric acid (1 N)/1.5 mL of acetic acid (0.5 N)/0.95 g of p-toluenesulfonic acid (0.1 N)/1 g of b-naphthalenesulfonic acid (0.1 N). The reaction mixture was stirred at ambient temperature for 4 h, filtered, washed with ample amount of water, followed by acetone. The sample was dried at 60 °C till a constant weight was reached. Amount of acid dopant (wt %) present on polyaniline salt was found out from the formula: ðweight of polyaniline salt weight of polyanilinebaseÞ 100:
The results of emeraldine form of polyaniline salts are reported in Table 3.
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2.3 Typical Experimental Procedure for the Reductive Amination In a typical experiment, anthraldehyde (206 mg, 1 mmol) and n-butyl amine (0.11 mL, 1.1 mmol) were dissolved in 4 mL of dichloromethane and methanol (DCM-MeOH) mixture (3:1). To this reaction mixture, PANI-HBF4 catalyst (5 wt % with respect to anthraldehyde) and NaBH4 (41 mg, 1.1 mmol) were added, stirred at room temperature, immediately checked the TLC and showed the formation of product. The reaction mixture was quenched with aq. NH4Cl solution (2 mL), filtered the solution and washed with 10 mL of DCM. The catalyst was recovered. The organic layer was separated, dried in Na2SO4 and concentrated the solution to get the product. The crude product was purified by silica gel column chromatography (eluent: hexane). The above experimental procedure was adopted for the preparation of amines reported in the present study. The authenticity of the products was confirmed from their 1H NMR and mass spectral data. 2.4 The Spectral Data for New Products 2.4.1 1-(Anthracen-9-Ylmethyl) Piperidine (3) Paleyellow solid, mp = 133 °C, IR (KBr): 3,446, 3,045, 2,935, 2,850, 2,756, 1,619, 1,440, 1,335, 1,150, 1,100, 892, 729 cm-1; 1H NMR (200 MHz, CDCl3): d = 8.43 (d, J = 9.4 Hz, 2H), 8.34 (s, 1H), 7.92 (dd, J = 7.03 Hz, 2H), 7.42 (m, J = 3.12 Hz, 4H), 4.35 (s, 2H), 3.45 (s, 2H), 2.54 (t, J = 4.38 Hz, 4H), 1.48 (m, J = 7.03 Hz, 4H); 13C NMR (200 MHz, CDCl3): d = 131.3, 130.4, 128.8, 127.1, 125.3, 124.7, 55.1, 54.7, 26.1, 24.5; MS (ESI): m/z = 276 (M ? H?); HRMS (ESI): m/z, (M ? H?) calculated for C20H22N: 276.1752; found 276.1744. 2.4.2 N-(Anthracen-9-Ylmethyl)-9-Ethyl-9H-Carbazol-3Amine (4) Yellow solid, mp = 216 °C, IR (KBr): 3,387, 3,048, 2,964, 2,865, 1,627, 1,576, 1,492, 14,68, 1,230, 1,085, 791, 732 cm-1; 1H NMR (200 MHz, CDCl3): d = 8.42 (s, 1H), 8.30 (d, J = 8.31 Hz, 2H), 7.99 (q, J = 6.99 Hz, 3H), 7.58 (S, 1H), 7.44 (m, J = 7.93 Hz, 4H), 7.37 (dd, J = 7.93 Hz, 1H) 7.31 (d, J = 8.12 1H), 7.14 (m, J = 8.69 Hz, 2H), 6.88 (dd, J = 8.69 Hz, 1H), 5.27 (s, 2H), 4.28 (q, J = 6.99 Hz, 2H), 1.40 (t, J = 7.18 Hz, 3H); 13C NMR (200 MHz, CDCl3): d = 131.7, 130.3, 128.6, 127.5, 126.o, 125.0, 124.7, 124.1, 120.1, 117.8, 108.7, 107.9, 95.8, 42.4, 37.1, 13.6; MS (ESI): m/z = 401 (M ? H?). HRMS (ESI): m/z, (M ? H?) calculated for C29H25N2: 401.2017; found 401.2013.
Novel Combination of Sodium Borohydride and Reusable PANI catalyst
2.4.3 N-((10-Methylanthracen-9-yl)Methyl)Aniline (5) Pale yellow solid, mp = 175 °C, IR (KBr): 3,402, 3,044, 2,872, 1,600, 1,502, 1,311, 749, 691 cm-1; 1H NMR (200 MHz, CDCl3): d = 8.29 (m, J = 3.21 Hz, 4H), 7.46 (m, J = 2.64 Hz, 4H), 7.22 (m, J = 3.40 Hz, 2H), 6.74 (m, J = 6.6 Hz, 3H), 5.10 (s, 2H), 3.45 (s, 1H), 3.13 (s, 3H); 13C NMR (200 MHz, CDCl3): d = 148.4, 130.1, 129.8, 129.3, 125.8, 125.3, 125.0, 124.7, 117.5, 112.5, 40.8, 14.3; MS (ESI): m/z = 298 (M ? H?); HRMS (ESI): m/z, (M ? H?) calculated for C22H19N: 298.1027; found 298.1019. 2.4.4 4-Methoxy-N-(10-Methylanthracen-9yl) Methyl)Aniline (6)
483 Table 1 Reductive amination of carbonyl compounds with NaBH4 catalyzed by PANI-HBF4 S.No
Carbonylcompound
Amine
H N CHO 1
96
NH2
CHO
H N
NH 2
2
96
N
CHO
H N 95
3
Yellow solid, mp = 148 °C, IR (KBr): 3,396, 2,963, 1,615, 1,510, 1,261, 1,094, 1,023, 867, 800, 746, 700 cm-1; 1H NMR (200 MHz, CDCl3): d = 8.28 (m, J = 3.39 Hz, 4H), 7.45 (m, J = 3.71 Hz, 4H), 6.76 (m, J = 9.06 Hz, 4H), 5.08 (s, 2H), 3.77 (s, 3H), 3.13 (s, 3H); 13C NMR (200 MHz, CDCl3): d = 152.1, 142.7, 131.1, 129.8, 129.7, 125.6, 125.1, 124.7, 124.5, 114.8, 113.5, 55.6, 41.6, 14.1; MS (ESI): m/z = 328 (M ? H?); HRMS (ESI): m/z, (M ? H?) calculated for C23H22NO: 328.1701; found 328.1691.
CHO 4
3 Results and Discussion
H N
H2 N
96
H N CHO
NH 2
5
96
H N
NH 2
96
O
H N CHO NH 2
7
CHO
94
H N
NH 2
8
95
CHO
N H N
9
R1 N R2
CHO
R
O
6
Initially, anthraldehyde was reacted with butylamine using NaBH4 in DCM-CH3OH (3:1) mixture at r.t. for 2 h and obtained anthryl methyl butyl amine, imine and alcohol mixture as products. In order to prepare amine in better yield without side products, we catalyzed the reaction with emeraldine form of PANI salt (PANI-HBF4) powder containing HBF4 as lewis acid dopant prepared by emulsion polymerization pathway and obtained the product immediately with 96% yield. Then the reaction of different anthraldehydes with amines (aliphatic primary and secondary amines, aromatic amines) was carried out using NaBH4/ PANI-HBF4 system in DCM-CH3OH (3:1) at room
R1
NaBH4/PANI-HBF4
R2
DCM-MeOH r.t., Immediate
N
N
N
CHO
H
Isolated Yield(%)
Product
95
H N
CHO 10
98
NH 2
R
Scheme 1 Reductive amination of anthraldehydes with NaBH4 catalyzed by PANI. R = –H, –CH3, R1 = –butyl, –phenyl, –anisyl, -9-ethyl-9H-carbazol-3-yl, R2 = –H, R1, R2 = –CH2–CH2–CH2– CH2–CH2
CHO 11
NH2
H N 98
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484
C. L. Devi et al.
Table 1 continued S.No
Carbonylcompound
CHO
Amine
Isolated Yield(%)
Product
N
H N
12
96
13 S
14
NH2
CHO
O
CHO
N
CHO
NH 2
15
NH 2
NH2
H N
94
H N
95
S
O
H N
N
90
HN 97
CHO
16
NH2
NH
We have applied this method successfully for the reductive amination of naphthaldehyde with butylamine, aniline and piperdine (Scheme 2), which gave the corresponding amines in excellent yields immediately (Table 1, entries 7–9). In order to check the versatility of the reductive amination of NaBH4/PANI-HBF4 system, benzaldehyde was reacted with series of amines (Scheme 3), the reactions went smoothly to afford the corresponding amines in quantitative yields (Table 1, entries 10–12). Further, we extended this reaction for heterocyclic aldehydes such as furfuraldehyde, thiophene-2-carboxaldehyde and pyridine2-carboxaldehyde with butyl amine (Scheme 3), which gave the corresponding amines in high yields (Table 1, entries 13–15). To explore the scope of this system [NaBH4-PANI] in the reaction of a, b-unsaturated aldehydes, we carried out the reaction of crotonaldehyde and cinnamaldehyde with aniline (Scheme 4) and obtained the corresponding amines in quantitative yields (Table 1, entries 16, 17) with the double bond intact. We also examined this method for reductive amination of ketone by carrying out the reaction of cyclohexanone
CHO 17
98
R1
HN
O NH2
18
N
90
CHO H N
O
NH2
R2
R1 NaBH4/PANI-HBF 4 R2
DCM-MeOH r.t., Immediate
HN 90
Scheme 2 Reductive amination of naphthaldehydes with NaBH4 catalyzed by PANI. R1 = –butyl, –phenyl, R2 = –H, R1, R2 = –CH2–CH2–CH2–CH2–CH2–
19
O 20
H N
N 90
R1
R-CHO
temperature. The reaction went immediately (within a minute) and gave anthryl methyl amines (Scheme 1) in excellent yields (95–96%) (Table 1, entries 1–6). HBF4 acid dopant present on PANI-HBF4 salt play a role in the reductive amination reaction, wherein, polyaniline salt acts as polymer supported acid catalyst. First, H? ion of HBF4PANI attack carbonyl group which interact with amine to form imine by losing water molecule. Again H? ion attack imine, forms immonium ion and then hydride ion from NaBH4 attack on carbon atom of the immonium ion and forms the reduced amine.
123
H N
N
R 1 NaBH4/PANI-HBF4 R2
DCM-MeOH r.t., Immediate
R2
R
Scheme 3 Reductive amination of benzaldehyde and heterocyclic aldehydes with NaBH4 catalyzed by PANI. R = –aryl, –heterocyclic, R1 = –butyl, –phenyl, R2 = –H, R1, R2 = –CH2–CH2–CH2–CH2– CH2– HN
NH2
R3
NaBH4/PANI-HBF4 R3
CHO DCM-MeOH r.t., Immediate
Scheme 4 Reductive amination of a, b-unsaturated aldehydes with NaBH4 catalyzed by PANI. R3 = –C6H5, –CH3
Novel Combination of Sodium Borohydride and Reusable PANI catalyst R2
O H N
N
R1
R1 NaBH4/PANI-HBF4 R2
DCM-MeOH r.t., Immediate
485 Table 3 The amount of acid dopant and number of dopant unit per aniline unit of PANI salts and yield of anthryl methylbutyl amine obtained using PANI salts in the reductive amination of anthraldehyde and butyl amine PANI
Amount of acid dopant present on unit per aniline of anthryl
Number of dopant polyaniline salt aniline unit methylbutyl
Isolated yield (wt %) amine (%)a
PANI-PTSA
33.3
0.24
95
Scheme 5 Reductive amination of cyclohexanone with NaBH4 catalyzed by PANI. R1 = –butyl, –phenyl, R2 = –H, R1, R2 = –CH2–CH2–CH2–CH2–CH2–
with aliphatic and aromatic amines. This method yielded the corresponding cyclohexylamine derivatives (Scheme 5) in high yields (Table 1, entries 18–20). Very recently Alinezhad et al. [25] have reported an efficient reductive amination. In this work, they reported reductive amination of aldehydes and ketones using sodium borohydride-silica phosphoric acid in THF solvent and the reaction took place from 2 to 31 min. In our method, we obtained the product immediately after mixing the reactants with NaBH4-PANI. For example, in our reaction with benzaldehyde and aniline, product was obtained immediately (95% yield) against 10 min (93% yield). Similarly, the reaction with cinnamaldehyde and aniline, we obtained the product immediately (98% yield) against 5 min (90% yield) of the reported one. The above observations indicate that the present method is more efficient than the reported ones. 3.1 Recyclability of Catalyst PANI-HBF4 In order to verify the recyclability of the PANI-HBF4 catalyst, we carried out reductive amination reaction using anthraldehyde and n-butyl amine as a model substrate. The catalyst was filtered from the reaction mixture, washed with water and then by DCM. Catalyst was dried and reused for five cycles. The reaction proceeded smoothly with a yield of 96–90% (Table 2). This result indicates that the activity of the catalyst was not affected on recycling.
Table 2 Results of recyclability of PANI-HBF4 catalyst in reductive amination of anthraldehyde with butylamine Run
Anthraldehyde (mg)
Butyl amine (mL)
Catalyst (mg)
Isolated yield (%)
1
1,000
0.53
50
96
2
740
0.40
37
95
3
680
0.36
34
95
4
600
0.32
30
93
5
500
0.27
25
91
6
320
0.17
16
90
PANI-NSA
34.1
0.23
95
PANI-H2SO4
17.4
0.20
94
PANI-HCl
13.3
0.38
93
PANI-AA
20.2
0.38
95
a
Anthraldehyde (1 mmol), n-butyl amine (1.1 mmol), DCM-MeOH mixture (4 mL), PANI salt catalyst (5 wt % with respect to anthraldehyde) and NaBH4 (1.1 mmol)
3.2 Catalytic Activity of PANI Salts In order to establish the use of PANI as catalyst in reductive amination reaction, Various PANI salts containing mineral acids such as sulphuric acid (PANI-H2SO4), hydrochloric acid (PANI-HCl); solid organic acid such as p-toulene sufonic acid (PANI-PTSA), b-Naphthalene sulphonic acid (PANI-NSA) and liquid organic acids such as acetic acid (PANI-AA) have been prepared by chemical polymerization method and the amount of dopants and number of dopant unit per aniline unit are reported in Table 3. These PANI salts are tried out as catalyst in the reductive amination of anthradehyde and butylamine, which resulted in anthrylmethylbutylamine in excellent yields (93–96%; see Table 3). These results indicate that PANI containing any protonic acid group prepared by emulsion/aqueous polymerization pathway can acts as catalyst in the reductive amination reactions.
4 Conclusion In conclusion, an efficient method has been developed for direct reductive amination of carbonyl compounds with amines using sodiumborohydride catalyzed by PANI. The advantages of this method are: simple reaction, rapid formation of product, versatility, use of little amount of easily synthesizable, stable, handlable, inexpensive, eco-friendly, stable and mild reusable catalyst. This methodology may open up new direction for the synthesis of various organic compounds. Acknowledgments The authors thank Dr. J. S. Yadav, Director, Indian Institute of Chemical Technology, for facilities and encouragement C.L.D, O.S.O and C.S.P thank the Council of Scientific and Industrial Research, New Delhi for the research fellowship, O.S.O
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486 also thank Academy of Sciences for Developing World (TWAS) for Award of fellowship and Ladoke Akintola University of Technology Ogbomoso (LAUTECH) Nigeria for granting sabbatical leave.
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