The synthesis of α-azidoesters and geminal triazides

Article abstract of DOI:10.1002/anie.201402433

Three simple methods for the synthesis of geminal triazides are described: Starting from 1) 3-oxocarboxylic acids, 2) iodomethyl ketones, or 3) terminal olefins, a range of triazidomethyl ketones can be constructed under mild oxidative reaction conditions by the use of IBX-SO₃K, a sulfonylated derivative of 2-iodoxybenzoic acid (IBX), and NaN₃ as an azide source. This is the first report of representatives of this novel class of triazide compounds: Despite their high nitrogen content, the geminal triazides are easy to handle, even when preparative-scale syntheses are performed. (Caution: These procedures still require protective measures!) The triazides are now broadly available for further studies regarding their properties and reactivity. Furthermore, we show how the method can be used to provide α-azidoesters, which are potential building blocks for amino acids. Either/or: Geminal triazides are rapidly constructed with broad scope by the use of oxocarboxylic acids, iodomethyl ketones, or terminal olefins as starting substrates in oxidative azidations with a mild derivative of 2-iodoxybenzoic acid and sodium azide. Along with this little-studied class of organic azides, α-azidoesters were also synthesized.

Full text of DOI:10.1002/anie.201402433

Angewandte
Chemie
DOI: 10.1002/anie.201402433
Organic Azides
The Synthesis of a-Azidoesters and Geminal Triazides**
Philipp Klahn, Hellmuth Erhardt, Andreas Kotthaus, and Stefan F. Kirsch*
Abstract: Three simple methods for the synthesis of geminal
triazides are described: Starting from 1) 3-oxocarboxylic acids,
2) iodomethyl ketones, or 3) terminal olefins, a range of
triazidomethyl ketones can be constructed under mild oxida-
tive reaction conditions by the use of IBX-SO3K, a sulfonylated
derivative of 2-iodoxybenzoic acid (IBX), and NaN3 as an
azide source. This is the first report of representatives of this
novel class of triazide compounds: Despite their high nitrogen
content, the geminal triazides are easy to handle, even when
preparative-scale syntheses are performed. (Caution: These
procedures still require protective measures!) The triazides are
now broadly available for further studies regarding their
properties and reactivity. Furthermore, we show how the
method can be used to provide a-azidoesters, which are
Scheme 1. Limitations of our previous azidation protocols (see
Ref. [7]) and goals of the current work.
potential building blocks for amino acids.
O
ver the last decade, the field of organic azides has
witnessed a tremendous “boom”,[1] owing in part to the
increased use of this class of compounds in the functionaliza-
tion of biomolecules through click chemistry,[2] but also due to
novel applications in materials science.[3] Therefore, the
question as to how organic azides can be constructed has
become a central theme of numerous studies.[1a,4–6] In this
context, our laboratory is engaged in the development of
methods for the direct azidation of enolizable carbonyl
compounds: We have recently shown that 1,3-dicarbonyl
compounds are easily functionalized with sodium azide under
oxidative conditions and in a highly chemoselective manner
(Scheme 1).[7] This in situ umpolung tolerates a great variety
of functional groups and, depending on the substitution
pattern, provides simple access to mono-azidated products 2
and di-azidated products 3. As an oxidizing agent with
a perfectly balanced oxidation power, IBX-SO3K was used,
a sulfonylated derivative of 2-iodoxybenzoic acid (IBX)[8]
developed in our group to make the azidation broadly
applicable.
ing in a safe way a-azidoesters 4, which may act as potential
precursors of a-amino acids,[9,10] and, for the first time,
preparative amounts of a-triazidocarbonyl compounds 5. The
geminal triazides 5 representing a novel class of compounds
are easily accessed starting from three different types of
substrates, namely 1) 3-oxocarboxylic acids 7, 2) iodomethyl
ketones 8, and 3) terminal olefins 9. It is now possible to use
the triazides in subsequent transformations in a controllable
way.
Due to their high nitrogen content, the development of
a broadly applicable method for the synthesis of the geminal
triazides 5 is a synthetic challenge. Besides the compounds
described herein, only a limited number of polyazidated
molecules have been reported with carbon atoms having
a similarly high degree of azide substitution.[11] For example,
Banert et al. generated the highly explosive tetraazidome-
thane C(N3)4 in 2007.[12] Hassner et al. described triazido-
methane HC(N3)3, the handling of which is also not straight-
forward.[13,14] Moreover, several polyazides from higher
homologues of Group 14 elements are known, some of
which are highly explosive, and have a quite variable degree
of azidation ranging from diazides up to hexaazides.[15] For
instance, in recent work Filippou et al. isolated and chacter-
ized Lewis base adducts of silicon tetraazide Si(N3)4 and the
anionic hexaazido compounds [Si(N3)6]2ꢀ and [Ge(N3)6]2ꢀ.[16]
Initially, we planned a decarboxylative strategy to con-
struct the desired a-azidoesters 4: Based on our previous
work, we expected that the reaction of malonic acid mono-
esters 6 with NaN3 in the presence of IBX-SO3K and
substoichiometric amounts of NaI in aqueous DMSO
would, upon decarboxylation, directly lead to the a-azido-
esters 4.[17] Indeed, the azidation of the free carboxylic acid 6
under these conditions at 608C results in the exclusive
formation of the decarboxylated product 4, and the azidated
As this method is based on the fact that the 1,3-dicarbonyl
moiety is easily enolizable, we were previously unable to
generate the azidated molecules 4 and 5 which do not contain
an additional carbonyl group adjacent to the a-azidocarbonyl
unit. Herein we report the azidation of organic carbonyl
compounds under mild oxidative reaction conditions provid-
[*] Dr. P. Klahn, H. Erhardt, Dr. A. Kotthaus, Prof. Dr. S. F. Kirsch
Organic Chemistry, Bergische Universitꢀt Wuppertal
Gaussstrasse 20, 42119 Wuppertal (Germany)
E-mail: sfkirsch@uni-wuppertal.de
[**] This project was supported by the DFG (KI 1289/2-2). We thank
RockwoodLithium for the kind donation of chemicals and A. Helfer
for the TGA/DSC measurements.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403433.
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Communications
malonic acid monoester was not isolated. Table 1 summarizes
the synthesis of a broad range of substituted a-azidoesters 4:
The experimental procedure is pretty simple, yields were
NaI used in the presence of an excess of NaN3 turned out to be
important factors (method A: IBX-SO3K (4.5 equiv), NaI
(50 mol%), NaN3, 238C, DMS/H2O). Using the combination
of IBX-SO3K/NaI as a mild oxidant was particularly crucial
for the formation of the triazide: All the alternative oxidation
and iodination reagents that we tested were unable to effect
the triple azidation. In some cases (e.g., with iodine) we only
obtained the product of the monoazidation (10), albeit in low
yields. Preliminary investigations regarding the mechanism
indicate that here, too, iodination of the enolizable oxocar-
boxylic acid 7a is the first step. Decarboxylation and
nucleophilic substitution with NaN3 then give monoazide 10
as the key intermediate. Under the reaction conditions,
incorporation of the further azides is likely to occur two
more times via iodination and substitution. As a matter of
fact, we demonstrated that both iodoacetophenone 8a and
monoazide 10 react in the presence of IBX-SO3K, NaI, and
NaN3 to give triazide 5a.
Table 1: Synthesis of a-azidoesters through decarboxylative oxidative
azidation of malonic acid monoester.[a]
Entry
Substrate
OR
R’
Yield [%][b]
1
2
3
4
5
6
7
8
9
6a
6b
6c
6d
6e
6 f
6g
6h
6i
OEt
OEt
CH2Ph
Me
iPr
sBu
93
98
83
83[c]
68
84
78
83
73
OMe
OMe
OMe
OMe
OMe
OMe
OMe
n-octyl
CH2-CH=CH2
CH2-CH=C(CH3)2
CH2-CꢁCH
CH2Ph
The direct characterization of the triazidomethyl moiety
of 5a was complicated due to the fact that analytical reference
data were not available for the class of geminal triazides, and
the determination of the accurate mass of the molecule ion
failed, too. The NMR and IR data revealed the acetophe-
none-related skeleton bearing azides attached to a quaternary
carbon atom [selected data for 5a: 13C NMR (CDCl3): d =
93.0 ppm, C(N3)3; IR (ATR): n˜ = 2110 cmꢀ1, N3; 15N NMR
(CDCl3): d = ꢀ140.0 (Nb), ꢀ147.5 (Ng), ꢀ281.9 ppm (Na)].
However, these data did not provide unequivocal support for
the triazide PhC(O)C(N3)3, since less azidated species such as
PhC(O)C(I)(N3)2 were also possible. To this end, we synthe-
sized the compounds 15N-5a and 15N-10 using 15N-labeled
Na15N3 (single terminal label). As shown in Figure 1,
a random mixture of 15N-5a and 15N-10 was then analyzed
using both 1H NMR and 15N NMR (D1 = 60 s) spectroscopy:
[a] Conditions: substrate 6 (1 equiv), IBX-SO3K (1.5 equiv), NaI
(20 mol%), NaN3, 608C, DMSO/H2O (2:1). [b] Yield of isolated product
after purification by column chromatography. [c] d.r.=1:1.
consistently high, and numerous types of functional groups
were tolerated. More detailed investigations regarding the
mechanism revealed that, with the example of 6a, oxidative
iodination takes place first at the enolizable position of the
malonic acid monoester. Spontaneous decarboxylization
followed by nucleophilic substitution of the iodide with the
azide anion then leads to the formation of the product (see the
Supporting Information (SI)). In contrast to malonic acid
monoesters, which are selectively converted into the mono-
azidated products 4, simple carbonyl systems (e.g., esters and
ketones) lacking the additional activating carbonyl group at
the 3-position fail to react under the conditions.
We then investigated the decarboxylative monoazidation
of 3-oxocarboxylic acids 7 with the example of 7a (Scheme 2).
We found that, aside from acetophenone and the desired
monoazidation product 10, the unexpected geminal triazide
5a is formed in significant amounts. After optimization of the
reaction conditions, we were able to increase the yield of
triazide 5a to 70%. Both the temperature and the amount of
1
The integration of the H NMR spectrum shows a ratio of
0.36:1 with regard to the scaffolds of 15N-5a and 15N-10; the
integration of the nitrogen resonances shows, upon deconvo-
lution and averaging over the two nitrogen atoms Ng and Na,
a ratio of 1.07:1 with regard to the azides.[18] These ratios
demonstrate that 5a is the threefold azidated compound
PhC(O)C(N3)3, which has a nitrogen content three times
higher than that of the monoazide 10. Nevertheless, triazide
5a and all the other triazides 5 in the present study were
converted into the corresponding tristriazoles by use of
cyclooctyne. The tristriazole derivatives were unambiguously
characterized by NMR and IR techniques as well as by mass
spectroscopy (see SI).[19]
3-Oxocarboxylic acids 7 proved to be cumbersome to
prepare and were prone to spontaneous decarboxylization to
the unreactive methylketones. Instead, we drew inspiration
from the conversion 8a!5a as depicted in Scheme 2, and the
easily accessible a-iodomethyl ketones 8 turned out to be the
ideal starting materials for the synthesis of geminal triazides 5.
Table 2 lists several a-iodomethyl ketones that were success-
fully polyazidated in the presence of IBX-SO3K and NaN3 in
aqueous DMSO at room temperature (method B). Addition
of NaI proved unnecessary in every instance, since the
substrates themselves served as the iodide source. While
iodine-containing alkylketones showed low reactivity under
Scheme 2. Formation of geminal triazides through decarboxylative
polyazidation.
2
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and IBX-SO3K were added to the reaction mixture and
finally the desired triazides 5 f–l were isolated. Of note,
due to its lower oxidative power, IBX-SO3K cannnot be
used to replace IBX in the initial formation of the
iodomethyl ketones. On the other hand, the use of IBX
instead of IBX-SO3K as the oxidizing agent in the poly-
zidation of the a-iodomethyl ketone with sodium azide
results in a strongly exothermic reaction that is uncon-
trollable, and ends in the complete decomposition of the
starting material and the evolution of gas. This observa-
tion underlines the importance of using IBX-SO3K for
the oxidative azidation step.
The class of geminal triazides 5 features an excep-
tionally high nitrogen content located at the a-carbonyl
carbon, thus making this class of compounds attractive
for potential applications as energetic materials.[21] To our
surprise, triazide 5a withstands temperatures up to 608C
in solution; the compound remains stable without sig-
nificant (or even explosive) decomposition.[22] We were
able to concentrate dilute solutions with the rotary
evaporator at temperatures between 30 and 358C without
problems. The triazides 5 were obtained as pure sub-
stances, small amounts of which were stored at ꢀ208C.
Nevertheless, extreme caution should be used in handling
the geminal triazides: Due to the inherent risk of
explosion, all experiments were performed with full
protective measures; for each operation, temperatures
were kept below 358C; the scale was kept small (0.2–
0.6 mmol).[23]
Figure 1. NMR study to determine the degree of azidation of 5a. A) 1H NMR
spectrum of a mixture of 15N-5a and 15N-10 with 15N-labeled azide groups
(single terminal label). B) 15N NMR spectrum of the same mixture.
Table 2: Synthesis of geminal triazides.
Furthermore, diazides were smoothly accessed from the
corresponding a-azidoketones through method C, from the a-
iodoketones through method B, and from the olefins through
method D.[24] Scheme 3 illustrates these transformations using
the example of diazide 11. These methods for the synthesis of
diazides bearing only one neighboring carbonyl group are
complementary to our earlier diazidation method which
affords diazides with two adjacent carbonyl groups from 1,3-
dicarbonyl compounds.[7]
Entry
Product
R
Yield [%][a] (B/D)[b,c]
1
2
3
4
5
6
7
8
5a
5b
5c
5d
5e
5 f
5g
5h
5i
Ph
77 (B)/43 (D)
30 (B)
61 (B)
59 (B)
58 (B)
37 (D)
37 (D)
47 (D)
47 (D)
48 (D)
48 (D)
32 (D)
2,4-(MeO)2C6H3
4-MeC6H4
4-PhC6H4
4-ClC6H4
2-MeOC6H4
3-MeOC6H4
2-ClC6H4
9
4-FC6H4
10
11
12
5j
5k
5l
2-naphthyl
5-methyl-2-furyl
2-pentyl
[a] Yield of isolated product after purification by column chromatography
[b] Method B: IBX-SO3K (3 equiv), NaN3, 238C, DMSO/H2O (2:1).
[c] Method D: 1) IBX (2 equiv), NIS (1.1 equiv), 358C, DMSO, 2) H2O,
3) IBX-SO3K (3 equiv), NaN3, 238C.
Scheme 3. Formation of geminal diazides through oxidative azidation.
the reaction conditions, iodinated arylketones 5b–e were
readily converted in good yields.
In conclusion, we have presented novel oxidative methods
for the introduction of azide groups to organic molecules. In
all cases, sodium azide was used as a cheap and readily
available azide source, while IBX-SO3K in combination with
iodides was the perfectly balanced oxidizing agent. We were
able to construct a-azidoesters starting from malonic acid
monoesters. More intriguingly, we described a broadly appli-
cable entry into geminal triazides, a neglected class of
Next we developed an alternative one-pot protocol that
provides the triazides 5 directly from terminal olefins 9
(Table 2, method D). To this end, the olefins were first
transformed into the iodomethyl ketones using IBX and N-
iodosuccinimide (NIS) as previously described by Moorthy
et al.[20] After addition of water followed by filtration, NaN3
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compounds, starting from either 3-oxocarboxylic acids, a-
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these nitrogen-rich molecules.
Received: February 14, 2014
Published online: && &&, &&&&
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Keywords: azides · energy-rich compounds ·
hypervalent iodine compounds · nitrogen · oxidation
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[22] In preliminary studies, TGA-DSC measurements (heating rate:
58Cminꢀ1) for 5a showed a thermal decomposition process,
which begins at Ton = 1148C (TP = 1408C) and liberates a signifi-
cant heat of decomposition (DHD = ꢀ1.4 kJgꢀ1). When droplets
[6] For selected examples of the azidation of 1,3-dicarbonyls, see:
a) Q. H. Deng, T. Bleith, H. Wadepohl, L. H. Gade, J. Am.
Chem. Soc. 2013, 135, 5356; b) M. V. Vita, J. Waser, Org. Lett.
2013, 15, 3246; c) M. J. Galligan, R. Akula, H. Ibrahim, Org.
Lett. 2014, 16, 600.
[7] T. Harschneck, S. Hummel, S. F. Kirsch, P. Klahn, Chem. Eur. J.
2012, 18, 1187.
4
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Angewandte
Chemie
of 5a (< 0.5 mg) are applied onto a hot heating plate (> 808C),
an immediate detonation takes place, accompanied by a flare
and a bang.
Chem. Soc. 1967, 89, 3931; f) R. H. McGirk, C. R. Cyr, W. D.
Ellis, E. H. White, J. Org. Chem. 1974, 39, 3851; g) S. I. Al-Khalil,
W. R. Bowman, M. C. R. Symons, J. Chem. Soc. Perkin Trans.
1 1986, 555; h) W. Ogilvie, W. Rank, Can. J. Chem. 1987, 65, 166;
i) K. Nishiyama, M. Oba, A. Watanabe, Tetrahedron 1987, 43,
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dron Lett. 2012, 53, 4195.
[23] We synthesized triazide 5a on a larger scale (9 mmol) only once.
Due to safety concerns we concentrated the dilute solution in
small portions (approximately 10 ꢅ 0.9 mmol).
[24] For alternative entries into geminal diazides, see: a) G. Schro-
eter, Chem. Ber. 1909, 42, 2336; b) M. O. Forster, R. Mꢁller, J.
Chem. Soc. Trans. 1910, 97, 1360; c) S. Gçtzky, Chem. Ber. 1931,
64, 1555; d) H. Bretschneider, N. Karpitschka, Monatsh. Chem.
1953, 84, 1091; e) L. Barash, E. Wasserman, W. A. Yager, J. Am.
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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.
Angewandte
Communications
Communications
Organic Azides
Either/or: Geminal triazides are rapidly
constructed with broad scope by the use
of oxocarboxylic acids, iodomethyl
P. Klahn, H. Erhardt, A. Kotthaus,
S. F. Kirsch*
&&&& — &&&&
ketones, or terminal olefins as starting
substrates in oxidative azidations with
a mild derivative of 2-iodoxybenzoic acid
and sodium azide. Along with this little-
studied class of organic azides, a-azido-
esters were also synthesized.
The Synthesis of a-Azidoesters and
Geminal Triazides
6
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These are not the final page numbers!