Reactions of substituted phenylnitromethane carbanions with aromatic nitro compounds in methanol: Carbanion reactivity, kinetic, and equilibrium studies

Article abstract of DOI:10.1002/kin.20864

The feasibility of carrying out nucleophilic addition from electron-deficient heteroaromatics has been addressed through a detailed investigation of the interaction of a two 7-substituted-nitrobenzofurazan (R = OMe 2a; R = Cl 2b) with a series of substituted-nitroaryl anions (X = 4-NO ₂ 1a; X = 3-NO₂ 1b; X = 4-CN 1c; X = 4-Br 1d), all reactions first lead to the quantitative formation of the σ-adducts 3a-d and 4a-d arising from covalent addition of the nucleophile to the C-5 carbon. The rate and equilibrium constants for the formation of σ-adducts 3a-d and 4a-d (k₅, K₅) together with the rate constants for their decomposition (k_-5) have been determined in methanol at 25C, allowing a determination of intrinsic rate constants, k₀ = 0.03, the lower k₀ value reflects the very strong salvation by methanol of the negative charge on the nitro group. The discovery of a linear correlation between the E and pK_a^MeOH parameters allows a calibration of the electrophilicity power of 2a and 2b, E = -11.67 and -10.29, respectively. Applying the general approach to nucleophilicity/electrophilicity recently developed by Mayr et al. through the relationship log k = s(E + N), a successful ranking of our nitroaryl anions 1a-d on the general nucleophilicity scale (N) has been carried out. The N values of 1a-d are found to cover a range from 15.78 to 16.69. The results are compared with previously reported data in water and DMSO.

Full text of DOI:10.1002/kin.20864

Reactions of Substituted
Phenylnitromethane
Carbanions with Aromatic
Nitro Compounds in
Methanol: Carbanion
Reactivity, Kinetic, and
Equilibrium Studies
BASIM H. ASGHAR
Department of Chemistry, Faculty of Applied Sciences, Umm Al-qura University, P.O. Box: 9569, Makkah, Saudi Arabia
Received 13 July 2013; revised 3 December 2013; 11 May 2014; accepted 12 May 2014
DOI 10.1002/kin.20864
Published online 27 June 2014 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The feasibility of carrying out nucleophilic addition from electron-deficient het-
eroaromatics has been addressed through a detailed investigation of the interaction of a two
7-substituted-nitrobenzofurazan (R = OMe 2a; R = Cl 2b) with a series of substituted-nitroaryl
anions (X = 4-NO2 1a; X = 3-NO2 1b; X = 4-CN 1c; X = 4-Br 1d), all reactions first lead to
the quantitative formation of the σ-adducts 3a–d and 4a–d arising from covalent addition of
the nucleophile to the C-5 carbon. The rate and equilibrium constants for the formation of
σ-adducts 3a–d and 4a–d (k5, K5) together with the rate constants for their decomposition
(k−5) have been determined in methanol at 25°C, allowing a determination of intrinsic rate
constants, k0 = 0.03, the lower k0 value reflects the very strong salvation by methanol of the
negative charge on the nitro group. The discovery of a linear correlation between the E and
pKaMeOH parameters allows a calibration of the electrophilicity power of 2a and 2b, E = −11.67
and −10.29, respectively. Applying the general approach to nucleophilicity/electrophilicity re-
cently developed by Mayr et al. through the relationship log k = s(E + N), a successful ranking
of our nitroaryl anions 1a–d on the general nucleophilicity scale (N) has been carried out. The
Corresponding to: Basim H. Asghar; e-mail: bhasghar@
uqu.edu.sa.
Supporting Information is available in the online issue at
www.wileyonlinelibrary.com.
C
ꢀ
2014 Wiley Periodicals, Inc.

478
ASGHAR
N values of 1a–d are found to cover a range from 15.78 to 16.69. The results are compared with
ꢀC
previously reported data in water and DMSO.
46: 477–488, 2014
2014 Wiley Periodicals, Inc. Int J Chem Kinet
addition rates. To that end, the kinetics of the σ-
adduct forming reactions of a series of nitro-substituted
phenylnitromethane carbanions to 7-methoxy-4-nitro-
benzofurazan 2a and 7-chloro-4-nitrobenzofurazan 2b
have been probed via 1H NMR studies in DMSO-d6. In
addition, kinetic measurements for the reactions of 2a
and 2b with anions derived from 4-nitrophenylnitro-
methane 1a, 3-nitrophenylnitromethane 1b, 4-cyano-
phenylnitromethane 1c, and 4-bromophenylnitro-
methane 1d were obtained in methanol to allow
comparison of the results with both the reactions of
nitromethane anions with 4-nitrobenzofurazan deriva-
tives [14] and the reactions of carbanions stabilized by
trifluoromethylsulfonyl groups [15–18].
Our results additionally have relevance to studies
of the nucleophilic substitution of hydrogen by the
vicarious substitution mechanism [19], a process that
allows the formation of neutral carbon–carbon bonded
species. There is evidence that the first step in this
pathway involves carbanion addition to give a σ-adduct
[20,21], and the present work can provide insight into
the factors governing the kinetics of this process.
INTRODUCTION
There is much current interest in comparing nucle-
ophilicities, particularly those of carbon nucleophiles
[1–3]. The reactions of carbanions stabilized by ester,
acyl, cyano, and nitro groups with a series of ben-
zyl cations have been studied to assess nucleophilici-
ties [4–6]. Although these studies provided useful rela-
tionships, they revealed that relative nucleophilicities
may be strongly influenced by the solvent, e.g., water
versus DMSO, and that often there is a poor correlation
between the nucleophilic reactivity and the pKa values
of the corresponding carbon acids [4–6]. This also in-
dicates that the nucleophilic reactivities of carbanions
can be strongly affected by substituent variation in a
methanol solution.
The nucleophilicities of many carbanions have been
measured, and values for some nitroalkane anions have
been assessed by measuring rate constants for their
reactions with benzhydrylium cations and quinone
methides in water [4], DMSO [4], and methanol–
acetonitrile [6]. Conversely, the electrophilicities of
some electrophiles and superelectrophiles, includ-
ing, respectively, 1,3,5-trinitrobenzene (TNB) and
4,6-dinitrobenzofuroxan (DNBF), have been deter-
mined by measuring the rate constants for their re-
actions with some standard nucleophiles, including N-
methylpyrrole and indole, in acetonitrile [7,8]. It is
known that nitroalkane anions may react to form an-
ionic σ-adducts [1], such as 1, with DNBF [9,10] and
TNB [11], and there is one report of the reaction of the
2-nitropropenide anion with 4-nitrobenzofurazan [12].
However, no rate constants have been reported for the
reactions of nitroaryl anions with substituted (C-7)
neutral electron-deficient heteroaromatic compounds
and it is not known whether the relative product corre-
sponds to a substitution product or σ-adduct formation.
Previous reports have largely concentrated on de-
termining the structures of the adducts formed, al-
though kinetic data are available for the reactions of
nitroalkane anions with TNB in methanol [13] and
with DNBF in water [9]. In a previous study, kinetic
and equilibrium measurements of the reactions of car-
banions derived from nitroalkane and benzyltriflone
anions with nitrobenzofurazan derivatives in methanol
were also investigated [14,15]
EXPERIMENTAL
7-Chloro-4-nitrobenzofurazan 2b (Sigma-Aldrich;
98%) and benzofurazan (Sigma-Aldrich; 97%)
were the purest available commercial samples. 4-
Nitrobenzofurazan was prepared by the nitration of
benzofurazan using 1 equiv of nitric acid (Sigma-
Aldrich; 70%) in 6 equiv of sulfuric acid 98% at 5°C:
mp 92°C (lit. mp 93°C [22]).
7-Methoxy-4-nitrobenzofurazan 2a, mp 113°C (lit.
mp 115°C [23]), was prepared by the reaction of 7-
chloro-4-nitrobenzofurazan with 1 equiv of sodium
methoxide in methanol at 40°C for 1 h. Solutions
of sodium methoxide were prepared by dissolving
clean sodium in methanol under nitrogen. Cloudi-
ness in the resulting solutions was removed by
centrifugation. Other reagents, such as 4-nitrobenzyl
bromide, 3-nitrobenzyl bromide, 4-cyanobenzyl bro-
mide, 4-bromobenzyl bromide, silver nitrite, triethyl-
amine, sodium methoxide, and diethyl ether (all
Aldrich products), were available of the highest quality
and were recrystallized or distilled before use when-
ever necessary. Methanol was used without further pu-
rification.
The present work is designed to extend our un-
derstanding of how nitro groups impact carbanion
International Journal of Chemical Kinetics DOI 10.1002/kin.20864

REACTIONS OF PHENYLNITROMETHANE CARBANIONS WITH AROMATIC NITRO COMPOUNDS
479
Scheme 1
NMR spectra of the parent compounds 2a and
2b were carried out in DMSO-d6 in the absence of
reagents, and then carried out in separate experiments
by adding the reagents 1a–d and following the spec-
tral changes toward formation of σ-adducts 3a–d and
4a–d.
UV–vis spectra and kinetic measurements were
made with a Shimadzu UV-2101 PC or SPECordꢀR
Analytik Jena spectrophotometer. First-order rate con-
stants, precise to 5%, were evaluated following the
appearance of the σ-adduct 3a–d 390 < λ < 410 nm
and 4a–d 418 < λ < 430 nm. All reactions in this study
obeyed the pseudo–first-order kinetics under condi-
tions of excess nitroaryl anions 5a–d. Pseudo–first-
order rate constants, kobs, were obtained from the cor-
relation ln(Aꢀ – At) against time, where Aꢀ refers to the
absorbance of the product 3a–d and 4a–d at the com-
pletion of the reaction and At refers to the absorbance
at time t.
Preparation of 4-Cyanophenylnitromethane
1c and 4-Bromophenylnitromethane 1d
The preparations of 1c and 1d were previously re-
ported [25] (Scheme 1). An example procedure fol-
lows: 4-Cyanobenzyl bromide (12.5 g) and silver ni-
trite (9.0 g) were refluxed in dry diethyl ether for 2 h.
Silver bromide was filtered, and the filtrate was con-
centrated. Recrystallization from ethanol gave crystals
of 1c, mp 46°C. Compound 1d was prepared from 4-
bromobenzyl bromide in a similar manner, mp 84°C.
RESULTS AND DISCUSSION
1H NMR Studies
1H NMR measurements were performed in DMSO-d6
to determine the structures of the intermediates and
products produced in the reactions under investiga-
tion. These processes are summarized in Scheme 2.
Spectral data for mixtures of the nitrobenzofurazan
derivatives with two molecular equivalents each of the
phenylnitromethanederivatives and triethylaminewere
recorded after mixing and the instantly formation of
products 3a–d and 4a–d.
The pseudo–first-order rate constants, kobs, for all
the reactions obeyed Eq. (1):
kobs = k5[5] + k
(1)
−5
From the slopes and intercepts of the plots kobs = k5
[5], the second-order rate constants k5 and first-order
rate constants k–5 were readily derived, respectively.
Spectra of 7-methoxy-4-nitrobenzofurazan 2a
(0.2 mol dm−3), phenylnitromethane derivatives
(0.7 mol dm−3), and triethylamine (0.5 mol dm−3
)
in DMSO-d6 were obtained. Data for the parent
molecules are listed in Table I, and chemical shifts of
the σ–adducts are presented in Table II. The methoxy
signal of 7-methoxy-4-nitrobenzofurazan 2a appeared
at δ 3.68 ppm.
The signals due to unreacted 7-methoxy-4-nitroben-
zofurazan were seen at δ 8.76, 7.10, and 4.22 ppm.
No change in the spectrum occurred after 24 h.
With 7-chloro-4-nitrobenzofurazan 2b (0.2 mol dm−3),
phenylnitromethane derivatives (0.5 mol dm−3), and
triethylamine (0.5 mol dm−3) in DMSO-d6 (Table II),
the spectra indicate rapid and irreversible decomposi-
tion, thus no products could be positively identified.
Preparation of 4-Nitrophenylnitromethane
1a and 3-Nitrophenylnitromethane 1b
The preparations of 1a and 1b were previously reported
[24] (Scheme 1). An example procedure follows: To
a mixture of silver nitrite (50 g, 0.325 mol) in dry
diethyl ether (500 mL), a solution of 4-nitrobenzyl
bromide (21.6 g, 0.1 mol) was added in dry diethyl
ether (800 mL) over 1 h. The mixture was stirred at
0°C for 24 h. After filtration of the silver salts, the
filtrate was evaporated via a rotary evaporator to afford
a solid residue, which was recrystallized from ethanol.
1a: mp 90–91°C. 1b: mp 91–92°C.
International Journal of Chemical Kinetics DOI 10.1002/kin.20864

480
ASGHAR
Scheme 2
Table I 1H NMR Dataa for Parent Molecules in DMSO-d6
δ, Chemical shift
Compound
H5
H6
Me
CHα
J5,6
7-Methoxy-4-nitrobenzofurazan, 2a
7-Chloro-4-nitrobenzofurazan, 2b
8.75
8.68
7.06
8.03
4.21
-
-
-
H6
8.6
7.6
CHα
-
-
-
-
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
H2
H4
H5
J4 ,5
J5 ,6
3-Nitrophenylnitromethane, 1b
7.89
H5
7.82
H6
7.75
Me
7.89
CHα
5.51
8.0
8.0
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
H2 ,6
H3 ,5
J2 ,3
4-Nitrophenylnitromethane, 1a
4-Cyanophenylnitromethane, 1c
4-Bromophenylnitromethane, 1d
-
-
-
-
-
-
-
-
-
5.60
5.48
5.30
7.80
7.78
7.45
7.75
7.70
7.69
8.0
8.0
8.0
aCoupling constants are in hertz.
Table II 1H NMR Dataa for σ-Adducts Formed with Phenylnitromethane Derivatives in DMSO-d6
Compound
δ, Chemical shifts
7-methoxy-4-nitrobenzofurazan, 2a
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
δ H5
δ H6
CHα
H2 ,6
H3 ,5
J5,6
JH Hα
JH H
J2 ,3
7
6
5
4-nitrophenylnitromethane, 1a
4-cyanophenylnitromethane, 1c
4-bromophenylnitromethane, 1d
5.01
4.90
4.80
δ H5
5.80
5.70
5.63
δ H6
6.40
6.25
6.20
CHα
7.77
7.74
7.41
7.74
7.70
7.68
6.2
6.2
6.2
1.2
1.2
1.2
0.8
0.8
0.8
8.8
8.8
8.8
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
H2
H4
H5
H6
J5 ,6
JH Hα
JH H
7
6
5
3-nitrophenylnitromethane, 1b
7-chloro-4-nitrobenzofurazan, 2b
4.99
5.75
6.35
7.91
7.84
7.77
7.90
8.2
1.2
0.8
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
δ H5
δ H6
CHα
H2 ,6
H3 ,5
J5,6
JH Hα
JH H
J2 ,3
7
6
5
4-nitrophenylnitromethane, 1a
4-cyanophenylnitromethane, 1c
4-bromophenylnitromethane, 1d
5.90
5.03
4.99
δ H5
6.86
6.50
5.96
δ H6
6.80
6.40
6.30
CHα
7.79
7.76
7.47
7.76
7.73
7.70
6.0
6.0
6.0
1.1
1.1
1.1
0.8
0.8
0.8
8.8
8.8
8.8
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
H2
H4
H5
H6
J5 ,6
JH Hα
JH H
7
6
5
3-nitrophenylnitromethane, 1b
4.51
5.77
6.39
7.94
7.88
7.79
7.93
8.2
1.1
0.8
aCoupling constants are in hertz.
Generally, the recorded results indicate that, for the
reactions of 2a and 2b with some selected nucleophiles,
5-adducts formation is kinetically favored.
This notice of the reactivity pattern has been
recorded for some reactions with methoxide ions
[23,28,30] in methanol, hydroxide ions [29] in water,
International Journal of Chemical Kinetics DOI 10.1002/kin.20864

REACTIONS OF PHENYLNITROMETHANE CARBANIONS WITH AROMATIC NITRO COMPOUNDS
481
sulfite ions [22] in water, and aliphatic amines [31] in
DMSO. The brief explanation of this phenomenon is
that the 5-adducts have fewer resonance forms avail-
able to delocalize negative charge than the 7-adducts,
leading to smaller kinetic barriers and also the smaller
thermodynamic stabilities [12,31]. It is interesting to
cite that neither 2a nor 2b showed no evidence for nu-
cleophilic attack at the substituted 7-position. This can
be explained through the steric and electrostatic repul-
sion between the entering carbon base and the leaving
group [23,29].
Kinetic and Equilibrium Measurements in
Methanol
These measurements were made spectrophotomet-
rically using phenylnitromethane anion derivatives,
Scheme 3
which were generated in situ from the parent phenyl-
nitromethane derivatives and sodium methoxide. Ow-
ing to the known reactions of 4-nitrobenzofurazans
with methoxide [20,22,27], it was necessary to reduce
the concentration of free methoxide ions to low val-
ues; this was accomplished by working with [phenyl-
nitromethane] ࣙ [MeO−] stoichiometries.
Kinetic measurements were made at 25°C, with
concentrations of 1a–d and methoxide in excess of
the concentrations of 2a and 2b. Under these condi-
tions, first-order kinetics were observed; the rate con-
stant was designated as kobs. These processes are in-
terpreted in terms of Scheme 3. It is known that the
first step, the equilibration of the phenylnitromethane
derivatives with methoxide, is rapid [13]. In view of the
1H NMR results and the known reactivity with other
nucleophiles [22–29], the process that we observed was
likely the formation of the 5-adduct. Therefore, Eq. (1)
will apply:
which the maximum shifted to 390 nm with increased
absorbance. Kinetic measurements obtained for the re-
action of 2a (5 × 10−5 mol dm−3) with 3-nitrophenyl
compound 1b (0.07 mol dm−3) and methoxide in
methanol are reported as specimen data in Table III.
A linear plot of kobs versus [4] gives a value for k5. The
plot, according to Eq. (1) and shown in Fig. 1, was lin-
ear, giving the values k5 = 3.50 × 102 dm3 mol−1 s−1
and k–5 = 1.10 × 10−3
s
−1. Combining these values
gave K5 = k5/k–5. The reactions with 1a, 1c, and 1d
resulted in values for k5 of 2.41 × 102, 5.31 × 102, and
8.35 × 102 dm3 mol−1 s−1, respectively. Values for k5,
k–5, and K5 are listed in Table IV. It should be noted
that the equilibrium constant [28] for the formation of
the 5-methoxy adduct was 22 dm3 mol−1; thus, at the
equilibrium methoxide concentrations used here, there
was no interference from this process. Methoxide may
also be added at the 7-position to give a dimethoxy
adduct. However, the rate constant, 14.5 dm3 mol−1
kobs = k5[5] + k
(1)
−5
s
−1, is sufficiently small that this process [23] cannot
7-Methoxy-4-nitrobenzofurazan, 2a
compete kinetically with carbanion addition at the 5-
1
The parent 2a exhibited a UV maximum at 375 nm,
ε = 9.1 × 103 dm3 mol−1 cm−1. In the presence of
the anions of 1a–d, a process, kobs, was observed in
position. As found from the H NMR spectra, there
was no evidence for elimination from the 5-adducts to
give alkene derivatives.
Table III Kinetic Data for the Reaction of 7-Methoxy Substrate 2aa with 1ba and Methoxide in Methanol at 25°C
[NaOMe] (mol dm−3
)
[MeO−]b (mol dm−3
)
[3-nitro anion]b (mol dm−3
)
kobs (s−1
)
c
eq
0.007
0.010
0.013
0.017
1.201 × 10−11
1.720 × 10−11
2.232 × 10−11
2.920 × 10−11
6.99 × 10−3
10.01 × 10−3
11.30 × 10−3
17.00 × 10−3
0.61 × 10−1
0.35 × 10−1
0.39 × 10−1
0.59 × 10−1
a[2a] = 5 × 10−5 mol dm−3a and [1b] = 0.07 mol dm−3
.
bCalculated using a value for KCH of 8.32 × 109 dm3 mol−1. Measured as a
c
color-forming process at 418 nm.
International Journal of Chemical Kinetics DOI 10.1002/kin.20864

482
ASGHAR
Figure 1 Plots according to Eq. (1), where k5 = 3.50 × 102 and 1.59 × 103, for the reactions of 7-methoxy-4-nitrobenzofurazan
2a with 3-nitrophenylnitromethane anion 1b and 7-chloro-4-nitrobenzofurazan 2b with 4-nitrophenylnitromethane anion 1a,
respectively.
Table IV Comparison of Kinetic and Equilibrium Data for Reaction of 4-Nitrobenzofurazan Derivatives with
Phenylnitromethane Anions in Methanol at 25°C
Phenylnitromethane Anion
k5 (dm3 mol−1 s−1
)
k
−
(s−1
)
K5 (dm3 mol−1
)
5
7-Methoxy-4-nitrobenzofurazan, 2a
1.30 × 10−3
4-Nitrophenylnitromethane, 1a
3-Nitrophenylnitromethane, 1b
4-Cyanophenylnitromethane, 1c
4-Bromophenylnitromethane, 1d
2.41 × 102
3.50 × 102
5.31 × 102
8.35 × 102
1.85 × 105
3.18 × 105
5.90 × 105
1.04 × 106
1.10 × 10−3
0.90 × 10−3
0.80 × 10−3
7-Chloro-4-nitrobenzofurazan, 2b
4-Nitrophenylnitromethane, 1a
3-Nitrophenylnitromethane, 1b
4-Cyanophenylnitromethane, 1c
4-Bromophenylnitromethane, 1d
1.59 × 103
2.31 × 103
3.46 × 103
5.31 × 103
3.98 × 10−3
3.9 × 105
2.5 × 10−3
9.2 × 105
-
-
-
-
Table V Kinetic Data for the Reaction of 7-Chloro Substrate 2ba with 1aa and Methoxide in Methanol at 25°C
b
c
[NaOMe] (mol dm−3
)
[MeO−]eq (mol dm−3
)
[4-nitro anion]b (mol dm−3
)
kobs (s−1
)
0.007
0.010
0.013
0.017
1.20 × 10−12
1.72 × 10−12
2.23 × 10−12
2.92 × 10−12
6.99 × 10−3
10.00 × 10−3
11.98 × 10−3
17.01 × 10−3
1.11 × 101
1.59 × 101
1.90 × 101
2.70 × 101
a[2b] = 5 × 10−5 mol dm−3a and [1a] = 0.07 mol dm−3
.
bCalculated using a value for KCH of 8.32 × 1010 dm3 mol−1. Measured as a
c
color-forming process at 430 nm.
ments were made in the range 418–430 nm, and values
for the rate constants were independent of the wave-
7-Chloro-4-nitrobenzofurazan, 2b
7-Chloro-4-nitrobenzofurazan, 2b, showed a UV ab-
sorption maximum at 337 nm, ε = 1.0 × 104 dm3
mol−1 cm−1. A reaction with the four phenylni-
length of measurement.
Data for the reaction of 2b (5 × 10−5 mol dm−3
)
with 1a (0.07 mol dm−3) and methoxide in methanol
are reported as specimen data in Table V. A linear
plot of kobs versus [4] gives the value for k5. A plot
tromethane anions 1a–d showed a rapid process, kobs
,
giving an increase in the absorbance. Kinetic measure-
International Journal of Chemical Kinetics DOI 10.1002/kin.20864

REACTIONS OF PHENYLNITROMETHANE CARBANIONS WITH AROMATIC NITRO COMPOUNDS
483
Figure 2 Correlations between log k5 of the reactions of aryl-substituted nitronates 1a–d with electrophiles 2a and 2b in
MeOH and the corresponding σ− values [30]. Correlation equations: (•) log k5 = −0.43σ− + 3.80 (r2 = 0.821), (ꢀ) log k5 =
−0.44σ− + 3.00 (r2 = 0.822).
according to Eq. (1), shown in Fig. 1, was linear, giving
trifluoromethane sulfonyl–stabilized carbanions with
quinone methides in DMSO have previously been in-
vestigated and showed the same trend [5,31]
the values k5 = 1.59 × 103 dm3 mol−1 s−1 and k–5
=
3.98 × 10−3
s
−1. The combination of these values
gave K5 = k5/k–5. The reactions with 1b, 1c, and 1d
led to k5 values of 2.31 × 103, 3.46 × 103, and 5.31 ×
103 dm3 mol−1 s−1, respectively. Values for k5, k–5, and
K5 are listed in Table IV. It is worth noting that changes
in the value of k5 were much bigger than those in the
value of k–5, which probably indicates a product-like
transition state for these reactions. Thus, the value of
k–5 was too small to be accurately determined.
Determination of Estimated Electrophilicity
Parameters E of 4-Nitro-7-X-benzofurazan
2a–b
According to Mayr and co-workers’ studies, it is
possible to describe the rates of a large variety of
electrophile–nucleophile combinations by the three pa-
rameter as shown in Eq. (2) [3,32,33] From this equa-
tion, it is clear that estimated E parameter expresses
the strength of the electrophile, whereas the N and s
parameters express the sensitivity to the nucleophile.
According Eq. (2), general electrophilicity (E) and
nucleophilicity (N) scales have been identified. These
scales have found to be interesting for predicting reac-
tivity [3,32,33]:
Hammett Correlations
Nucleophilic reactivities of phenolate and benzyl an-
ions usually correlate with the Hammett–Brown pa-
rameter, σ− (direct conjugation of the lone pair at the
reaction center with the variable substituents) [30]. Ac-
cordingly, the reactions of carbanions 1a–d with vari-
ous electrophiles 2a–b in MeOH follow the Hammett
− correlations with small negative reaction constants
log k = s (N + E)
(2)
σ
(ρ ࣈ −0.5), though with low correlation coefficients,
indicating that only a small part of the negative charge
in the carbanions is delocalized by the aromatic ring
(Fig. 2).
Relative reactivities of aryl-substituted nitronates
with benzhydrylium ions and quinone methides in
DMSO and water also the reactivities of benzene and
It has been found that the electrophilicity of the
series of neutral electron-deficient nitroaromatics and
heteroaromatics of widely different in terms of reac-
tivity and the structure is appropriately described by
Eq. (2). A most characteristic feature, however, was
the finding that the measured estimated E values are
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484
ASGHAR
Figure 3 Ranking of 7-methoxy and 7-chloro-4-nitrobenzofurazan on the E scale, as defined by Mayr et al.
linearly related to the pKαMeOH values for covalent
methoxylation of the Meisenheimer structures, defin-
ing a correlation as represented in Eq. (3) [34]:
in Fig. 3, making possible a comparison of their re-
activity with that of representative structures previ-
ously calibrated by Mayr. Referring to the set of
cationic and neutral electrophiles used to construct
the E scale estimated, the finding of E ca. −10.29
for 2b and −11.67 for 2a indicates that these two 7-X-
nitrobenzofurazans are two and three orders of mag-
nitude, respectively, more electrophilic than the refer-
ence trinitro compound (E = −13.19 for TNB); the
reactivity of 2b is found to approach that of the most
reactive neutral substrate studied by Mayr, namely,
benzylidenemalonitrile (E = −9.42), however, con-
siderably lower than that of DNTP (E = −4.67) and
E = −0.662 pKαMeOH − 1.53
(3)
Referring to methoxylation at an unsubstituted car-
bon C-5 to minimize steric effects, the following es-
timates of the E values (ꢁ 0.5 unit) for our 7-X-
nitrobenzofurazan are obtained from Eq. (3): E =
−11.67 for 2a (pKαMeOH = 15.33) and E = −10.29
for 2b (pKαMeOH = 13.23) [23]. This leads to their
ranking on the Mayr electrophilicity scale, as shown
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REACTIONS OF PHENYLNITROMETHANE CARBANIONS WITH AROMATIC NITRO COMPOUNDS
485
Table VI Comparison of the Reactivity Parameters N
and s for Carbanions 1a–d in Methanol, Water, and
DMSO
DNBF (E = −5.06), the two most reactive Meisen-
heimer electrophiles known to date. Anchoring to car-
bocationic reactivity, the electrophilicity of 2b is of the
same order as that of a benzhydrylium cation (E =
−10.04) but lower than that other positively charged
species such as methyl(phenyl)methyleneammonium
(E = −5.17) and the tropylium ion (E = −3.72).
N/s
Carbanion
Methanola
Waterb
DMSOb
1a
1b
1c
1d
15.78/0.59
15.95/0.59
16.29/0.59
16.69/0.58
13.58/0.52
14.25/0.46
13.23/0.52
16.29/0.75
18.06/0.71
16.96/0.73
Nucleophilicities Parameters of Carbanions
1a–d in Methanol
aThis work. bFrom [5].
A more comprehensive comparison of nucleophilic re-
activities can be based on Eq. (2), to determine the
nucleophilicity parameters N and s of the nucleophiles
1a–d, a plot of the rate constants (log k5) of the re-
actions of representative carbanions with reference
electrophiles 2a and 2b in methanol against the cor-
responding estimated E parameters. Evaluation of the
correlation lines according to Eq. (2) yields the N and s
parameters for carbanions 1a–d in methanol, as listed
in Table VI.
As expected, electron-withdrawing groups at the
paraposition of the aromatic ring decrease the nucle-
ophilicities N of the carbanions 1a–d. On the other
hand, the solvent effects, as previously reported for
the basicities of carbanions [35,36], have now been
found for their nucleophilicities. Because the stabi-
lization of most carbanions occurs to a large extent
through hydrogen bonding, the basicities as well as
the nucleophilicities of carbanions are usually smaller
in protic than in aprotic solvents, in line with the
observation that the nucleophilicities of carbanions 1a–
d are higher in DMSO than water and methanol. How-
ever, the decrease in nucleophilicity strongly depends
on the nature of the carbanions. As a consequence, the
carbanions 1a–d, which are the stronger nucleophiles
in DMSO, are the weaker nucleophiles in water. Mayr
et al. have recently demonstrated that the solvent ef-
fect on the carbanions (aromatic nitronates) is much
smaller (ꢀN = 3–6), It is thus indicated that the nu-
cleophilicities parameters of our carbanions 1a–d in
methanol are generally within in this range, which is
the case here (Fig. 4).
Figure 4 Comparison of the nucleophilicities of carbanions 1a–d in methanol, water, and DMSO.
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486
ASGHAR
Figure 5 Logarithmic plots of the rate constants versus equilibrium constants for the reactions of phenylnitromethane anions
at the 5-positions of 7-methoxy-4-nitrobenzofurazan 2a (ꢀ k5 values and ꢀ k– values) and 7-chloro-4-nitrobenzofurazan 2b
5
(ꢁ k5 values and × k– values).
5
From the kinetic studies and the NMR spectra for
the reactions of nitrobenzofurazans 2a,b with 1a–d,
the following comparisons and conclusions were in-
ferred: The results in Table IV show that the reactivity
order, in terms of k5 values, for each of the phenylni-
tromethane anions was 7-chloro-4-nitrobenzofurazan,
2b > 7-methoxy-4-nitrobenzofurazan, 2a. The higher
reactivity of the chloro derivative 2b than the methoxy
derivative 2a was consistent with the inductive effect,
σm 0.37, expected for chlorine meta to the reaction site
at the 5-position [37]. The reduced electrophilicity of
2a is attributed to resonance stabilization in the parent
between the nitro and methoxy ring substituents [22]
For 2b and 2a, the addition of carbanions occurred at
the 5-position. There was no evidence for the displace-
ment of chloride or methoxide by the carbon nucle-
ophiles, presumably due to steric factors, as shown in
Scheme 3. The usual behavior of 4-nitrobenzofurazan
and its derivatives in reactions with nucleophiles is that
5-adducts are kinetically favored [1,38].
Although the data are limited, I thought it is worth-
while to estimate a value for the intrinsic rate con-
stant k0, in the Marcus [39] sense, was determined by
suitable interpolation or extrapolation of the Bronsted
plots; it has been recognized that there is commonly an
inverse relationship between the intrinsic reactivity of
a carbon acid and the extent of the structural and sol-
vational reorganization, which is required to form the
conjugate carbanion. The greater the resonance stabi-
lization and therefore the sp2-hybridized character of
the resulting carbanion, the greater, in general, are the
structural and solvation changes involved in the ion-
ization process and the lower the intrinsic reactivity.
In practice, k0 is obtained by interpolation or ex-
trapolation of log k5 and log k–5 versus log K5 where
log K5 = 0 as shown schematically in Fig. 5. It is pos-
sible to estimate a value for the intrinsic rate constant
for the carbon–carbon bond forming reaction between
the phenylnitromethane derivatives anions with 2a or
2b. A plot of the values of log k5 and log k–5 ver-
sus log K5 gave k = 0.03, as shown in Fig. 5. The
value of 0.03 can be compared with values of k0, 0.20
and 0.05 found previously [13] for corresponding re-
actions of the nitroalkane anions with trinitrobenzene
and nitrobenzofurazans, respectively. The values of the
intrinsic rate coefficients are thought [40–44] to reflect
the amount of electronic-structural reorganization and
solvent reorganization accompanying a reaction; the
more reorganization required, the lower the reactivity.
The anions in methanol may be represented by Scheme
4 in which the negative charge is largely on the ni-
tro group and will be strongly solvated. The lower k0
value for the nitrobenzofurazan derivatives than trini-
trobenzene and phenylnitromethane derivatives than
nitroalkane anions may reflect the very strong solvation
by methanol of the negative charge on the nitro groups.
Bernasconi [40–44] has shown that the effects of a re-
mote substituent, such as R in the benzyl cyanides,
on values of intrinsic rate constants is expected to be
small.
The value of k5 for the reaction of 4-
nitrophenylnitromethane 1a was found to be lower than
that for nitromethane [14], thus indicating considerably
lower reactivity. These reaction results indicated that
factors that affect the reactivities include the extent of
charge delocalization and solvation by methanol, and
the anions may be represented by Scheme 4 in which
the negative charge is largely on the nitro group and
will be strongly solvated: high in the case of phenyl-
nitromethane, according to results with nitromethane
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REACTIONS OF PHENYLNITROMETHANE CARBANIONS WITH AROMATIC NITRO COMPOUNDS
487
Scheme 4
[14] and low in the case of benzyltriflone derivatives
(i.e., the negative charge is more localized on the car-
bon atom than in the nitroalkane anions) [15].
In agreement with this, it should be noted that Hib-
bert [45] found that little structural reorganization was
involved in the ionization of disulfonyl carbon acids in
water.
B.H.A. is indebted to King Abdul Aziz City for Science and
Technology (KAST) for funding project LGP-13–1 of this
study and to Dr. M. R. Crampton, Durham University, UK,
and Dr. Nizar El Guesmi, Faculte´ des Sciences de Monastir,
Tunisia, for their valuable advice.
Through comparison of the rate constant k5 values in
Table IV with the previously reported 1 corresponding
values for carbanion derivatives, it was concluded that
the 4-bromophenylnitromethane 1d was the most reac-
tive carbanion whereas the 4-nitrophenylnitromethane
1a was the least reactive. Therefore, the values of k5
decreased with increasing electron withdrawal by the
parasubstituent in the phenylnitromethane derivatives.
The carbanions will be stabilized by the presence of
Br, CN, and NO2 groups.
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