Multicomponent synthesis of 1,2,3-triazoles in water catalyzed by copper nanoparticles on activated carbon

Article abstract of DOI:10.1002/adsc.201000637

Copper nanoparticles on activated carbon have been found to effectively catalyze the multicomponent synthesis of 1,2,3-triazoles from different azide precursors, such as organic halides, diazonium salts, anilines and epoxides in water. The first one-pot transformation of an olefin into a triazole is also described. The catalyst is easy to prepare, very versatile and reusable at a low copper loading. Copyright

Full text of DOI:10.1002/adsc.201000637

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DOI: 10.1002/adsc.201000637
Multicomponent Synthesis of 1,2,3-Triazoles in Water Catalyzed
by Copper Nanoparticles on Activated Carbon
Francisco Alonso,a,* Yanina Moglie,a Gabriel Radivoy,b and Miguel Yusa,*
a
Departamento de Quꢀmica Orgꢁnica, Facultad de Ciencias and Instituto de Sꢀntesis Orgꢁnica (ISO), Universidad de
Alicante, Apdo. 99, 03080 Alicante, Spain
Fax : (+34)-96-590-3549; phone: (+34)-96-590-3548; e-mail: falonso@ua.es or yus@ua.es
Departamento de Quꢀmica, Instituto de Quꢀmica del Sur (INQUISUR-CONICET), Universidad Nacional del Sur
b
Avenida Alem 1253, 8000 Bahꢀa Blanca, Argentina
Received: August 13, 2010; Published online: December 1, 2010
Dedicated to Professor Carmen Nꢁjera on the occasion of her 60th birthday.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000637.
Abstract: Copper nanoparticles on activated carbon
have been found to effectively catalyze the multi-
component synthesis of 1,2,3-triazoles from differ-
ent azide precursors, such as organic halides, diazo-
nium salts, anilines and epoxides in water. The first
one-pot transformation of an olefin into a triazole
is also described. The catalyst is easy to prepare,
very versatile and reusable at a low copper loading.
ficient heterogeneous and potentially reusable cata-
lysts.[15] All the aforementioned methodologies, how-
ever, involve pre-formed organic azides, for which the
use of organic solvents (e.g., dioxane, toluene, DMF,
dichloromethane, hexane) is, in general, mandatory.
The in-situ generation of organic azides in the pres-
ence of the alkyne (three-component alkyne-azide cy-
cloaddition)[16] minimizes hazards derived from their
isolation and handling, at the same time this avoids
the time expenditure and waste generation of an addi-
tional synthetic step. This version is especially inter-
esting when performed under heterogeneous condi-
tions in neat water.[17] Despite the clear advantages of
heterogeneous catalysis, the long and tedious proce-
dures usually required for the heterogenization of
copper preclude the widespread utilization of this
Keywords: click chemistry; copper nanoparticles;
cycloaddition; heterogeneous catalysis; triazoles;
water
Since the paramount discovery by the groups of type of catalysts. Therefore, easy-to-prepare and ver-
Meldal[1] and Sharpless[2] of the copper(I)-catalyzed satile heterogeneous copper catalysts that can effi-
Huisgen[3] 1,3-dipolar cycloaddition of organic azides ciently catalyze the multicomponent 1,3-dipolar cyclo-
and alkynes, a plethora of methods have flourished addition of organic azides and alkynes in water are
around this reaction.[4] Enormous efforts have been welcome.
devoted in order to maximize the general efficiency
Our ongoing interest on the reactivity of active
of the process adapted to the multiple and manifold metals[18] led us to the application of active copper
applications of the resulting 1,2,3-triazoles.[4,5] For in- [from CuCl2·2H2O, Li, and 4,4’-di-tert-butylbiphenyl
stance, to reduce the amounts of copper in solution (DTBB, cat.) in THF] in reduction reactions.[19] More
should be a priority, particularly for biological appli- recently, we discovered that unsupported copper
cations, due to its potential toxicity.[6] In this sense, nanoparticles, generated as above but from anhydrous
heterogeneous catalysts offer several advantages over CuCl2, effectively catalyze the 1,3-dipolar cycloaddi-
the homogeneous counterparts, such as easy recovery, tion of azides and terminal alkynes in short reaction
easy recycling, and enhanced stability.[7] Charcoal,[8] times and in the absence of any stabilizing additive or
zeolites,[9] montmorillonite,[10] NHC-modified silica,[11] ligand.[20] Notwithstanding the superior catalytic activ-
polystyrene[12] or chitosan[13] are some of the supports ity when compared with other commercially available
used for copper(I) in the heterogeneous version of copper sources, the copper nanoparticles underwent
the title click reaction. Since the discovery that dissolution under the reaction conditions (Et3N, THF,
copper metal can be a source of the catalytic spe- 658C) and could not be reused. We wish to present
cies,[14] copper nanoparticles have also emerged as ef- herein our findings on the 1,3-dipolar cycloaddition of
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Multicomponent Synthesis of 1,2,3-Triazoles in Water Catalyzed by Copper Nanoparticles
alkynes and in-situ generated azides, from different
precursors, catalyzed by copper nanoparticles sup-
ported on activated carbon in water.
A variety of copper catalysts were prepared by ad-
dition of the support to a suspension of the recently
prepared copper nanoparticles, the latter readily gen-
erated from copper(II) chloride, lithium metal, and a
catalytic amount of DTBB (10 mol%) in THF at
room temperature. The catalysts were not subjected
to any pre-treatment. Benzyl bromide (1a) and phe-
nylacetylene (2a) were used as model substrates in
order to test the activity of the different catalysts
(Table 1). SiO2 (entry 1), Al2O3 (entry 2), Al silicate
(entry 6), magnetite (entry 9), graphite (entry 14),
MWCNT (entry 15), and activated carbon (entry 19)
led to yields ꢀ90% in ꢁ9 h at 708C. Activated
carbon, however, was shown to be more active (100%
yield, 3 h) and the sole catalyst providing a quantita-
tive yield of 3aa when reused in a second cycle
(entry 19). When the cycloaddition was performed
with activated carbon, in the absence of copper, a
Figure 1. TEM micrograph of CuNPs on active carbon.
Table 1. Three-component 1,3-dipolar azide-alkyne cycload-
dition catalyzed by copper on different supports.[a]
lower yield of the two regioisomeric triazoles was ob-
tained (entry 20).
The copper-on-activated-carbon catalyst was char-
acterized by different means. The copper content in
the catalyst, 1.6 wt%, was determined by inductively
coupled plasma mass spectrometry (ICP-MS). Analy-
sis by TEM revealed the presence of spherical nano-
particles dispersed on the active carbon with diame-
ters of ca. 6Æ2 nm (Figure 1). Energy-dispersive X-
ray (EDX) analysis on various regions confirmed the
presence of copper, with energy bands of 8.04,
8.90 keV (K lines) and 0.92 keV (L line). The XRD
diffractogram did not show any significant peak due
to the amorphous character of the sample, to the fact
that the crystal domains are <10 nm, and/or low
copper loading weight. XPS analysis showed two O
(1s) peaks at 532.2 and 534.2 eV, and three Cu (2p3/2)
peaks at 934.1, 936.4, and 945.7 eV. From these results
it can be inferred that the surface of the copper nano-
catalyst is mainly oxidized. All peaks corresponding
to the Cu (2p3/2) level appear at higher binding energy
when compared with those obtained with unsupport-
ed copper nanoparticles,[20] with the peak at 945.7 eV
being a satellite shake-up feature characteristic of
Cu2+ species.[21] The selected-area electron-diffraction
pattern (SAED) of the copper nanoparticles is also in
agreement with the presence of Cu2O and CuO. It is
worthy of note that mixed Cu/Cu-oxide[15c] and, very
recently, CuO nanostructures[22] have been found to
catalyze the 1,3-dipolar cycloaddition of azides and
terminal alkynes.
Entry Support [mol% Cu][b] T [8C] t [h] Yield [%][c]
1
2
3
4
5
6
7
8
SiO2 [1]
Al2O3 [1]
TiO2 [1]
MgO [1]
ZnO2 [1]
Al silicate [1]
Al [1]
70
70
70
70
70
70
70
70
70
70
70
25
25
70
70
70
4
9
90 (16)
100 (13)
74
16
57
100 (19)
18
17
100 (0)
80
0
33
31
90
100 (20)
100
0
24
24
24
6
24
24
9
14
24
24
24
7
6
7
24
24
3
MCM-10 [1]
magnetite [1]
graphite [5]
graphite [5][d]
graphite [5]
graphite [1]
graphite [1]
MWCNT[e] [5]
activated carbon [5]
9
10
11
12
13
14
15
16
17
18
19
20
activated carbon [5][d] 70
activated carbon [5]
activated carbon [1]
activated carbon [0]
25
70
70
30
100 (100)
24
50[f]
[a]
[b]
[c]
[d]
[e]
[f]
1a (1 mmol), NaN3 (1.1 mmol), and 2a (1 mmol) in H2O.
Amount of copper added to the support.
GLC yield; the yield after a second cycle in parenthesis.
Solvent-free reaction.
Multi-walled carbon nanotube.
As a 1:1.3 mixture of regioisomers; alkyne 19%; azide
31%.
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Table 2. Three-component 1,3-dipolar cycloaddition catalyzed by CuNPs/C using organic halides as the azide
precursors.[a]
[a]
Reaction conditions: 1 (1 mmol), 2 (1 mmol), NaN3 (1.1 mmol), CuNPs/C (0.5 mol%) in H2O (2 mL) at 708C.
Isolated yield.
Reaction in H2O-EtOH 1:1.
Reaction at 1008C.
[b]
[c]
[d]
[e]
2 mmol of 1a.
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Multicomponent Synthesis of 1,2,3-Triazoles in Water Catalyzed by Copper Nanoparticles
An array of activated organic halides (Table 2) was catalyst (Scheme 1). We were delighted to discover
subjected to the three-component reaction with phe- that epoxides reacted in water at 1008C in a three-
nylacetylene in water at 708C, using 0.5 mol% component mode,[24] while other protocols require the
CuNPs/C (Table 2, entries 1–6). Benzyl chloride react- sequential addition of reagents.[24a] As an example,
ed slower than the bromide counterpart albeit in ex- styrene oxide was regioselectively transformed into
cellent yield in both cases (entry 1). Benzyl bromides the corresponding 2-substituted triazol-1-yl alcohol in
bearing either electron-withdrawing or electron-do- high yield (Scheme 1). Diazonium salts, such as com-
nating groups reacted nicely to furnish the corre- mercially available phenyldiazonium tetrafluorobo-
sponding triazoles in near quantitative yields (en- rate, were utilized for the first time as potential sub-
tries 2 and 3). A single product was obtained for cin- stitutes of the less reactive aromatic halides
namyl bromide, with the intermediate azide not un- (Scheme 1). Even more attractive was the four-com-
dergoing a [3,3]-sigmatropic rearrangement leading to ponent strategy involving an aromatic amine and tert-
the secondary allylic azide[23] under the reaction con- butyl nitrite at 708C (Scheme 1). This result is re-
ditions (Table 2, entry 4). Some activated functional- markable if we take into account that, in the only
ized organic halides, such as a-chloroacetophenone published method for this transformation, t-BuONO
(entry 5) or ethyl a-bromoacetate (entry 6), were also was used together with TMSN3 (NaN3 in our case)
studied, with the former reacting more sluggishly. In- and applied sequentially in organic media.[25] It is
terestingly, not only activated but also deactivated noteworthy that the reactivity of both the diazonium
alkyl halides could be used as the azide precursors in salt and aniline were comparable to that of benzyl
the title reaction (entries 7 and 8). A solvent system bromide (2–3 h).
composed of 1:1 H2O-EtOH is, however, recommend-
We were intrigued by the possibility of using hal-
ed in order to attain optimum results. n-Nonyl iodide oalkynes, the derived azides of which might react
(entry 7, X=I) and cyclohexyl bromide (entry 8) re- either in an intramolecular or intermolecular fashion.
acted at 708C, whereas a temperature of 1008C was Our expectations came to reality by subjecting 6-
required for the more reluctant to react n-nonyl chlo- chlorohex-1-yne to the standard reaction conditions
ride (entry 7, X=Cl). The substrate 3-(2-bromoethyl)- (Scheme 2). The bicyclic triazole 6 was synthesized
1H-indole furnished the attractive doubly heterocyclic for the first time in a straight-forward manner using
product 3ia (entry 9). The methodology also proved click chemistry, while other reported procedures in-
to be effective for alkynes other than phenylacety- volved several synthetic steps.[26] Furthermore, the
lene, such as phenyl propargyl ether (2b) or N-propar- transformation of alkenes into triazoles was also de-
gylphthalimide (2c) (entries 10 and 11, respectively). vised by taking advantage of the azasulfenylation of
The successful reaction with trimethylsilylacetylene alkenes developed by Trost et al.[27] In this methodolo-
provides an indirect entry into the monosubstituted
triazoles (after proper desilylation), making unneces-
sary the handling of acetylene (entry 12). Moreover,
bistriazole 3ae was obtained in good yield from diyne
2e and two equivalents of benzyl bromide.
We next explored the possibility of using alterna-
tive substrates to the organic halides as azide precur-
sors which, being compatible with the standard reac-
tion conditions, could expand the versatility of the
Scheme 2. Intramolecular three-component click reaction
catalyzed by CuNPs/C in water.
Scheme 1. Reaction of phenylacetylene with different azide precursors catalyzed by CuNPs/C in water.
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Francisco Alonso et al.
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loading. Interestingly, 10 mol% Cu2O reached a maxi-
mum 90% conversion, albeit side products (10%)
were also obtained and its reutilization furnished the
corresponding triazole in 20% conversion after 24 h
as a ca. 4:1 mixture of regioisomers. Therefore, the
nanosized character of our catalyst makes all the dif-
ference.
Scheme 3. One-pot multicomponent synthesis of a 1,2,3-tria-
In conclusion, we have presented a new heteroge-
neous catalyst for the multicomponent Huisgen 1,3-di-
polar cycloaddition in water. The catalyst consists of
oxidized copper nanoparticles on activated carbon
zole from cyclohexene catalyzed by CuNPs/C.
gy, an alkene was treated with dimethyl(methylthio)- and it is readily prepared from commercially available
sulfonium tetrafluoroborate (DMTSF) at 08C to chemicals under mild conditions. The CuNPs/C, at a
room temperature, followed by the addition of a ni- low catalyst loading (0.5 mol%), manifested a high
trogen nucleophile at room temperature and stirring versatility as not only organic halides, but other azide
for 1–4 days. We applied a variation of this method in precursors, including epoxides, diazonium salts, ani-
which the alkene was directly mixed with the lines, or alkenes, could be successfully transformed
CuNPs/C, DMTSF, and NaN3 in MeCN to produce into the corresponding 1,2,3-triazoles. The catalyst is
the corresponding methylsulfanyl azide in only 1 h at reusable and seemingly operates under heterogeneous
room temperature. The subsequent reaction with the conditions. Further research to extend the substrate
alkyne represents, to the best of our knowledge, the scope and better understand the catalysis is under
first example of triazole synthesis from an alkene in way.
one pot (Scheme 3). The conditions and yield were
not optimized yet but it seems a promising route to
directly transform carbon-carbon double bonds into Experimental Section
triazoles bearing a versatile methylsulfanyl group.
It is worthwhile mentioning that all the reactions Typical Procedure for the Preparation of CuNPs/C
were carried out without air exclusion. Furthermore,
reactions at a higher dilution, such as 0.1M or even
0.01M, also afforded the expected triazoles in high
Anhydrous copper(II) chloride (135 mg, 1 mmol) was added
to a suspension of lithium (14 mg, 2 mmol) and 4,4’-di-tert-
butylbiphenyl (DTBB, 27 mg, 0.1 mmol) in THF (2 mL) at
yields, albeit longer reaction times were required (i.e.,
8 and 24 h, respectively, for benzyl bromide and phe-
nylacetylene). In addition, the catalyst could be easily
recovered by filtration and reused, leading to triazole
3aa in quantitative yield along five consecutive cycles.
No leaching of copper was detected after the fifth
cycle (ICP-MS). Nonetheless, in order to test the ro-
bustness of the catalyst and unveil the nature of the
catalysis, the reaction of benzyl bromide and phenyl-
acetylene was run up to a 100% conversion (<3 h),
and the resulting mixture containing the triazole was
subjected to additional heating until a total time of
24 h. Then, the catalyst and the triazole were filtered
off, the aqueous phase was extracted with ethyl ace-
tate and fresh starting materials were again added to
the resulting aqueous phase, which were allowed to
react at 708C for 24 h. A ca. 1:1 mixture of the corre-
sponding regioisomeric triazoles was obtained with
17% conversion, thereby indicating that, in this case,
the cycloaddition proceeded uncatalyzed under ther-
mal conditions. ICP-MS analyses of the resulting
aqueous phase gave <50 ppb of copper. These results
point to a process of heterogeneous nature.
room temperature under an argon atmosphere. The reaction
mixture, which was initially dark blue, rapidly changed to
black, indicating that the suspension of copper nanoparticles
was formed. This suspension was diluted with THF (18 mL)
followed by the addition of the activated carbon (1.28 g).
The resulting mixture was stirred for 1 h at room tempera-
ture, filtered, and the solid successively washed with water
(20 mL), THF (20 mL) and dried under vaccum.
General Procedure for Three-Component 1,3-Dipolar
Cycloaddition Catalyzed by CuNPs/C in Water
NaN3 (72 mg, 1.1 mmol), the azide precursor (organic
halide, diazonium salt, or epoxide, 1 mmol) and the alkyne
(1 mmol) were added to a suspension of CuNPs/C (20 mg,
0.5 mol% Cu) in H2O (2 mL). The reaction mixture was
warmed to 708C and monitored by TLC until total conver-
sion of the starting materials. Water (30 mL) was added to
the resulting mixture followed by extraction with EtOAc
(3ꢃ10 mL). The collected organic phases were dried with
MgSO4 and the solvent was removed under vacuum to give
the corresponding triazole, which did not require any further
purification (except compounds 3ia and 7).
Finally, we compared the CuNPs/C catalyst with
commercially available Cu, Cu2O, and CuO in the re-
action of benzyl bromide and phenylacetylene under
Acknowledgements
This work was generously supported by the Spanish Minister-
the standard conditions at 10 and 1 mol% catalyst io de Ciencia e Innovaciꢀn (MICINN; CTQ2007-65218 and
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Multicomponent Synthesis of 1,2,3-Triazoles in Water Catalyzed by Copper Nanoparticles
Consolider Ingenio 2010-CSD2007-00006), the Generalitat
Valenciana (GV; PROMETEO/2009/039). Y.M. acknowledg-
es the Vicerrectorado de Investigaciꢀn, Desarrollo e Innova-
ciꢀn of the Universidad de Alicante for a grant.
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