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Efficient organocatalytic multicomponent synthesis of (α-aminoalkyl)phosphonates
⁎Corresponding author. Tel.: +91 9441300060; fax: +91 877 2243909. gajulapallilavanya@gmail.com (Palakondu Lavanya)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
l-Proline has been used as an organocatalyst for an efficient synthesis of (α-aminoalkyl) phosphonates by treatment of aldehydes, amines and triethyl phosphate at room temperature. The products are formed in excellent yields (82–94%) within 30–45 min.
Keywords
Aldehydes
Amines
Triethyl phosphate
(α-Aminoalkyl)phosphonates
l-Proline
1 Introduction
Organocatalytic reactions have attracted much attention in recent years because they do not require toxic metal-based catalysts (Dalko and Moisan, 2004; Bertelesen and Jorgensen, 2009). These reactions thus contribute significantly to green chemistry. The (α-aminoalkyl)phosphonic acid and its derivatives possess various valuable medicinal properties including anticancer, anti-HIV and antibacterial activities (Alonso et al., 2000; Atheron et al., 1986). They are also applied as enzyme inhibitors and peptide mimics (Barlett et al., 1990; Kafarski and Lejczak, 1991). Hence, the synthesis of these compounds is an important goal for the organic chemists, and some methods have been developed for their preparation (Varaprasad and Jarsival, 1982; Qian and Huang, 1998; Ranu et al., 1999; Manabe and Kobayashi, 2000; Bhagat and Chakraborthi, 2007). In continuation of our work (Maddila and Jonnalagadda, 2012a,b; Sudhakar et al., 2010) on the development of useful synthetic methodologies, we have discovered an efficient method for organocatalytic synthesis of (α-aminoalkyl)phosphonates. These compounds were prepared by using different Lewis acids (Heydari et al., 2001; Saidi and Azizi, 2002; Ranu et al., 1999), heterogeneous and homogenous catalysts (Yadav et al., 2001; Kasthuraiah et al., 2007; Das et al., 2011). However, the immediate preparation of the catalysts, long reaction times, application of costly and toxic metal–based reagents, high temperature, unsatisfactory yields, harsh reaction conditions and limited applicability are problems in various methods of synthesis of the compounds.
In recent years l-proline has drawn much interest in different organic reactions due to its experimental simplicity, easy handling, low cost and excellent solubility in water and organic solvents. l-proline has been found to be a very efficient catalyst in different transformations in enamine based direct catalytic condensations. It has also been used as a catalyst for carbon homologation and in various multicomponent reactions in one-pot synthesis. Here, we report the organocatalytic synthesis of (α-aminoalkyl)phosphonates in the presence of l-proline.
2 Results and discussion
The synthesis of (α-aminoalkyl)phosphonates 4 was carried out by treatment of aldehydes 1, amines 2 and triethyl phosphate ((EtO)3P) 3 in the presence of l-proline as a catalyst at room temperature (Scheme 1). Initially, the reaction of benzaldehyde, aniline and triethyl phosphate was conducted in different solvents such as MeOH, tetrahydrofuran (THF), CHCl3, dichloroethane (DCE), dichloromethane (DCM) and acetonitrile (MeCN) (Table 1) at room temperature. Considering the yield, dichloroethane (DCE) was found to be the most effective solvent at room temperature. The reaction in DCE was completed within 30–45 min, and yield was 82–94%. Using other solvents, the reaction time was longer and the yield was less.
Synthesis of (α-aminoalkyl)phosphonates.
Entry
Solvent
Isolated yield (%)
Time (h)
1
MeOH
70
2.5–3.0
2
THF
56
4.0–5.0
3
CHCl3
52
3.0–4.0
4
DCE
83
0.5–0.75
5
DCM
48
6.0–6.5
6
MeCN
72
1.5–2.0
The catalytic activity of l-proline has also been compared with that of other catalysts used earlier for the preparation of (α-aminoalkyl)phosphonates (Table 2), and the present catalyst was most effective. Thus, subsequently DCE was used at room temperature to prepare a series of (α-aminoalkyl)phosphonates from various aldehydes and amines with triethyl phosphate in the presence of l-proline (Table 3). Both aromatic and aliphatic aldehydes were employed to prepare these compounds. The aromatic aldehydes and the anilines used for this conversion contained electron-donating as well as electron-withdrawing groups. Various functional groups such as –NO2, –OH, halogen and ester remained unchanged. The conversion was complete within 30–45 min, and the products were formed in excellent yields (82–94%). The structures of the products were established from their spectral [IR, 1H and 13C-NMR and MS] and analytical data.
Entry
Catalyst
Time (h)
Isolated yield
1
BF3.Et2O
1
79
2
TaCl5–SiO2
24
90
3
Mg(ClO4)2
5
85
4
PPA.SiO2
2
92
Entry
R1
R2
Time
Yield (%)b
Product
1
C6H5
C6H5
30
83
4a
2
C6H5
4-Me-C6H4
30
92
4b
3
C6H5
4-OH–C6H4
45
89
4c
4
4-OMe-C6H4
4-Me-C6H4
40
91
4d
5
4-Me-C6H4
4-MeOOC–C6H4
35
87
4e
6
2-NO2–C6H4
4-Me-C6H4
40
90
4f
7
4-NO2–C6H4
4-OH–C6H4
45
89
4g
8
4-iPr-C6H4
C6H5
35
86
4h
9
4-Cl,3-F–C6H3
4-Me-C6H4
40
93
4i
10
2,4-(Cl)2–C6H3
C6H5
45
90
4j
11
4-Br–C6H4
4-OH–C6H4
40
94
4k
12
iPr
C6H5
45
84
4l
13
Pentyl
C6H5
45
82
4m
l-proline is a less expensive, easy to handle and readily available amino acid. It has been used successfully to catalyze the synthesis mechanism of (α-aminoalkyl)phosphonates (Scheme 2). It activates the –C⚌N– bond of the imines (H2O generated by the reaction of aldehydes and amines) to produce (α-aminoalkyl)phosphonates). The yields of the products were greater than those obtained earlier by using different catalysts (Table 2).
Mechanism of (α-aminoalkyl)phosphonates.
3 Experimental
All the reactions were monitored by thin layer chromatography (TLC). The silica gel F254 plates were used for thin layer chromatography (TLC) in which the spots were examined under UV light and then developed by an iodine vapor. Column chromatography was performed with silica gel (BDH 100–200 mesh). Solvents were purified according to standard procedures. The spectra were recorded with the following instruments; NMR: Varian Gemini 400 MHz (1H); 75 MHz (13C) and 202.5 MHz (31P). ESIMS: VG-Autospec micromass, analyses of all the compounds were recorded in LCQ Fleet, Thermo Fisher Instruments Limited. High-resolution mass spectra (HR-MS) were recorded under electron impact (70 eV) condition using a MicroMass GCT CA 055 instrument. Organic extracts were dried over anhydrous Na2SO4.
3.1 General procedure for synthesis of (α-aminoalkyl)phosphonates
A mixture of an amine (12.0 mmol) and an aldehyde (10.0 mmol) in DCE (5 ml) was stirred for a few min at room temperature and P(OEt)3 (15.0 mmol) and l-proline (4.0 mmol) were added. The mixture was stirred for the appropriate time and the progress of the reaction was monitored by TLC. After completion of the reaction (determined by TLC), the solvent was evaporated, and H2O (10 ml) was added. The mixture was extracted with EtOAc (3 × 10 ml), and the extract was dried over anhyd. Na2SO4 and concentrated. The residue was purified by column chromatography to afford a pure (α-aminoalkyl)phosphonates.
3.2 Diethyl phenyl(phenylamino)methylphosphonate (4a)
IR (KBr): 3290, 1620, 1241 cm−1; 1H-NMR (CDCl3): δ 7.34 – 7.28 (m, 2H); 7.15 – 7.04 (m, 5H); 6.65 (t, 1H, J = 8.0); 6.54 (d, 2H, J = 8.0); 4.82 – 4.69 (m, 2H); 4.18 – 4.08 (m, 2H); 3.94 – 3.90 (m, 1H); 3.64 – 3.62 (m, 1H); 1.29 (t, 3H, J = 7.0); 1.12 (t, 3H, J = 7.0); 13C-NMR (CDCl3): δ 147.8, 136.2 (d, J = 10.0), 129.6, 128.7, 126.9, 117.2, 113.5, 62.8 (d, J = 6.0), 55.0 (d, J = 152.0), 16.0 (d, J = 10.5); ESI-MS: 342 ([M + Na]+). Anal. calc. for C17H22NO3P (319.23): C 63.92, H 6.97, N 4.35; found: C 63.94, H 6.96, N 4.37.
3.3 Diethyl {[(4-methylphenyl)amino](phenyl)methyl}phosphonate (4b)
IR (KBr): 3292, 1622, 1504, 1243 cm−1; 1H-NMR (CDCl3): δ 7.31 – 7.26 (m, 2H); 7.10 – 7.01 (m, 4H), 6.61 (t, 1H, J = 8.0), 6.51 (d, 2H, J = 8.0), 4.75 – 4.66 (m, 2H), 4.15 – 4.02 (m, 2H), 3.91 – 3.89 (m, 1H), 3.62 – 3.60 (m, 1H), 2.31 (s, 3H), 1.28 (t, 3H, J = 7.0), 1.12 (t, 3H, J = 7.0); 13C-NMR (CDCl3): δ 144.8, 137.2 (d, J = 10.0), 129.2, 127.9, 126.8, 126.0, 114.2, 63.0 (d, J = 6.0), 55.2 (d, J = 152.0), 23.7, 16.0 (d, J = 10.5); ESI-MS: 351 ([M + NH4]+). Anal. calc. for C18H24NO3P (333.36): C 64.85, H 7.26, N 4.20; found: C 65.34, H 7.20, N 4.27.
3.4 Diethyl (4-hydroxyphenylamino)(phenyl)methylphosphonate (4c)
IR (KBr): 3302, 1660, 1520, 1233 cm−1; 1H-NMR (CDCl3): δ 8.22 (d, 2H, J = 8.0 Hz), 7.30 – 7.24 (3H, m), 6.63 (d, 2H, J = 8.0 Hz), 6.38 (d, 2H, J = 8.0 Hz), 4.75 (d, 1H, J = 24.0 Hz), 4.52 (brs, 1H), 4.20 – 4.08 (m, 2H), 4.02 (m, 1H), 3.98 (brs, 1H), 3.92 (m, 1H), 1.30 (t, 3H, J = 7.0 Hz), 1.22 (t, 3H, J = 7.0 Hz); 13C-NMR (CDCl3): δ 150.2, 140.3, 136.2 (d, J = 14.0 Hz), 128.6, 126.9, 116.5, 115.1, 63.2 (d, J = 6.5 Hz), 57.2 (d, J = 152.0 Hz), 16.2 (d, J = 6.0 Hz); ESIMS: m/z 358[M + Na]+; HRMS (ESI): m/z 358.1050 [M + Na]+ (Calcd. for C17H22NO4PNa: m/z 358.1038).
3.5 Diethyl(p-toluidino)(4-methoxyphenyl)methylphosphonate (4d)
IR (KBr): 3290, 1618, 1532, 1346, 1240 cm−1; 1H-NMR (CDCl3): δ 7.46 (d, 2H, J = 8.0 Hz), 7.32 (d, 2H, J = 8.0 Hz), 7.15 (d, 2H, J = 8.0 Hz), 6.60 (d, 2H, J = 8.0 Hz), 5.52 (t, 1H, J = 10.0 Hz), 4.75 (dd, 1H, J = 24.0, 10.0 Hz), 4.18 – 4.07 (m, 2H), 3.96 (q, 2H, J = 7.02 Hz), 3.72 (s, 3H), 1.28 (t, 3H, J = 7.0 Hz), 1.10 (t, 3H, J = 7.0 Hz), 2.31 (s, 3H); 13C-NMR (CDCl3): δ 158.8, 144.5, 130.0, 128.4, 128.1, 126.8, 114.1, 113.2, 62.8 (d, J = 6.0 Hz), 56.6 (d, J = 152.0 Hz), 56.2, 24.7, 16.1 (d, J = 6.0 Hz), 16.0 (d, J = 6.0 Hz); 31P-NMR (CDCl3): δ 50.4 (s); ESI-MS m/z 363 (M)+.; HRMS: 386.1462 (M + Na)+. Elemental analysis for C19H26NO4P: Calcd: C, 62.80; H, 7.21; N, 3.85%. Found: C, 62.78; H, 7.20; N 3.87%.
3.6 Methyl 4-{[(diethoxyphosphoryl)(4-methylphenyl)methyl]amino}benzoate (4e)
IR (KBr): 3310, 1707, 1606, 1523, 1437, 1275 cm−1; 1H-NMR (CDCl3): δ 7.80 (d, 2H, J = 8.0), 7.32 (d, 2H, J = 8.0), 7.13 (d, 2H, J = 8.0), 6.60 (d, 2H, J = 8.0), 5.50 (t, 1H, J = 10.0), 4.79 (dd, 1H, J = 24.0, 10.0), 4.15 – 4.06 (m, 2H), 3.92 – 3.88 (m, 1H), 3.81 (s, 3H), 3.63 – 3.61 (m, 1H), 2.02 (s, 3H), 1.10 (t, 3H, J = 7.0), 0.88 (t, 3H, J = 7.0); 13C-NMR (CDCl3): δ 166.2, 151.0, 135.9, 133.4 (d, J = 10.0), 129.2, 128.1, 126.8, 118.2, 113.8, 63.1 (d, J = 6.0), 55.9 (d, J = 152.0), 50.5, 24.2, 16.2 (d, J = 10.5); ESI-MS: 392 ([M + H]+); HR-ESI-MS: 392.1630 ([M + H]+; C20H27NO5P+; calc. 392.1626).
3.7 Diethyl(p-toluidino)(2-nitrophenyl)methylphosphonate (4f)
IR (KBr): 3298, 1615, 1504, 1272, 1236 cm−1; 1H-NMR (CDCl3): δ 7.12 – 6.96 (m, 4H), 6.80 – 6.58 (m, 4H), 5.21 (dd, 1H, J = 24.0, 10.0 Hz), 4.72 (t, 1H, J = 10.0 Hz), 4.23 – 4.12 (m, 2H), 3.92 – 3.86 (m, 1H), 3.65 – 3.58 (m, 1H), 2.32 (s, 3H), 1.31 (t, 3H, J = 7.0 Hz), 1.12 (t, 3H, J = 7.0 Hz); 13C- NMR (CDCl3): δ 149.6, 144.3, 137.2 (d, J = 10.0 Hz), 134.8, 129.8, 127.9, 126.8, 123.5, 113.4, 63.6 (d, J = 6.5 Hz), 53.4 (d, J = 152.0 Hz), 24.6, 16.5 (d, J = 6.5 Hz), 16.2 (d, J = 6.5 Hz); 31P-NMR (CDCl3): δ 50.6 (s); ESI-MS m/z 379 (M + H)+.; HRMS: 401.1632 (M + Na)+. Elemental analysis for C18H23N2O5P: Calcd: C, 57.14; H, 6.12; N, 7.40%. Found: C, 57.11; H, 6.10; N, 7.43%.
3.8 Diethyl (4-hydroxyphenylamino)(4-nitrophenyl)methylphosphonate (4g)
IR (KBr): 3299, 1666, 1518, 1347, 1231 cm−1; 1H-NMR (CDCl3): δ 8.18 (d, 2H, J = 8.0 Hz), 7.61 (2H, d, J = 8.0 Hz), 6.60 (2H, d, J = 8.0 Hz), 6.35 (2H, d, J = 8.0 Hz), 4.72 (1H, d, J = 24.0 Hz), 4.50 (1H, brs), 4.20 – 4.05 (2H, m), 4.02 (1H, m), 3.98 (brs, 1H), 3.89 (1H, m), 1.30 (3H, t, J = 7.0 Hz), 1.21 (3H, t, J = 7.0 Hz); 13C-NMR (CDCl3): δ 150.5, 147.3, 138.8 (d, J = 14.0 Hz), 128.6, 123.3, 116.0, 115.1, 64.0 (d, J = 6.5 Hz), 63.2 (d, J = 6.5 Hz), 57.1 (d, J = 152.0 Hz), 16.1 (d, J = 6.0 Hz), 16.0 (d, J = 6.0 Hz); ESIMS: m/z 403 [M + Na]+; HRMS (ESI): m/z 403.1052 [M + Na]+ (Calcd. for C17H21N2O6PNa: m/z 403.1034).
3.9 Diethyl (4-isopropylphenyl)(phenylamino)methylphosphonate (4h)
IR (KBr): 3314, 1602, 1500, 1235 cm−1; 1H-NMR (CDCl3): δ 7.49 (d, 2H, J = 8.0 Hz), 7.16 (d, 2H, J = 8.0 Hz), 7.08 (t, 2H, J = 8.0 Hz), 6.67 (t, 1H, J = 8.0 Hz), 6.60 (d, 2H, J = 8.0 Hz), 4.85 (brs, 1H), 4.76 (d, 1H, J = 24.0 Hz), 4.18 – 4.01 (m, 2H), 3.90 (m, 1H), 3.63 (m, 1H), 2.83 (m, 1H), 1.25 (t, 3H, J = 7.0 Hz), 1.18 (d, 6H, J = 7.0 Hz), 1.05 (t, 3H, J = 7.0 Hz); 13C-NMR (CDCl3): δ 146.2 (d, J = 14.0 Hz), 133.0, 129.1, 128.0, 126.0, 118.2, 113.9, 62.9 (d, J = 6.5 Hz), 55.1 (d, J = 153.5 Hz), 33.5, 23.9, 16.8 (d, J = 6.5 Hz), 16.2 (d, J = 6.5 Hz); ESIMS: m/z 384 [M + Na]+; HRMS (ESI): m/z 362.1875 [M + H]+ (Calcd. for C20H29NO3P: m/z 362.1885).
3.10 Diethyl (3-chloro-4-fluorophenyl)(p-tolylamino)methylphosphonate (4i)
IR (KBr): 3303, 1616, 1519, 1239 cm−1; 1H-NMR (CDCl3): δ 7.50 (d, 1H, J = 8.0 Hz), 7.32 (brs, 1H), 7.04 (t, 1H, J = 8.0 Hz), 6.85 (d, 2H, J = 8.0 Hz), 6.42 (d, 2H, J = 8.0), 4.77 (brs, 1H), 4.62 (d, 1H, J = 24.0 Hz), 4.28 – 4.05 (m, 2H), 3.99 (m, 1H), 3.82 (m, 1H), 2.19 (s, 3H), 1.28 (t, 3H, J = 7.0 Hz), 1.19 (t, 3H, J = 7.0 Hz); 13C-NMR (CDCl3): δ 159.3, 143.5 (d, J = 14.0 Hz), 133.2, 130.0, 129.6, 127.9, 127.2 (d, J = 4.0 Hz), 121.1 (d, J = 16.0 Hz), 116.8 (d, J = 16.0 Hz), 113.5, 63.6 (d, J = 6.5 Hz), 63.0 (d, J = 6.5 Hz), 55.2 (d, J = 152.5 Hz), 20.2, 16.2 (d, J = 6.0 Hz), 16.0 (d, J = 6.0 Hz); ESIMS: m/z 386, 388 [M + H]+; HRMS (ESI): m/z 408.09.4 [M + Na]+ (Calcd. for C18H22ClFNO3PNa: m/z 408.0907).
3.11 Diethyl (2,4-dichlorophenyl)(phenylamino)methylphosphonate (4j)
IR (KBr): 3309, 1607, 1508, 1247 cm−1; 1H-NMR (CDCl3): δ 7.51 (d, 1H, J = 8.0 Hz), 7.40 (d, 1H, J = 2.0 Hz), 7.22 (dd, 1H, J = 8.0, 2.0 Hz), 7.06 (t, 2H, J = 8.0 Hz), 6.64 (t, 1H, J = 8.0 Hz), 6.48 (d, 2H, J = 8.0 Hz), 5.23 (dd, 1H, J = 24.0, 10.0 Hz), 4.82 (t, 1H, J = 10.0 Hz), 4.25 – 4.12 (m, 2H), 3.91 (m, 1H), 3.67 (m, 1H), 1.35 (t, 3H, J = 7.0 Hz), 1.11 (t, 3H, J = 2.0 Hz); 13C- NMR (CDCl3): δ 146.2 (d, J = 14.0 Hz), 130.2, 129.8, 128.2, 119.1, 113.5, 96.2, 63.4 (d, J = 6.5 Hz), 63.2 (d, J = 6.5 Hz), 51.6 (d, J = 155.2 Hz), 16.6 (d, J = 6.5 Hz), 16.2 (d, J = 6.5 Hz); ESIMS: m/z 387 [M]+, 388, 390, 392 [M + H]+; HRMS (ESI): m/z 410.0459 [M + Na]+ (calculated for C17H20Cl2NO3PNa: m/z 410.0455.
3.12 Diethyl(p-bromophenyl)(4-hydroxyphenylamino)methylphosphonate (4k)
IR (KBr): 3420, 1628, 1490, 1375, 1238 cm−1; 1H-NMR (CDCl3): δ 7.82 (d, 2H, J = 8.0 Hz), 7.32 (d, 2H, J = 8.0 Hz), 7.18 (d, 2H, J = 8.0 Hz), 6.64 (d, 2H, J = 8.0 Hz), 5.55 (t, 1H, J = 10.0 Hz), 4.80 (dd, 1H, J = 24.0, 10.0 Hz), 4.24 – 4.12 (m, 2H), 3.95 – 3.89 (m, 1H), 3.65 – 3.62 (m, 1H), 3.06 (brs, 1H), 1.36 (t, 3H, J = 7.0 Hz), 1.18 (t, 3H, J = 7.0 Hz); 13C-NMR (CDCl3): δ 147.7, 140.5, 135.4 (d, J = 10.0 Hz), 131.6, 129.5, 121.2, 116.9, 115.2, 64.3 (d, J = 6.5 Hz), 63.4, 63.2 (d, J = 6.5 Hz), 55.0 (d, J = 155.0 Hz), 16.6 (d, J = 6.0 Hz), 16.1 (d, J = 6.0 Hz); 31P-NMR (CDCl3): δ 50.4 (s); ESI-MS m/z 413, 415 (M + H)+.; HRMS: 415.1236 (M + H)+. Elemental analysis for C17H21BrNO4P: Calcd: C, 49.29; H, 5.11; N, 3.38%. Found: C, 49.32; H, 5.08; N 3.41%.
3.13 Diethyl 2-methyl-1-(phenylamino)propylphosphonate (4l)
IR (KBr): 2958, 1618, 1233, 1035 cm−1; 1H-NMR (CDCl3): δ 7.04 (m, 3H); 6.59 (d, 2H, J = 8.0), 4.82 – 4.69 (m, 2H), 4.18 – 4.08 (m, 2H), 3.94 – 3.90 (m, 1H), 3.64 – 3.62 (m, 1H), 2.02 (m, 1H), 1.29 (t, 3H, J = 7.0); 1.12 (t, 3H, J = 7.0), 1.02 (s, 6H); 13C-NMR (CDCl3): δ 147.6, 129.6, 117.2, 113.5, 62.6 (d, J = 6.0), 55.0 (d, J = 152.0), 29.8, 19.4, 16.0 (d, J = 10.5); ESI-MS: 308 ([M + Na]+). Anal. calc. for C14H24NO3P (285.23): C 63.94, H 6.96, N 4.37; found: C 63.92, H 6.97, N 4.35.
3.14 Diethyl 1-(phenylamino)pentylphosphonate (4m)
IR (KBr): 3310, 1624, 1248, 1038 cm−1; 1H-NMR (CDCl3): δ 7.02 (m, 3H); 6.57 (d, 2H, J = 8.0), 4.80 – 4.68 (m, 2H), 4.18 – 4.08 (m, 2H), 3.94 – 3.89 (m, 1H), 2.84 – 2.69 (m, 1H), 1.52 (m, 2H), 1.34 – 130 (m, 4H), 1.28 (t, 3H, J = 7.0), 1.12 (t, 3H, J = 7.0), 0.96 (t, 3H, J = 7.0); 13C-NMR (CDCl3): δ 147.6, 129.8, 117.2, 113.5, 62.7 (d, J = 6.0), 55.2 (d, J = 152.0), 28.3, 22.5, 20.8, 16.0 (d, J = 10.5), 14.8; ESI-MS: 322 ([M + Na]+). Anal. calc. for C15H26NO3P (299.35): C 62.94, H 6.97, N 4.37; found: C 62.92, H 6.95, N 4.39.
Conclusion
In conclusion, we have developed a simple and an efficient organocatalytic synthesis of (α-aminoalkyl)phosphonates 4 by a one-pot reaction of aldehydes, amines and triethyl phosphate [(Et3O)P] using l-proline as a catalyst at room temperature. The mild reaction conditions, utilization of a metal-free catalyst, rapid conversion, excellent yields, and wide applicability are the notable advantages of the present method.
Acknowledgement
The authors are thankful to the authorities of Annamacharya Institute of Technology & Sciences, J.N.T.University, Tirupati, India and School of Chemistry, University of KwaZulu-Natal, Westville campus, Durban, South Africa for the facilities and encouragement.
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