The 'inverse electron-demand' Diels-Alder reaction in polymer synthesis. Part 4.1 the preparation and crystal structures of some bis(1,2,4,5-tetrazines)

Article abstract of DOI:10.1039/a607646g

Reaction of 3,6-bis(3,5-dimethylpyrazolyl)-1,2,4,5-tetrazine with mono- and di-amines gives rise to nucleophilic substitution of one or both of the pyrazolyl substituents, and reaction with diamines under appropriate conditions can lead to bis(3-amino-1,2

Full text of DOI:10.1039/a607646g

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The ‘inverse electron-demand’ Diels–Alder reaction in polymer
1
synthesis. Part 4. The preparation and crystal structures of some
bis(1,2,4,5-tetrazines)
Christopher Glidewell, Philip Lightfoot, Brodyck J. L. Royles and
David M. Smith*
School of Chemistry, University of St. Andrews, Purdie Building, St. Andrews, Fife,
UK KY16 9ST
Reaction of 3,6-bis(3,5-dimethylpyrazolyl)-1,2,4,5-tetrazine with mono- and di-amines gives rise to
nucleophilic substitution of one or both of the pyrazolyl substituents, and reaction with diamines under
appropriate conditions can lead to bis(3-amino-1,2,4,5-tetrazines), e.g. 12a, 12b and 13. The crystal
structures of two of these (12a and 13) show electronic interaction between the tetrazine rings and the
amino groups, but none between the tetrazine and pyrazole rings. In 12a there is an extensive network of
N ᎐H ؒ ؒ ؒ N hydrogen bonds.
As part of our ongoing investigation of the applicability of the
inverse electron-demand’ Diels–Alder reactions to polymer
synthesis, we have previously described methods for the syn-
typically undergo [4 ϩ 2] cycloaddition below 100 ЊC to give the
‘
corresponding pyridazines in high yield, with concomitant
elimination of molecular nitrogen. [These pyridazines in turn
are capable of a further Diels–Alder reaction to afford benzen-
oid rings, although high reaction temperatures and/or very
reactive dienophiles are required, especially in intermolecular
2
3
thesis of novel series of bis(1,2,4-triazines) and bisalkynes
which may serve as potential monomers. The use of the ‘nor-
mal’ Diels–Alder reaction as a polymerisation process has
been well described for both aliphatic and aromatic systems, the
reaction between bisalkynes and bis-maleimides, bis(tetra-
1
processes (Scheme 1).] We have already shown that some sim-
ple 1,2,4,5-tetrazines undergo Diels–Alder reactions at both
reactive sites in diethynyl-aromatic compounds and bis(enol
trimethylsilyl ethers): bis(1,2,4,5-tetrazines) are thus attractive
synthetic targets, since they offer the prospect of irreversible
Diels–Alder polymerisation reactions occurring rapidly under
mild conditions and in high yield.
cyclones) or bis-α-pyrones yielding high molecular mass poly-
4
(
aromatics). On the other hand, although ‘inverse electron-
demand’ Diels–Alder reactions of simple azaheterocycles (e.g.
pyridazines, 1,2,4-triazines and 1,2,4,5-tetrazines) are well
5
known, the use of this type of cycloaddition in polymerisation
1
has not been recorded. Our previous work has shown that
Bis(1,2,4,5-tetrazines) are virtually unknown: indeed, the
6
bis-dienophiles as reaction partners for bis(1,2,4-triazines) may
not be easily accessible.
Of the above ring systems which undergo ‘inverse electron-
demand’ Diels–Alder reactions, the 1,2,4,5-tetrazines (being the
most electron-deficient) are the most reactive. Tetrazines will
only well-authenticated example reported in the literature is
6,6Ј-diphenyl-3,3Ј-bi-1,2,4,5-tetrazinyl 2. The structure of this
compound, prepared (albeit in low yield) in a linear sequence
(Scheme 2) from oxalyl dibenzoylhydrazide 1, has been con-
O
O
O
O
Cl Cl
Cl Cl
PCl5
H2NNH2
NOx
N
Ph
Ph
Ph
Ph
Ph
HN NH HN NH
N
N
N
N
N
R
N
N
N
N
R1
1
R
R
N
<100 ˚C
N
H
H
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
R
R1
Ph
Ph
Ph
N
H
H
–
N2
R1
2
?
R2
N
N
N
N
R1
N
N
N
Ph
room temp.
2.0 equiv.
X
R
R
Ph
N
R
R
N
N
(
X = NMe2 or OEt)
N
N
3
R2
>200 ˚C
1.0 equiv.
Ph
–
N2
N
N
N
N
N
N
N
N
R1
Ph
Ph
Ph
N
N
R
R
XH
R2
Scheme 1
Scheme 2
J. Chem. Soc., Perkin Trans. 2, 1997
1167

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N
N
N
N
N
N
N
N
N
N
v
N
N
H2N
NH2
6
4
iii
i
N
N
N
N
N
N
N
N
N
N
vi
N
NH(CH2)5CH3
N
NH2
iv
8
vii
5
N
N
N
N
N
N
N
N
CH3(CH2)5NH
NH(CH2)5CH3
N
N
N
ii
1
0
9
N
N
N
N
N
N
N
N
N
N
N
N
N
N
NHNH2
N
O
O
N
N
N
N
H
N
H
1
1
7
Scheme 3 Reagents and conditions: i, NH3 (g), PhMe, 20 ЊC; ii, isophthaloyl dichloride, DMAP, PhCl, 130 ЊC; iii, hexylamine, PhMe, 20 ЊC; iv,
piperidine, PhMe, 80 ЊC; v, NH (l), sealed tube, 20 ЊC; vi, hexylamine (excess), 20 ЊC; vii, hydrazine hydrate, PhMe, 20 ЊC
3
8
firmed † by X-ray crystallography. Compound 2 exhibits typical
[
gaseous ammonia is bubbled through a suspension of com-
pound 4 in toluene at room temperature was very interesting
from our perspective, since this unsymmetrically substituted
tetrazine 5 bears functionality suitable for further elaboration
(Scheme 3).
4 ϩ 2] cycloaddition reactions at one or both of the tetrazine
rings, according to the proportions of reagents employed.
A synthetic route of this type, involving construction of both
heterocyclic moieties in a single step, is unlikely to give bis-
tetrazines in good yield, and we have envisaged the coupling of
Preliminary acylation studies, using N-benzoylation of 5 as a
model, showed that the amino group of 5 was similar in reactiv-
(
suitably functionalised) mono-tetrazines to be a more expedi-
1
2
ent route to such molecules. However, whilst a variety of
methods for the preparation of mono-tetrazines exists, there is
no general procedure which allows a broad range of substitu-
ents to be incorporated. Furthermore, the majority of these
procedures rely on a dimerisation step to form the six-
membered ring-skeleton and therefore give rise to symmetric-
ity to that of a 2-aminopyrimidine, and a reaction temper-
ature of 130 ЊC in the presence of 4-(N,N-dimethylamino)-
pyridine (DMAP: 1.0 equiv.) was necessary for benzoylation to
occur. Acylation of 5 with isophthaloyl dichloride (0.5 equiv.)
under such conditions led to formation of the corresponding
bis-tetrazine 7, although the purification of this compound
proved difficult (it appears as a single streak on TLC and its
purity is not improved by reprecipitation from a variety of solv-
ents or by chromatography), and a sample of analytical purity
was not obtained. Accordingly, this line of investigation was
not pursued further.
9
ally substituted tetrazines; whereas bis-tetrazines, by their very
nature, are unsymmetrically substituted. The synthesis of
1
,2,4,5-tetrazines where only one of the substituents is capable
of reaction (i.e. those required for efficient coupling) is gener-
1
0
ally a more difficult challenge. We now report the results of
our initial investigations in this area.
Initial experiments revealed that addition of 1 equiv. of
amines such as hexylamine (at 20 ЊC) and piperidine (at 80 ЊC)
to a suspension of compound 4 in toluene also brings about the
displacement of one of the dimethylpyrazolyl units. Further-
more, if the amine is used both as solvent and reagent, then
both of the dimethylpyrazolyl units may be replaced: elevated
temperatures are not essential for the double displacement to
Results and discussion
3
,6-Bis(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine 4, which is
simply prepared from triaminoguanidine and acetylacetone
with subsequent oxidation of the resulting dihydrotetrazine
1
1
11
intermediate, was the starting compound for these studies.
occur, as had been implied by previous workers. Indeed, dis-
1
1
The observation
that 3-amino-6-(dimethylpyrazol-1-yl)-
placement of both groups is also found when 4 is dissolved in
liquid ammonia and the mixture allowed to stand at room tem-
perature (in a sealed tube). This affords 1,2,4,5-tetrazine-3,6-
diamine 6 in excellent yield, and represents a much more
straightforward procedure than any reported to date for the
tetrazine 5 is obtained exclusively and in excellent yield when
7
†
An earlier report, claiming the synthesis of this compound via the
thermolysis of the azo(phenyltetrazine) 3, is open to question. The
product is clearly different from 2, both in physical characteristics and
chemical reactivity, and no evidence is presented in the paper even to
support the structure 3 for the precursor.
1
1,13
preparation of 6 (Scheme 3).
All attempted reactions of 4 with sulfur nucleophiles
(thiophenol, thiophenoxide and benzenesulfinate ion) resulted
1
168
J. Chem. Soc., Perkin Trans. 2, 1997

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(
CH2)n
N
N
N
N
N
N
N
N
N
N
N
NH
HN
N
1
1
2a : n = 2
2b : n = 3
i
N
N
N
N
N
N
N
N
N
N
N
N
N
N
ii
N
N
N
N
N
N
N
N
4
13
iii
N
N
N
N
H
N
H2N
N
H
NH2
1
4
Scheme 4 Reagents and conditions: i, H N(CH ) NH (0.5 equiv.), PhMe, 20 ЊC; ii, piperazine (0.5 equiv.), PhMe, 80 ЊC; iii, ethylenediamine, 20 ЊC
2
2
n
2
a
either in decomposition or in no reaction taking place. This was
somewhat disappointing, as oxidation of the sulfur atom would
have permitted the incorporation of electron-withdrawing sub-
stituents, and these might in turn have facilitated ‘inverse
electron-demand’ Diels–Alder reactions.
Extension of the above reactions with amines, involving
treatment of a suspension of compound 4 in toluene with the
appropriate primary diamine (0.5 equiv.) at room temperature,
produces the expected bis(tetrazines) 12a and 12b in good yield.
When piperazine is used it is necessary to increase the temper-
ature to 80 ЊC before the bis(tetrazine) 13 can be isolated. Reac-
tion of compound 4 with ethylenediamine (neat) at room tem-
perature, however, leads to the formation almost exclusively of
the mono-tetrazine 14.
Table 1 Selected molecular dimensions (d/Å, angle/Њ)
Compound 12a
Bond length
C1᎐N1
C2᎐N2
C2᎐N3
C1᎐N4
N7᎐C8
1.363(4)
1.337(4)
1.322(4)
1.351(4)
1.454(4)
N1᎐N2
N3᎐N4
C1᎐N7
C2᎐N5i
C8᎐C8
1.312(3)
1.326(3)
1.333(4)
1.414(4)
1.527(6)
Angle
C8᎐N7᎐C1᎐N1 Ϫ178.1(3)
N2᎐C2᎐N5᎐C5 Ϫ130.4(4)
Compound 13
Bond length
C1᎐N1
C2᎐N2
C2᎐N3
C1᎐N4
N7᎐C8i
N7᎐C9
1.359(4)
1.337(4)
1.324(4)
1.360(4)
1.456(4)
1.460(4)
N1᎐N2
N3᎐N4
C1᎐N7
C2᎐N5
C8᎐C9
1.316(3)
1.336(3)
1.335(4)
1.409(4)
1.507(6)
N
N
N
N
MeO2C
CO2Me
1
5
Angle
An alternative strategy which we have also investigated is
centred on the coupling of two units of the tetrazinedicarboxy-
C8᎐N7᎐C1᎐N1 Ϫ178.1(3)
N2᎐C2᎐N5᎐C5 Ϫ121.4(4)
N2᎐C2᎐N5᎐N6 48.8(4)
C9 ᎐N7᎐C1᎐N4 Ϫ167.5(3)
C8᎐N7᎐C1᎐N4
4.8(5)
9.7(4)
1
4
lic ester 15 either via amide linkages or by transesterification.
Reaction of compound 15 with primary amines at room tem-
perature leads to the rapid and vigorous decomposition of the
tetrazine ring system (disappearance of the red colour!) with
loss of molecular nitrogen. This is presumably the result of
nucleophilic attack by the amino group at one of the ring car-
bons in a similar way to that observed during alkaline or acidic
i
i
C9 ᎐N7᎐C1᎐N1
Compound 15
Bond length
ii
C1᎐N1
C1᎐N2
1.343(5)
1.328(5)
N1᎐N2
C1᎐C2
1.327(5)
1.513(6)
Angle
1
5,16
hydrolysis.
The products of this decomposition process are
O2᎐C2᎐C1᎐N1
O2᎐C2᎐C1᎐N2 Ϫ164.7(5)
C3᎐O1᎐C2᎐C1
13.5(7)
O1᎐C2᎐C1᎐N1 Ϫ165.2(4)
O1᎐C2᎐C1᎐N2 16.5(6)
numerous and complex, and are as yet unidentified. GC–MS
analysis of the product mixture from the reaction with hexyl-
amine, however, suggests that one product may be hexyl iso-
cyanate, thus suggesting that the desired nucleophilic attack at
the ester carbonyl may at least be one of the primary processes
involved. Efforts to achieve the transesterification of 15 using
ethylene glycol (0.5 equiv.) have been similarly unsuccessful
under all conditions tried, and have resulted only in recovery of
starting materials.
175.9(4)
a
Symmetry codes: (i) 1 Ϫ x, Ϫy, 1 Ϫ z; (ii) 1 Ϫ x, Ϫy, Ϫz.
1
7
confirmed in all respects the structure previously reported.
Given that there are so few properly authenticated examples of
bis(1,2,4,5-tetrazines), an important result of the structure
analyses for compounds 12a and 13 has been the definitive
proof of their constitutions.
Only a modest number of structure determinations have been
reported for symmetrically disubstituted 1,2,4,5-tetrazines, and
it is striking that, with only one exception containing chiral
X-Ray crystallography
The structures of the bis-tetrazines 12a and 13, as well as that
of the mono-tetrazine 15 have been determined by single crystal
X-ray diffraction: in addition, the structure of 1,2,4,5-tetrazine-
1
8
substituents,
the molecules all lie across centres of
1
7,19–24
inversion.
Similarly, in compound 15, the molecules lie
3
,6-diamine 6 has been redetermined, and this analysis has
across centres of inversion at the centres of the tetrazine rings,
J. Chem. Soc., Perkin Trans. 2, 1997
1169

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Fig. 3 Perspective view of a molecule of compound 15, showing the
atom-numbering scheme; atoms labelled with an asterisk occupy the
symmetry positions (1 Ϫ x, Ϫy, Ϫz)
Fig. 1 Perspective view of a molecule of compound 12a, showing the
atom-numbering scheme; atoms labelled with an asterisk occupy the
symmetry positions (1 Ϫ x, Ϫy, 1 Ϫ z)
Fig. 4 Perspective view of part of the crystal structure of 12a, showing
the N᎐H ؒ ؒ ؒ N hydrogen bonds: the origin is at the top left, with the
[
100] and [001] directions horizontal and vertical, respectively
behaves as a strong π-acceptor, and that substituents of types
OR and NR act as strong π-electron donors towards this ring.
2
In both 12a and 13 it is noteworthy that the tetrazine ring is
essentially coplanar with the CNRRЈ fragment of the central
spacer unit, as judged by the C᎐N᎐C᎐N torsional angles about
the C1᎐N7 bond (Table 1). In contrast, the 3,5-dimethyl-
pyrazolyl rings are twisted away from the tetrazine plane by
some 50Њ: at the same time, the bonds C2᎐N5 in both 12a and
1
1
3 are long, much longer in fact than the upper quartile value,
.382 Å, for C(aryl)᎐NR bonds. Thus there is no evidence for
2
25
any conjugation between the 6π tetrazine ring and the 6π pyra-
zolyl units, in contrast to the strong interactions involving the
aliphatic amino substituents.
In the mono-tetrazine compound 15, the molecules are
almost planar. The exocyclic C᎐C bond length is well above the
2
5
upper quartile value, 1.494 Å, for such bonds in esters of aryl
carboxylic acids, presumably reflecting the proximity of two
electron-acceptor fragments, although the C᎐O bond lengths
are all typical of those found in esters.
Stacking modes and hydrogen-bonding motifs
In each of 12a, 13 and 15, the molecular axes are aligned
roughly parallel, along a single direction: this phenomenon has
also been observed in 6,6Ј-diphenyl-3,3Ј-bi(1,2,4,5-tetrazinyl),
Fig. 2 Perspective view of a molecule of compound 13, showing the
atom-numbering scheme; atoms labelled with an asterisk occupy the
symmetry positions (1 Ϫ x, Ϫy, 1 Ϫ z)
8
where the intermolecular interactions include both π-facial
stacking and edge-to-face packing of the phenyl rings as well as
C᎐H ؒ ؒ ؒ N hydrogen bonding. In 6-phenyl-1,2,4,5-tetrazine-3-
carbaldehyde benzoylhydrazone, which contains similarly
and in the bis(tetrazines) 12a and 13 the molecules again lie
across centres of inversion, in these examples at the centres on
the diamino spacer units.
8
elongated molecules, a herringbone type of stacking is found.
Molecular dimensions and conformations‡
The molecular conformations of 12a and 13 are too remote
from coplanarity for any significant π-stacking or edge-to-face
stacking to be feasible, and in compound 13 there are no inter-
molecular distances significantly shorter than the sum of the
van der Waals’ radii. Although the molecules of 13 contain a
large number of potential hydrogen-bond acceptors, evidently
none of the C᎐H bonds is a sufficiently good donor to generate
any C᎐H ؒ ؒ ؒ N hydrogen bonds. In compound 12a, however,
there is an extensive hydrogen-bonded network, generated by
the action of the symmetry elements in propagating a single
type of hydrogen bond throughout the structure.
In each of 12a, 13 and 15 (Figs. 1–3), the C᎐N and N᎐N bond
lengths (Table 1) are consistent with extensive electronic
delocalisation: in 15 and in other symmetrically disubstituted
1
7,19–24
tetrazines,
this delocalisation is a necessary inference from
the centrosymmetry of the tetrazine ring. Similarly, the lengths
of the exocyclic bonds C1᎐N7 in both 12a and 13 are signifi-
cantly less than the lower quartile value, 1.363 Å, determined
for C(aryl)᎐NR bonds involving planar nitrogen atoms, using
2
2
5
crystallographic data available in 1987; and in compound 13 it
is noteworthy that the piperazine nitrogen atoms are essentially
planar [sum of bond angles, 359.6(4)Њ]. These observations are
Nitrogen atom N7 (Fig. 1) in the asymmetric unit of 12a at
(x, y, z) acts as hydrogen-bond donor to the pyrazolyl nitrogen
atom N6 in the unit at (1.5 Ϫ x, Ϫy, 0.5 ϩ z), in a molecule
1
7
wholly consistent with the view that the 1,2,4,5-tetrazine unit
related to the first by the action of the 2 screw axis at (0.75, 0,
z): the N7 atom in this second molecule in turn acts as
‡
In this and the following section, the atom numbering is non-
1
systematic, and is as shown in Figs. 1–3.
1
170
J. Chem. Soc., Perkin Trans. 2, 1997

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3
drotetrazine (29.30 g, 108 mmol) in dichloromethane (500 cm ).
Stirring was continued for 3 h at room temp., then the solvent
and excess nitrogen dioxide were removed by evaporation at
reduced pressure. The red solid residue was re-dissolved in
3
dichloromethane (400 cm ) and this solution was washed with
3
saturated aqueous sodium hydrogen carbonate (200 cm ) then
3
water (200 cm ) before being dried (MgSO ). Evaporation of
4
the solvent under reduced pressure gave a bright-red solid
which was recrystallised from ethyl acetate to afford pure 4
1
1
(
22.10 g, 82 mmol, 76%), mp 225–226 ЊC (lit., 226 ЊC).
3
-Amino-6-(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine, 5
This was prepared from the tetrazine 4 (10.00 g, 37 mmmol)
1
1
according to the method of Coburn et al. Yield 6.79 g (96%);
Fig. 5 Section of the crystal structure of 15, showing molecules
centred at z = 0.5: hydrogen atoms are omitted for the sake of clarity.
The origin is at the top left, with the [001] direction vertical; the direc-
tion of view is approximately along [1 Ϫ 10].
1
1
mp 217–219 ЊC (lit., 218 ЊC). δ [300 MHz, (CD ) SO at 30 ЊC]
H
3 2
2
.23 and 2.40 (each 3H, s, 2 × Me), 6.18 (1H, s, 4Ј-H), 8.19–8.28
(2H, br s, NH ); δ [75.4 MHz, (CD ) SO] 12.3 and 13.5
2
C
3 2
(
1
2 × Me), 108.5 (C-4Ј), 141.5 (C-5Ј), 150.1 (C-3Ј), 157.2 (C-6),
63.2 (C-3); m/z 191 (M , 80%), 121 (100), 106 (24) and 80
ϩ
ؒ
hydrogen-bond donor to the N6 atom in the unit at (x, y, 1 ϩ z)
(
17); λmax/nm 264 (ε 26 700), 386 (1900), 520 (640).
(
Fig. 4). The N ؒ ؒ ؒ N distance is 3.025(4) Å. Repetition of this
26,27
C(8) motif
generates a spiral of hydrogen-bonded molecules
Aroylation of compound 5
parallel to the [001] direction. At the same time, the symmetry-
related atom N7* of the initial molecule centred at (0.5, 0, 0.5)
similarly acts as hydrogen-bond donor to the atom N6 in a unit
at (Ϫ0.5 ϩ x, y, 0.5 Ϫ z), and repetition of this interaction gen-
erates a second spiral of hydrogen-bonded molecules around
With benzoyl chloride. A mixture of the aminotetrazine 5
0.764 g, 4 mmol), benzoyl chloride (0.562 g, 4 mmol) and 4-
(
(
N,N-dimethylamino)pyridine (DMAP; 0.480 g, 4 mmol) in
3
chlorobenzene (10 cm ) was heated under reflux for 5 h, then
cooled; the solvent was evaporated under reduced pressure and
the red solid residue dissolved in dichloromethane (30 cm ).
This solution was washed with dilute brine§ (2 × 30 cm ), dried
(
red solid foam (0.86 g), the spectral data of which were consist-
ent with 3-benzamido-6-(3,5-dimethylpyrazol-1yl)-1,2,4,5-tetra-
zine (yield, 73%). νmax/cm 3380 (N᎐H str.), 1700 (C᎐O), 1645
the 2 screw axis at (0.25, 0, z), of opposite hand to the first.
3
1
Units in these two spirals are, of course, connected by the cen-
tral N7᎐C8᎐C8*᎐N7* fragments of the molecules and the
overall result is the generation of a continuous two-dimensional
network of molecules, with the individual nets lying parallel to
3
MgSO ) and concentrated under reduced pressure to leave a
4
4
4
26,27
(
010) and built up from a series of R (38) rings.
Ϫ1
The molecules of compound 15 are all aligned along the long
(N᎐H bend) and 1599; δ 2.30 and 2.66 (each, 3H, s, 2 × Me),
H
axis of the unit cell, and their centres occupy the Wyckoff b
sites; thus there are almost square arrays of molecules centred
at z = 0, 0.5, 1.0 and so on. Each of these arrays exhibits a
herringbone type of stacking (Fig. 5), but with no significant
intermolecular contacts closer than the sum of van der Waals’
radii.
6
.15 (1H, s, 4Ј-H), 7.40–7.62 (3H, m, Ar-H) and 8.01–8.12 (2H,
ϩ
ؒ
m, Ar-H), 9.86–9.98 (1H, br s, NH); m/z 295 (M , 18%), 121
(77), 105 (100), 95 (12).
With isophthaloyl dichloride. A procedure similar to the
above was followed, using the tetrazine 5 (1.91 g, 10 mmol),
isophthaloyl dichloride (1.02 g, 5 mmol) and DMAP (1.22 g, 10
mmol) in chlorobenzene (25 cm ). Double volumes of dichlo-
3
Experimental
romethane and dilute brine were used during the work-up, and
the crude product was purified by dissolution in the minimum
volume of dichloromethane and reprecipitation by dropwise
addition to diethyl ether. This gave a pink powder which
according to the spectral data was predominantly 3,3Ј-
Melting points were determined on an Electrothermal 9100
apparatus and are uncorrected; IR spectra, recorded on a
Perkin-Elmer 1710 FT spectrophotometer, are those of Nujol
mulls; UV–VIS spectra, recorded on a Philips PU-8730 spec-
trophotometer, are those of solutions in acetonitrile. H and
NMR spectra are those obtained at either 300 or 200 MHz and
5.1 or 50.3 MHz, respectively, for solutions in CDCl unless
(isophthaldiamido)bis[6-(3,5-dimethylpyrazol-1-yl)-1,2,4,5-
1
13
C
tetrazine] 7; however all attempts at recrystallisation or chroma-
Ϫ1
tography failed to effect purification of this material. νmax/cm
7
3
3
2
7
385 (N᎐H str.), 1702 (C᎐O), 1648 (N᎐H bend) and 1601; δ
.11 and 2.48 (each 6H, s, 4 × Me), 5.98 (2H, s, 2 × 4Ј-H),
.45–7.56 (1H, t, J 9, 5-H), 8.14–8.28 (4H, d, J 9, 4,6-H; also br
᎐
H
indicated otherwise; chemical shifts are expressed relative to
SiMe (δ = δ = 0) and coupling constants (J) in Hz. Mass
4
H
C
spectra were obtained using a VG Autospec HR instrument,
under electron impact unless indicated otherwise. Dichloro-
methane was dried by distillation from calcium hydride, and
toluene was dried by storage over sodium wire; other com-
mercially available starting materials were used as received.
Light petroleum refers to the fraction of bp 40–60 ЊC unless
indicated otherwise.
s, 2 × NH), 9.02–9.10 (1H, br t, 2-H); δ 13.5 and 14.4 (4 × Me),
C
1
(
1
11.4 (pyrazole C-4), 127.9 (Ar CH), 129.4 (Ar CH), 133.0
2 × Ar CH), 133.2 (2 × Ar quat.), 143.7 (2 × pyrazole C-5),
53.9 (2 × pyrazole C-3), 158.4 (tetrazine C-6), 159.9 (tetrazine
ϩ
C-3) and 164.6 (C᎐O); m/z (FAB) 513 (MH , 100%) (Found:
᎐
m/z 513.1961. C H N O requires m/z 513.1972).
2
2
21 14
2
3
-(3,5-Dimethylpyrazol-1-yl)-6-(hexylamino)-1,2,4,5-tetrazine, 8
3
,6-Bis(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine, 4
Hexylamine (0.202 g, 2 mmol) was added to a vigorously stirred
slurry of the tetrazine 4 (0.54 g, 2 mmol) in dry toluene (10
cm ). The resulting suspension was stirred at room temp. for 4 h
and the solvent then removed under reduced pressure to leave a
thick, red syrup. This was dissolved in dichloromethane (30
Dihydro-3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine was
prepared from triaminoguanidine (35.15 g, 0.25 mol) and
acetylacetone (50.06 g, 0.5 mol) according to the method of
3
1
1
11
Coburn et al. Yield 29.38 g (86%), mp 150.5–152 ЊC (lit.,
50 ЊC). The crude material was used directly in the next stage.
A stock solution of nitrogen dioxide (from a lecture bottle) in
dichloromethane was prepared with a concentration of ca. 10
1
3
3
cm ) and the solution washed with water (2 × 20 cm ), dried
MgSO ) and concentrated (reduced pressure). The remaining
(
4
Ϫ3
3
mmol cm . This solution (32.5 cm , 325 mmol) was added
dropwise, over 5 min, to a rapidly stirred solution of the dihy-
§ A saturated solution diluted with an equal volume of water.
J. Chem. Soc., Perkin Trans. 2, 1997
1171

View Article Online
3
(
,5-dimethylpyrazole was separated by Kugelrohr distillation
bp 110 ЊC at 15 mmHg) to leave a red tar, the NMR and mass
spectra of which are consistent with its formulation as 8. δH
as a red powder (0.201 g, 92%), mp 133–135 ЊC. The compound
could not be obtained in analytical purity, even after repeated
recrystallisation from ethyl acetate, possibly because of con-
tamination by small amounts of polymeric material; however,
the spectroscopic properties are in accord with the proposed
structure. δ (CD OD) 2.91 and 3.53 (each 4H, t, J 6, 4 × CH );
0
1
3
.86 (3H, t, J 7, Me of C H ), 1.29–1.43 (6H, m, 3 × CH ),
6
1
3
2
.62–1.72 (2H, m, CH ), 2.32 and 2.52 (each 3H, s, 2 × Me),
2
.54–3.63 (2H, m, CH NH), 6.07 (1H, s, 4Ј-H), 6.16 (1H, bs,
2
H
3
2
NH); δ 13.2, 13.5 and 13.8 (3 × Me), 22.3, 26.3, 28.8 and 31.2
δ [(CD ) SO] 41.1 and 44.7 (4 × CH ), 160.8 (C-3 and 6); m/z
C 3 2 2
ϩ
C
ؒ
(
4 × CH ), 41.5 (NHCH ), 109.4 (C-4Ј), 141.7 (C-5Ј), 151.8 (C-
198 (M , 100%), 169 (96), 111 (12), 85 (16).
2
2
ϩ
ؒ
3
Ј), 157.4 (C-3), 161.2 (C-6); m/z 275 (M , 14%), 121 (15), 95
(100), etc.
1,2,4,5-Tetrazine-3,6-diamine, 6
The tetrazine 4 (4.00 g, 14.8 mmol) was placed inside a glass
pressure vessel with a tap closure and the system cooled to
Ϫ20 ЊC. Liquid ammonia (30 cm ) was introduced, the tap
closed, and the mixture swirled to effect complete dissolution.
The solution was left to stand at room temp. for 18 h, then
cooled once again to Ϫ20 ЊC before the tap was opened (care!).
The excess of ammonia was allowed to evaporate and the
residual crimson–red solid triturated with diethyl ether (30
3
-(3,5-Dimethylpyrazol-1-yl)-6-piperidino-1,2,4,5-tetrazine, 9
3
Piperidine (0.16 g, 1.8 mmol) was added to a stirred slurry of
the tetrazine 4 (0.50 g, 1.8 mmol) in dry toluene (15 cm ) and
the mixture was heated under reflux for 1 h then allowed to
cool. The solvent was evaporated under reduced pressure to
leave a thick red syrup which was dissolved in diethyl ether (40
3
3
3
cm ). The solution was washed with water (4 × 25 cm ), dried
MgSO ) and concentrated (reduced pressure) to give com-
3
3
(
cm ), filtered off, washed with diethyl ether (2 × 30 cm ) and
sucked dry. Sublimation (190 ЊC at 0.4 mmHg) gave pure 6
as a bright-red powder (1.98 g, 97%) which decomposes
4
pound 9 as a crimson red solid (0.36 g, 76%), mp 108–111 ЊC
(Found: C, 55.5; H, 6.6; N, 37.6. C H N requires C, 55.6; H,
12 17 7
1
5
6
.6; N, 37.8%). δH 1.62–1.80 (6H, m, 2 × β- and γ-CH2 of
above 350 ЊC but does not melt below 400 ЊC (lit., subl. 200–
240 ЊC; mp >300 ЊC). δ [(CD ) SO] 6.76 (4H, br s, 2 × NH );
piperidine), 2.33 and 2.52 (each 3H, s, 2 × Me), 3.94–4.02 (4H,
m, 2 × α-CH of piperidine), 6.09 (1H, s, 4Ј-H); δ 13.8 and 14.2
H
3
2
ϩ
2
ؒ
δ [(CD ) SO] 162.0; m/z 112 (M , 72%), 43 (100) and 42 (78).
2
C
C 3 2
(
1
6
2 × Me), 24.9 (2 × γ-CH ), 25.9 (2 × β-CH ), 45.2 (2 × α-CH ),
For confirmation of structure, see the X-ray crystallography
section above.
2
2
2
09.8 (C-4Ј), 142.2 (C-5Ј), 152.3 (C-3Ј), 158.0 (C-3), 161.6 (C-
ϩ
ؒ
); m/z 259 (M , 22%), 244 (3), 121 (7), 110 (43), 96 (100), 95
(87) and 84 (24).
N,NЈ-Bis[6-(3,5-Dimethylpyrazol-1-yl)-1,2,4,5-tetrazin-3-yl]-
ethane-1,2-diamine, 12a
3
,6-Bis(hexylamino)-1,2,4,5-tetrazine 10
The tetrazine 4 (2.70 g, 10 mmol) was added in a single portion
(slight exotherm observed) to a stirred solution of ethylenedi-
amine (0.30 g, 5 mmol) in dry toluene (50 cm ). The slurry was
The tetrazine 4 (0.300 g, 1.1 mmol) was added, in a single por-
tion, to hexylamine (3 cm ) (slight exotherm!) and stirring was
3
3
continued at room temp. for 3 h, during which time a red solid
crystallised. The slurry was diluted with toluene (10 cm ) and
stirred for 18 h at room temp., and the orange–red solid was
3
3
filtered off, washed sequentially with dry toluene (30 cm ) and
3
the red crystals filtered off, then washed sequentially with tolu-
diethyl ether (2 × 30 cm ), sucked dry, and recrystallised from
3
3
ene (3 cm ) and light petroleum (10 cm ) to leave pure 10 (0.27
g, 87%), mp 138 ЊC (Found: C, 60.0; H, 10.0; N, 29.6. C H N
acetic acid–propan-2-ol (1:1) to give pure 12a (1.98 g, 97%) as
bright-red blocks, mp 222–224 ЊC (decomp.) (Found: C, 47.35;
H, 5.0; N, 47.9. C H N requires C, 47.1; H, 4.9; N, 48.0%).
1
4
24
8
requires C, 60.0; H, 10.1; N, 29.95%). δH 0.90 (6H, t, J 8.1,
16
20 14
2
2
2
× Me), 1.26–1.45 (12H, m, 6 × CH ), 1.60–1.72 (4H, m,
λmax/nm 273 (ε 48 300), 404 (2800), 519 (1100); δ [(CD ) SO]
2
H 3 2
× CH ), 3.46 (4H, m, 2 × NHCH ), 5.06–5.15 (2H, br t,
2.25 and 2.41 (each 6H, s, 4 × Me), 3.80 (4H, s, 2 × CH ), 6.21
2
2
2
× NH); δ 13.9 (Me), 22.5, 26.3, 29.3 and 31.4 (8 × CH ), 41.7
(2H, s, 2 × H-4Ј), 8.80–9.10 (2H, br s, 2 × NH); δ 12.5 and 13.5
C
2
C
ϩ
ؒ
(NHCH ), 160.5 (C-3 and 6); m/z 280 (M , 100%), 127 (88), 85
(4 × Me), 39.4 (2 × CH ), 109.8 (2 × C-4Ј), 141.6 (2 × C-5Ј),
2
2
(29), etc.
150.3 (2 × C-3Ј), 157.1 (2 × C-6), 161.7 (2 × C-3); m/z (CI) 409
ϩ
(MH , 100%), 408 (10).
3
-Hydrazino-6-(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine 11
Hydrazine monohydrate (98%; 0.19 g, 3.7 mmol) was added to
N,NЈ-Bis[6-(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazin-3-yl]-
propane-1,3-diamine, 12b
a rapidly stirred slurry of the tetrazine 4 (1.00 g, 3.7 mmol) in
3
toluene (30 cm ). The red suspension was stirred at room temp.
This compound was prepared from the tetrazine 4 (2.47 g, 9.1
mmol) and 1,3-diaminopropane (0.34 g, 4.55 mmol) using the
above method. Recrystallisation from acetic acid–propan-2-ol
(1:1) gave pure 12b (1.18 g, 61%) as fine, red needles, mp 203–
205 ЊC (decomp.) (Found: C, 48.4; H, 5.3; N, 46.3. C H N
for 3 h (during which time it became orange–red), then filtered
3
off, washed sequentially with toluene (2 × 10 cm ) and diethyl
3
ether (2 × 15 cm ) prior to being dried in vacuo. This gave 11 as
an orange–red solid (0.66 g, 86%), mp 146–147 ЊC (from ethyl
1
7
22 14
acetate) (Found: C, 40.95; H, 4.7; N, 54.3. C H N requires C,
requires C, 48.3; H, 5.25; N, 46.4%). λmax/nm 272 (ε 48 600), 403
(2400), 520 (1100); δ 2.11–2.28 (2H, m, central CH ), 2.31 and
7
10
8
4
3
1
6
1
0.8; H, 4.9; N, 54.3%). δ 2.37 and 2.58 (each 3H, s, 2 × Me),
H
H
2
.60–4.60 (3H, br s, NHNH ), 6.13 (1H, s, 4Ј-H); δ 13.5 and
2.52 (each 6H, s, 4 × Me), 3.73–3.87 (4H, m, 2 × NHCH ), 6.08
2
C
2
3.6 (2 × Me), 110.0 (C-4Ј), 142.2 (C-5Ј), 152.5 (C-3Ј), 158.3 (C-
(2H, s, 2 × H-4Ј), 7.29–7.40 (2H, br t, 2 × NH); δ 13.4 and 13.6
C
ϩ
ϩ
), 163.0 (C-3); m/z (ESI)¶ 229 (M ϩ Na , 31%), 207 (MH ,
(4 × Me), 28.2 (central CH ), 38.8 (2 × NHCH ), 109.7 (2 × C-
2
2
00).
4Ј), 141.9 (C-5Ј), 152.2 (C-3Ј), 157.5 (2 × C-6), 161.4 (2 × C-3);
ϩ
ؒ
m/z 422 (M , 44%), 232 (19), 121 (100), 106 (29), 95 (30), etc.
3
,6-Bis-(2-aminoethylamino)-1,2,4,5-tetrazine 14
The tetrazine 4 (0.300 g, 1.1 mmol) was added, as a single por-
tion, to ethylenediamine (3.5 cm ) (slight exotherm!) and the
N,NЈ-Bis[6-(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazin-3-yl]-
piperazine, 13
3
resulting solution stirred at room temp. for 4 h. The excess of
diamine was evaporated under reduced pressure until a solid
was seen to precipitate out. Toluene (5 cm ) was added and the
A suspension of the tetrazine 4 (2.70 g, 10 mmol) and piper-
azine (0.41 g, 5 mmol) in dry toluene (50 cm ) was heated to
80 ЊC for 3 h, then allowed to cool. The orange–red product was
3
3
precipitate collected by filtration, washed with light petroleum
collected by filtration, then washed sequentially with dry tolu-
ene (20 cm ) and diethyl ether (2 × 20 cm ), sucked dry, and
3
3
3
(bp 60–80 ЊC; 10 cm ) and dried at 40 ЊC in vacuo to give 13
recrystallised from ethanol–acetic acid (2:1) giving pure 13
(
1.78 g, 82%) as fine, red needles, mp 271–273 ЊC (decomp.)
¶
This was obtained on a VG Platform using a methanol–water (9:1)
solvent system and a cone voltage of 3.85 kV.
(Found: C, 49.8; H, 5.1; N, 45.2. C18 14 requires C, 49.8; H,
H N
22
1
172
J. Chem. Soc., Perkin Trans. 2, 1997

View Article Online
Table 2 Summary of crystal data, data collection and refinement details
2a
1
13
15
(a) Crystal data
Empirical formula
C H N
C H N
C H N O
4
1
6
20 14
18 22 14
6
6
4
Molar mass
Colour, habit
Crystal size, mm
Crystal system
a/Å
b/Å
c/Å
α(Њ)
408.43
Red, plate
0.40 × 0.30 × 0.05
Orthorhombic
15.747(5)
16.912(3)
7.218(4)
90
434.47
198.14
Red, plate
0.40 × 0.35 × 0.05
Orthorhombic
21.233(6)
6.602(7)
5.837(4)
90
Red, needle
0.50 × 0.20 × 0.15
Monoclinic
7.273(3)
13.218(5)
10.793(4)
90
β(Њ)
90
90
1922(1)
Pbca
4
100.62(4)
90
1019.8(7)
90
90
818(1)
Pbca
4
γ(Њ) 3
V/Å
Space group
Z
P2 /a
1
2
F(000)
856
1.411
0.092
456
1.415
0.091
408
1.608
0.137
Ϫ3
Dcalc/g cm
Ϫ1
µ/mm
(b) Data acquisition
Unit-cell reflcns (2θ-rangeЊ)
Max. 2θ (Њ) for reflcns
hkl range of reflcns
Variation in 3 standard reflcns
Reflcns measured
20 (7.1–12.0)
50.0
0,18; 0,20; 0,8
<0.2%
1984
25 (23.4–24.7)
50.0
0,8; 0,15; Ϫ12,12
<0.2%
25 (23.3–24.9)
50.0
0,25; 0,7; Ϫ6,4
<1.0%
1037
2029
Unique reflcns
1984
1878
889
Rint
—
0.031
0.095
Reflcns with I > 3σ(I)
900
1245
506
(c) Structure solution and refinement
Solution method
Direct
137
Direct
146
Direct
65
No. of variables in LS
Abs. corr. transmission
Factors: max., min.
Sec. extinction. coeff (×10 )
R, Rw
—
0.659
0.041, 0.030
Ϫ0.14, 0.17
0.000
1.000, 0.765
1.479
0.047, 0.042
Ϫ0.28, 0.24
0.000
—
1.823
0.061, 0.050
Ϫ0.27, 0.22
0.005
6
Ϫ3
Density range in final ∆-map/e Å
Final shift/error ratio
2
8
5
2
.1; N,45.1%). λmax/nm 285 (ε 56 500), 418 (2200), 524 (1000); δH
direct methods using SIR92 and refined by full-matrix least-
29
.37 and 2.60 (each 6H, s, 4 × Me), 4.26 (8H, s, 4 × CH ), 6.13
squares on F, using the TEXSAN system. An absorption cor-
rection was applied for compound 13 using the Ψ-scan method;
no correction was necessary for either 12a or 15. All hydrogen
atoms were located from difference maps, and were included in
the refinements as riding atoms in idealised positions with iso-
tropic displacement parameters; all non-hydrogen atoms were
refined anisotropically. The figures were prepared using
2
(2H, s, 2 × H-4Ј); δC 13.7 and 13.8 (4 × Me), 43.2 (4 × CH ),
2
1
10.0 (2 × C-4Ј), 142.1 (2 × C-5Ј), 152.4 (2 × C-3Ј), 157.2
ϩ
(2 × C-6), 160.6 (2 × C-3); m/z (CI) 435 (MH , 100%), 434 (22).
Reactions of dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate 15
The diester 15 was obtained, essentially by the published pro-
cedure, as described in Part 3.
1
4
1
30
ORTEPII; selected geometric parameters are given in Table 1.
Attempted amide formation. Treatment of the diester 15 with
or 2 equiv. of hexylamine or 1 equiv. of ethylenediamine led
Refined atomic coordinates, displacement parameters and
full lists of bond lengths and angles have been deposited at the
Cambridge Crystallographic Data Centre.||
1
to rapid decomposition of the tetrazine ring, as evidenced by
vigorous evolution of gas and disappearance of the red colour.
GC–MS showed a complex product mixture (at least nine com-
pounds), one of which corresponded to hexyl isocyanate [m/z
Acknowledgements
ϩ
ؒ
1
27 (M , 3%), 112 (18), 99 (100), 84 (27), 56 (58), etc.].
We thank the EPSRC for generous financial support (to
B. J. L. R.), Mrs S. Williamson for the microanalyses and Mr
C. Millar for the mass spectra.
Attempted transesterification. Attempted reaction of the
diester 15 with ethylene glycol under a variety of reaction con-
ditions gave only unchanged starting materials.
|
| For details of the CCDC deposition scheme, see ‘Instructions for
X-Ray structure determination
Authors’, J. Chem. Soc., Perkin Trans. 2, 1997, Issue 1. Any request to
the CCDC for this material should quote the full literature citation and
the reference number 188/67.
Crystals of compounds 12a, 13 and 15 suitable for single-
crystal X-ray diffraction were grown from solutions in acetic
acid–propan-2-ol (1:1), ethanol–acetic acid (2:1), and ethyl
acetate, respectively. Table 2 summarises the details of the crys-
tal data, the data collection, and the refinements. The system-
atic absences allowed unique assignment of all the space
References
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1
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J. Chem. Soc., Perkin Trans. 2, 1997
1173

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0 S. C. Fields, M. H. Parker and W. R. Erickson, J. Org. Chem., 1994,
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Paper 6/07646G
Received 11th November 1996
Accepted 12th February 1997
1
1
174
J. Chem. Soc., Perkin Trans. 2, 1997