Stable organic azides based on rigid tetrahedral methane and adamantane structures as high energetic materials

Article abstract of DOI:10.1039/b713792c

A four-folded azidation of tetrakis(4-iodophenyl)methane and -adamantane leads to stable organic azides, but yet energetic materials, measured by differential scanning calorimetry (DSC). The rigid and symmetrical structures can be useful for new polymer and nanomaterial developments in material sciences as well as bioconjugations, after 1,3-dipolar cycloaddition reactions with terminal alkynes to 1,2,3-triazoles. The Royal Society of Chemistry.

Full text of DOI:10.1039/b713792c

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Stable organic azides based on rigid tetrahedral methane and adamantane
structures as high energetic materials†
Christine I. Schilling and Stefan Bra¨se*
Received 7th September 2007, Accepted 20th September 2007
First published as an Advance Article on the web 5th October 2007
DOI: 10.1039/b713792c
A four-folded azidation of tetrakis(4-iodophenyl)methane
and -adamantane leads to stable organic azides, but yet ener-
getic materials, measured by differential scanning calorimetry
(DSC). The rigid and symmetrical structures can be useful
for new polymer and nanomaterial developments in material
sciences as well as bioconjugations, after 1,3-dipolar cycload-
dition reactions with terminal alkynes to 1,2,3-triazoles.
was obtained in two steps from tritylchloride.16,17 Its analogue,
1,3,5,7-tetrakis(4-iodophenyl)adamantane 2,18,19 was synthesized
from 1,3,5,7-tetraphenyladamantane in one step.20
The aryl azides 3d, 4a–d were prepared via Ullmann-type
coupling reactions,21 using CuI as catalyst22 to activate the aryl
halides for the nucleophilic substitution. The reaction rate is
influenced by factors like ligands, for example L-proline or
diamines, and solvent systems.8
The first attempts to obtain 1,3,5,7-tetrakis(4-azidophenyl)-
adamantane 4d using L-proline as catalyst gave lower substituted
homologues 4a–c as side products (Scheme 1, entries 1–3) in
DMSO–H2O (5 : 1) as solvent. Different ligands and solvents
were tested. Optimized conditions delivered tetraazide 4d with
yields up to 34%. N,Nꢀ-Dimethylethylenediamine (1.2 equiv.) as
ligand, NaN3 (16 equiv.), sodium ascorbate (0.4 equiv.) and CuI
(0.8 equiv.) in DMSO–H2O (10 : 1) were added to a solution
of tetraiodoaryl 2 in DMSO. The similarity of the molecular
structures, the thermal resistance of the adamantane core and the
well known insolubility of high symmetrical compounds made the
purification and characterization of all derivatives very difficult.
All samples were purified by flash chromatography (cyclohexane–
CH2Cl2, 5 : 1) and characterized by 1H and 13C NMR analysis.
The research of highly energetic materials, especially polyazides,
has attracted the attention of chemists in the last few years.
Inorganic azides of the type M(N3)n were investigated1,2 as well
as numerous organic compounds.3 Banert et al.4 reported a re-
markable example of explosive polyazides, the tetraazidomethane.
A nitrogen-rich compound as an intermediate stage in a one-pot
synthesis, based on the tetraphenylmethane structure, was recently
reported by Chen et al.5
Nowadays, aryl azides are increasingly used in organic
synthesis,6 due to the versatile transformations of the azide
functional group. Anilines and nitrenes7 can be generated and
azides are useful for the preparation of different heterocycles.8,9
Polyazido organic compounds have high relative heats of forma-
tion measurable by differential scanning calorimetry. One azido
group typically adds about 364 kJ mol−1 of endothermicity to the
organic frame.2,10
Furthermore, the significant Huisgen-variant of the 1,3-dipolar
cycloaddition reaction, a metal catalyzed azide/alkyne ‘click’
reaction,11,12 leads to 1,4- or 1,5-substituted heterocycles.
Tetrafunctionalized molecules with a rigid structure, like the
newly developed symmetrical polyazides are promising com-
pounds for polymerization processes, material and macromolecu-
lar sciences,13 as well as for the investigation of novel nanomate-
rials. The 1,4-disubstituted 1,2,3-triazoles, easily accessible from
polyazides, found applications prevalently in drug discovery and
bioconjugations.7
However, structures like the tetrasubstituted polyazides of tetra-
phenylmethane 3d and its analogue, the 1,3,5,7-tetraphenyl-
adamantane 4d, have not yet been synthesized.‡ Herein, we
report stable aryl azides that, unlike alkyl azides,2,14 combine
high energetic properties with non-explosive ones under ambient
conditions (25 ◦C, 1 atm).
The synthesis of the polyazides is based on iodoarylated
compounds in both cases: the tetrakis(4-iodophenyl)-methane 115
Universita¨t Karlsruhe (TH), Institut fu¨r Organische Chemie, Fritz-Haber-
Weg 6, 76131 Karlsruhe, Germany. E-mail: braese@ioc.uka.de; Fax: (+)49
(0)721 608 8581; Tel: (+)49 (0)721 608 2902
† Electronic supplementary information (ESI) available: Experimental
procedures and detailed characterizations of all structures. See DOI:
10.1039/b713792c
Scheme 1 Ullmann-type coupling with iodoaryl 1 and 2.
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We found that tetrasubstituted compounds 3d and 4d can best
be prepared using a solvent ratio of DMSO–H2O 5 : 1 and 20 : 1
respectively (Scheme 1). All other tested solvents like EtOH,
THF, t-BuOH and other common solvent mixtures suffered from
solubility problems.
Thus, tetraiodoaryl 1 yielded 1,3,5,7-tetrakis(4-azidophenyl)-
methane 3d under the conditions optimized for the preparation of
4d (100 ◦C in DMSO–H2O 20 : 1). Polyazide 3d was isolated after
24 h by flash chromatography (cyclohexane–CH2Cl2, 5 : 1) in 41%
yield (Scheme 1, entry 6).
Furthermore, all isolated azides containing the adamantane
core 4a–d as well as the methane tetraazide 3d were analyzed
by differential scanning calorimetry and thermal gravimetric
measurements were performed.
Obviously, the tetrasubstituted azides 3d (Fig. 1, Table 1)
and 4d showed the highest exothermicity. A thermal energy of
700 kJ mol−1 (1445 kJ kg−1) for polyazide 3d and 741 kJ mol−1
(1225 kJ kg−1) for compound 4d were measured while heating.
Tetrakis(4-azidophenyl)methane 3d gave a higher reported heat
than polyazide 4d. The reason may be due to a lower molecular
weight and consequently a lower C–N ratio in the core structure.
Despite its high energetic potential, we never had problems in
isolating this product,6 compared to an analogue, the unstable
azidobenzene (344 kJ mol−1 in liquid phase).23 The decomposition
of this azide was measured by Waddell et al. and led to molecular
explosions upon irradiation in diluted solutions of azidobenzene.24
The samples of the polyazides were heated (5.0 K min−1) in an
aluminium pan under a N2-atmosphere. Here, an empty pan acted
as a reference.
Thermal gravimetric measurements were performed as further
evidence for the molecular structure. Analysis of compound 4d
showed a stepwise mass loss of 19%, 20% and 11% relative to
its molecular mass of 604.3 g mol−1 caused by the fragmentation
of the molecule. Derivative 4a could also be identified by mass
spectrometry (FAB-MS) probably due to the relatively small
exothermic effect during thermal decomposition caused by its
sole azide functional group. The high thermal energy of all other
compounds leads to a complete fragmentation of the sample
during ionization. Thus, characterization by mass spectrometry
was unsuccessful
Furthermore, a Cu(I)-catalyzed 1,3-dipolar cycloaddition re-
action, named as ‘click’ reaction, was performed with com-
pound 4d and prop-2-yn-1-ol (12 equiv.), as
a terminal
alkyne, to form 1,3,5,7-tetrakis(4-(4-hydroxymethyl-1,2,3-triazol-
1-yl)phenyl)adamantane 5d‡ in up to 11% yield (Scheme 2) after
purification by flash chromatography (CH2Cl2–MeOH, 5 : 1).7,10
Other conditions, like higher temperatures (100 ◦C) and longer
reaction times (48 h), were required for a successful addition
despite the mild conditions reported in previous investigations.7,8
Fig. 1 DSC analysis of tetrakis(4-azidophenyl)methane 3d.
Depending on the molecule structure, different N2-eliminations
were detected, visible in several TDecomp., Tmax and DH (Table 1). The
data for all polyazides are listed in Table 1. Increasing the number
of azide functions in a molecule, e.g. mono- 4a, di- 4b, tri- 4c and
tetrasubstituted 4d aryl azide derivatives of the adamantane core,
goes along with an enhancement of the exothermicity (Table 1).
Hence, the expected drift for the exothermic effect could be
shown.
Table 1 Data of DSC analysis for all polyazides
a
b
TDecomp.
Tmax
R DH
Azide
◦C
◦C
kJ mol−1
3d
4a
4b
175.9
181.8
166.3
311.6
167.9
344.0
174.1
195.6
204.7
198.4
331.4
199.2
355.1
193.9
700
73
430
4c
4d
435
741
a Thermal decomposition starts. b Temperature with a maximum energy.
c Endothermicity was detected more often during the measurement,
depending on the structure.
Scheme 2 ‘Click’ reaction with polyazide 4d.
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Attempts using less equivalents of alkyne for the cycloaddition
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1
The compounds were characterized by H and 13C NMR after
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Conclusions
In summary, we have successfully synthesized polyazides, based
on adamantane and methane cores. Despite the high energetic
properties of tetrasubstituted azides (for safety issues, please
see ref. 5), measured by differential scanning calorimetry, all
synthesized azides were stable and could be handled without
special precautions.
Further ‘click’ reactions with tetrasubstituted azides led to rigid
tetrahedral 1,2,3-triazoles with a variety of applicable functional
groups for bioconjugations or material sciences.
We thank Dr Stefan Lo¨bbecke, Fraunhofer Institute for Chemi-
cal Technology ICT, Karlsruhe, for the DSC and thermogravimet-
ric analysis. We acknowledge the DFG center for functionalized
nanostructure (CFN, C5.1) for funding.
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Notes and references
‡ Some elected spectral data of 3d, 4d, 5d. For experimental procedures
and detailed characterizations of all structures see ESI. 1,3,5,7-Tetrakis(4-
azidophenyl)methane (3d): 1H NMR (400 MHz, CDCl3): d = 6.9 (dd, 3J =
4
3
4
8.8 Hz, J = 4.9 Hz, 8 H, Aro–H), 7.13 (dd, J = 8.8 Hz, J = 4.9 Hz,
8 H, Arm–H) ppm. 1,3,5,7-Tetrakis(4-azidophenyl)adamantane (4d): 1H
NMR (400 MHz, CDCl3): d = 2.10 (s, 12 H, Ad-CH2), 7.02 (d, 3J = 8.64,
8 H, Aro–H), 7.45 (d, 3J = 8.64, 8 H, Arm–H) ppm. 1,3,5,7-Tetrakis(4-
(4-hydroxymethyl-1,2,3-triazol-1-yl)phenyl)adamantane (5d): 1H NMR
(400 MHz, CDCl3): d = 2.15 (s, 12 H, Ad–CH2), 4.61 (s, 6 H, –CH2OH),
7.53 (d, 3J = 8.8 Hz, 8 H, Aro–H), 7.60 (d, 3J = −8.7 Hz, 8 H, Arm–H),
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=
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The Royal Society of Chemistry 2007
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