Copper(I) Complexes Bearing Carbenes beyond Classical N-Heterocyclic Carbenes: Synthesis and Catalytic Activity in "click Chemistry"

Article abstract of DOI:10.1002/adsc.201500453

The synthesis and characterization of abnormal N-heterocyclic carbene, cyclic (alkyl)(amino)carbene, and mesoionic carbene copper(I) complexes are reported. These organometallic species are obtained via a versatile and inexpensive synthetic pathway using

Full text of DOI:10.1002/adsc.201500453

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DOI: 10.1002/adsc.201500453
Copper(I) Complexes Bearing Carbenes Beyond Classical
N-Heterocyclic Carbenes: Synthesis and Catalytic Activity in
“Click Chemistry”
Yannick D. Bidal,a Mathieu Lesieur,a Mohand Melaimi,b Fady Nahra,a
David B. Cordes,a Kasun S. Athukorala Arachchige,a Alexandra M. Z. Slawin,a
Guy Bertrand,b and Catherine S. J. Cazina,*
a
EaStCHEM School of Chemistry, University of St Andrews, St Andrews, KY16 9ST, U.K.
Fax: (+44)-(0)1334-463808; e-mail: cc111@st-andrews.ac.uk
UCSD-CNRS Joint Research Laboratory (UMI 3555), Department of Chemistry and Biochemistry, University of
b
California, San Diego, La Jolla, California 92093-0358, USA
Received: May 7, 2015; Revised: August 24, 2015; Published online: October 14, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500453.
Abstract: The synthesis and characterization of ab- [3+2] cycloaddition of alkynes with azides
normal N-heterocyclic carbene, cyclic (alkyl)(ami- (CuAAC). Outstanding catalytic properties were ob-
no)carbene, and mesoionic carbene copper(I) com- served for the abnormal NHC- and triazolylidene-
plexes are reported. These organometallic species based copper(I) complexes.
are obtained via a versatile and inexpensive synthetic
pathway using readily available reactants, namely Keywords: abnormal carbenes; click chemistry;
copper oxide and iminium salts. The catalytic activity [3+2] cycloaddition; copper; N-heterocyclic car-
of this series of complexes was evaluated in the benes; remote carbenes
Introduction
parts in catalysis.[8a,11] So far, reports on copper com-
plexes bearing such ligands are scarce[12,13] and have
The isolation of an N-heterocyclic carbene (NHC, I) mainly focused on triazol-5-ylidene (IV) deriva-
by Arduengo and co-workers in the early 1990s led to tives.[14] This might be explained by the fact that all
major breakthroughs in organometallic chemistry.[1] In reported synthetic methodologies involve the highly
the course of developing more efficient transition reactive and air- and moisture-sensitive free carbene
metal-based catalytic systems, NHCs have evolved species. In contrast, NHC-Cu(I) complexes are readily
into workhorses as ligands for organometallic com- accessible,[1e–i,15,16] as shown by the versatile, green and
plexes. The efficiency of these ligands for transition cost/user-friendly synthetic strategy as demonstrated
metal catalysts is due to their strong s-electron-donat- with 1CuCl (Scheme 1).[17]
ing properties and highly tunable steric features.[2] In
this context, the synthesis of even more electron-do-
nating ligands is a topic of significant interest in the
organometallic and homogeneous catalysis communi-
ties. During the last decade, the library of stable
cyclic carbenes has been enriched by new species such
as the abnormal carbenes (aNHC, II),[3] cyclic (alkyl)-
(amino)carbenes (CAAC, III),[4] mesoionic carbenes
(MIC, IV),[5] and bis(amino)cyclopropenylidenes
(BAC, V),[6] all of them being more electron-donating
than NHCs (Figure 1).[7,8,9,10]
It has already been shown that in certain instances
metal complexes supported by these novel types of
carbene (II–IV) outperform their NHC (I) counter- Figure 1. Carbenes considered as ligands in this work.
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Scheme 1. Synthesis of [Cu(Cl)(SIMes)] 1CuCl (Mes=mesi-
tyl).
Herein, we report the generalization of this ap-
proach to the preparation of aNHC-, CAAC- and
MIC-Cu(I) complexes. The catalytic activity of these
complexes was evaluated in the [3+2] cycloaddition
of alkynes and azides (CuAAC).[18]
Results and Discussion
We began our study with the metallation of the C-2
substituted imidazolium salt 2HCl, which is the classical
precursor of free aNHC II.[3] When 2HCl was reacted
with copper oxide in refluxing toluene for 24 h,
a clean reaction was observed (Scheme 2). Interest-
ingly 2CuCl was obtained in analytically pure form in
98% yield after simple filtration through Celite and
removal of the volatiles under vacuum. This protocol
represents a dramatic improvement over the previous-
ly reported procedure by Mandal and co-workers
using the free aNHC II.[12]
Scheme 2. Synthesis of complexes 2CuCl
[Dipp=2,6-(di-isopropyl)phenyl].
, 3CuCl and 4CuCl
Similarly, 3CuCl was prepared from the correspond- After recrystallization from dichloromethane/diethyl
ing cyclic iminium 3HCl and copper oxide in refluxing ether, 4CuCl was isolated in a 95% yield.
1,4-dioxane for 24 h (Scheme 2). After work-up and
recrystallization from dichloromethane/pentane, 3CuCl the use of chloride salts, which are not always readily
was obtained in 75% yield. accessible for non-classical NHCs. However, it was
Noteworthy, this synthetic approach necessitates
Conventional heating conditions used for the prep- possible, for all ligands investigated in this study, to
aration of 2CuCl and 3CuCl did not result in full conver- exchange the BF4 anion for ClÀ using Amberlite 402
sion of 4HCl even after 24 h in refluxing solvents. How- IRA Cl-resin.
ever, under microwave irradiation[19] for 2.5 h at 808C
in acetonitrile, the copper complex 4CuCl, bearing
The atom connectivity in copper complexes 2CuCl
CuCl, and 4CuCl was unambiguously established by
,
3
a 1,2,3-triazol-5-ylidene IV, was cleanly obtained. single crystal X-ray diffraction studies (Figure 2). All
Figure 2. Molecular representations of aNHC, CAAC, MIC copper(I) complexes 2CuCl, 3CuCl, and 4CuCl, respectively. Hydro-
gen atoms are omitted for clarity.[22]
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Copper(I) Complexes Bearing Carbenes Beyond Classical N-Heterocyclic Carbenes
Table 1. %VBur and selected bond lengths (Š) and angles (8)
(esd) for complexes 2CuCl–4CuCl
of these reactions was monitored by gas chromatogra-
phy (Figure 4).
.
For the three sets of substrates, excellent conver-
sions were observed within 5 h with all catalysts,
except with [Cu(Cl)(CAAC)] 3CuCl (maximum conver-
sion: 48%). In all cases, aNHC, MIC and BAC copper
complexes 2CuCl, 4CuCl and 5CuCl, respectively, are acti-
vated much faster than the classical NHC analogue
Complex
2CuCl
3CuCl
4CuCl
[a]
%VBur
Cu C
40.5
1.871 (7)
173.53 (17)
44.8
1.895 (6)
174.18 (17)
33.3
1.876 (4)
174.84 (14)
À
À
À
Cl Cu C
[a]
À
Calculated with Cu C=1.9 Š.
1CuCl. The most challenging reaction, phenyl azide
with 1-hexyne, allows a better discrimination of the
catalysts. The aNHC and MIC complexes 2CuCl and
complexes adopt a slightly distorted linear geometry 4CuCl clearly outperform the BAC analogue 5CuCl; 50%
À
À
with Cl Cu C1 angles between 173.53(17)8 and conversion is observed after 0.3, 0.6, and 2.1 h, respec-
174.84(14)8 (Table 1). The Cu C1 bond lengths are tively; note that this conversion is reached only after
À
shorter [1.871(7)–1.895(6) Š] than those found in the 4.0 h with the NHC complex 1CuCl. The main differ-
corresponding unsaturated NHC complexes, such as ence lies in the duration of the initiation period,
[Cu(Cl)(IMes)] and [Cu(Cl)(IPr)][20] [1.956(10) Š and
1.953(8) Š, respectively]; however these bond distan-
2
CuCl <4CuCl !5CuCl !1CuCl
Since the aNHC and MIC complexes 2CuCl and 4CuCl
.
ces are similar to those observed for saturated NHCs performed almost equally well, the latter was chosen
such as in [Cu(Cl)(SIPr)] [1.896(7) Š].[16a,20,21]
to study the scope of the reaction, because of its
The steric hindrance of these ligands was assessed easier preparation. Using the same experimental con-
using the SambVca application (Table 1).[23] ditions as above, a series of six azides and seven ter-
[Cu(Cl)(aNHC)] 2CuCl possesses a moderately congest- minal alkynes were tested, and the reactions were
ed metal centre (%VBur =40.5) despite the presence monitored to assess the time required to reach com-
of the bulky 2,6-(diisopropyl)phenyl N-substituent. pletion (Scheme 3).
The CAAC ligand in 3CuCl is the more sterically de-
The reactions reached completion within 0.5 to 6 h
manding of this series with a %VBur of 44.8. However, with isolated yields ranging from 95 to 99%. As ex-
it remains a relatively small ancillary ligand compared pected, the range of functionalities tolerated is broad
to the commonly used [Cu(Cl)(IPr)] (%VBur
=
and includes nitro (6e), nitrile (6g), ether (6h), car-
49.6).[21a,24] Surprisingly, the triazolylidene ligand in bonyl (6j), alcohol (6k) and amine (6l) functional
4CuCl is significantly smaller (%VBur =33.3). Notewor- groups. The slowest reaction was observed for the for-
thy, because of the dissymmetric environment around mation of triazole 6g, possibly because of the metal
the carbene centre in 2CuCl–4CuCl, these complexes fea- interacting with the cyano group, which is away from
ture both a heavily congested and a more accessible the azide moiety. However, even with this difficult
quadrant, which may very well induce particularly in- substrate, 6g is quantitatively formed within 6 h,
teresting behaviour in catalysis.[25]
showcasing the efficiency of the [Cu(Cl)(MIC)] 4CuCl.
The catalytic activity of complexes 2CuCl–4CuCl was At the other extreme, when 2-ethynylpyridine is used,
studied for the [3+2] cycloaddition of azides with al- heterocycle 6i was obtained quantitatively in only
kynes (CuAAC), which represents the flagship trans- 30 min! In this case, and in contrast with the cyano
formation of “click chemistry”.[26] The results are derivative, the pyridine might template the metal
compared with those obtained with the [Cu(Cl)- close to the reaction centre.
[15a,26b,27]
(SIMes)] 1CuCl
,
and the recently reported
[Cu(Cl)(BAC)] 5CuCl (Figure 3).[28] Three different
substrate sets were chosen. The reactions were con- Conclusions
ducted at room temperature, without solvent, and
using 0.5 mol% of catalyst loading. The kinetic profile In summary, we have generalized the use of Cu2O for
the preparation of a series of copper(I) chloride com-
plexes, featuring strongly basic carbenes. This meth-
odology circumvents the preparation of highly air-sen-
sitive free carbenes. [Cu(Cl)(aNHC)] 2CuCl and
[Cu(Cl)(MIC)] 4CuCl complexes were activated signifi-
cantly faster than their NHC analogues, when pro-
moting [3+2] cycloadditions of a variety of azides
with alkynes. Further investigations into the catalytic
potential of this series of copper complexes are cur-
rently underway in our laboratories.
Figure 3. Previously reported [Cu(Cl)(SIMes)] 1CuCl and
[Cu(Cl)(BAC)] 5CuCl
.
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Yannick D. Bidal et al.
Figure 4. Kinetic profiles of the [3+2] cycloaddition reactions catalyzed by complexes 1–5CuCl (lines are visual aids and not
curve fits). Reaction conditions: Catalyst (0.5 mol%), azide (1.0 mmol), alkyne (1.1 mmol), 258C, solvent-free.
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Copper(I) Complexes Bearing Carbenes Beyond Classical N-Heterocyclic Carbenes
Scheme 3. Scope of the [3+2] cycloaddition of azides and alkynes promoted by complex 4CuCl. Reaction conditions: azide
(1.0 mmol), alkyne (1.1 mmol), 4CuCl (0.5 mol%), 258C, solvent-free. Isolated yields are given, average of two reactions.
a pad of Celite and all volatiles were removed under
vacuum. The residue was dissolved in CH2Cl2 (1 mL) and
Et2O (10 mL) was added. The product was collected by fil-
Experimental Section
tration and washed with Et2O (2”10 mL).
General Considerations
Chloro[1,3-bis(2,6-diisopropylphenyl)-2,4-diphenyl-imida-
zol-5-ylidene]copper(I) (2CuCl): yellow/green solid; yield:
398 mg (98%); 1H NMR (300 MHz, CDCl3, 298 K, TMS):
d=7.50–7.40 (m, 5H, CHAr), 7.22–7.16 (m, 7H, CHAr), 7.06
All reactions were performed under inert atmospheres using
standard Schlenk and glove-box techniques unless stated
otherwise. Solvents were dispensed from a solvent purifica-
tion system. All other reagents were used without further
purification. Microwave-assisted reactions were carried out
3
(t, JH,H =8.3 Hz, 2H, CHAr), 6.88–6.85 (m, 2H, CHAr), 2.72–
2.53 (m, 4H, CH-CH3), 1.43 (d, 3JH,H =6.8 Hz, 6H, CH3),
1
in a CEM Discover microwave. H, 19F-{1H} and 13C-{1H} nu-
3
3
0.96 (d, JH,H =6.8 Hz, 6H, CH3), 0.82 (overlapped d, JH,H
=
clear magnetic resonance (NMR) spectra were recorded on
a Bruker AVANCE 300 or a Bruker AVANCE 400 Ultra-
shield spectrometer using the residual solvent peak as refer-
ence[29] (CDCl3: dH =7.26 ppm, dC =77.16 ppm; C6D6, dH =
7.16 ppm, dC =128.06 ppm and CD3CN, dH =1.94 ppm, dC =
1.32 and 118.26 ppm) at 298 K. Gas chromatography (GC)
analyses were performed on an Agilent 7890A apparatus
equipped with a flame ionization detector and a (5%
phenyl)-methylpolysiloxane column (30 m, 320 mm, film:
0.25 mm).
6.8 Hz, 6H, CH3), 0.80 (overlapped d, 3JH,H =6.8 Hz, 6H,
CH3); 13C-{1H} NMR (75 MHz, CDCl3, 298 K, TMS): d=
159.5 (C-Cu), 145.0 (CIV), 144.6 (CIV), 144.4 (CIV), 141.7
(CIV), 135.8 (CIV), 131.1 (CHAr), 131.0 (CHAr), 130.6 (CHAr),
130.5 (CHAr), 130.4 (CIV), 129.5 (CHAr), 129.0 (CHAr), 128.3
(CHAr), 128.2 (CHAr), 128.0 (CHAr), 125.3 (CHAr), 124.6
(CIV), 123.7 (CIV), 28.9 (CHCH3), 28.6 (CHCH3), 25.9
(CH3), 23.7 (CH3), 23.5 (CH3), 22.6 (CH3); elemental analy-
sis calcd. (%) for C39H44ClCuN2 : C 73.22, H 6.93, N 4.38;
found: C 73.15, H 7.02, N 4.45.
Chloro[2-(2,6-diisopropylphenyl)-3,3-(dimethyl)-2-azaspi-
ro[4.5]dec-1-ylidene]copper(I) (3CuCl): pale pink solid; yield:
220 mg (75%). H NMR (300 MHz, C6D6, 298 K): d=7.16–
Synthesis of 2CuCl, 3CuCl and 4CuCl
1
7.09 (m, 1H, CHAr), 7.00–6.98 (m, 2H, CHAr), 2.66 (septet,
In a glove-box, a vial was charged with the ligand salt, Cu2O
(0.65 equiv.) and the appropriate volume of solvent. The re-
action mixture was stirred during the given time period at
the indicated temperature (either under microwave or ther-
mal heating). The reaction mixture was filtered through
3JH,H =6.8 Hz, 2H, CHCH3), 2.00–1.94 (m, 2H, H2CCy), 1.57–
3
1.54 (m, 2H, H2CCy), 1.41 (d, JH,H =6.8 Hz, 6H, CH3CH),
1.38 (s, 2H, CH2), 1.13–1.00 (m, 12H, H2CCy and CH3CH),
0.78 [s, 6H, C(CH3)2]; 13C-{1H} NMR (75 MHz, C6D6,
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Yannick D. Bidal et al.
298 K): d=249.9 (C-Cu), 145.3 (CIV), 134.6 (CIV), 130.1
(CHAr), 124.9 (CHAr), 79.8 (CIV), 59.2 (CIV), 45.7 (CH2), 36.0
(H2CCy), 29.3 (CH-CH3), 29.1 (CH3), 27.2 (CH3CH), 25.2
(H2CCy), 22.5 (H2CCy), 22.2 (CH3CH); elemental analysis
calcd. (%) for C23H35ClCuN: C 65.07, H 8.31, N 3.30; found:
C 64.93, H 8.35, N 3.39.
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Chloro[1-(2,6-diisopropylphenyl)-3-(methyl)-4-(4-tert-bu-
tylphenyl)-1,2,3-triazol-5-ylidene]copper(I) (4CuCl): colour-
1
less solid; yield: 438 mg (95%); H NMR (400 MHz, CDCl3,
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3
298 K, TMS): d=7.69 (d, JH,H =8.4 Hz, 2H, CHAr), 7.56 (d,
3
3JH,H =8.4 Hz, 2H, CHAr), 7.52 (t, JH,H =8.1 Hz, 1H, CHAr),
3
7.30 (d, JH,H =8.1 Hz, 2H, CHAr), 4.24 (s, 3H, NCH3), 2.31
3
(septet, JH,H =6.8 Hz, 2H, CHCH3), 1.37 (s, 9H, CH3), 1.30
3
3
(d, JH,H =6.8 Hz, 6H, CH3CH), 1.16 (d, JH,H =6.8 Hz, 6H,
CH3CH); 13C-{1H} NMR (101 MHz, CDCl3, 298 K, TMS):
d=166.4 (Cu-Ctriazolylidene), 153.6 (CIV), 148.6 (CIV), 144.9
(CIV), 135.7 (CIV), 131.1 (CHAr), 129.2 (CHAr), 126.5 (CHAr),
124.1 (CHAr), 123.9 (CIV), 37.7 (N-CH3), 34.9 (CIV), 31.2
(CHCH3), 28.6 (CH3), 24.3 (CH3CH), 24.2 (CH3CH); ele-
mental analysis calcd. (%) for C25H33ClCuN3 : C 63.27, H
7.01, N 8.85; found: C 63.15, H 7.16, N 8.72.
General Procedure for Catalysis
[5] G. Guisado-Barrios, J. Bouffard, B. Donnadieu, G. Ber-
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A vial was charged with the azide (1.00 mmol), alkyne
(1.10 mmol) and the appropriate catalyst (0.5 mol%). The
reaction mixture was stirred at 258C for the appropriate
amount of time in solvent-free conditions. The conversion
was monitored by GC analysis.
[6] V. Lavallo, Y. Canac, B. Donnadieu, W. W. Schoeller,
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Supporting Information
Synthesis and characterization of ligands and copper com-
plexes 2CuCl–4CuCl, 3D mapping and calculations of %VBur
,
[8] For reviews on non-conventional carbenes, see: a) M.
Albrecht, Chem. Commun. 2008, 3601–3610; b) O.
Schuster, L. Yang, H. G. Raubenheimer, M. Albrecht,
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Soleilhavoup, G. Bertrand, Angew. Chem. 2010, 122,
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d) A. Poulain, M. Iglesias, M. Albrecht, Curr. Org.
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synthesis and characterization of substrates and triazoles
1
(6), general procedures for catalysis, H and 13C-{1H} NMR
spectra of all ligands, complexes, substrates and catalysis
products.
Acknowledgements
The authors gratefully acknowledge the Royal Society (Uni-
versity Research Fellowship to CSJC) and the DOE (DE-
FG02-13ER16370) for financial support. We also thank the
EPSCR National Spectrometry Service Centre in Swansea for
HR-MS analyses.
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