Combined dual substituent constant and activation parameter analysis assigns a concerted mechanism to alkaline ethanolysis at phosphorus of Y-substituted phenyl diphenylphosphinates

Article abstract of DOI:10.1039/b712427a

Second-order rate constants have been measured for reactions of Y-substituted phenyl diphenylphosphinates (1a-h) with EtO^-K ⁺ in anhydrous ethanol. A linear Bronsted-type plot is obtained with β_Lg = -0.54, a typical β_Lg value for reactions which proceed through a concerted mechanism. The Hammett plots correlated with σ^o and σ^- constants are linear but exhibit many scattered points, while the corresponding Yukawa-Tsuno plot results in excellent linear correlation with r = 0.41. The r value of 0.41 indicates that the leaving group departs at the rate-determining step (RDS) whether the reactions proceed through either a concerted or a stepwise mechanism. However, a stepwise mechanism in which departure of the leaving group occurs at the RDS is excluded since the incoming EtO^- ion is much more basic and a poorer leaving group than the leaving aryloxide. The ΔH^? values determined in the current reactions are strongly dependent on the nature of the substituent Y, while the ΔS ^? values remain constant on changing the substituent Y in the leaving group, i.e., from Y = H to Y = 4-NO₂ and Y = 3,4-(NO ₂)₂. These ΔH^? and ΔS ^? trends also support a concerted mechanism. The Royal Society of Chemistry.

Full text of DOI:10.1039/b712427a

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Combined dual substituent constant and activation parameter analysis assigns
a concerted mechanism to alkaline ethanolysis at phosphorus of Y-substituted
phenyl diphenylphosphinates†
Ik-Hwan Um,* Jee Eun Park and Young-Hee Shin
Received 13th August 2007, Accepted 18th September 2007
First published as an Advance Article on the web 1st October 2007
DOI: 10.1039/b712427a
Second-order rate constants have been measured for reactions of Y-substituted phenyl
−
+
diphenylphosphinates (1a–h) with EtO K in anhydrous ethanol. A linear Brønsted-type plot is
obtained with bLg = −0.54, a typical bLg value for reactions which proceed through a concerted
o
−
mechanism. The Hammett plots correlated with r and r constants are linear but exhibit many
scattered points, while the corresponding Yukawa–Tsuno plot results in excellent linear correlation with
r = 0.41. The r value of 0.41 indicates that the leaving group departs at the rate-determining step (RDS)
whether the reactions proceed through either a concerted or a stepwise mechanism. However, a stepwise
mechanism in which departure of the leaving group occurs at the RDS is excluded since the incoming
−
‡
EtO ion is much more basic and a poorer leaving group than the leaving aryloxide. The DH values
determined in the current reactions are strongly dependent on the nature of the substituent Y, while the
‡
DS values remain constant on changing the substituent Y in the leaving group, i.e., from Y = H to Y =
‡
‡
4
-NO
2
and Y = 3,4-(NO
2
) . These DH and DS trends also support a concerted mechanism.
2
Introduction
result in a linear Brønsted-type plot with bnuc = 0.47, while those
of aryl dimethylphosphinothioates with phenoxide exhibit much
better Hammett correlation with r than r constants. Their
LFER study has led them to conclude that the reaction proceeds
through a concerted mechanism. This conclusion has been further
Certain organophosphorus compounds are widely used as pesti-
cides, neurotoxins and other biologically active substances. Thus,
there has been considerable recent and continuing interest in
phosphoryl transfer and related reactions due to their importance
in the environment, as well as in biological processes. Numerous
studies have been performed to enhance the rate of decomposition
of toxic organophosphorus compounds.
of methods have been developed, from use of metal ions as
Lewis acid catalysts to use of highly reactive a-nucleophiles.
Various metal ions including alkali metal ions engender significant
catalytic effects in dephosphorylations of neurotoxin simulants.
−
o
1e
18
15
supported by studies of the primary O and secondary N kinetic
1
–14
1e
isotope effects.
o
On the contrary, Buncel and coworkers have shown that r
constants result in much better Hammett correlation than r
constants for alkaline ethanolysis of aryl dimethylphosphinates.
2
–10
Accordingly, a range
−
2a
2–5
6–10
A similar result has been reported for alkaline hydrolysis of
aryl diphenylphosphinates and imidazole catalyzed hydrolysis
2–5
12
of aryl diphenylphosphinates. Thus, these reactions have been
−
The a-nucleophiles (e.g., oximates, HOO and o-iodosylbenzoate)
concluded to proceed through a stepwise mechanism with a pen-
tacoordinate intermediate, in which formation of the intermediate
have also exhibited highly enhanced nucleophilicity under mild
6–10
conditions.
2a,12
is the rate-determining step (RDS).
However, systematic kinetic studies have been pursued much less
intensively. Therefore, mechanisms for phosphoryl transfer and
related reactions have not been completely elucidated but remain
controversial (i.e., a concerted versus a stepwise mechanism).
In a series of important studies by Williams et al., reactions
We have recently shown that aminolysis of Y-substituted phenyl
diphenylphosphinates and diphenylphosphinothioates results in
o
−
better Hammett correlation with r constants than with r
1,2,11,12
13
constants. Traditionally, such a result has been interpreted as
14–17
evidence of a stepwise mechanism.
However, in this case,
of 4-nitrophenyl diphenylphosphinate with aryloxides have been
we concluded that the reaction proceeds through a concerted
11
concluded to proceed through a concerted mechanism. The
evidence consisted mainly of the absence of a break (or curvature)
in the Brønsted-type plot obtained for the reactions with a series
13
mechanism. This was because the Yukawa–Tsuno plots for the
same reactions exhibit significantly better correlation than the
o
13
Hammett plots correlated with r constants alone.
of aryloxides whose pK
a
values straddle the basicity of the leaving
We have extended our kinetic study to reactions of Y-substituted
phenyl diphenylphosphinates (1a–h) with ethoxide anion in anhy-
drous ethanol at various temperatures (Scheme 1). The kinetic
data have been analyzed using the dual-parameter Yukawa–
Tsuno equation together with the activation parameters (i.e.,
11
4
-nitrophenoxide. Similarly, Hengge et al. found that reactions
of 4-nitrophenyl dimethylphosphinothioate with aryloxides also
Division of Nano Sciences and Department of Chemistry, Ewha Womans
University, Seoul, 120-750, Korea. E-mail: ihum@ewha.ac.kr; Fax: +82-2-
‡
‡
DH and DS ) associated with the reactions. Application of
this combined approach permits unambiguous assignment of the
reaction mechanism here.
3
277-2844; Tel: +82-2-3277-2349
†
Electronic supplementary information (ESI) available: Kinetic condi-
tions and experimental results. See DOI: 10.1039/b712427a
This journal is © The Royal Society of Chemistry 2007
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Scheme 1
Results and discussion
All reactions in this study obeyed pseudo-first-order kinetics.
Pseudo-first-order rate constants (kobsd) were determined from
the equation ln(A
[
∞
− A
EtO K ] were linear passing through the origin, indicating that
the contribution of EtOH to kobsd is negligible. Thus, the rate
law is given by eqn 1. The second-order rate constants (kEtO−
were determined from the slopes of the linear plots of kobsd vs.
t
) = −kobsdt + C. The plots of kobsd vs.
−
+
)
Fig. 1 Brønsted-type plot for reactions of Y-substituted phenyl diphenyl-
−
+
−
+
phosphinates (1a–h) with EtO K in the presence of 18-crown-6-ether
[
EtO K ] and summarized in Tables 1 and 2. The uncertainty in
◦
−
+
(
18C6) in anhydrous EtOH at 25.0 ± 0.1 C. [18C6]/[EtO K ] = 5.0.
the kEtO− is estimated to be less than 3% from replicate runs. Kinetic
conditions and results are detailed in the electronic supplementary
information (ESI)†.
The magnitude of the Brønsted coefficients bnuc and bLg has been
used as a measure of reaction mechanisms including transition-
state structures of various types of reactions.
reported that bnuc (or −bLg) changes from 0.9 ± 0.1 to 0.3 ± 0.1
for reactions which proceed through a stepwise mechanism with a
change in the RDS. Hupe and Jencks found curved Brønsted-type
plots for reactions of aryl acetates with thiolate nucleophiles (i.e.,
15–17
−
+
It has often been
Rate = kobsd[Sub], where kobsd = kEtO−[EtO K ]
(1)
Effect of leaving group basicity on reactivity
As shown in Table 1, the second-order rate constant increases as
bLg = ca. −0.9 for strongly basic leaving group but b = −0.33
Lg
19
the leaving group becomes less basic, i.e., it increases from 7.51 ×
for weakly basic leaving group). A change in the RDS has been
−
3
−1 −1
−1 −1
19
10
M
s
to 0.170 and 21.4 M
s
as the pK
a
of the conjugate
suggested to be responsible for the curved Brønsted-type plots.
acid of the leaving aryloxide decreases from 15.76 to 13.26 and
.75, respectively. The effect of leaving group basicity on reactivity
is illustrated in Fig. 1. The Brønsted-type plot is linear with bLg
0.54.
The linear Brønsted-type plot with a bLg value of −0.54 obtained in
the present system is typical for reactions which proceed through
a concerted mechanism (e.g., bLg = −0.52 for reactions of aryl
9
=
1e
−
dimethylphosphinothioates with phenoxide and b = −0.66 for
Lg
13a
reactions of aryl diphenylphosphinate with piperidine ). Thus,
Table 1 Summary of second-order rate constants for reactions of Y-
−
one might suggest that the current reactions of 1a–h with EtO
−
+
substituted phenyl diphenylphosphinates (1a–h) with EtO K in the
◦
proceed through a concerted mechanism.
presence of 18-crown-6-ether (18C6) in anhydrous EtOH at 25.0 ± 0.1 C.
−
+
[
18C6]/[EtO K ] = 5.0
Hammett versus Yukawa–Tsuno plots
a
−1 −1
Entry
Y
pK
a
(Y–PhOH)
k
EtO− /M
s
To get more conclusive information on the reaction mechanism,
1
1
1
1
1
1
1
1
a
b
c
d
e
f
H
4-Cl
15.76
14.90
14.64
13.26
13.04
12.66
11.98
9.75
0.00751
0.0419
0.0641
0.170
0.563
0.265
1.09
o
−
Hammett plots have been constructed using r and r constants in
−
Fig. 2A and 2B, respectively. One might expect that r constants
3-COMe
4-COMe
4-CN
4-CHO
4-NO
3,4-(NO
o
would result in a better Hammett correlation than r constants if
the P–OAr bond rupture is involved in the RDS. On the contrary,
b
b
b
o
−
r constants would exhibit a better correlation than r constants if
g
h
2
the P–OAr bond rupture occurs after the RDS. In fact, as shown in
2
)
2
21.4
o
Fig. 2, r constants result in only slightly better Hammett correla-
a
b
pK
a
data in anhydrous ethanol were taken from ref. 18. pK
(in EtOH) = 1.30pK
a
values were
(in H O) +
−
2
o
2
−
tion than r constants (i.e., R = 0.981 for r and R = 0.979 for r
calculated from the relationship, pK
.70. (see ref. 18).
a
a
2
constants). Thus, one cannot obtain any conclusive information
on the reaction mechanism from these Hammett plots.
2
Table 2 Summary of kinetic results for reactions of phenyl diphenylphosphinates (1a), 4-nitrophenyl diphenylphosphinates (1g) and 3,4-dinitrophenyl
−
+
−
+
diphenylphosphinates (1h) with EtO K in the presence of 18-crown-6-ether in anhydrous EtOH at 5 different temperatures. [18C6]/[EtO K ] = 5.0
2
−1 −1
1
1
0 kEtO− /M
s
◦
◦
◦
◦
◦
‡
−1
‡
−1
−1
5.0 C
20.0 C
25.0 C
35.0 C
45.0 C
DH /kcal mol
DS /cal mol
K
1
1
1
a
g
h
0.378
59.1
1480
0.580
84.1
1760
0.751
109
2140
1.41
169
3280
2.85
273
4680
11.3 ± 0.2
8.4 ± 0.4
6.5 ± 0.2
−30 ± 1
−30 ± 1
−30 ± 1
3
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the latter mechanism, two different transition-state structures
are possible, i.e., TS
2
represents the transition-state structures
in the rate-determining formation of the intermediate and TS
3
applies to the rate-determining leaving group departure from the
intermediate to yield the products.
Since leaving group departure is not advanced in TS
not bear a partial negative charge on the O atom of the leaving
ArO moiety that the r value indicates. Accordingly, TS is not
2
, it does
2
o
−
Fig. 2 Correlation of log kEtO− with r (A) and r (B) for reactions
likely to be the TS structure in this study. One can also exclude
−
+
of Y-substituted phenyl diphenylphosphinates (1a–h) with EtO K in
TS on the basis of the fact that the incoming ethoxide is much
3
◦
the presence of 18-crown-6-ether in anhydrous EtOH at 25.0 ± 0.1 C.
more basic and a poorer leaving group than the leaving aryloxide.
Thus, one can suggest that the current reactions proceed through
−
+
o
−
[
18C6]/[EtO K ] = 5.0. The r and r values were taken from ref. 20.
a concerted mechanism with TS . This is consistent with the
preceding argument on the basis of the linear Brønsted-type plot
with bLg = −0.54.
1
We have recently shown that the dual-parameter Yukawa–Tsuno
equation (eqn 2) is highly effective to elucidate ambiguities in reac-
13,21
tion mechanisms of phosphinyl transfer and related reactions.
Thus, a Yukawa–Tsuno plot has been constructed for the reactions
Activation parameters and reaction mechanism
−
of 1a–h with EtO . As shown in Fig. 3, the Yukawa–Tsuno plot
2
‡
now results in an excellent correlation (i.e., R = 0.997) with q =
To further probe the above argument, activation parameters (DH
‡
1.98 and r = 0.41.
and DS ) have been determined from rate constants measured at 5
different temperatures for the reactions of phenyl, 4-nitrophenyl,
and 3,4-dinitrophenyl diphenylphosphinates (1a, 1g, and 1h, re-
spectively) with ethoxide ion. The kinetic results are summarized in
Table 2. The Arrhenius plots shown in Fig. 4 exhibit excellent linear
Y
H
o
−
o
log k /k = q[r + r(r − r )]
(2)
‡
‡
correlations, indicating that the DH and DS values determined
in this study are accurate and reliable.
Fig. 3 Yukawa–Tsuno plot for reactions of Y-substituted phenyl di-
−
+
phenylphosphinates (1a–h) with EtO K in the presence of 18-crown-6-
◦
−
+
ether in anhydrous EtOH at 25.0 ± 0.1 C. [18C6]/[EtO K ] = 5.0.
The r value in the Yukawa–Tsuno equation represents the
Fig.
4 Arrhenius plots for reactions of phenyl diphenylphosphi-
resonance demand of the reaction center or the extent of resonance
nate (1a), 4-nitrophenyl diphenylphosphinate (1g) and 3,4-dinitrophenyl
diphenylphosphinate (1h) with EtO K in the presence of 18C6 in
anhydrous EtOH. [18C6]/[EtO K ] = 5.0.
22
contribution. Thus, the fact that r = 0.41 in the current reactions
indicates that a partial negative charge develops on the O atom
of the leaving aryloxide in the rate-determining transition state,
which can be delocalized on the substituent Y through resonance
interaction. Accordingly, one can suggest that the departure of the
leaving group occurs definitely in the RDS in the present system
whether the reactions proceed through a concerted mechanism
−
+
−
+
‡
Importantly, Table 2 shows that DH decreases significantly
(i.e., ca. 5 kcal mol ) on changing the substituent from H (1a)
to 3,4-(NO
substituent Y on DH is illustrated in Fig. 5. The correlation of
DH with r constants of the substituent Y exhibits an excellent
linearity with a large slope, indicating that DH is strongly
−
1
‡
2
)
2
(1h) while DS remains constant. The effect of the
‡
‡
−
with a transition-state structure similar to TS
1
or through a
‡
stepwise mechanism with a pentacoordinate intermediate. For
This journal is © The Royal Society of Chemistry 2007
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‡
−1
−1
and DS = ca. −50 cal mol
K
for aminolysis of phenyl
24c
cyclopropanecarboxylates in CH CN ). Thus, one can suggest
that the DS values determined in the present reactions also
3
‡
support a concerted mechanism.
Conclusion
We conclude that the current reactions of 1a–h proceed through a
concerted mechanism with a transition-state structure similar to
TS on the basis of the following arguments:
1
(
1) The linear Brønsted-type plot with bLg = −0.54 observed for
−
reactions of 1a–h with EtO is typical for reactions which proceed
through a concerted mechanism.
(
2) The Yukawa–Tsuno plot for reactions of 1a–h with EtO−
exhibits an excellent correlation with r = 0.41, indicating that the
departure of the leaving group occurs in the RDS.
Fig. 5 Plot of DH versus r− constants for reactions of phenyl
‡
‡
(
3) The DH values are strongly dependent on the nature of
diphenylphosphinate (1a), 4-nitrophenyl diphenylphosphinate (1g), and
,4-dinitrophenyl diphenylphosphinate (1h) with EtO K in the presence
of 18C6 in anhydrous EtOH at 25.0 ± 0.1 C.
substituent Y in the leaving group, which is only possible when the
leaving group departure is involved in the RDS.
−
+
3
◦
‡
−1
−1
(
4) The DS values are ca. −30 cal mol
K
regardless of the
substituent Y, which also support a concerted mechanism.
influenced by the electronic nature of the substituent Y. Such a
strong dependence of DH on the substituent Y in the leaving
group can be expected only for reactions in which the departure
of the leaving group occurs in the RDS.
‡
Experimental
23
Materials
‡
The above argument is complemented by the DS values. One
might expect that the transition state would become earlier as
Y-substituted phenyl diphenylphosphinates (1a–h) were syn-
12b,25
the substituent Y changes from H to 3,4-(NO
2
)
2
on the basis
of a normal Hammond effect. If the current reaction proceeds
through TS , in which the departure of the leaving group is not
thesized by modification of literature reported previously.
15
−
+
EtO K stock solution was prepared by dissolving potassium
metal in anhydrous ethanol under nitrogen and was stored in
2
‡
−
+
advanced, DS should be dependent mainly on the degree of bond
the refrigerator. The concentration of EtO K stock solution was
determined by titration with standard HCl solution. 18-Crown-6-
ether (18C6) was recrystallized from acetonitrile and dried over
formation between the incoming ethoxide and the P-electrophilic
‡
site. In this case, DS should be less negative for the reaction of
1
h (Y = 3,4-(NO
2
)
2
) than for that of 1a (Y = H). However, as
P
2
O in vacuo. The anhydrous ethanol used was further dried over
5
‡
shown in Table 2, the DS value remains constant, indicating that
magnesium and distilled under N .
2
the reactions do not proceed through TS or a normal Hammond
2
effect is not operative in the current system.
On the contrary, if the reaction proceeds through a concerted
mechanism with a transition-state structure similar to TS
Kinetics
‡
1
, DS
Kinetic studies were performed with a Scinco S-3100 UV–vis
spectrophotometer for slow reactions (t1/2 ≥10 s) or with an Ap-
plied Photophysics DX-17 MV stopped-flow spectrophotometer
for fast reactions (t1/2 <10 s) equipped with a constant temperature
circulating bath. The reactions were followed by monitoring
should be influenced by the degree of both bond formation and
bond rupture. In this case, one might expect that DS would be
‡
independent of the electronic nature of the substituent Y. This
‡
is because the change in DS upon bond formation would be
‡
compensated by the change in DS upon bond rupture, whether
the appearance of the leaving aryloxide at a fixed wavelength
−
the reactions proceed through an early or late transition state. In
corresponding to the maximum absorbance (kmax) of Y–C
6
H
4
O .
‡
fact, Table 2 shows that DS remains constant upon changing
All the reactions were carried out under pseudo-first-order con-
ditions in the presence of a complexing agent, 18C6 to sequester
‡
the substituent Y. Besides, the current DS value of −30 ±
−
1
−1
+
+
−
+
1
cal mol
K
is comparable to those reported for reactions
K and avoid possible catalysis by K ion (i.e., [18C6]/[EtO K ] =
5.0). Typically, the reaction was initiated by adding 5 lL of
a 0.02 M substrate solution in MeCN by a 10 lL gastight
syringe to a 10 mm quartz UV cell containing 2.50 mL of the
thermostated reaction mixture made up of anhydrous ethanol
‡
which proceed through a concerted mechanism (e.g., DS
=
−
1
−1
−
24.8 and −36.6 cal mol
K
for hydrolysis of 4-nitrophenyl
1g
‡
diethyl phosphate and ethyl phosphate, respectively, DS = −29.3
and −34.3 cal mol for aminolysis of 2,4-dinitrophenyl
diphenylphosphinate and diphenylphosphinothioate, respectively
−
1
−1
K
−
+
and an aliquot of EtO K stock solution. All the solutions
were transferred by Hamilton gastight syringes under nitrogen.
Generally, the concentration of EtO K was varied over the range
13b
‡
in 80 mol% H
2
O–20 mol% DMSO and DS = ca. −25 cal
−
1
−1
−
+
mol
K
for reactions of aryl N-phenyl thiocarbamates with
24a
‡
−3
benzylamines in CH
3
CN ). However, the current DS value
1–100 × 10 M, while the substrate concentration was 2–4 ×
−
5
−
+
is much less negative than those reported for reactions which
10 M. Usually 5 different concentrations of EtO K solution
‡
proceed through a stepwise mechanism (e.g., DS = ca. −60 cal
were used to determine the kEtO− value from the slope of the linear
−
1
−1
24b
−
+
mol
K
for pyridinolysis of aryl furan-2-carbodithioates
plot of kobsd vs. EtO K concentration.
3
542 | Org. Biomol. Chem., 2007, 5, 3539–3543
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Products analysis
E. Buncel, Langmuir, 2006, 22, 9009–9017; (e) D. Churchill, J. C. F.
Cheung, Y. S. Park, V. H. Smith, G. W. vanLoon and E. Buncel,
Can. J. Chem., 2006, 84, 702–708; (f) V. K. Balakrishnan, E. Buncel and
G. W. vanLoon, Environ. Sci. Technol., 2005, 39, 5824–5830; (g) V. K.
Balakrishnan, X. Han, G. W. VanLoon, J. M. Dust, J. Toullec and E.
Buncel, Langmuir, 2004, 20, 6586–6593.
Y-substituted phenoxide was liberated quantitatively and identi-
fied as one of the reaction products by comparison of the UV–vis
spectra after the completion of the reactions with those of the
authentic samples under the same reaction conditions.
9 (a) L. D. Quin, A Guide to Organophosphorus Chemistry, Wiley, New
York, 2000; (b) T. G. Wu and J. U. Qiu, Environ. Sci. Technol., 2006, 40,
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