Mechanism and linear free energy relationships in michael-type addition of 4-substituted anilines to activated olefin in acetonitrile

Article abstract of DOI:10.1002/kin.20811

Kinetic studies for the Michael-type reactions of ethyl-3-(4′-N,N- dimethylaminophenyl)-2-(nonafluorobutane)sulfonylpro-penoate 1 with 4-X-substituted anilines 2a-e (X = OCH₃, CH₃, H, F, and Cl) have been investigated in acetonitrile

Full text of DOI:10.1002/kin.20811

Mechanism and Linear
Free Energy Relationships
in Michael-Type Addition
of 4-Substituted Anilines
to Activated Olefin in
Acetonitrile
N. DHAHRI,1 T. BOUBAKER,1 R. GOUMONT2
1Laboratoire CHPNR, Faculte´ des Sciences de Monastir, Universite´ de Monastir, 5019 Monastir, Tunisia
2Institut Lavoisier de Versailles UMR 8180, Universite´ de Versailles 45, 78035 Versailles Cedex, France
Received 5 January 2013; revised 12 June 2013; 8 July 2013; accepted 11 July 2013
DOI 10.1002/kin.20811
Published online 3 September 2013 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Kinetic studies for the Michael-type reactions of ethyl-3-(4ꢀ-N,N-dimethylam-
inophenyl)-2-(nonafluorobutane)sulfonylpro-penoate 1 with 4-X-substituted anilines 2a–e
(X = OCH3, CH3, H, F, and Cl) have been investigated in acetonitrile at 20◦C. A quadratic
dependence of the pseudo–first-order rate constants (kobsd) versus [2a–e] has been observed
and has been interpreted in terms of a dimer nucleophile mechanism. The finding of a relatively
large negative ρ value (−3.09) for the Hammett plot suggests that the intermediate (I ) is highly
zwitterionic in nature. A linear correlation (r2 = 0.9989) between the Hammett’s substituent
constants σ and nucleophilicity parameters N of 4-X-substituted anilines in acetonitrile has
been observed. The electrophilicity parameters E of the olefin 1 is evaluated, using the corre-
lations σ versus N aꢁnCd log k versus σ and compared with the electrophilicities of analogously
Michael acceptors. 2013 Wiley Periodicals, Inc. Int J Chem Kinet 45: 763–770, 2013
and reported in the literature [1–4]. For example,
Bernasconi and co-workers performed the reaction
of benzylidenemalononitrile with piperidine and mor-
pholine in 50% Me2SO and 50% H2O. The reac-
tions have been found to proceed through a typi-
cal Michael-type nucleophilic addition (Scheme 1) in
which the formation of the zwitterionic intermediate,
T , is the rate-determining step with a fast subsequent
decomposition of this intermediate into the substituted
INTRODUCTION
The mechanism of the nucleophilic addition of amines
to an activated olefin has been extensively studied
Correspondence to: T. Boubaker; e-mail: boubaker taoufik@
yahoo.fr.
Supporting Information is available in the online issue at
www.wileyonlinelibrary.com.
C
ꢀ
2013 Wiley Periodicals, Inc.

764
DHAHRI, BOUBAKER, AND GOUMONT
CN
H
H
H
CN
CN
H
CN
CN
Ar
R2N
CN
Ar
+
R2NH
+
R2NH
Ar
+
T
Scheme 1
H
CO2Et
δ- δ-
CO2Et
H
H
CO2Et
CO2Et
Cβ
Cα
+
XArCH2NH2
YAr
XArCH2NH
CO2Et
YAr
CO2Et
YAr
H
δ+
δ+
XArCH2N
H
H
X = OMe, Me, H and Cl
Y = OMe, Me, H, Cl and Br
Scheme 2
product [1a]. The reactions of benzylidenediethyl-
malonates with substituted benzylamines in acetoni-
trile have been investigated by Oh and co-workers
[3]. In this latter case, the results have been inter-
preted in terms of concerted mechanism, in which the
2.5
2.0
1.5
1.0
0.5
0.0
t = 0 min
–
–
N Cα and the H Cβ bonds are formed simultaneously
leading to the neutral final product in a single step
(Scheme 2).
t = 40 min
In this paper, we report on the data obtained from a
detailed kinetic study of the nucleophilic addition of
4-X-substituted anilines 2a–e (X = OCH3, CH3, H, F,
and Cl) to ethyl-3-(4ꢀ-N,N-dimethylaminophenyl)-2-
(nonafluorobutane)sulfonylpro-penoate 1 in acetoni-
trile solution at 20◦C as shown in Eq. (1). As will be
seen, the observed aniline concentration dependence
of kobsd in the present system can be interpreted in
terms of a “dimer nucleophile” mechanism. This study
also allows the determination of the electrophilicity
parameter E of olefin 1 and the comparison of this
E value with that of structurally similar olefins. The
effect of the substituent X on the reactivity of anilines
will also be discussed.
t = 0 min
200
350
400
450
500
550
Wavelength (nm)
Figure 1 Time-dependence of the electronic absorption
spectrum of ethyl-3-(4ꢀ-N,N-dimethylaminophenyl)-2-(non-
afluorobutane)sulfonylpro-penoate 1 (3 × 10−5 mol dm−3
)
in the presence of aniline 2b (10−1 mol dm−3) in CH3CN at
20◦C. Curves 1–10 are recorded at 0, 1, 2, 3, 4, 5, 10, 20, 30,
and 40 min, respectively.
X
XC6H4NH
SO2C4F9
SO2C4F9
CH3CN
+
20 °C
CO2Et
CO2Et
N
N
3a-e
1
NH2
2a-e
(2a) X = OMe, (2b) X = Me, (2c) X = H
(2d) X = F, (2e) X = Cl
(1)
EXPERIMENTAL
of ethyl(nonafluorobutanesulfonyl)acetate [6,7] and
1 equiv of 4-N,N-dimethylaminobenzaldehyde were
dissolved in toluene (20 dm−3). Then piperidine
(0.05 dm−3) and glacial acetic acid (0.1 dm−3) were
To prepare ethyl-3-(4ꢀ-N,N-dimethylaminophenyl)-2-
(nonafluorobutane)sulfonylpro-penoate, 1 [5] 1 equiv
International Journal of Chemical Kinetics DOI 10.1002/kin.20811

MICHAEL-TYPE ADDITION OF 4-SUBSTITUTED ANILINES TO OLEFIN IN ACETONITRILE
765
been kinetically studied in acetonitrile at 20◦C. The
kinetic determinations were carried out under pseudo–
first-order conditions using a large excess of aniline.
In all experiments, only one step corresponding to
the quantitative formation of the substitution products
3a–e (355 nm < λmax < 365 nm) was observed. On the
other hand, the UV–visible spectroscopy analysis at
different times shows a clear isobestic point, evidenc-
ing the cleanness of the reactions and the fact that the
rising absorption is due to the absorbance of the prod-
uct 3a–e. A typical experiment is illustrated in Fig. 1,
which shows the set of UV–visible absorption spectra,
describing the progressive conversion of the 1 to the
product 3b.
0.035
0.03
0.025
0.02
0.015
0.01
0.005
0
OCH3
CH3
0
0.1
0.2
0.3
0.4
0.5
0.6
[4-X-aniline] (mol dm-3)
Figure 2 Dependence of the pseudo–first-order rate con-
stants, kobsd, on the aniline concentration, for the reactions
of ethyl-3-(4ꢀ-N,N-dimethylaminophenyl)-2-(nonafluorobu-
tane)sulfonylpro-penoate 1 with anilines 2a (o) and 2b (•) in
CH3CN at 20◦C.
Pseudo–first-order rate constants (kobsd) were de-
termined from the slope of the plot of ln (A −At)
∞
against time, where A and At are the equilibrium
∞
absorbance and the absorbance after time t, respec-
tively (the correlation coefficient for the linear regres-
sion was usually higher than 0.9934). The kinetics
of these reactions has been studied using different
aniline concentrations, while the other experimental
conditions remain unchanged. Anilines 2a–e concen-
added. The mixture was refluxed for 3–4 h with a water
trap (Dean-Stark), until no further water separated. The
solvent was removed under reduced pressure, and the
residue was taken up in Et2O. The Et2O was washed
with HCl (50 dm−3; 5%), then washed several times
with water, followed by drying (MgSO4). The solvent
was evaporated, and the residue was purified by re-
crystallization from pentane/ethyl acetate or by column
chromatography [5]:
trations and pseudo–first-order rate constants (kobsd
)
for all the experiments are summarized in Tables SI–
SV (see the Supporting Information). The analysis of
the data from Tables SI–SV (in the Supporting Infor-
mation) reveals that the plot of kobsd versus [Aniline]
SO2 C4 F9
C4 F9 SO2 CH2 CO2 E t
O
CO2 E t
N
toluene, acetic acid
piperidine, 3h
N
The kinetic study was performed using a Shimadzu
UV–visible model 1650 PC spectrophotometer. All the
reactions were carried out under pseudo–first-order
conditions in the presence of excess aniline. Gener-
ally, the anilines 2a–e concentration was varied over
the range 2 × 10−2 to 8 × 10−1mol dm−3, whereas the
exhibited an upward curvature, indicating a kinetic or-
der greater than unity in the aniline. Figure 2 shows
the behavior for the nucleophilic addition of anilines
2a and 2b to ethyl-3-(4ꢀ-N,N-dimethylaminophenyl)-
2-(nonafluorobutane)sulfonylpro-penoate 1.
olefin 1 concentration was 3 × 10−5mol dm−3
.
Reaction Mechanism
RESULTS AND DISCUSSION
Kinetic Investigations
The aniline concentration dependence of kobsd
with [Aniline] displayed in Fig. 2 for the reac-
tion of olefin 1 with 4-X-substituted anilines 2a–
e shows clear evidence of a nonlinear behavior.
This situation has been rarely reported in the case
of the Michael-type nucleophilic addition to an
The nucleophilic addition of a series of 4-X-substituted
anilines 2a–e to ethyl-3-(4ꢀ-N,N-dimethylaminophen-
yl)-2-(nonafluorobutane)sulfonylpro-penoate
1 has
International Journal of Chemical Kinetics DOI 10.1002/kin.20811

766
DHAHRI, BOUBAKER, AND GOUMONT
activated alkene. However, quadratic dependences of
rate constant on [amine] in aprotic solvents have been
observed for nucleophilic aromatic substitution reac-
tions [8–10]. Nudelman and co-workers, for exam-
ple, have reported that the kinetics of reactions be-
tween 2,4-dinitrochlorobenzene and aniline in toluene
at 40◦C is characterized by a parabolic dependence
on the aniline concentration. This kinetic behav-
ior is interpreted in terms of the aggregation of
aniline [9c].
-1
-1.5
-2
OCH3
CH3
H
F
-2.5
-3
Cl
-3.5
-4
Taking into account that the amine aggregates
are known to be more nucleophilic than the free
amine in aprotic media [9–12], Scheme 3 is proposed
to interpret the interaction of the activated olefin 1
with anilines 2a–e in acetonitrile. In this scheme,
the zwitterionic intermediate (I ) is formed in the
addition step through the interaction of the dimer
of the aniline 2a–e:2a–e with olefin 1. We note
that the formation of this dimer can be explained
in terms of the intermolecular hydrogen bond as
described in equilibrium (2). Conversion of inter-
mediate I to product 3a–e can therefore occur via
the uncatalyzed pathway or via the base catalyze
pathway:
-4.5
-2
-1.5
-1
-0.5
0
log [4-X-aniline]
Figure 3 Effect of the aniline concentration on the observed
pseudo–first-order rate constant, kobsd, for the reactions of
ethyl-3-(4ꢀ-N,N-dimethylaminophenyl)-2-(nonafluorobutane)-
sulfonylpro-penoate 1 with 4-X-substituted anilines 2a–e in
CH3CN at 20◦C. Correlation equations: a log kobsd = −0.49
+ 1.95 log [2a] (r2 = 0.9998), b log kobsd = −0.95 + 1.91
log [2b] (r2 = 0.9997), c log kobsd = −1.42 + 1.93 log [2c]
(r2 = 0.9993), d log kobsd = −1.58 + 1.94 log [2d] (r2 =
0.9995), and e log kobsd = −2.08 + 1.97 log [2e] (r2
=
0.9963).
H
K
I I I I I I I I I
2 XC6H4NH2
XC6H4N
H
NC6H4X
H
H
2
2a-e:2a-e
(2a) X = OMe, (2b) X = Me
(2c) X = H, (2d) X = F, (2e) X = Cl
(2)
Based on Scheme 3, the general expression for the
observed first-order rate constant, kobsd, for the for-
mation of the addition products 3a–e, as described
under the assumption that the zwitterions I are low-
concentration intermediates, is given by Eq. (3), where
K is the equilibrium constant for the monomer:dimer
equilibrium according to Eq. (2):
According to Eq. (4), excellent straight lines with
zero intercepts were obtained when we plotted the
kobsd versus [2a–e]2 (see Figs. S1 and S2 in the Sup-
porting Information). On the other hand, plotting log
kobsd versus log [2a–e] affords nice straight lines (r2 >
0.9963) with a slope close to 2 within experimental
error for all the anilines studied (see Fig. 3), indi-
cating a second-order dependence of the aniline con-
centration and is not catalyzed by either aniline. The
k values that correspond to the intercepts of Fig. 3
are presented in Table I with some other important
data.
The linearity of the plots log kobsd against log [2a–e]
gives clear evidence for the formation of the zwitteri-
onic intermediate (I ) and indicates that the aniline
dimers exist predominantly as the nucleophilic partner
rather than the free aniline. Therefore, the present re-
sult also unambiguously suggests that the reactions (1)
proceed through a dimer nucleophile mechanism.
k1 k2 K[2a − e]2
kobsd
=
(3)
k
−1 + k2
The variation of kobsd with the aniline concentra-
tion observed in the present work (Fig. 2) can now
be explained from Eq. (3). This equation shows that
when k–1 ꢂ k2, i.e., the formation of the zwitterionic
intermediate (I ) is rate limiting, kobsd would exhibit a
linear response with second order on the aniline con-
centration as shown in Eq. (4):
kobsd = k1K[2a − e]2 = k[2a − e]2
(4)
International Journal of Chemical Kinetics DOI 10.1002/kin.20811

MICHAEL-TYPE ADDITION OF 4-SUBSTITUTED ANILINES TO OLEFIN IN ACETONITRILE
767
H
I I I I I I I I I
H
NH2C6H4X
XC6H4N+
SO2C4F9
SO2C4F9
CO2Et
H
XC6H4N
H
k1
H
I I I I I I I I I
+
NC6H4X
H
H
CO2Et
k-1
N
N
1
2a-e:2a-e
I +
H
I I I I I I I I I
H
NH2C6H4X
XC6H4NH
XC6H4N+
SO2C4F9
SO2C4F9
CO2Et
k2
H
+
XC6H4NH2
CO2Et
N
N
I+
3a-e
Scheme 3
Table I Kinetic Data for Addition Reactions of
4-X-Substituted Anilines 2a–e with
Ethyl-3-(4ꢀ-N,N-dimethylaminophenyl)-2-
(nonafluorobutane)sulfonylpro-penoate 1 in CH3CN
at 20◦C
0
2a, X = OCH3
-0.5
X
NH2
2b, X = CH3
2a-e
-1
-1.5
-2
4-X-Substituted
Aniline
k (mol−1 dm3 s−1 a
)
σb
N
2c, X = H
2d, X = F
OCH3
CH3
H2N
3.26 × 10−1
−0.27 13.42c
−0.17 13.19c
2a
2e, X = Cl
H2N
-2.5
-0.4
1.11 × 10−1
3.80 × 10−2
2.63 × 10−2
8.31 × 10−3
2b
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
H
F
σ
H2N
Figure 4 Plot of log k for the reactions of 4-X-substituted
anilines 2a–e with ethyl-3-(4ꢀ-N,N-dimethylaminophenyl)-
2-(nonafluorobutane)sulfonylpro-penoate 1 in CH3CN at
20◦C versus Hammett’s substituent constants σ. The σ val-
ues are taken from [13].
2c
0
12.64c
12.36d
11.92c
H2N
2d
0.06
0.23
Cl
H2N
2e
Hammett Correlation
aThis work.
The examination of the data in Table I shows that the
reactivity of 4-X-substituted anilines 2a–e addition to
the olefin 1 appears to be significantly dependent on
the electronic nature of the substituents, i.e., the rate
constants decrease regularly from X = OCH3 to X = Cl
leading to a nice Hammett correlation (Fig. 4), which
is defined by the following equation:
bValues of σ substituent constants are taken from [13].
cThe N values are taken from [14].
d The value is calculated by using Eq. (6).
It is interesting to note that this kind of mech-
anism involving dimer formation has been reported
in the literature, especially for SNAr reactions of ni-
troactivated substrates with amines in nonpolar aprotic
solvents, for example, the reaction of 1-halo-2,
4-dinitrochlorobenzene with aniline in toluene and
benzene/n-hexane mixtures [9,10,15,16], the reac-
tion of 2,4-dinitrofluorobenzene with O-anisidine in
benzene [9b], and the reaction of 2,4- and 2,6-
dinitroanisole with cyclohexylamine in benzene [9a].
ꢀ
ꢁ
log k(20 ◦C) = −1.40 − 3.09σ r2 = 0.9954 (5)
The large negative ρ value of −3.09 deduced from
the slope of the log k versus Hammett’s substituent
constants σ is clearly consistent with a significant de-
velopment of a positive charge on the aniline nitrogen
atom in the transition state. On the other hand, the large
International Journal of Chemical Kinetics DOI 10.1002/kin.20811

768
DHAHRI, BOUBAKER, AND GOUMONT
0.6
0.4
0.2
0
NH2
negative ρ value suggests that the intermediate (I ) is
highly zwitterionic [17]. We note in particular that this
ρ value is the first one to be reported in the literature for
the study of the nucleophilic amine addition to olefins
in acetonitrile.
In comparison, it is interesting to note that the mag-
nitude of ρ = −3.09 obtained in the present work
is more negative than those observed in numerous
systems in which the formation of carbon–nitrogen
bond is a rate-determining step [14,18,19]. The data in
Table II are illustrative in this regard.
NH2
Cl
2e
NH2
H
NH2
2c
CH3
2b
OCH3
-0.2
-0.4
2a
We consider that the ρ value of −3.09 may re-
flect an intrinsic difference between the lone pair on
the nitrogen atom in the nucleophilic reactivity of free
aniline 2a–e and the autoassociates to aniline solu-
11.5
12
12.5
13
13.5
14
N
Figure 5 Correlation between the Hammett’s substituent
constants σ and nucleophilicity parameters N of 4-X-
substituted anilines (X = OCH3, CH3, H, and Cl). (For σ
and N values, see Table I.)
–
tion (2a–e:2a–e), formed by an N H···N hydrogen
bond as shown in Eq. (2). This interpretation agrees
with the previous observation of Nudelman and co-
workers that, in aprotic solvents, autoassociation of
amines are more nucleophilic than the free amine [11].
On the other hand, this greater sensitivity to substituent
changes of the rates of nucleophilic addition may be
related to the higher basicity of the dimer (2a–e:2a–e)
than of the free aniline 2a–e, implying the development
of a large positive charge in the transition state for for-
mation of the corresponding zwitterionic intermediate.
Therefore, one can suggest that the reactions (1) pro-
ceed through a dimer nucleophile mechanism in which
the formation of the zwitterionic intermediate (I ) is
the rate-determining step in most cases.
Correlations with Nucleophilicity
Parameter N
When the Hammett constants (σ) of 4-substituted ani-
lines 2a–e [13] are plotted against their nucleophilic-
ity parameter N, recently reported by Mayr and co-
workers [20], a linear correlation is found (Fig. 5) and
the corresponding relationship is given by Eq. (6). A
similar correlation between N and σ+ parameters of
arenes has been observed by Mayr and co-workers [21].
Interestingly, the plot of Eq. (6) can be used to estimate
Table II Comparison of ρ Values for SNAr and Michael-Type Addition Reactions with 4-X-Substituted Anilines in
Various Solvents
Electrophile
4-X-Substituted Aniline
Solvent
Me2SO
ρ
Reference
[18]
Cl
X = H, I, Cl, CN
−1.97
N
O
N
NO2
OCH3
X = OCH3, CH3, H, F, Cl
X = H, I, Cl
MeOH
Me2SO
−1.54
−1.40
[14a]
[14b]
SO2CF3
CF3O2S
SO2CF3
OCH3
X = OCH3, CH3, H, F, Cl
MeOH
MeCN
−1.44
−1.84
[14c]
[19]
CF3O2S
SO2CF3
NO2
N
Cl
X = OCH3, CH3, H, Cl, Br, COCH3
MeOH
MeCN
−1.11
−3.20
O2N
NO2
SO2C4F9
CO2Et
X = OCH3, CH3, H, F, Cl
This work
N
1
International Journal of Chemical Kinetics DOI 10.1002/kin.20811

MICHAEL-TYPE ADDITION OF 4-SUBSTITUTED ANILINES TO OLEFIN IN ACETONITRILE
769
the unknown nucleophilicity parameter N values of 4-
X-substituted anilines in acetonitrile at 20◦C. On this
basis, the N values of 4-nitroaniline (N = 10.18), 4-
cyanoaniline (N = 10.54), 4-iodoaniline (N = 12.00),
4-fluoroaniline (N = 12.36), and 4-hydroxyaniline
(N = 13.67) have been assessed using the Hammett’s
substituent constant σ values of the nitro group, cyano
group, iodine atom, fluorine atom, and the hydroxy
group, respectively:
It should be noted that the nucleophile-specific slope
parameter (s = 1.02) of the correlation Eq. (7) is
larger than those of 4-methoxyaniline (s = 0.73), 4-
methylaniline (s = 0.69), aniline (s = 0.68), and 4-
chloroaniline (s = 0.60) estimated in acetonitrile by
Mayr and co-workers [20]. The reason for this varia-
tion is not clear at this stage.
CONCLUSIONS
σ = 4.14 − 0.33N (r2 = 0.9989)
(6)
In the present work, the addition reaction of 4-X-
substituted anilines 2a–e (X = OCH3, CH3, H, F,
and Cl) to ethyl-3-(4ꢀ-N,N-dimethylaminophenyl)-2-
(nonafluorobutane)sulfonylpro-penoate 1 in acetoni-
trile at 20◦C is investigated. The analysis of the kinetic
results has revealed that the most reasonable explana-
tion for the observed quadratic dependence kobsd ver-
sus the aniline concentration is in terms of a dimer
nucleophile mechanism. We note that the proposed
mechanism in the present study constitutes the first ex-
ample observed in Michael-type reactions. The large
magnitudes of the ρ value (−3.09) suggest the exis-
tence of autoassociates to aniline solution, formed by
The good correlation between the Hammett con-
stants σ of 4-substituted anilines 2a–e with their nu-
cleophilicity parameter N shown in Fig. 5 can be
employed to obtained more information about the nu-
cleophilic reactivities. Combining Eqs. (5) and (6), i.e.,
substituting σ in Eq. (5) by the expression of σ reported
in Eq. (6), leads to the new Eq. (7), confirming that the
rate constants log k are linearly related to the nucle-
ophilicity parameter N of anilines 2a–e:
log k(20 ◦C) = −14.20 + 1.02 N
(7)
–
an N H···N hydrogen bond. On the other hand, we
have shown that the combination of log k versus σ
and σ versus N correlations constitutes an interesting
and useful approach for evaluating the electrophilicity
parameter E of olefin 1.
Electrophilicity Parameter E for Olefin 1
Recently, Mayr and co-wokers have demonstrated that
the linear free energy relationship (Eq. (8)) [22–28]
can also be employed for describing the rates of typi-
cal Michael addition [29–33]. In this equation, k is the
second-order rate constant, s is the nucleophile-specific
slope parameter, N represents the nucleophilicity pa-
rameter, and E is the electrophilicity parameter:
SUPPORTING INFORMATION
Tables SI–SV present pseudo–first-order rate con-
stants, kobsd, values for the reactions of olefin 1 with the
4-X-substituted anilines 2a–e in acetonitrile at 20◦C.
Figures S1–S2 show plots of kobsd versus [4-X-aniline]2
for the reactions of 4-X-substituted anilines 2a–e with
the olefin 1 in acetonitrile at 20◦C.
log k(20 ◦C) = s E + s N
(8)
As can be seen, there is an analogy between
Mayr’s Eq. (8) and Eq. (7). However, the electrophilic-
ity parameter E of olefin 1 can be estimated us-
ing the intercept (sE = −14.20) and the slope (s =
1.02) values of Eq. (7). The E value of −13.92 is
thus obtained. The comparison of the electrophilic-
ity E of ethyl-3-(4ꢀ-N,N-dimethylaminophenyl)-2-
(nonafluorobutane)sulfonylpro-penoate 1 with the E
values recently reported for a few Michael acceptors
reveals that the olefin 1 exhibits an electrophilicity
that compares well with that of p-(dimethylamino)
benzylidene-indan-1,3-dione 4 (E = −13.56) [30],
benzylidenemalonitrile 5 (E = −13.30) [31], and
quinone methides 6 (E = −13.39) [32]. Most im-
portantly, our olefin 1 is much stronger electrophile
than the analogously diethyl 4-(dimethylamino)
benzylidene malonate 7 (E = −23.10) [33].
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