Light-sensitive and recoverable n-heterocyclic carbene copper(I) complex in homogeneous catalysis

Article abstract of DOI:10.1002/adsc.201401140

The synthesis of a light-responsive N-heterocyclic carbene copper(I) complex by introducing a nitrobenzospiropyran unit into the carbene ligand was developed. The solubility of this complex was controlled through reversible conversion between its netural and ionic states using ultraviolet light irradiation. Taking advantage of such a light-sensitive property facilitated its recovery and reuse in copper(I)-catalyzed homogeneous oxidation and "click" reactions.

Full text of DOI:10.1002/adsc.201401140

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DOI: 10.1002/adsc.201401140
Very Important Publication
Light-Sensitive and Recoverable N-Heterocyclic Carbene
Copper(I) Complex in Homogeneous Catalysis
Guodong Zhang,a,b Rui Lang,a,b Wenlong Wang,a Hui Lv,a Liyuan Zhao,a
Chungu Xia,a and Fuwei Lia,*
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research Institute of LICP, Lanzhou Institute
of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou 730000, Peopleꢀs Republic of China
Fax : (+86)-931-496-8129; e-mail: fuweili@licp.cas.cn
Graduate University of Chinese Academy of Sciences, Beijing, 100049, Peopleꢀs Republic of China
b
Received: December 7, 2014; Revised: January 21, 2015; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201401140.
such as temperature variation,[3] bubbling of CO2
and irradiation with ultraviolet (UV) light.[6] These
methods could not only maintain the typical advan-
tages of homogeneous catalysis, but also allow for
good recycling of catalyst from reaction mixture.[5]
Since the first isolation of a “free” N-heterocyclic
carbene (NHC) by Arduengo in 1990, NHC-transition
metal complexes have attracted considerable atten-
tion owing to their high stability and outstanding ap-
plication in homogeneous catalysis.[7] However, the
synthesis of the NHC requires a multi-step procedure,
and separation of NHC-metal complexes from reac-
tion mixture is usually difficult because of their good
solubility. Therefore, numerous efforts have been
made to modify NHC ligands to increase their recy-
cling possibilities.[8] For example, increasing the water-
solubility by introducing carbonate or sulfonate to
[4]
Abstract: The synthesis of a light-responsive N-het-
erocyclic carbene copper(I) complex by introducing
a nitrobenzospiropyran unit into the carbene ligand
was developed. The solubility of this complex was
controlled through reversible conversion between
its netural and ionic states using ultraviolet light ir-
radiation. Taking advantage of such a light-sensitive
property facilitated its recovery and reuse in cop-
per(I)-catalyzed homogeneous oxidation and
“click” reactions.
Keywords: carbenes; copper; homogeneous cataly-
sis; ligand design; sustainable chemistry
Homogeneous catalysis has provided a number of ad- NHC ligands to prepare catalysts suitable for biphasic
vantages over heterogeneous catalysis, such as high systems is an excellent method.[9] In our previous
efficiency and selectivity. However, homogeneous cat- study, we have prepared a series of pH-responsive ter-
alysis, especially for organometallic catalysis, has not tiary amine-functionalized NHC-metal complexes.
been widely spread due to the intricate recycling These catalysts were monophasic/biphasic switchable
problems and thus also metal contamination of the in the reaction/recycling system by adding external
products.[1]
acid. Although this pH-responsive strategy is effective
Several strategies have been developed with the in the separation of product and catalyst, it also
aim to recycle homogeneous catalysts. For example, caused the formation of massive inorganic salts which
immobilizing catalysts on supports definitely facili- brought about mass transfer problems.[10]
tates the recovery procedure by simple filtration, but
With the continuing interest in catalyst recycling
a reactivity loss is always present due to the insolubili- and green catalysis in our group, here we report
ty of the supported catalyst. Another method is to use a much more simple and convenient catalyst recycling
a biphasic reaction system. Since the catalyst and method by introducing a light-responsive polarity
product dissolve in different phases, the catalyst is changing group onto the ligand instead of adding ex-
separated easily from the product, however, the mass ternal chemicals. The polarity and solubility of this
transfer limitation also impedes the reactivity.[2]
catalyst were evidently changed under light irradia-
A facile solution to overcome the above-mentioned tion, thus facilitating the catalyst recycling process.
limitations is to carry out the reaction in a single We chose nitrobenzospiropyran (SP) as the light-
phase and to transfer the catalyst or products into the active group due to its rapid and reversible photo-
second phase by alteration of the external conditions, transition property between neutral and ionic states
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Guodong Zhang et al.
Scheme 1. Syntheses of SP-functionalized Cu(I)-NHC catalyst (5). Reagents and conditions: (i) ethyl chloroformate, MeCN,
reflux; (ii) NaN3, CH3OH, H2O, reflux; (iii) CuCl, t-BuOK, THF, 708C, 12 h; (iv) SPCC, CHCl3, 708C, in the dark.
under the irradiation of UV light without any photo- 3. The target complex 5a was formed almost quantita-
products.[11] This SP-functionalized NHC-Cu complex tively after stirring complex of 4 with SPCC {1’,3’-
is capable of catalyzing both the oxidative reaction of dihydro-3’,3’-dimethyl-1’-propargyl-6-nitrospiro[2H-1-
benzyl alcohol and click reactions under homogene- benzopyran-2,2’-(2H)-indole]} in chloroform at 708C
ous condition, and the catalyst could be easily recy- by way of a click reaction. As shown in Scheme 1,
cled and reused for several times without significant complex 5a was totally dissolved in ethyl acetate
loss of activity.
(EA) – the upper layer. Under the influence of UV ir-
The synthetic pathway to the SP-functionalized radiation, 5b (the ionic form) was gradually formed
Cu(I)-NHC complex is depicted in Scheme 1. The and transferred to the bottom layer (glycol). Addi-
synthesis of complex 1 was reported in our previous tionally, petroleum ether (PE) was added to acceler-
work.[10] Treatment of 1 with ethyl chloroformate in ate the transfer rate. In the absence of UV light or in
acetonitrile at reflux gave the chloromethyl group- the dark, complex 5b converted back to its original
functionalized SIPr precursor [SIPr=N,N’-bis(2,6-di- neutral form (5a), thus demonstrating that this light-
ACHTUNGTRENNUNGisopropylphenyl)imidazolidin-2-ylidene] 2, which was controlled transformation was reversible and switcha-
subsequently reacted with sodium azide and so gave ble.
rise to the azide group functionalized-SIPr precursor
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Light-Sensitive and Recoverable NHC Copper(I) Complex
Figure 1. (A) Vis-UV spectrum of SP-functionalized Cu(I)-NHC complex (5) depending on irradiation time. (B) Vis-UV
spectrum of the upper solvent.
To further prove this light-switchable property of Table 1. NHC-Cu-catalyzed benzyl alcohol oxidation reac-
tion.[a]
complex 5, a UV adsorption experiment was per-
formed (Figure 1A). A homogeneous solution of dis-
solving 5a (0.25 mg) in 3 mL EA was tested. After
UV irradiation for 15 seconds, a new peak at 586 nm
appeared, indicating that 5b was formed. On prolong-
ing the irradiation time, this peak gradually increased
and reached its maximum value after 1 min, indicating
that the rapid and sensitive UV-irradiated isomeriza-
tion had occurred. As shown in Figure 1B, when the
solvent was changed to an EA/glycol mixture, com-
plex 5a was assumed to totally convert to 5b, and be
completely transferred to the lower layer after 2 minꢀ
UV irradiation; However, when the upper layer was
sucked out for UV-vis adsorption analysis, the peak at
326 nm was observed, indicating that small amount of
Entry Catalyst [mol%] Temp. [oC] Time [h] Yield [%]
1
2
3
5
5
5
50
80
80
6
6
12
39
53
89
[a]
Reaction
conditions:
5a
(0.05 mmol),
TEMPO
(0.05 mmol), 6a (1 mmol), O2 (1 atm), 808C for 12 h,
1 mL ethyl acetate and 1 mL glycol as solvent.
Cu(I)-NHC complex still remained in the upper layer. for this transformation, and the recovery of the cata-
To accelerate the transfer speed and degree, different lyst would be convenient in the presence of UV light.
amounts of PE were added in the EA/glycol mixture.
As shown in Table 1, the oxidation of 4-bromoben-
When the solvent ratio was 2:1:1 (PE:EA:glycol), the zyl alcohol (6a) was carried out using 5 mol% Cu(I)-
peak at 326 nm nearly disappeared, suggesting that NHC complex and TEMPO as catalysts and an EA/
the concentration of complex 5 in the upper layer was glycol mixture as solvent. Initially, the reaction was
below the detection limit. Based on the above obser- performed at 508C for 6 h under 1 atm of oxygen, and
vations, PE was proven to facilitate the transferring of the desired 4-bromobenzyl aldehyde (7a) was formed
complex 5 between the two phases (EA/glycol), and (entry 1). Both higher reaction temperature (808C,
was used in the following recycling tests.
entry 2) and longer duration (12 h, entry 3) increased
The selective oxidation of alcohols is one of the the yield of 7a, such that the starting material (6a)
most fundamental transformations in organic synthe- was totally converted to 7a with 89% isolated yield.
sis since the corresponding products play important
The recycling experiment was carried out to prove
roles in the total synthesis of natural products and the light-sensitive property of our catalyst and its sta-
fine chemicals.[12] Among the reported methodologies, bility under the reaction conditions. As shown in Fig-
the Cu(I)/TEMPO catalytic system has attracted spe- ure 2A, at room temperature, a liquid/liquid bi-layer
cial attention due to its high efficiency.[13] The mecha- system was formed when EA and glycol were simulta-
nism of this reaction has been thoroughly investigat- neously added, and the substrate and catalyst were
ed, and according to literature reports, an electron- dissolved in EA (the upper layer). At elevated tem-
donating ligand will significantly increase the reactivi- perature (808C), the two layers began to mix into one
ty.[14] Therefore, we anticipated that our SP-function- phase, so the reaction was conducted under homoge-
alized Cu(I)-NHC complex was a suitable candidate neous conditions with high efficiency. Afterwards, the
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Scheme 2. NHC-Cu-catalyzed benzyl alcohol oxidation. Re-
action conditions: 5a (0.05 mmol), TEMPO (0.05 mmol), 6
(1 mmol), O2 (1 atm), 808C for 12 h, 1 mL ethyl acetate and
1 mL glycol as solvent.
demonstrated the quick and reversible switch of com-
plex 5 in the presence of UV radiation. As shown in
Figure 2B, the catalyst could be used in the benzyl al-
cohol oxidation reaction five times with only a little
loss of reactivity.
Various substituted benzyl alcohols were tested
under the above optimized conditions. As shown in
Scheme 2, para-substituted benzyl alcohols bearing an
electron-withdrawing group (bromide) furnished ex-
cellent isolated yields (7a). Benzyl alcohols with elec-
tron-donating groups were also reactive (7b and 7c),
along with benzyl alcohol (7d) which gave 86% isolat-
ed yield. ortho-Substituted benzyl alcohols (7e and
7f) also reacted smoothly with no significant steric
hindrance. In conclusion, the SP-functionalized Cu(I)-
NHC complex showed a high efficiency and good sub-
strate tolerance in the oxidative transformation of
benzyl alcohols.
Figure 2. (A) Photographs of the recycling test; (B) recy-
cling test. Reaction conditions: 5a (0.05 mmol), TEMPO
(0.05 mmol), 6 (1 mmol), O2 (1 atm), 808C for 12 h, 1 mL
ethyl acetate and 1 mL glycol as solvent.
To further extend the application scope of our SP-
reaction system was cooled down to room tempera- functionalized Cu(I)-NHC catalyst, the azide-alkyne
ture, whereupon 5a and product were still dissolved in cycloaddition reaction was performed using benzyl
the upper layer. After UV irradiation and addition of azide and an alkyne as substrates as shown in
PE, 5b was quickly formed and transferred to the Scheme 3. The reaction was carried out at 708C, with
bottom layer due to its high polarity. Furthermore, an 5 mol% catalyst loading. When phenylacetylene was
accurate ICP analysis of the upper solvent was per- used as starting material, the desired triazole product
formed, and only
a
trace amount of Cu (10a) was isolated with 93% yield. Phenylacetylenes
(1.03 mmolLÀ1) was found in the residual material bearing electron-donating and electron-withdrawing
which proved the efficiency of our system. As groups (tert-butyl and fluoride) on the para-position
a result, the product and the catalyst were easily sepa- gave 89% and 82%, yields, respectively (10b and 10c).
rated by sucking out the upper layer. In the next run, An alkyl propargyl alcohol gave 83% isolated yield
additional EA and substrate were added in the ab- (10d). To validate the recyclability of 5a in click reac-
sence of UV light, thus 5b switched back to 5a and tions, the reuse investigation was performed in the
dissolved in the upper layer. The above procedure model reaction using benzyl azide and phenylacety-
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Light-Sensitive and Recoverable NHC Copper(I) Complex
ed by suction filtration and HCl was introduced to the
upper solvent to afford the desired aldehyde.
General Procedure for Recycling Test
Benzyl alcohol (1 mmol), TEMPO (0.05 mmol), 5a
(0.05 mmol), and ethyl acetate:glycol (1:1; 2 mL) were intro-
duced to a Schlenk tube under 1 atm O2. The reaction was
carried out at 808C for 12 h. After completion of the reac-
tion, the mixtuire was exposed to UV light and 2 mL of PE
were introduced to the tube. The upper solvent was collect-
ed by suction filtration, HCl was introduced to the upper
solvent to afford the desired aldehyde. Then 1 mL of ethyl
acetate was added to the tube and the tube was left in the
dark for 30 min. After that, benzyl alcohol and TEMPO
were added again to the tube and the above procedure was
repeated.
Scheme 3. NHC-Cu-catalyzed azide-alkyne cycloaddition re-
action. Reaction conditions: 5a (0.05 mmol), 8 (1 mmol), 9
(1 mmol), 708C for 12 h, 1 mL ethyl acetate and 1 mL glycol
as solvent.
General Procedure for NHC-Cu-Catalyzed Azide-
Alkyne Cycloaddition Reaction
Benzyl azide (1 mmol), alkyne (1 mmol), 5a (0.05 mmol),
and ethyl acetate:glycol (1:1; 2 mL) were introduced to
a Schlenk tube. The reaction was carried out at 708C for
12 h. After completion of the reaction, the mixture was ex-
posed to UV light and 2 mL of PE were introduced to the
tube. The upper solvent was collected by suction filtration
to afford the desired triazole. All the products can be ob-
tained with NMR purity by this procedure.
lene as substrates with a catalyst loading of 5 mol%
for 12 h and an 85% isolated yield was achieved even
after four times (see the Supporting Information for
the detailed procedure). Based on above results, we
conclude that 5a is a suitable and recoverable catalyst
for azide-alkyne cycloaddition reactions.
In summary, we have successfully developed a new
synthetic method for the light-tagged Cu(I)-NHC
complex by introducing an SP unit into NHC skele-
ton. This homogeneous catalyst could be used for five
times without significant loss of reactivity in the oxi-
dative transformation of benzyl alcohols using light-ir-
radiated polarity variation. This recovery procedure is
cleaner and more convenient compared with the tra-
ditional protocols, since no external chemical has to
be added. Thus reported synthetic strategy of SP-
functionalized Cu(I)-NHC complex and its sustaina-
ble applications in homogeneous oxidation and click
reactions opens a new door for homogeneous catalyst
recovery and green catalysis. Current experiments in
our laboratory are directed at the synthesis of new re-
cyclable transition metal-NHC complexes and their
clean catalysis.
Acknowledgements
We thank the Chinese Academy of Sciences and the National
Natural Science Foundation of China (21133011, 21373246
and 21403258) for generous financial support.
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Benzyl alcohol (1 mmol), TEMPO (0.05 mmol), 5a
(0.05 mmol), and ethyl acetate:glycol (1:1; 2 mL) were intro-
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Light-Sensitive and Recoverable N-Heterocyclic Carbene
Copper(I) Complex in Homogeneous Catalysis
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Adv. Synth. Catal. 2015, 357, 1 – 7
Guodong Zhang, Rui Lang, Wenlong Wang, Hui Lv,
Liyuan Zhao, Chungu Xia, Fuwei Li*
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