Dual reactivity of 1-chloro- and 1-bromo-3,5-dinitrobenzenes in aromatic nucleophilic substitution

Article abstract of DOI:10.1016/j.mencom.2017.03.018

Aromatic nucleophilic substitution reactions in 1-halo-3,5-dinitrobenzenes (where the halogen is bromine or chlorine) can occur with replacement of either a nitro group or a halogen atom, depending on the nature of anionic nucleophile Nu^– as a Lewis base (hard, soft or intermediate), as well as on the polarity of the dipolar aprotic solvent.

Full text of DOI:10.1016/j.mencom.2017.03.018

Mendeleev
Communications
Mendeleev Commun., 2017, 27, 160–162
Dual reactivity of 1-chloro- and 1-bromo-3,5-dinitrobenzenes
in aromatic nucleophilic substitution
Mikhail D. Dutov,*a Svyatoslav A. Shevelev,a Vladimir N. Koshelev,b David R. Aleksanyan,b
Olga V. Serushkina,a Olga D. Neverova,a Evgeniya V. Kolvinab and Egor S. Bobrovc
a N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow,
Russian Federation. Fax: +7 499 135 5328; e-mail: dutov@ioc.ac.ru
b I. M. Gubkin Russian State University of Oil and Gas, 119991 Moscow, Russian Federation.
Fax: +7 499 507 8877
c D. I. Mendeleev University of Chemical Technology of Russia, 125047 Moscow, Russian Federation.
Fax: +7 499 978 8657
DOI: 10.1016/j.mencom.2017.03.018
NO2
O2N
NO2
NO2
Aromatic nucleophilic substitution reactions in 1-halo-3,5-di-
nitrobenzenes (where the halogen is bromine or chlorine)
can occur with replacement of either a nitro group or a
halogen atom, depending on the nature of anionic nucleophile
Nu– as a Lewis base (hard, soft or intermediate), as well as
on the polarity of the dipolar aprotic solvent.
Nu–
+
Hal
Nu O2N
Hal
Hal = Br, Cl
Nu
NuH = 1,2,4-triazole, ArSH, PhOH, CF3CH2OH, PhC(Me)=NOH
This paper reports a part of our studies on aromatic nucleophilic
substitution (SNAr) in the series of 1-X-3,5-dinitrobenzenes
(X = NO2, SO2R, N-azolyl),1–6 in which activation of substitu-
tion is determined only by the inductive effect (–I) of substituents.
Herein, specific features of 1-chloro- and 1-bromo-3,5-dinitro-
benzenes in the SNAr reaction are studied. It is known that the
substitution rate and the pathway in the SNAr reaction are generally
determined by the first stage, i.e., reversible formation of anionic
ipso-s-complexes, which is usually determined by the enthalpy
factor.7,8
Only the halogen atom (F, Cl, Br, I) is replaced in 1-halo-
2,4-dinitrobenzenes, irrespective of the reaction conditions and
the character of the nucleophile nature, which is associated
with the high thermodynamic stability of the corresponding
ipso-s-complex that is caused by considerable delocalization of
the negative charge due to –M and –I effects of the o/p-nitro
groups. In the case of 1-halo-3,5-dinitrobenzenes, activation of
substitution decreases abruptly due to the absence of conjugation.
In 1-fluoro-3,5-dinitrobenzene, the fluorine atom is replaced on
treatment with alkoxides,9 however, the rate of replacement
on treatment with the MeO–/MeOH system is ~5 orders smaller
than that of 1-fluoro-2,4-dinitrobenzene under the same condi-
tions.8(a) At the same time, unlike in 1-fluoro-3,5-dinitrobenzene,
only a nitro group is replaced in 1-chloro- and 1-bromo-3,5-di-
nitrobenzenes treated with sodium methoxide in MeOH10–13 or
PhCH2OH + KOH in tetramethylurea.14 The only example of
nucleophilic substitution of a chlorine atom in 1-chloro-3,5-di-
nitrobenzene on treatment with morpholine in DMSO at 100°C,
which gives a mixture of a chlorine atom replacement (35%) and
a nitro group replacement (14%) products, is reported.15
As noted above, reactions of 1a and 1b with alkoxides, which
are hard Lewis bases, with replacement of a nitro group are
reported in literature. It may be assumed that carbon atoms
bound to chlorine or bromine atoms are softer Lewis acids than
a carbon atom bound to a nitro group, since the C–Hal moiety
(Hal = Cl, Br) is more polarizable (the +M effect of a halogen)
and is bound to a less electronegative atom than in the C–NO2
moiety. Therefore, hard Lewis bases would more easily form
an ipso-s-complex with the carbon atom in a C–NO2 moiety.
It may be expected that softer Lewis acids would be capable of
forming an ipso-s-complex with a C–Hal moiety.
As concerns the effect of solvents, their ability to solvate an
ipso-s-complex would favour the thermodynamic stability of
these complexes and thus affect the course of the reaction. In
accordance with the hard and soft acids and bases principle,16
dipolar aprotic solvents (DAS) are more capable of solvating
easily polarizable (soft) large anions. In this study we used the
following DAS listed in ascending order by polarity and basicity:
MeCN, N-methylpyrrolidone (NMP) and hexamethylphosphor-
triamide (HMPA).
The following nucleophiles (Nu–) with various properties
were used as Lewis bases: (1) 1,2,4-triazolate anion, a hard
Lewis base due to its high basicity, difficult oxidation and low
polarizability of the nucleophilic center (which is within the
aromatic system); (2) phenoxide, a softer Lewis base as compared
to aliphatic alkoxides (lower basicity, easier oxidation and higher
polarizability of the nucleophilic center); (3) arenethiolates, soft
Lewis bases (easy oxidation, low basicity and high polarizability of
the nucleophilic center); (4) acetophenone oximate Ph(Me)C=NO–
,
a Lewis base with intermediate properties since its basicity is
close to that of phenoxide (pKa 11 for acetophenone oxime17(a) and
pKa 10 for phenol18), at the same time oximates possess higher
nucleophilicity;17(b)–(d) (5) 2,2,2-trifluoroethoxide CF3CH2O–, a
softer anion than the non-fluorinated analogue as it has a much
smaller basicity.
We assumed that, due to the weakening of activation of the
halogen atoms in 1-bromo-3,5-dinitrobenzene 1a and in 1-chloro-
3,5-dinitrobenzene 1b under the effect of meta nitro groups (only
the –I effect), the reaction pathway may be affected by external
factors, i.e., the nature of the nucleophile and the nature of the
solvent. In this work, the direction of nucleophilic substitution in
1
We used the NuH + K2CO3 system, molar ratio 1a (1b):NuH:
6
a,b is considered in terms of hard and soft Lewis acids and bases.1
K2CO3 of 1:1:2, in order to generate Nu– from NuH. The reac-
© 2017 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
– 160 –

Mendeleev Commun., 2017, 27, 160–162
Hal
Table 1 Reactions of 1-halo-3,5-dinitrobenzenes with nucleophiles.
Hal
N
K2CO3
N
Products, HPLC ratios
Starting compounds
+
N
and isolated yields (%)
N
NH
N
O2N
NO2
Entry
1-halo-
3,5-dinitro- NuH
benzene
O2N
1a,b
2a,b
a Hal = Br
b Hal = Cl
MeCN
NMP
Resin
HMPA
1
2
3
4
5
6
7
1a
1b
1a
1b
1a
1b
1a
1,2,4-triazole
1,2,4-triazole
PhSH
2a (20)
2b (18) Resin
Resin
Resin
3 (44)
3 (41)
4 (38)
4 (42)
5a + 7, ~1:1
(12, 19)
5b + 7, ~1:1
(15, 11)
6a + 8,
~1:1
6b + 8,
~1:1
9a + 10, ~1:1
(30, 41)
Scheme 1
Resin
Resin
Resin
Resin
5a (60)
3 (43)
3 (45)
4 (40)
4 (35)
5a + 7,
~6:1
always occurred incompletely. No principal differences between
the bromo and chloro derivatives were found (except for the
diversity of admixtures). The choice of the solvents followed
from previous experience.19
PhSH
4-ClC6H4SH
4-ClC6H4SH
PhOH
The reaction of 1a,b with 1,2,4-triazole in both NMP and HMP
A
gave only resinous products. On using MeCN, the main products
2a,b of the nitro group replacement were isolated (Scheme 1).
Reactions of 1a,b with arenethiols (Scheme 2) in NMP or
HMPA resulted only in halogen substitution products to give
known4,20 1-arylsulfanyl-3,5-dinitrobenzenes 3, 4 in 35–45%
yields.
8
9
1b
1a
1b
1a
1b
PhOH
5b (66) 5b + 7,
~6:1
6a + 8,
~6:1
CF3CH2OH
CF3CH2OH
6a (75)
10
11
12
6b (80) 6b + 8,
~6:1
9a (66)
PhC(Me)=NOH 9a (60)
The reaction of 1a,b with phenol in NMP led to both 5a,b
and 71 in an approximate ratio of 6:1, i.e., the product of the
PhC(Me)=NOH 9b (65) 9b (61) 9b + 10, ~1:1
(29, 39)
NO2
R
K2CO3
+
1a,b
HS
R
tions were carried out at 80°C or, in the case of highly nucleo-
philic ones, at 20 and 80°C (Table 1).†
O2N
S
In our experiments the nucleophilic substitution could proceed
in a more complex manner than in 1,3,5-trinitrobenzene itself and
3 R = H
4 R = Cl
Scheme 2
†
1H NMR spectra were recorded on a Bruker AMX-300 instrument.
Chemical shifts in DMSO-d6 are reported relative to TMS. Mass spectra
(EI, 70 eV) were obtained on a Finnigan MAT INCOS 50 spectrometer
with a direct inlet system. The melting points were determined on a Boetius
hot stage using Koffler’s technique (the heating rate was 4 K min–1). The
course of the reactions and the purity of the compounds were monitored
by HPLC (MeCN–H2O, 3:2; reversed phase C18), Waters 1525 pump
with Waters 2996 photodiode detector. Column chromatography was
carried out using 0.035–0.070 mm silica gel. All chemicals used in this
study were commercially available.
Method A. Reactions in MeCN. A mixture of a nucleophile (0.001 mol),
K2CO3 (0.002 mol), MeCN (5 ml) and 1-halo-3,5-dinitrobenzene 1a,b
(0.001 mol) was refluxed with stirring for 20–40 h until the consumption
of the starting compounds stopped (the reactions of all the nucleophiles
did not go to completion, and we judged that the reactions ceased when
the HPLC chromatogram did not change anymore). The reaction mixture
was cooled to room temperature and filtered. The solvent was evaporated
in vacuo. The residue was dissolved in dichloromethane (30–50 ml)
and filtered through a thin layer of silica gel. The filtrate was partially
concentrated in vacuo to a volume of 5–10 ml. After cooling, the solid
was filtered off.
Method B. Reactions in N-methylpyrrolidone. A mixture containing
1-halo-3,5-dinitrobenzene 1a,b (0.001 mol), a nucleophile (0.001 mol),
K2CO3 (0.002 mol) and NMP (5 ml) was stirred for 18–19 h at room
temperature (or heated to 80°C and kept for 11–12 h) until the reaction
stopped (HPLC). The reaction mixture was poured into ice water with
vigorous stirring, kept for 10–15 min, and filtered in vacuo. The product
was dried, dissolved in chloroform, and filtered through a short pad of
silica gel, then recrystallized from ethanol.
Method C. Reactions in HMPA. A mixture containing 1-halo-3,5-dinitro-
benzene 1a,b (0.001 mol), a nucleophile (0.001 mol), K2CO3 (0.002 mol)
and HMPA (5 ml) was stirred for 16–17 h at room temperature (or heated
to 80°C and kept for 10–11 h) until the reaction stopped (HPLC). The
reaction mixture was poured into ice water with vigorous stirring, kept for
10–15 min, and filtered in vacuo. The residue was separated by column
chromatography.
1-(5-Chloro-3-nitrophenyl)-1,2,4-triazole 2b. Yield 18% (method A,
1
MeCN, 40 h), mp 101–103°C. H NMR, d: 8.23 (s, 1H), 8.35 (s, 1H),
8.65 (s, 1H), 8.71 (s, 1H), 9.65 (s, 1H). MS, m/z: 226/224 [M]+. Found (%):
C, 42.88; H, 2.46; N, 24.83. Calc. for C8H5ClN4O2 (%): C, 42.58; H, 2.24;
N, 24.99.
1-Bromo-3-nitro-5-phenoxybenzene 5a. Yield 60% (method A, MeCN,
20 h), yellow oil. 1H NMR, d: 7.19 (d, 2H, 2J 7.9 Hz), 7.29 (t, 1H,
2J 7.3 Hz), 7.51 (t, 2H, 2J 7.7 Hz), 7.62 (s, 1H), 7.71 (s, 1H), 8.12 (s, 1H).
MS, m/z: 295/293 [M]+. Found (%): C, 49.31; H, 2.52; N, 4.93. Calc. for
C12H8BrNO3 (%): C, 49.01; H, 2.74; N, 4.76.
1-Chloro-3-nitro-5-phenoxybenzene 5b.Yield 66% (method A, MeCN,
22 h), yellow oil. 1H NMR, d: 7.15 (d, 2H, 2J 8 Hz), 7.29 (t, 1H,
2J 7.3 Hz), 7.41 (t, 2H, 2J 7.7 Hz), 7.51 (s, 1H), 7.68 (s, 1H), 8.02 (s, 1H).
MS, m/z: 251/249 [M]+. Found (%): C, 57.61; H, 3.00; N, 5.47. Calc. for
C12H8ClNO3 (%): C, 57.73; H, 3.23; N, 5.71.
1-Bromo-3-nitro-5-(2,2,2-trifluoroethoxy)benzene 6a. Yield 75%
(method A, MeCN, 23 h), mp 55–57°C. 1H NMR, d: 5.10 (q, 2H,
2J 8 Hz), 7.65 (s, 1H), 7.84 (s, 1H), 7.95 (s, 1H). MS, m/z: 301/299 [M]+.
Found (%): C, 32.25; H, 1.91; N, 4.77. Calc. for C8H5BrF3NO3 (%):
C, 32.03; H, 1.68; N, 4.58.
1-Chloro-3-nitro-5-(2,2,2-trifluoroethoxy)benzene 6b. Yield 80%
(method A, MeCN, 24 h), mp 61–63°C. 1H NMR, d: 5.00 (q, 2H,
2J 8 Hz), 7.54 (s, 1H), 7.73 (s, 1H), 7.89 (s, 1H). MS, m/z: 257/255 [M]+.
Found (%): C, 37.48; H, 1.69; N, 5.63. Calc. for C8H5ClF3NO3 (%):
C, 37.60; H, 1.97; N, 5.48.
O-(3-Bromo-5-nitrophenyl) acetophenone oxime 9a. Yield 60%
(method A, MeCN, 16 h), 78% (method B, NMP, 20°C, 18 h), 66%
(method B, NMP, 80°C, 11 h), 35% (method C, HMPA, 20°C, 17 h),
30% (method C, HMPA, 80°C, 10 h), mp 79–81°C. 1H NMR, d: 2.46 (s,
3H), 7.51–7.53 (m, 3H), 7.84–7.86 (m, 2H), 7.95 (s, 1H), 8.05 (s, 2H).
MS, m/z: 336/334 [M]+. Found (%): C, 49.89; H, 3.08; N, 8.54. Calc. for
C14H11BrN2O3 (%): C, 50.17; H, 3.31; N, 8.36.
O-(5-Chloro-3-nitrophenyl) acetophenone oxime 9b. Yield 65%
(method A, MeCN, 18 h), 72% (method B, NMP, 20°C, 19 h), 61%
(method B, NMP, 80°C, 12 h), 38% (method C, HMPA, 20°C, 16 h),
29% (method C, HMPA, 80°C, 11 h), mp 72–74°C. 1H NMR, d: 2.47 (s,
3H), 7.52–7.55 (m, 3H), 7.83–7.89 (m, 2H), 7.95 (s, 1H), 8.05 (s, 2H).
MS, m/z: 292/290 [M]+. Found (%): C, 57.99; H, 4.03; N, 9.38. Calc. for
C14H11ClN2O3 (%): C, 57.84; H, 3.81; N, 9.64.
1-(3-Bromo-5-nitrophenyl)-1,2,4-triazole 2a. Yield 20% (method A,
1
MeCN, 38 h), mp 107–109°C. H NMR, d: 8.35 (s, 1H), 8.41 (s, 1H),
8.62 (s, 1H), 8.69 (s, 1H), 9.60 (s, 1H). MS, m/z: 270/268 [M]+. Found (%):
C, 35.62; H, 1.99; N, 20.93. Calc. for C8H5BrN4O2 (%): C, 35.81; H, 1.87;
N, 20.67.
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Mendeleev Commun., 2017, 27, 160–162
nitro group replacement predominated. In HMPA, such a ratio
Similar results were obtained in the case of trifluoroethoxide
anion: in HMPA, both possible substitution processes are
observed in ~1:1 ratio.
In conclusion, dual reactivity of 1-bromo and 1-chloro-3,5-di-
nitrobenzenes in aromatic nucleophilic substitution has been
becomes ~1:1, i.e. the rate of halogen substitution becomes
higher. In MeCN, only the nitro group is replaced to give
products 5a (60%) and 5b (66%) (Scheme 3).
Hal
NO2
identified. Regioselectivity (only one reaction pathway) is observe
d
either in the case of nucleophiles that are hard Lewis bases
(1,2,4-triazolate anion, non-substituted alkoxides) – only a nitro
group is replaced, or soft Lewis bases (arenethiolates) – only
a halogen atom is replaced. In the case of intermediate Lewis
bases, both pathways occur in NMP and HMPA, whereas in
MeCN, only a nitro group is replaced.
K2CO3
+
1a,b + ROH
O2N
OR O2N
OR
5a,b, 6a,b
7, 8
a Hal = Br
b Hal = Cl
5, 7 R = Ph
6, 8 R = CF3CH2
Scheme 3
This work was supported by the Russian Foundation for Basic
Research (grant no. 13-03-01276).
The substitution with trifluoroethanol in HMPA afforded a
mixture of 1-halo-3-nitro-5-(2,2,2-trifluoroethoxy)benzene 6a
or 6b and 1,3-dinitro-5-(2,2,2-trifluoroethoxy)benzene 82 in
~1:1 ratio. In MeCN, only products of the nitro group replace-
ment 6a (75%) and 6b (80%) were obtained (see Scheme 3).
The reactions of 1a,b with acetophenone oxime in NMP
(Scheme 4) occurred with replacement of the nitro group to give
O-(3-halo-5-nitrophenyl) derivatives 9a,b. No halogen replace-
ment product 1021 was detected. The reactions proceeded with
complete conversion of the starting compounds 1, both at 80°C
and at 20°C. In HMPA, both types of products 9 and 10 were
obtained in ~1:1 ratio. In MeCN, only products of the nitro group
substitution, 9a (76%) or 9b (80%), were formed.
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Scheme 4
The results obtained demonstrate that the nucleophilic sub-
stitution pathway depends on the nucleophile and the solvent
nature. They can be rationalized in view of the Lewis Hard and
Soft Acids and Bases (HSAB) principle: the softer base the
anionic nucleophile (Nu–) is, the higher the degree of halogen
replacement, since a carbon atom bound to a halogen is a softer
Lewis acid than a carbon atom bound to a nitro group. The
reaction pathway depends considerably on the properties of
dipolar aprotic solvents: other conditions being equal, an increase
in the solvent polarity (in the series MeCN < NMP < HMPA)
favours the replacement of a halogen atom.
It is noteworthy in the data obtained (see Table 1) that only
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latter depending on the nucleophile and solvent properties. The
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Received: 12th July 2016; Com. 16/4993
– 162 –