744
Chemistry Letters Vol.37, No.7 (2008)
Transesterification of Phosphotriester Paraoxon Catalyzed by ZnCl2
Supported on Silica Gel
Ismail O. Kadyꢀ
Department of Chemistry, East Tennessee State University,
Johnson City, TN 37614, USA
(Received April 7, 2008; CL-080353; E-mail: kadyi@etsu.edu)
ZnCl2 supported on silica gel has shown to be an effective
progress was monitored spectro-photometrically by observing
catalyst for transesterification of the organophosphate insecti-
cide, paraoxon, in the presence of 2-phenylethanol. The kinetics
of the reaction in acetonitrile was studied. Formation of the
product diethyl 2-phenylethyl phosphate was confirmed by
31P NMR.
the rate of formation of p-nitrophenol (ꢀmax ¼ 312 nm, "max ꢂ
10000 Mꢃ1 cmꢃ1).13 Periodically (10 min intervals) the catalyst
was allowed to settle to the bottom of the flask, 150-mL sample
of the solution was withdrawn, diluted to 3 mL in acetonitrile,
and the UV-vis spectrum was recorded using a Hitachi U-2000
spectrophotometer.14 The reaction was followed to at least five
half-life times and found to exhibit pseudo-first-order kinetics.
The observed rate constant (kobs) was evaluated from the plot
of absorbance (after background adjustment) versus time by
fitting it into a standard exponential model (Figure 1).
Processes involving phosphoryl transfer are of enormous
importance in biological systems. In spite of the fact that phos-
photriesters are not natural products, they are readily processed
by the microbial Pseudomonas diminuta phosphotriesterase
(PTE).1–4 PTE, which contains two zinc centers in its active site,
catalyzes the hydrolysis of a variety of phosphotriesters through
a mechanism that is still not fully understood. Such phospho-
triesters include insecticides and pesticides such as parathion,
malathion, and paraoxon that accumulate in soil and are relative-
ly stable at low temperature and low pH. These phosphotriesters
are structurally related to chemical warfare agents, such as
sarin, soman, and nerve agent VX. Therefore, considerable effort
has been devoted toward developing methods for controlled
degradation of such organophsphorous contaminants.
It is well known that transesterification reactions can gener-
ally be catalyzed by acids and bases.5 Although there have been
few reports in recent years indicating that some metal complexes
can catalyze transesterification of phosphotriesters,6–10 there
have been no reports on the catalysis of such reactions by simple
Lewis acids such as ZnCl2. In our continuous effort in develop-
ing catalysts for phosphoryl-transfer reactions by metal ions
and metal complexes, we found ZnCl2 supported on silica gel
to be efficient in catalyzing phosphotriesters transesterification.
In this regard, we wish to present the results of our work involv-
ing catalysis of the transesterification of diethyl p-nitrophenyl
phosphate (paraoxon) by SiO2-supported ZnCl2 (Scheme 1).
A solution of diethyl p-nitrophenyl phosphate (paraoxon,
2 mM) in anhydrous acetonitrile (100 mL) was stirred at 50 ꢁC
in the presence of 2-phenylethanol (10 mM) and ZnCl2/SiO2
(0.2 g, 2 mM)11,12 under dry argon atmosphere. Reaction
31P NMR spectroscopy of the reaction mixture confirmed
the formation of diethyl 2-phenylethyl phosphate product and
showed complete disappearance of paraoxon starting material
after 90 min of reaction time. In order to confirm that the sup-
ported catalyst has different activity than the non-supported
one, control reactions were carried out using non-supported
ZnCl2 and/or SiO2. Neither ZnCl2 nor SiO2 alone failed to give
any reaction beyond background. However, when the reaction
was carried out in the presence of unsupported ZnCl2 and silica
gel only minimal transesterification was observed (5% conver-
sion was obtained in the presence of 2 mM ZnCl2 and 0.2 g
SiO2, after 3 h). Thus, this solution was used for background
correction when studying the reaction kinetics.
Catalyst turnover was confirmed by periodically monitoring
the integrated spectra of a solution containing 4.0 mM paraoxon,
10.0 mM 2-phenylethanol, and 0.2 mM catalyst, over a period
of 2 h. The time profiles of the disappearance of paraoxon
and appearance of the product diethyl 2-phenylethyl phosphate
indicated more than 20 turnovers of the catalyst.15
1.2
1.0
0.8
0.6
0.4
0.2
0.0
O
P
NO2
ZnCl2/SiO2
O
O
HOCH2CH2Ph
O
+
O
P
Paraoxon
0
20
40
60
80
Ph
Reaction time/min
O
O
HO
NO2
O
+
Figure 1. Release of p-nitrophenol from the reaction of
paraoxon and 2-phenylethanol, in the presence of ZnCl2/SiO2
(1 mmol/g) at 25 ꢁC.
Scheme 1.
Copyright ꢀ 2008 The Chemical Society of Japan