Beyond click-chemistry: Transformation of azides with cyclopentadienyl ruthenium complexes

Article abstract of DOI:10.1021/om200295c

The cyclopentadienyl Ru complexes Cp*RuCl(cod) (cod = 1,5-cyclooctadiene), Cp*RuCl(PPh₃)₂, and [CpRuCl₂]₂ (Cp = η⁵-1-methoxy-2,4-di-tert- butyl-3-neopentylcyclopentadienyl) are able to catalyze the decomposition of benzyl azides to give 1,3,5-triphenyl-2,4-diazapenta-1,4-diene ("hydrobenzamide"), benzyl-benzylideneamine, and benzonitrile. Reactions with the catalyst precursor [CpRuCl₂]₂ are particularly fast and give hydrobenzamide with high selectivity. A similar coupling reaction is observed for other benzylic azides but not for (2-azidoethyl)benzene and ethyl-4-azidobutanoate. If the reactions are performed in the presence of water, benzylic azides are converted into aldehydes. Mononuclear tetrazene complexes are formed in stoichiometric reactions of [CpRuCl₂]₂ with benzyl azide and (2-azidoethyl)benzene.

Full text of DOI:10.1021/om200295c

ARTICLE
pubs.acs.org/Organometallics
Beyond Click-Chemistry: Transformation of Azides with
Cyclopentadienyl Ruthenium Complexes
Julie Risse, Rosario Scopelliti, and Kay Severin*
Institut des Sciences et Ingꢀenierie Chimiques, Ecole Polytechnique Fꢀedꢀerale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
S Supporting Information
b
ABSTRACT: The cyclopentadienyl Ru complexes Cp*RuCl-
(cod) (cod = 1,5-cyclooctadiene), Cp*RuCl(PPh3)2, and
[Cp∧RuCl2]2 (Cp∧ = η5-1-methoxy-2,4-di-tert-butyl-3-neo-
pentylcyclopentadienyl) are able to catalyze the decomposi-
tion of benzyl azides to give 1,3,5-triphenyl-2,4-diazapenta-
1,4-diene (“hydrobenzamide”), benzyl-benzylideneamine,
and benzonitrile. Reactions with the catalyst precursor
[Cp∧RuCl2]2 are particularly fast and give hydrobenzamide
with high selectivity. A similar coupling reaction is observed
for other benzylic azides but not for (2-azidoethyl)benzene
and ethyl-4-azidobutanoate. If the reactions are performed in
the presence of water, benzylic azides are converted into
aldehydes. Mononuclear tetrazene complexes are formed in
stoichiometric reactions of [Cp∧RuCl2]2 with benzyl azide and (2-azidoethyl)benzene.
’ INTRODUCTION
numbers (TONs) of more than 50 after 12 h for reactions
performed in benzene at 50 °C. Intrigued by this process, we
analyzed the reaction products by NMR and GC-MS. It was found
that a mixture of three products was formed: 1,3,5-triphenyl-2,4-
diazapenta-1,4-diene (“hydrobenzamide”, A), benzyl-benzylide-
neamine (B), and benzonitrile (C) (Scheme 1).
Ruthenium-catalyzed azideÀalkyne cycloaddition (RuAAC)
reactions were reported in 2005 by Jia and Fokin.1 This
catalytic process allows the synthesis of 1,5-disubstituted
triazoles from azides and alkynes. Due to its unique regio-
selectivity, RuAAC complements the very popular copper-
catalyzed azideÀalkyne cycloaddition (CuAAC), which gives
rise to 1,4-disubstituted triazoles.2 Meanwhile, the scope and
limitations of RuAAC reactions have been investigated in
some detail,3 variants have been developed,4 and numerous
applications have been reported.5 The most popular catalyst
precursors for RuAAC are Cp*RuCl(cod) (Cp* = penta-
methylcyclopentadienyl; cod = 1,5-cyclooctadiene) and
Cp*RuCl(PPh3)2. These complexes are a source of the
[Cp*RuCl] fragment, which is believed to be the catalytically
active species.3b
It has been reported that Cp*RuCl(cod) reacts with two
equivalents of (2-azidoethyl)benzene to give a highly stable
and catalytically inactive tetraazadiene complex.3b This finding
led to the recommendation that the “azide should not be added
to the catalyst before the alkyne”.3b We observed that cyclopen-
tadienyl Ru complexes are also able to convert azides in a catalytic
fashion. Details about this new catalytic transformation are
reported below.
Next, we examined whether other Ru complexes are able to
catalyze this transformation as well. The RuII complex Cp*RuCl-
(PPh3)2 gave similar results to those observed for Cp*RuCl-
(cod): after 14 h, the majority of benzyl azide had reacted to give
a mixture of A, B, and C with no special selectivity (Table 1,
entries 1 and 2). The RuIII complex [Cp*RuCl2]2, on the other
hand, gave only a low conversion (entry 3). We then investigated
the catalytic activity of [Cp∧RuCl2]2 (Cp∧ = η5-1-methoxy-2,4-
di-tert-butyl-3-neopentylcyclopentadienyl). This complex is ea-
sily accessible by reaction of RuCl3(solv)n with tert-butyl
acetylene.6 Its Cp∧ (“Cp-roof”) ligand is sterically more demand-
ing than the classic Cp* ligand, which imparts a unique reactivity
to Cp∧Ru complexes.7 When subjected to the standard reaction
conditions (2 mol % Ru, benzene, 50 °C), we observed a
dramatically enhanced activity and selectivity: complete conver-
sion of benzyl azide was observed after only 2 h, with the
main product being 1,3,5-triphenyl-2,4-diazapenta-1,4-diene (A)
(entry 4). Commonly used Ru complexes without a cyclo-
pentadienyl ligand or the CuI complex CuBr(IMes) (active in
CuAAC)8 gave no measurable conversion of benzyl azide
(entries 5À9).
’ RESULTS AND DISCUSSION
During investigations in the context of RuAAC we observed
that Cp*RuCl(cod) led to a conversion of benzyl azide, even if no
alkyne was present. The process was clearly catalytic with turnover
Received: April 6, 2011
Published: May 25, 2011
r
2011 American Chemical Society
3412
dx.doi.org/10.1021/om200295c Organometallics 2011, 30, 3412–3418
|

Organometallics
ARTICLE
A transition metal-catalyzed conversion of benzyl azide into
1,3,5-triphenyl-2,4-diazapenta-1,4-diene (A) is—to the best of our
knowledge—unprecedented. A noncatalytic version of this reac-
tion is known, but the conditions are very harsh (flash vacuum
pyrolysis at >400 °C).9 1,3,5-Triaryl-2,4-diazapenta-1,4-dienes are
interesting
synthetic
intermediates
because
they
can be converted into 1,3-diazabuta-1,3-dienes,10 R-amino-
phosphonates,11 dibenzylamines,12 or amarines.13 The latter are
valuable precursors for the synthesis of 1,2-diamino-1,2-
diarylethanes13,14 and imidazoles.9 Traditionally, 1,3,5-triphenyl-
2,4-diazapenta-1,4-diene (A) is prepared by reaction of benzalde-
hyde with liquid ammonia.10b,15 Our Ru-catalyzed method starting
from benzyl azide thus represents an interesting alternative.
The scope and the limitations of the new reaction were
examined in a series of experiments. The [Cp∧RuCl2]2-
catalyzed conversion of benzyl azide into hydrobenzamide A
proceeds well in aprotic organic solvents such as tetrahydro-
furan, dioxane, diethyl ether, acetonitrile, hexane, benzene,
and toluene. Likely side products are dinitrogen and
ammonia.9 The latter was detected indirectly with a pH paper.
In methanol, the reaction was less selective and a significant
amount of benzyl-benzylideneamine (B) was observed by
NMR spectroscopy. It is worth noting that benzyl-benzylide-
neamine can be synthesized in good yields from benzyl azide
with the catalyst tetrathiomolybdenate.16
Scheme 1. Ru-Catalyzed Conversion of Benzyl Azide
Table 1. Catalytic Activity of Different Ru and Cu Complexes for the Conversion of Benzyl Azidea
entry
catalyst
time [h]
conv [%]
yield A [%]
yield B [%]
yield C [%]
1
2
3
4
5
6
7
8
9
Cp*RuCl(cod)
14
14
14
2
78
86
16
100
0
30
24
3
32
39
7
10
11
4
Cp*RuCl(PPh3)2
[Cp*RuCl2]2
[Cp∧RuCl2]2
89
9
3
3
RuHCl(PPh3)3(CO)
(p-cymene)RuCl2(PPh3)
[(p-cymene)RuCl2]2
RuCl2(PPh3)3
12
12
12
12
12
9
9
0
9
9
9
0
9
9
9
0
9
9
9
CuBr(IMes)
0
9
9
9
a Reaction conditions: [BnN3] = 200 mM, [Ru] or [Cu] = 4.0 mM (2 mol %), C6H6, 50 °C. Mesitylene was used as internal standard. Yields of A and B
were calculated from the NMR spectra. Yields of C were determined by GC-MS.
Table 2. Ru-Catalyzed Synthesis of 1,3,5-Triaryl-2,4-diazapenta-1,4-dienesa
a Reaction conditions: [substrate] = 200 mM; [Ru] = 4.0 mM (2 mol %). NMR yields were determined using mesitylene as internal standard.
3413
dx.doi.org/10.1021/om200295c |Organometallics 2011, 30, 3412–3418

Organometallics
ARTICLE
Scheme 2. Synthesis of the Tetrazene Complexes 1 and 2
Table 3. Selected Bond Distances (Å) for Complexes 1 and 2
1
2
Ru1ÀN1
Ru1ÀN4
Ru1ÀCl1
N1ÀN2
N2ÀN3
N3ÀN4
1.998(4)
1.953(4)
2.3852(12)
1.304(6)
1.314(6)
1.328(5)
1.993(3)
1.947(3)
2.3892(9)
1.313(4)
1.325(4)
1.338(4)
1,3,5-Triphenyl-2,4-diazapenta-1,4-diene was formed from benzyl
azide in a crude yield of 97% after 9 h using 2 mol %
Cp∧RuCl(nbd) (THF, 50 °C). The yield was significantly
lower when the reaction was carried out in dioxane at 80 °C.
These results point to a possible advantage of the RuII
complex Cp∧RuCl(nbd) compared to the RuIII complex
[Cp∧RuCl2]2. However, it has to be taken into account that
the synthesis of the catalyst precursor Cp∧RuCl(nbd) is more
complicated. Therefore, reactions with [Cp∧RuCl2]2 seem
more appealing from a synthetic point of view.
We have also attempted reactions with (2-azidoethyl)benzene
and ethyl-4-azidobutanoate as substrates. Very low conversions
(<5%) were observed after 24 h, indicating that the catalytic
conversion is limited to benzyl azides.
In order to obtain information about possible intermediates,
stoichiometric reactions between [Cp∧RuCl2]2 and benzyl azide
or (2-azidoethyl)benzene were carried out (Scheme 2). In both
cases, mononuclear RuII complexes with tetrazene chelate
ligands were obtained (1 and 2). Plausible reducing agents for
these transformations are the azides. Both complexes are green,
air-stable compounds. Their structures were evidenced by NMR
spectroscopy, elemental analyses, and single-crystal X-ray ana-
lyses (Figure 1).
Overall, the structures of 1 and 2 are similar. Both com-
pounds are mononuclear half-sandwich complexes with a Cp∧
ligand, a chloro ligand, and a tetrazene ligand. The bond
lengths of the tetrazene ligand are of special interest. When
coordinated to Ru, tetrazene ligands can exist as neutral
tetraazadienes or as dianionic ligands.17 An example of the
former is found for the complex RuCl2(PMe3)2[(mes)-
NNNN(mes)] (mes = mesityl), which features short N1À
N2 and N3ÀN4 bonds (1.307(4) and 1.313(4) Å) and a
longer N2ÀN3 bond (1.338(4) Å).18 The tetrazene ligand in
(p-cymene)Ru[(2,4,6-t-Bu3C6H2)NNNN(mes)], on the other
hand, should be described as a dianionic ligand with longer
N1ÀN2 and N3ÀN4 bonds (1.365(4) and 1.358(3) Å) and a
short N2ÀN3 bond (1.288(4) Å).19
Figure 1. Graphical representation of the molecular structure of com-
plexes 1 (top) and 2 (bottom) in the crystal. Thermal ellipsoids are at the
50% probability level. Hydrogen atoms are not shown for clarity.
The substrate scope was investigated using different azides.
Preliminary tests had shown that the reactions are slightly faster in
ethers. Therefore, THF and dioxane were used as solvents. All
benzylic azides that we have tested were converted with good
selectivity into the corresponding 1,3,5-triaryl-2,4-diazapenta-1,4-
dienes using 1 mol % of complex [Cp∧RuCl2]2 (2 mol % Ru) as
catalyst precursor (Table 2). The crude yields range between 77%
and 85%; the isolated yields are somewhat lower due to loss of
productduringpurification(selective precipitation). The reactions
were performed at slightly elevated temperature (50 or 80 °C), but
they also proceed at room temperature. Benzyl azide, for example,
was completely converted within 6 h at room temperature in THF
(2 mol % Ru). We have also examined the catalytic activity of
Cp∧RuCl(nbd) (nbd = norbornadiene),6a which can be regarded
as a “Cp-roof” analogue of the commonly used Cp*RuCl(cod).
For complexes 1 and 2, the situation is different: one can
observe an increase in the bond lengths from N1ÀN2 (1.304(6)
and 1.313(4) Å) to N2ÀN3 (1.314(6) and 1.325(4) Å) and
finally N3ÀN4 (1.328(5) and 1.338(4) Å) (Table 3). It is
conceivable that this distortion is induced by the likewise highly
asymmetric Cp∧ ligand.
3414
dx.doi.org/10.1021/om200295c |Organometallics 2011, 30, 3412–3418

Organometallics
ARTICLE
Scheme 3. Reaction of 1 with Benzyl Azide
Table 4. Ru-Catalyzed Conversion of Benzylic Azides into
Aldehydesa
a Reaction conditions: [azide] = 100 mM; [Ru] = 5.0 mM (5 mol %),
[H2O] = 4.0 M. The yields were determined by GC-MS using
mesitylene as internal standard.
The isolation of complex 3 is evidence that the Cp∧Ru
fragment is able to mediate the transformation of benzyl azide
into phenylmethanimine. Phenylmethanimine is a direct pre-
cursor for hydrobenzamide A, a reaction that takes place even
without catalyst.10b A key role of the catalyst therefore seems to
be the azide to imine conversion. However, further studies
would be needed for a more detailed proposal about the
catalytic cycle.
Figure 2. Graphical representation of the molecular structure of com-
plex 3 in the crystal. Thermal ellipsoids are at the 50% probability level.
Hydrogen atoms are not shown for clarity. Selected bond lengths (Å)
and angles (deg): Ru1ÀN1 2.106(3), N1ÀC20 1.242(4); C20ÀN1À
Ru1 144.5(3), N1ÀRu1ÀCl1 79.78(8), N1ÀRu1ÀCl1# 82.04(7).
The proposition of arylmethanimines as key intermediates
suggested that it might be possible to convert benzyl azides into
benzaldehydes if the reaction is performed in the presence of
water. This turned out to be the case. Best results were obtained
when the catalyst precursor [Cp∧RuCl2]2 was first dissolved in a
mixture of water and acetonitrile (1:1) under heating. A yellow
catalyst stock solution was obtained, which was used for sub-
sequent reactions. Using 2.5 mol % [Cp∧RuCl2]2 (5 mol % Ru)
we were able to convert different benzylic azides into the
corresponding aldehydes with yields between 73% and 86%
(Table 4). A similar reaction has been described for the MoIV
complex MoO2(Et2NCS2)2, but more forcing conditions are
required.22 The hydrolytic decomposition of benzyl azide was
also tested with [Cp*RuCl2]2 as catalyst precursor, but only a low
conversion of the substrate was observed after 24 h. This finding
corroborates the importance of the sterically very demanding
Cp∧ ligand for these transformations.
The formation of aldehydes can be explained by hydrolysis of
the intermediate imines. Alternatively, one could imagine hydro-
lysis of 1,3,5-triaryl-2,4-diazapenta-1,4-dienes, which are formed
in a Ru-catalyzed process. Indeed, the hydrolysis of hydrobenza-
mide derivatives has been known for a long time.23 To test
whether hydrolysis of hydrobenzamide is occurring under our
reaction conditions, a solution of hydrobenzamide was stirred in
a mixture of water and acetonitrile (1:20) at 75 °C. After 4 h, 80%
Tetrazene complexes are likely intermediates during the trans-
formation of the catalyst precursor [Cp∧RuCl2]2 into the cataly-
tically active species. This assumption is supported by the fact that
the color of the reaction mixture changes from brown to green
after addition of the azide. However, the green color does not
persist and quickly changes to red. We have therefore examined
the reaction of complex 1 with additional benzyl azide (Scheme 3).
As observed for the catalytic reaction, the color of the reaction
mixture changed from green to red. An NMR spectroscopic
investigation of the mixture indicated the formation of several
complexes. Attempts to separate one of these products on a
preparative scale failed. However, we were able to isolate small
amounts of crystals from the mixture.20 A crystallographic analysis
revealed the formation of the dinuclear imine complex 3
(Figure 2). It is interesting to note that addition of (2-azi-
doethyl)benzene to a solution of complex 2 did not result in a
reaction (dioxane, 80 °C). This finding is in line with the inability
of [Cp∧RuCl2]2 to catalyze the conversion of nonbenzylic azides.
The metal-induced transformation of benzyl azide into a
metal-imine complex has already been observed in a few cases.21
It is assumed that these complexes are formed via benzyl azide
complexes, which liberate dinitrogen to give nitrene complexes.
The latter tautomerize to give the imine complexes.21a A similar
reaction pathway may be operational in our case, but a direct
involvement of the tetrazene ligand cannot be excluded.
3415
dx.doi.org/10.1021/om200295c |Organometallics 2011, 30, 3412–3418

Organometallics
ARTICLE
of the initial compound was hydrolyzed. These experiments
suggest that imine and hydrobenzamide hydrolysis are both
plausible reaction pathways.
32.21 (C1, C2, C3), 33.90, 34.73 (C4, C5), 37.60 (C7), 57.77 (C9),
62.84 (C8), 71.78 (NÀCH2), 94.27, 102.79, 107.37 (C10, C11, C12),
127.27, 127.49, 128.15, 128.23, 128.34, 130.06, 136.30, 137.29 (CÀAr),
C6 and C13 not observed. Anal. Calcd (%) for RuClN4OC33H47: C,
60.76; H, 7.26; N, 8.58. Found: C, 60.74; H, 7.36; N, 8.54. Single crystals
were obtained from a solution of cold toluene/pentane.
’ CONCLUSION
We have shown that cyclopentadienyl Ru complexes are able
to catalyze the decomposition of benzylic azides. The reaction is
particularly fast and selective if [Cp∧RuCl2]2, a Ru complex with
a sterically very demanding cyclopentadienyl ligand, is used as
catalyst precursor. The decomposition of benzylic azides should
be considered as a potential side reaction in RuAAC, in particular
if alkynes with a low intrinsic reactivity are employed. From a
synthetic point of view it is interesting that reactions with
[Cp∧RuCl2]2 provide 1,3,5-triaryl-2,4-diazapenta-1,4-dienes
with high selectivity. To the best of our knowledge, this is the
first report of a transition metal-catalyzed reaction of this kind.
The process may be of interest for synthetic organic chemistry,
because it offers an alternative, base-free route to an important
class of compounds.
Cp∧RuCl(PhCH2CH2NNNNCH2CH2Ph) (2). (2-Azidoethyl)benzene
(60 mg, 0.40 mmol) was added to a solution of [Cp∧RuCl2]2 (80 mg,
0.17 mmol) in THF (3 mL). The solution was stirred at room
temperature for 6 h. After removal of the solvent, the residue was
purified by flash chromatography on silica gel, using CH2Cl2 as eluant.
1
The product was isolated as a green powder (87 mg, 72%). H NMR
(400 MHz, CDCl3): δ 1.02 (s, 3H, t-Bu), 1.16 (s, 3H, t-Bu), 1.50 (s, 3H,
2
t-Bu), 1.88 (m, 1H, CH2ÀCp∧), 3.11 (d, JHH = 15 Hz, 1H,
CH2ÀCp∧), 3.39À3.52 (m, 2H, PhÀCH2), 3.57À3.64 (m, 1H,
PhÀCH2), 3.73 (s, 3H, OCH3), 3.77À3.88 (m, 1H, PhÀCH2),
4.62À4.69 (m, 2H, NÀCH2, HÀCp∧), 4.76À4.86 (m, 2H, NÀCH2),
5.05 (m, br, 1H, NÀCH2), 7.25À7.29 (m, 2H, Ar), 7.33À7À39 (m, 8H,
Ar). 13C NMR (400 MHz, CDCl3): δ 31.87, 32.28, 32.35 (C1, C2, C3),
34.00, 34.79 (C4, C5), 35.63 (PhÀCH2), 36.86 (PhÀCH2) 37.01 (C7),
57.70 (C9), 63.08 (C8) 69.95 (NÀCH2), 70.78 (NÀCH2), 93.70,
102.09, 106.29 (C10, C11, C12), 126.37, 126.41, 128.61, 129.06, 129.13,
139.06, 139.77 (CÀAr), C6 and C13 not observed. Anal. Calcd (%) for
RuClN4OC35H51: C, 61.78; H, 7.55; N 8.23. Found: C, 62.03; H, 7.44;
N, 8.01. Single crystals were obtained from a MeOH/CH2Cl2 solution
by slow evaporation.
’ EXPERIMENTAL SECTION
General Procedures. All experiments were performed inside a
glovebox under an atmosphere of dinitrogen. Thoroughly dried and
deoxygenated solvents were used. The complexes [Cp∧RuCl2]2,6 CuBr-
(IMes),8 RuHCl(PPh3)3CO,24 Cp*RuCl(PPh3)2,3b [Cp*RuCl2]2,25
Cp*RuCl(cod),26 [(p-cymene)RuCl2]2,27 (p-cymene)RuCl2(PPh3),27
and RuCl2(PPh3)328 were prepared according to literature procedures.
Benzyl azide was purchased from Alfa Aesar. The other azides were
synthesized in analogy to a reported procedure from the corresponding
benzylic bromides.29 2-(Bromomethyl)naphthalene, methyl 4-(bromo-
methyl)benzoate, and 4-(tert-butyl)benzyl bromide were purchased
from Aldrich, and 3,5-dimethylbenzyl bromide was from Acros.
1H and 13C spectra were recorded on a Bruker Advance DPX 400
spectrometer using the residual protonated solvents (1H, 13C) as
internal standards. All spectra were recorded at room temperature.
Column chromatography was performed using silica gel 60 (230À400
mesh, 0.04À0.063 nm) from Fluka. The GC measurements were
performed with a GC/MS using a Varian 3800 spectrometer coupled
to a Varian 2200 mass spectrometer. Elemental analyses were performed
with an EA 1110 CHN instrument.
Crystallographic Analyses. The data collections for the three
crystal structures (1, 2, and 3) were measured at low temperature using
Mo KR radiation. An Oxford Diffraction Sapphire/KM4 CCD was
employed for 1, while the remaining samples were measured on a Bruker
APEX II CCD. Both diffractometers have a kappa geometry goniometer.
Data reduction were carried out by Crysalis PRO30 (1) and EvalCCD31
(2, 3) and then corrected for absorption.32 The solutions and refine-
ments were performed by SHELX.33 The structures were refined using
full-matrix least-squares based on F2 with all non hydrogen atoms
anisotropically defined. Hydrogen atoms were placed in calculated
positions by means of the “riding” model. Twinning problems were
discovered in the case of 1. The twinning by reticular merohedry was
analyzed by the TWINROTMAT routine of PLATON.34 A HKLF5 file
was then generated and used in the refinement of the structure,
obtaining final BASF parameters of 0.0032(7), 0.029(7), 0.097(10),
and 0.0034(8).
Synthesis and Characterization of Complexes 1 and 2.
General Procedure for the Synthesis of 1,3,5-Triaryl-2,4-
diazapenta-1,4-dienes. The catalyst precursor [Cp∧RuCl2]2
([Ru]final = 2 mol %) was added to a solution of the respective benzylic
azide in dioxane or THF ([azide]final = 200 mM). The mixture was
stirred at the given temperature (Table 2). After the given time
(Table 2), the solvent was removed. The product was isolated as
described below.
1,3,5-Triphenyl-2,4-diazapenta-1,4-diene. The reaction was per-
formed with 193 mg (1.36 mmol) of benzyl azide in THF. After
removal of the solvent, the residue was dissolved in EtOH (6 mL), and
the resulting solution was dropped into water (20 mL). After a few
minutes of stirring, a white suspension formed. The product was
isolated by filtration, washed with water (10 mL), and dried in vacuo to
give a white solid (95 mg, 70%). The spectroscopic data for the product
correspond to those previously described.9 1H NMR (400 MHz,
CDCl3): δ 6.01 (s, 1H, CH), 7.29À7.33 (m, 1H, Ar), 7.37À7.47 (m,
8H, Ar), 7.53À7.55 (m, 2H, Ar), 7.87À7.90 (m, 4H, Ar), 8.61 (s, 2H,
NCH). 13C NMR (400 MHz, CDCl3): δ 92.69 (CH), 127.28, 127.86,
128.57, 131.07, 136.03, 141.76 (CÀAr), 160.72 (NCH). Anal. Calcd
(%) for C21H18N2: C, 84.53; H, 6.08; N, 9.39. Found: C, 84.87; H,
5.97; N, 9.36.
Cp∧RuCl(PhCH2NNNNCH2Ph) (1). Benzyl azide (54 mg, 0.38 mmol)
was added to a solution of [Cp∧RuCl2]2 (86 mg, 0.19 mmol) in toluene
(3 mL). The solution was stirred at room temperature for 6 h. After
removal of the solvent, the residue was purified by flash chromatography
on silica gel, using CH2Cl2 as eluant. The product was isolated as a green
1
powder (65 mg, 52%). H NMR (400 MHz, CDCl3): δ 1.08 (s, 3H,
t-Bu), 1.13 (s, 3H, t-Bu), 1.45 (s, 3H, t-Bu), 1.95 (d, br, 2JHH = 15 Hz, 1H,
2
CH2ÀCp∧), 3.11 (d, JHH = 16 Hz, 1H, CH2ÀCp∧), 3.81 (s, 3H,
OCH3), 4.81 (s, 1H, CH), 5.51 (d, 2JHH = 16 Hz, 1H, NÀCH2), 5.74 (d,
2JHH = 9 Hz, 1H, NÀCH2), 5.78 (d, 2JHH = 10 Hz, 1H, NÀCH2), 6.09
2
(d, JHH = 14 Hz, 1H, NÀCH2), 7.25À7.36 (m, 8H, Ar), 7.52À
7.54 (m, 2H, Ar). 13C NMR (400 MHz, CDCl3): δ 32.04, 32.21,
3416
dx.doi.org/10.1021/om200295c |Organometallics 2011, 30, 3412–3418

Organometallics
ARTICLE
1,3,5-Tri(4-tert-butylphenyl)-2,4-diazapenta-1,4-diene. The reac-
tion was performed with 145 mg (766 μmol) of 4-tert-butylbenzyl azide
in dioxane. After removal of the solvent, the residue was dissolved in
EtOH (9 mL). Brine (2 mL) was then added dropwise to precipitate the
product followed by H2O (1 mL). The product was isolated by filtration,
washed with water (12 mL), and dried in vacuo to give an off-white solid
(72 mg, 61%). 1H NMR (400 MHz, CDCl3): δ 1.31 (s, 9 H, t-Bu), 1.35
(s, 18H, t-Bu), 5.97 (s, 1H, CH), 7.37À7.39 (m, 2H, Ar), 7.43À7.47 (m,
6H, Ar), 7.80À7.82 (m, 4H, Ar), 8.57 (s, 2H, NCH). 13C NMR (400
MHz, CDCl3): δ 31.22, 31.35, 34.49, 34.93 (t-Bu), 92.61 (CH), 125.39,
125.49, 126.81, 128.49, 133.50, 139.10, 150.48, 154.37 (CÀAr), 160.20
(NCH). Anal. Calcd (%) for C33H42N2: C 84.93, H 9.07, N 6.00. Found:
C 84.64, H 9.37, N 5.96.
’ ASSOCIATED CONTENT
S
Supporting Information. X-ray analysis data in cif for-
b
mat. This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: kay.severin@epfl.ch.
’ ACKNOWLEDGMENT
1,3,5-Tri(3,5-dimethylphenyl)-2,4-diazapenta-1,4-diene. The reac-
tion was performed with 188 mg (980 μmol) of 3,5-dimethylbenzyl
azide in dioxane. After removal of the solvent, the residue was suspended
in EtOH (9 mL). Brine (3 mL) was then added dropwise to precipitate
the product followed by H2O (4 mL). The product was isolated by
filtration, washed with water (12 mL) and i-PrOH (6 mL), and dried in
vacuo to give a white solid (77 mg, 62%). 1H NMR (400 MHz, CDCl3):
δ 2.33 (s, 6H, CH3), 2.37 (s, 12H, CH3), 5.88 (s, 1H, CH), 6.95 (s, 1H,
Ar), 7.10 (s, 2H, Ar), 7.13 (s, 2H, Ar), 7.51 (s, 4H, Ar), 8.52 (s, 2H,
NCH). 13C NMR (400 MHz, CDCl3): δ 21.15, 21.41 (CH3), 93.17
(CH), 125.03, 126.56, 129.47, 132.69, 136.06, 138.03, 138.11, 141.70
(CÀAr), 160.82 (NCH). Anal. Calcd (%) for C27H30N2: C, 84.77; H,
7.90; N, 7.32. Found: C, 84.64; H, 7.94; N, 7.42.
1,3,5-Tri(4-methylbenzoate)-2,4-diazapenta-1,4-diene. The reac-
tion was performed with 177 mg (922 μmol) of methyl(4-azido-
methyl)benzoate in THF. After removal of the solvent, the residue
was dissolved in EtOH (9 mL). Brine (5 mL) was then added dropwise
to precipitate the product followed by H2O (15 mL). The product was
isolated by filtration, washed with water (9 mL) and i-PrOH (4 mL), and
dried in vacuo to give an off-white solid (90 mg, 62%). 1H NMR (400
MHz, CDCl3): δ 3.92 (s, 3H, CH3), 3.95 (s, 6H, CH3), 6.09 (s, 1H,
CH), 7.61 (d, 2H, J = 8.28 Hz, Ar), 7.94 (d, 4H, J = 8.40 Hz, Ar), 8.08 (d,
2H, J = 8.36 Hz, Ar), 8.12 (d, 4H, J = 8.32 Hz, Ar), 8.64 (s, 2H, NCH).
13C NMR (400 MHz, CDCl3): δ 52.15, 52.33 (CH3), 91.84 (CH),
127.31, 128.62, 129.90, 130.04, 132.44, 139.47, 145.85 (CÀAr), 160.51
(NCH), 166.53, 166.80 (CO). Anal. Calcd (%) for C27H24N2O6: C,
68.63; H, 5.12; N, 5.93. Found: C, 68.88; H, 5.28; N, 5.99.
1,3,5-Trinaphthyl-2,4-diazapenta-1,4-diene. The reaction was per-
formed with 152 mg (830 μmol) of 2-azidoethylnaphthalene in dioxane.
After removal of the solvent, the residue was suspended in EtOH
(12 mL). Water (10 mL) was then added dropwise to precipitate the
product. The product was isolated by filtration, washed with water
(10 mL) and EtOH (6 mL), and dried in vacuo to give a light brown solid
(82 mg, 66%). The product has already been prepared, but no
characterization was reported in the literature.12 1H NMR (400 MHz,
CDCl3): δ 6.30 (s, 1H, CH), 7.48À7.58 (m, 6H, Ar), 7.74À7.76 (m,
1H, Ar), 7.84À7.94 (m, 9H, Ar), 8.07 (s, 1H, Ar), 8.20À8.23 (m, 4H,
Ar), 8.86 (s, 2H, NCH). 13C NMR (400 MHz, CDCl3): δ 92.85 (CH),
124.26, 125.50, 126.00, 126.07, 126.15, 126.49, 127.34, 127.69, 127.90,
128.22, 128.44, 128.46, 128.73, 130.87, 133.08, 133.18, 133.46, 133.74,
134.95, 139.27 (CÀAr), 161.09 (NCH). Anal. Calcd (%) for C33H24N2:
C, 88.36; H, 5.39; N, 6.25. Found: C, 88.06; H, 5.42; N, 6.49.
General Procedure for the Catalytic Conversion of Azides
into Aldehydes. A stock solution of the catalyst was prepared by
dissolving [Cp∧RuCl2]2 (20 mg, 45 μmol) in a mixture of acetonitrile
(500 μL) and water (500 μL) at 75 °C for 3 h ([Ru] = 45 mM). An aliquot
of the catalyst stock solution (100 μL) was added to a solution of the
respective benzylic azide in a mixture of acetonitrile and water (0.8 mL,
50:1) (final conc: [azide] = 100 mM; [Ru] = 5 mmol, [H2O] = 4.0 M). The
reaction was stirred at 75 °C. After 5À8 h (Table 4), the yield of the
aldehyde was determined by GC-MS using mesitylene as internal standard.
This work was supported by the Swiss National Science
Foundation and by the EPFL. We thank Marjorie Sonnay and
Laurent Joye for their help with synthetic work.
’ REFERENCES
(1) Zhang, L.; Chen, X.; Xue, P.; Sun, H. H. Y.; Williams, I. D.;
Sharpless, K. B.; Fokin, V. V.; Jia, G. J. Am. Chem. Soc. 2005, 127,
15998–15999.
(2) Meldal, M.; Tornøe, C. W. Chem. Rev. 2008, 108, 2952–3015.
(3) (a) Hou, D.-R.; Kuan, T.-C.; Li, Y.-K.; Lee, R.; Huang, K.-W.
Tetrahedron 2010, 66, 9415–9420. (b) Boren, B. C.; Narayan, S.;
Rasmussen, L. K.; Zhang, L.; Zhao, H.; Lin, Z.; Jia, G.; Fokin, V. V.
J. Am. Chem. Soc. 2008, 130, 8923–8930. (c) Rasmussen, L. K.; Boren,
B. C.; Fokin, V. V. Org. Lett. 2007, 9, 5337–5339. (d) Majireck, M. M.;
Weinreb, S. M. J. Org. Chem. 2006, 71, 8680–8683.
(4) Johansson, J. R.; Lincoln, P.; Nordꢀen, B.; Kann, N. J. Org. Chem.
2011, 76, 2355–2359.
(5) For selected examples see: (a) Montagu, A.; Roy, V.; Balzarini, J.;
Snoeck, R.; Andrei, G.; Agrofoglio, L. A. Eur. J. Med. Chem. 2011,
46, 778–786. (b) Tietze, D.; Tischler, M.; Voigt, S.; Imhof, D.;
Ohlenschl€ager, O.; G€orlach, M.; Buntkowsky, G. Chem.—Eur. J. 2010,
16, 7572–7578. (c) Moumnꢀe, R.; Larue, V.; Seijo, B.; Lecourt, T.;
Micouin, L.; Tisnꢀe, C. Org. Biomol. Chem. 2010, 8, 1154–1159.
(d) Takasu, K.; Azuma, T.; Takemoto, Y. Tetrahedron Lett. 2010,
51, 2737–2740. (e) Van Poecke, S.; Negri, A.; Gago, F.; Van Daele, I.;
Solaroli, N.; Karlsson, A.; Balzarini, J.; Van Calenbergh, S. J. Med. Chem.
2010, 53, 2902–2912. (f) Kee, J.-M.; Villani, B.; Carpenter, L. R.; Muir,
T. W. J. Am. Chem. Soc. 2010, 132, 14327–14329. (g) Odlo, K.; Fournier-
Dit-Chabert, J.; Ducki, S.; Gani, O. A. B. S. M.; Sylte, I.; Hansen, T. V.
Bioorg. Med. Chem. 2010, 18, 6874–6885. (h) Chemama, M.; Fonvielle,
M.; Arthur, M.; Valꢀery, J.-M.; Etheve-Quelquejeu, M. Chem.—Eur.
J. 2009, 15, 1929–1938. (i) Nulwala, H.; Takizawa, K.; Odukale, A.;
Khan, A.; Thibault, R. J.; Taft, B. R.; Lipshutz, B. H.; Hawker, C. J.
Macromolecules 2009, 42, 6068–6074. (j) Oppilliart, S.; Mousseau, G.;
Zhang, L.; Jia, G.; Thuꢀery, P.; Rousseau, B.; Cintrat, J.-C. Tetrahedron
2007, 63, 8094–8098. (k) Tam, A.; Arnold, U.; Soellner, M. B.; Raines,
R. T. J. Am. Chem. Soc. 2007, 129, 12670–12671.
(6) (a) Dutta, B.; Solari, E.; Gauthier, S.; Scopelliti, R.; Severin, K.
Organometallics 2007, 26, 4791–4799. (b) Gauthier, S.; Solari, E.; Dutta,
B.; Scopelliti, R.; Severin, K. Chem. Commun. 2007, 1837–1839.
(7) (a) Dutta, B.; Curchod, B. F. E.; Campomanes, P.; Solari, E.;
Scopelliti, R.; Rothlisberger, U.; Severin, K. Chem.—Eur. J. 2010,
16, 8400–8409. (b) Dutta, B.; Scopelliti, R.; Severin, K. Organometallics
2008, 27, 423–429. (c) Dutta, B.; Scolaro, C.; Scopelliti, R.; Dyson, P. J.;
Severin, K. Organometallics 2008, 27, 1355–1357.
(8) Broggi, J.; Díez-Gonzꢀalez, S.; Petersen, J. L.; Berteina-Raboin, S.;
Nolan, S. P.; Agrofoglio, L. A. Synthesis 2008, 1, 141–148.
(9) Chou, C.-H.; Chu, L.-T.; Chiu, S.-J.; Lee, C.-F.; She, Y.-T.
Tetrahedron 2004, 60, 6581–6584.
(10) (a) Abbiati, G.; Contini, A.; Nava, D.; Rossi, E. Tetrahedron
2009, 65, 4664–4670. (b) Hunter, D. H.; Sim, S. K. Can. J. Chem.
1972, 50, 669–677.
3417
dx.doi.org/10.1021/om200295c |Organometallics 2011, 30, 3412–3418

Organometallics
ARTICLE
(11) (a) Chen, M.-H.; Chen, Z.; Song, B.-A; Bhadury, P. S.; Yang, S.;
Cai, X.-J.; Hu, D.-Y.; Xue, W.; Zeng, S. J. Agric. Food. Chem. 2009,
57, 1383–1388. (b) Long, N.; Cai, X.-J.; Song, B.-A.; Yang, S.; Chen, Z.;
Bhadury, P. S.; Hu, D.-Y.; Jin, L.-H.; Xue, W. J. Agric. Food Chem. 2008,
56, 5242–5246. (c) Hu, D.-Y.; Wan, Q.-Q.; Yang, S.; Song, B.-A.;
Bhadury, P. S.; Jin, L.-H.; Yan, K.; Liu, F.; Chen, Z.; Xue, W. J. Agric.
Food Chem. 2008, 56, 998–1001. (d) Kaboudin, B.; Moradi, K. Tetra-
hedron Lett. 2005, 46, 2989–2991.
(12) Azizi, N.; Akbari, E.; Amiri, A. K.; Saidi, M. R. Tetrahedron Lett.
2008, 49, 6682–6684.
(13) (a) Corey, E. J.; K€uhnle, F. N. M. Tetrahedron Lett. 1997,
38, 8631–8634. (b) Hunter, D. H.; Sim, S. K. Can. J. Chem. 1972,
50, 678–689.
(14) (a) Busacca, C. A.; Bartholomeyzik, T.; Cheekoori, S.;
Grinberg, N.; Lee, H.; Ma, S.; Saha, A.; Shen, S.; Senanayake, C. H.
J. Org. Chem. 2008, 73, 9756–9761. (b) Braddock, D. C.; Redmond,
J. M.; Hermitage, S. A.; White, A. J. P. Adv. Synth. Catal. 2006, 348,
911–916.
(15) Williams, O. F.; Bailar, J. C., Jr. J. Am. Chem. Soc. 1959, 81,
4464–4469.
(16) Ramesha, A. R.; Bhat, S.; Chandrasekaran, S. J. Org. Chem.
1995, 60, 7682–7683.
(17) Tetrazene ligands can also coordinate as monoanionic radicals
as observed for an Fe complex: Cowley, R. E.; Bill, E.; Neese, F.;
Brennessel, W. W.; Holand, P. L. Inorg. Chem. 2009, 48, 4828–4836.
(18) Danopoulos, A. A.; Hay-Motherwell, R. S.; Wilkinson, G.;
Cafferkey, S. M.; Sweet, T. K. N.; Hursthouse, M. B. J. Chem. Soc.,
Dalton Trans. 1997, 3177–3184.
(19) Danopoulos, A. A.; Wilkinson, G.; Sweet, T. K. N.; Hursthouse,
M. B. J. Chem. Soc., Dalton Trans. 1996, 3771–3778.
(20) Attempts to obtain larger amounts of crystalline 3 failed, and
further analyses (NMR, elemental analysis) were therefore not
performed.
(21) (a) Albertin, G.; Antoniutti, S.; Castro, J. J. Organomet. Chem.
2010, 695, 574–579. (b) Albertin, G.; Antoniutti, S.; Baldan, D.; Castro,
J.; Garꢀcia-Fontꢀan, S. Inorg. Chem. 2008, 47, 742–748. (c) Albertin, G.;
Antoniutti, S.; Bacchi, A.; Celebrin, A.; Pelizzi, G.; Zanardo, G. Dalton
Trans. 2007, 661–668.
(22) Maddani, M.; Prabhu, K. R. Tetrahedron Lett. 2008, 49,
4526–4530.
(23) Strain, H. H. J. Am. Chem. Soc. 1927, 49, 1558–1571.
(24) Levison, J. J.; Robinson, S. D. J. Chem. Soc. A 1970, 2947–2954.
(25) Tilley, T. D.; Grubbs, R. H.; Bercaw, J. E. Organometallics
1984, 3, 274–278.
(26) K€olle, U.; Kang, B.-S.; Thewalt, U. J. Organomet. Chem. 1990,
386, 267–273.
(27) Hodson, E.; Simpson, S. J. Polyhedron 2004, 23, 2695–2707.
(28) La Placa, S. J.; Ibers, J. A. Inorg. Chem. 1965, 4, 778–783.
(29) Colombano, G.; Travelli, C.; Galli, U.; Caldarelli, A.; Chini,
M. G.; Canonico, P. L.; Sorba, G.; Bifulco, G.; Tron, G. C.; Genazzani,
A. A. J. Med. Chem. 2009, 53, 616–623.
(30) CrysAlis PRO version 171.34.49; Agilent Technologies, 2010.
(31) Duisenberg, A. J. M.; Kroon-Batenburg, L. M. J.; Schreurs,
A. M. M. J. Appl. Crystallogr. 2003, 36, 220–229.
(32) Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51, 33–38.
(33) SHELX: Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64,
112–122.
(34) PLATON: Spek, A. L. Acta Crystallogr. 2009, D65, 148–155.
3418
dx.doi.org/10.1021/om200295c |Organometallics 2011, 30, 3412–3418