Synthesis of some heptazine derivatives

Article abstract of DOI:10.1007/s10593-019-02614-2

[Figure not available: see fulltext.] New derivatives of heptazine were prepared from 2,5,8-trichloroheptazine by a Friedel–Crafts reaction and Pd-catalyzed amination. New triamino-substituted heptazine derivative represents a soluble carbon nitride monomeric unit suitable for the assembly of metal–organic and covalent–organic frameworks. Aromatic substituents in heptazine ring can be displaced by an alkylamine in a pseudo-nucleophilic substitution reaction.

Full text of DOI:10.1007/s10593-019-02614-2

DOI 10.1007/s10593-019-02614-2
Chemistry of Heterocyclic Compounds 2019, 55(12), 1281–1284
Synthesis of some heptazine derivatives
1
1
1
Kristīne Krūkle-Bērziņa , Kārlis Bērziņš , Kirill Shubin *
1
Latvian Institute of Organic Synthesis,
2
1 Aizkraukles St., Riga LV-1006, Latvia; е-mail: kir101@osi.lv
Published in Khimiya Geterotsiklicheskikh Soedinenii,
019, 55(12), 1281–1284
Submitted November 25, 2019
Accepted December 3, 2019
2
New derivatives of heptazine were prepared from 2,5,8-trichloroheptazine by a Friedel–Crafts reaction and Pd-catalyzed amination. New
triamino-substituted heptazine derivative represents a soluble carbon nitride monomeric unit suitable for the assembly of metal–organic
and covalent–organic frameworks. Aromatic substituents in heptazine ring can be displaced by an alkylamine in a pseudo-nucleophilic
substitution reaction.
Keywords: heptazine, Friedel–Crafts reaction, Pd-catalyzed amination.
Heptazine is a monomeric unit of a carbon nitride.1
Compounds based on this motif are promising photo-
catalysts2 and support materials for heterogeneous
Herein, we report a synthesis of new 4,4',4''-
(1,3,4,6,7,9,9b-heptaazaphenalene-2,5,8-triyl)trianiline (4)
as a novel tridentate ligand, using Friedel–Crafts reaction
of 2,5,8-trichloroheptazine (1) with chlorobenzene,
followed by Buchwald–Hartwig amination and Boc
deprotection (Scheme 1).
,3
4
catalysts. Monomeric structures were reported as efficient
5
thermally activated delayed fluorescence emitters and are
capable of self-organization to liquid crystals with
6
semiconducting properties. Surprisingly, despite of the
First, construction of 2,5,8-trichloroheptazine (1) core
was performed by a known method. Commercially
general interest in heptazine derivatives, quite a few of
them are described in the literature.
available melamine was calcined in air to result in nitrous
polymer – melon.2
1–23
The latter was hydrolyzed by
Extremely high reactivity of 2,5,8-trichloroheptazine (1)
limits chemical transformations to obtain its derivatives.
The set of published reactions includes almost exclusively
dissolving in boiling alkali to afford tripotassium salt of
cyameluric acid.6 Finally, chlorination of cyameluric acid
,24
6
–11
7,8,25
nucleophilic substitution
with some examples of
No transition metal-
was performed using PCl
5
in boiling POCl
3
.
Yet the
2
,4,12,13
electrophilic substitution.
yields are far from satisfactory, other methods for melon
26
catalyzed reactions are known for heptazine. On the other
hand, expansion of the synthetic toolbox will allow access
to new heptazine derivatives, suitable for the preparation of
metal–organic or covalent–organic frameworks. While
metal–organic frameworks are formed by coordination
bonds of polydentate ligands with metals, covalent–organic
synthesis utilizing, e.g., melamine and dicyandiamide,
7,28 29
ammonium thiocyanate,2
or urea afforded similar
results. Compound 1 possesses extremely low solubility in
organic solvents. Nevertheless, the reactivity of compound 1
even in a solid form toward substitution of chlorine atoms
is extremely high: it is unstable in open air due to
hydrolysis. However C–C bond construction on heptazine
seems rather challenging: the only reliable method found in
frameworks are porous polymers, in which organic
components are linked through covalent bonds.1
4,15
A
13,30–32
variety of potential applications for such materials include
literature is Friedel–Crafts reaction catalyzed by AlCl
We were pleased to find, that the slurry of 2,5,8-tri-
chloroheptazine (1) and AlCl react smoothly with neat
chlorobenzene even at a room temperature.
3
.
1
5
16
capture of CO
2
,
photocatalysis,
electrochemical
sensors,1
7
sorption,
18
light-responsive
materials,
19
3
catalysis.20
0
009-3122/19/55(12)-1281©2019 Springer Science+Business Media, LLC
1281

Chemistry of Heterocyclic Compounds 2019, 55(12), 1281–1284
Scheme 1
Table 1. Optimization of the reaction conditions
of Buchwald–Hartwig amination of compound 2*
excess of TFA in CH
Cl
at room temperature to result in
2
2
target amine 4 smoothly.
1
Yield
Downfield shift of the aryl proton signals in H NMR
spectrum of compound 2 hinted about a possible electron-
acceptor nature of the heptazine ring. We have investigated
nucleophilic substitution of chlorine atoms in para position
of phenyl groups in derivative 2 by the reaction in neat
piperidine and pyrrolidine. A complete conversion of
starting material was achieved, however, the structures of
obtained products were quite surprising (Scheme 2).
Catalyst
Base
Solvent
Time
of product 3, %
Pd (dba)
2
3
Cs
Cs
2
2
CO
3
3
PhMe
PhMe
PhMe
PhMe
6 days
4 days
traces
traces
traces
traces
15
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
CO
K
2
CO
3
13 days
13 days
Na
2
CO
3
Pd
Pd
2
2
(dba)
3
3
Cs
Cs
Cs
2
CO
CO
CO
3
3
3
1,4-Dioxane 5 days
(dba)
2
2
THF
5 days
18 h
5
Scheme 2
Pd(OAc)
2
1,4-Dioxane
THF**
24
Pd(OAc)
2
Cs
2
CO
3
18 h
68
*
Reaction conditions: tris(4-chlorophenyl) derivative 2 (1.0 eqiuv), tert-
butyl carbamate (4.5 equiv), catalyst (30 mol %), rac-BINAP (40 mol %),
and base (4.7 equiv) in dry solvent were heated to reflux.
*
* Reaction was performed at 100°C.
With tris(4-chlorophenyl) derivative 2 in hand, we
investigated the reaction of chlorobenzene moiety in a Pd-
33
catalyzed Buchwald–Hartwig amination (Table 1). Initially,
optimization study was performed using rac-BINAP as
ligand and varying different Pd sources (Pd(OAc)
2
and
Instead of substitution in phenyl ring, we observed a
cleavage of two 4-chlorophenyl fragments with the
insertion of two cyclic amine moieties into these positions.
The third 4-chlorophenyl group remained intact, without
any traces of nucleophilic substitution of chlorine. HRMS
spectra displayed signals with m/z 422 and 450, which
Pd
2
(dba)
3 2
), bases (Cs CO
3
, K
2
CO
3
, and Na
2
CO
3
), and
solvents (THF, PhMe, and 1,4-dioxane). The best result
was obtained under conditions given in the last entry.
With optimal conditions, other ligands were studied as
well (Table 2). However, none of them improved the
conversion and yield of product 3. Finally, reactions with
acetamide and benzamide did not provide any products.
Resulting reaction mixtures consisted mainly of insoluble
products. Deprotection of Boc groups was performed using
+
correspond to molecular ions [M+H] for derivatives 5 and
1
6, respectively. H NMR spectra show the presence of one
para-substituted phenyl group and two aliphatic amino
groups per each molecule. 13C NMR spectra contain four
characteristic signals corresponding to a trisubstituted
heptazine core, where two substituents are identical and the
third differs: 170.5–172.7 (C-2), 160.7–161.4 (C-3a,9a),
Table 2. Ligand screening for the Buchwald–Hartwig
amination of compound 2*
1
56.1–156.4 (C-5,8), and 154.6–155.3 ppm (C-6a).
Our hypothesis for the mechanism of this unusual
Ligand
Conversion**, %
Ligand
Conversion**, %
rac-BINAP
JohnPhos
70 (68***)
traces
30
Sphos
Xantphos
Xphos
traces
36
transformation is based on a well-described ability of
heptazines12 and analogous polymeric carbon nitrides to
generate long-living radical species. Internal structure of
the phenyl-substituted heptazine resembles that of redox-
34
DavePhos
42
Me4tButylXphos
traces
Ph
3
P
traces
active carboxylic esters, activated for metal-catalyzed or
photoinduced radical decarboxylation.3
5–37
We assume, that
*
Reaction conditions: tris(4-chlorophenyl) derivative 2 (1.0 equiv), tert-
butyl carbamate (4.5 equiv), Pd(OAc) (30 mol %), ligand (40 mol %),
(4.7 equiv) in dry THF were heated to reflux for 18 h.
* Conversion estimated by HPLC.
** Isolated yield of product 3.
2
once molecule 2 is in an excited state a series of internal
bond rearrangements eventually lead to a radical cleavage
of aryl–heptazine bond. Finally, heptazine core is left in an
and Cs
*
*
2
CO
3
1
282

Chemistry of Heterocyclic Compounds 2019, 55(12), 1281–1284
activated state, capable of reaction with an alkylamine,
and decantation of the resulting solution. The organic
solution was concentrated to a half of its volume (approx.
however, a detailed reaction mechanism is still under
investigation.
250 ml) and washed with saturated aqueous NH Cl solution
4
In conclusion, we have synthesized new 2,5,8-tris(4-chloro-
phenyl)-1,3,4,6,7,9,9b-heptaazaphenalene using Friedel–
Crafts reaction of 2,5,8-trichloroheptazine with chloro-
benzene. The synthesized derivative was successfully
subjected to Buchwald–Hartwig amination followed by
Boc group deprotection to a novel triaminoheptazine
derivative – 4,4',4''-(1,3,4,6,7,9,9b-heptaazaphenalene-2,5,8-
triyl)trianiline. The reaction of 2,5,8-tris(4-chlorophenyl)-
(50 ml). Organic layer was evaporated under reduced
pressure. Yield 1.5 g (68%), yellow powder, mp >300°C
1
(decomp.). H NMR spectrum (CDCl
3
), δ, ppm (J, Hz):
8.48 (6H, d, J = 8.9, H Ar); 7.47 (6H, d, J = 8.9, H Ar);
1
6.79 (3H, br. s, NH); 1.53 (27H, s, C(CH
spectrum (DMSO-d ), δ, ppm (J, Hz): 9.92 (3H, br. s, NH);
8.31 (6H, d, J = 9.0, H Ar); 7.68 (6H, d, J = 9.0, H Ar);
3 3
)
). H NMR
6
13
1.51 (27H, s, C(CH
3
)
3
). C NMR spectrum (CDCl ),
3
1
,3,4,6,7,9,9b-heptaazaphenalene with cyclic alkylamines
δ, ppm: 174.4; 158.1; 152.1; 144.5; 132.3; 128.5; 117.6;
+
resulted in substitution in heptazine ring with the loss of
two aryl groups.
81.6; 28.4. Found, m/z: 747.3373 [M+H] . C39
Calculated, m/z: 747.3367.
H
43
N
10
6
O .
4
,4',4''-(1,3,4,6,7,9,9b-Heptaazaphenalene-2,5,8-triyl)-
Experimental
trianiline (4). The reaction flask was charged with amide 3
1H and 13С NMR spectra were recorded on a Bruker
Avance 400 spectrometer (400 and 101 MHz, respec-
tively), internal standard – residual solvent signals. HRMS
analyses were performed on a Waters Synapt GII Q-ToF
UPLC/MS system (ESI). Reaction conversion was
estimated by UPLC on Waters Acquity UPLC H-class
instrument, column Waters Acquity UPLC BEH-C18,
(0.8 g, 1.1 mmol) and TFA (1.6 ml, 20.9 mmol, 20 equiv)
in CH
2
Cl (1.6 ml). The reaction mixture was stirred at
2
room temperature overnight and then concentrated under
reduced pressure to result dark-red powder, which was
suspended in 10% aqueous MeOH and heated under reflux
(50°C) for 48 h to remove excess TFA. The insoluble
powder was collected by filtration from the hot solution,
washed on filter with EtOAc (3×25 ml) and dried for 24 h
at 100°C at atmospheric pressure. Yield 0.125 g (26%),
2
.1 × 50 mm, 1.7 mm. Melting points were determined on a
Mettler Toledo TGA/DSC 2 apparatus.
1
Reagents, starting materials, and solvents were obtained
from commercial sources and used as received.
dark-red powder, mp >300°C (decomp.). H NMR spectrum
(DMSO-d ), δ, ppm (J, Hz): 8.10 (6H, d, J = 8.8, H Ar);
6
13
2
,5,8-Tris(4-chlorophenyl)-1,3,4,6,7,9,9b-heptaaza-
6.64 (6H, d, J = 8.8, H Ar); 6.30 (6H, br. s, NH
2
). C NMR
phenalene (2). 2,5,8-Trichloroheptazine (1) (5.4 g, 19.5 mmol)
was added to a suspension of AlCl (12.8 g, 96.0 mmol,
.9 equiv) in PhCl (53 ml) at 0°C. The mixture was stirred
spectrum (CDCl
3
), δ, ppm: 171.6; 157.0; 154.6; 131.8;
+
3
121.3; 113.0. Found, m/z: 447.1812 [M+H] . C24
Calculated, m/z: 447.1794.
H
19
10
N .
4
at 0°C for 30 min and then at room temperature overnight.
The reaction mixture was poured into ice water and stirred
vigorously until the color changed into yellow. Next, the
resulted suspension was heated to 100°C for 30 min,
cooled to room temperature, precipitated solids were
2-(4-Chlorophenyl)-5,8-di(pyrrolidin-1-yl)-1,3,4,6,7,9,9b-
heptaazaphenalene (5). The 10-ml pressure tube was charged
with 2,5,8-tris(4-chlorophenyl)-1,3,4,6,7,9,9b-heptaaza-
phenalene (2) (0.1 g, 0.2 mmol) and pyrrolidine (1.0 ml,
12.0 mmol, 60 equiv) and sealed. The reaction mixture was
collected by filtration, washed with H
pentane (25 ml), and dried in a desiccator over P
reduced pressure for 16 h. Yield 8.8 g (89%), light-yellow
2
O (25 ml) and
stirred at 100°C overnight, poured into H
extracted with CH Cl (3 × 25 ml). Organic layers were
pooled together, washed with H O (3×10 ml), dried over
Na SO , and concentrated under reduced pressure. The
resulted crude product was purified by flash chromato-
graphy on SiO , eluent CH Cl –MeOH, gradient from 0 to
10:1. Yield 0.03 g (36%), light-brown oil, solidifies upon
2
O (100 ml) and
2
O
5
under
2
2
2
1
powder, mp >300°C (decomp.). H NMR spectrum (CDCl
3
),
2
4
δ, ppm (J, Hz): 8.51 (6H, d, J = 8.6, H Ar); 7.51 (6H, d,
1
J = 8.7, H Ar). H NMR spectrum (DMSO-d
6
), δ, ppm
2
2
2
(
J, Hz): 8.31 (6H, d, J = 8.7, H Ar); 7.69 (6H, d, J = 8.7,
13
1
H Ar). C NMR spectrum (CDCl
other signals are too weak and not visible. C NMR
spectrum (DMSO-d ), δ, ppm: 170.1; 153.1; 151.5; 149.3;
31.0; 129.3. Found, m/z: 504.0298 [M+H] . C24
Calculated, m/z: 504.0292.
3
), δ, ppm: 132.1; 129.3;
standing. H NMR spectrum (CDCl
(2H, d, J = 8.7, H Ar); 7.39 (2H, d, J = 8.7, H Ar); 3.80–
3.68 (8H, m, NCH ); 2.00–1.93 (8H, m, NCH CH ).
C NMR spectrum (CDCl ), δ, ppm: 172.7; 160.7; 156.1;
154.6; 139.5; 133.9; 131.4; 128.5; 47.8; 25.2. Found, m/z:
3
), δ, ppm (J, Hz): 8.44
1
3
6
2
2
2
+
13
1
H
3
13Cl N
7
.
3
+
Tri-tert-butyl [1,3,4,6,7,9,9b-heptaazaphenalene-2,5,8-
triyltris(benzene-4,1-diyl)]tricarbamate (3). A 50-ml
pressure tube with a screw cap was charged with com-
pound 2 (1.5 g, 3.0 mmol), tert-butyl carbamate (1.57 g,
422.1611 [M+H] . C20
H
21
N Cl. Calculated, m/z: 422.1608.
9
2-(4-Chlorophenyl)-5,8-di(piperidin-1-yl)-1,3,4,6,7,9,9b-
heptaazaphenalene (6) was obtained by the procedure
described for compound 5. Yield 0.03 g (34%), yellow powder,
1
1
3.4 mmol, 4.5 equiv), Pd(OAc)
rac-BINAP (0.741 g, 1.2 mmol, 40 mol %), and Cs
4.5 g, 13.8 mmol, 4.7 equiv). The tube was purged with
2
(0.2 g, 0.9 mmol, 30 mol %),
mp >300°C (decomp.). H NMR spectrum (CDCl
3
), δ, ppm
2
CO
3
(J, Hz): 8.43 (2H, d, J = 8.7, H Ar); 7.41 (2H, d, J = 8.7,
H Ar); 3.98–3.88 (8H, m, NCH ), 1.67–1.60 (8H, m, CH );
),
(
2
2
13
Ar. Next, anhydrous THF (15 ml) was added, vial was
sealed and heated at 100°C for 18 h. Then the reaction
mixture was cooled to room temperature and extracted with
EtOAc (5×100 ml) by suspending solid residue in EtOAc
1.27–1.22 (4H, m, CH
2
). C NMR spectrum (CDCl
3
δ, ppm: 170.5; 161.4; 156.4; 155.3; 139.5; 133.9; 131.3;
+
128.6; 45.8; 45.7; 26.3; 24.5. Found, m/z: 450.1934 [M+H] .
C
22
H
25
N Cl. Calculated, m/z: 450.1934.
9
1
283

Chemistry of Heterocyclic Compounds 2019, 55(12), 1281–1284
This work was supported by the European Regional
Development Fund (agreement No. 1.1.1.1/16/A/288).
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