.
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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Angew. Chem. Int. Ed. 2015, 54, 10975 –10979