Copper-catalyzed CuAAC/intramolecular C-H arylation sequence: Synthesis of annulated 1,2,3-triazoles

Article abstract of DOI:10.3762/bjoc.8.202

Step-economical syntheses of annulated 1,2,3-triazoles were accomplished through copper-catalyzed intramolecular direct arylations in sustainable one-pot reactions. Thus, catalyzed cascade reactions involving [3 + 2]-azide-alkyne cycloadditions (CuAAC) and C-H bond functionalizations provided direct access to fully substituted 1,2,3-triazoles with excellent chemo- and regioselectivities. Likewise, the optimized catalytic system proved applicable to the direct preparation of 1,2-diarylated azoles through a one-pot C-H/N-H arylation reaction.

Full text of DOI:10.3762/bjoc.8.202

Copper-catalyzed CuAAC/intramolecular
C–H arylation sequence: Synthesis of
annulated 1,2,3-triazoles
Rajkumar Jeyachandran1, Harish Kumar Potukuchi1,2 and Lutz Ackermann*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 1771–1777.
1Institut für Organische und Biomolekulare Chemie,
Georg-August-Universität, Tammannstrasse 2, 37077 Göttingen,
Germany and 2Lehrstuhl für Organische Chemie I, Lichtenbergstrasse
4, 85747 Garching, Germany
doi:10.3762/bjoc.8.202
Received: 04 July 2012
Accepted: 19 September 2012
Published: 16 October 2012
Email:
Lutz Ackermann* - lutz.ackermann@chemie.uni-goettingen.de
This article is part of the Thematic Series "C–H Functionalization".
Guest Editor: H. M. L. Davies
* Corresponding author
Keywords:
© 2012 Jeyachandran et al; licensee Beilstein-Institut.
License and terms: see end of document.
cascade reaction; C–H functionalization; copper; heteroarenes;
triazoles
Abstract
Step-economical syntheses of annulated 1,2,3-triazoles were accomplished through copper-catalyzed intramolecular direct aryla-
tions in sustainable one-pot reactions. Thus, catalyzed cascade reactions involving [3 + 2]-azide–alkyne cycloadditions (CuAAC)
and C–H bond functionalizations provided direct access to fully substituted 1,2,3-triazoles with excellent chemo- and regioselectivi-
ties. Likewise, the optimized catalytic system proved applicable to the direct preparation of 1,2-diarylated azoles through a one-pot
C–H/N–H arylation reaction.
Introduction
Transition-metal-catalyzed C–H bond functionalizations are C–H bond functionalizations could be combined with the
increasingly viable tools for step-economical syntheses of Huisgen [51] copper(I)-catalyzed [52,53] [3 + 2]-azide–alkyne
various valuable bioactive compounds [1-3], which avoid the cycloaddition (CuAAC)[54], while C–H bond arylations of
preparation and use of preactivated substrates [4-16]. This 1,2,3-triazoles were previously only accomplished with more
streamlining of organic synthesis has predominantly been expensive palladium [55-62] or ruthenium [63-66] catalysts.
accomplished with palladium [4-16], rhodium [17-19] or ruthe- Notably, this strategy allowed for the atom-economical syn-
nium [20-22] complexes [4-16]. However, less expensive thesis of fully substituted 1,2,3-triazoles in a highly regioselec-
nickel, cobalt, iron or copper catalysts bear great potential for tive fashion [54,67]. While the research groups of Rutjes [68] as
the development of economically attractive transformations [23- well as Sharpless [69] elegantly devised alternative approaches
50]. In this context, we previously reported on the use of cost- exploiting 1-haloalkynes [70], we became interested in
effective copper(I) catalysts for direct arylations of 1,2,3-tria- exploring a single [71-73] inexpensive copper catalyst for one-
zoles. Thus, we showed that intermolecular copper-catalyzed pot reaction sequences comprising a 1,3-dipolar cycloaddition
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Scheme 1: Copper-catalyzed step-economical C–H arylation-based cascade reaction.
along with an intramolecular C–H bond arylation; in particular, pensive CuI for the formation of up to one C–C and three C–N
because of the notable biological activities exerted by fully bonds in a site-selective fashion (Scheme 1).
substituted 1,2,3-triazoles [74-88]. As a consequence, we wish
to present herein novel cascade reactions, in which cost-effec- Results and Discussion
tive copper(I) compounds serve as the catalyst for two mecha- We initiated our studies by exploring reaction conditions for the
nistically distinct transformations for the synthesis of fully key copper-catalyzed intramolecular direct C–H bond arylation,
substituted annulated 1,2,3-triazoles as well as for twofold employing substrate 3a (Table 1). Notably, the envisioned C–H
N–H/C–H bond arylations. Notable features of our strategy bond functionalization occurred readily with the aryl iodide 3a
include (i) the development of a chemoselective C–H arylation- when catalytic amounts of CuI were used, even at a reaction
based three-component reaction, as well as (ii) the use of inex- temperature as low as 60 °C, with optimal yields being obtained
Table 1: Optimization studies for the intramolecular direct arylation of triazole 3a.a
entry
base
ligand
T [°C]
isolated yield [%]
1
2
3
LiOt-Bu
LiOt-Bu
LiOt-Bu
–
–
–
140
120
100
82
97
91
4
LiOt-Bu
–
80
93
5
6
LiOt-Bu
LiOt-Bu
K3PO4
K3PO4
K3PO4
K3PO4
–
60
20
72
<2b
5b
–
7
DMEDA
140
140
140
140
8
N,N-dimethylglycine
2,2-bipyridyl
1,10-phenanthroline
5b
9
4b
10
11
aGeneral reaction conditions: 3a (1.00 mmol), CuI (10 mol %), ligand (10 mol %), DMF (3.0 mL).
bBy 1H NMR spectroscopy.
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Beilstein J. Org. Chem. 2012, 8, 1771–1777.
Scheme 2: Copper-catalyzed sequential catalysis with alkyne 1a.
at 80 °C (Table 1, entries 1–6). While the transformation Subsequently, we explored the extension of this approach to the
proceeded efficiently with LiOt-Bu as the stoichiometric base, development of a chemoselective three-component one-pot
K3PO4 only led to unsatisfactory results, even when additional reaction. Thus, we found that alkyl bromides 2 could be directly
stabilizing ligands were used (Table 1, entries 7–10).
employed as user-friendly substrates for the in situ formation of
the corresponding organic azides (Scheme 3). Notably, the
catalytic system proved broadly applicable, and a variety of
organic electrophiles 2, thereby, delivered differently decorated
N-substituted 1,4-dihydrochromeno[3,4-d][1,2,3]triazoles 4.
With optimized reaction conditions for the intramolecular direct
arylation in hand, we tested the possibility of its implementa-
tion in a sequential synthesis of 1,4-dihydrochromeno[3,4-
d][1,2,3]triazole (4b, Scheme 2). We were delighted to observe
that the desired reaction sequence consisting of a copper- Importantly, performing the one-pot reaction in a sequential
catalyzed 1,3-dipolar cycloaddition and an intramolecular C–H fashion was not found to be mandatory. Indeed, our strategy
bond arylation converted alkyne 1a to the desired product 4b turned out to be viable in a nonsequential manner by directly
with high catalytic efficacy.
employing equimolar amounts of the three substrates. Hence,
Scheme 3: Copper-catalyzed reaction sequence using alkyl bromides 2. General reaction conditions: 1 (1.00 mmol), 2 (1.00 mmol), NaN3
(1.05 mmol), CuI (10 mol %), DMF (3.0 mL), LiOt-Bu (2.00 mmol); yields of isolated product. a60 °C in the first step.
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Beilstein J. Org. Chem. 2012, 8, 1771–1777.
inexpensive CuI allowed the direct assembly of aryl iodides 1, zoles through cascade reactions consisting of copper(I)-
alkyl bromides 2 and NaN3 with excellent chemo- and regiose- catalyzed [3 + 2]-azide–alkyne cycloadditions (CuAAC) and
lectivities (Scheme 4). Thereby, a variety of annulated 1,2,3- intramolecular C–H bond arylations. Notably, the optimized
triazoles 4 were obtained, featuring six- or seven-membered copper catalyst accelerated two mechanistically distinct trans-
rings as key structural motifs. It is particularly noteworthy that formations, which set the stage for the formation of up to one
the copper-catalyzed transformation enabled the formation of C–C and three C–N bonds in a chemo- and regioselective
one C–C and three C–N bonds in a chemoselective manner, and fashion, and also allowed for twofold C–H/N–H bond aryla-
thereby provided atom- and step-economical access to annu- tions on various azoles.
lated carbo- as well as O- and N-heterocycles.
Experimental
Finally, we found that the catalytic system also proved to be General information
applicable to the one-pot copper-catalyzed direct arylation of Catalytic reactions were carried out under an inert atmosphere
various azoles 5 through N–H/C–H bond cleavages with aryl of nitrogen using predried glassware. All chemicals were used
iodides 6 as the organic electrophiles (Scheme 5).
as received without further purification unless otherwise speci-
fied. DMF was dried over CaH2. Alkynes 1 [89-92] and tria-
zoles 3 [93] were synthesized according to previously described
Conclusion
In summary, we have reported on the use of inexpensive methods. CuI (99.999%) was purchased from ABCR with the
copper(I) complexes for step- and atom-economical sequential following specifications: Ag <3 ppm, Ca = 2 ppm, Fe = 1 ppm,
catalytic transformations involving direct C–H bond arylations. Mg <1 ppm, Zn <1 ppm. Yields refer to isolated compounds,
Thus, CuI enabled the synthesis of fully substituted 1,2,3-tria- estimated to be >95 % pure, as determined by 1H NMR. Thin-
Scheme 4: Nonsequential cascade synthesis of fully substituted triazoles 4. General reaction conditions: 1 (1.00 mmol), 2 (1.00 mmol), NaN3
(1.05 mmol), CuI (10 mol %) DMF (3.0 mL), LiOt-Bu (2.00 mmol); yields of isolated product.
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Beilstein J. Org. Chem. 2012, 8, 1771–1777.
Scheme 5: Copper-catalyzed one-pot twofold C–H/N–H arylation with azoles 5. aReaction performed at 120 °C.
layer chromatography (TLC) was carried out on silica gel 60
F254 aluminum plates (Merck). Chromatography: Merck silica
gel 60 (40–63 μm). NMR: Spectra were recorded on Varian
Unity 300, Mercury 300 or Inova 500 in the solvent indicated;
chemical shifts (δ) are given in parts per million (ppm). All IR
spectra were taken on a Bruker FTIR Alpha device. MS: EIMS-
spectra were recorded with Finnigan MAT 95, 70 eV; high-
resolution mass spectrometry (HRMS) with APEX IV 7T
FTICR, Bruker Daltonic. Melting points were determined with
a Stuart melting-point apparatus SMP3, Barlworld Scientific;
values are uncorrected.
Supporting Information
Supporting Information containing all experimental details
and analytical data of new compounds as well as their 1H
and 13C spectra are provided.
Supporting Information File 1
Experimental procedures, characterization data, and NMR
spectra for new compounds.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-8-202-S1.pdf]
General procedure for the synthesis of triazoles 4
NaN3 (1.05 equiv), CuI (10 mol %), LiOt-Bu (2.00 equiv), Acknowledgements
alkyne 1 (1.00 equiv) and alkyl bromide 2 (1.00 equiv) were Support by the DFG and the DAAD (fellowship to H.K.P.) is
dissolved in DMF (3.0 mL) and stirred at 80 °C for 20 h. Then, gratefully acknowledged.
H2O (50 mL) was added at ambient temperature, and the
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