An abnormal N-heterocyclic carbene-copper(I) complex in click chemistry

Article abstract of DOI:10.1002/adsc.201300343

Herein we report the synthesis of a copper(I) chloro complex using an abnormal N-heterocyclic carbene (aNHC) salt, 1,3-bis(2,6-diisopropylphenyl)-2,4- diphenylimidazolium. The Cu(aNHC) complex efficiently catalyzed Huisgen 1,3-dipolar cycloaddition reacti

Full text of DOI:10.1002/adsc.201300343

FULL PAPERS
DOI: 10.1002/adsc.201300343
An Abnormal N-Heterocyclic Carbene–Copper(I) Complex in
Click Chemistry
Samaresh Chandra Sau,a Sudipta Raha Roy,a Tamal K. Sen,a Dinesh Mullangi,a
and Swadhin K. Mandala,*
a
Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata, Mohanpur – 741252,
India
Fax : (+91)-33-4809-2033; e-mail: swadhin.mandal@iiserkol.ac.in
Received: April 22, 2013; Revised: August 21, 2013; Published online: October 9, 2013
Dedicated to Prof. K. N. Ganesh on the occasion of his 60th birthday.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300343.
Abstract: Herein we report the synthesis of a cop- at room temperature with the high TON value of
per(I) chloro complex using an abnormal N-hetero- 19,800. The catalyst also exhibits high efficiency in
cyclic carbene (aNHC) salt, 1,3-bis(2,6-diisopropyl- the reaction between sterically hindered azides and
phenyl)-2,4-diphenylimidazolium. The CuClACTHNUGRTENUNG(aNHC) alkynes under solvent-free conditions at room tem-
complex efficiently catalyzed Huisgen 1,3-dipolar cy- perature. Furthermore, a number of internal alkynes
cloaddition reactions (click reactions) of azides with was successfully tested in this copper-catalyzed cyclo-
alkynes to give 1,4-substituted 1,2,3-triazoles in ex- addition reaction for synthesis of 4,5-disubstituted
cellent yields at room temperature within short reac- triazoles.
tion time under solvent-free conditions. The catalyst
successfully activated benzyl azide and phenylacety- Keywords: abnormal carbene; click reaction; cop-
lene under the low catalyst loading of 0.005 mol% per(I) complex; internal alkynes; sterically hindered
resulting in a nearly quantitative yield of the product azides and alkynes
Introduction
Scheme 1 for numbering). In 2001, Crabtree, Faller
and co-workers first reported a new mode of NHC
The isolation of normal N-heterocyclic carbenes binding to a metal ion termed as the abnormal mode
(nNHCs) by Arduengo and co-workers[1] more than of metal binding of NHC through the C-4 centre on
two decades ago has led to numerous exciting discov- treatment of 2-pyridylmethylimidazolium salt with
eries in organometallic chemistry. The N-heterocyclic IrH5ACTHNUTRGNEUNG
(PPh3)2.[3] It is now well established that the ab-
carbenes also have been used as nucleophiles for or- normal N-heterocyclic carbenes have excellent poten-
ganocatalytic reactions efficiently catalyzing a number tial towards different catalytic reactions.[4,5] However,
of organic transformations.[2] The normal NHCs bind the isolated abnormal carbenes have long been con-
to the metal centre through the C-2 carbon atom (see sidered as transient species till the report by Bertrand
Scheme 1. Synthetic scheme leading to abnormal N-heterocyclic carbene copper(I) complex 1.
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An Abnormal N-Heterocyclic Carbene–Copper(I) Complex in Click Chemistry
and co-workers in 2009 when for the first time an ab- quiring the use of any external hazardous organic sol-
normal N-heterocyclic carbene (aNHC) was isolated vent. The catalyst remained active in 10 successive
by chemically blocking the C-2 position and the iso- catalytic cycles towards aliphatic as well as aromatic
lated abnormal NHC was characterized by single crys- azide substrates. Furthermore, the catalyst successful-
tal X-ray diffraction.[6] This particular report led to ly catalyzed the reaction between sterically hindered
the anticipation that the access to this type of abnor- azides and alkynes at room temperature under sol-
mal NHC would lead to new cornerstones in the field vent-free conditions. The synthesis of 4,5-disubstituted
of organometallic and organocatalysis.[7] Metal coordi- triazoles was also achieved by activating different in-
nated aNHCs had earlier documented excellent po- ternal alkynes using catalyst 1 within short reaction
tential to be utilized as catalysts, resulting the activa- times.
À
À
tion of unreactive bonds such as C H, H H and, in
some cases, the aNHC coordinated metal complexes
have outperformed the catalytic activity as compared Results and Discussion
to their normal NHC counterparts (nNHC).[8] In this
regard, earlier we had shown that the abnormal N- The imidazolium salt, IA (Scheme 1) was employed
heterocyclic carbene [1,3-bis(2,6-diisopropylphenyl)- for the current study as an aNHC precursor to pre-
2,4-diphenyl-5-ylidene] can be used very efficiently in pare the copper complex. Complex 1 was prepared by
metal-free ring-opening polymerization of three dif- treatment of the aNHC salt (IA) with potassium bis-
ferent cyclic esters leading to the development of the
ACHTUNGTREN(NGNU trimethylsilyl)amide and copper(I) chloride in THF.
best organocatalyst in terms of activity for ring-open- During the reaction, the colour of the reaction mix-
ing polymerization catalysis among any NHCs.[2c] Re- ture changes from colourless to green. Analytically
cently, we have used the same aNHC to synthesize pure compound of 1 was obtained by recrystallization
À
halobridged C H activated palladium dimers for of the dried reaction mixture from a dichloromethane
Suzuki–Miyaura cross-couplings of challenging aryl (DCM)/pentane mixture yielding light green coloured
chloride substrates delivering excellent yields at room crystals of 1 in nearly 70% yield. The complex 1 was
temperature under
a
very low-catalyst loading characterized by NMR spectroscopy, X-ray crystallog-
1
(0.005 mol%).[5] Now with the present study, we raphy, and elemental analysis. The H NMR spectrum
report the synthesis and characterization of an aNHC of 1 featured the absence of the characteristic singlet
coordinated copper(I) chloro complex 1 using 1,3- at d=8.8 ppm arising from C-5(H) of imidazolium
bis(2,6-diisopropylphenyl)-2,4-diphenyl-imidazolium
salt, IA confirming the abnormal mode of copper
salt IA. Ther copmplex I is capable of transforming binding. The 13C NMR spectrum revealed a singlet
a range of versatile click reaction, also known as peak at d=159.4 ppm assigned to the C-5 carbon res-
Huisgen 1,3-dipolar cycloadditions, under solvent-free onance bound to the copper centre. This NMR chemi-
condition. The Huisgen 1,3-dipolar cycloaddition[9,10] cal shift value is comparable to that reported with
originally introduced by Sharpless and co-workers in a copper carbene complex CuCl
ACHTUNGTRENNUNG
2001,[11] has been popularized as “click” reaction[12] methyl-1,4-diphenyl-1,2,3-triazol-5-ylidene)
and very often applied to chemical synthesis, biology, a metal-bound carbene carbon appeared around d=
and material sciences.[13] The preparation of cyclic 161.3 ppm in the 13C NMR spectrum.[12e] Finally, the
compounds from the click reaction and their applica- molecular structure determined by the X-ray crystal-
tion as ligands for transition metal chemistry have lographic study (see Figure 1) confirmed the atom
been reported by various research groups.[14] Howev- connectivity of 1 as depicted in Scheme 1. Compound
¯
er, the activation of sterically hindered azides and al- 1 crystallizes in the triclinic space group P1. The ge-
kynes in click reactions has been limited under sol- ometry around the copper ion is linear exhibiting the
À
vent-free conditions, which usually require elevated abnormal mode of NHC binding in 1. The Cu C and
temperatures.[12e] On the other hand, the synthesis of the Cu Cl bond lengths were found to be 1.882(2) ꢁ
À
4,5-disubstituted triazoles can be achieved by using in- and 2.1031(10) ꢁ, respectively. These values compare
ternal alkyne substrates which usually require long well with those observed [1.881(7) ꢁ and 2.106(2) ꢁ,
time and thermal activation.[12c,15–18] Thus the activa- for Cu C and Cu Cl, respectively] in a related NHC
À
À
tion of sterically hindered azides and alkynes as well copper(I) complex, CuClACTHNUTRGNE(NUG IPr) [IPr=1,3-bis(2,6-diiso-
as the activation of internal alkynes through click re- propylphenyl)imidazol-2-ylidene].[19] The C Cu Cl
actions at room temperature under solvent-free condi- bond angle was determined as 173.65(6)8 indicating
tions within short reaction times has remained among a nearly linear geometry around the Cu(I) centre in
the great challenges in this area. In the present study, complex 1. As a part of our ongoing interest to devel-
the catalyst 1 yielded the Huisgen 1,3-dipolar cycload- op aNHC based homogeneous catalyst for different
dition product, 1,4-substituted 1,2,3-triazoles in excel- organic transformations,[2c,5] we first evaluated the cat-
lent yields at room temperature within very short re- alytic activity of aNHC copper(I) complex 1 for the
action times. The catalysis went smoothly without re- click reaction of benzyl azide with phenylacetylene
À
À
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1
was monitored by H NMR spectroscopy which re-
veals that within first 15 min, the reaction reaches to
completion (Figure 2).
As a control experiment, we also carried out the
catalytic reaction maintaining identical reaction con-
ditions using CuCl as catalyst (loading 1 mol%) under
solvent-free conditions with benzyl azide and phenyl-
acetylene as substrates. Even after 60 min, CuCl pro-
duced only a trace amount of 4a (less than 5%), an
observation also noted in earlier studies.[12e,g] Further-
more, we investigated the scope of the reaction by
carrying out reactions of benzyl azide and 4-bromo-
benzyl azide with a variety of alkynes using 1 mol%
of 1 as catalyst under solvent-free conditions and the
results of the catalytic reactions are summarized in
Table 1. The reactions were carried out at room tem-
perature in the absence of any solvent with a catalyst
loading of 1.0 mol%. Also a few representative sub-
strates were cyclized using the low catalyst loading of
0.1 mol% resulting in nearly quantitative yields under
solvent-free conditions which led to the products 4a
(60 min, 99%), 4o (3 h, 91%) and 4s (7 h, 90%) re-
quiring longer times as compared with the 1 mol%
catalyst loading conditions. During the catalysis under
Figure 1. Perspective ORTEP view of the molecular struc-
ture of 1. Thermal ellipsoids are drawn with 50% probabili-
ty. Hydrogen atoms and lattice held solvent molecule (di-
chloromethane) have been omitted for the sake of clarity.
Selected bond lengths (in ꢁ) and bond angles (8) for 1:
À
À
À
À
Cu1 C5, 1.882(2); Cu1 Cl1, 2.1031(10); C5 Cu1 Cl1,
À
À
À
À
173.65(6);
127.05(15).
N1 C5 Cu1,
128.31(14);
C4 C5 Cu1, solvent free condition, the initial liquid reaction mix-
ture solidifies after the consumption of substrates (see
the Supporting Information, video clips). Analytically
pure product was isolated as a solid material after ini-
without adding any solvent. The reaction went on tial work-up with dichloromethane/water. The results
smoothly leading to quantitative conversion into presented in Table 1 establish that the catalyst 1 is
product 4a (Figure 2). The progress of the reaction very active in the click reaction. It successfully works
for both electron-rich as well as electron-poor al-
kynes. Also the catalyst has sufficient functional
group tolerance as indicated in the case of products
(4c, 4d, 4g, 4h, 4o, 4r, 4s, 4t, 4u, 4x, 4y, 4z, 4aa, 4ab,
4ac and 4ae; Table 1). Cycloadditions with phenylace-
tylene as well as with electron-deficient fluoro-substi-
tuted phenylacetylene proceeded smoothly to give the
corresponding 1,2,3-triazoles in almost quantitative
yield in a short period of time (4a, 4d, 4e, and 4h;
Table 1 within 30 min). The reactions are compara-
tively slow (reaction completion time 45 min to 2 h)
for electron-rich alkynes such as alkyl-substituted al-
kynes, and trimethylsilylacetylene (4i, 4j, 4k, 4l, 4m,
4n, 4o, 4p, 4v and 4w; Table 1). Functional groups
such as hydroxy, amine, amide, ester, acid and differ-
ent halides were tolerated (4r, 4s, 4t, 4, 4x, 4y, 4z, 4aa,
4ab, 4ac and 4ae; Table 1), during the reaction requir-
ing relatively longer reaction times (2–5 h). Our cata-
lyst also survives 2-ethynylpyridine in the reaction
medium to afford the corresponding triazole product
(4ad, Table 1) after 3 h at room temperature. We then
investigated the scope of this catalytic system to acti-
vate electron-rich as well as electron-poor alkynes in
the presence of aryl azides instead of aliphatic azides.
Results are presented in Table 2. All reactions pro-
ceeded smoothly to completion within short reaction
Figure 2. Plot of conversion [%] versus time (min) in the
coupling of phenylacetylene (1.73 mmol) with benzyl azide
(1.57 mmol) at 258C using catalyst 1 (1 mol%).
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An Abnormal N-Heterocyclic Carbene–Copper(I) Complex in Click Chemistry
Table 1. Solvent-free route to triazoles by click chemistry using 1 as catalyst and aliphatic azide as substrate.[a]
Entry
Azide
Alkyne
Product
Time
Yield[b] [%]
1
2
3
4
5
6
7
8
R1 =H
R1 =H
R1 =H
R1 =H
R1 =Br
R1 =Br
R1 =Br
R1 =Br
R2 =H
4a
4b
4c
4d
4e
4f
4g
4h
20 min
30 min
20 min
5 min
30 min
40 min
30 min
10 min
99
98
98
98
98
97
98
98
R2 =Me
R2 =OMe
R2 =F
R2 =H
R2 =Me
R2 =OMe
R2 =F
9
R1 =H
R1 =H
R1 =H
R1 =Br
R1 =Br
R1 =Br
n=2
n=4
n=6
n=2
n=4
n=6
4i
4j
4k
4l
4m
4n
45 min
45 min
45 min
1 h
1 h
1 h
98
98
98
98
98
98
10
11
12
13
14
15
16
17
R1 =H
R1 =H
R1 =Br
X=Si
X=C
X=C
4o
4p
4q
45 min
2 h
3 h
97
97
96
18
19
20
21
R1 =H
R1 =H
R1 =Br
R1 =Br
R3 =H
4r
4s
4t
4u
2 h
2 h
3 h
3 h
96
96
96
96
R3 =Me
R3 =H
R3 =Me
22
23
R1 =H
R1 =Br
4v
4w
45 min
1 h
99
97
24
25
26
R1 =H
R1 =H
R1 =H
R3 =Cl
4x
4y
4z
5 h
5 h
4 h
75[c]
73[c]
98[d]
R3 =Br
R3 =NH2
27
R1 =H
R3 =NH2
4aa
2 h
93
times and the triazoles 6a–6l were isolated in excel- cally crowded azides such as 2,6-dimethylazide (6c,
lent yield with high purity after simple work-up with 6d, 6e and 6k; Table 2) and mesityl azide (6f, 6g, 6h
dichloromethane/water (see Experimental Section). and 6l; Table 2) substrates. Encouraged by these re-
Table 2 showcases that in the presence of catalyst 1, sults, we attempted reactions of sterically hindered al-
phenyl azides react and yield the triazole products kynes with sterically hindered azides using catalyst 1.
(6a, 6b, 6i and 6j; Table 2) in very short reaction The results are shown in Scheme 2. Generally for
times. The reactions are comparatively slow for steri- such challenging sterically hindered substrates, one
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Table 1. (Continued)
Entry
Azide
Alkyne
Product
Time
Yield[b] [%]
28
29
R1 =H
R1 =H
R3 =OMe
R3 =OH
4ab
4ac
15 min
15 min
97
83
30
R1 =H
R1 =H
4ad
4ae
3 h
5 h
96
31
93[c]
[a]
Reaction conditions: azide (1.57 mmol), alkyne (1.73 mmol), catalyst 1 (1.0 mol%), 258C.
Isolated yield after initial work-up with dichloromethane/water.
Reaction temperature was 708C.
Reaction was carried out in presence of water at 258C for 4 h. The resulting compound precipitates from water and the
compound was dissolved in mixture of cold (08C) ethanol and few drops of concentrated HCl (12M) to isolate 4z·HCl.
[b]
[c]
[d]
needs t-BuOH/water and a stoichiometric amount of yield 9b in 78% yield. The high activity of catalyst
CuSO4/sodium ascorbate to prepare the triazole 9a by 1 in the Huisgen cycloaddition prompted us to test its
the reaction of mesitylacetylene with mesityl azide reactivity with more challenging internal alkyne sub-
employing Sharpless conditions.[11]
strates.
Although several copper catalysts are being used
In the present study, the reaction using 1 mol% of
1 as catalyst took 5 h at room temperature under sol- regularly to carry out click reactions using the termi-
vent-free conditions to yield 9a in 95% yield, and 24 h nal alkynes but only few studies have been reported
at room temperature under solvent-free conditions to for the activation of internal alkynes leading to the
Table 2. Solvent-free route to triazoles by click chemistry using 1 as catalyst and aryl azide as substrate.[a]
Entry
Azide
Alkyne
Product
Time
Yield[b] [%]
1
2
3
4
5
6
7
8
R1 =R2 =R3 =H
R4 =H
R4 =Me
R4 =H
R4 =Me
R4 =F
6a
6b
6c
6d
6e
6f
6g
6h
10 min
15 min
6 h
7 h
7 h
5 h
6 h
6 h
98
98
94
95
93
95
91
92
R1 =R2 =R3 =H
R1 =R2 =Me, R3 =H
R1 =R2 =Me, R3 =H
R1 =R2 =Me, R3 =H
R1 =R2 =R3 =Me
R1 =R2 =R3 =Me
R1 =R2 =R3 =Me
R4 =H
R4 =Me
R4 =F
9
R1 =R2 =R3 =H
6i
1 h
95
10
11
12
R1 =R2 =R3 =H
6j
6k
6l
1 h
9 h
8 h
94
91
92
R1 =R2 =Me, R3 =H
R1 =R2 =R3 =Me
[a]
Reaction conditions: azide (1.57 mmol), alkyne (1.73 mmol), catalyst 1 (1.0 mol%), 258C.
Isolated yield after initial work-up with dichloromethane/water.
[b]
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An Abnormal N-Heterocyclic Carbene–Copper(I) Complex in Click Chemistry
Scheme 2. Reaction of sterically hindered alkynes with sterically hindered azides to form triazoles in the presence of 1 as
a catalyst. Reaction conditions: azide (1.57 mmol), alkyne (1.73 mmol), catalyst 1 (1.0 mol%), 258C.
4,5-substituted triazole.[12c,15–18] In a preliminary ex- catalytic cycles without adding any extra catalysts (see
periment, we noticed that the treatment of dimethyl Experimental Section), however, after each catalytic
acetylenedicarboxylate or diethyl acetylenedicarboxy- cycle, a fresh batch of substrates was added. The
late with phenyl or benzyl azide in the presence of result of this experiment indicated a sustained activity
1 mol% catalyst 1 yields the corresponding triazole of catalyst 1 for 10 successive cycles (see the Support-
(12a, 12b, and 12c, Scheme 3) in good yield within ing Information for details). Furthermore, we have
short reaction times under solvent-free conditions at
room temperature. We next carried out the reaction
using less activated alkyne substrates (diphenylacety-
lene, 3-hexyne, 1-phenyl-1-butyne) with 1 mol% of
1 as catalyst. The triazoles (12d, 12e, and 12f,
Scheme 3) were formed in fair to good yields after
heating at 708C. The reaction using diphenylacetylene
was complete within 5 h but for other two internal
alkyne substrates, we needed 24 h to obtain good
yields. In the case of unsymmetrical internal alkynes,
we obtained a mixture of two isomers in 2:1 ratio
(12g and 12g’ as well as 12h and 12h’).
We observed no decomposition of the catalysts or
precipitation of metallic copper during all the reac-
tions using 1 as catalyst. The stability of the catalyti-
cally active species was subjected to further investiga-
tion by performing the in situ recycling experiments.
One of the major problems in homogeneous catalysis
is the inability for recycling the catalysts as they are
inseparable from the reaction mixture. Earlier at-
tempts by Fukuzawa, Li, Sarkar and co-workers have
shown that the activity of copper(I) NHC catalysts in
click reactions gradually decreases as the number of
catalytic cycles increases and beyond six cycles the
catalysts were not tested.[12d–f] Herein, we checked the
longevity of the catalyst 1 by performing several cata-
lytic runs into the same reaction pot to check whether
the catalyst remains live[5,20] for several catalytic
cycles. We performed 10 successive catalytic runs by
using 1 mol% of catalyst loading and two different
sets of substrates such as benzyl azide and phenylace-
tylene as well as phenyl azide and phenylacetylene.
Scheme 3. Reaction of azides with internal alkynes to form
4,5-disubstituted triazoles in the presence of 1 as catalyst.
Reaction conditions: azide (1.57 mmol), alkyne (1.73 mmol),
The catalytic reaction was continued for 10 successive catalyst 1 (1.0 mol%).
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performed the kinetic experiments using benzyl azide
To compare the activity of the present catalyst with
and phenylacetylene substrates for each of the 10 suc- those reported in literature, it may be noted that
cessive catalytic cycles. From this measurement, it can there have been few reports of copper catalysts to tol-
be observed that 20 min is enough to obtain quantita- erate a variety of functional groups.[10e,12i–m] The cata-
tive conversion of the substrates into the product for lytic results presented by catalyst 1 (Table 1, Table 2,
the first six catalytic cycles, however, for the 7th–10th Scheme 2, and Scheme 3), we can conclude that the
catalytic cycles, the reaction becomes slow requiring present catalyst is a versatile one which can survive
40 min to obtain similar conversion (see the Support- a range of functional groups and can activate sterical-
ing Information). This result also encouraged us to ly hindered substrates as well as it can activate inter-
check the longevity of the catalyst in the case of click nal alkynes. For example, Fukuzawa and co-workers
reactions between sterically hindered alkynes with have demonstrated that the catalytic reaction using
sterically hindered azides. We performed 5 successive 3 mol% copper(I) 1,2,3-triazol-5-ylidene complex
catalytic runs by using 1 mol% of catalyst 1 to acti- gave 9a in 71% yield within 30 min at 1008C under
vate mesitylacetylene and mesityl azide under sol- neat conditions and yielded 9b in 63% yield in 18 h at
vent-free conditions at room temperature. The result 1008C under solvent-free conditions.[12e] In the present
demonstrates that the catalyst 1 stays active for 5 cat- study, the reaction using only 1 mol% of 1 as catalyst
alytic cycles towards cyclization of mesitylacetylene took 5 h at room temperature under solvent-free con-
and mesityl azide (see the Supporting information). ditions to afford 9a in 95% yield, and 24 h at room
This sustained catalytic activity in consecutive catalyt- temperature under solvent-free condition to afford 9b
ic runs prompted us to check the catalystꢂs ability to in 78% yield, respectively. This result clearly supports
execute the click reaction under lower catalyst load- that the present catalyst is more active than earlier re-
ing. The lower catalyst loading test was performed ported[12e,n,o] catalysts in cyclizing the sterically hin-
using benzyl azide and phenylacetylene as substrates dered challenging substrates at ambient temperature.
under solvent-free conditions at room temperature Furthermore, Nolan and co-workers have earlier
(Table 3). The result in Table 3 indicates that the cata- shown that the reaction using 5 mol% [(SIMes)CuBr]
lyst 1 is active for coupling of benzyl azide and phe- [SIMes=N,N’-bis(2,4,6-trimethylphenyl)-4,5-dihydroi-
nylacetylene with a catalyst loading as low as midazol-2-ylidene] catalyst for 48 h at 708C under
0.005 mol% resulting in a nearly quantitative yield of neat conditions gave 12e in 80% yield.[12c] In the pres-
the product at room temperature which leads to ent study, we used a lower loading of catalyst
a high TON value of 19,800 (Table 3). Also we have 1 (1 mol%) to obtain a similar yield of 12e (80%) at
been able to scale-up the reaction at room tempera- 708C under solvent-free conditions in a faster way (in
ture as well as at 08C. The reaction was carried out 24 h).
using 50 mg (0.078 mmol) of catalyst 1, phenylacety-
lene (1.2 mL, 10.73 mmol), and benzyl azide (1.3 mL,
9.76 mmol) at 08C under solvent-free conditions Conclusions
which required a longer time (3 h) as compared to
that required (20 min) for the similar scale reaction at In summary, we have developed a versatile and highly
room temperature.
efficient catalytic system bearing the abnormal N-het-
erocyclic carbene for the Huisgen 1,3-dipolar cycload-
dition of azides and alkynes at room temperature.
This catalysis can be performed under solvent-free
conditions to get a variety of pure triazoles in good to
excellent yields within short reaction times. Electron-
rich, electron-deficient and different functionalized al-
kynes and aliphatic as well as aromatic azides were
successfully used by this catalytic protocol resulting in
nearly quantitative yield. The catalyst exhibits its ac-
tivity at very low catalyst loadings of up to
0.005 mol% at room temperature resulting in a high
TON value of 19,800. The catalyst can successfully
catalyze the reaction between sterically hindered
azides and sterically hindered alkynes under solvent-
free conditions at ambient temperature. Furthermore,
the catalyst exhibits efficacy for the more challenging
internal alkyne substrates demonstrating its utility in
click chemistry for versatile substrates.
Table 3. Click reactions of benzyl azide and phenylacetylene
using catalyst 1 at different loadings.[a]
Entry Catalyst [mol%] Time [min] Yield[b] [%] TON
1
2
3
4
5
0.50
0.25
0.10
0.01
0.005
30
45
60
180
240
99
99
99
99
99
198
396
990
9900
19800
[a]
Reaction conditions: azide (1.57 mmol), alkyne
(1.73 mmol), catalyst 1 (1.0 mol%), 258C.
Isolated yield after initial work-up with dichloromethane/
water.
[b]
2988
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Adv. Synth. Catal. 2013, 355, 2982 – 2991

An Abnormal N-Heterocyclic Carbene–Copper(I) Complex in Click Chemistry
was applied. The lattice parameters were determined from
Experimental Section
least-squares analysis, and reflection data were integrated
using the program SHELXTL.[23] The crystal structure was
solved by direct method using SHELXS-97 and refined by
full-matrix least-squares refinement on F2 with anisotropic
displacement parameters for non-H atoms using SHELXL-
97.[24] CCDC 881468 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif (see also
the Supporting Information).
Experimental Materials and Instrumentation
All manipulations were performed under a dry and oxygen-
free atmosphere (argon) using standard Schlenk techniques
or inside a glovebox maintained at below 0.1 ppm of O2 and
H2O levels, utilizing oven-dried (1308C) glassware after
evacuation while hot prior to use. All solvents were distilled
from Na/benzophenone prior to use. All other chemicals
were purchased from Sigma–Aldrich and used as received.
The HR-MS data were obtained using a Q-Tof Micromass,
Waters instrument. Elemental analyses were performed in
a Perkin–Elmer 2400, Series II, CHNS/O analyser. The
melting point was measured in a sealed glass tube on
a Bꢃchi B-540 melting point apparatus. Analytical TLC was
performed on a Merck 60 F254 silica gel plate (0.25 mm
thickness). NMR spectra were recorded on a JEOL ECS
General Procedure for the [3+2]Cycloaddition of
Azides and Terminal Alkynes
Alkyne (1.73 mmol), azide (1.57 mmol), and catalyst
1 (10 mg, 1 mol%) were loaded in a 25-mL Schlenk flask.
The reaction mixture was stirred at room temperature.
After an appropriate period of time, the reaction was
quenched by dissolving the reaction mixture in dichlorome-
thane (10 mL). Subsequently, a simple aqueous work-up was
carried out by adding 10 mL of water, and the organic layer
was extracted with dichloromethane. The water layer was
washed twice by adding two 5 mL portions of dichlorome-
thane. The white solid was dried under high vacuum for 12 h
which evaporated the excess volatile alkyne substrate yield-
ing NMR pure isolated product. In case of non-volatile
alkyne substrates, the excess alkynes were removed by
column chromatographic separation. All reported yields are
isolated yields and are average of at least two runs.
400 MHz spectrometer and on
a Bruker Avance III
500 MHz spectrometer. All chemical shifts were reported in
ppm using tetramethylsilane as a reference. Chemical shifts
(d) downfield from the reference standard were assigned
positive values. Abnormal N-heterocyclic carbene salt 1,3-
bis(2,6-diisopropylphenyl)-2,4-diphenyl-imidazolium
salt
(IA, Scheme 1) was prepared starting from N,N’-bis(2,6- dii-
sopropylphenyl)benzamidine[21] following a modified litera-
ture protocol[6] Benzyl azide, 4-bromobenzyl azide, phenyl
azide, 2,6-dimethylphenyl azide, mesityl azide, 2,6-diisopro-
pylphenyl azide, 1-ethynyl-2,6- diisopropylbenzene and me-
sitylacetylene were prepared according to literature proce-
dures.[12n,14c,22]
General Procedure for the [3+2]Cycloaddition of
Sterically Hindered Azides and Sterically Hindered
Alkynes
Synthesis of Complex 1
Under an argon atmosphere, THF (10 mL) was added at
À788C to a mixture of 1,3-bis(2,6-diisopropylphenyl)-2,4-di-
phenylimidazolium salt IA (308 mg, 0.50 mmol), copper(I)
chloride (50 mg, 0.5 mmol) and potassium bis(trimethylsily-
l)amide (200 mg, 1 mmol). After 30 min at ~À788C, the
mixture was warmed to room temperature and stirred for
12 h. Solvent was evaporated under reduced pressure and
the residue was extracted with dichloromethane (3ꢄ20 mL).
The analytically pure light green title compound was ob-
tained by recrystallization from CH2Cl2/pentane. yield:
224 mg (0.35 mmol, 70%); mp 231–2338C. 1H NMR
(500 MHz, CDCl3, 258C, TMS): d=7.49–7.40 (m, 4H), 7.21–
7.18 (m, 8H), 7.05 (t, J=8 Hz, 2H), 6.85 (d, J=7.5 Hz, 2H),
2.68–2.63 (m, 2H), 2.58–2.54 (m, 2H), 1.41 (d, J=7 Hz,
6H), 0.95 (d, J=7 Hz, 6H), 0.80 (t, J=6.8 Hz, 12H);
13C NMR (125 MHz, 258C, TMS): d=159.4, 144.9, 144.5,
144.4, 141.6, 135.7, 131.1, 130.9, 130.5, 130.4, 130.3, 129.4,
129, 128.2, 128.1, 127.9, 125.2, 124.5, 123.5, 28.8, 28.6, 25.8,
23.7, 23.4, 22.5; HR-MS: m/z=639.256, calcd. for
C39H45ClCuN2 [M+H]+: 639.258; elemental analysis: calcd.
for C39H44ClCuN2: C 73.22, H 6.93, N 4.38; found: C 72.80,
H 6.88, N 4.17.
Alkyne (1.73 mmol), azide (1.57 mmol), and catalyst
1 (10 mg, 1 mol%) were loaded in a 25-mL Schlenk flask.
The reaction mixture was stirred at room temperature.
After an appropriate period of time, the reaction was
quenched by dissolving the reaction mixture in dichlorome-
thane (10 mL). Subsequently, a simple aqueous work-up was
carried out by adding 10 mL of water, and the organic layer
was extracted with dichloromethane. The water layer was
washed twice by adding two 5 mL portions of dichlorome-
thane. The crude product was purified by flash chromatogra-
phy on silica gel yielding NMR pure isolated product. All
reported yields are isolated yields and are average of at
least two runs.
General Procedure for the [3+2]Cycloaddition of
Azides and Internal Alkynes
Alkyne (1.73 mmol), azide (1.57 mmol), and catalyst
1 (10 mg, 1 mol%) were loaded in a 25-mL Schlenk flask.
The reaction was allowed to proceed at room temperature
or at 708C. After an appropriate period of time, the reaction
was quenched by dissolving the reaction mixture in dichloro-
methane (10 mL). Subsequently, a simple aqueous work-up
was carried out by adding 10 mL of water, and the organic
layer was extracted with dichloromethane. The water layer
was washed twice by adding two 5 mL portions of dichloro-
methane. The crude product was purified by flash chroma-
tography on silica gel yielding NMR pure isolated product.
X-ray Crystallographic Details
The single-crystal X-ray diffraction data of the crystal were
collected on a Bruker Kappa APEX-II CCD DUO diffrac-
tometer at 100(2) K using graphite-monochromated Mo Ka
radiation (l=0.71073 ꢁ). Multiscan absorption correction
Adv. Synth. Catal. 2013, 355, 2982 – 2991
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2989

Samaresh Chandra Sau et al.
FULL PAPERS
All reported yields are isolated yields and are average of at
least two runs.
Acknowledgements
SKM thanks SERB, India (DST, No. SR/S1/IC-25/2012) for
financial support. SCS thanks the UGC, New Delhi, for a re-
search fellowship. SRR thanks the IISER-Kolkata and CSIR
for RA fellowship. TKS thanks the CSIR, New Delhi, for re-
search fellowships and DM was a summer fellow in SKMꢀs
laboratory (from Department of Chemistry, University of
Pondicherry, Puducherry – 605014) and thanks the Indian
Academy of Science for a summer fellowship. Authors also
thank the NMR and X-ray facilities of IISER-Kolkata.
Procedure for Catalyst Longevity Experiment
between Benzyl Azide and Phenylacetylene
In a Schlenk flask, the compound 1 (10 mg, 1 mol%), phe-
nylacetylene (1.73 mmol) and benzyl azide (1.57 mmol)
were taken and stirred at 258C. The reaction mixture was
monitored by 1H NMR spectroscopy by taking aliquots of
the reaction mixture after 40 min intervals and the reaction
was stopped when the consumption of substrates was com-
plete. During this period, a solid product was formed. Again
a
fresh batch of liquid substrates phenylacetylene
(1.73 mmol) and benzyl azide (1.57 mmol) was added for
the next catalytic cycle without adding any further catalyst
into the reaction vessel. This procedure was repeated for
a total of ten consecutive catalytic runs.
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In a Schlenk flask, the compound 1 (10 mg, 1 mol%), phe-
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Rate of Conversion
A
1
round-bottom flask was charged with compound
(10 mg, 1 mol%), phenylacetylene (1.73 mmol), and
benzyl azide (1.57 mmol) at 258C. The reaction mixture was
1
monitored by H NMR spectroscopy after taking aliquots of
the reaction mixture after each 2 min interval and the reac-
tion was stopped when the substrate consumption was com-
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