Kinetic studies of reactions of 4-nitrobenzofuroxan with carbanions derived from benzyltriflones in methanol

Article abstract of DOI:10.1002/kin.20621

This paper reports the rate measurements for the reactions of carbanions derived from benzyltriflones, 2, with 4-nitrobenzofuroxan, 4, in methanol, to give anionic σ-adducts. ¹H NMR studies in DMSO-d₆ indicate that the products formed by the reaction of 2 and 4 in the presence of triethylamine are consistent with the products formed by the elimination of trifluoromethylsulfinic acid from σ-adducts, initially formed by a carbanion attack at the 5 position of 4. The low value of β, which is the slope of the linear plot of log k₅ versus pK_a, provides evidence for the high steric requirements of the benzyltriflone anions.

Full text of DOI:10.1002/kin.20621

Kinetic Studies of Reactions
of 4-Nitrobenzofuroxan with
Carbanions Derived from
Benzyltriflones in Methanol
BASIM H. ASGHAR
Department of Chemistry, Faculty of Applied Sciences, Umm Alqura University, P.O. Box 9569, Makkah, Saudi Arabia
Received 12 June 2011; revised 12 August 2011, 12 September 2011; accepted 30 September 2011
DOI 10.1002/kin.20621
Published online in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: This paper reports the rate measurements for the reactions of carbanions derived
from benzyltriflones, 2, with 4-nitrobenzofuroxan, 4, in methanol, to give anionic σ-adducts.
1H NMR studies in DMSO-d6 indicate that the products formed by the reaction of 2 and 4
in the presence of triethylamine are consistent with the products formed by the elimination
of trifluoromethylsulfinic acid from σ-adducts, initially formed by a carbanion attack at the
5 position of 4. The low value of β, which is the slope of the linear plot of log k5 versus pKa,
provides evidence for the high steric requirements of the benzyltriflone anions. ꢀ 2012 Wiley
C
Periodicals, Inc. Int J Chem Kinet 1–9, 2012
including N-methylpyrrole and indole, in acetonitrile
[7–9]. The vicarious nucleophilic substitution [10] re-
action that proceeds via addition of α-chlorocarbanions
to nitroarenes followed by faster base-induced β-
eliminations was used for the determination of elec-
trophilicity of nitroarenes was studied [11,12].
This study assesses the effects of a trifluoromethyl-
sulfonyl group on carbanions in their σ-adduct-
forming reactions with nitrobenzofurazan deriva-
tives. It is known that the SO2CF3 group has a
strong electron-withdrawing nature, and, as judged by
Hammett σ values [13], this group is more effective
than NO2 as a ring substituent in stabilizing anionic
σ-adducts. Thus, adduct 1 is more stable than the
trinitro analogue in methanol by a factor of 106 [14].
However, compared with phenylnitromethane, benzyl-
triflone has been shown [15] to be less acidic by two
pKa units in DMSO. This difference is further mag-
nified in water owing to the excellent solvation of the
nitronate ion by water [16]. By conducting kinetic,
INTRODUCTION
Currently, there is considerable interest in the quantita-
tive measurement of nucleophilic and electrophilic re-
activities, particularly in carbon–carbon bond-forming
reactions [1–3]. Mayr and coworkers have used the
reactions involving a series of benzhydryl cations to
assess the nucleophilicities of carbanions stabilized by
acyl, ester, cyano, and nitro groups [4–6]. Their results,
although providing useful relationships, show that rel-
ative nucleophilicities may be strongly influenced by
the solvent, for example, methanol versus DMSO
[4–6]. Conversely, the electrophilicities of some su-
perelectrophiles, including 4,6-dinitrobenzofuroxan,
have been determined by measuring the rate constants
of their reactions with some standard nucleophiles,
Corresponding to: Basim H. Asghar; e-mail: Basim15@
yahoo.com.
c
ꢀ
2012 Wiley Periodicals, Inc.

2
ASGHAR
thermodynamic, and NMR studies, researchers have
previously reported [16–18] that in carbanions de-
rived from benzyltriflones, the negative charge remains
largely on the exocyclic Cα carbon atom, where it is
stabilized by polarization effects.
In a previous study [19], the reactions of the car-
banion from bis(phenylsulfonyl)methane were studied
and the rate constants for the formation of the initial
σ-adducts with some 4-nitrobenzofurazan derivatives
and 4-nitrobenzofuroxan in methanol were reported
along with the methoxide-catalyzed elimination of
phenylsulfinic acid.
In another study, the results of kinetic measure-
ments of the reactions of carbanions derived from
ring-substituted benzyltriflone anions, 3a–e, and nitro-
gen nucleophile with nitrobenzofurazan derivatives, in
methanol and DMSO, respectively, were studied [20–
30] to obtain a comparison of these results with the
previous results obtained with 4-nitrobenzofuroxan.
Here, we report the results of 1H NMR studies
in DMSO-d6 of the σ-adduct-forming reactions of
carbanion 3 derived from benzyltriflone 2 with elec-
trophile 4. The pKa values of the benzyltriflones 3b,
3d, and 3e in methanol are available in the literature
[23], whereas pKa values for 3a and 3c were measured
according to previously reported methods [29,30].
2c were prepared as previously reported [16,32] by
refluxing the appropriately substituted benzylbromide
with potassium triflinate in acetonitrile for 6 h us-
ing potassium iodide as a catalyst; 2a: mp 98◦C,
1
yield = 63%, H NMR (DMSO-d6, 400 MHz) 5.15
(2H, s, CH2), 7.30 (2H, d, J = 8 Hz, H-2 and H-6),
7.20 (2H, d, J = 8 Hz, H-3 and H-5); 2c: mp 94◦C,
1
yield = 44%, H NMR (DMSO-d6, 400 MHz) 5.28
(2H, s, CH2), 7.43 (2H, d, J = 8.2 Hz, H-2 and H-
6), 7.65 (2H, d, J = 8.2 Hz, H-3 and H-5). Com-
pounds 6a–e were prepared with a mixture of 4-
nitrobenzofuroxan, 4, with 2a–e in the presence of
triethylamine in DMSO-d6 or methoxide ions in
methanol. The product does not correspond to that ex-
pected [23] for the σ-adduct, 5a–e, but is attributable
to 6a–e, the products of elimination of trifluoromethyl-
sulfinic acid from 5a–e. Solutions of sodium methox-
ide were prepared by dissolving clean sodium in
AnalaR methanol under nitrogen. Solutions contain-
ing very low equilibrium concentrations of methox-
ide ions were prepared using buffers prepared from
4-bromophenol whose pKa value [33] in methanol is
13.61.
1H NMR spectra in DMSO-d6 were recorded at
24◦C using a Bruker Avance-400 MHz instrument.
−
CHSO2CF3
CH2SO2CF3
H
OMe
SO2CF3
F3CO2S
-
R1
R1
SO2CF3
1
2 a R1 = 4-MeO
b R1 = 4-Me
c R1 = 4-Cl
3 a R1 = 4-MeO
b R1 = 4-Me
c R1 = 4-Cl
d R1 = 4-Br
e R1 = 3-CF3
d R1 = 4-Br
e R1 = 3-CF3
O
N
O
N
NO2
4
EXPERIMENTAL
UV–visible spectra and kinetic measurements were
carried out at 25◦C with a Specord Analytik Jena
spectrophotometer. First-order rate constants, precise
to 5%, were evaluated by using standard methods.
4-Nitrobenzofuroxan, 4, was prepared by nitration of
benzofuroxan [31]. Benzyltriflones 2b, 2d, and 2e
were available from previous work [30], but 2a and
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KINETIC STUDIES OF REACTIONS OF 4-NITROBENZOFUROXAN WITH CARBANIONS
3
For 2a and 2c, it was not possible to obtain high con-
version into the respective carbanions using methanol
alone. Accordingly, the basicity of the medium was
increased using methanol–DMSO mixtures [34]. As
previously reported [22], the sodium methoxide con-
centration was kept constant at 0.025 mol dm−3 and
the ratio of DMSO to methanol was varied. JM values,
relating to base addition, are available for solutions of
sodium methoxide in methanol–DMSO [34]. The JM
values were obtained from the reaction of methoxide
ions with α-cyanostilbenes, as shown in Eq. (2) [35].
This reaction involves the formation of a carbanion
that resembles those formed by the ionization of the
benzyltriflones. Hence, these results were used to re-
flect the basicities of the media used in the present
work. The results presented in Table II give a value of
pKa = 20.66 0.1 for 2a and pKa = 18.66 0.1 for
2c, which were obtained by using Eq. (3).
RESULTS AND DISCUSSION
1H NMR Results
The 1H NMR spectrum of a mixture of 0.04 mol
dm−34-nitrobenzofuroxan, 4, with 0.05 mol dm−3
2a–e, and 0.l mol dm−3 triethylamine, was measured
within 25 min of mixing. Interestingly, the spectrum
does not correspond to that expected [23] for the σ-
adduct, 5a–e, but is attributable to 6a–e, the prod-
uct of elimination of trifluoromethylsulfinic acid from
5a–e.
Chemical shifts for 4 with 2a–e in the presence of
triethylamine are listed in Table I. The shifts of H6,
H7, and Hα are similar to those of the related alkene
[23] formed by the elimination of nitrous acid from
the nitroethane adduct of 4. Further justification that
the reaction occurs at the 5 position rather than the
7 position is presented in a subsequent section.
O
H
O
H7
H5′
H6
H
N
F3CO2S
N
H6′
NEt3
R
+
HSO2CF3
O
O
1
N
N
H3′
H
−
−
H
NO2
NO2
R1
H2′
H
5a–e
R1 = 4-MeO, 4-Me, 4-Cl, 4-Br, 3-CF3
6a–e
pKa Values in Methanol
Because kinetic measurements were performed in
methanol, it was necessary to determine the pKa values
of the benzyltriflones in this solvent. Carbanions 3a and
3c were generated from the benzyltriflones 2a and 2c
by the reaction with sodium methoxide in methanol,
as shown in Eq. (1). Values of the ionization ratio,
[3]/[2], were determined spectrophotometrically using
the strong UV absorbance of the benzyltriflone an-
ions according to Eq. (3). Absorption maxima were at
279 nm for 3a and 290 nm for 3c.
MeO
NC
C
C
H
CH
C
−
OMe
+
-
NC
R
R
(2)
−
CHSO2CF3
CH2SO2CF3
K
−
MeOH
+
(1)
OMe
+
R
R1
1
2 a R1 = 4-MeO
c R1 = 4-Cl
3 a R1 = 4-MeO
c R1 = 4-Cl
International Journal of Chemical Kinetics DOI 10.1002/kin

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ASGHAR
Table II Ionization of 2a,c (4.2 × 10−5 mol dm−3) in
Methanol–DMSO Mixtures Containing Sodium
Methoxide (0.025 mol dm−3
)
2a, 4-MeO
2c, 4-Cl
Abs,
DMSO
(mol%)
Abs
JMa
(279 nm)
pKab
290 nm
pKab
0
3.9
8.1
15.32
15.70
16.10
16.60
17.10
18.45
18.72
19.02
19.64
20.19
20.88
21.42
22.30
–
–
–
–
–
–
–
–
–
–
–
–
0.00095 18.66
–
–
–
–
–
–
12.5
17.2
33.3
36.3
39.4
46.1
53.3
61.0
67.0
70.0
–
–
–
–
0.33
–
0.63
0.79
0.855
–
18.67
–
18.62
18.69
18.65
–
0.019
0.073
0.215
0.52
0.84c
–
20.66
20.66
20.67
20.66
–
–
–
–
–
0.88c
aCalculated from values in [36].
bCalculated from Eq. (3).
cTaken as a value for limiting absorbance at high basicity.
[3]
pKa = JM − log
[2]
(3)
Kinetic Measurements
Kinetic measurements of the reaction of 4-
nitrobenzofuroxan, 4, with carbanions 3a–e generated
in situ by the reaction of the benzyl triflones 2a–e with
methoxide ions were carried out in methanol. Because
the methoxide ions themselves will react with 4 to give
σ-adducts [37,38], the concentration of methoxide was
maintained at a very low level, ca. 10−5 mol dm−3
,
using bromophenol buffers [23,33]. Under these con-
ditions, the UV–visible spectra show that in the case
of 4, the absorbance of the parent molecule at 403 nm,
ε = 8 × 103 dm3 mol−1 cm−1, is gradually replaced
by a much stronger absorption band in the visible re-
gion. The absorption maxima of the products formed
(presented in parentheses) with each carbanion is as
follows: 3a (411 nm), 3b (413 nm), 3c (314 nm), 3d
(415 nm), and 3e (419 nm), with molar absorptivity
values of ε = (2–3) × 104 dm3 mol−1 cm−1
.
It is known that σ-adducts formed by attack of either
methoxide [35] or carbanion [22,23] at the unsubsti-
tuted ring positions of 4 have maxima at ca. 335 nm,
ε = 1.5 × 104 dm3 mol−1 cm−1. Hence, the new bands
observed are not simply due to this process. The shift
to longer wavelength and an increase in absorbance
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETIC STUDIES OF REACTIONS OF 4-NITROBENZOFUROXAN WITH CARBANIONS
5
−
CHSO2CF 3
CH2SO2CF3
−
K
+
MeOH
+
OMe
R1
R1
2
3
O
N
−
O
CHSO2CF3
H
F3CO2S
N
N
k5
k
O
N
+
O
−5
H
−
H
NO2
R1
NO2
R1
5
3
4
−
kel
OMe
O
N
H
H
R1
+
O
HSO2CF3
N
−
NO2
6
Scheme 1 Reactions of benzyltriflone anions with 4-nitrobenzofuroxan in methanol.
are compatible with the increased possibilities of de-
localization in the products of elimination of triflu-
oromethylsulfinic acid, 6, from the initially formed
adducts, 5.
This indicates that nucleophilic attack by carbanions
on the nitrobenzofuroxan is rate limiting so that the
slopes of plots of kobs versus [3] give values of k5. A
summary of the kinetic data is presented in Table IV.
It should be noted that if the elimination step was rate
determining, then the values of kobs would be expected
to show dependences on both carbanion and methoxide
concentrations, which is not the case. In addition, the
reactions are of the first order in the benzofuroxan con-
centration, showing that the formation of carbanions
3a–e from 2a–e and methoxide is not rate determin-
ing. If this were the case, then a zero-order dependence
on the benzofuroxan concentration would be expected.
Hence, the interpretation of our results is shown
in Scheme 1. Measurements at the absorption max-
ima of the products showed good first-order kinetics.
The values of the rate constants, kobs, are presented in
Table III. The magnitude of the change in absorbance
was independent of the carbanion concentration, in-
dicating that the reactions went to completion. There
is no spectroscopic evidence for buildup in the con-
centration of the σ-adduct intermediates, 5a–e, and
the application of the steady-state principle leads to
Eq. (4). The results presented in Table III indicate that
the values of kobs show a linear dependence on the equi-
librium concentrations of the carbanion present but are
not directly dependent on the equilibrium of methox-
ide concentrations. These dependences correspond to
the condition kel[MeO−] ≥ k−5, so that Eq. (4) reduces
to Eq. (5).
COMPARISON
1. The values of k5 decrease as the electron-
withdrawing ability of the substituent in the ben-
zyltriflones increases. The carbanions will be sta-
bilized by the presence of Br and CN groups.
Although only five substituents were measured,
it is worth drawing Hammett plots. Values of
log k5 versus σ− are shown in Fig. 1 and give a
ρ value of −1.00.
kel[MeO−]
kel[MeO−] + k
kobs = k5[3]
kobs = k5[3]
(4)
(5)
−5
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ASGHAR
Table III Kinetic Results for Reactions of 4a with 3a–e in Methanol at 25◦C
[2a] (mol dm−3
)
[MeO−]eq (mol dm−3 b
)
[3a] (mol dm−3 c
)
kobs (s−1
)
k5 (dm3 mol−1 s−1 d
)
0.0107
0.0087
0.0070
2.58 × 10−5
2.58 × 10−5
2.58 × 10−5
5.02 × 10−11
4.06 × 10−11
3.29 × 10−11
2.87 × 10−5
2.31 × 10−5
1.88 × 10−5
5.71 × 105
5.70 × 105
5.71 × 105
[2b] (mol dm−3
0.0107
0.0087
)
[MeO−]eq (mol dm−3 b
2.58 × 10−5
)
[3b] (mol dm−3 c
1.352 × 10−10
1.089 × 10−10
0.884 × 10−10
)
kobs (s−1
)
k5 (dm3 mol−1 s−1 d
4.07 × 105
)
5.5 × 10−5
4.4 × 10−5
3.6 × 10−5
2.58 × 10−5
4.06 × 105
0.0070
2.58 × 10−5
4.07 × 105
[2c] (mol dm−3
0.0107
0.0087
)
[MeO−]eq (mol dm−3 b
)
[3c] (mol dm−3
5.05 × 10−9
4.06 × 10−9
3.28 × 10−9
)
kobs (s−1
)
k5 (dm3 mol−1 s−1 d
)
2.58 × 10−5
2.58 × 10−5
2.58 × 10−5
8.99 × 10−4
7.23 × 10−4
5.80 × 10−4
1.78 × 105
1.78 × 105
1.77 × 105
0.0070
[2d] (mol dm−3
0.0107
0.0087
)
[MeO−]eq (mol dm−3 b
2.58 × 10−5
)
[3d] (mol dm−3 c
)
kobs (s−1
)
k5 (dm3 mol−1 s−1 d
1.58 × 105
)
5.507 × 10−9
4.457 × 10−9
3.603 × 10−9
0.8 × 10−3
0.7 × 10−3
0.6 × 10−3
2.58 × 10−5
1.57 × 105
0.0070
2.58 × 10−5
1.58 × 105
[2e] (mol dm−3
0.0107
0.0087
)
[MeO−]eq (mol dm−3 b
)
[3e] (mol dm−3 c
3.64 × 10−8
2.95 × 10−8
2.40 × 10−8
)
kobs (s−1
)
k5 (dm3 mol−1 s−1 d
)
2.58 × 10−5
2.58 × 10−5
2.58 × 10−5
3.5 × 10−3
2.9 × 10−3
2.3 × 10−3
9.6 × 104
9.8 × 104
9.6 × 104
0.0070
aConcentration is 3.3 × 10−5 mol dm−3
.
bEquilibrium methoxide concentrations in bromophenol buffers.
cCalculated using pKa values determined in this work.
d kobs/[3].
Figure 1 Hammett plots for the reaction of 4-nitrobenzofuroxan, 4, with a series of benzyltriflone carbanions in methanol at
25◦C. Values of log k5 versus σ− give ρ values of −1.00.
2. The Brønsted plot in Fig. 2 of log k5 versus
pKa values of the benzyltriflones provides fur-
ther evidence for the high steric requirement
of the benzyltriflone anions. Thus, the slope
β of the plot has the value of 0.26 for 4-
nitrobenzofuroxan, 4, and this low value obtained
for βnuc is compatible with a steric effect with
a large separation of reactants in the transition
state.
3. The kinetic data in Table III allows the deter-
mination of the rate constants for the nucle-
ophilic attack of the carbanion 3 with 4 and are
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KINETIC STUDIES OF REACTIONS OF 4-NITROBENZOFUROXAN WITH CARBANIONS
7
Figure 2 Brønsted-type plots of log k versus pK values for reaction of 4-nitrobenzofuroxan, 4, with a series of benzyltriflone
a
5
carbanions in methanol at 25◦C, with slope, β, of 0.26 for 4.
Figure 3 Plot of values of pK versus σ− gives a good straight line with slope −3.8 for reaction of 4-nitrobenzofuroxan, 4,
a
with a series of benzyltriflone carbanions in methanol at 25◦C.
mol−1
[22].
s
−1, which is close to the diffusion limit
in accord with the reaction at position 5. The
usual behavior of 4-nitrobenzofuroxan and its
derivatives in reactions with nucleophiles is that
5-adducts are kinetically favored, whereas the
isomeric 7-adducts are thermodynamically more
stable [22,23,30,31,37,38].
O
N
NC
O
4. Although the negative charge is likely to be local-
ized at the carbon center [39] in both carbanions
3 and 4-cyanobenzylcyanide, the values obtained
for k5 are considerably lower than those for the
corresponding reactions of ring-substituted ben-
zylcyanide anions with 4-nitrobenzofuroxan, 4.
For example, the value of the rate constant for
formation of 7 in methanol is 1.4 × 108 dm3
C
N
NC
H
H
−
NO2
7
The lower reactivity of the benzyltriflone carban-
ions may result from their large bulk, which will
International Journal of Chemical Kinetics DOI 10.1002/kin

8
ASGHAR
Table IV Summary of Kinetic and Acidity Data for the
Reaction of 4-Nitrobenzofuroxan, 4, with a Series of
Benzyltriflone Carbanions in Methanol at 25◦C
reactivity by a factor of 15 on going from
4-nitrobenzofuroxan to 4-nitrobenzofurazan,
which is likely to result from the increased
electrophilicity of 4-nitrobenzofuroxan, 4, as a
result of the presence of the N O group [41].
2. The reaction proceeds via initial formation of σ-
adduct intermediates, 5a–e, which is a relatively
rapid process, followed by the elimination of tri-
fluoromethylsulfinic acid from the adduct to give
the corresponding alkenes, 6a–e.
3. The measurements do not allow the determina-
tion of the rate constant for the elimination of
HSO2CF3 from the σ-adduct formed, but the fail-
ure to observe the σ-adduct precursors suggests
that elimination is a relatively facile process.
4. The plot of pKa of the benzyltriflones versus
sigma minus values represents the plot for a
molecule in which a negative charge develops
adjacent to the aromatic ring.
k5 (dm3 mol−1 s−1
4-Nitrobenzofurazanb
)
Carbanion
pKaa
4
4-MeO
4-Mec
4-Cl
20.66
20.23
18.66
18.62
17.80
19.50
17.50
16.00
–
5.71 × 105
4.07 × 105
1.78 × 105
1.58 × 105
9.67 × 104
–
2.95 × 104
–
4-Brc
3-CFc3
Hc
10.00 × 103
6.10 × 103
1.5 × 104
4.5 × 103
1.5 × 103
4-CFb3
4-CNb
–
–
aValues for corresponding acids.
bFrom [29].
cFrom [30].
sterically inhibit their approach to the reaction
center. We expect the decrease in reactivity on
going from 7 to 5 may be attributed to increas-
ing the strong electron-withdrawing effect of the
trifluoromethylsulfonyl group (SO2CF3) that is
well known [40].
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1
From both the H NMR spectra and the kinetic stud-
ies of the reactions of 4-nitrobenzofuroxan, 4, with
ring-substituted benzyltriflone anions, 3, the following
conclusions are inferred:
1. The results of the kinetic studies show
that the rate constants are higher for
4-nitrobenzofuroxan,
nitrobenzofurazan. The decrease in the
4,
than
for
4-
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETIC STUDIES OF REACTIONS OF 4-NITROBENZOFUROXAN WITH CARBANIONS
9
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