Electrophilic Amination of Methylbenzenes with the System NaN 3-AlCl3-HCl. Effects of the Solvent, Crown Ether, and Substrate Structure

Article abstract of DOI:10.1023/A:1026057212359

Using mesitylene and o-xylene as examples, it was shown that the solvent nature strongly affects the conversion of methylbenzenes in electrophilic amination with the system NaN₃-AlCl₃-HCl. Two-parameter correlations were found between the substrate conversion, on the one hand, and dielectric constant and number of heavy atoms in the solvent, on the other. A considerable solvent effect on the regioselectivity of amination of o-xylene was observed. The presence of 18-crown-6 weakly affects the ratio of isomeric amines, but the conversion of o-xylene sharply decreases. Solid aromatic substrates, such as durene and pentamethylbenzene can also be involved in electrophilic amination with the system NaN₃-AlCl₃-HCl. On the basis of the experimental data and the results of quantum-chemical calculations, participation of a nitrenium intermediate in this reaction was postulated.

Full text of DOI:10.1023/A:1026057212359

Russian Journal of Organic Chemistry, Vol. 39, No. 5, 2003, pp. 672 679. Translated from Zhurnal Organicheskoi Khimii, Vol. 39, No. 5, 2003,
pp. 718 724.
Original Russian Text Copyright
2003 by Borodkin, Elanov, Popov, Pokrovskii, Shubin.
Electrophilic Amination of Methylbenzenes
with the System NaN3 AlCl3 HCl. Effects of the Solvent,
Crown Ether, and Substrate Structure*
G. I. Borodkin1,2, I. R. Elanov1, S. A. Popov1,2,
L. M. Pokrovskii1, and V. G. Shubin1
1
Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Division, Russian Academy of Sciences,
pr. Akademika Lavrent’eva 9, Novosibirsk, 630090 Russia
2
Novosibirsk State University, Novosibirsk, Russia
Received November 1, 2002
Abstract Using mesitylene and o-xylene as examples, it was shown that the solvent nature strongly affects
the conversion of methylbenzenes in electrophilic amination with the system NaN3 AlCl3 HCl. Two-param-
eter correlations were found between the substrate conversion, on the one hand, and dielectric constant and
number of heavy atoms in the solvent, on the other. A considerable solvent effect on the regioselectivity of
amination of o-xylene was observed. The presence of 18-crown-6 weakly affects the ratio of isomeric amines,
but the conversion of o-xylene sharply decreases. Solid aromatic substrates, such as durene and pentamethyl-
benzene can also be involved in electrophilic amination with the system NaN3 AlCl3 HCl. On the basis of
the experimental data and the results of quantum-chemical calculations, participation of a nitrenium inter-
mediate in this reaction was postulated.
Direct electrophilic amination [3 11] is a promis-
ing method for preparation of aromatic amines. It is
more advantageous than common multistep proce-
dures for building up a Carom NR2 fragment, which
imply introduction of a temporary functional group or
atom [12 14]. A convenient reagent for direct electro-
philic amination is sodium azide which is used under
conditions of acid catalysis [1, 2, 5]. Olah et al. [5]
studied electrophilic amination of benzene and its
derivatives with the system NaN3 AlCl3 HCl. The
best results were obtained when excess aromatic
substrate was used as solvent. When the reaction was
carried in hexane, the yield of aromatic amine was
lower, while 1,2-dichloroethane and nitromethane
were shown [5] to be inappropriate solvents because
of strong complex formation with the catalytic system.
According to our data [2], the amination of mesitylene
with the system NaN3 AlCl3 HCl in hexane and
1,2-dichloroethane affords 2,4,6-trimethylaniline in
moderate (40%) and high (86%) yield, respectively.
electrophilic amination of methylbenzenes with the
system NaN3 AlCl3 HCl. Scheme 1 shows the most
probable mechanisms of this reaction (cf. [2, 5]).
According to Olah et al. [5], in the first stage
sodium azide reacts with aluminum chloride; there-
fore, solvents capable of effectively coordinating to
AlCl3 should hinder the process. In fact, using
mesitylene as substrate we found that solvents with
a large donor number (greater than 2 according to
Gutmann) [15], such as tetrahydrofuran, sulfolane,
nitrobenzene, and nitromethane, afford low conversion
of aromatic substrate (Table 1). Protonation of
AlCl2N3 species gives diazonium ion I which loses
nitrogen molecule to form nitrenium ion II. The reac-
tion of methylbenzenes with ion I could give two
different -complexes A and B. According to the
results of AM1 quantum-chemical calculations,
-complexes A are less stable than B (Fig. 1); there-
fore, the formation of complexes like B seems to
be more energetically favorable. Complexes B are
characterized by a very long H2N N bond ( 3 ),
and they resemble -complexes C in this respect.
Complexes C could be formed by reaction of nitre-
nium ion II with methylbenzenes. It should be noted
The goal of the present work was to examine the
effects of substrate structure and solvent nature on
____________
*
For preliminary communications, see [1, 2].
1070-4280/03/3905-0672$25.00 2003 MAIK Nauka/Interperiodica

ELECTROPHILIC AMINATION OF METHYLBENZENES
673
Scheme 1.
that the geometric parameters of -complexes A, B,
= (0.29 0.08) + (0.067 0.010)
(0.107 0.023)NCl;
and C within the corresponding series are similar;
therefore, we can expect that reactions of methylben-
zenes with the system NaN3 AlCl3 HCl follow
a common mechanism.
(1)
(2)
r = 0.97, s = 0.08, n = 9;
According to published data [3], the ground state
of the NH+2 cation is triplet (3NH+2); its energy is lower
by 125.9 kJ/mol than the energy of the singlet state
1NH+2) [18]. Taking into account that the formation of
-complexes C requires singlet state of the nitrenium
ion, the substrate conversion should depend on the
probability of singlet triplet inversion of that cation.
This probability should increase with increase in the
number of heavy atoms in the solvent molecule and
increase in their atomic number [3]. As follows from
the data in Table 1, the conversion of mesitylene and
o-xylene decreases in the series CH2Cl2 > CHCl3 >
= (0.40 0.10) + (0.059 0.012)
(0.143 0.027)NCl;
r = 0.96, s = 0.1, n = 7.
Figures 2 and 3 show correlations between the
experimental mole fractions of tri- and dimethyl-
anilines and those calculated by Eqs. (1) and (2),
respectively. A considerable contribution of the term
including the dielectric constant
(which charac-
terizes the solvent polarity) is consistent with the
assumed ionic reaction mechanism.
CCl4, ClCH2CH2Cl > ClCH2CCl3, and CH2Cl2
>
Singlet nitrenium ion NH+2 is likely to be a very
1
CH2Br2. This is consistent with the assumed patricipa-
tion of NH+2 ion as reactive intermediate.
strong electrophile; therefore, the regioselectivity in
the amination of substituted benzenes with the system
NaN3 AlCl3 HCl should be low. This aspect of the
process was studied using o-xylene as substrate. The
mole fractions of 2,3- and 3,4-dimethylanilines given
in Table 1 indicate that the regioselectivity is in fact
low. By varying the solvent, the ratio of 2,3- and 3,4-
dimethylanilines can be changed from 0.56 to 0.90.
Here, the fraction of 3,4-dimethylaniline is always
greater than the fraction of 2,3-dimethylaniline. These
data are in agreement with the results of AM1
quantum-chemical calculations, according to which
-complex C-2 is more stable than C-1 (Fig. 1).
Presumably, the rate-determining stage is formation
of -complex C. Taking into account its polar
character, the conversion of methylbenzenes should
depend not only on the number of heavy atoms (N)
in the solvent molecule but also on the dielectric
constant ( ). This follows from the existence of two-
parameter correlations (1) and (2) for the reactions
with mesitylene and o-xylene, respectively, where
is the mole fraction of 2,4,6-trimethylaniline or
2,3- and 3,4-dimethylanilines in the mixture with
mesitylene or o-xylene, and NCl is the number of
chlorine atoms in the solvent molecule.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 5 2003

674
BORODKIN et al.
Hf
1042.4
1053.6
979.3
968.7
933.4
A-1
A-2
A-3
A-4
A-5
Hf
870.3
865.5
801.9
793.2
758.0
B-1
B-2
B-3
B-4
B-5
Hf
831.2
819.5
761.7
752.1
717.3
C-1
C-2
C-3
C-4
C-5
Fig. 1. Heats of formation Hf (kJ/mol) of -complexes A C and bond lengths ( ) therein (AM1 calculation).
Fig. 2. Correlation between the calculated and experi-
mental conversion of mesitylene in the amination with the
system NaN3 AlCl3 HCl.
Fig. 3. Correlation between the calculated and experi-
mental conversion of o-xylene in the amination with the
system NaN3 AlCl3 HCl.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 5 2003

ELECTROPHILIC AMINATION OF METHYLBENZENES
675
It was interesting to compare the regioselectivity
observed in the amination of o-xylene and in other
electrophilic substitution reactions with the same sub-
strate (Table 2). It is seen that the regioselectivity in
the amination of o-xylene changes to a lesser extent
than in the nitration and bromination; in some cases,
the isomer ratio is reversed. Presumably, this is the
result of change of the reaction mechanism. In the
nitration with the NO+2 BF4 sulfolane system, the
fraction of 1-nitro-2,3-dimethylbenzene is larger than
that of 1-nitro-3,4-dimethylbenzene, while in the
nitration with HNO3 H2SO4 (54%) and HNO3 Ac2O
the reverse situation is observed. It is known that
nitronium tetrafluoroborate is a strong electrophile;
therefore, the high regioselectivity but low substrate
selectivity is explained by the fact that the rate-deter-
mining stage is formation of -complex rather than
-complex [21], as in the other nitration reactions.
Table 1. Conversions of mesitylene and o-xylene ( )a
in the amination with the system NaN3 AlCl3 HCl and
solvent parameters ( , DN)
DN
[15]
Solvent
[16]
TMAb
DMAc
Hexane
1.89
1.92
2.02
4.70
8.9
2.23
10.4
8.2
7.93d
6.7
0
0
0
0.40
0.44
0.47
0.23
0.65
0.10
0.86
0.42
0.31
0.44
0.17
<0.04
<0.04
0
0.45 (0.80)
Heptane
Cyclohexane
CHCl3
CH2Cl2
CCl4
ClCH2CH2Cl
Cl2CHCHCl2
ClCH2CCl3
CH2Br2
C6F6
THF
0.36 (0.83)
0.65 (0.67)
0
0.80 (0.68)
0.28 (0.90)
0.18e
0
0.1
0.48 (0.56)
The activity of a reagent in electrophilic aromatic
substitution can be estimated from the Brown Stock
relation between the substrate selectivity and regio-
selectivity [30]. Santiago and Houk [31] compared
the substrate selectivities and regioselectivities for
108 electrophilic substitution reactions of toluene and
obtained correlation (3):
7.32
35
44
20.0
4.4
14.8
2.7f
PhNO2
Sulfolane
CH2Cl2 MeNO2 38.6f
(1: 1, v/v)
0.02
a
stands for the mole fraction of 2,4,6-trimethylaniline or di-
methylaniline, taking into account that 2 equiv of the aromatic
substrate was involved with respect to NaN3; = 2x/(x + y),
where x and y are the mole fractions of ArNH2 and ArH,
respectively. Given are averaged values determined from the
1H NMR and GC MS data.
2,4,6-Trimethylaniline.
2,3- and 2,4-Dimethylanilines; their molar ratio is given in
parentheses.
logfp
=
0.17 + 1.38Sf; r = 0.91.
(3)
Here, fp is a factor characterizing partial rate of
hydrogen substitution in the para position of toluene,
and Sf is a selectivity factor which is equal to the ratio
fp/fm for toluene. The correlation was improved by
exclusion of the data for nitration of toluene with
NO+2 BF4 in polar aprotic solvents and some other
data (total of 8 points); as a result, correlation (4) was
obtained [31]:
b
c
d
e
At 40 C [17].
The ratio of 2,3- and 3,4-dimethylanilines was not determined,
for the 1H NMR spectrum contained foreign signals.
Parameters of MeNO2.
f
logfp
=
0.08 + 1.37Sf; r = 0.95.
(4)
with PrOH H2SO4, where the reactive intermediate
is assumed to be isopropyl cation (as in the alkylation
i-PrBr GaBr3), the ratio of 1-isopropyl-2,3-dimethyl-
benzene to 1-isopropyl-3,4-dimethylbenzene is con-
siderably smaller than unity (Table 2). Reduction of
this ratio in going from amination to alkylation may
be due to greater steric hindrance to electrophilic
attack by Me2CH+, as compared to NH+2.
The effective size of NH+2 ion can be increased,
e.g., via complex formation with crown ethers. The
results of AM1 quantum-chemical calculations
showed that the cavity of 18-crown-6 readily acco-
modates NH+2 ion (Fig. 4), thus forming a quite stable
complex ( Hf = 296.6 kJ/mol). However, addition
Calculation by Eqs. (3) and (4) of logfp for amina-
tion of toluene with the system NaN3 AlCl3 HCl [5]
gives values of 0.87 and 0.95, respectively, which
approach the experimental value (0.97). The low
values of logfp and Sf for the reaction under study
(0.97 and 0.75, respectively) suggest that the nitre-
nium ion is a very active intermediate species whose
reactivity is comparable to those observed in the
alkylation with MeBr GaBr3, EtBr GaBr3, and
i-PrBr GaBr3 (the parameters logfp and Sf are equal
to 1.072 and 0.842, 0.780 and 0.587, and 0.703 and
0.554, respectively [32]). In the alkylation of o-xylene
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 5 2003

676
BORODKIN et al.
Table 2. Isomer ratios of 1-X-2,3-dimethyl- (A) and 1-X-3,4-dimethylbenzenes (B) in electrophilic substitution
in o-xylene
X
Electrophilic system
Temperature,
C
Molar ratio A: B
Reference
a
NO2
HNO3 54% H2SO4
25
0
25
25
0.50
1.56
0.54
3.93
[19]
[20]
[19]
[21]
90% HNO3 98% H2SO4
HNO3 Ac2Oa
NO+2 BF4 sulfolane
NO
Br
NO CF3COOH
38
25
25
25
0.61b
0.78
0.78
0.70
[22]
[23]
[23]
[23]
Cl2 AlCl3 MeNO2 o-xylene
Cl2 FeCl3 MeNO2
Cl2 FeCl3 o-xylene
Br2 FeCl3 MeNO2
Br2 AcOH
Br2 AcOH H2O
Br2 CF3COOH
Br2 SO2
Br2 ClCH2CH2Cl
Br2 Tl(OCOMe)3 CCl4
HBrO AcOH
25
25
25
25
9
2.13
0.41
0.33
0.14
0.07
0.18
0.18
0.72
[24]
[25]
[25]
[26]
[26]
[26]
[27]
[25]
20
c
25
i-Pr
PrOH 80% H2SO4
i-PrOH 80% H2SO4
70 80
70 80
0.15 0.19
0.15 0.19
[28]
[28]
2-Bu
t-Bu
BuOH 85% H2SO4
2-BuOH 80% H2SO4
25 30
70 80
0.1
0.1
[28]
[28]
t-BuOH 80% H2SO4
i-BuOH 80% H2SO4
2-Methylbutene AlCl3 MeNO2
t-BuBr SnCl4 MeNO2
70 80
70 80
25
0.01 0.02
0.01 0.02
[28]
[28]
[29]
[29]
0
0
25
a
b
c
The concentration of HNO3 was not given.
Calculated from the kinetic data.
Heating in boiling carbon tetrachloride.
of even small amounts of 18-crown-6 to the aminating
system sharply reduces the conversion of o-xylene,
while the regioselectivity changes only slightly
(Table 3). Presumably, the conversion decreases as
a result of reduction of the acidity of the medium,
which hampers formation of reactive intermediate
according to Scheme 1. Binding of NH+2 ion into
a complex with crown ether is unlikely to be the
reason for its reduced reactivity since the positive
charge on the nitrogen changes insignificantly, from
0.489 to 0.428. It should be noted that, by contrast,
addition of 18-crown-6 to the system benzene
CF3COOH (which is obviously characterized by lower
acidity than the system NaN3 AlCl3 HCl) in the
photochemical generation of NH+2 from 1-aminopyri-
dinium or 1-aminoquinolinium salts increases the
yield of aniline [33].
According to the results of our experimental study
of the amination of mesitylene and o-xylene, the
maximal yield of aminoarenes is achieved in such
solvents as CH2Cl2 and ClCH2CH2Cl. We anticipated
that the use of these solvents will allow us to involve
in electrophilic amination solid substrates (under
normal conditions) and obtain the corresponding aryl-
amines in a sufficiently high yield. It should be em-
phasized that Olah and co-workers [5] used excess
liquid aromatic substrate and that solid arenes have
not been brought so far into electrophilic amination
with the system NaN3 AlCl3 HCl [1, 2, 5]. We per-
formed electrophilic amination of durene and penta-
methylbenzene (which are solid substances at the
reaction temperature) in one of the above solvents.
As in the reactions with mesitylene and o-xylene,
from durene in methylene chloride we obtained the
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 5 2003

ELECTROPHILIC AMINATION OF METHYLBENZENES
corresponding monoamination product, 2,3,5,6-tetra-
677
methylaniline, in 30% yield. The yield of penta-
methylaniline was appreciably larger (52%).
Thus, using polymethylbenzenes as examples, we
have demonstrated a strong effect of the solvent on
the substrate conversion and regioselectivity of
electrophilic amination with the system NaN3 AlCl3
HCl. The use of such solvents as methylene chloride
and 1,2-dichloroethane makes it possible to involve
in this reaction aromatic compounds which are solid
substances under standard conditions.
EXPERIMENTAL
1
The H NMR spectra were recorded on Bruker
WP-200-SY, AC-200, and AM-400 spectrometers in
CDCl3 using the solvent ( 7.24 ppm) and HMDS
( 0.04 ppm) as internal references. Gas chromatog-
raphic mass spectrometric data were obtained on
a Hewlett Packard G1800A setup consisting of an HP
5890 Series II gas chromatograph and an HP 5971
mass-selective detector; energy of ionizing electrons
70 eV; oven temperature programming from 50 C
(2 min) to 280 C at a rate of 10 deg/min; injector
temperature 280 C; ion source temperature 173 C;
30-mm 0.25-mm HP-5MB capillary column (5% of
diphenylsiloxane and 95% of dimethylsiloxane, film
thickness 0.25 m); carrier gas helium, flow rate
1 ml/min. Quantitative analysis of the reaction mix-
tures was performed by the internal normalization
technique. AM1 quantum-chemical calculations [34]
were performed with the aid of HyperChem 5.02 soft-
ware with full geometry optimization of the cations.
Sodium azide of pure grade was dried over P2O5
under reduced pressure; aluminum chloride of pure
grade (Merck) was sublimed under reduced pressure;
reagent-grade o-xylene and mesitylene were distilled
over molecular sieves; durene, pentamethylbenzene,
and 2,3-dimethylaniline (all of pure grade) were dis-
tilled. 18-Crown-6 was synthesized at the pilot factory
at the Novosibirsk Institute of Organic Chemistry and
was dried over P2O5. Hydrogen chloride was prepared
by the procedure described in [35] and was dried
by passing through concentrated sulfuric acid and
a column charged with P2O5. The solvents: CH2Cl2,
CH2Br2 [36], hexane, heptane, cyclohexane, 1,2-di-
chloroethane, CHCl3, CCl4, PhNO2, sulfolane [16],
MeNO2, and Cl2CHCHCl2 [37] were purified by
standard procedures. 1,1,1,2-Tetrachloroethane was
synthesized and purified as described in [17]. Tetra-
hydrofuran of pure grade was distilled over metallic
sodium. Hexafluorobenzene (pure) was dried over
molecular sieves.
Fig. 4. Structure of the complex of 18-crown-6 with NH2+
ion, optimized by the AM1 method.
Amination of methylbenzenes (general proce-
dure). A mixture of 3.65 mmol of sodium azide and
3.65 mmol of anhydrous aluminum chloride in 3.5 ml
of appropriate dry solvent was stirred for 20 h at
room temperature. Methylbenzene, 7.3 mmol, was
added, and dry hydrogen chloride was passed through
the mixture at 20 25 C over a period of 2 h (until
nitrogen no longer evolved). The mixture was poured
onto ice, treated with excess concentrated aqueous
sodium hydroxide, and extracted with diethyl ether
(3 15 ml). The extract was dried over MgSO4, and
the solvent was distilled off.
In experiments with crown ether, 18-crown-6 was
added to the system in an amount specified in Table 3
(with respect to sodium azide) before passing HCl.
Methylene chloride was used as solvent.
The composition of the amination products was
1
determined from the H NMR and GC MS data. The
Table 3. Effect of 18-crown-6 on the isomer ratio of
2,3- (A) and 3,4-dimethylanilines (B) and on the substrate
conversion ( ) in the amination of o-xylene with the
system NaN3 AlCl3 HCl in CH2Cl2 (20oC)
a
Ratio 18-crown-6 NaN3
Ratio A: Ba, b
0
0.67
0.59
0.65
0.65
0.54
0.05
0.1
0.5
0.61
c
<0.02
a
b
c
Molar ratio.
Determined from the GC MS data.
The conversion of toluene was very low.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 5 2003

678
BORODKIN et al.
amination products of o-xylene were identified by
comparison with authentic samples which were pre-
pared by the procedure reported in [5] and purified
by thin-layer chromatography on Al2O3. The structure
of 2,3-dimethyl- and 3,4-dimethylanilines was deter-
11. Pirkuliev, N.Sh., Brel, V.K., Akhmedov, N.G.,
Zefirov, N.S., and Stang, P.J., Mendeleev Commun.,
2001, no. 5, p. 172.
12. Comprehensive Organic Chemistry, Barton, D. and
Ollis, W.D., Eds., Oxford: Pergamon, 1979, vol. 2.
Translated under the title Obshchaya organicheskaya
khimiya, Moscow: Khimiya, 1982, vol. 3, p. 168.
1
mined by H NMR spectroscopy using an available
authentic sample of 2,3-dimethylaniline.
13. Methoden der organischen Chemie (Houben Weyl),
The results of the amination of o-xylene with the
system NaN3 AlCl3 HCl in CH2Cl2 were checked for
reproducibility. The mean deviation in the GC MS
determination of 2,3- and 3,4-dimethylanilines was
0.1%. The mean deviation in the determination of
the conversion of mesitylene (in hexane, heptane,
CH2Cl2, CHCl3, CCl4, ClCH2CH2Cl, and ClCH2CH3)
Stuttgart: Georg Thieme, 1957, vol. XI/1, p. 3.
14. Krohn, K., Nachr. Chem. Tech. Lab., 1987, vol. 35,
p. 1047; Belfield, A.J., Brown, G.R., and Foubis-
ter, A.J., Tetrahedron, 1999, vol. 55, p. 11399.
15. Gutmann, V., Coordination Chemistry in Non-
aqueous Solutions, New York: Springer, 1968.
Translated under the title Khimiya koordinatsionnykh
soedinenii v nevodnykh rastvorakh, Moscow: Mir,
1971, p. 22.
1
by H NMR and GC MS was 13%.
This study was financially supported by the Rus-
sian Foundation for Basic Research (project no. 02-
03-32431).
16. Gordon, A.J. and Ford, R.A., The Chemist’s Compa-
nion, New York: Wiley, 1972. Translated under the
title Sputnik khimika, Moscow: Mir, 1976, pp. 11,
437.
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