Expanding the Scope of Chelating Triazolylidenes: Mesoionic Carbenes from the 1,5-“Click”-Regioisomer and Catalytic Synthesis of Secondary Amines from Nitroarenes

Article abstract of DOI:10.1002/chem.201603901

Chelating 1,2,3-triazolylidenes have been established as privileged ligands in homogeneous catalysis. We present herein a new approach towards chelating 1,2,3-triazolylidene ligands based on the 1,5-regioisomer of the corresponding triazole, which can be obtained through simple click chemistry. The new ligands are compared to their 1,4-regioisomeric counterparts through coordination to the ruthenium p-cymene fragment. The complexes are characterized structurally and spectroscopically and are employed as (pre)catalysts in the reductive condensation of nitroarenes and primary alcohols to yield secondary amines. The activity of chelating mesoionic carbene ligands obtained from the two different regioisomers of the triazoles are compared and contrasted in catalysis. The performance of the ruthenium complexes with mesoionic carbenes could be improved through the choice of the employed base and reaction conditions, giving rise to the most effective systems thus far. The results presented here prove the utility of chelating mesoionic carbenes as an extremely potent class of ligands for the synthesis of secondary amines from nitroarenes.

Full text of DOI:10.1002/chem.201603901

DOI: 10.1002/chem.201603901
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&
Ligand Design |Hot Paper|
Expanding the Scope of Chelating Triazolylidenes: Mesoionic
Carbenes from the 1,5-“Click”-Regioisomer and Catalytic Synthesis
of Secondary Amines from Nitroarenes
[
a]
Lisa Suntrup, Stephan Hohloch, and Biprajit Sarkar*
Abstract: Chelating 1,2,3-triazolylidenes have been estab-
lished as privileged ligands in homogeneous catalysis. We
present herein a new approach towards chelating 1,2,3-tria-
zolylidene ligands based on the 1,5-regioisomer of the corre-
sponding triazole, which can be obtained through simple
click chemistry. The new ligands are compared to their 1,4-
regioisomeric counterparts through coordination to the
ruthenium p-cymene fragment. The complexes are charac-
terized structurally and spectroscopically and are employed
as (pre)catalysts in the reductive condensation of nitroarenes
and primary alcohols to yield secondary amines. The activity
of chelating mesoionic carbene ligands obtained from the
two different regioisomers of the triazoles are compared and
contrasted in catalysis. The performance of the ruthenium
complexes with mesoionic carbenes could be improved
through the choice of the employed base and reaction con-
ditions, giving rise to the most effective systems thus far.
The results presented here prove the utility of chelating
mesoionic carbenes as an extremely potent class of ligands
for the synthesis of secondary amines from nitroarenes.
Introduction
type cross coupling reactions of aryl bromides and aryl chlor-
[9]
ides with boronic acids. One of the advantages we pointed
out in that contribution is that, in contrast to the 1,4-regioiso-
mer, the methylation (or functionalization) of the triazole to
the triazolium salt occurs in the a-position to the carbene-
One of the most utilized reactions in contemporary chemistry
is the copper-catalyzed [3+2] cycloaddition between azides
[
1]
and alkynes (CuAAC) yielding 1,2,3-triazoles. This reaction is
the best known example of a “click” reaction. The ease of per-
forming this reaction, in combination with the huge substrate
scope, is the main reasons for its popularity in chemistry today.
[9]
carbon atom. This offers great opportunities for the steric
and electronic fine tuning of such carbenes, as well as the in-
troduction of functional groups that interfere with methylation
[9]
1
,2,3-Triazoles have gained considerable attention in the last
reactions (e.g., pyridines).
[
2]
years in coordination chemistry. One prominent use of tria-
zoles in coordination/organometallic chemistry is the methyla-
tion of triazoles to obtain 1,2,3-triazolium salts, which can be
Having tested the utility of chelating mesoionic carbenes de-
rived from the 1,4-regioisomer of the click reaction as ligands
in various catalytic transformations, and in particular transfer
[3]
[10]
used as precursors to generate mesoionic carbenes. Metal
complexes of 1,2,3-triazole-derived mesoionic carbenes (so
called 1,2,3-triazolylidenes) have shown remarkable proper-
hydrogenation reactions, we were interested in expanding
the scope of chelating mesoionic carbenes derived from the
1,5-regioisomers. Additionally, we aimed to compare and con-
trast the two related ligand classes in a challenging catalytic
reaction to generate useful chemical products from readily
available starting materials.
[
4]
ties and unique catalytic potential towards various transfor-
[
5,6]
mations in the last years.
However, if one reconsiders the
uncatalyzed version of the cycloaddition reaction between al-
kynes and azides, there are two regioisomers possible, the 1,4-
The formation of carbon–nitrogen bonds is one of the most
[
7]
[11]
regioisomer and the 1,5-regioisomer. (Scheme 1).
Even though many effective protocols have emerged to
important reactions in organic synthesis.
The traditional
route for CÀN bond formation involves the condensation of
amines with aldehydes or ketones, in the presence of Lewis
acid catalysts, to form imines first, which can then be reduced.
[
8]
obtain the 1,5-regioisomer selectively, these triazoles have
been rather neglected in the literature to date. We have re-
cently published a first approach for the synthesis of 1,5-tria-
zole-derived mesoionic carbenes and shown that their palladi-
um complexes are efficient (pre)catalysts for Suzuki–Miyaura-
[12]
Other routes use the oxidation of amines. In terms of metal-
catalyzed reactions, one of the most prominent routes for this
[13]
reaction is the “borrowing hydrogen” concept. In such a reac-
tion, an alcohol and an amine are coupled to give the corre-
sponding N-alkylated product. This route thus provides an effi-
cient catalytic way to synthesize secondary amines.
[
a] L. Suntrup, Dr. S. Hohloch, Prof. Dr. B. Sarkar
Institut fꢀr Chemie und Biochemie, Freie Universitꢁt Berlin
Fabeckstraße 34–36, 14195 Berlin (Germany)
E-mail: biprajit.sarkar@fu-berlin.de
An innovative approach for the synthesis of secondary
amines from readily available starting materials was first intro-
duced by the group of Akiko Uno in 1966, in which nitroben-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201603901.
Chem. Eur. J. 2016, 22, 1 – 12
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Scheme 1. The two possible regioisomers obtained in the [3+2] cycloaddition reaction between organic azides and alkynes.
III
I
II III
zene and benzyl alcohol were reacted in the presence of po-
tassium hydroxide under extreme conditions of 250 to
tallic Ir /Au or Pd /Ir complexes used for that purpose deliv-
[16]
ered the unreduced imine as the main product (Scheme 2).
[
12a]
2808C.
This reaction, although ingenious, is stoichiometric,
Recently, we have reported on the use of metal complexes
of mesoionic carbenes for the catalytic transfer hydrogenation
and suffers from low product yield and harsh reaction condi-
tions. In the last years, ruthenium-based systems have been re-
of nitroarenes to yield aniline and azobenzene depending on
[
14]
[10d]
ported as efficient catalysts for this reaction. In the catalytic
reaction, the concepts of catalytic transfer hydrogenation to
the conditions and catalysts applied to this reaction.
Based
on our previous results that showed that ruthenium complexes
of chelating mesoionic carbenes were highly efficient catalysts
for transfer hydrogenation reactions, we were interested in
combining that catalytic capacity with the hydrogen-borrow-
ing concept to develop catalytic synthetic routes for secondary
amines starting from readily available starting materials such
as nitroarenes and benzyl alcohols.
[
15]
reduce nitroarenes is combined with the borrowing hydro-
gen process. Thus, the nitroarenes are first reduced to the cor-
responding anilines using a primary alcohol as hydrogen
source (Scheme 2, Part1). During this transformation, the pri-
mary alcohol is converted into the aldehyde, which subse-
quently reacts with the anilines formed to yield imines. These
imines can then be reduced in a final transfer hydrogenation
step to the corresponding secondary amines (Scheme 2,
Part 2). By using the catalytic protocol, nitrobenzenes and
benzyl alcohols could be converted into secondary amines
2
Herein, we present the synthesis of the new ligands L and
L by using the 1,5-regioisomer of the triazoles, and compare
them to their 1,4-regioisomer derived counterparts, L and L
(Scheme 3).
4
1
3
[
14]
with high selectivities and yield. Metal complexes of N-heter-
ocyclic carbene based ligands were also used for the afore-
mentioned catalytic transformation. However, the heterobime-
Half sandwich complexes of the type [Ru(Cym)Cl(L)](PF )
6
1
2
3
[with Cym=p-cymene and L=L (for Ru-1), L (for Ru-2), L
(for Ru-3), and L (for Ru-4)] are presented for all ligands and
4
Scheme 2. Reductive coupling of nitroarenes and primary alcohols towards secondary amines.
Scheme 3. Ligands used in this work.
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5
their structural and spectroscopic properties are investigated.
Furthermore, the catalytic properties of such complexes are ex-
plored in the condensative transfer hydrogenation of nitroar-
enes with primary alcohols for the synthesis of secondary
amines.
the mono-substituted iodomethylene triazolium salt HL [I]
(Scheme 5, left).
1
The identity and purity of all ligands were established by H
13
and C NMR spectroscopy and mass spectrometry. All ligands
display the characteristic low-field signal for the triazole-5H in
1
their H NMR spectra, unambiguously proving the formation of
4
the desired triazolium salts. In case of H L [OTf] , the identity
2
2
Results and Discussion
was further confirmed by the occurrence of the methylene
protons of the bridging CH group at d=8.08 ppm (see experi-
2
Synthesis and Crystal Structures
mental section).
1
The triazolium salt HL [OTf] from the 1,4-regioisomers were
After establishing the ligands described above, piano-stool
type ruthenium cymene complexes Ru-1–4 were synthesized
(Scheme 6).
[
5p]
synthesized by following reported routes. To obtain the tria-
3
zolium salt H L [BF ] , the corresponding 1,4-regioisomer of the
2
4 2
bitriazole was first synthesized by following a reported proce-
The synthesis of Ru-1 was performed as reported previous-
ly. Complexes Ru-2–4 were all synthesized by following
[
17]
[5]
dure,
followed by a methylation using Meerwein’s salt at
[9]
room temperature similar to previously reported conditions.
a
transmetallation protocol previously reported by our
[10d]
For the synthesis of the 1,5-triazole-derived carbene, the 1,5-
group.
Formation of the desired complexes was indicated
diphenyl-substituted triazole precursor T1 was synthesized by
by the absence of the low-field signal of the triazole-5H in the
[
8a]
1
following a metal-free procedure. To obtain the chelating
H NMR spectra of the corresponding complexes. Furthermore,
2
4
ligand HL [BF ], we exploited the fact that nucleophilic substi-
on complexation of the ligand L , the methylene protons in
4
tution on pyridines readily occurs at high temperatures, as has
the proton NMR spectra split into two signals. By coordination
[18]
4
been shown with similar imidazolylidene systems before. On
of L to the ruthenium center, the two methylene protons
heating 2-bromopyridine with the 1,5-triazole precursor T1 at
become chemically inequivalent and this results in the obser-
vation of two pseudo doublets for the two methylene protons
at d=7.23 ppm and d=6.51 ppm, both integrating for only
one proton (JAB =13.6 Hz). Formation of the desired complexes
1
608C for two days and subsequent aqueous anion exchange,
2
we isolated the desired triazolium salt HL [BF ] in reasonable
4
yields (Scheme 4, right). However, trying to incorporate a picolyl
group by using picolyl bromide always resulted in the forma-
tion of the desired product together with various inseparable
side products. It is known that, in selected cases, a pyridine ni-
trogen can be alkylated more easily than the triazole nitroge-
13
was further established by C NMR spectroscopy showing the
carbene-C signals at d=170.5, 178.3, and 164.3 ppm for Ru-2,
Ru-3, and Ru-4, respectively; thereby being in the same range
as for comparable ruthenium-triazolylidene complexes (see Ex-
[
5a]
[5n,10d]
n; therefore, we believe that the picolyl bromide most likely
alkylates itself (Scheme 4, left).
perimental Section and the Supporting Information).
In
addition, mass spectrometric measurements displayed the mo-
lecular peaks for the cationic parts of the complexes.
4
To obtain the triazolium salt H L [OTf] , we first reacted diio-
2
2
domethane with silver triflate under exclusion of light to gen-
erate ditriflatomethane in situ, which was then reacted with
the 1,5-triazole precursor T1 to give the desired bis-triazolium
Single crystals of Ru-2 and Ru-3 could be obtained by slow
diffusion of n-hexane into a diluted solution of the respective
complex in dichloromethane at room temperature. X-ray dif-
fraction studies show that Ru-2 crystallizes in the monoclinic
4
salt H L [OTf] (Scheme 5, right). It has to be mentioned here
2
2
that the prior activation of the diiodomethane with silver tri-
flate is desirable, because the direct reaction of the 1,5-triazole
T1 with diiodomethane results in the formation of mixtures of
space group C2/c, and Ru-3 in P2 /n. By slow diffusion of n-
1
hexane into a concentrated solution of Ru-4 in dichlorome-
thane at 88C, we were able to obtain single crystals of Ru-4 as
a dichloromethane solvate that was suitable for X-ray diffrac-
4
the desired triazolium salt H L [I] and what we believe to be
2
2
2
4
Scheme 4. Reaction conditions leading to the formation of HL [BF ].
4
Scheme 5. Reaction yielding H
Chem. Eur. J. 2016, 22, 1 – 12
2
L [OTf]
2
(right) and side product formation for the unactivated reaction without the use of silver(I) triflate (left).
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Scheme 6. Complexes Ru-2–4 synthesized during this work.
[
10d]
tion studies. Complex Ru-4 crystallizes in the triclinic space
group P 1¯ as a dichloromethane solvate. All complexes exhibit
triazolylidenes.
The pyridyl moiety in Ru-2 exhibits a bond
length of 2.113(3) ꢁ for N2ÀRu1. Comparison of the bond
lengths within the triazolylidene rings shows a delocalized
bonding situation within the ring systems. The angles on the
carbene centers C1 and C2 are slightly reduced compared with
those of free triazolium salts. Overall, the bond lengths and
angles for Ru-3 are similar to the diisopropylphenyl analogue,
a typical piano-stool geometry (Figure 1).
6
The para-cymene ligand is bound in a h -mode showing
a distance of 1.710(1), 1.724(1), and 1.725(1) ꢁ between the
centroid of the cymene ligand and the ruthenium center for
Ru-2, Ru-3, and Ru-4, respectively.
All complexes exhibit a C1ÀRu1 or C2ÀRu1 distance be-
tween 2.03 and 2.05 ꢁ for the Ru–C(triazolylidene) (Table S1 in
the Supporting Information) and are therefore in the same
range as comparable triazolylidene complexes with chelating
[10d]
which was reported by us in 2013.
The structure of Ru-3 is
also similar to the structure of a ruthenium complex with a bi-
[19]
triazole-2-ylidene reported previously.
Figure 1. ORTEP representation of Ru-2 (upper left), Ru-3 (upper right), and Ru-4·CH
omitted for clarity. Thermal ellipsoids are shown at 50% probability.
2
Cl
2
(bottom). Hydrogen atoms, solvent molecules and counter ions are
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In the case of Ru-4, the two triazolylidene units are bridged
by the C5 methylene linker, which displays single bonds to the
ring nitrogen atoms. The N1-C5-N2 bond angle is 109.0(2)8, dis-
Table 2. Optimization regarding the employed base.
[
a]
Entry
Base
Mole fraction [%]
3
1
2
3
4
playing a nearly perfect sp hybridization of the linker. The two
triazolylidene units are tilted by 32.3(2)8 with respect to each
other. The dihedral angles between the N-phenyl rings and the
triazolylidene rings are 42.2(3)8 and 60.9(2)8, whereas the C-
phenyl rings are tilted by 56.8(2)8 and 74.4(3)8 towards the tria-
zole planes. Further crystallographic details for Ru-2–4 are
given in the Supporting Information (Tables S1 and S2).
1
2
3
4
KOH
K CO
2 3
KOtBu
0
0
7
0
0
11
19
0
90
73
65
0
10
17
9
KOtAmyl
>99
[
a] Reaction conditions: base (0.6 equiv), benzyl alcohol (2.0 mmol), nitro-
benzene (0.2 mmol), Ru-3 (3 mol%), 1208C 24 h; mixture composition de-
termined based on GC-MS using hexadecane as internal standard.
Catalysis
We next turned out attention to the use of the synthesized
complexes as precatalysts for the conversion of nitroarenes
and benzyl alcohols into secondary amines. In doing so, we
were interested in comparing the activity of these new cata-
lysts to ruthenium-cymene based catalysts containing N-heter-
ganic bases as well as potassium tert-butanolate (Table 2, en-
tries 1–3) led to N-benzalaniline (3) as the main product. By
using a solution of potassium tert-amylate (1.7m in toluene),
the desired N-benzylaniline (4) was delivered in extremely high
selectivity.
[
16]
[14]
ocyclic carbenes as well as other ligands. Furthermore, we
were interested in comparing the catalytic performance of
a 1,4- versus a 1,5-disubstituted-triazole-derived triazolylidene
ligand.
We ascribe this vast difference to two different effects: the
solubility of the whole system and especially of the base is in-
creased, and a pre-activation of the catalyst occurs via the
base, given that mixing the base and ruthenium complexes re-
sults in a strong change of color (see the Supporting Informa-
tion). Adding toluene to the reactions with other bases had no
influence on the product formation. Furthermore, we reduced
the catalyst loading, reaction temperature and reaction time
(Table 3). Reducing the reaction time to 7 h still gave excellent
To start our catalytic studies, an initial screening was carried
out using the N-alkylation of nitrobenzene towards N-benzyla-
niline as the benchmark reaction (Scheme 7), using 3 mol% of
the catalyst and 0.6 equivalents of KOtBu in benzyl alcohol at
1208C. The results are summarized in Table 1.
[a]
Table 3. Optimization towards temperature and catalyst loading.
[
b]
Entry
Cat. loading [mol%]
T [8C]
t [h]
Yield [%]
1
2
3
4
5
6
7
3
3
3
3
1
1
4
120
120
120
100
120
100
120
24
12
7
12
12
12
3
85
95
92
78
92
75
Scheme 7. Reductive coupling of nitrobenzene and benzyl alcohol.
Table 1. First survey to determine most active (pre)catalyst for optimiza-
tion.
[
c]
73
[
a] Reaction conditions: Ru-3 (3 mol%), KOtAmyl (0.6 equiv), benzyl alco-
[
a]
Entry
Complex
Mole fraction [%]
hol (2.0 mmol), nitrobenzene (0.2 mmol). [b] Yield of N-Benzylaniline de-
termined based on GC-MS analysis using internal standard. [c] Aniline and
N-benzalaniline were also observed.
1
2
3
4
1
2
3
4
Ru-1
Ru-2
Ru-3
Ru-4
0
0
0
0
0
0
0
0
>99
>99
89
0
0
11
0
>99
[
(
a] Reaction conditions: KOtBu (0.6 equiv), benzyl alcohol (1 mL), toluene
1 mL), nitrobenzene (0.2 mmol), catalyst (3 mol%), 1208C, 24 h; mixture
yields without loss in selectivity (entry 3), whereas a reaction
over 24 h resulted in a slightly lower yield (entry 1). The forma-
tion of the twofold N-alkylated product over longer periods is
composition determined based on GC-MS analysis using hexadecane as
internal standard.
[14a]
a possibility.
Unfortunately, we were not able to detect the
twofold N-alkylated product by GC, and hence we cannot rule
out the operation of other side reactions of the mono-N-alky-
lated products with components present in the catalytic reac-
tion mixture. Decreasing the reaction temperature to 1008C
(entry 4) still gave reasonable yields over 12 h. Furthermore,
we could reduce the catalyst loading to 1 mol% and still get
yields of over 90% after 12 h (entry 5). Lowering both temper-
ature and catalyst loading (entry 6) as well as reducing the re-
action time to 3 h (entry 8), resulted in moderate yields.
These results pointed out that Ru-3 is likely the most potent
pre)catalyst in this series; therefore, for further optimizations,
(
Ru-3 was employed. We then reduced the amount of benzyl
alcohol to 10 equivalents with regard to the nitrobenzene to
minimize the twofold N-alkylation resulting in tertiary amines.
Screening of different potassium-based bases revealed
a strong influence of base on the product distribution: inor-
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From these results, we decided that the reaction conditions
utilized in Table 3, entry 3 are the most convenient for labora-
tory applications, and these conditions were used to re-evalu-
ate the ruthenium complexes Ru-1–4 under these optimized
conditions (Scheme 8, Table 4).
Scheme 8. Optimized conditions for benchmark reaction.
Figure 2. Composition of reaction mixture over time.
[
a]
Table 4. Comparison of complexes Ru-1–4 under optimized conditions.
[
b]
Entry
(Pre)catalyst
Yield [%]
consumption of nitrobenzene and the formation of N-benzyla-
niline up to four hours (Figure 2).
1
2
3
4
5
6
Ru-1
Ru-2
Ru-3
Ru-4
[Ru(p-Cym)Cl
no catalyst
85
92
95
70
40
Between the first and second hour, we could also identify
phenyl(phenylamino)methanol (Scheme 2, Part II) by mass
spectrometry, which presented as a broad signal in the gas
chromatogram. Accumulation of aniline and N-benzalaniline in
the reaction mixture was observed after 2.5 and 3 h, respec-
tively, probably due to reduced catalyst activity over time.
This would suggest that the reaction of aniline and benzal-
dehyde is fairly fast compared with the preceding reduction of
nitrobenzene to aniline. To verify this assumption, aniline was
reacted by using only 1 mol% catalyst loading of complex Ru-
3; under these conditions, the secondary amine was formed
quantitatively within one hour (Scheme 9 and the Supporting
Information).
2 2
] (1.5 mol%)
[c]
30
[a] Reaction conditions: KOtAmyl (0.6 equiv), benzyl alcohol (2.0 mmol),
nitrobenzene (0.2 mmol), catalyst (3 mol%), 1208C, 7 h. [b] Yield of N-ben-
zylaniline determined based on GC-MS analysis using hexadecane as in-
ternal standard. [c] Azobenzene observed as side product.
All catalysts exhibit good to excellent yields under these
conditions towards the desired amine. The low yield with Ru-4
Table 4, entry 4) could be ascribed to the high acidity of the
(
methylene bridge of the ligand. Although we cannot rule out
the operation of steric effects, the low product yield with Ru-4,
in comparison with Ru-3, which also has two phenyl substitu-
ents on the ligand backbone, likely indicates the aforemen-
tioned acidity of the methylene bridge as the reason for the
lower activity of Ru-4. It is conceivable that the catalyst might
not be stable over the course of the reaction in the presence
of potassium tert-amylate. The ruthenium precursor without
any ligands delivers only 40% product yield (entry 5) under
the same conditions. This result shows the necessity of the
MIC ligands for the high activity of these ruthenium catalysts.
Interestingly, product formation also occurs in the absence
of ruthenium catalysts (Table 4, entry 6). Given that this could
only be observed when potassium tert-amylate was used, this
must be due to this particular base and is not completely sur-
Scheme 9. Conversion of aniline under catalytic conditions. Determined
based on GC-MS analysis by using hexadecane as internal standard.
We also got indications of the likely operation of the homo-
geneous pathway of the reaction by adding a drop of Hg ac-
cording to the amalgam poisoning test; this did not result in
decreased activity of the catalyst (see the Supporting Informa-
tion, Section 4.3).
[
12a]
prising considering the work by Uno et al.
To screen whether our catalytic system would tolerate func-
tional groups on the nitrobenzene, we converted the me-
thoxy-substituted substrate 7 and the chloro-substituted 9
under the optimized conditions. Both nitro-compounds could
be transformed in satisfying isolated yields (Scheme 10).
We were also able to use 4-nitroaniline (11) as a substrate in
this reaction to obtain the di-amine-di-hydrochloride 12 with-
out increasing the catalyst loading (Scheme 10). In this case,
isolation of the product was achieved through formation of
the corresponding hydrochloride to reduce the effort of purifi-
According to our proposed reaction pathway (Scheme 2),
the reaction can be subdivided into the reduction of nitroben-
zene to aniline (Scheme 2, Part I) and the condensation of ani-
line with benzaldehyde, followed by a final reduction step
(Scheme 2, Part II). The progress of the reaction could be ob-
served by tracking the composition of the reaction mixture by
using GC-MS. During the first two hours of the reaction, there
was almost no accumulation of aniline or N-benzalaniline, but
there was an inversely proportional correlation between the
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Table 4). However, Ru-2, which contains a pyridyl-triazolylidene
ligand derived from a 1,5-regioisomer of triazole, delivers com-
parable catalytic yield (Table 4). As noted above, the efficiency
of Ru-4 is probably lower because of the sensitivity of the
methylene groups in the ligand backbone towards bases,
which can lead to catalyst degradation. Hence, it is to be ex-
pected that functionalization of those methylene groups
would lead to more efficient catalysts based on such ligands in
the future. A comparison of the (pre)catalysts presented here
with similar Ru-NHC complexes shows a much higher catalytic
efficiency for the present complexes towards the formation of
[16]
secondary amines. Furthermore, to our knowledge, in overall
terms, the (pre)catalysts presented here are capable of catalyz-
ing the formation of secondary amines from nitroarenes and
benzyl alcohols in shorter reactions times and under milder re-
action conditions compared with the best reported catalysts
[14]
for this transformation.
Conclusions
Scheme 10. Conversion of: a) 1-methoxy-4-nitrobenzene, b) 1-chloro-4-nitro-
benzene (isolated as the corresponding hydrochloride), and c) 4-nitroaniline
We were able to extend the synthetic diversity offered by the
,5-regioisomer of the [3+2] cycloaddition product between
1
(isolated as the corresponding hydrochloride) under catalytic conditions.
azides and alkynes to generate new chelating mesoionic car-
benes. Besides the synthesis of a new chelating pyridyl-triazo-
Yields refer to isolated products.
2
lylidene ligand L , we were also able to explore a new synthet-
cation from a column to a simple precipitation. We found this
to be a convenient way to save time during the work up.
To further investigate the scope of this reaction, two addi-
tional primary alcohols were tested in the reductive condensa-
tion of nitrobenzene (Scheme 11). The use of (4-methoxy)ben-
zyl alcohol resulted in a smooth reaction with a satisfactory
yield of 89%, whereas 1-hexanol gave in a reduced yield of
ic route for the synthesis of chelating bitriazolylidene ligands
4
(such as L ). In addition to the introduction of the new ligands,
we also presented the synthesis and characterization of their
corresponding half-sandwich metal complexes using the ruthe-
nium cymene metal fragment. We found that, in case of meth-
ylene-linked bitriazolylidene ligands, not only the absence of
the triazolium-5H gives good indication for complex synthesis,
but the protons of the methylene linker also act as probes be-
cause they become diastereotopic in the reaction product. In
addition to the new complexes Ru-2 and Ru-4, the correspond-
ing complexes derived from the 1,4-regioisomer of triazoles
are presented and, in case of complex Ru-3, characterized. The
four complexes were then tested for their catalytic activity in
the condensative reduction of nitroarenes by using primary al-
cohol as the hydrogen source, yielding secondary amines as
the final product. Extremely short reaction times could be used
by introducing potassium tert-amylate; to our knowledge, this
protocol exceeds all other reported protocols for this reac-
56%. This can be attributed to the lower reactivity of 1-hexa-
nol to form the corresponding aldehyde compared with ben-
zylic substrates, but we still find this to be a good example of
the broad applicability of the present protocol.
[14]
tion.
These results once again demonstrate the diversity offered
by triazolylidene ligands, and highlight the ease of synthetic
procedures available to build interesting ligand systems and
also to extend the multiple catalytic applications that metal
complexes containing 1,2,3-triazolylidene ligands can facilitate.
In future, we will explore the possibilities of generating new li-
gands (especially tripodal ligands) that can be achieved
through the new approach of using 1,5-disubstituted triazoles
as starting materials for mesoionic carbenes. Work focusing on
these topics is being pursued in our laboratories. The use of
the 1,5-disubstituted triazoles as precursors for mesoionic car-
benes thus provides a valuable addition to this rapidly expand-
ing ligand class.
Scheme 11. Conversion of: a) (4-methoxy)benzyl alcohol, and b) 1-hexanol
under catalytic conditions. Yields refer to isolated products.
Thus, it is seen that, of the four complexes tested here, Ru-3,
containing a bicarbene ligand derived from a 1,4-regioisomer
of triazoles, delivers the best catalytic yield (Table 1 and
Chem. Eur. J. 2016, 22, 1 – 12
www.chemeurj.org
7
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

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3
,3’-Methylenebis(1,5-diphenyl-1H-1,2,3-triazol-3-ium) ditriflate
Experimental Section
4
(H L [OTf] ): Diiodomethane (1.0 equiv, 2.5 mmol, 665 mg, 0.2 mL)
2
2
Materials and physical methods
was dissolved in anhydrous n-hexane (10 mL) and silver(I) triflate
2.1 equiv, 5.25 mmol, 1.34 g) was added. The mixture was capped
(
[
RuCymCl ] was purchased from ABCR. All the reagents were used
2 2
in a Schlenk flask under argon and heated to 708C shielded from
light for 5 h. In the meantime, in a second Schlenk flask, 1,5-tria-
zole T1 (2.5 equiv, 6.25 mmol, 1.39 g) was suspended in anhydrous
toluene (10 mL). When the reaction with silver triflate was com-
plete, the mixture was directly filtered over Celite into the triazole-
containing toluene solution. The Celite was washed once again
with n-hexane (5 mL) and the resulting reaction mixture (toluene/
n-hexane 1:1.5) was heated to reflux overnight. The reaction mix-
ture was then cooled to RT and poured into diethyl ether (400 mL).
The resulting colorless precipitate was collected by filtration and
washed with diethyl ether (50–100 mL). The product was obtained
1
as supplied. HL [OTf] and Ru-1 were synthesized by following re-
ported procedures.
[5o,p]
All reactions were carried out by using
standard Schlenk-line techniques under an inert atmosphere of ni-
trogen (Linde, HiQ Nitrogen 5.0, purity>99.999%). Anhydrous sol-
vents were obtained with a MBRAUN MB-SPS-800 solvent system.
All solvents were degassed by using standard techniques prior to
1
13
1
use. H NMR and C{ H} NMR spectra were recorded with a Jeol
ECS 400 spectrometer or a JEOL ECZ 400R spectrometer at 208C.
Chemical shifts are reported in ppm (relative to the TMS signal)
[20]
with reference to the residual solvent peaks. Multiplets are re-
ported as singlet (s), doublet (d), triplet (t), quartet (q), quintet
1
as a colorless solid. Yield: 1.53 g (2.00 mmol, 81%); H NMR
(
quint), and combinations thereof. Mass spectrometry was per-
(
400 MHz, [D ]acetone): d=9.69 (s, 2H; triazole-5H), 8.09 (s, 2H; N-
6
formed with an Agilent 6210 ESI-TOF. GC-MS analysis was per-
formed with an Agilent 7820A GC System combined with an Agi-
lent 5977E GC/MSD (Column: Agilent HP-5 ms UI, 30 mꢂ250 mm,
CH -N), 7.82–7.79 (m, 2H; aryl-H), 7.77–7.73 (m 8H; aryl-H), 7.68–
2
7
4
5
6
.63 (m, 2H; aryl-H), 7.60–7.54 (m, 4H; aryl-H), 7.50–7.46 ppm (m,
13
1
H; aryl-H); C{ H} NMR (100 MHz, [D ]acetone): d=145.6 (triazole-
6
0
1
.25 mm, p/n 190915–433UI, Method: 508C to 3008C, heating rate
5 Kmin , inlet temperature 3008C). Purity of benzyl alcohol, ni-
C), 135.1, 133.3, 132.6, 132.1, 130.9, 129.9, 127.4, 123.3 (all aryl-C),
À1
2
+
5.8 ppm (N-CH -N); MS (ESI): m/z calcd for [C H N ] : 228.1026;
2
29 24
6
trobenzene and hexadecane for catalysis experiments was checked
by GC-MS analysis.
found 228.1044.
Synthesis of the complexes
Synthesis of ligands
In a 100 mL-Schlenk flask, the corresponding ligand precursor
2.0 equiv, 0.2 mmol) was mixed with basic silver(I) oxide (7.0 equiv,
0.7 mmol, 163 mg) and a chloride source, potassium chloride
20 equiv, 2.0 mmol, 155 mg), and was dissolved in anhydrous ace-
tonitrile (10 mL). The mixture was stirred under the exclusion of
light for 2 days, then the black suspension was filtered and all sol-
vents were removed under high vacuum. The resulting white-yel-
lowish solid was then dissolved in dichloromethane (15 mL) and
(
3
,4-Diphenyl-1-(pyridin-2-yl)-1H-1,2,3-triazol-3-ium tetrafluoro-
2
borate (HL [BF ]): 1,5-Triazole T1 (1.2 equiv, 3.0 mmol, 665 mg) was
4
(
mixed with 2-bromopyridine (1 equiv, 2.5 mmol, 390 mg, 230 mL) in
a screw-capped vial and the mixture was stirred and heated for 3
days at 1608C. The mixture was subsequently cooled to RT, dis-
solved in dichloromethane (10 mL) and slowly added to diethyl
ether (400 mL). The brown precipitate was collected by filtration
and immediately dissolved in methanol (15 mL) and a large excess
of ammonium tetrafluoroborate (2 g) was added. The mixture was
stirred for 15 min and then diluted with water (250 mL) to cause
precipitation of the desired solid. The product was recrystallized
from dichloromethane/n-hexane (1:3) at À188C. The product was
[Ru(p-Cym)Cl2]2 (1.0 equiv, 0.1 mmol, 60 mg) was added and the
mixture stirred for 2 days under the exclusion of light. The precipi-
tated silver(I) chloride was filtered through a pad of Celite and all
volatiles were removed. The crude product was then dissolved in
methanol (5 mL). KPF6 (8.0 equiv, 0.8 mmol, 147 mg) was added
and the solution was stirred for another 20 min before water
(80 mL) was slowly added to cause precipitation of the desired
complex. The yellow solids were filtered and dried under air. The
complexes were obtained as powders in moderate yields of 56%
and higher.
obtained as crystalline colorless plates. Yield: 521 mg (1.35 mmol,
1
5
4%); H NMR (400 MHz, [D ]DMSO): d=10.23 (s, 1H; triazole-5H),
6
8
.86–8.85 (m, 1H; py-H), 8.37–8.33 (m, 2H; py-H), 7.90–7.89 (m, 1H;
py-H), 7.78–7.68 (m, 5H; Ph-H), 7.61–7.47 ppm (m, 5H; Ph-H);
1
3
1
C{ H} NMR (100 MHz, [D ]DMSO): d=150.4 (triazole-5C), 143.1,
6
2
2
1
35.8, 132.7, 131.8, 131.7, 130.8, 130.1, 127.8, 127.7, 126.9, 115.6 (all
[Ru(Cym)Cl(L )](PF ) (Ru-2): Synthesized from HL [BF ] (2.0 equiv,
6 4
+
aryl-C); MS (ESI): m/z calcd for [C H N ] : 299.1297; found:
0.20 mmol, 77 mg). Yield: 110 mg (0.15 mmol, 77%); orange solid;
19
15
4
1
2
99.1323.
H NMR (400 MHz, [D ]acetone): d=9.61 (d, J=5.6 Hz, 1H; py-H),
6
8
.49–8.47 (m, 2H; py-H), 7.86 (m, 1H; py-H), 7.75–7.61 (m, 10H;
1
,1’-Diphenyl-3,3’-dimethyl-1H,1’H-[4,4’-bi(1,2,3-triazole)]-3,3’-
3
aryl-H), 5.84 (d, J=6.2 Hz, 1H; Cym-H), 5.75 (d, J=6.2 Hz, 1H; Cym-
H), 5.68 (d, J=6.1 Hz, 1H; Cym-H), 5.21 (d, J=6.0 Hz, 1H; Cym-H),
diium ditetrafluoroborate (H L [BF ] ): Synthesized by following
2
4 2
[18]
a reported procedure.
88 mg, 1.0 mmol) was dissolved in anhydrous dichloromethane
15 mL) and trimethyloxonium tetrafluoroborate (2.5 equiv, 372 mg,
.5 mmol) was added. The mixture was stirred at RT for three days,
then methanol (3 mL) was added and the mixture was stirred for
5 min before it was slowly poured into diethyl ether (400 mL).
The white precipitate was filtered, washed with diethyl ether
100 mL) and dried to give the desired product as a white powder.
The corresponding bitriazole (1.0 equiv,
2
6
.56 (hept, J=6.8 Hz, 1H; CH(CH ) ), 2.09 (s, 3H; CH ), 1.03 (d, J=
3 2 3
2
(
2
.9 Hz, 3H; CH(CH ) ), 0.98 ppm (d, J=7.0 Hz, 3H; CH(CH ) );
3 2
3 2
13
1
C{ H} NMR (176 MHz, [D ]acetone): d=171.6 (carbene-C), 158.4,
6
1
43.1, 132. 7, 131.8, 131.7, 130.8, 130.1, 127.8, 127.73, 126.9, 115.6
(all aryl-C), 91.6, 88.1, 87.1, 84.2 (Cym-C), 31.9 (CH(CH ) ), 22.7 (CH ),
1
3 2
3
+
2
5
2.0, 19.0 ppm (CH(CH ) ); MS (ESI): m/z calcd for [C H N ClRu] :
3 2
29 28
4
69.1046; found: 569.1090.
(
1
3
3
Yield: 368 mg (0.75 mmol, 75%); H NMR (400 MHz, [D ]DMSO): d=
[Ru(Cym)Cl(L )](PF ) (Ru-3): Synthesized from H L [BF ] (2.0 equiv,
6
6
2
4 2
1
6
0.18 (s, 2H; triazole-5H), 8.12–8.10 (m, 4H; aryl-H), 7.85–7.83 (m,
H; aryl-H), 4.58 ppm (s, 6H; N-CH3); C{ H} NMR (100 MHz,
0.2 mmol, 98 mg). Yield: 82 mg (0.11 mmol, 56%); yellow solid.
13
1
1
H NMR (400 MHz, [D ]acetone): d=8.15–8.09 (m, 4H; aryl-H), 7.83–
6
[
(
D ]DMSO): d=135.5 (triazole-5C), 132.4, 131.2, 130.6, 127.3, 122.0
7.77 (m, 6H; aryl-H), 4.9–4.87 (m, 8H; N-CH and 2ꢂCym-H), 4.83
6
3
all aryl-C), 39.8ppm (N-CH , overlay with solvent); MS (ESI): m/z
(d, J=6.0 Hz, 2H; Cym-H), 1.94 (s, 3H; CH ), 0.64 ppm (d, J=7.2 Hz,
3
3
2
+
13
1
calcd for [C H N ] : 159.0791; found: 159.0795.
6H; CH(CH3)2 not observed due to solvent overlay); C{ H} NMR
1
8
18
6
&
&
Chem. Eur. J. 2016, 22, 1 – 12
www.chemeurj.org
8
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
1
(
700 MHz, CD Cl ): d=178.3 (carbene-C), 139.0, 138.3, 131.1, 129.7,
H NMR (400 MHz, D O): d=7.41–7.35 (m, 8H; aryl-H), 7.31–7.29
2
2
2
1
2
25.8 (all aryl-C), 90.5, 87.4 (Cym-C), 40.0 (N-CH ), 31.3 (CH(CH ) ),
(m, 2H; aryl-H), 7.25 (dd, J=7.8, 1.6 Hz, 4H; aryl-H), 7.10 (s, 4H;
benzyl-H), 4.51 (s, 4H; ammonium-H). Solubility was too low for
C NMR spectroscopy. MS (ESI): m/z calc. for [C H N ] :
20 22 2
3
3 2
2.1 (CH3), 18.3 ppm (CH(CH3)2); MS (ESI): m/z calcd for
+
13
2+
[
C H N ClRu] : 587.1264; found: 587.1256.
2
8
30
6
4
4
145.0886; found: 145.0895.
[
Ru(Cym)Cl(L )](PF ) (Ru-4): Synthesized from H L [OTf] (2.0 equiv,
6
2
2
0
.20 mmol, 150 mg). Yield: 131 mg (0.15 mmol, 75%); green solid.
N-(4-Methoxybenzyl)aniline (13): Obtained from (4-methoxy)ben-
zyl alcohol (2.0 mmol, 0.25 mL) after 7 h reaction time after column
chromatography (SiO2, n-hexane/ethyl acetate, 95:5). Yield:
1
H NMR (400 MHz, CD Cl ): d=7.80–7.75 (m, 4H; Aryl-H), 7.51–7.40
2
2
(
m, 16H; Aryl-H), 7.23 (d, J=13.6 Hz, 1H; N-CH -N), 6.51 (d, J=
2
1
1
6
3
3.6 Hz, 1H; N-CH -N), 4.89 (d, J=6.0 Hz, 2H; Cym-H), 4.70 (d, J=
37.9 mg (0.18 mmol, 89%); colorless solid; H NMR (400 MHz,
2
.0 Hz, 2H; Cym-H), 2.13 (hept, J=7.2 Hz, 1H; CH(CH ) ), 1.67 (s,
CDCl ): d=7.31–7.30 (m, 2H; aryl-H), 7.23–7.18 (m, 2H; aryl-H),
3
2
3
13
1
H; CH ), 0.84 ppm (d, J=7.2 Hz, 6H; CH(CH ) ); C{ H} NMR
6.92–6.89 (m, 2H; aryl-H), 6.74 (tt, J=7.3, 1.2 Hz, 1H; aryl-H), 6.68–
6.64 (m, 2H; aryl-H), 4.27 (s, 2H; benzyl-H), 3.96 (br. s, 1H; amine-
3
3 2
(
700 MHz, CD Cl ): d=164.3 (carbene-C), 149.0, 135.1, 131.9, 131.0,
2 2
[21]
1
30.2, 129.6, 128.6, 127.9, 125.5 (all aryl-C), 90.7, 86.7 (Cym-C), 66.3
H), 3.82 (s, 3H; CH3).
(
N-CH -C), 31.3 (CH(CH ) ), 22.5 (CH ), 18.4 ppm (CH(CH ) ); MS (ESI):
2 3 2 3 3 2
N-Hexylaniline (14): Obtained from 1-hexanol (2.0 mmol, 0.25 mL)
+
m/z cacld for [C H N ClRu] : 725.1733; found: 725.1741.
3
9
36
6
after 7 h reaction time after column chromatography (SiO , n-
2
hexane/ethyl acetate, 95:5). Yield: 19.8 mg (0.11 mmol, 56%); color-
1
Catalysis
General setup: In a typical reaction, the substrate (1.0 equiv,
less oil; H NMR (400 MHz, CDCl
6
(
2
3
): d=7.19–7.15 (m, 2H; aryl-H),
.69 (tt, J=7.3, 1.1 Hz, 1H; aryl-H), 6.62–6.59 (m, 2H; aryl-H), 3.59
br. s, 1H; amine-H), 3.10 (t, J=7.2 Hz, 2H; alkyl-H), 1.65–1.56 (m,
H; alkyl-H), 1.44–1.26 (m, 6H; alkyl-H), 0.90 ppm (t, J=7.2 Hz, 3H;
0
.2 mmol) was added to a 25 mL-Schlenk flask filled with the re-
spective catalyst (1–3 mol%), base (0.6 equiv), and hexadecane
1.0 equiv, 0.2 mmol, 50 mL) as internal standard, and the respective
alcohol (10 equiv, 2.0 mmol). The mixture was then heated to 100–
208C for the indicated time.
[22]
alkyl-H).
(
1
X-ray crystallography
Optimized reactions: In a 25 mL-Schlenk flask, the respective com-
plex (3 mol%, 6 mmol) was dissolved in benzyl alcohol (10 equiv,
X-ray quality crystals of Ru-2 and Ru-3 were obtained by layering
diluted solutions in dichloromethane with n-hexane at RT. X-ray
quality crystals of Ru-4 were obtained by layering concentrated
solutions in dichloromethane with n-hexane at 88C. X-ray data of
Ru-2 were collected with a Bruker Smart AXS. Data were collected
2
1
0
.0 mmol, 0.2 mL). Potassium tert-amylate (0.6 equiv, 0.12 mmol,
.7m in toluene) and hexadecane as internal standard (1.0 equiv,
.2 mmol, 50 mL) were added and the mixture was stirred for 5 min
at RT before nitrobenzene (1.0 equiv, 0.2 mmol, 20 mL) was also
added to the flask. The mixture was heated to 1208C for 7 h. Sam-
ples for GC-MS analysis were taken directly from the mixture and
filtered over a short pad of silica using ethyl acetate as eluent.
at 140(2) K using graphite-monochromated MoKa radiation (l =
a
0
.71073 ꢁ). The strategy for the data collection was evaluated by
using the Smart software. The data were collected by the standard
omega scan techniques”, and were scaled and reduced by using
“
N-Benzylaniline (4): Obtained from nitrobenzene (0.20 mmol,
Saint+ software. The structures were solved by direct methods
2
0 mL) after 7 h reaction time after column chromatography (SiO2,
using SHELXS-97 and refined by full matrix least-squares with
SHELXL-97, refining on F . X-ray data of Ru-3 and Ru-4 were col-
lected with a Bruker D8 Venture system at 100(2) K using graphite-
2
n-hexane/ethyl acetate 95:5). Yield: 34.4 mg (0.19 mmol 94%); col-
orless oil. H NMR (400 MHz, CDCl ): d=7.38–7.32 (m, 4H; aryl-H),
1
3
7
7
.29–7.25 (m, 1H; aryl-H), 7.20–7.15 (m, 2H; aryl-H), 6.71 (t, J=
.33 Hz, 1H; aryl-H), 6.64 (d, J=7.5 Hz, 2H; aryl-H), 4.33 (s, 2H;
monochromated MoKa radiation (l =0.71073 ꢁ). The strategy for
a
the data collection was evaluated by using APEX2. The data were
collected by using “omega+phi scan techniques”, and were scaled
and reduced by using APEX2 and SADABS software. The structures
were solved by intrinsic phasing using APEX2 and refined by full
benzyl-H), 4.02 ppm (br. s, 1H; amine-H); GC-MS (EI): m/z (%): 77.1,
[14d]
9
1.1, 122.1, 196.1, 213.2.
N-Benzyl-4-methoxyaniline (8): Obtained from 1-nitro-4-methoxy-
benzene (0.20 mmol, 42.6 mg) after 7 h reaction time after column
chromatography (SiO , n-hexane/ethyl acetate 95:5). Yield: 38.5 mg
2
matrix least-squares using SHELXL-2014/7, refining on F . Non-hy-
[23]
drogen atoms were refined anisotropically. CCDC 1012084 (Ru-
2), 1020050 (Ru-3), and 1442699 (Ru-4) contain the supplementary
crystallographic data for this paper. These data are provided free
of charge by The Cambridge Crystallographic Data Centre.
2
1
(
0.17 mmol, 83%); yellow oil; H NMR (400 MHz, CDCl ): d=7.41–
3
7
1
CH3).
.28 (m, 5H; aryl-H), 6.81 (d, J=16.0 Hz, 2H; aryl-H), 6.63 (d, J=
5.9 Hz, 2H; aryl-H), 4.31 (s, 2H; benzyl-H), 3.76 ppm (s, 3H;
[14d]
N-Benzyl-4-chloroaniline hydrochloride (10·HCl): Obtained from
1
7
-nitro-4-chlorobenzene (0.20 mmol, 31.2 mg) as a white solid after Acknowledgements
h reaction time. The crude product was dissolved in ethanol and
ethereal HCl was added dropwise until the product was completely
We are grateful to the Deutsche Forschungsgemeinschaft
DFG), the Fonds der chemischen Industrie (FCI) and the Freie
precipitated. The solid was then washed with diethyl ether and
(
1
dried to give the product. H NMR (400 MHz, D O): d=7.51–7.38,
2
Universitꢃt Berlin for financial support of this work.
7
.30–7.23, 4.83 (s, 2H, overlay with solvent signal), 4.60 ppm (s, 2H,
13
NH ). Solubility was too low for C NMR spectroscopy. MS (ESI): m/
z calc. for [C H ClN] : 218.0731; found: 218.0719.
2
+
1
3
13
Keywords: regioselectivity · carbene ligands · carbenes · click
chemistry · ruthenium
1
,4-Di-(N-benzylamino)benzene dihydrochloride (12·2HCl): Ob-
tained from 4-nitroaniline (0.20 mmol, 27.6 mg) after 12 h reaction
time. The crude product was dissolved in ethanol and ethereal HCl
was added dropwise until the product was completely precipitat-
ed. The solid was then washed with diethyl ether and dried to give
the product. Yield: 66.2 mg (0.18 mmol, 92%); colorless solid;
[
1] a) V. V. Rostovtsev, L. G. Green, V. V. Fokin, K. B. Sharpless, Angew. Chem.
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b) C. W. Tornøe, C. Christensen, M. Meldal, J. Org. Chem. 2002, 67, 3057–
3064.
Chem. Eur. J. 2016, 22, 1 – 12
www.chemeurj.org
9
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
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Full Paper
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Published online on && &&, 0000
Chem. Eur. J. 2016, 22, 1 – 12
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
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These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
Ru getting into gear: The use of the
&
Ligand Design
1,5-regiomer of the azide–alkyne cyclo-
L. Suntrup, S. Hohloch, B. Sarkar*
& – &&
addition reaction for the development
of chelating mesoionic carbene ligands
is reported (see scheme). The corre-
sponding ruthenium complexes are
shown to be extremely potent catalysts
for the synthesis of secondary amines.
&
Expanding the Scope of Chelating
Triazolylidenes: Mesoionic Carbenes
from the 1,5-“Click”-Regioisomer and
Catalytic Synthesis of Secondary
Amines from Nitroarenes
&
&
Chem. Eur. J. 2016, 22, 1 – 12
www.chemeurj.org
12
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!