Significant and differential acceleration of dephosphorylation of the insecticides, paraoxon and parathion, caused by alkali metal ethoxides.

Article abstract of DOI:10.1039/b310055c

In the reaction of paraoxon with alkali metal ethoxides, ion-paired EtO-M+ species are more reactive than the dissociated EtO- with the reactivity order EtO-Li+ EtO-Na+ > EtO-K+ > EtO-, while in the reaction of parathion, the reactivity follows the order

Full text of DOI:10.1039/b310055c

Significant and differential acceleration of dephosphorylation of the
insecticides, paraoxon and parathion, caused by alkali metal ethoxides†
Ik-Hwan Um,* Sang-Eun Jeon, Mi-Hwa Baek and Hye-Ran Park
Department of Chemistry, Ewha Womans University, Seoul 120-750, Korea. E-mail: ihum@mm.ewha.ac.kr;
Fax: 822 3277 2844; Tel: 822 3277 2349
Received (in Cambridge, UK) 20th August 2003, Accepted 21st October 2003
First published as an Advance Article on the web 7th November 2003
In the reaction of paraoxon with alkali metal ethoxides, ion-
paired EtO2M+ species are more reactive than the dis-
the dissociated EtO2 and ion-paired EtO2M+, respectively, and
the association constant, Ka = [EtO2M+]/[EtO2][M+].
sociated EtO2 with the reactivity order EtO2Li+
>
Rate = kEtO2[EtO2][1] + kEtOM[EtO2M+][1]
obs/[EtO2] = kEtO2 + KakEtOM[EtO2]
(2)
(3)
EtO2Na+ > EtO2K+ > EtO2, while in the reaction of
parathion, the reactivity follows the order EtO2K+ > EtO2
> EtO2Na+ > EtO2Li+.
k
If eqns. (2) and (3) are valid for the present reaction, the plot
of kobs/[EtO2] vs. [EtO2] should be linear and pass through a
common intercept regardless of the nature of the M+ ions. In
fact, as shown in the inset of Fig. 1A, all the plots of kobs/[EtO2]
vs. [EtO2] are linear with a common intercept. Therefore, the
kEtO2 and KakEtOM values have been determined from the
intercept and the slope of the linear plots, respectively. Since the
Ka values of EtO2Li+, EtO2Na+ and EtO2K+ have been
reported to be 212, 102 and 90 M21 for this series,12 the
corresponding kEtOM values can be calculated. The kEtO2 and
The need for improved methods of environmental decontamina-
tion has provided the impetus for numerous studies aimed at
enhancing the rate of dephosphorylation of toxic organophos-
phorus compounds.1–10 Previous studies have included the use
of such highly reactive a-nucleophiles as oximates,1,2 hydrogen
peroxide anion,3 o-iodosylbenzoate and its derivatives4 or the
use of metal ions as Lewis acid catalysts.5–10 Buncel et al. have
performed a systematic study into the effect of alkali metal ions
on the reaction of 4-nitrophenyl diphenylphosphinate with
alkali metal ethoxides (EtO2M+) in anhydrous ethanol (EtOH),
and found that the reaction is catalyzed by alkali metal ions in
the order Li+ > Na+ > K+.6 The catalytic effect of these M+
ions has been explained by a 4-membered transition-state (TS),7
TS involving 3 to 5 M+ ions8 or a TS complexed with dimeric
EtO2M+ species for the reaction performed in high EtO2M+
concentrations.9
k
EtOM values determined in this way are summarized in Table 1.
Note, the magnitude of kEtOM and kEtO2 values is in the order
kEtOLi > kEtONa > kEtOK > kEtO2, indicating that the ion-paired
EtO2M+ is more reactive than the dissociated EtO2. In this case
alkali metal ions clearly behave as catalysts for the dephosphor-
ylation. The catalysis by these M+ ions could plausibly arise
either through increasing the electrophilicity of the P-center of
the PNO bond or by increasing the nucleofugality of the
4-nitrophenoxide in 1a. However, the present result alone
cannot distinguish these two possibilities.
(1)
We have performed the reactions of 1a (paraoxon) and 1b
(parathion) with EtO2M+ in anhydrous EtOH to obtain further
information on the catalytic role of M+ ions in the detoxification
reaction of the insecticides as shown in eqn. (1). Replacing the
O atom of the PNO bond in 1a by an S atom (PNO ? PNS) would
be expected to provide valuable information on the catalytic
role of M+ ions. Clearly, the degree of complexation by M+ ions
to a PNO center should differ markedly from complexation to
the analogous PNS center due to the difference in polarizability
between the PNO and PNS bonds.
As shown in Fig. 1A, the plots of kobs vs. [EtO2M+] exhibit
upward curvature. Such upward curvature is most remarkable
for the reaction with EtO2Li+. The corresponding plot for the
reaction with EtO2K+ in the presence of 18-crown-6-ether
(18C6) is linear with a significant decrease in the reactivity.
It has been reported that EtO2M+ exists as dimers or other
aggregates in the high concentration region ( > 0.1 M).11
However, at concentrations below 0.1 M as used in the present
study, EtO2M+ has been suggested to exist only as dissociated
or ion-paired species.11 Since both dissociated EtO2 and ion-
paired EtO2M+ could react with the substrates, 1, one can
derive rate equations (2) and (3),7 in which kEtO2 and kEtOM
represent the second-order rate constants for the reaction with
Fig. 1 Kinetic data for the reactions of 1a (paraoxon, A) and 1b (parathion,
B) with EtO2Li+ (5), EtO2Na+ (?), EtO2K+ (2) and EtO2K+ in the
presence of 18C6 ( < ) in anhydrous EtOH at 25 ± 0.1 °C.
Table 1 Summary of second-order rate constants for various ethoxide
species from ion-pair treatment of kinetic data for the reaction of paraoxon
(1a) and parathion (1b) with EtO2M+ in anhydrous EtOH at 25.0 ± 0.1
°C
105kEtO2/M21 s21
105kEtOM/M21 s21
EtO2M+
1a
1b
1a
1b
EtO2Li+
EtO2Na+
EtO2K+
124 ± 19.2
129 ± 6.9
122 ± 6.2
5.30 ± 0.2
5.09 ± 0.1
4.78 ± 0.2
4350 ± 10
3250 ± 10
2400 ± 10
2.34 ± 0.1
3.40 ± 0.1
6.11 ± 0.1
† Electronic Supplementary Information (ESI) available: kinetic data. See
http://www.rsc.org/suppdata/cc/b3/b310055c/
3016
CHEM. COMMUN., 2003, 3016–3017
This journal is © The Royal Society of Chemistry 2003

View Article Online
In order to further define the mechanism of catalysis by the
M+ ions, the study was extended to the reaction of 1b with
EtO2M+. As shown in Fig. 1B, the plot of kobs vs. [EtO2K+]
exhibits upward curvature, while the one for the reaction with
EtO2K+ in the presence of 18C6 is linear. Interestingly, the
corresponding plots for the reactions with EtO2Na+ and
EtO2Li+ exhibit downward curvature.
The kinetic data have been analyzed using eqn. (3) and are
illustrated in the inset of Fig. 1B. All the plots of kobs/[EtO2] vs.
[EtO2] are linear and pass through a common intercept.
Accordingly, the kEtO2 and kEtOM values have been determined
using the same method outlined above for the reaction of 1a,
and kinetic parameters are summarized in Table 1.
It is shown that the rate constants decrease in the order kEtOK
> kEtO2 > kEtONa > kEtOLi, indicating that the ion-paired
EtO2K+ is more reactive than the dissociated EtO2 while
EtO2Na+ and EtO2Li+ are less reactive than EtO2 in the
reaction of 1b. This is in contrast with the result obtained from
the reaction of 1a (see above), but appears to be in accordance
with the so-called hard and soft acids and bases principle.
Furthermore, as shown in Table 1, 1a is much more reactive
than 1b. One can notice that the kEtO2 value for the reaction of
1a with the dissociated EtO2 is ca. 20–30 times larger than the
one for 1b, while the kEtOLi value, as but one example, for 1a is
up to ca. 2 3 103 times larger than that for 1b. On the one hand,
the results with the dissociated EtO2 illustrate the intrinsic
reactivity difference due to the change in the P-center
electrophilicity on going from PNO to PNS. However, the much
greater reactivity difference for the ion-paired EtO2M+ reflects
the nature of the interaction between the M+ ion and the site of
complexation.
In such a model the catalytic effect would increase with
increasing charge density of M+ ions for the reaction of 1a. To
confirm this argument the catalytic effect (e.g., kEtOM/kEtO2) of
EtO2M+ has been correlated with the reciprocal radius of M+
ions for the reaction of 1a. As shown in Fig. S1 (ESI), log kEtOM
/
kEtO2 increases linearly with increasing 1/r value, in which r
represents the radius of the M+ ion. However, if the negative
charge develops at the S atom of the PNS bond in the transition
state, it is expected to be dispersed by the polarizable S atom.
Accordingly, the complexation of M+ ions with the S atom of
the PNS bond would not be as strong as that with the O atom of
the PNO bond and, in turn, the enhancement of P-electro-
philicity would not be as great for PNS as for PNO, which
accounts for the significant difference in the catalytic effect of
the M+ ions in the dephosphorylation of 1a and 1b.
We are grateful for the grant (R01-1999-00047) from the
Korea Science and Engineering Foundation.
Notes and references
1 (a) F. Terrier, E. Le Guevel, A. P. Chatrousse, G. Moutiers and E.
Buncel, Chem. Commun., 2003, 600; (b) E. Buncel, C. Cannes, A. P.
Chatrousse and F. Terrier, J. Am. Chem. Soc., 2002, 124, 8766.
2 (a) I. H. Um, J. Y. Hong and E. Buncel, Chem. Commun., 2001, 27; (b)
I. H. Um and E. Buncel, J. Am. Chem. Soc., 2001, 123, 11111.
3 J. Toullec and M. Mohamed, Chem. Commun., 1996, 221.
4 H. Morales-Rojas and R. A. Moss, Chem. Rev., 2002, 102, 2497.
5 (a) S. G. Srivatsan and Sandeep Verma, Chem. Commun., 2000, 515; (b)
Helmut Sigel, Claudia A. Blindauer, Antonín Holy´ and Hana Dvor-
áková, Chem. Commun., 1998, 1219; (c) Robert A. Moss, Kathryn
Bracken and Jing Zhang, Chem. Commun., 1997, 563.
6 E. Buncel, E. J. Dunn, R. A. B. Bannard and J. G. Purdon, J. Chem. Soc.,
Chem. Commun., 1984, 162.
7 R. Nagelkerke, G. R. J. Thatcher and E. Buncel, Org. Biomol. Chem.,
2003, 1, 163.
8 E. Buncel, R. Nagelkerke and G. R. J. Thatcher, Can. J. Chem., 2003,
81, 53.
9 V. K. Balakrishnan, J. M. Dust, G. W. vanLoon and E. Buncel, Can. J.
Chem., 2001, 79, 157.
10 J. S. W. Tsang, A. A. Neverov and R. S. Brown, J. Am. Chem. Soc.,
2003, 125, 1559.
If the effect of M+ ions originates from an increase in the
nucleofugality by complexation of M+ ions with the O atom in
the leaving 4-nitrophenoxide, one might expect that the
catalytic effect of M+ ions should be similar for both reaction of
1a and 1b since the leaving group is identical for these
substrates. However, in fact, the effect of M+ ions for the
reactions of 1a and 1b is opposite, indicating that the catalytic
effect shown by M+ ions is clearly not due to an increase in the
nucleofugality.
An alternative argument might hold that the two reactions are
not comparable because they might proceed via different
mechanisms and/or rate-determining steps. However, a differ-
ence in the reaction mechanism is not considered to be
responsible for the contrasting metal ion effect, since the
reactions of 1a and 1b have been shown to proceed through a
similar transition state.13,14
11 V. Pechanec, O. Kocian and J. Zavada, Collect. Czech. Chem.
Commun., 1982, 47, 3405.
12 J. Barthel, G. Baderand and M. Raach-Lenz, Z. Phys. Chem., 1973, 84,
100.
13 J. E. Omakor, I. Onydo, G. W. vanLoon and E. Buncel, J. Chem. Soc.,
Perkin Trans. 2, 2001, 324.
14 S. A. Ba-Saif, A. M. Davis and A. Williams, J. Org. Chem., 1989, 54,
5483.
Consequently, we propose that the difference in the M+ ion
effect arises from differences in complexation of the M+ ion
with the PNX (X = O, S) site. Therefore, the 4-membered
transition state shown below, where EtO2 is aligned with the P-
electrophilic site and the M+ complexes to X of PNX as well as
to the incoming EtO2, is considered to be responsible for the
enhanced reactivity of EtO2M+.
CHEM. COMMUN., 2003, 3016–3017
3017