Catalytic Azide Reduction in Biological Environments

Article abstract of DOI:10.1002/cbic.201100719

In the quest for the identification of catalytic transformations to be used in chemical biology and medicinal chemistry, we identified iron(III) meso-tetraarylporphines as efficient catalysts for the reduction of aromatic azides to their amines. The reaction uses thiols as reducing agents and tolerates water, air, and other biological components. A caged fluorophore was employed to demonstrate that the reduction can be performed even in living mammalian cells. However, in vivo experiments in nematodes (Caenorhabditis elegans) and zebrafish (Danio rerio) revealed a limitation to this method: the metabolic reduction of aromatic azides.

Full text of DOI:10.1002/cbic.201100719

DOI: 10.1002/cbic.201100719
Catalytic Azide Reduction in Biological Environments
[
a]
[b]
[c]
[c]
[a]
[d]
Pijus K. Sasmal, Susana Carregal-Romero, Alice A. Han, Craig N. Streu, Zhijie Lin, Kazuhiko Namikawa,
[
e]
[d]
[b]
[a]
Samantha L. Elliott, Reinhard W. Kçster, Wolfgang J. Parak, and Eric Meggers*
In the quest for the identification of catalytic transformations
to be used in chemical biology and medicinal chemistry, we
identified iron(III) meso-tetraarylporphines as efficient catalysts
for the reduction of aromatic azides to their amines. The reac-
tion uses thiols as reducing agents and tolerates water, air, and
other biological components. A caged fluorophore was em-
ployed to demonstrate that the reduction can be performed
even in living mammalian cells. However, in vivo experiments
in nematodes (Caenorhabditis elegans) and zebrafish (Danio
rerio) revealed a limitation to this method: the metabolic re-
duction of aromatic azides.
Introduction
[5,6]
peptide backbones.
However, the use of catalytic metal
complexes in living systems is limited. Bradley and co-workers
0
The last two decades have witnessed a steadily growing inter-
est for metal complexes in medicinal chemistry and chemical
recently reported that Pd nanoparticles trapped within poly-
styrene microspheres can enter cells and mediate a variety of
[
1]
0
biology. Unique properties such as structural complexity, un-
usual reactivities, adjustable ligand-exchange kinetics, fine-
tuned redox activities, photo-reactivity, the availability of radio-
isotopes, and distinct spectroscopic signatures render metal
complexes highly attractive scaffolds for the modulation, sens-
Pd -catalyzed reactions, such as, Suzuki–Miyaura cross-cou-
[7]
II
plings, whereas our group disclosed a Ru -catalyzed cleavage
of allylcarbamates within living mammalian cells, albeit with
[8,9]
low turnover numbers.
Clearly, designing catalysts that retain their catalytic activity
in a biological environment is a highly formidable challenge
owing to the presence of millimolar concentrations of nucleo-
philic thiols that are prone to deactivate metal-containing cata-
[
2]
ing, and imaging of biological processes. Surprisingly, the im-
portant ability of metal-containing compounds to catalyze
chemical transformations has not yet been exploited thor-
oughly for applications in the molecular life sciences, but
promises to point at an exciting future, in which catalytic
rather than stoichiometric events trigger biological processes
in an amplified fashion due to turnover capability. It can be
envisioned that such catalysts could be employed to amplify
signals by turning over substrates multiple times, catalytically
[10]
lysts, especially under protic and aerobic conditions.
We
here wish to introduce a novel catalyst/substrate system that
enables high turnover numbers and might become a valuable
tool for signal amplification in chemical biology.
label or deactivate target biomolecules, and catalytically acti- Results and Discussion
vate prodrugs. For example, the group of Francis developed
Iron–porphyrin-catalyzed reduction of aromatic azides
[3]
a series of novel metal-catalyzed protein modifications,
Cowan and co-workers reported a metal-catalyzed oxidative
We decided to seek a robust catalyst for the reduction of or-
ganic azides to their amines, as azides are absent from biologi-
cal systems and, thus, fulfill the requirement of bio-orthogonal-
ity, as has been demonstrated for the well-established in vitro
and in vivo reactions involving organic azides, such as click
[
4]
inactivation of enzymes, whereas Suh et al. used cobalt(III)–
cyclen complexes as mediators for the hydrolytic cleavage of
[
a] Dr. P. K. Sasmal, Z. Lin, Prof. Dr. E. Meggers
Fachbereich Chemie, Philipps-Universitꢀt Marburg
Hans-Meerwein-Strasse, 35043 Marburg (Germany)
E-mail: meggers@chemie.uni-marburg.de
[11,12]
chemistry and the Staudinger ligation.
Furthermore, as aro-
matic amines are important functional groups in many fluoro-
phores and drugs, so the catalytic reduction of azides to pri-
mary amines opens potential applications for employing
[
b] Dr. S. Carregal-Romero, Prof. Dr. W. J. Parak
Fachbereich Physik and WZMW, Philipps-Universitꢀt Marburg
Renthof 7, 35037 Marburg (Germany)
[13]
azides as catalytically activatable caging groups. In fact, in
the course of screening a variety of metal complexes, we
discovered that the 5,10,15,20-tetraphenyl-21H,23H-porphine
(TPP)-containing complex, [Fe(TPP)]Cl, smoothly catalyzes the
reduction of aromatic azides to their respective amines in the
[
c] A. A. Han, Prof. C. N. Streu
Department of Chemistry and Biochemistry, St. Mary’s College of Maryland
St. Mary’s City, MD 20686 (USA)
[d] Dr. K. Namikawa, Prof. Dr. R. W. Kçster
Zoologisches Institut, Technische Universitꢀt Braunschweig
Spielmannstrasse 8, 38106 Braunschweig (Germany)
combined presence of aliphatic thiols, protic solvents, and
[14–17]
air.
For example, the reaction of p-chlorophenylazide (1a)
[
e] Prof. S. L. Elliott
Department of Biology, St. Mary’s College of Maryland
St. Mary’s City, MD 20686 (USA)
with five equivalents of b-mercaptoethanol as reducing agent
and 1 mol% of [Fe(TPP)Cl] in dichloromethane/methanol (95:5)
under air at 308C provided p-chloroaniline (2a) in 86% isolated
yield (Scheme 1A). This reduction works similarly well for elec-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cbic.201100719.
ChemBioChem 0000, 00, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
1
&
ÞÞ
These are not the final page numbers!

Figure 1. Comparison of different metalloporphyrins for the catalytic reduc-
tion of aromatic azides with thiols. Shown are isolated yields of 2a. TPPPy:
5,10,15,20-tetra(N-methyl-4-pyridyl)porphine, TPPCl: 5,10,15,20-tetra(o-di-
chlorophenyl)-21H,23H-porphine, TPPE: 5,10,15,20-tetraphenyl-21H,23H-por-
phine modified with ethylene glycol chains, TPPF: 5,10,15,20-tetra(penta-
fluorophenyl)-21H,23H-porphine, PEt: 2,3,7,8,12,13,17,18-octaethyl-21H,23H-
porphine, TPPOMe: 5,10,15,20-tetra(4-methoxyphenyl)-21H,23H-porphine.
Reaction conditions: azide 1a (1m), b-mercaptoethanol (5 equiv), and metal-
loporphyrin (1 mol%) were reacted in CH
2 2
Cl /MeOH (95:5) for 30 min at
308C. See the Supporting Information for more details.
Reduction under biologically relevant conditions
Scheme 1. [Fe(TPP)]Cl-catalyzed reduction of aromatic azides to amines with
thiols. A) Reduction of simple aromatic azides. B) [Fe(TPP)]Cl-catalyzed for-
mation of the anticancer drug candidate MS-275. [a] Only 0.05 mol% [Fe-
To evaluate the iron–porphyrin-catalyzed azide reduction
under more biologically relevant conditions, we synthesized
bisazide 3, which upon reduction of the two azides can be
converted to the well-known fluorophore, rhodamine 110 (4),
thus allowing a simple fluorescence readout for the determina-
(TPP)]Cl used in the reaction.
tron-poor (1b!2b, 76%) and electron-rich (1c!2c, 83%;
[8,19]
1
d!2d, 93%) aromatic azides, and even with a reduced cata-
tion of yields and turnover numbers (Scheme 2).
For exam-
lyst loading of just 0.05 mol% [Fe(TPP)Cl] (1a!2a, 76%), re-
flecting a turnover number of 1520. This catalytic azide reduc-
tion is also applicable to more complicated substrates, as re-
vealed by the smooth reduction of 1e to the anticancer drug
[
18]
candidate MS-275 (2e) (Scheme 1B).
It is worth noting that other tested metalloporphyrins of
II
II
III
II
Cu , Co , Mn , and Ru did not work as catalysts under these
biologically relevant reaction conditions as displayed in
Figure 1, whereas electron-deficient derivatives of [Fe(TPP)Cl]
at least gave inferior results. In contrast, the reduced iron(II)
porphyrin [Fe(TPP)] (1 mol%) catalyzed the conversion of 1a
to 2a under argon with the same yield (86%) compared to the
corresponding oxidized iron(III) complex [Fe(TPP)Cl]. This
Scheme 2. Conversion of rhodamine bisazide 3 to rhodamine 110 (4) by [Fe-
TPP)Cl]-catalyzed azide reduction.
(
II
strongly indicates that the Fe -containing complex is the active
ple, the incubation of 3 with 5 mol% [Fe(TPP)Cl] in dimethyl
sulfoxide/phosphate buffer (1:1, 20 mm, pH 7.25) at 378C for
1 h gave no fluorescence signal (0.2ꢀ0.1%; Table 1, entry 1),
whereas the addition of 5 mm l-cysteine (Cys) led to an in-
creased fluorescence signal and corresponding yield (13ꢀ2%;
Table 1, entry 2). These results indicate that the thiol is needed
as reducing agent for the iron-catalyzed reduction of the azide.
Consistently, the additional presence of the reducing agent,
ascorbate (10 mm), or a higher Cys concentration (15 mm)
increased the yield further to 40ꢀ3% and 77ꢀ5%, respective-
ly (Table 1, entries 3 and 4). A stronger reducing agent such as
thiophenol in the presence of ascorbate even led to an almost
complete reduction of 3 to rhodamine 110 (85ꢀ5%) with only
catalyst and reacts with the arylazide to form an intermediate
IV
[14]
Fe –nitrene complex,
which is subsequently converted to
the reduced amine by thiols. Nevertheless, we prefer the usage
of the iron(III) complex, [Fe(TPP)Cl], due to its favorable solubil-
ity properties, especially, in polar and protic solvents. Thus, it
can be concluded that the [Fe(TPP)Cl]-catalyzed conversion of
aromatic azides to their respective amines is a very efficient
and mild method that tolerates biologically relevant reaction
conditions, such as, the combination of protic solvent, thiols,
and air.
&2
&
www.chembiochem.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemBioChem 0000, 00, 1 – 5
ÝÝ
These are not the final page numbers!

Table 1. Influence of biorelevant reaction conditions on the catalytic re-
[
a]
duction of rhodamine bisazide 3 by [Fe(TPP)Cl].
[
b]
[
Fe(TPP)Cl] [mol%]
Thiol
Ascorbate [mm]
Yield [%]
1
2
3
4
5
6
7
5
5
5
5
5
5
–
–
–
–
10
10
10
0.2ꢀ0.1
13ꢀ2
Cys (5 mm)
Cys (5 mm)
Cys (15 mm)
PhSH (5 mm)
Cys (5 mm)
Cys (5 mm)
40ꢀ3
77ꢀ5
[
d]
85ꢀ5
41ꢀ2
[
c]
10
10
2.3ꢀ0.5
[
(
[
a] Bisazide 3 (0.5 mm) was treated with the indicated amounts of [Fe-
TPP)Cl] in DMSO/phosphate buffer (1:1, 20 mm, pH 7.25) at 378C for 1 h.
b] Standard deviations from three independent experiments. [c] Reaction
performed in DMEM instead of phosphate buffer. [d] In the absence of
[Fe(TPP)Cl] the yield is only 1%.
5
mol% [Fe(TPP)Cl] (Table 1, entry 5). Gratifyingly, [Fe(TPP)Cl] is
quite tolerant towards other biological components commonly
present in culture medium: entry 6 in Table 1 demonstrates
that the replacement of phosphate buffer by cell culture
medium did not reduce the yields. It is important to note that
aromatic azides are not significantly reduced in the presence
of biologically relevant concentrations of organic thiols
(Table 1, entry 7). Thus, it can be concluded that [Fe(TPP)Cl] is
a highly suitable catalyst for the reduction of aromatic azides
Figure 2. [Fe(TPP)Cl]-induced reduction of bisazide 3 to fluorescent rhoda-
mine 110 inside HeLa cells. A) Superimposed phase contrast and confocal
fluorescence images. HeLa cells were pre-incubated with bisazide 3
[
20]
in a biological environment.
(
(
100 mm) for 25 min, washed with PBS, and subsequently treated with [Fe-
TPP)Cl] (10 mm). See the Supporting Information for more details. B) Quanti-
Experiments in living mammalian cells
fied time-dependent increase of fluorescence emission within the cellular
cytosol of HeLa cells upon [Fe(TPP)Cl] addition (*) and the corresponding
time-dependent fluorescence in the absence of catalyst (*). The image anal-
ysis was performed on regions of interest (ROIs) for eight individual HeLa
cells with the software ImageJ. The fluorescence evolution was analyzed for
eight different HeLa cells. The data for the catalytic reaction were fitted to
Encouraged by these results, we next used caged fluorophore
3
to investigate the [Fe(TPP)Cl]-catalyzed reduction of azides in
living mammalian cells. Accordingly, cultured HeLa cells were
incubated with 3 (100 mm) for 25 min and washed with phos-
phate-buffered saline (PBS); subsequent addition of fresh
medium ensured that 3 is located only within the cells and not
in the culture medium. Following the addition of [Fe(TPP)Cl]
ꢁ
1
an exponential equation with the exponent being the rate constant [s ] of
the intensity increase.
(
10 mm) to the medium, cellular fluorescence was monitored
by live-cell imaging with a confocal fluorescence microscope
Figure 2A). Indeed, Figure 2B reveals that the cellular green
(
fluorescence of rhodamine 110 developed quickly with a 28-
fold increase in fluorescence over a time period of 10 min.
In vivo experiments in nematodes and zebrafish
Next, we investigated the scope of this method in vivo. Un-
fortunately, experiments with nematodes, Caenorhabditis ele-
gans, as well as the zebrafish, Danio rerio, revealed that caged
fluorophore 3 develops a fluorescence signal even in the
absence of [Fe(TPP)Cl] catalyst. For example, adult C. elegans
incubated with bisazide 3 (25 mm) for 20 min revealed strong
fluorescence intensities in intestinal cells surrounding the
lumen (Figure 3A). Similarly, the incubation of 2.5-day-old
transparent zebrafish embryos with bisazide 3 (100 mm) for
Figure 3. Activation of bisazide 3 in vivo. A) Adult C. elegans nematode in-
cubated with bisazide 3 (25 mm) in M9 buffer with 1% DMSO for 20 min,
before being pelleted, washed with M9 buffer (1 mL), concentrated to
100 mL by decanting, placed on a slide, and treated with one drop of 10%
azide, followed by fluorescence imaging. B) 2.5-day-old transparent zebrafish
embryos incubated with bisazide 3 (100 mm) for 30 min followed by fluores-
cence imaging.
rently, bisazide 3 is quickly metabolized to rhodamine 110 in
vivo without the need for [Fe(TPP)Cl]. This observation is con-
sistent with other reports on the reductive metabolism of aro-
3
0 min led to a strong fluorescence in head and trunk regions
Figure 3B). In both nematode and zebrafish, the addition of
catalyst had no significant effect on the fluorescence. Appa-
(
[21]
matic azides in vivo.
ChemBioChem 0000, 00, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chembiochem.org
&
3
&
ÞÞ
These are not the final page numbers!

cells were washed with PBS (2ꢁ1 mL) and taken-up in fresh
medium (1 mL). [Fe(TPP)Cl] (10 mL, 1 mm DMSO stock, preheated
for 5 min at 708C for complete dissolution) was added to reach
a final concentration of 10 mm, and the mixture was agitated mod-
erately for a few seconds before live-cell imaging.
Conclusion
We here reported the discovery of the efficient and mild [Fe-
(TPP)Cl]-catalyzed reduction of aromatic azides to their respec-
tive amines. This reaction uses thiols as the reducing agent
and tolerates water, air, and other biological components, thus
rendering it a rare example of a catalytic reaction that can be
performed under physiological conditions and even in living
mammalian cells. However, the apparent in vivo metabolic re-
duction of aromatic azides reveals a limitation of this method.
The application of this new catalytic reaction to cellular imag-
ing and catalytic prodrug release are underway in our labora-
tory.
Fluorescence microscopy of cellular experiments: All confocal
laser scanning microscope (CLSM) images were recorded with
a LSM 510 META confocal microscope (Zeiss, Oberkochen, Germa-
ny). Images were acquired from phase-contrast and green fluores-
cence channels (488 nm excitation wavelength and BP 505–
5
30 nm emission wavelength filter). Time series of images were
recorded in order to analyze the increase in green fluorescence
intensity in the cellular cytosol of HeLa cells. Images of the cells
before the addition of catalyst were taken as a control.
Image analysis of cellular experiments: CLSM images of the
green fluorescence channels were recorded at various times after
catalyst addition and in the absence of catalyst (control). The
images were analyzed with the software ImageJ (version 1.44p) as
recently published. Briefly, the cellular area was drawn in the
same position for eight cells in every CLSM image. Each drawn
area defines a region of interest (ROI), which was analyzed with
the program after the final image was acquired. After selecting
Experimental Section
Synthesis: Azide substrates 1-azido-4-chlorobenzene (1a), 1-azido-
[24]
4
-nitrobenzene (1b), 1-azido-4-methylbenzene (1c), and 1-azido-4-
methoxybenzene (1d), and poly(ethylene glycol)-modified iron(III)
porphyrin [Fe(TPPE)Cl] were synthesized according to literature re-
[22,23]
ports.
See the Supporting Information for the synthesis of sub-
a ROI in the first image at time t , this ROI was then transferred to
1
strate 1e and bisazide 3.
the next image at time t until t using a ROI manager, in this way
2
n
Reduction of arylazides in organic solvents: Azide 1a (60 mg,
keeping the size and position of the ROI constant for every image
of the time series. A background value was measured in an area of
the same size in the surrounding medium outside of the cellular
cytosol. The background intensity was found to be negligible. The
net intensities were used to study the time dependence of the
green fluorescence emission in the cell cytosol after catalyst addi-
tion.
0
.392 mmol), [Fe(TPP)Cl] (2.76 mg, 0.0039 mmol, 1 mol%) and b-
mercaptoethanol (137 mL, 1.96 mmol) were dissolved in CH Cl /
2
2
MeOH (95:5, 0.4 mL) and stirred for 0.5 h at 308C. Next, the reac-
tion mixture was subjected to silica gel chromatography (hexane/
EtOAc, 4:1!3:1) to provide amine 2a (43 mg, 86%). With a re-
duced catalyst loading of 0.05 mol%, the reaction was stirred at
3
08C for one week to give amine 2a (76%). See the Supporting
In vivo experiments in nematodes and zebrafish: See the Sup-
porting Information for details.
Information for additional examples with different substrates.
Reduction of rhodamine bisazide 3 under biologically relevant
conditions
[
Fe(TPP)Cl] as catalyst: Bisazide 3 (0.5 mm), [Fe(TPP)Cl] (0.025 mm), Acknowledgements
(+)-sodium l-ascorbate (10 mm), and PhSH (5 mm) were mixed
thoroughly with a micropipette in DMSO/phosphate buffer (1:1,
0 mm, pH 7.25) and shaken for 1 h at 378C and 50 rpm in a KEM-
Lab Vortex Mixer (J-KEM Scientific, St. Louis, MO, USA). [Fe(TPP)Cl]
0.025 mm) was added as a DMSO stock solution (1 mm), which
was preheated to 708C for 5 min for complete dissolution. The
yield of rhodamine 110 (85ꢀ5%) was determined by measuring
the generated green fluorescence.
We thank the Philipps-University Marburg (Germany) for financial
support. Z.L. thanks the China Scholarship Council for a stipend.
S.C.R. is grateful to the Junta Andalucia (Spain) for a fellowship.
Parts of this work were supported by the German Research Foun-
dation (FOR630, grant no. ME-1805/3-1 to E.M.; SPP1313, grant
no. PA794/4-1 to W.J.P.; grant no. KO1949/4-1 to R.W.K.).
2
(
[
Fe(TPPE)Cl] as catalyst: Bisazide
3
(0.5 mm), [Fe(TPPE)Cl]
(
0.025 mm), (+)-sodium l-ascorbate (10 mm), and PhSH (5 mm)
Keywords: azide reductions · catalysis · cellular chemistry ·
fluorescence · iron porphyrins
were mixed thoroughly with a micropipette in DMSO/phosphate
buffer (1:3, 20 mm, pH 7.25) and then shaken for 1 h at 378C and
5
0 rpm using a KEM-Lab Vortex Mixer (J-KEM Scientific). The yield
[
1] a) U. Schatzschneider, N. Metzler-Nolte, Angew. Chem. 2006, 118, 1534–
537; Angew. Chem. Int. Ed. 2006, 45, 1504–1507; b) T. W. Hambley,
of rhodamine 110 (91ꢀ4%) was determined by measuring the
generated green fluorescence. See the Supporting Information for
more details and related experiments according to Table 1.
1
Dalton Trans. 2007, 4929–4937; c) E. J. Merino, A. K. Boal, J. K. Barton,
Curr. Opin. Chem. Biol. 2008, 12, 229–237; d) A. Mukherjee, P. J. Sadler,
Wiley Encyclopedia of Chemical Biology, Wiley, Hoboken, 2009, 1–47;
e) C. G. Hartinger, P. J. Dyson, Chem. Soc. Rev. 2009, 38, 391–401; f) E.
Meggers, Chem. Commun. 2009, 1001–1010; g) E. A. Hillard, G. Jaouen,
Organometallics 2011, 30, 20–27; h) G. Gasser, I. Ott, N. Metzler-Nolte, J.
Med. Chem. 2011, 54, 3–25; i) W.-H. Ang, A. Casini, G. Sava, P. J. Dyson,
J. Organomet. Chem. 2011, 696, 989–998.
Reduction of bisazide 3 in HeLa cells: HeLa cells were cultured in
Dulbecco’s Modified Eagle Medium (DMEM, Sigma–Aldrich) supple-
mented with 10% fetal bovine serum (FBS, Biochrom), glutamine
ꢁ1
(
2 mm), and penicillin/streptomycin (100 mgmL ). One day prior to
5
experiments, approximately 1ꢁ10 cells were plated in medium
2 mL) on standard-bottom dishes (35/12/1.5 mm). Rhodamine bis-
(
[
[
2] K. L. Haas, K. J. Franz, Chem. Rev. 2009, 109, 4921–4960.
3] J. M. Antos, M. B. Francis, Curr. Opin. Chem. Biol. 2006, 10, 253–262.
azide 3 (10 mL, 20 mm DMSO stock) was added to HeLa cells, brief-
ly agitated to ensure a homogeneous final concentration (100 mm)
and incubated for 25 min at 378C. After removal of the medium,
[4] L. Hocharoen, J. A. Cowan, Chem. Eur. J. 2009, 15, 8670–8676.
[5] T. Y. Lee, J. Suh, Chem. Soc. Rev. 2009, 38, 1949–1957.
&4
&
www.chembiochem.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemBioChem 0000, 00, 1 – 5
ÝÝ
These are not the final page numbers!

[
6] For pioneering work in this direction, see also a) A. Schepartz, B. Cue-
noud, J. Am. Chem. Soc. 1990, 112, 3247–3249; b) D. Hoyer, H. Cho,
P. G. Schultz, J. Am. Chem. Soc. 1990, 112, 3249–3250.
[18] T. Suzuki, T. Ando, K. Tsuchiya, N. Fukazawa, J. Med. Chem. 1999, 42,
3001–3003.
[19] E. A. Novikova, L. F. Avramenko, T. F. Grigorenko, I. D. Laznikova, V. G.
Syromyatnikov, L. I. Khmil, Russ. J. Org. Chem. 1998, 34, 1762–1767.
[20] The better water-soluble poly(ethylene glycol)-functionalized catalyst
[Fe(TPPE)Cl] can be used with a reduced DMSO content, although
some DMSO is necessary to dissolve the rhodamine bisazide 3 sub-
strate. See the Experimental Section for more details.
[
[
[
7] R. M. Yusop, A. Unciti-Broceta, E. M. V. Johansson, R. M. Sꢂnchez-Martꢃn,
M. Bradley, Nat. Chem. 2011, 3, 239–243.
8] C. Streu, E. Meggers, Angew. Chem. 2006, 118, 5773–5776; Angew.
Chem. Int. Ed. 2006, 45, 5645–5648.
9] For recent examples of catalytically active metal complexes in medicinal
chemistry see: a) D. Salvemini, Z.-Q. Wang, J. L. Zweier, A. Samouilov, H.
Macarthur, T. P. Misko, M. G. Currie, S. Cuzzocrea, J. A. Sikorski, D. P. Riley,
Science 1999, 286, 304–306; b) S. J. Dougan, A. Habtemariam, S. E.
McHale, S. Parsons, P. J. Sadler, Proc. Natl. Acad. Sci. USA 2008, 105,
11628–11633.
[
10] It is our experience that not the individual components, but rather the
combination of thiols, protic solvent and air causes serious problems
for most metal-containing catalysts.
[
11] E. M. Sletten, C. R. Bertozzi, Angew. Chem. 2009, 121, 7108–7133;
Angew. Chem. Int. Ed. 2009, 48, 6974–6998.
12] For a recently reported photochemical Ru -catalyzed azide reduction,
II
[
see: Y. Chen, A. S. Kamlet, J. B. Steinman, D. R. Liu, Nat. Chem. 2011, 3,
146–153.
[
13] For recent applications based on the reduction of aromatic azides, see:
a) H. Abe, J. Wang, K. Furukawa, K. Oki, M. Uda, S. Tsuneda, Y. Ito, Bio-
conjugate Chem. 2008, 19, 1219–1226; b) Z. Pianowski, K. Gorska, L.
Oswald, C. A. Merten, N. Winssinger, J. Am. Chem. Soc. 2009, 131, 6492–
6
1
497; c) A. R. Lippert, E. J. New, C. J. Chang, J. Am. Chem. Soc. 2011, 133,
0078–10080; d) M. Rçthlingshçfer, K. Gorska, N. Winssinger, Org. Lett.
2012, 14, 482–485.
[
14] For the interaction and activation of organic azides by transition metal
complexes, see: a) S. Cenini, E. Gallo, A. Caselli, F. Ragaini, S. Fantauzzi,
C. Piangiolino, Coord. Chem. Rev. 2006, 250, 1234–1253; b) T. G. Driver,
Org. Biomol. Chem. 2010, 8, 3831–3846.
[21] a) F. Kamali, A. Gescher, J. A. Slack, Xenobiotica 1988, 18, 1157–1164;
b) D. Nicholls, A. Gescher, R. J. Griffin, Xenobiotica 1991, 21, 935–943.
[22] a) M. Hu, J. Li, S. Q. Yao, Org. Lett. 2008, 10, 5529–5531; b) S. W. Kwok,
J. R. Fotsing, R. J. Fraser, V. O. Rodionov, V. V. Fokin, Org. Lett. 2010, 12,
4217–4219; c) P. S. Branco, A. M. M. Antunes, M. M. Marques, M. P.
Chiarelli, A. M. Lobo, S. Prabhakar, Chem. Res. Toxicol. 1999, 12, 1223–
1233.
II
[
15] In Co –porphyrin-catalyzed nitrene transfer reactions with aromatic
azides, the reduction of the azides to their respective amines was re-
ported as a side reaction. For an example, see: F. Ragaini, A. Penoni, E.
Gallo, S. Tollari, C. Li Gotti, M. Lapadula, E. Mangioni, S. Cenini, Chem.
Eur. J. 2003, 9, 249–259.
[23] J. W. Long, I. K. Kim, R. W. Murray, J. Am. Chem. Soc. 1997, 119, 11510–
11515.
[
[
16] For an efficient reduction of organic azides with dithiols, see: a) I. L.
Cartwright, D. W. Hutchinson, V. W. Armstrong, Nucleic Acids Res. 1976,
[24] L. L. Del Mercato, A. Z. Abbasi, W. J. Parak, Small 2011, 7, 351–363.
3, 2331–2339; b) H. Bayley, J. R. Standring, J. R. Knowles, Tetrahedron
Lett. 1978, 19, 3633–3634.
17] Aliphatic azides are not reduced under these conditions as has been
tested, for example, for 2-phenethylazide.
Received: November 15, 2011
Published online on && &&, 0000
ChemBioChem 0000, 00, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chembiochem.org
&
5
&
ÞÞ
These are not the final page numbers!

COMMUNICATIONS
P. K. Sasmal, S. Carregal-Romero,
A. A. Han, C. N. Streu, Z. Lin,
K. Namikawa, S. L. Elliott, R. W. Kçster,
W. J. Parak, E. Meggers*
A tolerant catalyst: The iron meso-tet-
raarylporphin-catalyzed reduction of ar-
omatic azides with thiols is described
and analyzed. The discovered system
tolerates the presence of water, air, and
other biological components, and can
even be performed in living mammalian
cells, thus providing a novel tool for
signal amplification in molecular life
sciences.
&& – &&
Catalytic Azide Reduction in Biological
Environments
&6
&
www.chembiochem.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemBioChem 0000, 00, 1 – 5
ÝÝ
These are not the final page numbers!