Superelectrophilic activation of 1-nitronaphthalene in the presence of aluminum chloride. Reactions with benzene and cyclohexane

Article abstract of DOI:10.1039/c8ob02653j

1-Nitronaphthalene smoothly reacts with benzene and undergoes selective reduction with cyclohexane in the presence of aluminum chloride to give 2,4,4-triphenyl-3,4-dihydronaphthalen-1(2H)-one oxime and 5,6,7,8-tetrahydro-1-naphthylamine, respectively. The mechanistic aspects of these and related reactions are discussed on the basis of DFT, providing insight into the protonation behavior of 1-nitronaphthalene coordinated to AlCl₃.

Full text of DOI:10.1039/c8ob02653j

Organic &
Biomolecular Chemistry
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Superelectrophilic activation of
1
-nitronaphthalene in the presence of aluminum
Cite this: Org. Biomol. Chem., 2018,
6, 9129
1
chloride. Reactions with benzene and
cyclohexane†
Received 26th October 2018,
Accepted 14th November 2018
DOI: 10.1039/c8ob02653j
a
b
a,b
Zhongwei Zhu, Alexander M. Genaev, George E. Salnikov
and
a,c
rsc.li/obc
Konstantin Yu. Koltunov
*
5
1
-Nitronaphthalene smoothly reacts with benzene and undergoes a change in the reaction pathway. In this connection, it is
selective reduction with cyclohexane in the presence of aluminum noteworthy that in addition to a strong Lewis acidity, a cata-
chloride to give 2,4,4-triphenyl-3,4-dihydronaphthalen-1(2H)-one lytic amount of protic superacid, HAl Cl4n or H O–Al Cl3n
n
2
n
oxime and 5,6,7,8-tetrahydro-1-naphthylamine, respectively. The (H ≈ −18), is normally present in such reaction media due to
o
mechanistic aspects of these and related reactions are discussed the in situ reaction between the excess of AlCl and traces of
3
6
on the basis of DFT, providing insight into the protonation behavior water in the starting materials. Besides, the proton acidity
of 1-nitronaphthalene coordinated to AlCl
3
.
could increase further if water is generated as a co-product in
the course of reaction.
Given the above results, we report herein experimental and
theoretical (DFT) studies on superelectrophilic activation of 1
Diprotonation of nitroarenes, nitroalkenes and aci-nitroalk-
anes in liquid superacids is an efficient approach to involve
them in Friedel–Crafts type transformations leading to a large
in the presence of AlCl . This approach provides a new and
3
1
variety of useful derivatives. However, apart from the usual
selective reaction of 1 with benzene, as well as with cyclo-
hexane, another weak nucleophile. Similar reactions of the
parent compound 2 and 1-naphthylamine have also been
briefly examined for the purpose of comparison.
difficulties encountered in handling vast amounts of Brønsted
superacids, an often-observed drawback of such reactions is
the lack of their selectivity. 1-Nitronaphthalene (1), for
instance, reacts with benzene in a 30-fold molar excess of
When reacted with benzene in the presence of a 5-fold
triflic acid (H
o
= −14) for 2 h at 0–5 °C to afford a mixture of
molar excess of AlCl , 1 was completely converted into triphe-
3
2
-phenyl-, 4-phenyl- and 3,4-diphenyl-1-naphthylamines in
nylated oxime 5 and tetracyclic oxime 6 just within 30 min at
2
about 50% overall yield. The parent nitrobenzene (2) reacts
with benzene similarly, but even less satisfactorily, to give a
mixture of mono-, di- and triphenylated anilines, wherein the
conversion is poor (<20% for 48 h at 80 °C) and the yields are
25 °C (Scheme 2). After which, the major product 5 was easily
isolated in 60% yield by crystallization. Upon prolonging the
reaction time for up to 7 days, compound 6, in its turn,
becomes the major product which can also be isolated by
means of crystallization. It is apparent that under the applied
conditions oxime 5 is a kinetically preferred product, while the
formation of oxime 6 is mainly a result of thermodynamically
controlled electrophilic transalkylation. Clearly, the elimin-
ation of water during the conversion of 1 into 5 strongly
increases the proton acidity of the reaction medium, which
3
moderate. The key intermediates of these reactions were
1
c
recognized to be superelectrophilic O,O-diprotonated forms 3
and 4 (Scheme 1), despite the whole mechanism not being
known.
On the other hand, it is known that readily available and
easy to use Lewis acids, such as AlCl , can mediate reactions
3
proceeding via dicationic superelectrophiles no less effectively
4
than liquid superacids. Moreover, in some cases this leads to
a
Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia.
E-mail: koltunov@catalysis.ru
b
Vorozhtsov Novosibirsk Institute of Organic Chemistry, Pr. Lavrentieva 9,
Novosibirsk, 630090, Russia
c
Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk, 630090, Russia
†
Electronic supplementary information (ESI) available: Experimental and com-
putational details and NMR spectra of all compounds (PDF). See DOI: 10.1039/
c8ob02653j
Scheme 1 Proposed direction of diprotonation of nitroarenes 1 and 2
in triflic acid.
2,3
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Scheme 2 Reaction of 1 with benzene. The molar percentage of pro-
1
ducts 5 and 6 is given based on H NMR spectroscopic data.
Scheme 4 Reactions of 2 with benzene and cyclohexane.
also promotes the ultimate production of 6. Indeed, we have
shown that compound 5 itself is practically not converted into
6
in the presence of aluminum chloride. However, 5 reacts of less than a 2-fold molar excess of AlCl
completely sup-
3
with benzene and aluminum chloride for 10 days at room presses them.
temperature to give 6, provided there is prior saturation of the
According to spectroscopic and theoretical studies, alumi-
reaction mixture with gaseous HCl.
num chloride coordinates with only one oxygen atom of nitro
Notably, the structures of 5 and 6 are determined unam- compounds regardless of the reagent ratio. Our DFT calcu-
9
biguously by NMR (ESI†). It is noteworthy that the upfield lations also show that the coordination of each oxygen atom in
−
1
shifted signal at δH = 6.59 ppm corresponded to the H5 1 with AlCl
protons of oxime 5 in the H NMR spectrum. According to our to monocoordination of 1 with Al
conformational analysis, H5 resides in the shielding cone of presence of several equivalents of AlCl
is quite unfavorable (by 4.5 kcal mol ) compared
3
1
0
1
2
Cl
6
(ESI†). Hence, in the
, 1 can exist as
3
one of the phenyl groups of the gem-diphenyl moiety in the O-monocoordinated species, which may undergo catalytic pro-
most stable conformation of 5 (ESI†). Remarkable also is an tonation to form a number of isomeric superelectrophiles 9.
unusual hindered rotation of the phenyl group in 6 at room
Depending on the balance between the stability (concen-
temperature. The H and C NMR spectra of 6 contain broad tration in the reaction media) and reactivity, intermediates 9
1
13
signals of hydrogen and carbon ortho-atoms of the phenyl could give the corresponding products with the nucleophiles.
#
group. The estimated NMR rotational barrier (ΔG at −5 °C) is In order to estimate the relative stabilities, electrophilicities,
−
1
1
1.6 kcal mol , which coincides with the value calculated by and positions of electrophilic centers for the most feasible
7
DFT/PBE/Λ1 (ESI†).
model structures 9a–h, we have calculated their relative ener-
Furthermore, the reaction of 1 with cyclohexane catalyzed gies, the energies of the lowest unoccupied molecular orbital
by a five- to eight-fold molar excess of AlCl for a few hours at (εLUMO), the squares of the coefficients of carbon atoms at the
3
2
1
00–110 °C resulted in the formation of 5,6,7,8-tetrahydro-1- LUMO of electrophilic centers (c
•
) and the atomic charges (q
•
)
naphthylamine (7) (Scheme 3). Under the reaction conditions localized at carbon atoms (Table 1).
cyclohexane exists in equilibrium with methylcyclopentane.
The computed relative energies of isomers 9 showed the fol-
Both alkanes are able to react with strong electrophiles as lowing favorable order of formation : 9a ≫ 9h > 9e > 9g > 9f >
+
hydride donors and thereby release isomeric cations C
6
H
11
.
9d > 9c > 9b, corresponding to the protonation of the
The latter reacts further with excess of cyclohexane/methyl- O-coordinated 1 at the vacant oxygen atom, followed by the
8
cyclopentane to give dicycloalkanes C H –C H ultimately.
positions 8, 5, 7, 6, 4, 3, and 2. The significant values of both
c and q predict electrophilic reaction centers to be C4 or C2
• •
6
11
6
11
2
The reaction of 2 with benzene in the presence of AlCl
3
3
appeared to be similar to that mediated by triflic acid, leading and C5 or C7 for 9a and 9h, respectively. Similarly, one can
to a complex mixture consisting mainly of mono-, di- and tri- easily predict electrophilic centers for other isomers, which
phenylated anilines (GC-MS data). In contrast, the reaction of however can be excluded from mechanistic considerations as
2
with cyclohexane is rather more selective, making it possible they are relatively too unstable.
to obtain 4-chloroaniline (8) in good isolated yield (Scheme 4).
From all these observations, it is possible to propose a
It should be noted that a 4–8 fold molar excess of alumi- plausible mechanism for the reaction of 1 with benzene invol-
num chloride is not essential and a decrease in the loading is ving an initial intermediacy of 9a′ (Scheme 5). It should be com-
possible. This, however, slows down the reactions, and the use mented on that another key intermediate formed in the reac-
tion, dicationic species 10, is responsible for the production of
either of the resulting structures 11 and 12, which are trans-
formed into oximes 5 and 6 after quenching with water.
The reaction of 1 with cyclohexane may, in principle,
proceed via three alternative pathways. The first one involves
the participation of intermediate 9h, which can react with
cyclohexane to give 5,6,7,8-tetrahydro-1-nitronaphthalene
Scheme 3 Reaction of 1 with cyclohexane.
initially (by analogy with similar reactions of quinoline and
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Table 1 PBE/cc-pVDZ calculated energies of the LUMO (εLUMO), the
2
square of the coefficients on carbon atoms at the LUMO (c
i
), NBO
a
i
charges (q ) and PBE/Λ1 relative energies of structures 9a–h
Relative energy,
kcal mol
b
c
2 c
−1
Structure, q
•
and (c
•
)
ε
LUMO, eV
−
−
−
9.12
9.66
9.30
0.0
28.3
26.3
Scheme 5 Proposed mechanism for the reaction of 1 with benzene.
−
−
−
9.36
9.34
9.29
25.7
21.4
25.2
−
−
9.32
8.83
23.6
15.1
Scheme 6 Proposed mechanism for the reaction of
cyclohexane.
1
with
a
The calculations were performed with thermochemical ΔG (298.15 K)
1
1 b
and Grimme’s D3(BJ) dispersion corrections.
for the most stable stereoisomers (see the ESI). These parameters are
shown for the most significant values of q and c .
i i
The data are provided
c
standable in view of the different self-determined proton
acidities in these two reactions. And lastly, the reaction of 1
with cyclohexane can include the intermediate formation of
2
4
-chloro-1-naphthylamine by analogy with the conversion of 2
1
2
isoquinoline)
followed by the nitro group reduction. into 8. In contrast to 8, 4-chloro-1-naphthylamine can easily
However, the latter step is not in line with the reaction of 2 lose the chlorine (as is the case with 4-chloro-1-naphthol) to
with cyclohexane leading to the formation of chlorine- give back 1-naphthylamine and thereby 7.
1
4
containing product 8. It is more likely, therefore, that inter-
mediate 9a′ is again involved in the reaction as shown in ably via nucleophilic attack of Cl on intermediate 4′ resulting
Scheme 6. This pathway implies, de facto, the intermediacy of in the formation of 4-chloronitrosobenzene coordinated to
Finally, the reaction of 2 with cyclohexane proceeds prob-
−
1
-naphthylamine, which undergoes ionic hydrogenation to AlCl
give 7 (cf. ref. 12 and 13). A separate experiment showed that In conclusion, an excess of aluminum chloride can be suc-
-naphthylamine was indeed converted into 7, provided there cessfully used instead of triflic acid to mediate the reaction of
3
followed by its reduction (Scheme 7).
1
is prior saturation of the reaction mixture with gaseous HCl, 1 with benzene. The new approach alters the reaction pathway
but at a somewhat slower rate than 1 (ESI†). This is under- and affords oximes 5 and 6 in good yields. It is also shown
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3
4
5
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cyclohexane.
2
with
2
5, 325.
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3
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(
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(
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There are no conflicts to declare.
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This work was conducted within the framework of the budget
project #AAAA-A17-117041710083-5 for the Boreskov Institute
of Catalysis. A. M. G. and G. E. S. gratefully acknowledge the
support of the Russian Fund for Basic Research (Grant No. 17-
8
9
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0 Normally, a direct comparison of energies of any two
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Academy of Sciences for the spectral measurements and
Cluster of the Information Computation Center, Novosibirsk
State University (http://www.nusc.ru/) for computing resources.
1
1
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(
(
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