A 6π Azaelectrocyclization Strategy towards the 1,5,9-Triazacoronenes

Article abstract of DOI:10.1002/adsc.201901433

We present the instance of two aromatic double bonds and an imine double bond involved thermal 6π-azaelectrocyclization and, on this basis, a one-step synthesis of triazacoronenes (TACs) from triphenylene-1,5,9-triamines and aldehydes under nonacidic cond

Full text of DOI:10.1002/adsc.201901433

Title: A 6π Azaelectrocyclization Strategy towards the 1,5,9-
Triazacoronenes
Authors: Yixun Sun, Xiaogang Wang, Guodong Shen, Tian Yang, Yihui
Yang, Jing Li, Mingyu Yang, Huaming Sun, and JunFa wei
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10.1002/adsc.201901433
Advanced Synthesis & Catalysis
Very Important Publication
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
A 6π Azaelectrocyclization Strategy towards the 1,5,9-
Triazacoronenes
Yi-Xun Sun,a Xiao-Gang Wang,a Guo-Dong Shen,a Tian Yang,a Yi-Hui Yang,a Jing
Li,a Ming-Yu Yang,a Hua-Ming Sun,a and Jun-Fa Weia *
a
School of Chemistry and Materials Science, Key Laboratory for Macromolecular Science of Shaanxi Province,
Shaanxi Normal University, Xi’an 710062, China
E-mail of corresponding author: weijf@snnu.edu.cn
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Bischler–Napieralski
cyclization
of
triamide
Abstract. We present the instance of two aromatic double
bonds and an imine double bond involved thermal 6π- precursors, which were obtained by acylation of
azaelectrocyclization and, on this basis,
a one-step
triamine (3, Table 1) with acyl chlorides or
anhydrides, in a molten salt ionic liquid of AlCl3–
NaCl at 220 °C or in POCl3–P2O5 at 120 °C as a
complementary method.[3c]
synthesis of triazacoronenes (TACs) from triphenylene-
1,5,9-triamines and aldehydes under nonacidic conditions.
This method has several advantages such as simplicity,
high yields, and extensive substrate scope. A plausible
reaction mechanism has been proposed with several
experimental supports. A typical derivative shows a unique
dimer holding together via a π-π interaction and six H-
bonds and a zigzag superstructure stabilized by three
centered H-bonds and Br···π interaction between the
adjacent dimers.
Keywords: triazacoronenes; electrocyclic reaction; fused-
ring systems; schiff bases; nitrogen heterocycles
Figure 1. Coronene and its triaza derivatives
As a continuance of our interest in seeking new
method to access (hetero)nanographenes, we found
that the Pictet–Spengler cyclization for synthesizing
TACs from the triphenylene triamine and aldehydes
proceed comfortably not only under acidic, but also
under neutral or even alkaline conditions. We deduce
this type of nonacid-catalyzed heterocycle-forming
Heterocyclic nanographenes (HNGs) constitute a
unique class of two-dimensional organic materials
with important physicochemical properties and have
become attractive objects in modern heterocyclic
chemistry.[1] 1,5,9-Triazacoronene (TAC, 1 and 2,
Figure 1) is a distinctive aza-version of coronene,[2] in
which the hub ring is alternately surrounded by three
pyridine rings and three benzene rings. Such a
“suprabenzene-like” polyaza nanographene possess
highly delocalized but highly electron-poor π-system,
endowing it with some attractive properties such as
stable electrogenerated chemiluminescences,[3] acid-
induced spectroscopic changes,[2g] and multiple
electrogenerated chemiluminescence emissions.[2h]
reaction comprising
a
6π-azaelectrocyclization
process. A systemic literature survey by SciFinder
reveals that no thermal electrocyclic reactions was
reported about aza 6π-electron systems where two
members of double bonds are both a part of aromatic
rings.[4] Herein, we would like to report the first
example of thermal electrocyclic reaction of aza 6π-
system involving two aromatic C=C bonds and on
this basis, a convenient and efficient method for the
synthesis of TACs.
Initially, we chose the reaction of the unsubstituted
triamine 3 (Table 1) and 4-tert-butylbenzaldehyde
under the condition we developed for the synthesis of
TACs to test the adaptability of our TfOH-catalyzed
threefold Pictet–Spengler cyclization. The result
shows that the reaction proceeds smoothly to deliver
the desired TAC 2d in a moderate yield (65 %)
In 2010, we designed and realized the synthesis of
nona-substituted TACs (1, Figure 1) from
triphenylene-1,5,9-triamines and aldehydes through a
TfOH-catalyzed threefold Pictet–Spengler cyclization
[3a]
in DMF at 100 °C.
This method was recently
proved to be able to conveniently access π-extending
TAC derivatives with a minor modification.[3b] Very
recently, Tan group synthesized trisubstituted
triazacoronenes (2, Figure 1) and their parent via a
1
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10.1002/adsc.201901433
Advanced Synthesis & Catalysis
(Table 1, entry 1), suggesting that alkoxys on
triamine is not indispensible to access TACs via our
TfOH-catalyzed methodolgy.[3a,3c] To optimize the
reaction conditions, we conducted the reaction in
absence of air because we conjectured the electron-
deficient nature of pyridine ring(s) preformed should
retard the subsequent ring closure for this Friedel-
Crafts-type heterocycle-forming reaction. As
expected, the reaction under an argon atmosphere and
then bubbling air gave an improved yield (84 %)
(entry 2). Switching DMF to DMSO, the reaction
gave the product in a comparable yield (entry 3).
amino)naphthalene (DMAN) (entries 7 and 8), and
found the reaction also works with yields of 70 % and
66 %. Therefore, the Pictet–Spengler cyclization is
further excluded, because the formation of
carbonium-ion crucial to Friedel-Crafts-like reactions
cannot occur in an alkaline system. At the oxidation
stage, air can be replaced by DDQ or CAN, resulting
in a significantly shortened reaction time while
comparable yields (entries 9 and 10).
Table 2. Scope of the 6π azaelectrocyclization reactions a
Table 1. Optimization of reaction conditions a
En
try
R1
R2
Product Yieldd
(%)
82e
1-stage
2-stage
1
2
3
4
5
6
7
8
9
10
11b
12
13b
14
15b
16
17
18b
19
20
21
22c
23c
24c
25c
26c
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C6H5
4-Me-C6H4
2-Me-C6H4
4-t-Bu-C6H4
4-CH3O-C6H4
2-CH3O-C6H4
3,4-(CH3O)2C6H3
2-(C6H5)-C6H4
2-F-C6H4
2a
2b
78e
Solvent/Additive/
Atmosphere/
Oxidant/Te
mperature
(°C) /Time
(h)
Yieldb
(%)
94
90
88e
91
81
84
85
86e
87
84e
82
86
92
78
90e
89e
81e
87e
82e
62
55
64
42
25
67
62
80
76
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
2p
2q
2r
2s
Entry
Temperature (°C)
1c
2c
3c
4
DMF/TfOH/Air/120
DMF/TfOH/Ar/120
DMSO/TfOH/Ar/120
DMSO/None/Ar/120
DMSO/None/Ar/150
DMSO/None/Ar/150
DMSO/Lutidine/Ar/150
DMSO/DMAN/Ar/150
DMSO/None/Ar/150
DMSO/None/Ar/150
-
65
84
82
72
90
88
70
66
85
83
Air/120/2
Air/120/2
Air/120/2
Air/120/2
Air/120/2
Air/120/2
Air/120/2
DDQ/rt/0.2
CAN/rt/0.2
4-Cl-C6H4
2-Cl-C6H4
4-Br-C6H4
2-Br-C6H4
5
6d
7e
8f
9g
3-Br-C6H4
6-Br-3,4-(CH3O)2C6H2
6-F-3,4-(CH3O)2C6H2
4-NO2-C6H4
4-CN-C6H4
4-CF3-C6H4
4-pyridinyl
2-thiophenyl
n-propyl
10h
a
All the reactions were carried out in Schlenk tubes using
0.18 mmol of triamine 3, 0.90 mmol of 4-tert-
butylbenzaldehyde, and 5 mL of the solvent, 120 °C or
b
150 °C for 24 h in the 1-stage. Isolated yields by column
chromatography. TfOH (1 mol %). The reaction was
carried out in the dark and the Schlenk tube was shielded
2t
c
d
2u
2v
2w
2x
2y
2z
1a
1b
1c
1d
e
f
light with silver paper. 0.6 equiv of 2,6-lutidine. 0.6
n-hexyl
cyclohexyl
CO2Et
CF3
4-t-Bu-C6H4
4-Br-C6H4
g
equiv of DMAN (1,8-bis(dimethylamino)-naphthalene).
3.3 equiv of DDQ. h 6.0 equiv of CAN.
27 OMe
28 OMe
29b OMe
It was surprising that in the absence of TfOH, the
reaction also proceeded to give the desired product
with 72 % yield (entry 4). This unexpected result
proves that such an acid-free reaction is dissimilar to
the traditional Pictet–Spengler cyclization. The yield
was further increased to 90 % when elevating the
reaction temperature to 150 °C (entry 5). To eliminate
the interference from light, we carried out the
reaction in the dark and shielded light with silver
paper, the reaction gives an almost indistinguishable
yield (entry 6), which show that the reaction is not
sensitive to light. To eliminate the interference from
any acidic substances, we carried out the reaction in
the presence of 2,6-lutidine or 1,8-bis(dimethyl-
4-CN-C6H4
4-NO2-C6H4
30 OMe
a
Reaction conditions: triamine 3 or 4 (0.18 mmol or 0.11
mmol, 1 equiv), aldehyde (0.90 mmol or 0.55 mmol, 5
equiv), in DMSO (5 mL) was stirred at 150 °C for 24 h
b
under argon atmosphere, then bubbling air. The reaction
was carried out at 130 °C. c The reaction was carried out at
80 °C for 12 h, then at 150 °C for 12 h. d Isolated yields by
column chromatography, unless otherwise noted. e Yields
by filtration and washing with water and acetone, products
are pure enough for NMR analysis.
2
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10.1002/adsc.201901433
Advanced Synthesis & Catalysis
Under the optimized conditions (Table 1, entry 5),
0.042Å from the least-squares plane defined by 18
atoms of core, parallel virtually to each other with a
trifling dihedral of 1.17 ° and an average interplanar
distance of 3.63 Å. All veratryl rings are twisted from
the core plane in the range of 67.24–93.06° while the
the scope of such a non-acid catalyzed reaction was
then examined using a variety of (hetero)aromatic
and aliphatic aldehydes (Table 2).As shown in Table
2, the reaction was found to be not significantly
affected by the substituents on the aromatic aldehydes. bromine atoms position out of the dimer. Most
The aromatic aldehydes bearing an electron-donating
group (2b-2g) or electron-withdrawing group (2i-2n,
2q-2s), sterically hindered aldehydes (2h) and
heteroaromatic aldehydes (2t-2u), give the
corresponding TACs in good yields. It is noteworthy
that aliphatic aldehydes (2v-2x) give the desired
TACs in satisfying yields, perhaps due to that the
side-reactions, i. e., aldol condensation of aliphatic
aldehyde and tautomerization of imine intermediate
to enamine are weakened under neutral conditions.
So, this reaction provides a much better tool for the
synthesis of TACs bearing aliphatic arms.[3a,3c] More
remarkably, aliphatic aldehydes bearing with strongly
electron-withdrawing group, such as ethyl glyoxylate
and trifluoroacetaldehyde, furnished the desired
product (2y-2z) in 42 % and 25 % yields, respectively.
interestingly, two aromatic cores are overlapped
greatly with a tiny slippage of 0.46 Å; the pyridine
rings of one molecule are superimposed oppositely to
those of the dimerized partner. These structural
features should be attributed to the strong π-π
interaction between two aromatic cores and
especially, six C–H···N interactions between each
nitrogen atom of one molecule and each 2-hydrogen
atom of the veratryl groups of another molecule. The
length of the six hydrogen bonds ranges from 2.40-
2.87 Å (Figure 2, b) and Figure S1, SI).
a)
A
control
experiment
shows
that
trifluoroacetaldehyde gave
a
complex mixture
without any detectable product using our TfOH-
catalyzed threefold Pictet–Spengler cyclization in
DMF at 120 °C. The hexamethoxy triamine 4 works
well to deliver the desired products (1a-1d) in good
yields, indicating the extensive scope of this non-
acidic concerted reaction.
Since [1,1'-biphenyl]-2-amine can be regard as the
substructure of triamine 3, it could be envisaged to
proceed this aza 6π-electrocyclization with aldehydes
such as 4-tert-butylbenzaldehyde. However, no
desired phenanthridine was detected in the reaction
mixture other than the Schiff base. In contrast, the
reaction works well when using N3',N3'-dimethyl-
[1,1'-biphenyl]-2,3'-diamine as the substrate to
deliver the wanted substituted phenanthridine in the
yield of 67 % (Scheme 1 and Figures S85, S86, S125,
SI). It seems that the sluggish azaelectrocyclization of
biphenyl 6π-systems involving an imine double bond
and two aromatic double bonds were promoted by
placing proper substituents. Similar trends are found
in the normal 6π-electrocyclization reactions of 1,3,5-
(hetero)hexatrienes. [4d, 5]
b)
Figure 2. Molecular structure of 2o: (a) top view and (b)
side view
The unit cell comprises two dimers, which link
with each other centrosymmetrically via two sets of
intermolecular three centered (C–O)2···H(C)
hydrogen bonds between a rim H atom of one
molecule and two OCH3 units of another (bond
length 2.53 and 2.80 Å); two adjacent dimers
between two neighboring cells associate reciprocally
in a similar manner (bond length 2.49 and 2.76 Å)
(Figure S2, SI). The dimers pack into zigzag stacking
geometry along c-axis and slipped π-stacked columns
along a-axis by two intermolecular Br···π interactions
between the Br atom and the aromatic plane of an
adjacent molecule with the atom-plane distances of
3.19 and 3.55Å (Figure S3-6, SI).
Scheme 1. The synthesis of 6-(4-(tert-butyl)phenyl)-N,N-
dimethylphenanthridin-9-amine
The X-ray study reveals that in crystal of 2o[6]
(Figure 2), two crystallographically distinct
molecules form a face-to-face dimer in a cross-
embedded manner. In the dimer, two aromatic planes,
with mean plane deviations (MPDs) of 0.037Å and
A plausible mechanism is shown in Scheme 2
though an exact mechanism of the reaction requires
3
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10.1002/adsc.201901433
Advanced Synthesis & Catalysis
further investigation. Initially, triamine 3 condenses
with an aldehyde to produce the tri-Schiff base, i. e.,
triimine A. Then, the triimine undergoes a thermal
6π-azaelectrocyclization with the formation of the
intermediate B, followed by a rapid [1,5]-H shift to
recover the aromaticity of the triphenylene
framework,
This process repeated three times to generate the
intermediate C, which is dehydrogenated to achieve
the aromatization of three dihydropyridine moieties,
delivering the product 2d by oxidant or air.
A more solid evidence for this mechanism was
obtained from the reaction of triamine 3 with
cyclohexanone in which a triannulated product C′′
(Scheme 3), a close analogue of immediate C, was
achieved in 45 % yield under the argon atmosphere.
The structure of C′′ was fully confirmed by
spectroscopic
characterization
and
X-ray
crystallographic analysis (Figures S7, S87, S88,
S126, SI).[6] In this case, the dehydrogenative
aromatization after the 6π-electrocyclization and
[1,5]-H migration is forbidden as no hydrogen atom
exists on the spiro-quaternary centers derived from
the carbonyl carbon of cyclohexanone. Thus, we
suggest that the key step of this transformation
involve
a
mechanism containing the 6π-
azaelectrocyclization.
Scheme 3. The synthesis of triannulated product C′′
In summary, we report an efficient synthetic
approach for trisubstituted triazacoronenes using a 6π
azaelectrocyclization strategy as the heterocycle-
forming reaction. This method shows several
advantages such as simplicity, high yields, and
extensive substrate scope, and thus provides a new
tool for the synthesis of heteroatom-doped
nanographenes. Compared to Bischler–Napieralski
cyclization, this non-acid catalyzed cyclization
furnishes directly TACs in a single step from
aldehydes and triamine precursors. A plausible
Scheme 2. Proposed pathway of the reaction (R = 4-t-
BuC6H4). The photographs were taken under the irradiation
of 365 nm UV lamp.
To gain insights into this mechanism, we tried to
isolate the related intermediates for spectroscopic
identification. Unfortunately, attempts to get the
intermediate A in pure form failed since it is labile
during purification. The triimine A was observed via
MS spectrometry in the reaction mixture of 3 with 4-
tert-butylbenzaldehyde in the presence of activated
4A Molecular Sieve in DMSO at room temperature
(Scheme S7 and Figure S9, SI). Noticeably, direct
oxidation of A formed in situ in DMSO by DDQ
gave a complex mixture without any detectable
product (Scheme S7 and Figure S10, SI). In sharp
contrast, after A formed in situ in DMSO was simply
preheated under an Ar atmosphere, the similar
oxidation delivered the wanted product 2d rapidly
(Scheme S8 and Figure S12, SI). It was noteworthy
that the fluorescence of A showed significantly
variation after simple heating treatment, which was
observed under the irradiation of 365 nm UV lamp.
Moreover, a monodehydrogenated species C′ was
observed via MS in the reaction mixture before
oxidation (Scheme S8 and Figure S11, SI). These
results suggest that intermediate C, derived from
triimine A via the thermal 6π electrocyclization and
the subsequent [1,5]-H shift, should be a key
intermediate in such a transformation.
reaction
mechanism
involving
a
6π
azaelectrocyclization, a [1,5]-H migration and an
oxidative aromatization for the formation of TACs
has been proposed with some experimental supports.
X-ray structural study for a typical derivative shows a
dimer holding together via a π-π interaction and
unique six H-bonds and a zigzag superstructure
stabilized by three centered H-bonds and Br···π
interaction between the adjacent dimers.
Experimental Section
General Procedure for 6π-Azaelectro-
cyclization Reaction of Triamine with
Aldehydes
To a dried 25mL Schlenk tube, were added: triamine 3 or 4
(50.0 mg, 0.18 mmol or 0.11 mmol), aldehyde (5.0 equiv,
0.90 mmol or 0.55 mmol). The content was purged with
Argon. DMSO (5mL) was then added, and the reaction
mixture was heated at 130-150°C for 24 h. Then bubbling
air to the resulting mixture by air pump at 120°C (checked
by TLC). After the reaction was complete, cooled to rt. and
water (15 mL) was added, extracted with CH2Cl2 (2 
30mL), washed with brine (2  30 mL). The organic layer
4
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10.1002/adsc.201901433
Advanced Synthesis & Catalysis
was dried by anhydrous MgSO4 and concentrated under
reduced pressure to give crude product which was purified
by silica gel column chromatography. Some compounds
were purified by filtration due to its poor solubility,
washed with water and acetone (2 × 5 ml), and dried under
vacuum to afford pure products.
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Acknowledgements
This work was financially supported by the National Foundation
of Natural science in China (Grant Nos. 21572124, 21272145,
21602128, 21702131 and 21871169) and the Natural Science
Basic Research Plan in Shaanxi Province of China (Program Nos.
2016ZDJC-03 and 2017JQ2010) and Fundamental Research
Funds For the Central Universities (SNNU. 2019TS040).
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Advanced Synthesis & Catalysis
COMMUNICATION
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1,5,9-Triazacoronenes
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