Nanosized zinc oxide as a catalyst for the rapid and green synthesis of β-phosphono malonates

Article abstract of DOI:10.1016/j.tet.2008.03.095

The design and development of a nano heterogeneous catalyst for the direct addition of P(O)-H bonds across various alkylidenes under solvent-free conditions is described. This is a mild, rapid, and efficient protocol to generate P-C bonds.

Full text of DOI:10.1016/j.tet.2008.03.095

Tetrahedron 64 (2008) 5519–5523
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Nanosized zinc oxide as a catalyst for the rapid and green
synthesis of
b-phosphono malonates
*
Mona Hosseini-Sarvari , Samane Etemad
Department of Chemistry, Faculty of Science, Shiraz University, Shiraz 71454, Islamic Republic of Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The design and development of a nano heterogeneous catalyst for the direct addition of P(O)–H bonds
across various alkylidenes under solvent-free conditions is described. This is a mild, rapid, and efficient
protocol to generate P–C bonds.
Received 16 December 2007
Received in revised form 12 March 2008
Accepted 27 March 2008
Available online 3 April 2008
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
catalysts. They showed that metal oxides can activate P(O)H groups
so that deprotonation of the P–H bond occurs in the presence of
Pioneering work on P–C bond formation was carried out by
Arbusov in the early 20th century, culminating in the well-known
Michaelis–Arbusov reaction.1 In the following decades the chem-
istry of phosphonates developed relatively slowly because of the
difficulty in formation of the C–P bond. Its renaissance came after
1959 with the discovery of naturally occurring aminophosphonic
acids2 and new biologically active phosphonates.3
a weaker base such as KOH.
Bearing this in mind, and during the course of our previous
studies on ZnO as catalyst in many organic reactions,16 herein, we
decided to develop a new catalyst for conjugate addition of
phosphorus nucleophile to
a
,
b
-unsaturated malonates (Scheme 1).
O
R2
Phosphonoaceticacid,4 forexample, hasbeenshowntoinhibitthe
replication of cytomegalovirus and herpes virus by interacting
directly with the virus-induced DNA polymerase. Phosphono-
formate5 has shownactivityincell cultures againstHTLV-III (thevirus
(EtO)2P
Ar
R2
R1
+
nanoflake-ZnO (5 mol %)
solvent free, 50 °C
HP(O)(OEt)2
Ar
R1
2
1a-j
3a-j
implicated in AIDS), and a derivative of b
-phosphonomalonic acid6
Scheme 1.
was used as an inhibitor of ras farnesyl protein transferase in studies
directed toward the development of new antitumor agents. Amongst
various methods togenerateP–Cbonds, theadditionofP(O)–Hbonds
across alkenes12 is one of the most utilized. There are three general
approaches: (a) the phospha-Michael reaction of activated alkenes,
mostcommonlypromotedbyalkalinemetalalkoxides,7,8 ortheuseof
tetramethylguanidine(TMG),9 Lewisacids,10 microwaves,11 ortheuse
of bases;12 (b) addition to unactivated olefins promoted by radical
initiators such as AIBN;13 (c) hydrophosphorylation of unactivated
alkenes catalyzed by transition metals.14
The possibility of performing the addition of a phosphorus
compound, containing a labile P–H bond, to double and triple
bonds using a solid base was first shown by Koenig and co-
workers,15 who used an Al2O3/KOH system in the Pudovik reaction
of various secondary phosphines and phosphites. In recent years,
Enders and Tedeschi12a were also involved in the study of C–P bond
formation using various metal oxides system as solid support
We first examined the effect of different metal oxides on the
addition of diethyl phosphite (2) to malonate 1a. The results are
summarized in Table 1.
According to Table 1, the best results were obtained with ZnO. In
all cases where the desired phosphonate 3a was formed in poor
yield, the unreacted starting materials remained unchanged (TLC).
The presence of a catalyst appeared to be essential for the activation
of the P–H bond toward deprotonation. No reaction could be
observed when no catalyst was used (entry 7).
Recently, nanocrystalline inorganic oxides gained interest
because of their different topical characteristics.17–22 Nanocrystal-
line semiconducting II–IV metal oxides have been efficiently used
as gas sensors and photocatalysts, whereas the catalytic properties
of these materials are poorly explored.23–25 Undoubtedly, one of the
most important semiconductor materials is zinc oxide (ZnO). When
used as catalyst, the activity of the nano ZnO material is expected to
be enhanced with reference to the commercial compound, not only
because of their increased surface area, but also because of the
changes of surface properties such as surface defects.26
* Corresponding author. Tel.: þ98 711 2284822; fax: þ98 711 2286008.
E-mail address: hossaini@susc.ac.ir (M. Hosseini-Sarvari).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.03.095

5520
M. Hosseini-Sarvari, S. Etemad / Tetrahedron 64 (2008) 5519–5523
Table 1
Screening of different metal oxides for the Michael addition of diethyl phosphite (2)
to 2-[(4-chlorophenyl) methylene]malononitrile (1a) to form the adduct 3aa
NC
CN
NC
CN
H
HP(O)(OEt)2
+
P(O)(OEt)2
Cl
3a
Cl
2
1a
Entry
Catalyst
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
ZnO
Fe2O3
Basic-Al2O3
CuO
MgO
2
18
18
18
18
18
24
80
20
30
67
Trace
Trace
0
TiO2
No catalyst
a
Conditions: all reactions were carried out at 50 ꢀC under solvent-free conditions
with 5 mol % of the catalyst.
Figure 2. SEM of the synthesized nano-flake ZnO.
furnace at temperature 100 ꢀC.29 In the present study, the procedure
is very simple and a large amount of Zn(OAc)2 and urea was refluxed
in a shorter time. Under the prevailing condition, it is likely that
nanoflake ZnO was prepared in large scale.
Two kinds of ZnO crystals [commercial ZnO, CM-ZnO, and
nanoflake prepared ZnO, NF-ZnO (20–30 nm)29] were screened in
the reaction between diethyphosphite (2) and 2-[(4-chlorophenyl)
methylene] malononitrile (1a) (Table 2). As shown in Table 2, NF-
ZnO was found to be more effective than CM-ZnO in mediating the
phospha-Michael addition under solvent-free conditions. In order
to examine the solvent effect and in our quest for the deployment of
b
Isolated yields.
We have recently reported that nanosized ZnO was an effective
catalyst for Knoevenagel and reduction reaction of double bond.27
Therefore in continuation of our interest to develop environmen-
tally benign protocols we report herein our results with nanosized
ZnO that efficiently catalyzed the phospha-Michael addition.
In this study, we prepared nanosized ZnO catalyst and used it
successfully in the synthesis of b-phopsphono malonates. The XRD
pattern of the synthesized ZnO particles is shown in Figure 1. The
nanoscopic nature of the crystalline ZnO particles is responsible for
the broadness of XRD peaks. The peak broadening is used toestimate
the average ZnO crystal grain size in terms of the Debey–Scherrer
equation.28 The average particle size of the ZnO sample thus esti-
mated is in the range of 20–30 nm. The scanning electron micro-
graph (SEM) of the synthesized ZnO particles is shown in Figure 2. It
can be seen that well defined and discrete ZnO nanoflake was
formed. Similar particle sizes and shapes have been reported earlier
for ZnO particles prepared by hydrolysis of Zn(OAc)2 using urea
under hydrothermal conditions.29 However, unlike in the earlier
case, individual small nanoparticles were not observed for the par-
ticles synthesized by this method. This is probably due to the dif-
ference in the procedure used for the preparation of these materials.
Wu and co-workers have used 40 ml of the solution of Zn(OAc)2 and
urea using a Teflon-lines autoclave and heated in an electronic
Table 2
Reaction between 2-[(4-chlorophenyl) methylene]malononitrile (1a) with diethyl
phosphite (2) catalyzed by different crystallite of ZnO at 50 ꢀCa
Entry
Catalyst
Solvent
Time (min)
Yieldb (%)
1
2
3
4
5
6
CM-ZnOd
NF-ZnOe
NF-ZnO
NF-ZnO
NF-ZnO
NF-ZnO
None
None
CH2Cl2
CH3CN
THF
120
30
180
180
180
180
80
98
43c
40c
40c
0
H2O
a
Conditions: (1a) (1.0 mmol), (2) (1.0 mmol), catalyst (5.0 mol %) were mixed at
50 ꢀC.
b
Isolated yields.
c
The unreacted starting materials remained unchanged (TLC).
‘CM-ZnO’ means commercial ZnO.
‘NF-ZnO’ means nanoflake ZnO.
d
e
Figure 1. X-ray diffraction pattern of the synthesized nano-flake ZnO sample.

M. Hosseini-Sarvari, S. Etemad / Tetrahedron 64 (2008) 5519–5523
5521
Table 3
Synthesis of b
-phosphono malonates catalyzed by NF-ZnO at 50 ꢀC in solvent-free condition
Entry
Substrate
Product
Type of catalyst
Time (h)
Yielda (%)
O
P(OEt)2
CN
CN
CN
CM-ZnOb
NF-ZnOc
2
0.5
80
98
1
3a
3b
3c
Cl
CN
Cl
O
P(OEt)2
CN
CN
CN
CM-ZnO
NF-ZnO
10
2.5
83
98
2
3
CN
O
P(OEt)2
CN
CN
CM-ZnO
NF-ZnO
20
5
87
98
CN
Me
CN
Me
O
P(OEt)2
CN
CN
CM-ZnO
NF-ZnO
40
5
30
90
CN
4
3d
MeO
CN
MeO
O
P(OEt)2
CN
CN
CM-ZnO
NF-ZnO
30
10
50
98
CN
5
6
7
8
9
3e
3f
O2N
Me
CN
O2N
Me
O
P(OEt)2
CN
CN
CM-ZnO
NF-ZnO
8
1
74
98
CN
CN
O
P(OEt)2
Cl
CN
CM-ZnO
NF-ZnO
8.5
0.5
76
98
Cl
CN
3g
3h
3i
CN
CN
O
S
P(OEt)2
CN
CM-ZnO
NF-ZnO
28
5
75
98
S
CN
NC
NC
NC
O
P(OEt)2
CN
S
CM-ZnO
NF-ZnO
28
3.5
83
98
CN
S
NC
O
P(OEt)2
CO2Et
CN
CM-ZnO
NF-ZnO
28
7.5
50
98
10
3jd
CO2Et
Cl
CN
Cl
a
Isolated yields.
b
c
‘CM-ZnO’ means commercial ZnO.
‘NF-ZnO’ means nanoflake ZnO.
d
Mixture of diastereoisomers, ratio is 50:50, and determined by 1H NMR.
a benign reaction medium, the reaction was explored in CH2Cl2,
CH3CN, THF, and water. The reaction in solvents required relatively
longer reaction times and afforded moderate yields of the product.
Subsequently, NF-ZnO with average diameters of 20–30 nm and
In this study, the activity clearly indicates that the particle size in
the nanoregime helps to expedite the reaction. This is in agreement
with ourrecentreportwhere anincreasein theactivity wasobserved
in the case of nanoparticles as compared to the bulk ZnO catalyst.27
CM-ZnO were employed in the synthesis of various
malonates. As summarized in Table 3, NF-ZnO catalyzed this
reaction to generate the corresponding -phosphono malonates in
high yields (mostly higher than 90%). The catalytic activity of the
synthesized nanosized ZnO particles was compared with the
commercially available ZnO catalyst. The reactions were carried out
in solvent-free conditions for both catalysts at 50 ꢀC.
b
-phosphono
In the synthesis of b-phosphono malonates NF-ZnO can activate
P(O)H groups so that deprotonation of the P–H bond occurs in the
presence of Lewis basic sites (O2ꢁ). Subsequently, a Lewis acid base
interaction between the hard Zn2þ cation and one of the nitrile
b
nitrogen atom of malonates was formed. Thus, the Lewis base (O2ꢁ
of the catalyst activates P–H bond, and the Lewis acid moiety (Zn2þ
activates malonates.
)
)

5522
M. Hosseini-Sarvari, S. Etemad / Tetrahedron 64 (2008) 5519–5523
1
To conclude, we have shown that nanoflake ZnO is a highly
active catalyst for the synthesis of -phosphono malonates in good
4.75 (m, 5H, CH(CN)2 and –OCH2CH3), 3.79 (dd, 1H, JHH¼7.0 Hz,
2JHP¼22.1 Hz, –CHP), 3.71 (s, 3H, –OCH3), 1.20 (t, 3H, J¼7.0 Hz,
–OCH2CH3), 0.96 (t, 3H, J¼7.1 Hz, –OCH2CH3); 13C NMR (CDCl3,
b
yields. The title compounds are of chemical and medicinal interest.
The advantages of this environmentally benign and safe protocol
include a simple reaction set-up, does not require specialized
equipment, mild reaction conditions, high product yields, short
reaction times, and the limitation of solvents.
3
3
62.9 MHz):
d
16.2 (d, JCP¼6.6 Hz, OCH2CH3), 16.3 (d, JCP¼6.5 Hz,
OCH2CH3), 35.8, 38.3, 54.8 (d, JCP¼144.2 Hz, CHP), 57.5 (d,
2JCP¼7.0 Hz, OCH2CH3), 58.8 (d, 2JCP¼7.3 Hz, OCH2CH3), 113.0, 113.2,
115.9, 122.6, 129.9, 172.5. Anal. Calcd for C15H19N2O4P: C, 55.90; H,
5.94. Found: C, 55.82; H, 5.91%.
2. Experimental
2.2.4. [1-(4-Nitrophenyl)2,2-dicyanoethyl] phosphonic
acid diethyl ester (3e)
2.1. Preparation of NF-ZnO
IR (neat) 2255, 2204 (CN), 1255 (P]O); 1H NMR (CDCl3,
In a typical experiment for the synthesis of NF-ZnO, zinc acetate
dehydrate (10 mmol) and CO(NH2)2 (0.2 mol) were dissolved in
200 mL deionized water at room temperature to form a transparent
solution. Then the mixture was refluxed for 12 h. It was cooled by
cold water to stop the reaction. The product was centrifuged and
washed with deionized water and absolute ethanol, and dried at
80 ꢀC for 8 h. The NF-ZnO was obtained by calcining the precursor
in a furnace in air at 400 ꢀC for 2 h.29
250 MHz):
d
8.68 (d, 2H, J¼7.1 Hz, Ar–H), 7.22 (d, 2H, J¼7.1 Hz, Ar–
H), 4.62 (dd, 1H, 1JHH¼7.8 Hz, 2JHP¼9.2 Hz, –CH(CN)2), 4.02–4.16 (m,
4H, –OCH2CH3), 3.77 (dd, 1H, 1JHH¼7.9 Hz, 2JHP¼21.6 Hz, –CHP), 1.32
(t, 3H, J¼7.2 Hz, –OCH2CH3), 1.12 (t, 3H, J¼7.0 Hz, –OCH2CH3); 13C
3
NMR (CDCl3, 62.9 MHz):
d
16.1 (d, JCP¼6.6 Hz, OCH2CH3), 16.2 (d,
3JCP¼6.6 Hz, OCH2CH3), 25.1, 44.0 (d, JCP¼144.2 Hz, CHP), 64.0 (d,
2JCP¼7.0 Hz, OCH2CH3), 64.6 (d, JCP¼7.1 Hz, OCH2CH3), 110.7, 111.1
2
123.8, 124.4, 130.5, 137.0. Anal. Calcd for C14H16N3O5P: C, 49.86; H,
4.78. Found: C, 49.81; H, 4.70%.
2.2. General procedure for synthesis of
malonates
b-phosphono-
2.2.5. [1-(3-Methylphenyl)2,2-dicyanoethyl] phosphonic
acid diethyl ester (3f)
Malonates (1 mmol) and diethylphosphonate (1 mmol) were
added to NF-ZnO (5 mol %, 0.004 g) in a test tube. The mixture was
stirred in an oil bath at 50 ꢀC, and the progress of the reaction was
monitoredbyTLC. After the reactionwas complete, EtOAcwas added
to the reaction mixture and centrifuged to separate the catalyst. The
organic solvent was removed under reduced pressure. After purifi-
cation by chromatography on silica gel (solvent: n-hexane/ethyl
acetate, 60:40) the product was obtained. The structure of the
products was confirmed by 1H NMR, IR and comparison with au-
thentic samples obtained commercially or prepared by reported
methods. Products, 3b, 3h, and 3j are known and were characterized
by 1H NMR, IR, and mass spectral data, which were found to be
identical with those described in Refs. 12b, 30, and 31, respectively.
IR (neat) 2298, 2254 (CN), 1257 (P]O); 1H NMR (CDCl3,
1
250 MHz):
d
7.12–7.31 (m, 4H, Ar–H), 4.51 (dd, 1H, JHH¼6.0 Hz,
2JHP¼9.0 Hz, –CH(CN)2), 3.65–4.19 (m, 4H, –OCH2CH3), 3.53 (dd, 1H,
1JHH¼8.2 Hz, JHP¼22.4 Hz, –CHP), 2.29 (s, 3H, –CH3), 1.24 (t, 3H,
2
J¼7.1 Hz, –OCH2CH3), 1.03 (t, 3H, J¼7.0 Hz, –OCH2CH3); 13C NMR
3
(CDCl3, 62.9 MHz):
d
16.0 (d, JCP¼5.8 Hz, OCH2CH3), 16.2 (d,
3JCP¼5.9 Hz, OCH2CH3), 21.3, 25.0, 44.5 (d, JCP¼143.9 Hz, CHP), 63.3
2
2
(d, JCP¼7.3 Hz, OCH2CH3), 64.2 (d, JCP¼7.1 Hz, OCH2CH3), 111.2,
111.5, 126.2, 129.2, 129.9, 130.1, 130.2, 139.2. Anal. Calcd for
C15H19N2O3P: C, 58.82; H, 6.25. Found: C, 58.81; H, 6.22%.
2.2.6. [1-(3-Chlorophenyl)2,2-dicyanoethyl] phosphonic
acid diethyl ester (3g)
IR (neat) 2260, 2227 (CN), 1259 (P]O); 1H NMR (CDCl3,
2.2.1. [1-(4-Chlorophenyl)-2,2-dicyanoethyl] phosphonic
acid diethyl ester (3a)
1
250 MHz):
d
7.24–7.44 (m, 4H, Ar–H), 4.62 (dd, 1H, JHH¼6.7 Hz,
2JHP¼8.4 Hz, –CH(CN)2), 3.65–4.16 (m, 4H, –OCH2CH3), 3.58 (dd, 1H,
IR(neat)2260, 2221 (CN),1249 (P]O); 1H NMR (CDCl3, 250 MHz):
1JHH¼7.7 Hz, 2JHP¼22.0 Hz, –CHP), 1.29 (t, 3H, J¼7.1 Hz, –OCH2CH3),
1
2
d
7.32–7.44 (s, 4H, Ar–H), 4.47 (dd, 1H, JHH¼7.5 Hz, JHP¼9.0 Hz,
1.1 (t, 3H, J¼7.1 Hz, –OCH2CH3); 13C NMR (CDCl3, 62.9 MHz):
d 16.0
–CH(CN)2), 4.05–4.14 (m, 4H, –OCH2CH3), 3.54 (dd, 1H, 1JHH¼7.5 Hz,
2JHP¼20.2 Hz, –CHP), 1.29 (t, 3H, JHH¼7.2 Hz, –OCH2CH3), 1.11 (t, 3H,
(d, 3JCP¼6.1 Hz, OCH2CH3),16.2 (d, 3JCP¼5.5 Hz, OCH2CH3), 25.2, 43.9
2
JHH¼7.0 Hz, –OCH2CH3); 13C NMR (CDCl3, 62.9 MHz):
d 16.1 (d,
(d, JCP¼143.9 Hz, CHP), 63.5 (d, JCP¼7.2 Hz, OCH2CH3), 64.3 (d,
2JCP¼7.0 Hz, OCH2CH3), 111.0, 111.3, 127.4, 129.6, 129.7, 130.6, 132.5,
135.0. Anal. Calcd for C14H16ClN2O3P: C, 51.47; H, 4.94. Found: C,
51.45; H, 4.90%.
3JCP¼5.0 Hz, OCH2CH3), 16.2 (d, JCP¼5.3 Hz, OCH2CH3), 25.4, 44.0
3
2
(d, JCP¼144.6 Hz, –CHP), 64.5 (d, JCP¼7.0 Hz, OCH2CH3), 63.6 (d,
2JCP¼7.1 Hz, OCH2CH3), 111.0, 111.2, 128.7, 129.6, 130.6, 138.2. Anal.
Calcd for C14H16ClN2O3P: C, 51.47; H, 4.94. Found: C, 51.65; H, 4.99%.
2.2.7. (2,20-Dicyano-1-thiophene-3-ylethyl) phosphonic
acid diethyl ester (3i)
2.2.2. [1-(4-Methylphenyl)-2,2-dicyanoethyl] phosphonic
acid diethyl ester (3c)
IR (neat) 2202 (CN), 1238 (P]O); 1H NMR (CDCl3, 250 MHz):
IR (neat) 2252, 2229 (CN), 1257 (P]O); 1H NMR (CDCl3,
d
7.42 (s, 1H, Ar–H), 7.16 (d, 1H, J¼2.7 Hz, Ar–H), 7.00 (d, 1H,
250 MHz):
d
7.28 (d, 2H, J¼7.1 Hz, Ar–H), 7.12 (d, 2H, J¼7.1 Hz, Ar–
J¼2.7 Hz, Ar–H), 4.34 (dd, 1H, 1JHH¼7.4 Hz, 2JHP¼9.0 Hz, –CH(CN)2),
H), 4.43 (dd, 1H, 1JHH¼7.8 Hz, 2JHP¼8.6 Hz, –CH(CN)2), 3.68–4.13 (m,
4H, –OCH2CH3), 3.51 (dd, 1H, 1JHH¼7.8 Hz, 2JHP¼21.2 Hz, –CHP), 2.29
3.76–3.94 (m, 4H, –OCH2CH3), 3.65 (dd, 1H, JHH¼6.4 Hz,
1
2JHP¼22.1 Hz, –CHP), 1.12 (t, 3H, J¼7.0 Hz, –OCH2CH3), 0.91 (t, 3H,
(s, 3H, –CH3), 1.27 (t, 3H, J¼7.1 Hz, –OCH2CH3), 1.06 (t, 3H, J¼7.1 Hz,
J¼7.1 Hz, –OCH2CH3); 13C NMR (CDCl3, 62.9 MHz):
d 16.0 (d,
–OCH2CH3); 13C NMR (CDCl3, 62.9 MHz):
d
16.1 (d, JCP¼6.1 Hz,
3JCP¼5.09 Hz, OCH2CH3), 16.2 (d, JCP¼5.1 Hz, OCH2CH3), 25.6, 39.8
3
3
3
2
OCH2CH3), 16.2 (d, JCP¼6.6 Hz, OCH2CH3), 21.1, 25.6, 44.37 (d,
(d, JCP¼146.3 Hz, CHP), 63.4 (d, JCP¼7.2 Hz, OCH2CH3), 64.2 (d,
2
JCP¼144.8 Hz, CHP), 63.3 (d, JCP¼7.2 Hz, OCH2CH3), 64.2 (d,
2JCP¼6.9 Hz, OCH2CH3), 111.2, 111.6, 124.3, 126.1, 127.7, 128.4. Anal.
2JCP¼7.0 Hz, OCH2CH3), 111.2, 111.4, 126.5, 129.0, 130.1, 140.0. Anal.
Calcd for C12H15N2O3PS: C, 48.32; H, 5.07. Found: C, 48.40; H, 5.82%.
Calcd for C15H19N2O3P: C, 58.82; H, 6.25. Found: C, 58.80; H, 6.20%.
2.2.3. [1-(4-Methoxyphenyl)2,2-dicyanoethyl] phosphonic
acid diethyl ester (3d)
Acknowledgements
IR (neat) 2275, 2221 (CN), 1257 (P]O); 1H NMR (CDCl3,
We gratefully acknowledge the support of this work by the
Shiraz University Research Council.
250 MHz):
d
7.28 (d, 2H, J¼7.1 Hz, Ar–H), 7.12 (d, 2H, J¼7.1 Hz, Ar–H),

M. Hosseini-Sarvari, S. Etemad / Tetrahedron 64 (2008) 5519–5523
5523
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