Reactivity of Nitroalkyl Anions Addition to Substituted Benzylidenecyanoacetates: Electrophilicity Parameters and Free Energy Relationships

Article abstract of DOI:10.1002/kin.21132

The kinetics of the reactions of benzylidenecyanoacetates 1a–d (X = H, Me, OMe, and NMe₂) with nitroalkyl anions 2a–c have been studied in aqueous solution at 20°C. The second-order rate constants are used to evaluate the electrophilicity param

Full text of DOI:10.1002/kin.21132

Reactivity of Nitroalkyl
Anions Addition to
Substituted
Benzylidenecyanoacetates:
Electrophilicity Parameters
and Free Energy
Relationships
K. AZAIEZ, N. DHAHRI, T. BOUBAKER
Laboratoire C.H.P.N.R, Faculte´ des Sciences de Monastir, Universite´ de Monastir, 5019, Monastir, Tunisie
Received 11 July 2017; revised 18 September 2017; accepted 19 September 2017
DOI 10.1002/kin.21132
Published online 26 October 2017 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The kinetics of the reactions of benzylidenecyanoacetates 1a–d (X = H, Me, OMe,
and NMe2) with nitroalkyl anions 2a–c have been studied in aqueous solution at 20°C. The
second-order rate constants are used to evaluate the electrophilicity parameter E of these series
of Michael acceptors 1a–d according to the linear free enthalpy relationship log k (20°C) = sN
(E + N). The measured E values were found to cover a domain of reactivity, ranging from −10.07
for the most reactive electrophile 1a (X = H) to −14.04 for the less electrophile 1d (X = NMe2).
Mayr’s approach was found to correctly predict the rate constants for the reactions of these
series of olefins 1a–d with the hydroxide ion in water and 50% water–50% acetonitrile at 20°C.
Analysis of the kinetic measurements using the Bro¨nsted relationship shows that βnuc values
remain remarkably constant for changes in the nature of the substituent X. A notable finding of
this work is perhaps provided by the observed large changes in the electrophilicity parameter
E on going from benzylidenecyanoacetates 1 to their analogues benzylidenemalonates 3 (ꢀE
ꢀ 9.06–10.48), whereas the replacement of second CO2Et group by the CN group in 1 to give
benzylidenemalononitriles 4 has little effect on electrophilic reactivity, i.e., ꢀE ꢀ 0.65–0.95.
Correspondence to: Taoufik Boubaker; e-mail: boubaker
taoufik@yahoo.fr.
Supporting Information is available in the online issue at
www.wileyonlinelibrary.com.
ꢀC
2017 Wiley Periodicals, Inc.

848
AZAIEZ, DHAHRI, AND BOUBAKER
This abnormal pattern in the E values has been attributed to the resonance interaction and
salvation effects. On the other hand, the effect of benzylidenecyanoacetate substituents on
the electrophilic reactivity was examined quantitatively on the basis of the electrophilicity
parameter E, leading to linear correlation of E with Hammett–Brown substituent constants
(σp+). More importantly, the four electrophiles have comparable log ko values, whꢀiCch are located
at a relatively low level, i.e., log ko ࣘ 4–5, in the intrinsic reactivity scale.
2017 Wiley
Periodicals, Inc. Int J Chem Kinet 49: 847–858, 2017
allows the quantification of the electrophilicity param-
eter E of these series of Michael acceptors 1a–d and the
comparison of these E values with that of structurally
similar olefins.
INTRODUCTION
In recent years, Mayr and co-workers have paid par-
ticular attention to the reactivity of ordinary Michael
acceptors [1–3]. These represent an another class of
neutral electrophiles whose C-electrophilicity can be
quantitatively described by the linear free enthalpy re-
lationship (1) [4], where the E parameter measures
the strength of the electrophile, k is a second-order
rate constant, whereas N and sN characterize the nucle-
ophiles reactivity:
We have also succeeded in obtaining new quantita-
tive information on the electrophilic behavior of elec-
trophiles 1a–d in looking at the interaction of these
electrophiles with the hydroxide anion in water and
50% water–50% acetonitrile at 20°C. The effect of ni-
troalkyl anions 2a–c basicity on the reactivity of ben-
zylidenecyanoacetates will also be discussed. Interest-
ingly, analysis of the results in terms of the Marcus ap-
proach reveals that our series of benzylidenecyanoac-
etates 1a–d exhibits lower intrinsic reactivities.
log k (20◦C) = sN (E + N)
(1)
Continuing the experimental studies on the quan-
tification of electrophilicity parameter E of Michael
acceptors, we have recently reported a kinetic study
of the nucleophilic additions of the hydroxide an-
ion to benzylidenemalononitriles 4 in 50% water–
50% acetonitrile [5]. The most significant finding
of this work was the observation that the most re-
active structures studied, i.e. the electrophiles 4 ac-
tivated by electron acceptors (X = NO2, CN, and
CF3), have an intrinsic electrophilic reactivity, which
is similar to that of such highly electrophilic cationic
structures as the benzhydrylium ion 5 [6] and the 4-
methylbenzenediazonium 6 [7].
EXPERIMENTAL
Materials
Commercially (Aldrich) available products such as
nitromethane, nitroethane, and 2-nitro-propane were
used as received. In aqueous solutions, all nitroalkyl
anions 2a–c were generated by the treatment of the
corresponding acids with KOH.
The benzylidenecyanoacetates 1a–d used in this
work were prepared according to literature procedures
(Scheme 2) [9].
E = -7.70
E = -7.69
+
+
N2
o
o
CN
C
CN
(CH2)4N
N(CH2)4
Me
X
4
5
6
X
E
NO2 -7.55
CN -7.80
CF3 -7.90
To include other Michael acceptors into the com-
prehensive electrophilicity scale of Mayr [8], we have
synthesized a series of benzylidenecyanoacetates 1a–d
and investigated the kinetics of their reactions with a
three nitroalkyl anions 2a–c according to Scheme 1 in
aqueous solution at 20°C. As will be seen, this study
Kinetic Measurements
Kinetic measurements pertaining to the Michael ac-
ceptor/nitroalkyl anion couplings were performed
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REACTIVITY OF NITROALKYL ANIONS ADDITION TO SUBSTITUTED BENZYLIDENECYANOACETATES
849
CR1R2NO2
CO2Et
CO2Et
o
R2
NO2
o
Water / 20 °C
+
CN
CN
R1
X
X
2
1
7
1a X = H
1b X = Me
1c X = OMe
1d X = NMe2
2a R1 = H, R2 = H
2b R1 = H, R2 = CH3
2c R1 = CH3, R2 = CH3
Scheme 1 Reaction of the nitroalkyl anions 2a–c with the benzylidenecyanoacetates 1a–d.
O
CO2Et
H
CN
Toluène
H
C
+
CN
CH3CO2H / piperidine
H
CO2Et
X
X
1
(1a) : X = H, (1b) : = Me
(1c) : X = OMe, (1d) : X = NMe2
Scheme 2 Synthesis of the benzylidenecyanoacetates 1a–d.
3 × 10−5 mol L−1 concentration of Michael acceptor
1 and a large excess of the nitroalkyl anions 2a–c (6 ×
10−4 to 6 × 10−2 mol L−1). On the other hand, the UV–
visible spectroscopy analysis at different times shows
a clear isobestic point, evidencing the cleanness of the
reactions and the fact that the rising absorption is due to
the absorbance of the adduct 7. An illustrative example
is given in Fig. 1, which shows the set of UV–visible
absorption spectra describing the progressive conver-
sion of 1c to the product 7ca (X = OMe and R1 = R2
= H). All the reactions studied in the present work fol-
lowed a simple rate equation given by Eq. (2), where
k is the second-order rate constant for the nitroalkyl
anion 2 addition to the benzylidenecyanoacetate 1.
on a conventional spectrophotometer (Shimadzu;
UV–visible model 1650). The thermoelectrically
temperature-controlled cell holder (model TCC-240 A)
was maintained at 20 0.1°C. All kinetic runs were
carried out in triplicate under pseudo–first-order con-
ditions with a concentration of electrophile 1 of ca. 5 ×
10−5 mol L−1 and nitroalkyl anion 2 concentrations in
the range 6 × 10−4 to 6 × 10−2 mol L−1. The kobsd val-
ues were determined from the slope of the plot of ln (Aꢁ
– At) versus time. In a given experiment, the rates were
found to be reproducible to 3–5%. Correlation co-
efficients of the linear regressions were usually higher
than 0.99. Measured first-order rate constants kobsd ob-
tained at nitroalkyl anion concentrations are given for
all substrates in Tables S1–S3 in the Supporting In-
formation, together with the derived second-order rate
constants.
kobsd = k [Nitroalkyl anion]
(2)
The kinetic results are demonstrated in Fig. 2 for the
reactions of electrophiles 1a–d with 2-nitro-propane
anions 2c. The second-order rate constants, k, have
been determined from the slope of the linear plots
of kobsd versus [nitroalkyl anion] and summarized in
Table I, together with the pKa values of the conjugate
acids [10] and the nucleophilicity parameters N and
sN [11] of nitroalkyl anions 2 in aqueous solutions.
RESULTS AND DISCUSSION
Kinetics of the Reactions of
Benzylidenecyanoacetates 1a–d with
Nitroalkyl Anions 2a–c in Aqueous Solution
The nucleophilic addition of a series of nitroalkyl an-
ions 2a–c to benzylidenecyanoacetates 1a–d has been
kinetically studied in aqueous solution at 20°C. The re-
actions were followed spectrophotometrically by mon-
itoring the formation of the adducts 7 at their ab-
sorption maxima (238–352 nm). All experiments were
carried out under pseudo–first-order conditions with a
Electrophilicity Parameters for
Benzylidenecyanoacetates 1a–d
To investigate the effect of nucleophilicity in
the structure of nitroalkyl anions on reactivity,
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AZAIEZ, DHAHRI, AND BOUBAKER
1,5
CO2E t
CN
MeO
1c
CH2NO2
CO2E t
t = 0
1
o
CN
MeO
7ca
0,5
0
250
300
350
400
450
Wavelengh (nm)
Figure 1 Time dependence of the electronic absorption spectrum of electrophile 1c (5 × 10−5 mol L−1) in the presence of
nitromethane anion 2a (4 × 10−3 mol L−1) in aqueous solution at 20°C. [Color figure can be viewed at wileyonlinelibrary.com]
0,03
CO2Et
1a
X = H
CN
X
1
1c
X = OMe
0,02
1b
X = Me
0,01
1d
X = NMe2
0
0
0,02
0,04
0,06
0,08
-1
[2-nitro-propane anion] (mol L )
Figure 2 Influence of the concentration of the 2-nitropropane anion 2c on the observed pseudo–first-order rate constants kobsd
for addition to Michael acceptors 1a–d in aqueous solution at 20°C.
Mayr’s plots were constructed for the reactions of
benzylidenecyanoacetates 1a–d with the series an-
ions 2a–c in aqueous solution. As can be seen
in Fig. 3, excellent parallel straight lines with
a correlation coefficients >0.9977 are obtained
on plotting (log k)/sN versus nucleophilicity pa-
rameter N of nitroalkyl anions 2 (see Table I)
[11].
International Journal of Chemical Kinetics DOI 10.1002/kin.21132

REACTIVITY OF NITROALKYL ANIONS ADDITION TO SUBSTITUTED BENZYLIDENECYANOACETATES
851
Table I Values of Second-Order Rate Constants (k) for Reactions of Michael Acceptors 1a–d with Nitroalkyl Anions
2a–c in Aqueous Solution at 20°C
k c (mol−1 L s−1
)
a
b
Nucleophile
N/sN
pKa
1a
1b
1c
1d
NO2
H
o
12.06/0.53
11.25/0.52
10.69/0.56
10.22
8.60
7.74
1.07 × 101
4.05
1.67
1.05 × 10−1
H
2a
H
NO2
o
CH3
4.32
2.13
1.48
6.05 × 10−1
3.02 × 10−1
4.14 × 10−2
1.77 × 10−2
2b
NO2
H3C
o
7.14 × 10−1
CH3
2c
aNucleophilicity parameters N and sN from 11.
bpKa values were taken from 10.
cThese rate constants were derived in this work from the slope of plots of kobsd versus [nitroalkyl anion] (see Fig. 1 and Figs. S1 and S2 in
the Supporting Information).
4
NO2
H3C
o
3
2
NO2
H
o
CH3
2c
NO2
H
o
CH3
1a
1b
1c
2b
H
2a
1
0
-1
-2
-3
-4
1d
10,5
11
11,5
Nucleophilicity N
12
Figure 3 Plots of (log k (H2O, 20°C))/sN for the reactions of the Michael acceptors 1a–d with the reference nitroalkyl anions
2a–c versus their nucleophilicity parameters N. The identity of numbers is given in Table I.
Interestingly, these lines have nearly identical slopes
(in the range 0.94–1.03), indicating that Eq. (1) can be
employed for determining the electrophilicity param-
eters E of these family of Michael acceptors 1 be-
cause application of Mayr’s approach implies slopes
of 1.00 for the (log k)/sN versus N [12–16]. On this
basis, the values of the electrophilicity parameters E of
1 have been evaluated from the intercepts of these lines
International Journal of Chemical Kinetics DOI 10.1002/kin.21132

852
AZAIEZ, DHAHRI, AND BOUBAKER
E
E ~ 0.65 - 0.95
E ~ 9.06 - 10.48
CO2Et
CO2Et
CN
CN
CO2Et
CN
-8
X
X
X
1
4
3
H
-9,42
-9,98
Me
-10
-12
H
Me
-10,07
-10,93
OMe
-10,81
-11,65
OMe
-13,29
NMe2
-14
-16
-18
-20
-22
-24
NMe2
-14,04
-20,55
H
Me
-21,11
-21,47
OMe
NMe2
-23,10
Figure 4 Comparison of the electrophilic reactivities of benzylidenecyanoacetates 1 with the benzylidenemalonates 3 and
benzylidenemalononitriles 4.
with the abscissa axis. The reactivities thus obtained
in the present study of all benzylidenecyanoacetates
1a–d are found in a relatively small range (¯10.07
investigated by Dhahri and co-workers [5], reveals that
the replacement of a first CN group in each of the four
electrophiles 3 by a CO2Et group to give 1 results in a
decrease of 0.65–0.95 E unit in the electrophilic reac-
tivity. This is considerably less than that brought about
by the substitution of a CN group by a second CO2Et
group into 1 to form benzylidenemalonates 3a–d [1a]
(ꢀE ꢀ 9.06–10.48). Such behavior can, at least in part,
be understood in terms of an increased stabilization of
the ground state of benzylidenemalonates 3 through
resonance interactions between the electron-donating
substituent and the two carbonyl groups (resonance
structure A) compared to the Michael acceptors 1 (res-
onance structures B and C).
> E > 14.04). We note in particular that our elec-
¯
trophile 1d (X = NMe2) is the less electrophile in the
series (E = −14.04), which is approximately one order
of magnitude smaller to that of 1,3,5-trinitrobenzene
(E = −13.19) [17], the common reference aromatic
electrophile in nucleophilic addition or substitution
processes [18–22].
Comparison of the data obtained for benzyli-
denecyanoacetates 1a–d (Fig. 4) with those of ¯9.42 >
E > ¯13.29 reported for benzylidenemalononitriles 4a,
4c, and 4d by Lemek and Mayr [2] and 4b previously
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REACTIVITY OF NITROALKYL ANIONS ADDITION TO SUBSTITUTED BENZYLIDENECYANOACETATES
853
Table II Comparison between Experimental and Calculated Second-Order Rate Constants, k, for the Reactions of
Michael Acceptors 1a–d with the Hydroxide Ion in H2O and 50% H2O–50% MeCN at 20°C
b
Michael acceptor E parametera
OH− N /sN (solvent)
kexpc (M−1 s−1
)
kcalcdd (M−1 s−1
)
kexp/kcalcd
1a
1b
1c
1d
−10.07
−10.93
−11.65
−14.04
−10.07
−10.93
−11.65
−14.04
10.47/0.6 (H2O)
1.23 × 101
1.75
7
3.95
5.24 × 10−1
1.90 × 10−1
6.64 × 10−3
7.5
10.3
8.4
1.97
5.58 × 10−2
1a
1b
1c
1d
10.19/0.62 (50% H2O–50% MeCN)
4.77
1.87
1.18
4
3.48 × 10−1
1.24 × 10−1
4.10 × 10−3
5.4
5.6
3.8
7.01 × 10−1
1.56 × 10−2
aPresent work.
bNucleophilicity parameters are taken from 23.
cPresent work.
dValues calculated with Eq. (1) from E, sN, and N parameters listed in this table.
OEt
OEt
O
O
o
+
o
O
X
EtO
O
X
X
EtO
Et
Et
O
Et
3
A
O
O
Oo
O
O
CNo
CN
CN
+
o
+
o
X
X
B
C
1
Plots of log kexp versus log kcalc result in two paral-
lel lines exhibiting slopes equal or very close to unity
(see Fig. S3 in the Supporting Information). Tables
III summarizes the slope and intercept values that we
have obtained in the present study and compares these
data with the analogous parameters that have previ-
ously been reported for the reactions involving benzyli-
denemalononitriles 4 with the hydroxide ion in 50%
water–50% acetonitrile [5] and benzoselenadiazolium
cation 8 with secondary amines in acetonitrile [27].
As can be seen, two important features are emerging
from these results: First, all the slopes of these cor-
relations are remarkably close to unity, and, second,
the intercepts are different and seem to strongly de-
pend on the structure of the nucleophile and/or of the
electrophile implied in the reaction. This brings up the
questions of the physical meaning of these intercepts
and of how to relate this intercept to an already known
constant.
Comparison of Predictions Rate Constants
with Experimental Results
In Table II, we have summarized predictions based
on the Mayr’s approach [4] of the second-order rate
constants (kcalcd) associated with the reactions of ben-
zylidenecyanoacetates 1a–d with the hydroxide ion in
water and 50% water–50% acetonitrile at 20°C, us-
ing the electrophilicity parameters E of the four elec-
trophiles 1a–d determined in this work and the pre-
viously published nucleophilicity parameters N and
sN for the hydroxide ion in water and 50% water–
50% acetonitrile [23]. To compare these results with
those experimental values, we have investigated the ki-
netic reactions of this family of Michael acceptors 1a–
d with the hydroxide ion under the same conditions.
The resulting rate constants (kexp) are also listed in
Table II.
Examination of Table II shows that Eq. (1) pre-
dicts the rate constants for the addition of OH− to the
Michael acceptors 1a–d withing a factor of ࣘ10.3. In
all cases, the agreement between calculated and ex-
perimental rate constants is remarkably good because
Eq. (1) usually predicts rate constants of polar organic
reactions within a factor of 10–100 [24–30].
Correlation between Electrophilicity
Parameters E and σp+
The electrophilicity parameters E obtained for these
series of benzylidenecyanoacetates 1 have been plotted
International Journal of Chemical Kinetics DOI 10.1002/kin.21132

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AZAIEZ, DHAHRI, AND BOUBAKER
Table III Comparison of Slopes and Intercepts for log kexp versus log kcalcd Correlations of Some Representative
Electrophile–Nucleophile Combinations
Electrophile
Nucleophile
Solvent
Slope
Intercept
CN
CN
X
4
OH−
50% H2O-50% CH3CN
0.99a
1.20a
X = NMe2, OMe, Me, H
Cl, CF3, CN and NO2
CH3
+
N
Se
N
O
N
N
H
H
CH3CN
0.98b
−1.60b
NO2
8
N
H
CO2Et
CN
OH−
H2O
0.97c
1.03c
0.89c
0.69c
X
1
X = NMe2, OMe, Me, H
50% H2O-50% CH3CN
aFrom 5.
bFrom 27.
cPresent work.
against the Brown’s substituent constants σp+ values
[31]. As seen from Fig. 5, the plot of Eσp+ versus is
quite good and the corresponding relationship is the
following
co-workers have demonstrated that the E parameters
for a series of 2-benzylidene-indan-1,3-diones also
correlate well with σp+ values of their substituents
(E = −9.78 + 2.34σp+) [32].
E = −10.07 + 2.29σp+ (R2 = 0.9959)
(3)
Effect of Nitroalkyl Anion Basicity on
Reactivity and Intrinsic Reactivities
This result is consistent with a preferential stabi-
lization of the ground state of Michael acceptors 1a–d
through resonance interactions between the electron-
donating substituent X and the carbonyl group. In-
terestingly, the plot of Eq. (3) can be used to esti-
mate the unknown electrophilicity parameter E val-
ues of other substituted benzylidenecyanoacetates 1.
On this basis, the E values of benzylidenecyanoac-
etates 1e (X = Cl, E = −9.81), 1f (X = CF3,
E = −8.67), 1g (X = CN, E = −8.56), and 1e
(X = NO2, E = −8.26) have been assessed using the
σp+ value of the chloro atom (σp+ = 0.11), CF3 group
(σp+ = 0.61), CN group (σp+ = 0.66), and NO2 group
(σp+ = 0.79) [31], respectively.
In this study, when the logarithm of the second-order
rate constants k was plotted against the pKa of the con-
jugate acid of the nitroalkyl anion 2 as measured by
Turnbull and Maron in aqueous solution [10] (Table
I), a linear correlation is obtained (Fig. 6). Examina-
tion of Table IV shows that the slope of these lines
(βnuc values) remains constant for any given X sub-
stituent. This suggests that the transition state does not
vary significantly with variation of the substituents on
the Michael acceptor. We also note that the relatively
small βnuc values of 0.27–0.29 indicate that carbon–
carbon bond formation has made rather little progress
at the transition state. This observation of a constant
βnuc value with changing substituent X is supported by
the results of a kinetic study by Bordwell and Hughes,
who observed constant βnuc values of 0.27–0.29 for
reactions of 9-methylfluorenide anions with a substi-
tuted N−(p-methoxybenzyl)-N,N−dimethylanilinium
cation in dimethyl sulfoxide solution at 25°C [35].
Relationships between parameters E and σp+ have
been reported by many authors [5,16,32–34]. An
and co-workers, for example, have observed a lin-
ear correlation between the electrophilicity parame-
ters E of various iminium ions and the Brown’s σp+
parameters (E = −7.08 + 1.80σp+) [34]. Berger and
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REACTIVITY OF NITROALKYL ANIONS ADDITION TO SUBSTITUTED BENZYLIDENECYANOACETATES
855
-7
-8
CO2Et
CN
-9
-10
-11
-12
-13
-14
-15
X
1a
1
(X = H)
1b
(X = Me)
1c
(X = OMe)
1d
(X = NMe2)
-2
-1,5
-1
-0,5
0
0,5
+
σp
Figure 5 Correlation between the electrophilicity parameters E of benzylidenecyanoacetates 1a–d and Brown’s substituent
constants σp+. For E see Fig. 4, and σp+ values were taken from 31 (σp+ (H) = 0, σp+ (Me) = −0.31, σp+ (OMe) = −0.78, and
σp+ (NMe2) = −1.70).
Interestingly, our experimental βnuc values are com-
parable in magnitude to those reported for nucle-
ophilic addition to other Michael acceptors [36]. For
example, the βnuc value of 0.25 has been reported
for reactions of benzylidene Medrum’s acid 9 with
a series of alkoxide anions in 50% DMSO–50% wa-
ter (v/v) at 20°C [36a] and the βnuc value of 0.33 for
the reaction of various secondary amines addition to ni-
trostyrene 10 at 20°C in aqueous solution has also been
reported [36b].
From the plots of Fig. 6, values for the intrinsic
rate constants expressed as log ko for nitroalkyl anions
2a–c addition to 1a–d may be obtained. In fact, using
the classical definition of the intrinsic rate constant
(in the Marcus sense) given by Eq. (4) [37–39], i.e.,
ko = k when pKꢁa = pKa, in which pKꢁa corresponds
to the value for water addition to Michael acceptor
1 (Scheme 3)∗ and pKa is a value of the conjugate
acid of the nitroalkyl anion 2 [10], the log ko values for
1a–d were readily determined from Bro¨nsted plots (see
Fig. 6 and Table IV).
log ko = log k + βnuc(pKa − pKꢁa)
(4)
The low values of log ko obtained for this series of
Michael acceptors 1a–d are approximately the same
within experimental error. This indicates that the ko rate
constants for reactions of 1a–d with nitroalkyl anions
2a–c in aqueous solution are largely governed by the
extend of charge delocalization (resonance structures
B and C).
It has to be noted that lower intrinsic rate con-
stants have also been observed for nucleophilic ad-
dition to various activated olefins [40], for exam-
ple, log ko = 1.35 and −0.63 for reactions of wa-
ter with dicarbonylolefin 11 [40a] and nitrostyrene 10
[40a,41], respectively, in aqueous solution at 25°C.
Bernasconi and co-workers have discussed the ori-
gin of these lower log ko values and suggested that
these values are the reflection of a possible stabi-
lization of transition state by intramolecular hydrogen
∗
pKꢁa corresponds to the value for water addition to Michael ac-
ceptors 1a–d are evaluated in this work through the E = −3.20 –
0.662 pKa relationship (see [17]).
International Journal of Chemical Kinetics DOI 10.1002/kin.21132

856
AZAIEZ, DHAHRI, AND BOUBAKER
2
1,5
1
0,5
1a
0
1b
-0,5
1c
-1
-1,5
1d
-2
6
8
10
12
pKa
14
16
Figure 6 Bro¨nsted plots for reactions of benzylidenecyanoacetates 1a–d with nitroalkyl anions 2a–c in aqueous solution at
20°C. The values of k and pKa are given in Table I and pKꢁa in Table IV.
Table IV Bro¨nsted βnuc Values and Intrinsic Rate Constant (log ko) for the Addition of Nitroalkyl Anions 2a–c to
Michael Acceptors 1a–d in Aqueous Solution at 20°C
CO2Et
b
c
pKꢁaa
β
log ko
CN
nuc
X
1
1a (X = H)
10.38
11.67
12.76
16.37
1.07
1.03
0.97
0.94
0.27
0.29
0.28
0.29
1b (X = Me)
1c (X = OMe)
1d (X = NMe2)
aThe pKꢁa values were evaluated in this work through the E = −3.20 – 0.662 pKꢁa relationship [17].
bβnuc values are derived from the slopes of log k versus pKa values of the conjugate acid of the nitroalkyl anions 2.
clog ko is obtained from log k versus pKa, extrapolated to pKa = pKꢁa (see Fig. 6).
bonding to a carbonyl or nitro oxygen [40a,40c], e.g.,
12.
H3C
H
O
O
H
H
O
O
O
O
NO2
-
δ
CH3
CH3
O
H
H
CH3
+
δ
O
CH3
H
O
H
CH3
12
9
10
11
International Journal of Chemical Kinetics DOI 10.1002/kin.21132

REACTIVITY OF NITROALKYL ANIONS ADDITION TO SUBSTITUTED BENZYLIDENECYANOACETATES
857
OH
CO2Et
CO2Et
+
Ho
o
H2O
+
+
CN
CN
X
X
1
X = H, Me, OMe, NMe2
Scheme 3 Reaction of water with the benzylidenecyanoacetates 1a–d.
nikov, G.; Schimmel, H. J Am Chem Soc 2001, 123,
9500–9512.
CONCLUSION
7. Mayr, H.; Hartnagel, M.; Grimm, K. Liebigs Ann Re-
cueil 1997, 1997, 55–69.
We have shown that the electrophilic reactivity of ben-
zylidenecyanoacetates 1a–d can be quantitatively de-
scribed by Mayr’s equation (1). Based on this, the elec-
trophilicity parameters E of these Michael acceptors
1a–d are evaluated. The second-order rate constants
for the reactions of benzylidenecyanoacetates 1 with
the hydroxide ion have been measured in water and
50% water–50% acetonitrile and found to agree with
those calculated from the electrophilicity parameters E
determined in this work and the previously published
N and sN parameters for the hydroxide ion. Interest-
ingly, the large changes in electrophilicity parameters
E observed on going from benzylidenecyanoacetates
1 to benzylidenemalonates 3 can be regarded as a
measure of the gain in activation incurred by substi-
tution of the CN group by a second CO2Et group. The
lower log ko obtained in this work clearly indicates
that the cyano and carbonyl groups contribute very
effectively to the resonance stabilization. On the other
hand, we have shown that the correlation of E versusσp+
constitutes an interesting and useful approach for
evaluating the electrophilicity parameter E of other
Michael acceptors 1. Finally, the near unit slopes of
the correlations log kexp versus log kcalc observed in
the present work will be discussed in a forthcoming
paper.
8. For a list of electrophilicity parameters E and nu-
cleophilicity parameters N and sN, see Mayr, H. at
http://www.cup.lmu.de/oc/mayr/DBintro.html.
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212–218.
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7565–7576.
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International Journal of Chemical Kinetics DOI 10.1002/kin.21132