Are Cu(I)-mesoionic NHC carbenes associated with nitrogen additives the best Cu-carbene catalysts for the azide-alkyne click reaction in solution? A case study

Article abstract of DOI:10.1016/j.tetlet.2013.01.054

Copper(I)-catalyzed azide alkyne cycloaddition is now a widely used tool for the synthesis of elaborated compounds. Until now, few stable and efficient copper(I) catalysts are available despite the limitations inherent to approaches employing the in situ reduction of a Cu^II species to deliver the catalytic system. We report that the combination of a copper(I)-mesoionic N-heterocyclic carbene (MIC) with a phenanthroline derivative generates a highly active catalyst, functioning in aqueous alcoholic solvent, without the intervention of a reducing agent. This catalytic system surpasses related 'normal' N-heterocyclic carbene-based systems in their efficiency.

Full text of DOI:10.1016/j.tetlet.2013.01.054

Tetrahedron Letters 54 (2013) 1808–1812
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Are Cu(I)-mesoionic NHC carbenes associated with nitrogen additives the best
Cu-carbene catalysts for the azide–alkyne click reaction in solution? A
case study
Stephan Hohloch a, Biprajit Sarkar a, , Lionel Nauton b,c, Federico Cisnetti b,c, Arnaud Gautier b,c,
⇑
⇑
a Institut für Chemie und Biochemie, Anorganische Chemie, Freie Universität Berlin, Fabeckstraße 34-36, D-14195 Berlin, Germany
b Clermont Université, Université Blaise Pascal, Institut de Chimie de Clermont-Ferrand, BP 10448, F-63000 Clermont-Ferrand, France
c CNRS, UMR 6296, ICCF, F-63171 Aubiere, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Copper(I)-catalyzed azide alkyne cycloaddition is now a widely used tool for the synthesis of elaborated
compounds. Until now, few stable and efficient copper(I) catalysts are available despite the limitations
inherent to approaches employing the in situ reduction of a CuII species to deliver the catalytic system.
We report that the combination of a copper(I)-mesoionic N-heterocyclic carbene (MIC) with a phenan-
throline derivative generates a highly active catalyst, functioning in aqueous alcoholic solvent, without
the intervention of a reducing agent. This catalytic system surpasses related ‘normal’ N-heterocyclic car-
bene-based systems in their efficiency.
Received 25 October 2012
Revised 10 January 2013
Accepted 14 January 2013
Available online 30 January 2013
Keywords:
N-heterocyclic carbene
Mesoionic carbene
Abnormal carbene
CuAAC
Ó 2013 Elsevier Ltd. All rights reserved.
Click Chemistry
‘Click chemistry’ is defined as a paradigm for organic synthesis
that invokes only simple, high-yielding, and easily workable trans-
formations.1 This has facilitated an extraordinary increase in the
number and variety of molecules available for coordination and
organometallic chemistry (ligand design, sensing studies), mag-
netic switching, supramolecular chemistry, therapeutics, biology,
materials science, and catalysis.2 Among ‘click’ reactions, the cop-
per(I)-catalyzed azide alkyne cycloaddition (CuAAC)—a catalytic
version of the Huisgen cycloaddition—has received unrivalled
attention.3 The most popular CuAAC catalytic systems consist of
a combination of copper(II) and nitrogen-based ligands (TBTA,
THPTA, bathophenanthroline. . .) in the presence of a sacrificial
reducing reagent (ascorbic acid or TCEP [tris(2-carboxyethyl)-
phosphane]) to obtain the active copper(I) species (Scheme 1).4
Alternatively, well-defined stable copper(I) complexes includ-
ing N-, P-, S-, and C-based ligands can be employed. Therefore a
pre-reduction step is not required, which constitutes an obvious
advantage (Scheme 2).5,6
chemist’s toolbox due to their outstanding binding affinities to
main group elements and transition metals, including copper(I).
Indeed, the use of copper(I) complexes of NHC ligands as catalysts
for CuAAC—following the pioneering work by Nolan and Diéz-
Gonzaléz has turned out to be highly efficient.6 Importantly,
Cu(I)-NHCs are bench-stable complexes that are not easily
oxidized in solution under reaction conditions, allowing
reducing-agent free reaction procedures. Noteworthy, undesired
Cu(I)-induced free radical side reactions as well as back addition
of reduced ascorbic acid do not occur with Cu(I)-NHC catalyst, thus
allowing reaction to proceed with sensitive substrates such as
peptides.6l These discoveries have stimulated researches on the
catalytic potential of Cu(I)-MIC systems in the context of the click
chemistry and it turned out that these systems are excellent
catalysts.6o,p
Some of us have reported that the scope of the Cu(I)-NHCs,
especially with NHC = SIMes, can be extended by adding additional
aromatic N-donor ligands such as 1,10-phenanthroline or 4,7-di-
chloro-1,10-phenanthroline.6j,k Using the latter additive, the solu-
bility of the catalyst is improved in various solvents and the
cycloaddition rate increases dramatically. Therefore, the CuAAC
reactions could thus be performed under conditions other than
neat or ‘on water’. A lengthening of the chloride–copper bond—as
Normal and mesoionic (MIC), also called abnormal
N-heterocyclic carbenes, a-NHCs (NHCs derived from imidazole
and MIC or a-NHCs derived from 1,2,3-1H-triazole, respectively)
have recently gained a prominent place in the organometallic
evidenced both by DFT6k and X-ray crystallographic data6m
—
⇑
Corresponding authors. Tel.: +33 473407646 (A.G.).
E-mail addresses: Biprajit.Sarkar@fu-berlin.de (B. Sarkar), arnaud.gautier@univ-
reflects the decrease of this bond strength and thus an easier
hydrolysis affording the actual catalytic species. Naturally, we
bpclermont.fr (A. Gautier).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.01.054

S. Hohloch et al. / Tetrahedron Letters 54 (2013) 1808–1812
1809
SO3Na
NaO3S
OH
N
N
N
N
N
N
N
N
3
3
N
N
Bathophenanthroline
TBTA
THPTA
Scheme 1. Nitrogen ligands for Cu(II).
N(C18H37 2
)
R''
N
N
N
NMe2
R
N
N R'
[CuBr(PPh3)3]
R
N
R'
(C18H37)2N
SH
N(C18H37 2
)
imidazole-derived triazole-derived
a-NHCs
C186tren
NHCs
Scheme 2. Examples of N, P-, S-, and C-ligands for Cu(I).
were interested in testing the effect of combining Cu(I)-MICs and
several nitrogen donors, expecting a marked enhancement of the
catalytic efficiency.
phenathroline) is the best additive for both Cu(I)-MICs. The
catalytic system was tested on electron rich and poor alkynes as
depicted in Table 2.
We first tested the effect of such a combination with the Cu(I)-
a-NHCs (1) and (2),6m,7 combined with additives 3–6 to catalyze
the reaction of phenylacetylene 7 with benzyl azide 8 to form 1-
benzyl-4-phenyl-1,2,3-1H-triazole (9) in tert-butanol/water (2/1)
at room temperature (Scheme 3, Table 1).
The reactions works equally well with rich (entry 2) or poor (en-
try 3) alkynes. In the case of 2-ethynyl anisole an exothermic reac-
tion takes place with a completion (tlc) reached in few minutes.
To get more information about the compared efficiency of cop-
per(I) systems containing normal and mesoionic NHCs, the forma-
tion of triazole 9 in wet methanol was followed by 1H NMR with
the systems 1+5 and 12+5 and compared with the stable complex
According to the click criteria, no precaution was taken to ex-
clude oxygen. To discriminate between the catalysts, the reaction
time was fixed at 4 h (except for the blank experiments, entries
1–2) and the product (9) was recovered by a simple filtration.
Regardless the reaction time (entries 1–4), a low conversion
was observed with both Cu(I)-MICs 1 and 2 in solution. In the pres-
ence of 0.5 mol % of additive 3 the solution became immediately
yellow-orange, this color being indicative of the formation of a sol-
uble copper(I)–phenanthroline complex. This resulted in improved
isolated yields of 9 with 30% and 60% being recovered after only 4 h
reaction time (entries 5 and 6). After these experiments, the effect
of substituents on the phenanthroline ligands was considered.
Introduction of electron-donating methoxy groups (additive 4) re-
sulted in a deleterious effect (entries 7, 8). Indeed, the reaction
mixture turned rapidly greenish indicating an oxidation at the cop-
per center. On the contrary, additive 5, possessing two electron-
withdrawing chlorides allowed a rapid and high-yielding reaction
(86% and 81%, entries 9 and 10). Entries 11 and 12 show that
1,10-bipyridine 6 did not allow a good conversion. Overall, similarly
to the NHC series, these results point out that 5 (4,7-dichloro-1,10-
Table 1
Screening of the catalytic systema
Entry
Catalystb
Additivec
Time (h)
Isolated yield
1
2
3
4
5
6
7
8
1
2
1
2
1
2
1
2
1
2
1
2
None
None
None
None
3
3
4
4
5
5
6
6
24
24
4
4
4
4
4
4
4
20%
5%
Traces
Traces
62%
30%
17%
14%
86%
81%
33%
18%
9
10
11
12
4
4
4
a
b
c
All reaction performed in tert-butanol/water (2/1) at room temperature.
Catalyst loading: 0.5 mol %.
Additive: 0.5 mol %.
MeO
Cl
OMe
Additives:
Catalyst:
ICu
N
N
N
N
N
N
N
ICu
3
4
N
Cl
N
N
N
N
S
N
N
5
6
1
2
8
N
N
N
+
N3
7
9
Scheme 3. CuAAC reactions in solutions with NHC catalysts and N-donors as additives.

1810
S. Hohloch et al. / Tetrahedron Letters 54 (2013) 1808–1812
Table 2
Examples of synthesized triazolesa
Entry
Isolated yield (%)
N
N
N
1
2
91
9
N
N
N
MeO
86
10
Figure 2. Views of complexes 1+5 (left, DFT) and 13 (right, X-ray)6m (gray: carbon,
green chloride, blue: nitrogen, orange: copper and purple: iodide).
N
N
N
3
95
Table 3
Selected distances (Å) and %Vbur
11
O2N
1+5 (DFT)
13 (X-ray)6m
C–N
C–Cu
%Vbur (d = 2.8 Å)
%Vbur (d = 3.5 Å)
2.05
1.92
31.6
30.7
2.12
1.90
35.2
37.2
a
All reactions performed in tert-butanol/water (2/1) at room temperature during
24 h; catalyst and additive loading: 0.5 mol %.
13 (conversion vs time are plotted in Fig. 1).8,6k The complex 12
corresponds to the iodo derivative of the Fukuzawa’s catalyst
[CuCl(TMes)].6o
All the kinetic curves exhibit an inductive period after which an
acceleration of the catalysis is observed. The mesoionic systems
1+5 and 12+5 displayed higher reaction rates than the ‘classical’
NHC 13. This observation could result from the strong
r-donor
character of MICs.6q
CuI
N
N
N
N
N
Cl
1
Cl
Cl
or
Cu
versus
N
N
+
N
N
13
5
Cl
Cl
N
CuI
N
N
N
Methanol
Catalyst
+
N
N
N3
12
(0.5 to 1 mol-%)
7
8
9
Figure 1. Kinetics of the formation of 9 at 298 K in methanol.

S. Hohloch et al. / Tetrahedron Letters 54 (2013) 1808–1812
1811
I
Cl
I
N
N
Cl
Cu
N
N
N
N
R1
N
Cu
N
Cl
N
N
N
N
N
Cl
1+5
H
R2
R1
[1+5] +
18
H
H
R1
Cu
N
R1
14
N
N
N
N
N
Cu
N
N
N
N
N
N
R2
R2
17
R1
N
R1
N
Cu
N
N
Cu
N
N
N
N
N
N
R2
N
N
R2
15
16
Scheme 4. Catalytic cycle leading to the formation of 1,2,3-triazole.
Among the mesoionic systems 1+5 (1.5 mol %) is highly active
as a total conversion is reached before we were able to record
the first scan during the NMR experiment (2 min). However, mea-
surements become possible by lowering the amount of catalyst to
0.5 mol %. Under this condition, a total completion is observed
after 5 min. The second mesoionic system (12+5) is also an excel-
lent catalyst but its performance stays below.
responsible for all the inductive periods observed in Figure 1. The
resulting cationic copper(I) MIC–phenanthroline complex reacts
with an alkyne to form the neutral copper–acetylide species 14.
Association with an azide yields 15 which forms transiently metal-
lacycle 16, then triazolide 17,12 which upon protonation affords the
expected triazole 18 and regenerates the cationic catalyst [1+5]
(Scheme 4).
In order to accede to the catalyst’s structure we tried to grow X-
ray suitable crystals from an equimolar mixture of 1 and 5, but
unfortunately, the crystallization attempts were not successful.
Therefore, DFT calculations for the complex formed by the associ-
ation of 1 with 5 were performed in the gas phase9 and compared
with the X-ray structure of 13 (Fig. 2).6m
In both cases, the copper(I) atom adopts a distorted tetrahedral
geometry. The comparison of the Cu–N and Cu–C distances for the
two complexes shows a fairly similitude (Table 3). A simple glance
at the copper environment shows that the ‘wings’ of the NHC com-
plex 13 are oriented toward the metal whereas for the mesoionic
complex, the benzyl group does not efficiently shield the copper
centre. This results in a visibly more crowded environment for
the copper(I) in 13. The %Vbur parameter has been introduced to
measure the steric crowding—that is the accessibility of the metal
center—associated to carbenic ligands.10
In conclusion, we have reported that the association of 1,10-
phenanthroline, especially the 4,7-dichloro derivative, with a cop-
per(I) mesoionic N-heterocyclic carbene affords a catalytic system
that surpasses the performances of a ‘normal’ NHC. This phenom-
enon could be explained by the combination of a greater electron-
donating character of the carbenic ligand6q and a less crowded
environment of the catalytic center.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
01.054.
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.
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Dimethoxybenzene (0.125 mol Lꢀ1
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