Synthesis of [1,2,4]triazolo[4,3-b]-s-tetrazines with incorporated furazan ring

Article abstract of DOI:10.1007/s11172-012-0017-6

Furazancarboxylic hydrazides can serve as nucleophiles to substitute for one dimethylpyrazole fragment in bis(3,5-dimethylpyrazol-1-yl)-s-tetrazine, giving the corresponding N-[6-(3,5-dimethylpyrazol-1-yl)-s-tetrazin-3-yl] -4-R-furazan-3-carbohydrazides i

Full text of DOI:10.1007/s11172-012-0017-6

Russian Chemical Bulletin, International Edition, Vol. 61, No. 1, pp. 121—130, January, 2012
121
Synthesis of [1,2,4]triazolo[4,3ꢀb]ꢀsꢀtetrazines
with incorporated furazan ring
A. B. Sheremetev,a N. V. Palysaeva,a K. Yu. Suponitskii,b and M. I. Struchkovaa
aN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Eꢀmail: sab@ioc.ac.ru
bA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Eꢀmail: kirshik@yahoo.com
Furazancarboxylic hydrazides can serve as nucleophiles to substitute for one dimethylpyrꢀ
azole fragment in bis(3,5ꢀdimethylpyrazolꢀ1ꢀyl)ꢀsꢀtetrazine, giving the corresponding
N´ꢀ[6ꢀ(3,5ꢀdimethylpyrazolꢀ1ꢀyl)ꢀsꢀtetrazinꢀ3ꢀyl]ꢀ4ꢀRꢀfurazanꢀ3ꢀcarbohydrazides in good
yields. Dehydration of the indicated carbohydrazides in polyphosphoric acid for the first time
gave rise to [1,2,4]triazolo[4,3ꢀb]ꢀsꢀtetrazines containing the furazan ring as a substituent at the
triazole ring.
Key words: furazan, triazole, tetrazine, triazolo[4,3ꢀb]ꢀsꢀtetrazine, cyclocondensation,
nucleophilic substitution, solvent effects, Xꢀray diffraction analysis.
1,2,4,5ꢀTetrazine ring (sꢀtetrazine) can be annulated
with various fiveꢀ and sixꢀmembered heterocycles, that is in
detail summarized in the recent review.1 The bicyclic system
of [1,2,4]triazolo[4,3ꢀb]ꢀsꢀtetrazine is of potential interest
as a basic framework for the design of compounds exhibitꢀ
ing a wide range of practically useful properties; dependꢀ
ing on the type of substituents, they can be, for example,
biologically active compounds2 or highꢀenergy materials.3
Such a bicyclic system, as a rule, is formed by annulaꢀ
tion of a 1,2,4ꢀtriazole ring to the sꢀtetrazine one. In this
case, 3ꢀhydrazinoꢀsꢀtetrazines are used as the starting
compounds,4 which are cyclized by such reagents, as forꢀ
mic acid,5 ortho esters,5–8 imino esters,5 bromocyan,6 or
carbon disulfide.5 At the same time, hydrazinotetrazines
can be initially involved into the reaction with aldehydes
to form the corresponding hydrazones, which cyclize to
triazolo[4,3ꢀb]ꢀsꢀtetrazines upon the action of such oxiꢀ
dants, as Pb(OAc)4 or ButOCl,5 or thermally.9
According to the recent studies,4 hydrazinotetrazine
acyl derivatives can be obtained by nucleophilic substituꢀ
tion for the dimethylpyrazole fragment in the easily availꢀ
able 3,6ꢀbis(3,5ꢀdimethylpyrazolꢀ1ꢀyl)ꢀsꢀtetrazine 1a (see
Ref. 10), in which case hydrazides of benzoic and nicoꢀ
tinic acids were successfully used as the nucleophiles. In
continuation of our works on the construction of comꢀ
pounds including sꢀtetrazine and furazan (1,2,5ꢀoxadiꢀ
azole) rings,11 we considered it reasonable to study a posꢀ
sibility of using furazancarboxylic hydrazides 2 for the synꢀ
thesis of the corresponding triazolo[4,3ꢀb]ꢀsꢀtetrazines.
The Taft induction constants (*) of 4ꢀRꢀfurazanyl
groups as substituents are within the interval from 2.55 to
2.88 (with R = NH2 and NO2, respectively).12 This index
for such a substituent as a CF3 group is the closest to that
of furazanyl fragment (* = 2.6).13 Therefore, initially
we attempted to find conditions under which available
N´ꢀ[6ꢀ(3,5ꢀdimethylpyrazolꢀ1ꢀyl)ꢀsꢀtetrazinꢀ3ꢀyl]trifluoꢀ
romethylꢀ3ꢀcarbohydrazide 3 (see Ref. 4) could have cyꢀ
clize to the corresponding (trifluoromethyl)triazolotetrꢀ
azine 4a. Note that the preceding attempts to cyclize hyꢀ
drazinotetrazine acyl derivatives under mild conditions
failed,5 at the same time, cyclization of hydrazinoazine
acyl derivatives is widely used for the construction of
various [1,2,4]triazolo[4,3ꢀb]azines.14—17 In fact, the studies
performed showed that compound 3 is efficiently dehyꢀ
drated by a shortꢀtime heating in polyphosphoric acid, and
the desired bicycle 4a is formed in 80% yield (Scheme 1).
In the azole series, the furazan ring as a substituent is
known to have one of the most powerful electronꢀwithꢀ
drawing effects.18—21 Naturally, the nucleophilicity of
molecules bearing this substituent is decreased. Will the
dimethylpyrazole fragment in tetrazine 1 undergo nucleoꢀ
philic substitution by the furazancarboxylic hydrazides?
To answer this questiion, we studied a reaction of comꢀ
pound 1a with 3ꢀmethylfurazancarboxylic hydrazide 2a
(see Ref. 22) as the model reaction (Scheme 2). We started
with clarification of the issue whether the type of a solvent
influences the reaction result. A typical procedure consistꢀ
ed in heating a mixture of the equimolar amounts of reacꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 119—128, January, 2012.
1066ꢀ5285/12/6101ꢀ121 © 2012 Springer Science+Business Media, Inc.

122
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 1, January, 2012
Sheremetev et al.
Scheme 1
Table 1. Effect of solvents on the reaction between compounds
1a and 2a
a
Entry
Solvent
T
/C
 /h
Yield
of 5a (%)
Other products
(yield (%))
1
2
3
4
5
6
7
8
MeOH
EtOH
PriOH
DMFА
Benzene
Dioxane
Glyme
DMSO
[emim][BF4]b 80
THF
64
78
82
90
80
85
80
80
2
2
3
4
5
40
47
80
78
76
60
86
62
86
75
58
28
8a (29)
8b (20)
8с (5)
8d (10)
—
—
—
—
—
6
18
23
24
26
36
54
9
10
11
12
66
80
61
—
—
tants (the molar concentration of 0.1 mol L–1) in a solꢀ
vent (Table 1). The reaction was carried out until the startꢀ
ing reagents or one of them were completely consumed,
that was monitored by TLC (but no longer than 55 h).
It turned out that substitution for one of the dimethylꢀ
pyrazole fragments to form the product 5a is observed in
all the solvents studied. As it is seen from Table 1, from 2
to 55 h is required to bring the reaction to completion
depending on the type of a solvent. The nucleophilic subꢀ
stitution is the fastest in alcohols. However, in this case
alcohols are also involved as competing nucleophiles, that
leads to the formation of a mixture of the target product 5a
with the corresponding alkoxytetrazines 8a—c; formation
of methoxy derivative 8a has been observed earlier4 in
the reaction of compound 1a with (cyclo)alkylamines in
methanol.
MeNO2
HCCl3
1a (43) +
+ 2а (30)
1a (67) +
+ 2а (61) +
+ 8е (7)
—
13
14
H2O
80
82
55
55
6
MeCN
88
a The reaction time.
b [emim][BF4] is the 4ꢀethylꢀ1ꢀmethylimidazolium tetrafluoroꢀ
borate.
It should be noted that alkoxytetrazines 8a—c are well
soluble in weakly polar solvents, this enables their easy
isolation from compound 5a, which is soluble only in polar
X = H (a—e), Br (f, g)
R = OMe (a), OEt (b, f), OPri (c, g), NMe2 (d), OH (e)
Scheme 2

[1,2,4]Triazolo[4,3ꢀb]ꢀsꢀtetrazine and furazan
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 1, January, 2012
123
Table 3. The reaction result of hydrazides 2a—c with tetrꢀ
azines 1a,b in MeCN in the presence of K2CO3 at 80 C (the
molar ratio of reagents 1 : 1 : 1)
solvents. Thus, simple treatment of a mixture of comꢀ
pounds 5a and 8a—c with hot hexane, which dissolves
alkoxy derivative 8a—c, provides an opportunity to quanꢀ
titatively separate these products.
Despite the fact that DMF was freshly dried and disꢀ
tilled, the reaction of tetrazine 1a and hydrazide 2a was
accompanied by a side formation of the dimethylamine
derivative 8d (see Table 1, entry 4).
The poor solubility of compound 1a in water strongly
interferes with the reaction course. After heating for 55 h,
about half of unreacted starting reactants were isolated,
and, besides the target product, the hydroxy derivative 8e
was also formed (see Table 1, entry 13).
Entry
Tetrꢀ
azine
Furꢀ
azan
Reaction
time/min
Product
(yield (%))
21
22
23
24
25
1a
1a
1a
1b
1b
2а
2b
2c
2a
2b
20
30
10
60
120
5a (85)
5b (87)
5c (80)
5d (92)
5e (90)
fold) and increase in the yield of monosubstituted product
(see Table 2, entry 20); the disubstituted product was not
detected in this case, either.
The maximum yield (see Table 1, entry 9) was obꢀ
tained when the reaction was carried out in the ionic liqꢀ
uid (4ꢀethylꢀ1ꢀmethylimidazolium tetrafluoroborate,
[emim][BF4]), however, the reaction is slow in this medium.
No substitution for the second dimethylpyrazole fragꢀ
ment on the tetrazine ring was detected in any of the solꢀ
vents listed in Table 1, even when a threeꢀfold excess
of hydrazide 2a and an increase in the heating time
were used.
Attempts to obtain the acyl derivative 5a by a proꢀ
longed heating of hydrazinotetrazine 6 and 3ꢀmethylꢀ
furazancarboxylic ester 7 (see Scheme 2) at 100 C in
dioxane or toluene failed.
We found that the reaction of compounds 1a and 2a
significantly accelerates in the presence of potassium carꢀ
bonate (Table 3). Thus, in boiling acetonitrile, where in
the absence of K2CO3 it required 55 h, in the presence of
potassium carbonate it came to completion within 20 min
and gave the target product 5a in good yield (see Table 3,
entry 21). Even poorly soluble in acetonitrile tetrazine 1b
reacts with hydrazide 2a in the presence of a base to form
the corresponding substituted product 5d within 1 h (see
Table 3, entry 24).
It is known4,23 that introduction of a bromine atom in
position 4 of the pyrazole fragment facilitates substitution
for this fragment on the tetrazine ring by the action of
some nucleophiles. In fact, in ethanol and isopropanol the
reaction of 3,6ꢀbis(4ꢀbromoꢀ3,5ꢀdimethylpyrazolꢀ1ꢀyl)ꢀ
sꢀtetrazine 1b with hydrazide 2a (see Scheme 2) is faster,
however, amount of the side product 8f,g increases, too
(cf. Tables 1 and 2). Introduction of a bromine atom into
the molecule decreases the solubility of compound 1b in
a number of solvents. Apparently, this is the fact that strongly
retards substitution in such solvents as MeCN, glyme, and
chloroform (see Table 2, entries 15, 18, and 19). Only in
DMSO the switch from compound 1a to 1b is accompaꢀ
nied by significant acceleration of the reaction (about tenꢀ
The reaction is of general scope and can be extended to
hydrazides of other furazancarboxylic acids (compounds
2b and 2c, see Scheme 1, Table 3).
However, it should be noted that attempts to replace
the pyrazole fragment in triazolotetrazine 4b with hyꢀ
drazide 2a (Scheme 3) led to a complex mixture of comꢀ
pounds (more than 10 spots on a TLC pattern) in both the
presence and the absence of K2CO3, from which no target
product 9 was isolated. The triazolotetrazine system is
known to undergo destruction in the presence of bases.7
Scheme 3
Table 2. Effect of solvents on the reaction between compounds
1b and 2a
a
Entry
Solvent
T/C
 /h
Yield
of 5d (%) (yield (%))
P b
15
16
17
18
19
20
MeCN
EtOH
PriOH
CHCl3
Glyme
DMSO
80
80
80
65
80
80
55.00
0.66
2.00
54.00
18.00
2.50
7
40
63
5
33
75
1b (78)
8f (30)
8g (10)
1b (62)
1b (41)
—
a The reaction time.
b Additional product.

124
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 1, January, 2012
Sheremetev et al.
The structure of hydrazine disubstituted derivatives
a
5a—e was confirmed by the IR, 1H and 13C NMR spectroꢀ
scopic data, mass spectrometric data (Table 4), and Xꢀray
diffraction studies of compound 5a. In the IR spectra,
absorption bands of the hydrazine fragment are observed
in the region 3208—3232 cm–1, as well as strong absorpꢀ
tion bands at 1710 cm–1 characteristic of the C=O bond
vibrations. Electron impact produces stable molecular
ions, whereas characteristic fragmentation results from
desintegration of the tetrazine ring with the concurrent
elimination of the N2 molecules and cleavage of the N—N
bond to form ions of two nitriles (Scheme 4).
O(2)
C(3)
C(10)
N(6)
N(5)
C(4)
C(9)
C(6)
C(1)
C(2)
N(2)
C(7)
C(8)
N(4)
N(9)
N(1)
C(5)
N(3)
N(7)
N(8)
O(1)
N(10)
C(11)
b
Scheme 4
Fig. 1. (a) The general view of the symmetrically independent
molecule A of compound 5a in representation of atoms as therꢀ
mal ellipsoids of atomic displacements with 50% probability;
(b) comparison of conformations of independent molecules A
(the solid lines) and A´ (the dashed lines).
It should be noted that the similar observation has
been made earlier29—32 when the structure of the moleꢀ
cules bearing only dimethylpyrazole and aminoꢀtetrazinyl
fragments was studied. The torsional angle C(3)—N(3)—
N(4)—C(4) describing a mutual orientation of the methꢀ
ylfurazanylcarbamide and aminoꢀtetrazinyl fragments in
molecules A and A´ is 100.0(2) and 119.7(2), respectively.
Such an orientation is determined by a combined influꢀ
ence of the intramolecular forces and the crystal packing,
in which hydrogen bonding is present between the NH
protons and the carbonyl groups. The nitrogen atoms of
the hydrazine fragment (N(3), N(4) and N(3´), N(4´) in
molecules A and A´, respectively) are characterized by
more planar pyramidal geometry (the angles at these
atoms are 357.5, 359.7, 358.5, and 359.8 for N(3), N(4),
N(3´), and N(4´), respectively), and the lone pair of elecꢀ
trons (LPE) on the atom N(4)—N(4´) is in transꢀposition
to the bond N(3)—C(3) (N(3´)—C(3´)) (the torsional
angle 167 (148)), that indicates existence of the anoꢀ
meric interaction. It should be also noted that the
N(3)—N(4) bond is somewhat shorter (1.381(2) and
1.385(2) Å in A and A´, respectively) as campared to its
average value (1.425 Å).33
The signals observed in the NMR spectra of derivatives
5a—e agree with the literature data for the structurally
similar tetrazine24,25 and furazan26—28 derivatives.
The Xꢀray diffraction data show that the symmetricalꢀ
ly independent part of the unit cell contains two molecules
of compound 5a (A and A´) and three molecules of water
(Fig. 1). Each of the structures of symmetrically indepenꢀ
dent molecules A and A´ is characterized by three planar
fragments turned with respect to each other: dimethylpyrꢀ
azole, methylfurazanylcarbamide, and aminoꢀtetrazinyl.
The major difference in the structure of molecules A and
A´ is due to the turn of the pyrazole ring with respect to the
tetrazine one (the angle is 36.12(7) and 5.78(10), respecꢀ
tively) and is apparently determined by the effects of packꢀ
ing of the molecules in the crystal. Despite the difference
in the mutual orientation of the pyrazole and tetrazine
rings, the bond distances C(5)—N(9) (C(5´)—N(9´)) in
molecules A and A´ are equal (1.404(2) and 1.402(2) Å,
respectively), that indicates the absence of conjugation
between the rings even in molecule A´, where these rings
are virtually coplanar.
This is apparently caused by participation of the LPE
of the atoms N(3) (N(3´)) and N(4) (N(4´)) in the conjuꢀ
gation with the surrounding ꢀaccepting fragments (the
tetrazine ring and the carbonyl group), that leads to a lesser
repulsion between the "deficient" LPE of the atoms N(3)
(N(3´)) and N(4) (N(4´)) and, as a consequence, to the
shortening the bond N(3)—N(4).

[1,2,4]Triazolo[4,3ꢀb]ꢀsꢀtetrazine and furazan
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 1, January, 2012
125

126
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 1, January, 2012
Sheremetev et al.
Scheme 5
Due to the presence in the structure of compound 5a of the
gives rise to the product 4d in only 30% yield; some resinꢀ
ification is observed in this case.
labile NH protons and the oxygen and nitrogen atoms
possessing the protonꢀaccepting properties, a wide system of
hydrogen bonding is effected in the crystal packing. Due to
these hydrogen bonds, molecules are bound in planes parallel
to ab, which are additionally stabilized by numerous someꢀ
what shortened contacts N...O and N...N, frequently obꢀ
served in the crystal packings of furazan derivatives.34—37
We studied dehydration of the hydrazide fragment in
compounds 5 in order to prepare the corresponding triꢀ
azoloꢀtetrazines 4 (Scheme 5). Thus, heating of comꢀ
pound 5a in phosphorus oxychloride in the presence of
pyridine leads to a triazole ring closure, and the target
derivative 4c can be isolated in 22% yield. However, like in
the synthesis of compound 4a (see Scheme 1), the deꢀ
hydration is the most efficient in polyphosphoric acid. In
this case, the yield of triazole 4c increases virtually threeꢀ
fold, reaching 62%. It should be noted that the cyclization
takes place only at temperatures above 150 C; at lower
temperatures no annulation of the triazole ring occurs.
Introduction of a bromine atom into the pyrazole ring,
apparently, decreases the thermal stability of compounds,
and dehydration of compound 5d in polyphosphoric acid
Compound 5b, bearing an amino group, in polyphosꢀ
phoric acid at 150 C is completely consumed within
15 min. Five minutes after the reaction begins, the TLC
data show the presence in the reaction mixture of the
product and the starting compound. However, further
heating leads to a decrease in the content of both the
starting compound and the target product. Compound 4e
was isolated in no more than 2% yield. If the amino group
was preliminary acylated, the dehydration of compound
5f is accompanied by strong resinification: neither the corꢀ
responding cyclized product, nor the deacylated product 4e
were isolated.
Thermolysis of compound 5c in polyphosphoric acid
also leads to resinification of the reaction mixture, that
can be apparently attributed to the presence of a pyrrole
ring in the molecule, which is unstable under such condiꢀ
tions. We unsucceded in isolation of any individual comꢀ
pounds from the reaction mixture.
The structure of obtained triazolotetrazine derivatives 4
was confirmed by the IR, 1H and 13C NMR spectroscopic
data, as well as by the mass spectrometric data (Table 5).
Table 5. Spectral characteristics of compounds 4a—e
Comꢀ
pound
1H NMR,
13C NMR, 
MS,
m/z
IR,
/cm–1

C(1)
C(2) C(3) C(4) C(5) C(6)
Other C atoms
4a
2.34 (s, 3 H, C(6)Me); 135.9 (q, 150.8 150.5 144.1 112.2 154.2 13.5, 13.6
284 [M]+, 2937, 1591,
2
2.62 (s, 3 H, C(4)Me); JCF
=
(C(4)Me, C(6)Me); 265, 187,
1542, 1526,
1462, 1405,
1372, 1294,
1250, 1211,
1168, 1106,
1043, 1021,
966, 820
6.46 (s, 1 H, C(5)H) = 42.5 Hz)
117.7 (q, (CF3,
1JC,F = 271.2 Hz)
161, 121,
107
(to be continued)

[1,2,4]Triazolo[4,3ꢀb]ꢀsꢀtetrazine and furazan
Table 5 (continued)
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 1, January, 2012
127
Comꢀ
pound
1H NMR,
13C NMR, 
MS,
m/z
IR,
/cm–1

C(1)
C(2) C(3) C(4) C(5) C(6)
Other C atoms
4b
2.22 (s, 3 H, C(6)Me); 137.8
2.62 (s, 3 H, C(4)Me);
6.45 (s, 1 H, C(5)H);
150.9 150.7 143.7 111.2 152.9 13.3, 13.6 (C(6)Me,
C(8)Me)
—
3084, 1586,
1525, 1470,
1415, 1404,
1359, 1282,
1144, 1109,
1072, 1027,
1012, 974,
946, 838
12.23 (s, 1 H, C(1)H)
4c
4d
4e
2.32 (s, 3 H, C(8)Me); 136.6
2.67 (s, 3 H, C(6)Me);
2.84 (s, 3 H, C(4)Me);
6.43 (s, 1 H, C(5)H)
150.8 150.7 142.7 111.7 153.6 9.5 (C(8)Me);
298 [M]+, 1584, 1536,
1472, 1396,
121, 106, 95 1288, 1236,
1112, 1044,
13.5, 13.7 (C(4)Me, 213, 158,
C(6)Me);
143.9 (C(7));
151.9 (C(8))
1016, 996,
984, 968, 892
2.36 (s, 3 H, C(8)Me); 137.4
2.69 (s, 3 H, C(6)Me);
2.89 (s, 3 H, C(4)Me);
151.5 150.9 141.9 101.8 152.6 10.1 (C(8)Me);
—
2931, 1576,
1534, 1473,
1395, 1358,
1296, 1240,
1043, 1017,
997, 982, 895
13.0, 13.5 (C(6)Me,
C(4)Me);
143.2 (C(7));
152.5 (C(8))
2.68 (s, 3 H, C(6)Me);
2.87 (s, 3 H, C(4)Me);
6.42 (s, 1 H, C(7)H);
6.48 (s, 2 H, NH2)
—
—
—
—
—
—
—
299 [M]+,
214, 121,
106
—
The signals in the 13C NMR spectra were assign based on
the selective heteronuclear double resonance 1H—13C and
1H—15N HMBC, as well as based on the data on the
related compounds.24,25
spectrometer using a Bruker standard procedure. The HMBC
experiments were optimized for the spinꢀspin coupling constants
JH—C and JH—N equal to 8 Hz. Mass spectra were recorded on
Varian MAT CHꢀ6 and Varian MAT CHꢀ111 (70 eV) instruꢀ
ments. IR spectra were recorded on a Bruker AlphaꢀT specꢀ
trometer (KBr pellets). Reaction progress and products purity
were monitored by TLC on Sorbfil precoated plates, SiO2 40/100
silica gel was used for preparative chromatography. The starting
3ꢀRꢀ4ꢀfurazancarboxylic hydrazides 2 were obtained according
to the procedures described earlier.22,38 Solvents were dried acꢀ
cording to the standard procedures. Polyphosphoric acid was
purchased from Lancaster.
In conclusion, it was shown that furazancarboxylic
hydrazides can serve as nucleophiles and substitute for the
dimethylpyrazole fragment bonded to the sꢀtetrazine ring.
The success of thermal dehydration of 1ꢀacylꢀ2ꢀtetrazinylꢀ
hydrazines in polyphosphoric acid to form the correspondꢀ
ing [1,2,4]triazolo[4,3ꢀb]ꢀsꢀtetrazines depends on the type
of substituents on the molecule. Our work results in the
synthesis of the first representatives of [1,2,4]triazolo[4,3ꢀb]ꢀ
sꢀtetrazines containing an electronꢀwithdrawing group
(CF3 group or furazanyl fragment) as a substituent on the
triazole ring.
Xꢀray diffraction studies. Single crystals of compound 5a suitꢀ
able for Xꢀray diffraction experiment were obtained by slow conꢀ
centration of its solution in aqueous isopropanol. Crystals of 5a
(C11H12N10O2•1.5H2O) at 100 K are monoclinic, a = 8.928(2) Å,
3
b = 11.774(3) Å, c = 29.879(7) Å,  = 92.496(4), V = 3138.1(12) Å ,
Z = 8, space group P21/n,  = 0.114 mm–1, dcalc = 1.453 g cm–3
.
Intensities of 38951 reflections were measured on a SMART
APEX2 CCD diffractometer ((MoꢀK) = 0.71073 Å, graphite
monochromator, ꢀscan technique, 2 < 60). The starting arꢀ
ray of measured intensities was processed using the SAINT and
SADABS program, included into the APEX2 program packꢀ
age.39 The structure was solved by direct method and refined by
fullꢀmatrix least squares method in anisotropic approximation
for nonhydrogen atoms on F 2hkl. Hydrogen atoms were placed
into geometrically calculated positions except for the hydrogen
atoms of amino groups, whose positions were localized from the
Experimental
Melting points were measured on a Gallenkamp melting
point apparatus and were not corrected. 1H, 13C, 15N, and
19F NMR spectra on the natural content of isotopes were reꢀ
corded on a Bruker AMꢀ300 spectrometer (300.13, 75.7, 30.4,
and 282.4 MHz, respectively) in DMSOꢀd6 or CDCl3. Chemiꢀ
cal shifts 15N were measured relative to the external standard
MeNO2. 2D spectra were recorded on a Bruker Avance 600

128
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 1, January, 2012
Sheremetev et al.
differential syntheses of electron density and then normalized on
the distance of 0.90 Å. All the hydrogen atoms were refined
using the riding model (Uiso(H) = nUeq(s,N), where n = 1.5 for
the carbon atoms of methyl groups, n = 1.2 for the N atoms).
1388, 1312, 1148, 1104, 1084, 1044, 1024, 984, 952, 932, 820,
760, 704. 1H NMR (DMSOꢀd6), : 1.49 (d, 6 H, CHMe2,
J = 6.1 Hz); 2.25 (s, 3 H, C(3)Me); 2.51 (s, 3 H, C(5)Me); 5.47
(septet, 1 H, CHMe2, J = 6.1 Hz); 6.26 (s, 1 H, C(4)H).
13C NMR (DMSOꢀd6), : 165.3 (C(1)); 158.5 (C(2)); 151.3 (C(5));
142.2 (C(3)); 109.8 (C(4)); 73.56 (CHMe2); 21.3 (CHMe2); 13.2,
12.9 (C(3)Me; C(5)Me).
3ꢀ(3,5ꢀDimethylpyrazolꢀ1ꢀyl)ꢀ6ꢀ(N,Nꢀdimethylamino)ꢀsꢀ
tetrazine (8d). The yield was 10%, red finely crystalline comꢀ
pound, m.p. 141—142 C (cf. Ref. 4: m.p. 137—138 C).
Found (%): C, 49.37; H, 6.00; N, 44.66. C9H13N7 (M = 219.25).
Calculated (%): C, 49.30; H, 5.98; N, 44.72. MS, m/z: 219 [M]+,
122, 106. IR, max/cm–1: 3146, 3101, 2922, 2871, 2798, 1573,
1488, 1411, 1399, 1368, 1296, 1207, 1131, 1076, 1038, 1021,
974, 948, 845, 810, 755, 611, 576, 531. 1H NMR (CDCl3), :
2.25 (s, 3 H, C(3)Me); 2.46 (s, 3 H, C(5)Me); 3.30 (s, 6 H,
NMe2); 6.02 (s, 1 H, C(4)H). 13C NMR (CDCl3), : 156.2 (C(1));
151.2 (C(5)); 143.9 (C(2)); 141.5 (C(3)); 104.1 (C(4)); 36.7
(NMe2); 8.5, 8.1 (C(3)Me, C(5)Me).
The refinement included 9138 independent reflections (Rint
=
= 0.0609), number of refined parameters was 448. Convergence
of refinement for all the independent reflections wR2 = 0.1115,
GOOF = 1.014 (R1 = 0.0478 on 6163 reflections with I > 2(I)).
All the calculations were performed on IBM PC AT using the
SHELXTL program package.40 The structure was deposited with
the Cambridge Structural Database (CCDC 852363).
Preparation of N´ꢀ[6ꢀ(3,5ꢀdimethylpyrazolꢀ1ꢀyl)ꢀsꢀtetrazinꢀ
3ꢀyl]ꢀ4ꢀmethylfurazanꢀ3ꢀcarbohydrazide 5a in different solvents
(general procedure). A mixture of tetrazine 1a (0.27 g, 0.1 mmol)
and hydrazide 2a (0.14 g, 0.1 mmol) in the corresponding solꢀ
vent 10 mL) was stirred on heating until the reaction reached
completion (TLC monitoring, eluent CCl4—MeCN (3 : 1)). The
method for the isolation of product 5a depends on the type of
solvent used. For the reaction conditions and the product yields,
see Table 1. The data for compound 5a are summarized in Table 4.
A. When the reaction was carried out in alcohols and DMF,
formation of two product was observed. The starting comꢀ
pounds and the reaction products are well detectable by TLC:
Rf(8) > Rf(1) > Rf(2) > Rf(5). The reaction mixture was cooled
to 20 C and diluted with water (10 mL). An orange precipitate
formed was filtered off, washed with hexane (15 mL), and reꢀ
crystallized from the PriOH—H2O (1 : 1) mixture. The filtrate
was extracted with hexane (30 mL), the solvent was evaporated
to obtain a mixture of tetrazine 8a—d and 3,5ꢀdimethylpyrazole,
which was separated by column chromatography on SiO2 (eluent
dichloromethane).
B. When the reaction was carried out in THF, nitromethane,
benzene, or dioxane, no formation of side products was observed.
After the reaction was completed, the solvent was evaporated to
dryness, and the residue was recrystallized from PriOH—H2O
(1 : 1).
C. When the reaction was carried out in glyme or DMSO
(the reaction mixture was diluted with water (10 mL)), an orꢀ
ange precipitate formed was filtered off, washed with water
(2×3 mL), and recrystallized from PriOH—H2O (1 : 1).
D. After reflux in chloroform for 54 h, the reaction mixture
still contained about half of unreacted starting compounds. The
reaction mixture was cooled to 20 C, a formed precipitate of
product 5a was filtered off and recrystallized. The filtrate conꢀ
taining the starting compounds 1a and 2a was concentrated to
dryness. The residue was dissolved in hot EtOH (2 mL), the
solution was cooled to –5 C; a white precipitate formed was
filtered off to obtain methylfurazancarboxylic hydrazide 2a. The
filtrate was concentrated to dryness, and the residue was reꢀ
crystallized from PriOH—H2O (7 : 1) to yield the starting tetrꢀ
azine 1a.
3ꢀ(3,5ꢀDimethylpyrazolꢀ1ꢀyl)ꢀ6ꢀmethoxyꢀsꢀtetrazine (8a).
The yield was 29%, pink solid compound, m.p. 157—158 C
(cf. Ref. 4, m.p. 156—157 C). Found (%): C, 46.67; H, 4.93,
N, 40.71. C8H10N6O (M = 206.20). Calculated (%): C, 46.60,
H, 4.89, N, 40.76. MS, m/z: 206 [M]+ 121, 106, 94, 80, 67. IR,

max/cm–1: 3112, 3024, 2960, 2924, 1644, 1576, 1484, 1448,
1400, 1376, 1356, 1280, 1160, 1136, 1084, 1048, 1028, 1000,
972, 952, 824, 760, 680, 588, 572. 1H NMR (DMSOꢀd6), : 2.25
(s, 3 H, Me, C(3)); 2.50 (s, 3 H, Me, C(5)); 4.27 (s, 3 H, OMe);
6.29 (s, 1 H, H, C(4)). 13C NMR (DMSOꢀd6), : 166.1 (C(1));
158.8 (C(2)); 151.5 (C(5)); 142.4 (C(3)); 110.1 (C(4)); 56.78
(OMe); 13.0, 13.3 (C(5)Me; C(3)Me). 15N NMR (DMSOꢀd6),
: 293.1, 301.6, 353.5, 373.2.
3ꢀ(3,5ꢀDimethylpyrazolꢀ1ꢀyl)ꢀ6ꢀethoxyꢀsꢀtetrazine (8b). The
yield was 20%, pink amorphous compound, m.p. 114—115 C.
Found (%): C, 49.13; H, 5.55, N, 38.08. C9H12N6O (M =
= 220.23). Calculated (%): C, 49.08: H, 5.49: N, 38.16. MS, m/z:
220 [M]+ 121, 106, 94, 80. IR, max/cm–1: 3104, 2988, 2964,
2932, 1576, 1484, 1432, 1388, 1348, 1328, 1280, 1112, 1088,
1024, 988, 968, 952, 932, 828, 764, 660. 1H NMR (DMSOꢀd6),
: 1.49 (t, 3 H, CH2CH3, J = 7 Hz); 2.25 (s, 3 H, C(3)Me); 2.50
(s, 3 H, C(5)Me); 4.65 (q, 2 H, CH2Me, J = 7 Hz); 6.28 (s, 1 H,
C(4)H). 13C NMR (DMSOꢀd6), : 165.7 (C(1)); 158.7 (C(2));
151.5 (C(5)); 142.3 (C(3)); 110.0 (C(4)); 65.8 (OCH2Me); 14.13
(OCH2CH3); 13.4, 13.0 (C(3)Me; C(5)Me).
E. When the reaction was carried out in water, it was stopped
after heating tetrazine 1a and hydrazide 2a for 55 h at 80 C
(according to the TLC data, the reaction mixture still contained
much of the starting compounds). The mixture was cooled to
20 C, the poorly soluble in water compounds 1a and 2a were
filtered off, the aqueous filtrate was extracted with dichloꢀ
romethane (10 mL), whose concentration gave product 5a. The
aqueous solution after acidification with 1% HCl (3 mL) and
extraction with diethyl ether (2×10 mL) furnished 3ꢀ(3,5ꢀdiꢀ
methylpyrazolꢀ1ꢀyl)ꢀ6ꢀhydroxyꢀsꢀtetrazine (8e), the yield was
11%, dark red solid compound, m.p. 205 C (with decomp.).
Found (%): C, 43.82; H, 4.16; N, 43.67. C7H8N6O (M = 192.18).
Calculated (%): C, 43.75; H, 4.20; N, 43.73. IR, max/cm–1
:
3136, 3088, 3036, 2900, 1744, 1708, 1692, 1552, 1508, 1432,
1404, 1372, 1264, 1160, 1140, 1104, 1064, 1024, 992, 976, 836,
808, 780, 764. 1H NMR (DMSOꢀd6), : 2.25 (s, 3 H, C(3)Me);
2.39 (s, 3 H, C(5)Me); 6.20 (s, 1 H, C(4)H). 13C NMR (DMSOꢀd6),
: 152.0 (C(1)); 150.6 (C(5)); 147.7 (C(2)); 141.9 (C(3)); 108.8
(C(4)); 13.21 12.11 (C(3)Me; C(5)Me).
F. When the reaction was carried out in ionic liquid, a mixꢀ
ture of [emim][BF4] (3 mL), tetrazine 1a (0.1 g, 0.37 mmol),
and hydrazide 2a (0.05 g, 0.37 mmol) was heated for 24 h at
3ꢀ(3,5ꢀDimethylpyrazolꢀ1ꢀyl)ꢀ6ꢀ(isoꢀpropoxy)ꢀsꢀtetrazine
(8c). The yield was 5%, light pink amorphous compound, m.p.
47—48 C. Found (%): C, 51.36; H, 6.07; N, 35.80. C10H14N6O
(M = 234.26). Calculated (%): C, 51.29; H, 6.02; N, 35.88. MS,
m/z: 234 [M]+ 121, 106. IR, max/cm–1: 2976, 2928, 1576, 1476,

[1,2,4]Triazolo[4,3ꢀb]ꢀsꢀtetrazine and furazan
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 1, January, 2012
129
80 C. Then the reaction mixture was cooled to 30 C and dilutꢀ
ed with water (20 mL). The product 5a was extracted with diethꢀ
yl ether (2×50 mL), the extract was dried with magnesium sulꢀ
fate, concentrated, and the residue was recrystallized from
PriOH—H2O (1 : 1).
3,6ꢀBis(4ꢀbromoꢀ3,5ꢀdimethylpyrazolꢀ1ꢀyl)ꢀsꢀtetrazine (1b).
NꢀBromosuccinimide (1.95 g, 11 mmol) was added to a solution
of tetrazine 1a (1.47 g, 5.5 mmol) in hot MeCN (20 mL). The
reaction mixture was refluxed for 2 min, then cooled to 20 C.
A precipitate formed was filtered off and washed with MeCN
(10 mL) to obtain a reddish orange amorphous compound (2.11 g,
91%) , m.p. 262—263 C (cf. Ref. 4: m.p. 262 C). MS, m/z: 428
[M]+, 199, 120, 105. IR, max/cm–1: 1568, 1500, 1456, 1428,
1404, 1388, 1056, 1008, 936, 780. 1H NMR (CDCl3), : 2.39
(s, 3 H); 2.72 (s, 3 H). 13C NMR (CDCl3), : 159.1 (C(1)); 153.4
(C(5)); 141.1 (C(3)); 102.5 (CBr); 13.8, 12.8.
N´ꢀ[6ꢀ(4ꢀBromoꢀ3,5ꢀdimethylpyrazolꢀ1ꢀyl)ꢀsꢀtetrazinꢀ3ꢀ
yl]ꢀ4ꢀmethylfurazanꢀ3ꢀcarbohydrazide (5d). A mixture of tetrꢀ
azine 1b (0.27 g, 0.1 mmol) and hydrazide 2a (0.14 g, 0.1 mmol)
in the corresponding solvent (10 mL) was heated with stirring
until the reaction reached completion (TLC monitoring, eluent
CCl4—MeCN (3 : 1)). For the reaction conditions and the
product yields, see Table 2.
A (Carrying out the reaction in alcohols). The starting comꢀ
pounds were dissolved in alcohol on heating. The reaction
progress was monitored by TLC (eluent CCl4—MeCN (3 : 1)).
After the reaction reached completion, formation of two prodꢀ
ucts, 5d (Rf = 0.4) and 8f (Rf = 0.9), was observed. The reaction
mixture was cooled to 20 C and diluted with water (10 mL).
A precipitate formed was filtered off and washed with hexane to
obtain product 5d (0.08 g, 67%) as an orange amorphous powꢀ
der, m.p. 186—189 C (see also Table 4). The filtrate was exꢀ
tracted with hexane, the solvent was evaporated to yield a mixꢀ
ture of 3ꢀ(4ꢀbromoꢀ3,5ꢀdimethylpyrazolꢀ1ꢀyl)ꢀ6ꢀ(3ꢀethoxy)ꢀ
1,2,4,5ꢀtetrazine (8f) and 4ꢀbromoꢀ3,5ꢀdimethylpyrazole, which
was separated by column chromatography on SiO2 (eluent hexꢀ
ane), Rf = 0.8, Rf = 0, respectively.
D. After reflux in chloroform for 54 h, the mixture still conꢀ
tained about half of the starting compounds. The reaction mixꢀ
ture was cooled to 20 C, a formed precipitate of product 5d was
filtered off and recrystallized. The filtrate containing the starting
compounds 1b and 2a was concentrated to dryness. The residue
was recrystallized from aqueous ethanol (2 mL) to yield the startꢀ
ing tetrazine 1b.
Synthesis of compounds 5 in the presence of potassium carꢀ
bonate (general procedure). Potassium carbonate (1 mmol) was
added to a solution of 3,6ꢀbis(4ꢀXꢀ3,5ꢀdimethylpyrazolꢀ1ꢀyl)ꢀsꢀ
tetrazine (1a or 1b) (1 mmol) and 3ꢀRꢀ4ꢀfurazancarboxylic hydrꢀ
azide 2a—c (1 mmol) in anhydrous acetonitrile (10 mL). A dark
claretꢀcolored suspension formed was refluxed until the reaction
reached completion (TLC monitoring, eluent CCl4—MeCN
(1 : 1)). The reaction mixture was cooled to 20 C and diluted
with water (10 mL) to obtain a homogeneous solution. This soluꢀ
tion was neutralized with 3% aqueous HCl to pH 3, with the
color changing from dark brown to orange and a precipitate
forming. The product was filtered off, washed with water
(2×5 mL), dried in air, and recrystallized from 50% aq. PriOH.
The reaction time and the product yields are summarized
in Table 3.
N´ꢀ[6ꢀ(3,5ꢀDimethylpyrazolꢀ1ꢀyl)ꢀsꢀtetrazinꢀ3ꢀyl]ꢀ4ꢀmethylꢀ
furazanꢀ3ꢀcarbohydrazide (5a), orange amorphous powder, m.p.
193—194 C (PriOH—H2O).
N´ꢀ[6ꢀ(3,5ꢀDimethylpyrazolꢀ1ꢀyl)ꢀsꢀtetrazinꢀ3ꢀyl]ꢀ4ꢀaminoꢀ
furazanꢀ3ꢀcarbohydrazide (5b), orange amorphous powder, m.p.
257—258 C (PriOH).
N´ꢀ[6ꢀ(3,5ꢀDimethylpyrazolꢀ1ꢀyl)ꢀsꢀtetrazinꢀ3ꢀyl]ꢀ4ꢀ(pyrꢀ
rolꢀ1ꢀyl)furazanꢀ3ꢀcarbohydrazide (5c), orange amorphous powꢀ
der, m.p. 228—229 C (PriOH—H2O).
N´ꢀ[6ꢀ(4ꢀBromoꢀ3,5ꢀdimethylpyrazolꢀ1ꢀyl)ꢀsꢀtetrazinꢀ3ꢀ
yl]ꢀ4ꢀmethylfurazanꢀ3ꢀcarbohydrazide (5d), orange amorphous
powder, m.p. 189—192 C (PriOH—H2O).
N´ꢀ[6ꢀ(4ꢀBromoꢀ3,5ꢀdimethylpyrazolꢀ1ꢀyl)ꢀsꢀtetrazinꢀ3ꢀ
yl]ꢀ4ꢀaminofurazanꢀ3ꢀcarbohydrazide (5e), orange amorphous
powder, m.p. 238—239 C (PriOH).
3ꢀ(4ꢀBromoꢀ3,5ꢀdimethylpyrazolꢀ1ꢀyl)ꢀ6ꢀethoxyꢀsꢀtetrazine
(8f). The yield was 30%, pink amorphous compound, m.p.
100—102 C. Found (%): C, 36.16; H, 3.75; N, 28.04.
C9H11N6OBr (M = 299.13). Calculated (%): C, 36.14; H, 3.71;
N, 28.10. MS, m/z: 298 and 300 [M]+, 199, 120, 105. IR,
Cyclization of hydrazinotetrazine acyl derivatives (general proꢀ
cedure). A suspension of hydrazinotetrazine 5a (0.16 g, 0.5 mmol)
in polyphosphoric acid (0.8 g) was heated with stirring to 150 C.
The solution formed was stirred at 150 C for 5 min. Then the
reaction mixture was cooled to 100 C and poured into water.
A precipitate was filtered off, washed with water, and recrystalꢀ
lized. Spectral characteristics of compounds 4a—c are given
in Table 5.
6ꢀ(3,5ꢀDimethylpyrazolꢀ1ꢀyl)ꢀ3ꢀtrifluoromethylꢀ1,2,4ꢀtriꢀ
azolo[4,3ꢀb]ꢀsꢀtetrazine (4a). The yield was 56%, yellow amorꢀ
phous powder, m.p. 193—194 C (PriOH—H2O). 19F NMR
(DMSOꢀd6), : –63.6 (CF3). 15N NMR (DMSOꢀd6), : –179
(C(4)N), –79 (C(6)N). Found (%): C, 38.00; H, 2.41; N, 39.34.
C9H7F3N6 (M = 284.20). Calculated (%): C, 38.04; H, 2.48;
N, 39.43.
6ꢀ(3,5ꢀDimethylpyrazolꢀ1ꢀyl)ꢀ3ꢀ(3ꢀmethylfurazanꢀ4ꢀyl)ꢀ
1,2,4ꢀtriazoloꢀ[4,3ꢀb]ꢀsꢀtetrazine (4c). The yield was 57%, yellow
amorphous powder, m.p. 165—170 C (EtOH—H2O).
Found (%): C, 44.23; H, 3.43; N, 46.88. C11H10N10O (M =
= 298.26). Calculated (%): C, 44.30; H, 3.38; N, 46.96.
6ꢀ(4ꢀBromoꢀ3,5ꢀdimethylpyrazolꢀ1ꢀyl)ꢀ3ꢀ(3ꢀmethylfurazanꢀ
4ꢀyl)ꢀ1,2,4ꢀtriazoloꢀ[4,3ꢀb]ꢀsꢀtetrazine (4d). The yield was 30%,
yellow amorphous powder, m.p. 183—184 C (EtOH). 15N NMR
(DMSOꢀd6), : –176.9, –176.6, –130.7, –78.3, –56.1, –39.5,

max/cm–1: 2924, 2852, 1568, 1493, 1454, 1389, 1348, 1064,
1034, 961, 925, 789, 696, 657, 620, 588, 552. 1H NMR (DMSOꢀd6),
: 1.50 (t, 3 H, CH2CH3, J = 7 Hz); 2.28 (s, 3 H, C(3)Me); 2.51
(s, 3 H, C(5)Me); 4.70 (q, 2 H, CH2Me, J = 7 Hz). 13C NMR
(DMSOꢀd6), : 165.9 (C(1)); 158.4 (C(2)); 150.0 (C(5)); 140.1
(C(3)); 99.1 (C(4)); 66.0 (OCH2Me); 14.1, 14.0 (C(3)Me;
C(5)Me); 12.3 (OCH2CH3).
B. When the reaction was carried out in DMSO, the reaction
mixture was diluted with water (10 mL), an orange precipitate
formed was filtered off, washed with water (2×3 mL), and reꢀ
crystallized from PriOH–H2O (1 : 1).
C. When the reaction was carried out in acetonitrile or glyme,
tetrazine 1b and hydrazide 2a were heated at 80 C for 25 h
(TLC showed that the mixture still contained the starting
compounds). The reaction mixture was cooled to 20 C, a poorly
soluble tetrazine 1b was filtered off. The filtrate was diluted
with water (10 mL) and extracted with dichloromethanene,
the extract was dried with MgSO4 and concentrated to obtain
product 5d.

130
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 1, January, 2012
Sheremetev et al.
30.1, 35.3, 36.0, 59.5. Found (%): C, 35.12; H, 2.47; N, 37.02.
C11H9BrN10O (M = 377.16). Calculated (%): C, 35.03; H, 2.41;
N, 37.14.
18. I. V. Tselinskii, S. F. Mel´nikova, S. N. Vergisov, Khim.
Geterotsikl. Soedin., 1981, 321 [Chem. Heterocycl. Compd.
(Engl. Transl.), 1981, 17, 228].
19. I. V. Tselinskii, S. F. Mel´nikova, S. N. Vergisov, G. M.
Frolova, Khim. Geterotsikl. Soedin., 1981, 35 [Chem. Heteroꢀ
cycl. Compd. (Engl. Transl.), 1981, 17, 27].
The authors are grateful to A. S. Shashkov for recordꢀ
ing and discussion of 2D NMR spectra.
20. A. B. Sheremetev, Usp. Khim., 1999, 68, 154 [Russ. Chem.
Rev. (Engl. Transl.), 1999, 68, 137].
21. A. B. Sheremetev, N. N. Makhova, W. Friedrichsen, Adv.
Heterocycl. Chem., Academic Press, 2001, 78, 65.
22. A. Gasco, G. Rua, E. Menziani, V. Mortarini, A. Fundaro,
Farmaco, Ed. Sci., 1971, 26, 241.
This work was partially financially supported by the
Division of Chemistry and Material Sciences of the Rusꢀ
sian Academy of Sciences (Programs OKhNMꢀ04 and
PRANꢀ7), as well as the Russian Foundation for Basic
Research (Project No. 09ꢀ03ꢀ12230).
23. Z. Novak, B. Bostai, M. Csekei, K. Lorincz, A. Kotschy,
Heterocycles, 2003, 60, 2653.
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Received May 17, 2011;
in revised form November 17, 2011