A facile and highly efficient route to α-amino phosphonates via three-component reactions catalyzed by Mg(ClO4)2 or molecular iodine

Article abstract of DOI:10.1039/b602536f

Three-component reactions of aldehydes, amines, and diethyl phosphite catalyzed by Mg(ClO₄)₂ or molecular iodine afforded the corresponding α-amino phosphonates in excellent yields under mild reaction conditions. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b602536f

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www.rsc.org/obc | Organic & Biomolecular Chemistry
A facile and highly efficient route to a-amino phosphonates via
three-component reactions catalyzed by Mg(ClO4)2 or molecular iodine
Jie Wu,*a,b Wei Sun,a Hong-Guang Xiaa and Xiaoyu Suna
Received 20th February 2006, Accepted 6th March 2006
First published as an Advance Article on the web 28th March 2006
DOI: 10.1039/b602536f
Three-component reactions of aldehydes, amines, and diethyl phosphite catalyzed by Mg(ClO4)2 or
molecular iodine afforded the corresponding a-amino phosphonates in excellent yields under mild
reaction conditions.
(Scheme 1). To our delight, we observed the formation of product
Introduction
3a. Complete conversion and 79% isolated yield were obtained
In recent years, magnesium perchlorate has been utilized as a mild
after 24 hours. Further studies established that EtOH was the
Lewis acid, imparting high regio-, chemo- and stereo-selectivity in
best choice of solvent among the solvents (EtOH, THF, CH2Cl2,
various organic transformations.1 Moreover, it has been found to
CH3CN, toluene) screened. Next, we surveyed the temperature
retain its activity even in the presence of amines and also effectively
for the reaction shown in Scheme 1. When the temperature was
elevated to 40 ◦C, the reaction time was shortened to 16 hours.
activates nitrogen-containing compounds such as imines. Very
recently, we also found that it was highly effective as a catalyst for
The result was dramatically improved when the reaction was
the synthesis of quinolines via Friedla¨nder annulation.1a Herein,
we wish to report its application as a catalyst for the efficient
synthesis of a-amino phosphonate via three-component reactions
of aldehydes, amines, and diethyl phosphite under mild conditions.
a-Amino phosphonic acids, their phosphonate esters, and
short peptides incorporating this unit are excellent inhibitors
of a wide range of proteolytic enzymes.2 In addition, a-amino
phosphonate derivatives have broad applications due to their
antibacterial3 and antifungal4 activity, and as inhibitors of
phosphatase activity.5 Lewis acid-catalyzed addition of diethyl
phosphite to aldimine provides a useful method for the preparation
of a-amino phosphonate.6 Recently, three-component a-amino
phosphonate syntheses starting from aldehydes, amines, and
diethylphophite or triethyl phosphite catalyzed by Lewis acids7
and Brønsted acid,8 or under microwave conditions9 have been
reported. However, many of these procedures suffered from the
use of stoichiometric and/or toxic, relatively expensive reagents.
Since a-amino phosphonate derivatives are increasingly useful and
important in pharmaceuticals and industry, the development of a
simple, eco-benign, low cost protocol is still desirable. As a-amino
phosphonate synthesis via three-component reactions is among
the most important acid-mediated reactions, the development
of a reaction that uses a catalytic amount of a readily available
magnesium salt of low toxicity should be of great interest.
performed at 50 ◦C or 60 ◦C. Only 5 hours were needed for
completion and the isolation of desired compound 3a in 84%
yield. Then, we examined the catalytic requirement of Mg(ClO4)2
(1–10 mol%) for the reaction in ethanol at 50 ◦C. A similar result
was obtained (5 hours, 85% yield) when a catalytic amount of
5 mol% magnesium perchlorate was employed in the reaction.
Gratifyingly, 1 mol% of Mg(ClO4)2 was found to be sufficient for
catalysis of a-amino phosphonate synthesis although a prolonged
reaction time was needed for completion with slightly lower yield
(1 mol% of Mg(ClO4)2, 12 hours, 60% yield). The result also
provides evidence that there is no deactivation or inhibition of
the magnesium catalyst, which is of interest considering that the
reactants and/or the amine product might act as a Lewis base.
Moreover, it is noteworthy that this reaction could be performed
open to the air without loss of efficiency.
Scheme 1 Reaction of 1a, 2a, and diethyl phosphite in EtOH catalyzed
by Mg(ClO4)2 (10 mol%).
To demonstrate the generality of this method, we next investi-
gated the scope of this reaction under the optimized conditions
(EtOH, 5 mol% of Mg(ClO4)2, air, 50 ◦C) and the results are
summarized in Table 1. As shown in Table 1, this method is
equally effective for both aromatic aldehydes and amines. Various
substituted aromatic aldehydes 1a–1d reacted smoothly with
amines 2 to produce a range of a-amino phosphonate derivatives.
Complete conversion and excellent isolated yields were observed
for all substrates employed. This reaction is very clean and free
from side reactions. For example, almost quantitative yields of
products 3b and 3c were obtained when aldehyde 1a reacted
with anilines 2b and 2c (entries 2 and 3). The reactions also
Results and discussion
An initial study was performed by the treatment of benzaldehyde
1a, aniline 2a, and diethyl phosphite in EtOH in the presence of
a catalytic amount of Mg(ClO4)2 (10 mol%) at room temperature
aDepartment of Chemistry, Fudan University, 220 Handan Road, Shanghai,
200433, China. E-mail: jie_wu@fudan.edu.cn; Fax: 86 21 6510 2412; Tel: 86
21 5566 4619
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road,
Shanghai, 200032, China
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Table 1 Reaction of aldehyde 1, amine 2, and diethyl phosphite catalyzed
Table 2 Reaction of aldehyde 1, amine 2, and diethyl phosphite catalyzed
by Mg(ClO4)2 (5 mol%) in EtOHa
by Mg(ClO4)2 (5 mol%) under solvent-free conditionsa
Entry
Aldehyde 1
Amine 2
Reaction time/h Yield (%)b
1
2
3
4
5
6
1a
1a
1a
1a
1d
1d
2a
2b
2c
2d
2b
2d
5
12
5
12
24
24
99 (3a)
99 (3b)
99 (3c)
99 (3d)
90 (3k)
95 (3l)
Entry Aldehyde 1 Amine 2 Reaction time/h Yield (%)b
a Reaction conditions: aldehyde 1 (0.4 mmol), amine 2 (0.4 mmol, 1.0 eq.),
diethyl phosphite (0.4 mmol, 1.0 eq.), Mg(ClO4)2 (5 mol%), 50 ◦C, 3–24 h.
b Isolated yield.
1
2
1a
1a
1a
1a
1b
1b
1b
1b
1c
1c
1d
1d
1e
1e
2a
2b
2c
2d
2a
2b
2c
2d
2b
2d
2b
2d
2a
2d
5
6
12
12
12
6
85 (3a)
99 (3b)
99 (3c)
99 (3d)
99 (3e)
99 (3f)
99 (3g)
99 (3h)
99 (3i)
96 (3j)
75 (3k)
99 (3l)
— (3m)
— (3n)
3
4
5
aniline 2a, and diethyl phosphite in EtOH in the presence of
a catalytic amount of I2 (5 mol%) at room temperature under
an air atmosphere. Similar results were obtained for the reaction
scope investigation (Table 3). Under solvent-free conditions, the
reaction of aldehyde 1a, aniline 2a, and diethyl phosphite at room
temperature also proceeded smoothly to afford the corresponding
product 3a in excellent yield (4 h, 99% yield).
6
7
8
9
10
11
12
13
14
12
12
24
24
24
12
24
24
a Reaction conditions: aldehyde 1 (0.4 mmol), amine 2 (0.4 mmol, 1.0 eq.),
diethyl phosphite (0.4 mmol, 1.0 eq.), Mg(ClO4)2 (5 mol%), EtOH
(0.5 mL), 50 ◦C, 3–24 h. b Isolated yield.
Conclusions
In conclusion, we have described a convenient and efficient
synthetic protocol for the preparation of a-amino phosphonate
derivatives utilizing Mg(ClO4)2 or molecular iodine as the catalyst
via three-component reactions. This method not only provides an
excellent complement to a-amino phosphonate synthesis via three-
component reactions, but also avoids the use of hazardous acids
or expensive/toxic Lewis acids and harsh reaction conditions.
proceeded smoothly when benzylamine was employed as the
substrate (entries 4, 8, 10, and 12). However, no desired products
were isolated with aliphatic aldehyde since the reaction systems
were complicated (entries 13 and 14). We attribute this to the
slow formation and unstable nature of the imine formed from the
aliphatic aldehyde examined.
On the other hand, due to growing concern about the effect of
organic solvents on the environment as well as on the human body,
organic reactions without the use of conventional organic solvents
have attracted the attention of synthetic organic chemists.10,11 We
also tried the solvent-free reaction of 1a, 2a and diethyl phosphite
Table 3 Reaction of aldehyde 1, amine 2, and diethyl phosphite catalyzed
by molecular iodine (5 mol%) in EtOHa
◦
in the presence of Mg(ClO4)2 (5 mol%) at 50 C and found that
the desired product 3a was generated in almost quantitative yield
(5 h, 99% yield). Other substrates were then investigated and the
results are shown in Table 2. From Table 2, it can be seen that the
catalytic system under solvent-free conditions was highly effective
and all the products were furnished in excellent yields.
Moreover, we turned our attention to other Lewis acid catalysts
for this three-component reaction, such as molecular iodine. In
recent years, iodine has emerged as a very effective Lewis acid
catalyst for various organic transformations.12,13 Iodine is relatively
inexpensive compared to other Lewis acids including rare earth
metal triflates, and is more tolerant in comparison to typical
Lewis acids/bases. Iodine catalysis can also be easily adapted
to commercial applications, since its handling does not require
special precautions and it is readily reduced to relatively nontoxic
iodide during work-up procedures. Gratifyingly, quantitative
yield of product 3a was obtained by treatment of aldehyde 1a,
Entry
Aldehyde 1
Amine 2
Reaction time/h
Yield (%)b
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1b
1b
1b
1b
1c
1d
1e
1e
2a
2b
2c
2d
2a
2b
2c
2d
2b
2b
2a
2d
4
3
4
4
4
3
3
3
4
3
4
4
99 (3a)
93 (3b)
99 (3c)
93 (3d)
99 (3e)
88 (3f)
99 (3g)
99 (3h)
96 (3i)
75 (3k)
— (3m)
— (3n)
10
11
12
a Reaction conditions: aldehyde 1 (0.4 mmol), amine 2 (0.4 mmol, 1.0 eq.),
diethyl phosphite (0.4 mmol, 1.0 eq.), I2 (5 mol%), EtOH (0.5 mL), room
temperature, 3–12 h. b Isolated yield.
1664 | Org. Biomol. Chem., 2006, 4, 1663–1666
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Diethyl (phenylamino)(p-tolyl)methylphosphonate 3e. Color-
Experimental
1
less liquid. H NMR (500 MHz, CDCl3): d (ppm) 1.13 (t, J =
All reactions were performed in test tubes under an air atmo-
sphere at room temperature. Flash column chromatography was
performed as described by Still et al.14 using silica gel (60 A pore
7.0 Hz, 3H), 1.28 (t, J = 7.0 Hz, 3H), 2.30 (s, 3H), 3.68–3.70 (m,
1H), 3.92–3.95 (m, 1H), 4.08–414 (m, 2H), 4.71 (d, J = 24.0 Hz
1H), 4.75 (br s, 1H), 6.60 (d, J = 7.5 Hz, 2H), 6.66–6.69 (m, 1H),
7.07–7.13 (m, 4H), 7.33–7.35 (m, 2H).
˚
size, 32–63 lm, standard grade, Sorbent Technologies). Analytical
thin-layer chromatography was performed using glass plates pre-
coated with 0.25 mm 230–400 mesh silica gel impregnated with a
fluorescent indicator (254 nm). Thin layer chromatography plates
were visualized by exposure to ultraviolet light. Organic solutions
were concentrated on Bu¨chi R-200 rotary evaporators at ∼20 Torr
Diethyl (4-methoxyphenylamino)(p-tolyl)methylphosphonate 3f.
1
Colorless liquid. H NMR (400 MHz, CDCl3): d (ppm) 1.14 (t,
J = 7.0 Hz, 3H), 1.28 (t, J = 7.0 Hz, 3H), 2.30 (s, 3H), 3.67 (s, 3H),
3.69–3.71 (m, 1H), 3.92–3.93 (m, 1H), 4.09–4.14 (m, 2H), 4.66 (d,
J = 24.3 Hz, 1H), 4.80 (br s, 1H), 6.55 (d, J = 9.2 Hz, 2H), 6.68
(d, J = 9.2 Hz, 2H), 7.12 (d, J = 7.8 Hz, 2H), 7.31–7.34 (m, 2H).
◦
(house vacuum) at 25–35 C. Commercial reagents and solvents
were used as received. Proton nuclear magnetic resonance (1H
NMR) spectra were recorded in parts per million from internal
tetramethylsilane on the d scale and are referenced from the
residual protium in the NMR solvent (CHCl3: d 7.27, DMSO-d6:
d 2.50). Data is reported as follows: chemical shift [multiplicity
(s = singlet, d = doublet, q = quartet, m = multiplet), coupling
constant(s) in Hertz, integration, assignment].
Diethyl (4-fluorophenylamino)(p-tolyl)methylphosphonate 3g.
1
Colorless liquid. H NMR (400 MHz, CDCl3): d (ppm) 1.12 (t,
J = 7.0 Hz, 3H), 1.28 (t, J = 7.0 Hz, 3H), 2.31 (s, 3H), 3.67–3.70
(m, 1H), 3.91–3.92 (m, 1H), 4.10–4.14 (m, 2H), 4.63–4.70 (m, 1H),
4.83–4.85 (m, 1H), 6.52–6.54 (m, 2H), 6.75–6.80 (m, 2H), 7.12 (d,
J = 7.8 Hz, 2H), 7.33 (d, J = 7.8 Hz, 2H).
General procedure
Diethyl (benzylamino)(p-tolyl)methylphosphonate 3h. Color-
A mixture of aldehyde 1 (0.4 mmol), amine 2 (0.4 mmol, 1.0 eq.),
diethyl phosphite (0.4 mmol, 1.0 eq.), and Mg(ClO4)2 or I2
(5 mol%) in EtOH (0.5 mL) or under solvent-free conditions was
stirred at 50 ◦C (Method A: Mg(ClO4)2) or at room temperature
(Method B: I2) under an air atmosphere. After completion of
the reaction as indicated by TLC, the reaction mixture was
quenched with water (10 mL) and extracted with EtOAc (2 ×
10 mL). Evaporation of the solvent followed by purification
(chromatography column on silica gel) afforded pure a-amino
phosphonate 3. (All the products are known compounds. The
characterizations of these compounds are identical with the
1
less liquid. H NMR (400 MHz, CDCl3): d (ppm) 1.14 (t, J =
7.0 Hz, 3H), 1.27 (t, J = 7.0 Hz, 3H), 2.30 (s, 2H), 2.35 (s, 3H),
3.53 (d, J = 13.2 Hz, 1H), 3.78–3.82 (m, 2H), 3.96–4.06 (m, 3H),
7.26–7.31 (m, 9H).
Diethyl (4-chlorophenyl)(4-methoxyphenylamino)methylphos-
1
phonate 3i. Colorless liquid. H NMR (400 MHz, CDCl3): d
(ppm) 1.16 (t, J = 7.0 Hz, 3H), 1.29 (t, J = 7.0 Hz, 3H), 3.69
(s, 3H), 3.80–3.81 (m, 1H), 3.94–4.03 (m, 1H), 4.09–4.14 (m,
2H), 4.51–4.53 (m, 1H), 4.62–4.70 (m, 1H), 6.50–6.70 (m, 4H),
7.29–7.39 (m, 4H).
literature reports.7–9
)
Diethyl (benzylamino)(4-chlorophenyl)methylphosphonate 3j.
Colorless liquid. H NMR (400 MHz, CDCl3): d (ppm) 1.25 (t,
J = 7.0 Hz, 3H), 1.27 (t, J = 7.0 Hz, 3H), 3.50 (d, J = 13.2 Hz,
1H), 3.77 (d, J = 13.2 Hz, 1H), 3.97–4.10 (m, 6H), 7.23–7.37 (m,
9H).
Diethyl phenyl(phenylamino)methylphosphonate 3a. Colorless
liquid. 1H NMR (400 MHz, CDCl3): d (ppm) 1.10 (t, J = 7.0 Hz,
3H), 1.27 (t, J = 7.0 Hz, 3H), 3.66–3.67 (m, 1H), 3.90–3.92 (m,
1H), 4.10–4.12 (m, 2H), 4.72–4.80 (m, 1H), 4.87–4.89 (m, 1H),
6.58–6.69 (m, 3H), 7.07–7.08 (m, 2H), 7.29–7.31 (m, 3H), 7.45–
7.46 (m, 2H).
1
Diethyl furan-2-yl(4-methoxyphenylamino)methylphosphonate
3k. Colorless liquid. 1H NMR (500 MHz, CDCl3): d (ppm) 1.20
(t, J = 7.0 Hz, 3H), 1.34 (t, J = 7.0 Hz, 3H), 3.70 (s, 3H), 3.87–
3.89 (m, 1H), 4.03–4.07 (m, 1H), 4.16–4.20 (m, 2H), 4.23 (br s,
1H), 4.78 (d, J = 13.2 Hz, 1H), 6.30 (s, 1H), 6.36–6.37 (m, 1H),
6.62–6.64 (m, 2H), 6.72–6.74 (m, 2H), 7.37 (s, 1H).
Diethyl (4-methoxyphenylamino)(phenyl)methylphosphonate 3b.
1
Colorless liquid. H NMR (400 MHz, CDCl3): d (ppm) 1.11 (t,
J = 7.0 Hz, 3H), 1.28 (t, J = 7.0 Hz, 3H), 3.68 (s, 3H), 3.70–3.72
(m, 1H), 3.91–3.93 (m, 1H), 4.08–4.14 (m, 2H), 4.56 (br s, 1H),
4.68 (d, J = 24.3 Hz, 1H), 6.55 (d, J = 9.2 Hz, 2H), 6.69 (d, J =
9.2 Hz, 2H), 7.30–7.32 (m, 3H), 7.45 (d, J = 7.3 Hz, 2H).
Diethyl (benzylamino)(furan-2-yl)methylphosphonate 3l. Col-
1
Diethyl (4-fluorophenylamino)(phenyl)methylphosphonate 3c.
orless liquid. H NMR (400 MHz, CDCl3): d (ppm) 1.22 (t, J =
1
Colorless liquid. H NMR (400 MHz, CDCl3): d (ppm) 1.10 (t,
7.0 Hz, 3H), 1.31 (t, J = 7.0 Hz, 3H), 3.59 (d, J = 13.2 Hz,
1H), 3.85–3.88 (m, 2H), 4.03–4.15 (m, 5H), 6.37–6.38 (m, 2H),
7.29–7.30 (m, 6H).
J = 7.0 Hz, 3H), 1.28 (t, J = 7.0 Hz, 3H), 3.66–3.68 (m, 1H),
3.91–3.93 (m, 1H), 4.10–4.14 (m, 2H), 4.66–4.73 (m, 1H), 4.85–
4.87 (m, 1H), 6.52–6.54 (m, 2H), 6.76–6.80 (m, 2H), 7.30–7.33 (m,
3H), 7.44–7.45 (m, 2H).
Acknowledgements
Diethyl (benzylamino)(phenyl)methylphosphonate 3d. Color-
1
less liquid. H NMR (400 MHz, CDCl3): d (ppm) 1.12 (t, J =
Financial support from the National Natural Science Foundation
of China (20502004), Ministry of Education of China, the Science
and Technology Commission of Shanghai Municipality, and
Fudan University is gratefully acknowledged.
7.0 Hz, 3H), 1.28 (t, J = 7.0 Hz, 3H), 2.36 (s, 2H), 3.53 (d, J =
13.2 Hz, 1H), 3.79–3.82 (m, 2H), 3.99 (m, 1H), 4.04–4.08 (m, 2H),
7.26–7.42 (m, 10H).
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