NO2+ nitration mechanism of aromatic compounds: Electrophilic vs charge-transfer process

Article abstract of DOI:10.1021/jo991538u

The nitration of methylnaphthalenes with NO₂BF₄ and NOBF₄ was examined in order to shed light on the controversial aromatic nitration mechanism, electrophilic vs charge-transfer process. The NO₂⁺ nitration of 1,8-dimethylnaphthalene showed a drastic regioselectivity change depending on the reaction temperature, where ortho-regioselectivity at -78 °C and para- regioselectivity at 0 °C were considered to reflect the electrophilic and the direct or alternative charge-transfer process, respectively, because the NO⁺ nitration through the same reaction intermediates as in the NO₂⁺ nitration via a charge-transfer process resulted in para-regioselectivity regardless of the reaction temperature. The NO₂⁺ nitration of redox potential methylnaphthalenes higher than 1,8-dimethylnaphthalene gave a similar ortho-regioselectivity enhancement to 1,8-dimethylnaphthalene at lower temperature, thus reflecting the electrophilic process. On the other hand, the NO₂⁺ nitration of redox potential methylnaphthalenes lower than 1,8-dimethylnaphthalene showed para-regioselectivity similar to the NO⁺ nitration, indicating the direct or alternative charge-transfer process. In the presence of strong acids where the direct charge-transfer process will be suppressed by protonation, the ortho-regioselectivity enhancement was observed in the NO₂⁺ nitration of 1,8-dimethylnaphthalene, suggesting that the direct charge-transfer process could be the main process to show para- regioselectivity. These experimental results imply that the NO₂⁺ nitration proceeds via not only electrophilic but also direct charge-transfer processes, which has been considered to be unlikely because of the high energy demanding process of a bond coordination change between NO₂⁺ and NO₂. Theoretical studies at the MP2/6-31G(d) level predicted ortho- and para-regioselectivity for the NO₂⁺ nitration via electrophilic and charge- transfer processes, respectively, and the preference of the direct charge- transfer process over the alternative one, which support the experimental conclusion.

Full text of DOI:10.1021/jo991538u

2
972
J . Org. Chem. 2000, 65, 2972-2978
+
NO
2
Nitr a tion Mech a n ism of Ar om a tic Com p ou n d s: Electr op h ilic
vs Ch a r ge-Tr a n sfer P r ocess
,
†
†
†
†
†
Mutsuo Tanaka,* Eiko Muro, Hisanori Ando, Qiang Xu, Masahiro Fujiwara,
†
§
Yoshie Souma, and Yoichi Yamaguchi
Osaka National Research Institute, AIST, 1-8-31, Midorigaoka, Ikeda, Osaka 563-8577, J apan, and
Kansai Research Institute, Kyoto Research Park 17, Chudoji Minami-machi,
Shimogyo-ku, Kyoto 600-8813, J apan
Received October 1, 1999
The nitration of methylnaphthalenes with NO
2 4 4
BF and NOBF was examined in order to shed
light on the controversial aromatic nitration mechanism, electrophilic vs charge-transfer process.
+
The NO
on the reaction temperature, where ortho-regioselectivity at -78 °C and para-regioselectivity at 0
C were considered to reflect the electrophilic and the direct or alternative charge-transfer process,
2
nitration of 1,8-dimethylnaphthalene showed a drastic regioselectivity change depending
°
+
+
respectively, because the NO nitration through the same reaction intermediates as in the NO
nitration via a charge-transfer process resulted in para-regioselectivity regardless of the reaction
temperature. The NO
2
+
2
nitration of redox potential methylnaphthalenes higher than 1,8-
dimethylnaphthalene gave a similar ortho-regioselectivity enhancement to 1,8-dimethylnaphthalene
+
at lower temperature, thus reflecting the electrophilic process. On the other hand, the NO
2
nitration
of redox potential methylnaphthalenes lower than 1,8-dimethylnaphthalene showed para-regio-
+
selectivity similar to the NO nitration, indicating the direct or alternative charge-transfer process.
In the presence of strong acids where the direct charge-transfer process will be suppressed by
+
protonation, the ortho-regioselectivity enhancement was observed in the NO
2
nitration of 1,8-
dimethylnaphthalene, suggesting that the direct charge-transfer process could be the main process
+
to show para-regioselectivity. These experimental results imply that the NO
2
nitration proceeds
via not only electrophilic but also direct charge-transfer processes, which has been considered to
be unlikely because of the high energy demanding process of a bond coordination change between
+
NO
2
2
and NO . Theoretical studies at the MP2/6-31G(d) level predicted ortho- and para-
+
regioselectivity for the NO
2
nitration via electrophilic and charge-transfer processes, respectively,
and the preference of the direct charge-transfer process over the alternative one, which support
the experimental conclusion
+
In tr od u ction
The nitration of aromatic compounds is one of the well-
direct charge-transfer process in the NO
2
nitration, and
the direct charge-transfer process between aromatic
has been reported to be unlikely
recently because high energy is necessary for the bond
+
compounds and NO
2
known aromatic substitutions and has been studied
1
extensively for both scientific and industrial scope. The
+
5
coordination change between NO
other hand, the ipso attack of NO
2
and NO
(eq 3)
2
6-9
. On the
+
mechanism of aromatic nitration with NO
2
has been
+
2
has been
2
considered as a typical electrophilic substitution (eq 1),
but it was suggested that the direct charge-transfer
process (eq 2) could be likely with low redox potential
aromatic compounds, especially in photochemical nitra-
tion. However, there has been no clear evidence for the
reported showing ortho-regioselectivity via the 1,2-shift
of the NO group to the ortho- from the ipso-position of
the ipso σ-complex (eq 4).7 During this process, ho-
mocleavage of the C-NO bond of the ipso σ-complex
2
,8
3
4
2
sometimes occurs and has been recognized as an alterna-
†
§
tive charge-transfer process (eq 5) to produce the radical
Osaka National Research Institute.
Kansai Research Institute.
8,9
cation with NO
2
.
Despite many recent studies using
(1) Olah, G. A.; Malhotra, R.; Narang, S. C. Nitration; VCH: New
10
11
various nitration reagents such as NO
2
,
NO
2
-O
3
,
and
York, 1989.
1
2
+
(2) (a) Olah, G. A.; Narang, S. C.; Olah, J . A. Proc. Natl. Acad. Sci.
C(NO
2
)
4
,
the aromatic nitration mechanism with NO
2
U.S.A. 1981, 78, 3298. (b) Olah, G. A.; Narang, S. C.; Olah, J . A.;
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1
990, 833.
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(
L.; Radner, F. Acc. Chem. Res. 1987, 20, 53. (c) Schmitt, R. J .; Buttrill,
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1
993, 115, 3091. (b) Kim, E. K.; Bockman, T. M.; Kochi, J . K. J . Chem.
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1
0.1021/jo991538u CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/26/2000

+
NO
2
Nitration Mechanism of Aromatic Compounds
J . Org. Chem., Vol. 65, No. 10, 2000 2973
is still ambiguous due to the complexity of the reaction
systems.
via the same reaction intermediates, the radical cation
1
4
+
and NO
2
,
as in the NO
2
nitration via the charge-
transfer processes (eqs 2 and 5). Therefore, we expected
+
+
that the comparison of NO
2
with NO nitration could
+
shed light on the mechanism of the NO
2
nitration.
In this paper, we wish to report the results of the
In our previous study, we found that the reaction of
naphthalenes with NO produces binaphthyls (3) via
experimental and theoretical studies on the mechanism
+
+
of the NO
2
nitration of aromatic compounds, namely,
oxidation with four nitro products (1, 2, 4, and 5) and
the direct charge-transfer process (eq 2), the ipso attack
of NO (eq 3), the 1,2-shift of the NO group (eq 4), and
2 2
the alternative charge-transfer process (eq 5) as shown
+
the binaphthyl formation is prompted by strong acids (eq
1
3
+
6
). In this reaction, the NO nitration (eq 7) proceeds
(
9) (a) Clemens, A. H.; Helsby, P.; Ridd, J . H.; Al-Omran, F.; Sandall,
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3
43. (c) Coombes, R. G.; Golding, J . G.; Hadjigeorgiou, P. J . Chem.
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(
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F. Acta Chem. Scand. 1978, 32B, 749. (d) Eberson, L.; Radner, F. Acta
Chem. Scand. 1985, 39B, 357. (e) Eberson, L.; Radner, F. Acta Chem.
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5
(11) (a) Suzuki, H.; Mori, T. J . Chem. Soc., Perkin Trans. 2 1997,
1
6
5
265. (b) Suzuki, H.; Mori, T. J . Chem. Soc., Perkin Trans. 2 1996,
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1
1
4
(14) (a) Kim, E. K.; Kochi, J . K. J . Org. Chem. 1989, 54, 1692. (b)
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Trans. 2 1985, 467.
1
049.

2
974 J . Org. Chem., Vol. 65, No. 10, 2000
Tanaka et al.
Ta ble 1. Nitr a tion of 1,8-Dim eth yln a p h th a len ea
products, yield (%)
reagent
T (°C)
1
2
3
4
NOBF4
NOBF4
NO2BF4
NO2BF4
0
-78
0
31 (81:19)
8.9 (92:8)
8.2 (78:22)
10 (25:75)
0
0
0
0
0
5.1
1.4
0
6.9
18
7.2
-78
0
a
The nitration was carried out using NOBF4 (2.5 mmol) or
NO2BF4 (2.5 mmol), 1,8-dimethylnaphthalene (2.5 mmol for
NOBF4 or 10 mmol for NO2BF4), and a mixture of CH3CN (25 mL)
with CH2Cl2 (25 mL) as a solvent for 1 or 2 h (for NOBF4 at -78
°
C) under air (for NOBF4) or N2 (for NO2BF4) atmosphere. Isomer
ratio in parentheses is 1,8-dimethyl-4-/1,8-dimethyl-2-nitronaph-
thalene.
in eq 8 by the bold arrows, on the basis of the regio-
selectivity variation of NO
under various conditions.
2 4 4
BF and NOBF nitration
F igu r e 1. Temperature dependence of 1,8-dimethylnaphtha-
+
lene NO
2
nitration isomer ratio. The nitration was carried
BF (2.5 mmol), 1,8-dimethylnaphthalene (10
mmol), and a mixture of CH CN (25 mL) with CH Cl (25 mL)
as a solvent for 1 h under N atm. O and b represent 1,8-
out using NO
2
4
3
2
2
2
dimethyl-4- and 1,8-dimethyl-2-nitronaphthalene, respectively.
to reaction temperature.16 Taking into account the similar
yields for 1 around the regioselectivity change point, the
high energy demanding process of bond coordination
+
5
change between NO
2
2
and NO , and the para-regiose-
+
lectivity for the NO nitration, the regioselectivity change
seems to reflect the reaction mechanism change depend-
ing on the reaction temperature, where ortho- and para-
+
regioselectivities in the NO
2
nitration of 1,8-dimethyl-
naphthalene are derived from the electrophilic (eq 1) and
the charge-transfer processes (eqs 2 and 5), respectively.
The ortho-regioselectivity enhancement in the electro-
+
philic NO
2
nitration (eq 1) has been suggested to
+
originate via two steps that are the attack of NO
2
on
the ipso-position to form the ipso σ-complex6 (eq 3) and
∼9
2
then the 1,2-shift of the NO group to the ortho-position
from the ipso-position of the ipso σ-complex to produce
an ortho σ-complex resulting in ortho-regioselectivity (eq
7
,8
4
2
). For the 1,2-shift of the NO group, it has been also
Resu lts a n d Discu ssion
suggested that a part of the ipso σ-complex undergoes
homocleavage of the C-NO bond to produce the radical
cation and NO as an alternative charge-transfer process
(eq 5) in the NO2 nitration. Para-regioselectivity for
the reaction of the radical cation with NO as the reaction
intermediates for the direct charge-transfer process (eq
2) and the alternative one (eq 5) in the NO2 nitration
Nitr a tion Exp er im en ts. Electr op h ilic vs Ch a r ge-
Tr a n sfer P r ocesses. First, the regioselectivity of the
,8-dimethylnaphthalene nitration with NO
NOBF was examined at 0 and -78 °C in CH CN-CH
Cl . These results are summarized in Table 1. Remark-
able ortho-regioselectivity to the methyl group of 1,8-
2
2
+
8,9
1
2
BF
4
and
4
3
2
-
2
2
+
+
17
dimethylnaphthalene appeared in the NO
2
nitration at
has been reported. Therefore, we inferred that such a
drastic regioselectivity change in the NO2 nitration of
+
-
78 °C, although para-regioselectivity was observed in
+
other experiments. The NO
2
nitration of toluene also
1,8-dimethylnaphthalene was caused by a mechanism
change between the electrophilic (eq 1) and the direct
charge-transfer process (eq 2) or the alternative one (eq
showed the ortho-regioselectivity enhancement at lower
temperature where the isomer ratio ortho/meta/para was
1
5
18
6
4:3:33 and 73:1:26 at 30 and -78 °C, respectively.
The results of the detailed experiments for temperature
5).
+
(16) The NO + nitration has been known as a diffusion control
reaction; therefore, the NO
dependence of the NO
2
nitration regioselectivity of 1,8-
2
+
nitration may proceed immediately at
dimethylnaphthalene are depicted in Figure 1, where the
2
the diffusion surface when 1,8-dimethylnaphthalene is added to NO
BF solution with reaction heat production before formation of the
2
-
yields of 1 were almost constant at about 10% under
4
+
these conditions. As the change in NO
2
nitration regio-
homogenious solution. The local reaction heat production makes
accurate reaction temperature control impossible, which may be one
of the reasons for such a drastic regioselectivity change. (a) Coombes,
R. G.; Moodie, R. B.; Schofield, K. J . Chem. Soc. B 1968, 800. (b)
Hartshorn, S. R.; Moodie, R. B.; Schofield, K.; Thompson, M. J . J .
Chem. Soc. B 1971, 2447.
selectivity was quite drastic at about 0 °C, a number of
+
NO
2
nitrations were reiterated. Interestingly, para- and
ortho-regioselectivities appeared at random about 0 °C,
indicating that the regioselectivity was quite sensitive
(17) (a) Eberson, L.; Radner, F. Acta Chem. Scand. 1980, 34B, 739.
(
b) Eberson, L.; Hartshorn, M. P.; Radner, F. Acta Chem. Scand. 1994,
+
(
15) The NO
2
nitration of toluene was carried out according to the
48, 937. (c) Kochi, J . K. Angew. Chem., Int. Ed. Engl. 1988, 27, 1227.
(d) Butts, C. P.; Eberson, L.; Hartshorn, M. P.; Radner, F.; Robinson,
W. T.; Wood, B. R. Acta Chem. Scand. 1997, 51, 839.
+
NO
2
nitration procedures. Only nitrotoluene was formed with 81 and
8% in yields at 30 and -78 °C, respectively.
2

+
NO
2
Nitration Mechanism of Aromatic Compounds
J . Org. Chem., Vol. 65, No. 10, 2000 2975
Ta ble 2. Nitr a tion of Na p h th a len e Der iva tivesa
Ta ble 3. Tem p er a tu r e Dep en d en ce of Nitr a tion
Regioselective Ten d en cya
products, yield (%)
T
nitration reagent
redox potentialb
substrate
reagent (°C)
1
2
3
4
naphthaleneb
NOBF4
0
28 (97:3)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
2.5
0
0
substrate
naphthalene
NOFB4 NO2BF4
(V)
NOBF4 -78 trace(98:2)
NO2BF4 5.6(95:5)
NO2BF4 -78 2.7(94:6)
NOBF4 31 (67:13:10:8:2)
naphthalene NOBF4 -78 0.2(87:4:6:2:1)
NO2BF4 7.7(54:34:6:6:0)
NO2BF4 -78 8.8(48:47:3:2:0)
NOBF4 75 (89:11)
naphthalene NOBF4 -78 3.7(94:6)
NO2BF4 24 (88:12)
NO2BF4 -78 5.6(67:33)
NOBF4 31 (81:19)
naphthalene NOBF4 -78 8.9(92:8)
NO2BF4 8.2(78:22)
NO2BF4 -78 10 (25:75)
NOBF4 60 (90:10)
naphthalenef NOBF4 -78 3.2(97:3)
NO2BF4 17 (93:7)
NO2BF4 -78 5.7(97:3)
NOBF4 32 (86:14)
naphthalene NOBF4 -78 2.6(88:12)
NO2BF4 4.1(86:14)
NO2BF4 -78 1.2(94:6)
NOBF4 48 (90:3:7)
naphthalene NOBF4 -78 7.6(96:1:3)
NO2BF4 14 (87:9:4)
NO2BF4 -78 9.5(92:6:2)
0
0
0
0
+ -
+
+ -
-
0
1
1
1
1
1
1
-methylnaphthalene
1.74
1.67
1.65
1.63
1.60
1.60
1
1
1
1
1
1
-methyl-
0
3.1 48
2.8 0.5
9.8
11
0
0
0
0
0
6.9 1.4
18
7.2
0
0
2.1
15
0
,5-dimethylnaphthalene
,8-dimethylnaphthalene
,4-dimethylnaphthalene
,2-dimethylnaphthalene
,3-dimethylnaphthalene
+
-
c
+
-
0
0
0.4
0
0
0
+
+
+
+
,5-dimethyl-
0
+
+
d
a
+
and - represent para- and ortho-regioselective tendencies
0
at lower temperature, respectively. + - represents no meaningful
variation in regioselectivity by temperature. b Cited from ref 19.
All potentials are given in V vs the Ag/AgCl electrode.
0
5.1
,8-dimethyl-
0
e
0
0
0
0
0
0
ingful difference in regioselectivity under these condi-
tions.
,4-dimethyl-
0
The relationship between the regioselective tendency
of nitration with redox potential of the substrates cited
from the reference19 is shown in Table 3. While the para-
0
0
31
,2-dimethyl-
0
g
9.8 1.0
32
25
0
18
+
+
regioselectivity was observed in both NO
nitrations for the lower redox potential substrates than
,8-dimethylnaphthalene, the ortho-regioselectivity en-
2
and NO
0
2.0
0
31
0
0
0
1
,3-dimethyl-
0
h
+
hancement appeared at lower temperature in the NO
2
nitration when substrates had a higher redox potential
than 1,8-dimethylnaphthalene. As mentioned before, the
0
9.6
6.7
a
The nitration was carried out using NOBF4 (2.5 mmol) or
NO2BF4 (2.5 mmol), substrate (2.5 mmol for NOBF4 or 10 mmol
for NO2BF4), and a mixture of CH3CN (25 mL) with CH2Cl2 (25
ortho-regioselectivity enhancement has been considered
+
to be derived from the ipso attack of NO
the 1,2-shift of the NO
2
(eq 3) and then
+
2
2
group (eq 4) in the NO nitration
mL) as a solvent for 1 or 2 h (for NOBF4 at -78 °C) under the air
as the electrophilic process (eq 1).7 On the other hand,
it is clear that formation of the radical cation to give para-
regioselectivity via both the direct charge-transfer process
(eq 2) and the alternative one (eq 5) is easier with a lower
,8
b
(
for NOBF4) or N2 (for NO2BF4) atmosphere. Isomer ratio in the
c
parentheses is 1-nitro-/2-nitronaphthalene. Isomer ratio in the
parentheses is 1-methyl-4-/1-methyl-2-/1-methyl-5-/1-methyl-8-/1-
d
methyl-3-nitronaphthalene. Isomer ratio in the parentheses is
1
,5-dimethyl-4-/1,5-dimethyl-2-nitronaphthalene. e Isomer ratio in
20
redox potential substrate. Again, it was implied that
the parentheses is 1,8-dimethyl-4-/1,8-dimethyl-2-nitronaphtha-
+
the NO
2
nitration had a dual process showing ortho- and
lene. f Isomer ratio in the parentheses is 1,4-dimethyl-2-/1,4-
g
para-regioselectivity depending on the redox potential of
the substrates.
Dir ect vs Alter n a tive Ch a r ge-Tr a n sfer P r ocess.
To evaluate the two possible processes to show para-
dimethyl-5-nitronaphthalene. Isomer ratio in the parentheses is
,2-dimethyl-4-/1,2-dimethyl-8-nitronaphthalene. h Isomer ratio in
the parentheses is 1,3-dimethyl-4-/1,3-dimethyl-2-/1,3-dimethyl-
1
5
-nitronaphthalene.
regioselectivity by the radical cation formation (eqs 2 and
To evaluate the influence of the redox potential of the
+
5
) in the NO
2
nitration, the nitration of 1,8-dimethyl-
BF and NO SbF
substrates on the mechanism, nitration using NO
2 4
BF
naphthalene was carried out using NO
2
4
2
6
and NOBF with various methylnaphthalenes was ex-
4
in the presence of strong acids, where strong acids will
protonate 1,8-dimethylnaphthalene21 resulting in sup-
pression of the direct charge-transfer process (eq 2). As
shown in Table 4, ortho-regioselectivity enhancement was
amined at 0 and -78 °C. These results are tabulated in
+
Table 2. In the case of NO nitration, para-regioselec-
tivity was more preferable at lower temperature in all
+
substrates. However, the NO
2
nitration showed two
observed even at 20 °C in the presence of CF
3
SO
H regardless of the nitration reagents.22 In the case
SO , para-regioselectivity appeared. These results
H > CF SO H > H
, namely, protonation ability of the acids.
When the nitration of 1,8-dimethylnaphthalene using
NO BF was carried out at various CF SO H concentra-
3
H and
regioselective tendencies depending on the substrates.
One was that para-regioselectivity became preferable at
FSO
of H
3
+
2
4
lower temperature similar to the NO nitration, and
reflect the acid strength order, FSO
SO
3
3
3
2
-
another was that ortho-regioselectivity was enhanced at
2
3
+
4
lower temperature similar to the NO
2
nitration of 1,8-
dimethylnaphthalene. Naphthalene did not show a mean-
2
4
3
3
tions, the regioselectivity drastically changed in the
(
18) Another plausible explanation for the regioselectivity change
2
+
is the formation of dicationic species such as HNO
2
BF
as an electrophile.
might be a strong
HF‚BF
enough acid to produce HNO
basic 1,8-dimethylnaphthalene to NO
3
produced during the nitration using NO
2
4
(19) Eberson, L.; Hartshorn, M. P.; Persson, O. J . Chem. Soc., Perkin
Trans. 2 1995, 409.
2
+
2
. However, in the presence of excess
2
+
2
BF
4
, the formation of HNO
2
(20) (a) Feng, J .; Zheng, X.; Zerner, M. C. J . Org. Chem. 1986, 51,
4531. (b) Patz, M.; Fukuzumi, S. J . Phys. Org. Chem. 1997, 10, 129.
(21) (a) Birchall, T.; Gillespie, R. J . Can. J . Chem. 1964, 42, 502.
(b) Olah, G. A. J . Am. Chem. Soc. 1965, 87, 1103. (c) Bakoss, H. J .;
Ranson, R. J .; Roberts, R. M. G.; Sadri, A. R. Tetrahedron 1982, 38,
623.
(22) In control experiments, the ortho-regioselectivity was about 45%
3 3
over 0 °C in the presence CF SO H, which was close to the expected
statistical isomer ratio for 1,8-dimethylnaphthalene.
(23) Olah, G. A.; Prakash, G. K. S.; Sommer, J . Superacids; Wiley-
Interscience: New York, NY, 1985; Chapter 1.
seems to be unlikely. (a) Olah, G. A.; Rasul, G.; Aniszfeld, R.; Prakash,
G. K. S. J . Am. Chem. Soc. 1992, 114, 5608. (b) Olah, G. A.; Wang, Q.;
Orlinkov, A.; Ramaiah, P. J . Org. Chem. 1993, 58, 5017. (c) Olah, G.
A.; Lin, H. C. J . Am. Chem. Soc. 1971, 93, 1259. (d) Olah, G. A. Angew.
Chem., Int. Ed. Engl. 1993, 32, 767. (e) Summers, N. L.; Tyrrell, J . J .
Am. Chem. Soc. 1977, 99, 3960. (f) Olah, G. A.; Orlinkov, A.; Oxyzoglou,
A. B.; Prakash, G. K. S. J . Org. Chem. 1995, 60, 7348. (g) Weiske, T.;
Koch, W.; Schwarz, H. J . Am. Chem. Soc. 1993, 115, 6312. (h) Olah,
G. A.; Germain, A.; Lin, H. C.; Forsyth, D. A. J . Am. Chem. Soc. 1975,
9
7, 2928.

2
976 J . Org. Chem., Vol. 65, No. 10, 2000
Tanaka et al.
+
Ta ble 4. 1,8-Dim eth yln a p h th a len e NO2 Nitr a tion in th e
dimethylnaphthalene via alternative charge-transfer pro-
cess (eq 5) proceeds immediately in a solvent-cage, the
ortho-regioselectivity is expected to be similar to the
a
P r esen ce of Acid s
products, yield (%)
intracomplex formylation27 because the formed NO
2
reagent
acid
none
T (°C)
1
2
3
4
should be close to the ortho-position. This possibility
cannot be excluded although this process is meaningless
in view of the synthetic scope.28
NO2BF4
NO2BF4
NO2BF4
NO2BF4
NO2BF4
NO2BF4
NO2SbF6
NO2SbF6
NO2SbF6
NO2SbF6
20
-78
20
-78
20
20
20
20
20
12 (81:19)
10 (25:75)
7.3 (54:46)
3.4 (25:75)
12 (51:49)
12 (81:19)
4.9 (82:18)
3.5 (53:47)
5.4 (51:49)
4.9 (83:17)
0
0
0
0
0
0
0
0
0
0
13
7.2
40
5.9
19
12
8.8
5.6
11
9.4
0
0
0
0
0
0
0
0
0
0
none
CF3SO3H
CF3SO3H
FSO3H
H2SO4
Exp er im en ta l Con clu sion s. The results of the nitra-
+
tion experiments suggest that the NO
2
nitration of 1,8-
dimethylnaphthalene proceeds via not only electrophilic
eq 1) but also direct charge-transfer processes (eq 2)
none
(
CF3SO3H
FSO3H
H2SO4
depending on the reaction conditions, although the direct
charge-transfer process has been considered to be un-
20
a
likely because of the high energy demanding process of
The nitration was carried out using nitration reagent (2.5
mmol), 1,8-dimethylnaphthalene (10 mmol), acid (10 mmol), and
a mixture of CH3CN (25 mL) with CH2Cl2 (25 mL) as a solvent
for 1 h under N2 atm. Isomer ratio in the parentheses is
+
a bond coordination change between NO
2
and NO
2
. The
direct charge-transfer process (eq 2) seems to be favorable
with low redox potential substrates; however, the pos-
sibility of the alternative charge-transfer process (eq 5)
cannot be excluded.
1
,8-dimethyl-4-/1,8-dimethyl-2-nitronaphthalene.
Th eor etica l Stu d ies
In the nitration experiments, it is suggested that the
direct charge-transfer process (eq 2), the ipso attack of
+
NO
2
(eq 3), and the 1,2-shift of the NO
2
group (eq 4)
+
are likely processes in the NO
2
nitration of 1,8-dimeth-
ylnaphthalene, except for the alternative charge-transfer
process (eq 5). To evaluate these results, the energy
profile of four σ-complexes (ortho, meta, para, and ipso)
and the radical cation of 1,8-dimethylnaphthalene with
2
NO were calculated at the MP2/6-31G(d)//HF/6-31G(d)
level of theory with the zero-point energy corrections of
the HF level, using the Gaussian 94 ab initio program
package.29 The calculation results for the energy profile
and the spin density are summarized in Schemes 1-3,
where the total energy for 1,8-dimethylnaphthalene with
F igu r e 2. Acid dependence of 1,8-dimethylnaphthalene NO +
nitration isomer ratio. The nitration was carried out using
NO BF (2.5 mmol), 1,8-dimethylnaphthalene (10 mmol), and
a mixture of CH CN (25 mL) with CH Cl (25 mL) as a solvent
for 1 h under N atm at 20 °C. O and b represent 1,8-dimethyl-
- and 1,8-dimethyl-2-nitronaphthalene, respectively.
2
+
NO
2
is the standard energy potential.
Eva lu a tion of Equ a tion 2 (Sch em e 1). The spin
density of the 1,8-dimethylnaphthalene radical cation
2
4
3
2
2
2
4
(26) The appearance of CF
under the conditions where CF
thalene seemed to be derived from that HF‚BF
diffusion surface of added 1,8-dimethylnaphthalene also protonated
1,8-dimethylnaphthalene reflecting the nature of the diffusion control
3
SO
SO
3
H influence on the regioselectivity
H was less than 1,8-dimethylnaph-
formed locally at the
3
3
3
3 3
presence of 5 mmol CF SO H as shown in Figure 2. The
+
yields were almost constant at about 9%. The proton H
seems to neither suppress the alternative charge-transfer
process (eq 5) because of the charge repulsion with the
ipso σ-complex nor influence the regioselectivity via the
direct charge-transfer process (eq 2) that is considered
to proceed via an inner-sphere mechanism taking into
account the high energy demanding process of bond
+
16
NO
to a mixture of NO
gave 1 with 16% ortho-regioselectivity as shown in Figure 2. However,
when NO BF (2.5 mmol) was added to a mixture of 1,8-dimethylnaph-
thalene (10 mmol) with CF SO H (3 mmol) at 20 °C, 35% ortho-
regioselectivity was observed. On the basis of the diffusion concept,
the local H concentration at the diffusion (reaction) surface of added
NO BF corresponding to the latter condition might be higher than
2
nitration. The addition of 1,8-dimethylnaphthalene (10 mmol)
2
BF (2.5 mmol) with CF SO H (3 mmol) at 20 °C
4
3
3
2
4
3
3
+
2
4
+
3b,5,24
that at the diffusion surface of the added 1,8-dimethylnaphthalene
corresponding to the former condition because the local concentration
coordination change between NO
2
and NO
2
and the
2
5
unhindered donor nature of 1,8-dimethylnaphthalene.
of free 1,8-dimethylnaphthalene to quench H
+
at the diffusion surface
under the latter condition should be lower than under the former
Therefore, we concluded that the protonation of 1,8-
dimethylnaphthalene by strong acids suppressed the
direct charge-transfer process (eq 2), resulting in an
ortho-regioselectivity enhancement, which suggested that
the direct charge-transfer process (eq 2) could be the main
process showing para-regioselectivity.26
+
2
nitration. These results reflect the fact that
16
condition during the NO
+
the NO
procedure was similar to the NO2 nitration procedures. NO BF (2.5
2
nitration is a typical diffusion control reaction. The reaction
+
2
4
mmol) was added to a mixture of 1,8-dimethylnaphthalene (10 mmol),
CF SO H (3 mmol), dry CH CN (25 mL), and dry CHCl (25 mL). The
product yields for 1, 2, 3, 4, and 5 were 3.2, 2.9, 47, trace, and 0%,
3
3
3
3
+
respectively. In control experiments, the NO
2
nitration of 1 to give 2
+
did not show substrate-selectivity according to this NO
2
nitration
+
2
nitration of 1 with the general procedures
The remaining point to discuss is the influence of the
procedure although the NO
+
solvent-cage on the NO
2
nitration via the alternative
showed substrate-selectivity to 2-nitro-1,8-dimethylnaphthalene. There-
fore, the obtained regioselectivity reflects the original regioselectivity
in 1.
charge-transfer process (eq 5). If collapse of the radical
cation with NO formed from the ipso σ-complex of 1,8-
2
(27) (a) Tanaka, M.; Fujiwara, M.; Xu, Q.; Souma, Y.; Ando, H.;
Laali, K. K. J . Am. Chem. Soc. 1997, 119, 5100. (b) Tanaka, M.;
Fujiwara, M.; Xu, Q.; Ando, H.; Raeker, T. J . J . Org. Chem. 1998, 63,
4408. In the intracomplex formylation, regioslectivity is depending on
the structure of σ-complex as a reaction precursor.
(
24) Boughriet, A.; Wartel, M. J . Chem. Soc., Chem. Commun. 1989,
09.
25) Hubig, S. M.; Rathore, R.; Kochi, J . K. J . Am. Chem. Soc. 1999,
21, 617.
8
(
1
(28) Todres, Z. V. Tetrahedron 1985, 41, 2771.

+
NO
2
Nitration Mechanism of Aromatic Compounds
J . Org. Chem., Vol. 65, No. 10, 2000 2977
Sch em e 1
Sch em e 3
Sch em e 2
during this process was -47.6 kcal/mol, which was lower
than the energy for the ortho σ-complex (-46.1 kcal/mol)
by the effect of electron correlation; therefore, the 1,2-
shift of the NO group (eq 4) with a 7.2 kcal/mol transition
2
state energy barrier was recognized to be the likely
process.
Eva lu a tion of Equ a tion 5 (Sch em es 1 a n d 2). The
energy difference between the radical cation with NO
2
(
-33.7 kcal/mol, Scheme 1) and the ipso σ-complex (-53.3
kcal/mol, Scheme 2) was 19.6 kcal/mol. Although the
radical cation formation via the alternative charge-
+
transfer process in the NO
2
nitration (eq 5) has been
reported with several aromatic compounds,8 the calcula-
tion result indicates that the 1,2-shift of the NO group
eq 4) is the more favorable process than the alternative
,9
2
(
charge-transfer process (eq 5) on the basis of the com-
parison of the transition state energy barrier for the
former process (7.2 kcal/mol) with that for the latter
process (at least 19.6 kcal/mol) in the case of the ipso
σ-complex from 1,8-dimethylnaphthalene. This result
supports the fact that the direct charge-transfer process
(eq 2) could be the main process to give para-regioselec-
tivity and the alternative charge-transfer process (eq 5),
which have potential to show not only para- but also
was the highest at the para-position, thus predicting
+
para-regioselectivity for the NO
2
nitration via the
charge-transfer processes (eqs 2 and 5) as reported
before.17 Unfortunately, it was impossible to evaluate the
energy barrier for the direct charge-transfer process (eq
+
2
) between 1,8-dimethylnaphthalene with NO
2
and the
radical cation with NO
2
.
Eva lu a tion of Equ a tion 3 (Sch em e 2). The calcula-
tion predicted that the collapse of 1,8-dimethylnaphtha-
ortho-regioselectivity with a solvent cage-effect, is un-
+
+
lene with NO
2
proceeded without the transition energy
likely in the NO
2
nitration in accordance with the
+
30
barrier similar to the collapse of benzene with CH
3
.
experimental results.
The ipso σ-complex formation (-53.3 kcal/mol) was the
Th eor etica l Con clu sion s. In the case of 1,8-dimeth-
3
1
most favorable among the four σ-complexes, implying
ylnaphthalene, the calculation predicted that the ipso
+
that the ipso attack of NO
2
(eq 3) is a probable process.
+
attack of NO
2
2
(eq 3) and the 1,2-shift of the NO group
On the other hand, the formation of the ortho σ-complex
(
eq 4) are the likely processes, but the alternative charge-
(
-46.1 kcal/mol) was not preferable compared with the
+
transfer process (eq 5) is unlikely in the NO
2
nitration.
para σ-complex formation (-50.6 kcal/mol).
Eva lu a tion of Equ a tion 4 (Sch em e 3). The calcula-
Con clu sion s
tion of the energy profile for the 1,2-shift of the NO
to the ortho- from the ipso-position of the ipso σ-complex
eq 4) was also performed. The transition state energy
2
group
The results of both the experimental and theoretical
(
+
studies suggest that the NO
2
nitration of 1,8-dimethyl-
naphthalene proceeds via not only an electrophilic (eq
) but also a direct charge-transfer process (eq 2), which
has been considered to be unlikely because of the high
(
29) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
1
J ohnson, B. G.; Robb, M. A.; Cheesemen, J . R.; Keith, T. A.; Patterson,
G. A.; Montgomery, J . A.; Raghavachari, K.; Al-Laham, M. A.;
Zakrzewski, V. G.; Ortiz, J . V.; Foresman, J . B.; Cioslowski, J .;
Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala,
P. Y.; Chen, W.; Wong, M. W.; Andres, J . L.; Replogle, E. S.; Gomperts,
R.; Martin, R. L.; Fox, D. J .; Binkley, J . S.; Defrees, D. J .; Baker, J .;
Stewart, J . J . P.; Head-Gordon, M.; Gonzalez, C.; Pople, J . A. Gaussian
energy demanding process of the bond coordination
+
change between NO
2
and NO
2
. The probability of the
+
direct charge-transfer process in the NO
2
nitration (eq
2
) implies that the coordination of an electrophile with
9
4; Gaussian Inc.: Pittsburgh, PA, 1995.
30) Miklis, P. C.; Ditchfield, R.; Spencer, T. A. J . Am. Chem. Soc.
998, 120, 10482.
31) When the calculation was carried out at the HF/6-31G(d) level,
(
an aromatic compound can play a critical role in deter-
mining the reaction process between the electrophilic and
direct charge-transfer processes via an inner-sphere
mechanism, and the direct charge-transfer process seems
to be possible for some aromatic substitutions that have
been considered to proceed via an electrophilic process
such as the diazonium coupling reaction.32
1
(
which is known to have less precision than MP2/6-31G(d) level, the
heat of formation energy for σ-complexes were -66.9, -65.3, -59.8,
and -69.0 kcal/mol for ipso, ortho, meta, and para σ-complexes,
respectively, indicating that para σ-complex formation was the most
favorable. Such a discrepancy is sometimes caused by to calculation
precision.

2
978 J . Org. Chem., Vol. 65, No. 10, 2000
Tanaka et al.
Exp er im en ta l Section
stirring in N
2
under temperature control. When the substrate
was a solid, the solution of the substrate dissolved in CH
10 mL) was added to the mixture. After 1 h, the reaction
mixture was poured into ice-water and extracted with CHCl
2 2
Cl
All materials were of the highest available purity and used
(
without further purification. The products and their isomers
3
.
1
13
were identified by NMR ( H, C NMR, H-H, C-H COSY,
NOESY, and COLOC) with mass spectra after separation
using GPC. The yields were determined by GC with the
internal standard method, and the isomer distributions were
determined by GC and NMR.
The product yields and structures were determined according
to general procedures.
NO+ Nitr a tion P r oced u r es. NOBF
in a three-necked flask (300 mL) containing a mixture of dry
CH CN (25 mL) and dry CH Cl (15 mL) (25 mL of dry CH
CN and 25 mL of dry CH Cl for liquid substrates such as
-methylnaphthalene, 1,2-, and 1,3-dimethylnaphthalene). A
(2.5 mmol) was placed
4
3
2
2
3
-
The formation of coupling products, 3, 4, and 5 (eq 4) in the
+
2
2
NO
2
nitration can be evidence for the charge-transfer process,
BF always
, causing the coupling reaction as an impurity.
1
but it is known that commercially available NO
contains NOBF
Azide is known as a NO scavenger; however, to obtain a
2
4
substrate (2.5 mmol) was added to the mixture with vigorous
stirring in air under temperature control. When the substrate
was a solid, the solution of the substrate dissolved in CH Cl
2 2
(10 mL) was added to the mixture. After 1 h ( 2h in the case
of at -78 °C), the reaction mixture was poured into ice-water
and extracted with CHCl . The product yields and structures
3
4
+
33
+
clear regioselectivity variation to compare the NO
NO nitration as much as possible, the NO
2
with the
+
+
2
nitration was
conducted without azide. Therefore, the formation of coupling
products was not discussed in this study. Furthermore, the
conditions under which dinitro compounds 2 were not formed
were adopted in order to avoid complexity in the products
analysis. The formation of 5 was not detected in any experi-
ments, and the meta nitro product in 1 was not found in the
were determined according to general procedures.
Ca lcu la tion P r oced u r es. All the molecular orbital calcu-
lations were carried out with the GAUSSIAN 94 ab initio
program package. The geometrical optimization for all the
29
case of 1,8-dimethylnaphthalene.
present molecules was performed with the HF level of theory
and 6-31G(d) basis set. On the basis of the optimized geom-
etries, the single-point calculations of the energies were carried
out at the PM2 (frozen core)/6-31G(d) level with the zero-point
energy (ZPE) corrections of the HF level. All the optimized
geometries corresponding to a local minimum point have real
frequencies, except for a transition state with one imaginary
frequency. A Silicon Graphics Origin 2000 R10000 workstation
was used for calculations in this study.
+
NO
2
Nitr a tion P r oced u r es. NO
2
BF
4
(2.5 mmol) (with the
required amount of acid in some cases) was placed in a three-
necked flask (300 mL) containing a mixture of dry CH CN (25
mL) and dry CH Cl (15 mL) (25 mL of dry CH CN and 25
mL of dry CH Cl for liquid substrates such as 1-methylnaph-
thalene, 1,2-, and 1,3-dimethylnaphthalene). The mixture was
3
2
2
3
2
2
+
13
degassed three times to exclude the NO nitration, and then
a substrate (10 mmol) was added to the mixture with vigorous
(
32) Bockman, T. M.; Kosynski, D.; Kochi, J . K. J . Org. Chem. 1997,
2, 5811.
33) (a) Clemens, A. H.; Ridd, J . H.; Sandall, J . P. B. J . Chem. Soc.,
Ack n ow led gm en t. We thank Prof. Inoue (Osaka
University) for his significant advice on this work.
6
(
Perkin Trans. 2 1984, 1659. (b) Clemens, A. H.; Ridd, J . H.; Sandall,
J . P. B. J . Chem. Soc., Perkin Trans. 2 1984, 1667.
J O991538U