Mechanisms of abiotic degradation and soil-water interactions of pesticides and other hydrophobic organic compounds. Part 3. Nucleophilic displacement at the phosphorus centre of the pesticide fenitrothion [O,O-dimethyl O-(3-methyl-4-nitrophenyl) phosphorothioate] by oxygen nucleophil...

Article abstract of DOI:10.1039/b008615k

Rate constants for the reaction of the title compound, 1, with a number of oxygen nucleophiles, including structurally related phenoxides and oxygen-based α-nucleophiles, have been measured in aqueous solution at 25 deg C. A significant α-effect was observed, confirming participation of the nucleophile in the rate-limiting step of the reaction as well as indicating different transition states (TS) for the reaction of α-nucleophiles compared to normal ones. The Broensted-type correlation of log k_Nu vs. pK_a of nucleophiles shows a linear plot for the series of structurally related phenoxides in the pK_a range 5.4-10.0, straddling the pK_a of the leaving group (3-methyl-4-nitrophenoxide, pK_a 7.20), but is curved in the highly basic region corresponding to CF3CH2O^- and HO^- as nucleophiles. The slope of the linear portion of the plot (β_Nu) is 0.49 (R² 0.988). The linearity of the plot for the series of structurally related phenoxides is consistent with a concerted mechanism for nucleophilic attack at the P center of the substrate. A value of β_1g -0.39 (R² 0.973) is measured for the reaction of PhO^- with substituted phenyl dimethyl phosphorothioate esters. Combining the values of β_Nu and β_1g gives β_eq = 0.88; these parameters when considered together with the effective charge distribution in the TS, demonstrate that the TS for the symmetrical reaction (in which nucleophile = leaving group = 3-methyl-4-nitrophenoxide) has no significant phosphorylium ion character. The Leffler index points to a concerted reaction in which bond formation is slightly ahead of bound rupture in the TS. Data from the present study are compared with literature data for (thio)phosphoryl group transfer. We propose that, unless special structural and/or environmental features prevail, (thio)phosphoryl transfers between phenoxides are more likely to occur via a concerted mechanism. It is shown that the TS for the concerted transfer of (EtO)2P=O between two PhO^- moieties shows more pentacoordiante intermediate character than the symmetrical reaction of 1 due to differences in (i) basicity of PhO^- versus 3-CH3,4-NO2PhO^-, and (ii) abilities of O and S in the P=X (X = O, S) moiety to stabilize the incoming negative charge.

Full text of DOI:10.1039/b008615k

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Mechanisms of abiotic degradation and soil–water interactions
of pesticides and other hydrophobic organic compounds. Part 3.
Nucleophilic displacement at the phosphorus centre of the pesticide
fenitrothion [O,O-dimethyl O-(3-methyl-4-nitrophenyl) phosphoro-
thioate] by oxygen nucleophiles in aqueous solution: ꢀ-effect and
mechanism†
2
John E. Omakor,a Ikenna Onyido,*b Gary W. vanLoona and Erwin Buncel*a
a Department of Chemistry, Queen’s University, Kingston, Canada K7L 3N6.
E-mail: buncele@chem.queensu.ca
b Department of Chemistry and Center for Agrochemical Technology, University of Agriculture,
Makurdi, Nigeria
Received (in Cambridge, UK) 25th October 2000, Accepted 3rd January 2001
First published as an Advance Article on the web 29th January 2001
Rate constants for the reaction of the title compound, 1, with a number of oxygen nucleophiles, including
structurally related phenoxides and oxygen-based α-nucleophiles, have been measured in aqueous solution at
25 ЊC. A significant α-effect was observed, confirming participation of the nucleophile in the rate-limiting step
of the reaction as well as indicating different transition states (TS) for the reaction of α-nucleophiles compared
to normal ones. The Brønsted-type correlation of log kNu vs. pKa of nucleophiles shows a linear plot for the
series of structurally related phenoxides in the pKa range 5.4–10.0, straddling the pKa of the leaving group
(3-methyl-4-nitrophenoxide, pKa 7.20), but is curved in the highly basic region corresponding to CF3CH2OϪ
and HOϪ as nucleophiles. The slope of the linear portion of the plot (βNu) is 0.49 (R2 0.988). The linearity of
the plot for the series of structurally related phenoxides is consistent with a concerted mechanism for nucleophilic
attack at the P center of the substrate. A value of βlg Ϫ0.39 (R2 0.973) is measured for the reaction of PhOϪ with
substituted phenyl dimethyl phosphorothioate esters. Combining the values of βNu and βlg gives βeq = 0.88; these
parameters when considered together with the effective charge distribution in the TS, demonstrate that the TS for
the symmetrical reaction (in which nucleophile = leaving group = 3-methyl-4-nitrophenoxide) has no significant
phosphorylium ion character. The Leffler index points to a concerted reaction in which bond formation is slightly
ahead of bond rupture in the TS. Data from the present study are compared with literature data for (thio)phosphoryl
group transfer. We propose that, unless special structural and/or environmental features prevail, (thio)phosphoryl
transfers between phenoxides are more likely to occur via a concerted mechanism. It is shown that the TS for the
concerted transfer of (EtO) P᎐O between two PhOϪ moieties shows more pentacoordinate intermediate character
᎐
2
than the symmetrical reaction of 1 due to differences in (i) basicity of PhOϪ versus 3-CH3,4-NO2PhOϪ, and
(ii) abilities of O and S in the P᎐X (X = O, S) moiety to stabilize the incoming negative charge.
᎐
The present paper examines the nucleophilic reaction of a
Introduction
variety of oxygen nucleophiles at the P center of 1 in water at
25 ЊC. It is noted that a variety of species which can function as
oxygen nucleophiles abound in the aquatic, terrestrial and
atmospheric environment. This study also seeks to contribute
to ongoing discussion on the detailed mechanism of phos-
phoryl transfer reactions which occupy a central role in the
biochemistry of life processes.
Phosphates of nucleotides and other phosphorylated
compounds are energy-carriers in biological systems, while
phosphorylation of proteins and enzymes is crucial to control
mechanisms.3–5 Genetic engineering techniques are critically
dependent on the selective fission of phosphodiester bonds.5a
A new phosphoryl transfer process involving 1,3-diols has
been suggested, consequent upon the discovery of RNA
enzymes (ribozymes).3 As a consequence of the importance of
phosphate esters and phosphoryl transfer reactions in nature,
continuing interest has attached to elucidation of the mech-
anisms of nucleophilic displacements at P centers.5–9 Compar-
isons of data for acyl, carbonyl, sulfyl transfers and identity
as well as elimination reactions, with those of phosphoryl
While the vital role of organophosphorus and other classes of
pesticides in increasing the world’s food supplies over the last
five decades is firmly established,1a the possible toxicity to
humans and long-term environmental effects1b dictate that
thorough and in-depth studies of their degradation mech-
anisms be undertaken. To that end we have initiated studies
on the mechanism of abiotic degradation and soil–water
interactions of pesticides and other hydrophobic organic
compounds, with the ultimate goal of providing rate data to
aid in the assessment of the stability and persistence of these
compounds in the environment.
Fenitrothion, O,O-dimethyl O-(3-methyl-4-nitrophenyl)
phosphorothioate, 1, has found use as a broad spectrum
insecticide and acaricide,2a efficacious in controlling a range of
orchard, forest and field crop pests. It is also classified as an
anti-acetylcholinesterase neurotoxin.2b Hence the chemistry of
1 is important to a broad range of interests.
† For Part 2, see ref. 52.
324
J. Chem. Soc., Perkin Trans. 2, 2001, 324–330
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b008615k

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Table 1 Second-order rate constants (kNu) for the attack of several
oxygen nucleophiles at the phosphorus center of 1 in water at 25 ЊC
kNu/10Ϫ5 MϪ1 sϪ1 b
a
Entry
Nucleophile
pKa
1
2
3
4
5
6
7
8
HOϪ
15.74
12.4
11.6
9.99
9.44
9.38
7.72
6.70
279.0
44.7
59300
79.8
180.0
4.88
0.561
0.298
0.316
0.048
0.052
CF3CH2OϪ
HOOϪ
PhOϪ
OxϪ c
4-ClPhOϪ
4-CNPhOϪ
2,4,5-Cl3PhOϪ
9
2,3,5,6-F4PhOϪ
5.53
a Values of the pKa’s for the conjugate acids of the nucleophiles at
25 ЊC, obtained from the literature. b Each rate constant is the average
of duplicate runs with a deviation of 3%; data for 2,4,5-Cl3PhOϪ were
obtained by the initial rate method (see text and ref. 51) and are subject
to an uncertainty of 5%. c OxϪ = butane-2,3-dione monoximate.
There has been no report, to our knowledge, of a study of
nucleophilic displacement in phosphorothioate esters by a
series of structurally related phenoxides which forms the pivot
of the present paper. Such studies involving structurally related
phenoxides of pyridines have been reported for phosphates and
similar substrates.5d,7 Data from the present paper are subjected
to Williams’ method6,7 for elucidating mechanisms of nucleo-
philic displacements which has so far not been applied to
phosphorothioate substrates. The nucleophiles employed in this
study include some α-nucleophiles, with a view to ascertain-
ing whether the reaction of interest exhibits an “α-effect”.17
Observation of the α-effect is a satisfactory criterion for estab-
lishing the participation of the nucleophilic species in the
rate-limiting step of a bimolecular displacement reaction.17–19
Our results provide kinetic data for attack of oxygen-based
nucleophiles on the pesticide 1 and extend information available
for these processes at the P center. By comparison with existing
literature data, these results provide greater latitude for discuss-
ing (thio)phosphoryl transfers. Significantly, a concerted mech-
anism in which bond-making is slightly ahead of bond rupture
is established in the transition state for the symmetrical reaction
of 1.
transfers, continue to enhance our understanding of bond-
making and bond-breaking processes in organic/biochemical
reactions, especially in relation to what constitutes the rate-
limiting step and whether or not these processes are
concerted.5–8,10–12
Under certain reaction conditions, evidence exists for meta-
phosphate and metaphosphate-like intermediates in phos-
phoryl group transfers and related reactions.13,14 Jencks has
shown5d that the transition state for phosphoryl transfer to
anionic oxygen nucleophiles from pyridine is dissociative and
metaphosphate-like, showing less dissociative character with
increasing basicity of the oxygen nucleophile or the pyridine
leaving group. The solvolysis of the mono/dianion of chiral
p-nitrophenyl [18O,16O]phosphorothioate shows significant
racemization, thus implicating the formation of a free thio-
metaphosphate intermediate.15 Stereochemical evidence from
the nonenzymic hydrolysis of deoxyadenosine 5Ј-[β-17O]-β-
thiodiphosphate in H218O at pH 4.5 indicates that this process
also occurs with partial racemization.15d Alkoxyphosphoranes
and (aryloxy)phosphoranes, which are pentacoordinate phos-
phorus derivatives, and similar species have been demonstrated
as intermediates in phosphyl group transfers between strong
nucleophiles.13d,14b On the other hand, results from stereo-
chemical probes of mechanism16 and the work of Williams and
co-workers,6,7 the latter using Brønsted-type dependence on
nucleophile pKa and parameters derived therefrom as
criteria of mechanism, have demonstrated concertedness
in phosphoryl/phosphyl group transfers in which case both
nucleophile entry and leaving group departure occur in a single
transition state.
There is also the notion that whereas the formation of a
metaphosphate intermediate is facilitated by the absence of
competent potential acceptors, thiophosphate transfers in
solution could routinely occur via thiometaphosphate inter-
mediates.3b This view is predicated on the perceived greater
stability of thiometaphosphate over metaphosphate, which
receives experimental support in the gas phase detection of
the former20 and the isolation of trithiometaphosphate as its
tetraphenylarsonium salt.21 Rate and stereochemical differences
between nucleophilic substitution reactions of phosphates and
phosphorothioates have aided the inference of mechanistic
information regarding enzymic catalysis of phosphoryl trans-
fers.3 The recent work of Hengge3b on the hydrolysis of mono-
and dianions of p-nitrophenyl phosphorothioate, 2, provides
interesting and significant information on phosphorothioate
reactivity and mechanism of reaction.
Results
First-order rate constants, kobs, for the reaction of 1 with a
number of oxygen nucleophiles, to include the α-nucleophiles
hydroperoxide (HOOϪ) and butane-2,3-dione monoximate
(OxϪ), have been measured spectrophotometrically under
pseudo first-order conditions in water at 25 ЊC, with the nucleo-
philes in large excess over the substrate in each case. The corre-
sponding second-order rate constants were evaluated from
plots of kobs vs. [Nu], which were linear in all cases. The results,
together with pKa’s of the conjugate acids of the nucleophiles
employed, are presented in Table 1. A Brønsted-type plot of log
kNu vs. pKa of the conjugate acid of the nucleophile, using the
data of Table 1, is displayed as Fig. 1.
Eqn. (1) describes the reaction of 1 with nucleophiles; ko
defined in eqn. (2) corresponds to the intercept in the kobs vs.
[Nu] plot.
kobs = ko ϩ kNu[Nu]
(1)
(2)
Ϫ
Ϫ
ko = kH O ϩ kHO [HO ]
2
The kH O term may assume some significance for very weak
2
nucleophiles and at very low HOϪ concentrations. At high pH,
Ϫ
the intercept, if present, is dominated by the kHO term. These
situations apply provided the intercept is significant in
J. Chem. Soc., Perkin Trans. 2, 2001, 324–330
325

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Table 2 Second-order rate constants (klg) for the reaction of PhOϪ
with a series of substituted dimethyl phosphorothioates in water at
25 ЊC
a
Substrate
Leaving group
pKa
klg/10Ϫ5 MϪ1 sϪ1 b
3a
1
3c
3b
3-NO2-PhOϪ
8.36c
7.20d
6.46c
5.42c
4.22
8.23
24.1
54.9
3-CH3,4-NO2-PhOϪ
2-NO2,4-Cl-PhOϪ
2-Cl,4-NO2-PhOϪ
a Values for the pKa’s of the conjugate acids of the leaving groups,
obtained from the literature. b Estimated error 3%. c See ref. 5b. d See
ref. 22.
Fig. 1 Plot of log kNu vs. pKa (nucleophile) for the reaction of 1 with
oxygen nucleophiles in water at 25 ЊC.
Scheme 1
of p-nitrophenyl phosphate. By contrast, the reaction of
2-(2,4Ј-dinitrophenyl)-2-oxo-1,3,2-dioxaphosphinanes
with
various oxygen nucleophiles in water gives P–OAr fission
products only.19b Hydrolysis of 1 has been shown24 to proceed
by Me–O bond fission at low pH (р 7.5) and exclusively by way
of P–OAr fission at high pH (> 9).
The present study was undertaken in buffered systems in the
pH range 8.5–12.4. No products of Me–O fission were observed
by 31P NMR spectroscopy, consistent with the observation that
Me–O fission is insignificant in this region.24 The experimental
absorbances of 3-methyl-4-nitrophenoxide measured at infinite
reaction times for the reactions of entries 1–7 in Table 1 agreed
with theoretical values. For the nucleophiles 2,4,5-Cl3PhOϪ and
2,3,5,6-F4PhOϪ whose rates were determined by the initial rate
method, corresponding NMR experiments did not identify
products of Me–O or Ar–O fission. We conclude, therefore, that
the reaction of 1 with the nucleophiles listed in Table 1 proceeds
through P–OAr fission. It is noted, however, that the SNAr (Ar–
O fission) pathway is an important process in the aminolysis of
1.25
Fig. 2 Plot of log klg vs. pKa (leaving group) for the reaction of
PhOϪ with 1 and 3a–c in water at 25 ЊC. The line is defined by eqn. (4).
comparison to the associated experimental error. Significant
intercepts were observed in the plots of eqn. (1) for the nucleo-
philes CF3CH2OϪ, 4-ClPhOϪ, 4-CNPhOϪ, 2,4,5-Cl3PhOϪ and
OxϪ but not for HOϪ, HOOϪ, PhOϪ and 2,3,5,6-F4PhOϪ.
Polar substituent effects in nucleophilic attack at the P center
of 1 were evaluated by keeping the attacking nucleophile
(PhOϪ) constant while varying the leaving group by altering the
substituents on the leaving group phenyl moiety. Thus the rates
of the reaction of 1 and 3a–c with PhOϪ were investigated
under pseudo first-order conditions at 25 ЊC. The relevant
second-order rate constants are assembled in Table 2. Construc-
tion of a Brønsted-type dependence of log klg vs. pKa of the
conjugate acid of the leaving group results in the linear plot of
Fig. 2.
(ii) ꢀ-Effect
Data in Table 1 and Fig. 1 show that kNu values for the α-
nucleophiles HOOϪ and OxϪ are higher than the corresponding
ones for reactions involving normal O-nucleophiles of similar
pKa. One can estimate from Fig. 1 that HOOϪ and OxϪ behave
like nucleophiles with pKa values of ~ 17.5 and 12.7 rather than
their actual pKa values of 11.6 and 9.4, respectively. Rate
constant ratios comparing reactivities of α- and normal
nucleophiles, kα/kn, of 37 and 1300 for the pairs OxϪ/
4ClPhOϪ(pKa’s 9.44, 9.38) and HOOϪ/CF3CH2OϪ (pKa’s 11.6,
12.4), respectively, demonstrate the magnitude of the α-effect
observed in this study. The α-effect of 1300 observed for
the HOOϪ/CF3CH2OϪ pair conceivably incorporates solvation
factors at play in the reactivity of strongly basic anions noted
below.
A number of studies have demonstrated the α-effect in nucleo-
philic attack at the P centre.5c,7,17i,26,27 The magnitude of the
α-effect reported in pure water for oxygen nucleophiles reacting
at a P centre is 15 for the OxϪ/4-ClPhOϪ pair17i and in the range
of 10–200 for the HOOϪ/HOϪ pair.27 A value of 210 is calcu-
lated for the HOOϪ/HOϪ pair from data in Table 1, in good
agreement with the range reported previously.27 We note, how-
ever, that the magnitude of the α-effect is substrate-dependent.
Discussion
(i) Reaction pathways for nucleophilic attack on 1
Scheme 1 displays the three possible pathways for the attack of
nucleophiles on 1 and similar substrates. Fission of P–OAr and
Ar–O bonds, occurring at comparable rates, was observed by
Kirby and Younas19a in the reaction of methyl 2,4-dinitro-
phenyl phosphate monoanion with oxyanions in water; the
latter reaction is an SNAr displacement. The SNAr (Ar–O
fission) pathway has also been demonstrated23 in the aminolysis
326
J. Chem. Soc., Perkin Trans. 2, 2001, 324–330

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Although a consensus on the origin of the α-effect is
lacking,5c,28 it is considered that the transition-state structure
for reactions of the α-nucleophile is very different from that for
the reactions of the normal nucleophile.5c,17j The inference that
can be drawn from the present results is that the reaction of 1
with nucleophiles proceeds through either a two-stage mech-
anism with rate-limiting formation of the pentacoordinate
intermediate or a one-step concerted mechanism, as shown in
Scheme 2. The alkaline hydrolysis of Ph2P(S)–OPhX correlates
and the monoanion of MeO(OH)P(O)–OPhX,34 respectively,
in water. Clearly, these latter substrates are more sensitive to
polar effects and exhibit greater leaving group separation in
the TS of their reactions with phenoxides than 1. The value of
βlg = Ϫ0.39 compares with Ϫ0.30 obtained for the hydrolysis
of Me2P(S)–OPhX35 in water at 25 ЊC, and lies within the
range of βlg = Ϫ0.30 to Ϫ0.50 observed35,36 for the hydrolysis of
Me2P(S)–OPhX, MePhP(S)–OPhX, and Ph2P(S)–OPhX in
50% ethanol at 25 ЊC. These latter P᎐S substrates studied
᎐
by Istomin and co-workers,35,36 as well as 1, appear to exhibit a
lesser degree of bond cleavage in the TS of their reactions with
phenoxides than the P᎐O substrates reported by Williams and
᎐
co-workers.7,34 The values of βNu and βlg combined with βeq for
the P᎐O substrates studied by Williams and co-workers reveal
᎐
a concerted mechanism in which there is (i) more advanced
bond cleavage than bond-making in the TS for Ph2P(O)–OPhX,
such that the Ph2P(O) group acquires a positive character;7b
(ii) a TS for (EtO)2P(O)–OPhX that has more pentacoordinate
intermediate character than the reaction of Ph2P(O)–OPhX;7a,34
and (iii) a slight charge imbalance in the TS which results in
some phosphorylium ion character for (PhO)2P(O)–OPhX.7a
By contrast, Hollfelder and Herschlag37 found a value of
βlg = 1.1 for the hydrolysis of O-aryl phosphorothioate
dianions, similar to the value of Ϫ1.2 reported for their phos-
phate ester counterparts;38 this suggests a very small amount of
P–O bonding in the TS. Such a TS is dissociative in character,
resembling the (thio)metaphosphate intermediate. The salient
point here is that the βlg value for 1 differs radically from that
reported for O-aryl phosphorothioate dianions.37 Comparing
Scheme 2
well with Hammett σ-values which suggests that formation of a
pentacoordinate intermediate is rate-determining;29 however,
no definitive exclusion of an alternative concerted mechanism
with an early TS and minimal leaving group separation was
achieved in that system.
(iii) Brønsted-type correlations: ꢀNu and ꢀlg values
Brønsted-type correlations probe the properties of the transi-
tion states of reactions,30–33 and have proved an invaluable tool
for elucidating the mechanisms of nucleophilic displacements
at various electrophilic sites.5–11 For a series of structurally
related nucleophiles, particularly those with the same atom at
the nucleophilic site, Brønsted βNu and βlg values are useful in
estimating transition structures and evaluating their response to
changing substitution patterns and reaction conditions.6,31
The traditional Brønsted-type plot of log kNu vs. pKa
depicted in Fig. 1 can now be discussed. Inspection of Fig. 1
reveals that the plot is linear for nucleophiles with pKa < 11
while curvature at higher pKa’s is clearly evident. The curvature
at high pKa is consistent with Jencks’ observation5e,11a that the
reactivity of HOϪ and strongly basic alkoxides is character-
istically low due to abnormal solvation of these anions relative
to the more weakly basic phenolate anions, rather than a
change in the rate-determining step.
P᎐O and P᎐S substrates, we note that the β value for 1 is
᎐
᎐
Nu
similar to the values reported for (PhO)2P(O)–OPhX7a and
Ph2P(O)–OPhX,7b while the βlg value for 1 differs substantially
from the values obtained for (PhO)2P(O)–OPhX,7a Ph2P(O)–
OPhX,7b and the monoanion of MeO(OH)P(O)–OPhX34 but
is in the same range as βlg values for Me2P(S)–OPhX,35
MePhP(S)–OPhX,35,36 and (EtO)2P(O)–OPhX.34
(iv) Mechanism, effective charge distribution and transition-state
structure
A mechanism involving a distinct thiometaphosphate inter-
mediate is untenable in the present case since the observation
of an α-effect implies participation of the nucleophile in the
rate-limiting step of the reaction. The linear portion of the plot
in Fig. 1 for the phenoxides shows that their reaction with 1
proceeds by a concerted [SN2(P) type] mechanism with a single
transition state (see Path B, Scheme 2). Although the data do
not unequivocally rule out the presence of a highly reactive
pentacoordinate intermediate lying in a shallow well at the
energy maximum in the reaction coordinate,7 we will show
below, using the reaction map of Fig. 3, that such an inter-
mediate would be too unstable to have any real existence.
If the points for HOϪ and CF3CH2OϪ are excluded, it is seen
that requirements for a valid Brønsted-type plot, i.e. structural
similarity of nucleophiles and a pKa range that straddles
the basicity of the leaving group, are satisfied by the group of
phenoxides listed in Table 1. The solid line defined by the
phenoxides up to PhOϪ (pKa 10.0) is given by eqn. (3) from
The effective charge on the entering and leaving O atoms
in the TS of a concerted reaction can be determined from
the values of βeq, defined according to eqn. (5), βNu, and βlg,
log kNu = (0.49 0.01)pKa Ϫ 8.94 0.04
(3)
which βNu = 0.49 0.01 (R2 0.988) is obtained. The value of
0.49 for βNu can be compared with βNu = 0.53 and 0.46 obtained
from the reaction of phenoxides with 4-nitrophenyl diphenyl
phosphate (PhO)2P(O)–OPhX,7a and 4-nitrophenyl diphenyl-
phosphinate Ph2P(O)–OPhX,7b respectively in aqueous solution
at 25 ЊC; these reactions have been unambiguously shown to
proceed via the concerted mechanism.
Eqn. (4) defines the straight line obtained from the plot of
log klg vs. pKa (Table 2, Fig. 2), giving βlg = Ϫ0.39 0.04 (R2
0.973). This can be compared with βlg values of Ϫ0.87, Ϫ0.79,
βeq = βNu Ϫ βlg
(5)
according to the methodology of Williams6,7 and Jencks.5b–d
Two conditions are required to establish the validity of eqn. (5):
(i) reactions yielding the values of βNu and βlg must be the
microscopic reverse of each other; and (ii) the same rate-
limiting step is involved in measurement of both quantities. We
believe the system under study satisfies both requirements. The
effective charge on O of the attacking nucleophile and O of the
leaving group in going from the ground state to the TS,
∆ε(R→TS), is taken to be equal to βNu (0.49) and βlg (Ϫ0.39),
respectively. The charge imbalance of ϩ0.10 between attacking
and leaving O (see structure 4) is probably balanced by Ϫ0.10
charge unit from the thiophosphoryl group in order to maintain
log klg = (Ϫ0.39 0.04)pKa Ϫ 1.16 0.08
(4)
Ϫ0.51, and Ϫ0.64 obtained for the reaction of PhOϪ with
(PhO)2P(O)–OPhX,7a Ph2P(O)–OPhX,7b (EtO)2P(O)–OPhX,34
J. Chem. Soc., Perkin Trans. 2, 2001, 324–330
327

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above. What is intended here is to situate the findings of the
present study alongside trends in the literature in order to high-
light the incremental contribution of this work to the discussion
of nucleophilic reactivity at P centers.
The hydrolysis of Me2P(S)–OPhX,35,36 MePhP(S)–OPhX,35
and Ph2P(S)–OPhX11d correlates satisfactorily with σ or σЊ
rather than σϪ values while βlg values lying in the range of
Ϫ0.30 to Ϫ0.50 have been reported.35,36 These observations
present a mechanistic ambiguity. Correlation with σ or σЊ rather
than σϪ values suggests a stepwise mechanism in which form-
ation of the pentacoordinate intermediate is rate-limiting. On
the other hand, βlg values of Ϫ0.30 to Ϫ0.50 are consistent with
little-to-moderate separation of the leaving group in the TS of a
concerted reaction. It is noted that a strongly basic nucleophile
and a strongly basic (i.e. very poor) leaving group could con-
ceivably move the TS towards the top left-hand corner of Fig. 3.
Further experimentation involving Brønsted-type correlations
of the type reported in this paper could help clarify issues
regarding these substrates.
Activation parameters, solvent effects and divalent metal
ion effects on the hydrolysis of the mono/dianions of 2 have
been reported; the results implicate a thiometaphosphate
intermediate.3b It is possible that the unfavorable electrostatic
interactions between the nucleophile and anionic forms of 2,
and assistance of leaving group expulsion by the oxyanions in
2, combine to move the TS towards the bottom right-hand
corner of Fig. 3, away from the pentacoordinate intermediate
and concerted pathways. Both of these effects are absent with 1.
A wide spectrum of mechanistic possibilities exist in phos-
phoryl group transfers. Reaction via the metaphosphate
intermediate occurs under certain conditions;13a–c the presence
of the pentacoordinate intermediate when the transfer is
between strong nucleophiles13e,14b,41,42 and reaction via the con-
certed mechanism7,16,40,41 have been variously demonstrated.
Fig. 3 Reaction map for the transfer of the dimethylthiophosphoryl
group between nucleophile and aryloxide leaving groups. Bond form-
ation and bond fission are measured by αbf = βNu/βeq and αbr = βlg/βeq,
respectively (see text). The point J represents the position of the TS for
the symmetrical reaction in which nucleophile = leaving group = 3-
methyl-4-nitrophenoxide.
overall charge neutrality. βeq in the present system is ϩ0.88; this
represents the overall charge change on the nucleophile and
leaving group for the overall reaction. In the ground state, there-
fore, the phenoxy O will have a charge of Ϫ0.12. The effective
charge on the nucleophilic O in the TS, εTS, is given by eqn. (6),
P᎐O substrates reported in the literature which bear the closest
᎐
εTS = βNu ϩ εR
(6)
structural resemblance to 1 and which have been shown to react
with phenoxides are (EtO)2P(O)–OPhX,7a,34 (PhO)2P(O)–
OPhX,7a and (to a certain extent) Ph2P(O)–OPhX.7b Roughly
speaking, the same extent of bond-making occurs in the TS of
the concerted reaction of 1 as in these substrates. Conversely,
these substrates show greater P–O bond cleavage than 1. We can
in which εR is the charge on the nucleophile in the ground state
which is Ϫ1.0; for the present system, εTS = Ϫ0.51.
The Leffler index,32 αЈ, measures the extent of bonding in the
TS for any particular bond.‡ The extent of bond formation,
αЈbf = βNu/βeq, and the extent of leaving group bond rupture,
αЈbr = βlg/βeq, are calculated to be 0.56 and Ϫ0.44, respectively.
These values of αЈbf and αЈbr locate the position of the TS in the
reaction map (Fig. 3) as J along the tightness diagonal39 or
disparity reaction,31 only slightly displaced from the inter-
section of the synchronous route and the tightness diagonal,
thus confirming that the reaction under consideration is a con-
certed one in which bond formation is only slightly advanced
over bond rupture in the TS. This result is inconsistent with the
presence of a discrete pentacoordinate intermediate, since such
an intermediate will be too unstable to have any real existence at
this point of the energy surface. Structure 4 shows the charge
distribution for the symmetrical reaction with 3-methyl-4-
nitrophenoxide as entering and leaving group, for which it
is required that the effective charge on the O atom of the
nucleophile and leaving group, as well as the extent of bonding
to the nucleophile and the leaving group, be identical to the TS.
It is clear that 4 possesses very little or no phosphorylium
character; on the other hand the results point to a concerted
mechanism with a TS only slightly on the associative side as in
Fig. 3.
speculate, within the scope of literature data available, that P᎐O
᎐
and P᎐S substrates are more likely to react with phenoxides via
᎐
a concerted mechanism, unless special structural and/or
environmental circumstances dictate otherwise. There could,
however, be wide variations in extents of bond-making and
bond cleavage, looseness or tightness of the TS, greater
resemblance of the TS to a metaphosphate or pentacoordinate
intermediate, depending on the specific substrate and reaction
conditions.
P᎐O substrates are more reactive than their P᎐S counterparts
᎐
᎐
by two or more orders of magnitude.43 This order of reactivity
has been ascribed to one or more of the following: (i) poorer
electron-donating ability of the thioanion relative to the oxy-
anion in expelling the leaving group;11d (ii) the superior ability
of O over S to stabilize the developing positive charge at the P
center;11d (iii) lower electrophilicity of P in P᎐S relative to P᎐O
᎐
᎐
due to electronegativity differences, favouring O over S;43 (iv)
differences in the degree of polarization of thiophosphoryl and
phosphoryl bonds, favouring the latter;43c and (v) structural
differences between P᎐O and P᎐S moieties, to include inter-
᎐
᎐
atomic distances and van der Waals radii.44 The absence of
significant phosphorylium ion character in the TS of the con-
certed reaction of 1 with phenoxides provides support for the
(v) Comparison with some P᎐S and P᎐O systems
᎐
᎐
Part of the comparison of the results of the present study with
relevant P᎐S and P᎐O systems has already been undertaken
low electrophilicity of P in P᎐S substrates.
᎐
The concerted transfer of (EtO)2P(O) between two PhOϪ
moieties shows significantly greater pentacoordinate inter-
mediate character than the symmetrical reaction of 1. Assuming
that the electronic effects of ethoxy and methoxy groups on
(thio)phosphoryl transfer are roughly the same, then the differ-
᎐
᎐
‡ The symbol αЈ is used here for the Leffler index so as to avoid duplic-
ation with α-effect; however it is noted that the original notation for the
Leffler index was in fact α.
328
J. Chem. Soc., Perkin Trans. 2, 2001, 324–330

View Article Online
ence in the TS structures of the symmetrical reactions of 1 and
(EtO)2P(O)–OPhX can be ascribed to the higher basicity of
Pseudo first-order rate constants, kobs, were obtained by the
least squares method as slopes of plots of 1n(Aoo Ϫ At) vs. time
which showed excellent linearity over 3 half-lives in all cases.
The relevant second-order rate constants, kNu, were obtained by
application of eqn. (1). In the case of the nucleophiles 2,4,5-
Cl3-PhOϪ and 2,3,5,6-F4-PhOϪ (see entries 8 and 9 in Table 1)
whose rates of reaction were very slow, rates were followed up
to ca. 10% of the total reaction and the second-order rate con-
stants were determined by the initial rate method;51 the reaction
medium was not maintained at constant ionic strength7a in such
cases. For all the reactions, initial repetitive scans were first
carried out prior to the kinetic determination in order to obtain
the appropriate wavelength for monitoring the appearance of
the product 3-methyl-4-nitrophenoxide, 398 nm in all cases, as
well as to ensure the occurrence of isosbestic points which
exclude the presence of any long-lived intermediate with signifi-
cant absorption along the reaction pathway.
PhOϪ over 3-CH3,4-NO2PhOϪ and the differences in P᎐O and
᎐
P᎐S moieties, in terms of stabilization of the incoming negative
᎐
charge and associated effects noted above.
Experimental
Materials
Reagents and solvents were commercial products used as
received unless otherwise noted. Stock solutions of substrates
were prepared under N2 and stored in volumetric flasks which
were sealed with rubber septa, wrapped with aluminium foil
and kept in a refrigerator. Melting points (uncorrected) were
obtained with a Thomas-Hoover mp apparatus. An Accumet
825 MP pH meter with a glass electrode calibrated with
standard buffer solutions was used to measure pH. NMR
spectra were obtained on a Bruker 400 spectrometer operating
at 400 MHz. Kinetic experiments were performed on one of
the following UV–visible spectrophotometers: Varian Cary 3
double beam, Hewlett Packard 8452A diode ray or Perkin-
Elmer Lambda-5. Water was distilled, deionized, degassed and
filtered through a 0.22 µm filter. Commercial 1,4-dioxane was
distilled from anhydrous stannous chloride to remove peroxides,
followed by redistillation from sodium ribbon. Hydrogen
peroxide was standardized by the persulfate method.45
Phenol, 4-chloro- and 4-cyanophenol were recrystallized from
petroleum ether (30–60), while butane-2,3-dione monoxime was
recrystallized from chloroform. NaOH was purified according
to the method of Perrin and Armarego.46
Acknowledgements
This paper is dedicated to W. P. Jencks. We thank the Natural
Sciences and Engineering Research Council of Canada (E. B.)
and the Canadian International Development Agency (I. O.
and J. E. O.) for supporting this work.
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