RhIII-Catalyzed C-H Activation with Pyridotriazoles: Direct Access to Fluorophores for Metal-Ion Detection

Article abstract of DOI:10.1002/anie.201504757

The first C-H bond activation with pyridotriazoles as coupling partners is presented using a Rh^III catalyst. The pyridotriazoles can be used as new carbene precursors in C-H activation for direct access to novel fluorescent scaffolds. These tunable fluorophores can be applied for the detection of metal ions.

Full text of DOI:10.1002/anie.201504757

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201504757
German Edition: DOI: 10.1002/ange.201504757
CÀH Activation
III
À
Rh -Catalyzed C H Activation with Pyridotriazoles: Direct Access to
Fluorophores for Metal-Ion Detection**
Ju Hyun Kim, Tobias Gensch, Dongbing Zhao, Linda Stegemann, Cristian A. Strassert,* and
Frank Glorius*
[
8]
[9]
Abstract: The first CÀH bond activation with pyridotriazoles
compounds and N-tosylhydrazones have been employed
as precursors to carbenes. While diazo compounds are very
versatile reagents for structurally complex products, some are
potentially explosive and extremely difficult to handle on an
III
as coupling partners is presented using a Rh catalyst. The
pyridotriazoles can be used as new carbene precursors in CÀH
activation for direct access to novel fluorescent scaffolds. These
tunable fluorophores can be applied for the detection of metal
ions.
[10]
industrial scale.
Although significant progress has been
S
mall-molecule fluorophores have attracted atten-
tion because of their potential applications in
[
1]
fluorescent bioimaging,
diodes, and as chemosensors for the detection of
metal ions and biologically relevant species. Over
the past decades, several fluorescent scaffolds, such
as naphthalimides, fluoresceins, rhodamines, and
organic light-emitting
[
2]
[
3]
[
4]
BODIPYs, have been developed. In general,
many organic fluorescent dyes are composed of
conjugated polycyclic scaffolds possessing highly
mobile electrons, which can lead to a change in the
fluorescence properties during interaction with
[
5]
binding units. Transition-metal-catalyzed CÀH
bond activations are widely used to construct
diverse p-conjugated polycyclic ring systems, but
[
6]
are rarely employed in dye synthesis. In this
respect, novel designs and direct syntheses of new
fluorescent analogues by CÀH activation strategies
are highly desirable.
Carbene migratory insertion has been success-
fully merged with reactions in which the CÀH bond
Scheme 1. Directed CÀH bond activation with carbene precursors.
[
7]
metalation is the initial step. In recent years, diazo
achieved, the development of new stable carbene precursors
is still highly challenging. Recently, transition-metal catalyzed
denitrogenative transformations of triazoles or tetrazoles
[
*] Dr. J. H. Kim, T. Gensch, Dr. D. Zhao, Prof. Dr. F. Glorius
Organisch-Chemisches Institut
Westfälische Wilhelms-Universität Münster
Corrensstrasse 40, 48149 Münster (Germany)
E-mail: glorius@uni-muenster.de
have received attention for the construction of diverse N-
[11]
heterocyclic compounds.
It has been demonstrated that
these 1,2,3-triazoles share characteristics of diazo compounds
L. Stegemann, Priv.-Doz. Dr. C. A. Strassert
Physikalisches Institut and Center for Nanotechnology (CeNTech)
Westfälische Wilhelms-Universität Münster
Heisenbergstrasse 11, 48149 Münster (Germany)
E-mail: ca.s@uni-muenster.de
[12]
as a result of an equilibrium of closed/opened forms.
II
0
Despite the remarkable advances achieved with Rh , Ni , or
I
[13]
Cu catalysts, CÀH bond metalation coupled with metal
carbene migratory insertion using triazoles has not been
reported previously. In the CÀH activation field, transforma-
[
**] This work was supported by the Korea Research Foundation (NRF-
014R1A6A3A03057520, J.H.K), the European Research Council
under the European Community’s Seventh Framework Program
FP7 2007–2013)/ERC Grant Agreement 25936, the Alexander von
2
tions involving 1,2,3-triazole compounds have been limited to
[
14,15]
their use as directing groups (Scheme 1a).
(
III
We recently reported a Co -catalyzed CÀH bond func-
tionalization with diazo compounds, which provides access to
a new family of fluorescent extended p-systems (Sche-
Humboldt Foundation (D.Z.), and by the DFG (SFB-TTR 61, Projekt
C07). We thank Dr. Constantin G. Daniliuc for X-ray crystallographic
analysis, Daniel Paul for valuable NMR experiments, Dr. Wei Li, Dr.
Matthew Hopkinson, and Suhelen Vµsquez-CØspedes (all WWU
Münster) for helpful discussions.
[16]
me 1b).
Inspired by our previous work, we envisioned
that CÀH activation followed by insertion of 1,2,3-triazole-
derived diazo compounds as coupling partners might be
possible if the diazoimine form is present. In consideration of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504757.
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10975

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Angewandte
Communications
the structural differences between triazoles and diazo com-
pounds, new analogous 1,2,3-triazole compounds, facilitating
the open diazoimine form of triazole, need to be explored. In
this regard, pyridotriazoles are especially attractive owing to
the facile formation of the diazoimine form, which could be
trapped with CÀH bond metalated compounds to form
[11c,12,17]
a pyridiyl carbene intermediate.
Furthermore, the
pyridyl moiety on this new class of fluorescent compounds
may offer a site for metal coordination, which can be
potentially used for metal-ion sensors (Scheme 1c). Herein,
we describe the first CÀH bond activation with pyridotri-
III
azoles as innovative carbene precursors using the Cp*Rh
(
Cp* = h-C Me ) catalyst, which provides direct synthetic
5 5
access to novel fluorescent scaffolds for selective metal-ion
detection. In this transformation, there are some intricate
challenges:
1
) The position of the equilibrium between the cyclic triazole
and the open diazoimine form is governed by the
substitution pattern, solvent polarity, and temperature.
[17a]
Although N-fused triazoles show an equilibrium shift
towards the diazoimine form, the equilibria still favor the
bicyclic structures in all cases studied to date. The
equilibrium shift to the diazoimine form is essential to
avoid unexpected side reactions.
Scheme 2. Substrate scope of various 2-arylpyridines. For reaction
conditions, see Supporting Information.
2
3
) Both the pyridine and 1,2,3-triazole groups are strongly
that this transformation is not restricted to a benzene plat-
form and is applicable to extended p-systems 1n–1p only
requiring a slightly increased catalyst loading.
coordinating directing groups that can lead to competing
[
14,15,18]
undesired CÀH bond activation.
) Moreover, the pyridine groups in the substrate and
product can coordinate strongly to the metal center,
Furthermore, a wide range of pyridotriazoles were inves-
tigated as coupling partners. As shown in Scheme 3, the
coupling reactions between 2-phenylpyridine (1a) and sub-
stituted pyridotriazoles 2 proceeded smoothly to furnish the
corresponding products 4 in moderate to excellent yields.
To better understand the reaction mechanism, we con-
ducted a series of experiments (see Supporting Information).
A competition experiment between electronically different
arylpyridines 1a and 1j showed that an electron-rich substrate
is more favored than an electron-deficient one by a 3:1 ratio,
thus suggesting that the CÀH bond activation process may be
[
19]
which may poison the catalyst.
We began our investigation utilizing 2-phenylpyridine
1a) and pyridotriazole 2a as model substrates. In accordance
(
[
16]
with our previous work, this reaction was investigated with
a {Cp*Co } catalyst, but neither the cyclized nor the
alkylated products were detected (Table S1 in the Supporting
Information). Gratifyingly, it was found that employing the
III
[
{Cp*RhCl } ]/AgOAc combination or [Cp*Rh(CH CN) ]-
2 2 3 3
(
SbF6)2 in 2,2,2-trifluoroethanol (TFE) afforded only the
desired product 3a in 96% and 87% yield, respectively
Table S1), without any of the alkylated product (Sche-
(
III
me 1b). It is noteworthy that the Rh catalyst can conduct
the cyclization in this transformation, which indicates that the
pyridyl moiety might play a crucial role in the cyclization.
Further improvement of the yield (99%) was achieved in the
presence of 2.5 mol% of [Cp*Rh(CH CN) ](SbF ) at 1408C.
3
3
6 2
The structure of 3a was unambiguously confirmed by X-ray
[20]
crystallographic analysis. In addition, the reaction can be
conducted effectively on a gram scale (1 g of 3a, 97% yield).
Under the optimal conditions, the scope of this reaction
was explored with various 2-arylpyridines (Scheme 2). Mono
or di-substituted substrates bearing both electron-donating
groups (R = Me, OMe, NMe ) and electron-withdrawing
2
groups (R = Cl, Br, F, CF ) on the aryl moiety were well-
3
tolerated under the reaction conditions, and afforded the
corresponding products 3b–3l in good to excellent yields
ranging from 72% to 99%. Moreover, the reaction with
methyl substituents on the pyridine also proceeded efficiently
and afforded 3m in high yield. We were pleased to observe
Scheme 3. Substrate scope of various pyridotriazoles. For reaction
conditions, see Supporting Information.
1
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III
ionic rhodacycle I. Next, the Rh car-
bene intermediate III is generated by N2
loss from the in situ generated diazoimi-
ne II. Intermediate III undergoes migra-
tory insertion leading to rhodacyclic
intermediate IV. After proto-demetala-
tion to V, nucleophilic addition to the
activated ester carbonyl occurs to give VI
III
with the aid of {Cp*Rh } as a Lewis acid.
Upon elimination of alcohol from inter-
mediate VI, product 3 or 4 is formed with
regeneration of the catalyst. Notably, the
pyridyl group facilitates this transforma-
tion instead of poisoning the catalyst.
Presumably, chelation of rhodium
between the pyridyl and ester groups in
V activates the carboxylate for the nucle-
ophilic cyclization.
The pyridine and carbonyl moieties in
Scheme 4. Proposed reaction mechanism.
the products constitute binding sites for
cations, making them bidentate lumino-
phores with sensing capabilities. To dem-
onstrate their ability to discriminate between metal ions, we
have monitored the changes in the UV/Vis absorption and
electrophilic in nature. Interestingly, competition experiments
for the preference of electronically different triazole com-
pounds did not noticeably affect the reactivity. A deuterium
labeling experiment indicated that the CÀH activation step is
II
II
emission spectra in the presence of Cu and Zn owing to the
high binding affinity with ligands containing oxygen or
[22]
reversible under the conditions with the H/D scrambling
nitrogen donor atoms.
Upon addition of Zn(ClO ) , the absorption maxima
being observed. The KIE value (k /k = 1.7) from two
H
D
4 2
parallel reactions suggests that the cleavage of the CÀH
appear visibly blue-shifted, an effect that is more pronounced
with Cu(ClO ) , which causes a stronger hypsochromic shift
(Figures 1a,c, Figures S1–S10, and Table S3). This observa-
bond is not involved in the rate-limiting step. To probe the
4
2
1
order of events, H NMR spectroscopy and ESI-MS analyses
of the stoichiometric
reaction were conducted
(
see Supporting Informa-
tion). The results show
that CÀH activation and
rhodacycle
formation
with phenyl pyridine 1a
occur readily even at
room temperature. The
intermediate after car-
bene insertion in the
reaction of the cyclome-
talated phenyl pyridine
with pyridotriazole 2a
has also been character-
ized.
On the basis of the
these experiments and
[8,16,21]
previous reports,
plausible reaction
a
mechanism is proposed
in Scheme 4. The trans-
formation is initiated by
the coordination of sub-
strate to the cationic Rh
center,
followed
by
Figure 1. a) Absorption and b) emission spectra of compound 3a in the presence of Cu(ClO ) or Zn(ClO ) .
4
2
4 2
a reversible CÀH bond
Photos of 3a in a solution of CH Cl and with Cu(ClO ) and Zn(ClO ) under c) ambient light and d) UV light.
2
2
4
2
4 2
cleavage to produce cat-
For detailed conditions, see Supporting Information.
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.
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Communications
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Keywords: carbene · CÀH activation · fluorophore · rhodium ·
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Published online: July 23, 2015
Angew. Chem. Int. Ed. 2015, 54, 10975 –10979
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 10979