Reusable Pd@PEG Catalyst for Aerobic Dehydrogenative C?H/C?H Arylations of 1,2,3-Triazoles

Article abstract of DOI:10.1002/chem.201902901

Dehydrogenative C?H arylations of 1,2,3-triazoles were accomplished with the aid of a reusable palladium catalyst in PEG. The widely applicable oxidative palladium catalysis enabled the synthesis of fully decorated 1,2,3-triazoles with a broad functional-

Full text of DOI:10.1002/chem.201902901

A Journal of
Accepted Article
Title: Reusable Pd@PEG Catalyst for Aerobic Dehydrogenative C–H/
C–H Arylations of 1,2,3-Triazoles
Authors: Francesco Ferlin, Santhivardhana Reddy Yetra, Svenja
Warratz, Luigi Vaccaro, and Lutz Ackermann
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.201902901
Link to VoR: http://dx.doi.org/10.1002/chem.201902901
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COMMUNICATION
Reusable Pd@PEG Catalyst for Aerobic Dehydrogenative C–H/C–
H Arylations of 1,2,3-Triazoles
Francesco Ferlin,[a][b] Santhivardhana Reddy Yetra,[a] Svenja Warratz,[a] Luigi Vaccaro,[b] and Lutz
Ackermann*[a]
Abstract: Dehydrogenative C–H arylations of 1,2,3-triazoles were
accomplished with the aid of a reusable palladium catalyst in PEG.
The widely applicable oxidative palladium-catalysis enabled the
synthesis of fully decorated 1,2,3-triazoles with a broad functional
group tolerance and ample substrate scope. The sustainability of the
aerobic C–H arylation was reflected by the use of PEG as green
reaction medium and demonstrated by recycling studies of the
catalyst and the reaction medium.
In order to satisfy both recyclability and environmental concerns,
an efficient approach is to immobilize a homogeneous catalyst in
a liquid phase by dissolving it into a non-volatile and non-miscible
liquid, such as ionic liquids[8] or poly(ethylene glycols) (PEGs).[9]
This strategy combines the benefits of homogeneous catalysis
with the possibility to recover and reuse not only the catalyst but
also the reaction medium. Although ionic liquids[10] provide
notable advantages, the tedious preparation of ionic liquids is a
main disadvantage and their environmental safety is still debated
as especially the first generation of ionic liquids are critical in
terms of toxicity.[11] It should be kept in mind, that the reaction
media constitutes the largest part of the waste generated in
chemical productions.[12] Therefore, a farsighted choice of the
reaction medium plays a pivotal role in establishing sustainable
chemical methodologies.[13] Here, PEGs can play a key role for
substituting toxic and/or flammable solvents.[14] PEGs have a
negligible vapour pressure, are commercially available,
inexpensive, thermally stable, recoverable, and nontoxic, hence
serving as efficient reaction media for environmentally-friendly
and safe chemical reactions.[15] In recent years, PEGs have been
successfully utilized as reaction media for palladium-catalyzed
direct arylations of 1,2,3-triazoles with aryl bromides,[9d] with facile
recyclability of solvents and palladium catalysts. However, to the
best of our knowledge no palladium-catalyzed intramolecular C–
H arylation under oxidative conditions have been reported to date
using PEG as solvent. Within our program on sustainable C–H
activation,[16] we now report on the application of a catalyst
comprising Pd(OAc)2, Cu(OAc)2·H2O,[17] PEG, and O2 as an
effective and reusable catalyst for the intramolecular oxidative C–
H arylation of 1,2,3-triazoles. Notable features of our strategy
include (i) a robust and versatile methodology for aerobic C–H
activation (ii) PEG as green and recyclable reaction medium as
well as (iii) reusable palladium complexes as the catalyst.
1,2,3-Triazoles are structural motifs found in several molecules
endowed with specific properties, which feature applications in
various applied fields of pharmaceutical industries and materials
sciences.[1] As a consequence, there is continued strong demand
for the development of synthetic methods that provide an efficient
access to fully decorated 1,2,3-triazoles. Thus, hetero-fused
triazoles have received considerable attention for their
applications in drug discovery and chemical biology.[2]
During the last two decades transition metal-catalyzed
functionalizations of otherwise inert C−H bonds have been
identified as atom-economic tools to assemble complex organic
molecules with a wide range of different functionalities.[3] Recently,
the dehydrogenative double C−H bond activation catalyzed by
metal catalyst based on palladium,[4] and ruthenium,[5] among
others,[6] have attracted considerable attention because these
methods avoid the tedious lengthy synthesis of prefunctionalized
starting materials thus, providing a streamlining of molecular
syntheses. However, industrial applications of these
homogeneous precious metal catalysts remain challenging due to
the tedious procedures for their separation from the reaction
media. In fact, this step is associated with the production of waste
and consequently
a drawback in terms of chemical and
environmental costs. From a green chemistry perspective, the
development of recyclable and reusable[7] catalysts that allow for
efficient and selective oxidative C−H arylations of a wide range of
substrates is of major interest.
[a]
[b]
F. Ferlin, Dr. S. R. Yetra, Dr. S. Warratz, Prof. Dr. L. Ackermann
Institut für Organische und Biomolekulare Chemie
Georg-August-Universität Göttingen
Tammannstrasse 2, 37077 Göttingen, Germany
E-mail: Lutz.Ackermann@chemie.uni-goettingen.de
Homepage: http://www.ackermann.chemie.uni-goettingen.de/
F. Ferlin, Prof. Dr. L. Vaccaro
Laboratory of Green S.O.C., Dipartimento di Chimica, Biologia e
Biotecnologie
Università degli Studi di Perugia
Via Elce di Sotto, 8-06123 Perugia, Italy
Figure 1. C–H Arylation of 1,2,3-triazoles.
Supporting information for this article is given via a link at the end of
the document.
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At the outset of our studies, we probed the intramolecular C–H
arylation of triazole 1a. The influence of the PEG’s molecular
weight and the palladium source was tested (Table 1, entry 1-3).
High molecular weight PEG-20000 and Pd(OAc)2 emerged as the
best solvent and catalyst, respectively, with pivalic acid providing
optimal performance (entries 5-7 and Table S2 in the Supporting
Information).
results were obtained when using an oxidant consisting of
molecular oxygen and Cu(OAc)2·H2O, allowing for high yields of
isolated product 2a and reducing the amount of pivalic acid
needed (Table 1, entry 7). Notably, the dehydrogenative coupling
did not occur in the absence of the palladium catalyst (entry 10).
As shown in Table 2, the optimal reaction conditions (Table 1,
entry 7) proved to be ideal for the recyclability of the palladium
catalyst with only a slight decrease in the catalytic efficacy. We
also tested the stability and durability of the palladium catalysis in
its use for consecutive runs (Table 2). Therefore, we reused the
Pd/PEG mixture and solely added catalytic amounts of copper
acetate, pivalic acid and the starting materials. Thus, for at least
four consecutive reactions similar results were obtained. Although
results are relevant for a small scale laboratory experiment, we
explored the loss in efficiency of the system. Therefore, to better
understand the effective palladium loading of our catalytic system
we have analysed the solid materials (PEG/Pd) isolated by
precipitation after the reaction by ICP-AES. We were able to thus
show that the loading of palladium basically remained unchanged
(Table 2).
Table 1. Optimization studies for the dehydrogenative C–H
arylation of triazole 1a.a
Table 2. Recovery and reuse of catalytic system with ICP-AES measurements
PEG
MW
Yield
(%)[b]
Entry
[Pd]
Oxidant
Acid
for the palladium loading.
Pd(OAc)2
(5 mol%)
Cu(OAc)2·H2O
(1.0 equiv)
PivOH (5.0
equiv)
Recycling Studies
1
2
2000
20000
20000
20000
20000
20000
20000
20000
20000
20000
47
Loading [Pd] in PEG /
mmol
Pd(OAc)2
(5 mol%)
Cu(OAc)2·H2O
(1.0 equiv)
PivOH (5.0
equiv)
75%
58
52
0,01
0,005
0
50%
25%
0%
Pd(OAc)2
(5 mol%)
Cu(OAc)2·H2O
(20 mol %)/O2
PivOH (5.0
equiv)
3
Pd(OAc)2
(5 mol%)
AgOAc (20
mol %)/O2
PivOH (1.5
equiv)
29
36
43
1
2
3
4
1
2
3
4
4
Pd(OAc)2
(5 mol%)
Cu(OAc)2·H2O
(20 mol %)/O2
MesCO2H
(5.0 equiv)
5
Pd(OAc)2
(5 mol%)
Cu(OAc)2·H2O
(20 mol %) /O2
PivOH (0.5
equiv)
6
Pd(OAc)2
(5 mol%)
Cu(OAc)2·H2O
(20 mol %) /O2
PivOH (1.5
equiv)
7
72
Pd(OAc)2
(5 mol%)
Cu(OAc)2·H2O
(20 mol %) /air
PivOH (1.5
equiv)
8
62
22
Pd(OAc)2
(5 mol%)
Cu(OAc)2·H2O
(1.0 equiv)
---
Run
Yield (%)
72 %
Loading of Pd in PEG
0.011 mmol (0.2% w/w)
0.010 mmol (0.2% w/w)
0.010 mmol (0.2% w/w)
0.011 mmol (0.2% w/w)
9
1
2
3
4
74 %
70 %
68 %
Cu(OAc)2·H2O
(20 mol %)/O2
PivOH (1.5
equiv)
-
10
---
[a] Reaction conditions: 1a (1.0 mmol), Pd(OAc)2 (5.0 mol %),
Cu(OAc)2·H2O (20 mol %), PivOH (1.5 equiv), PEG 20000 (1 g), O2 (1 atm),
140 °C, 16 h. For each consecutive run: 1a (1.0 mmol), Cu(OAc)2·H2O (20
mol %), PivOH (1.5 equiv), O2 (1 atm), 140 °C, 16 h.
[a] Reaction conditions: 1a (0.5 mmol), [Pd], Oxidant, Acid, PEG
(500 mg), 140 °C, 20 h. [b] Isolated yield.
In order to guarantee both recyclability and catalytic efficacy we
further optimized the stoichiometry and the reaction time.
However, among a variety of terminal oxidants, Cu(OAc)2·H2O
enabled more effective oxidative C–H arylations (entries 4-9 and
Table S2 in the Supporting Information). Particularly notable
With the optimized conditions in hand, we explored the
substrate scope of the palladium-catalyzed C–H activation
(Scheme 1). Hence, diversely decorated 1,2,3-triazolo-fused-
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chromenes were regioselectively synthesized in good yields,
even when bearing sensitive halogen-substituents. Remarkably,
the palladium catalyst also proved applicable to the synthesis of
sterically hindered tetra-ortho-substituted biaryl 2e.
Under otherwise identical conditions, we likewise achieved
the synthesis of isoindoline-fused triazoles by dehydrogenative
intramolecular cyclization of 4-substituted N-benzyl-triazoles
(Scheme 2). This transformation was characterized by high
functional group tolerance even when an electron-withdrawing
group was present in the triazole moiety (compound 4d).
Scheme 1. Scope for the synthesis of triazole-fused chromenes.a
[a] Reaction conditions: 3 (0.5 mmol), PEG 20000 (500 mg), Cu(OAc)2·H2O
(20 mol %), PivOH (1.5 equiv), O2 (1 atm), 140 °C, 16 h.
We also performed the cyclodehydrogenative arylation to
provide step-economical access to substituted phenanthro[9,10-
d]triazoles 6 (Scheme 3).
Scheme 3. Synthesis of phenantrene-fused triazoles.a
[a] Reaction conditions: 1 (0.5 mmol), PEG 20000 (500 mg), Cu(OAc)2·H2O
(20 mol %), PivOH (1.5 equiv), O2 (1 atm), 140 °C, 16 h.
Scheme 2. Scope of isoindoline-fused triazoles.a
[a] Reaction conditions: 5 (0.5 mmol), PEG 20000 (500 mg), Cu(OAc)2·H2O
(20 mol %), PivOH (1.5 equiv), O2 (1 atm), 140 °C, 16 h.
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Encouraged by these results we extended our reusable
palladium C–H activation to the synthesis of bio-active derivatives
(Scheme 4). Thereby, we were able to access heterofused-
coumarins 7-8, azepinone-like 9 and steroid-based triazolo-fused
isoindoline 10.
Università degli Studi di Perugia and MIUR are acknowledged for
financial support to the project AMIS, through the program
“Dipartimenti di Eccellenza - 2018-2022.
Keywords: PEG • C–H Activation • Palladium • Oxidation •
Arylation
Scheme 4. Derivatization of bio-active triazole compounds.a
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Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
Francesco Ferlin, Santhivardhana
Reddy Yetra, Svenja Warratz, Luigi
Vaccaro, and Lutz Ackermann*
Page No. – Page No.
Reusable Pd@PEG Catalyst for
Aerobic Dehydrogenative C–H/C–H
Arylations of 1,2,3-Triazoles
Aerobic C–H arylations were accomplished in benign PEG solvent. The easily
reusable catalyst Pd(OAc)2 proved highly effective for the intramolecular oxidative
C–H arylation of 1,2,3-triazoles, thus, enabling a step-economical synthesis of bio-
active derivatives in a sustainable fashion.
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