Highly efficient and selective photocatalytic hydroamination of alkynes by supported gold nanoparticles using visible light at ambient temperature

Article abstract of DOI:10.1039/c3cc38985e

The direct hydroamination of alkynes driven by visible light can be achieved in high yield and selectivity at ambient temperature using supported gold nanoparticles (AuNPs) as photocatalysts. Aniline molecules interact with visible light activated AuNPs m

Full text of DOI:10.1039/c3cc38985e

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Highly efficient and selective photocatalytic
hydroamination of alkynes by supported gold
nanoparticles using visible light at ambient
temperature†
Cite this: Chem. Commun., 2013,
49, 2676
Received 16th December 2012,
Accepted 13th February 2013
DOI: 10.1039/c3cc38985e
Jian Zhao, Zhanfeng Zheng, Steven Bottle, Alison Chou, Sarina Sarina and
Huaiyong Zhu*
www.rsc.org/chemcomm
The direct hydroamination of alkynes driven by visible light can be
Recently, we found that the organic molecules adsorbed on the
achieved in high yield and selectivity at ambient temperature surface of AuNPs can be activated under the irradiation of visible
using supported gold nanoparticles (AuNPs) as photocatalysts. light due to the Surface Plasmon Resonance (SPR) effect.4 As a result,
Aniline molecules interact with visible light activated AuNPs mean- the energy of the incident light is absorbed by the conducting
while alkynes could be activated at active sites on the support. The electrons that exist at the surface of AuNPs. The resulting high
findings reveal a new green approach for synthesis of fine organic energy surface may facilitate the reaction of organic molecules
compounds and provide insight into catalyst design for the activa- adsorbed on it. It is known that aniline and derivatives can weakly
R
tion of C C triple bonds and amines.
bind to gold surfaces through the nitrogen lone pair.5 Thus, we
hypothesized that such amines might also be activated by visible
Hydroamination of alkynes, or the direct addition of an N–H bond light irradiation when bound to AuNPs. If the surface of AuNPs, or
to a CRC bond, is of great significance in synthetic chemistry. This the support to which the NPs are bound, also activates the CRC
reaction can offer an atom-economical route to various organic bond of alkynes at the same time,6 then the intermolecular hydro-
nitrogen molecules and provide a convenient route for the synthesis amination of alkynes might be realized at ambient temperature.
of numerous important fine chemicals and synthetic intermediates.1 This approach would be of considerable interest as an environmen-
Developing efficient and green systems for this addition has tally benign alternative to the traditional synthetic route.
attracted significant interest, particularly for the intermolecular
To verify our hypothesis, the addition of the N–H bond across
process. A gold-based complex has also been identified as an active CRC triple bonds was first investigated with the photocatalyst
catalyst for this process.2 Recently, supported AuNPs were found to AuNPs on nitrogen doped TiO2(B) nanofibres, labelled as Au/TiO2À
N
be active for the hydroamination of alkynes.2i High conversions and (Table 1). The highest conversion was achieved for 4-phenyl-1-butyne
a moderate selectivity for imines (55%) could be achieved after 22 h (90%) with a selectivity of 91% to the target product, imine (entry 1).
reaction at 100 1C.
The results in Table 1 illustrate the excellent performance of the
Hydroamination of alkynes involves nucleophilic attack to AuNP photocatalyst for this synthetic process. When electron with-
CRC by amines, the activation energy of which is very high drawing or donating groups are bonded to the aromatic ring of the
due to the electrostatic repulsion between the N lone pair and reactants (entries 5–11), high conversion and selectivity were still
the p system of alkynes.3 Intermolecular hydroamination is achieved. Amines with electron withdrawing groups in the aromatic
most challenging as it is even more kinetically and thermo- ring (entries 8 and 9) achieved significantly higher yields than those
dynamically problematic than the intramolecular process. with electron donating groups (entries 10 and 11). This was also
Higher temperatures (generally above 100 1C) are typically observed by Tanaka and co-workers in thermo-catalysis,2h who
necessary to achieve high yields. However, at high temperatures suggested that it was more probable for the amine to coordinate
the reaction equilibrium shifts towards lower conversions as with the cationic gold(I) species before the C–N formation. Very poor
the reaction entropy DS0 of the amine addition is negative,3 conversion for the aliphatic aminohexane was examined (entry 12).
which creates a dilemma.
This may be attributed to the increased basicity of aliphatic amines,
which can deactivate the catalysts.2i,7 The reaction with a secondary
amine was shown to be possible (entry 15). For the alkynes, however,
terminal alkynes gave the highest yields (entries 1–3) while there is
almost no reaction for non-terminal triple bonds (entries 13 and 14),
most likely because of the increased steric hindrance by the groups
attached to the CRC bond.2h,7
School of Chemistry, Physics and Mechanical Engineering, Queensland University of
Technology, Brisbane, Qld 4001, Australia. E-mail: hy.zhu@qut.edu.au;
Fax: +61 7 3138 1804; Tel: +61 7 3138 1581
† Electronic supplementary information (ESI) available: Preparation of gold
catalysts, activity of AuNPs on different supports, TEM, UV-Vis spectra, IES, etc.
See DOI: 10.1039/c3cc38985e
c
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Table 1 Photocatalytic hydroamination of alkynes with amines on Au/TiO2ÀN
(under visible light)a
Entries Alkynes (R1)
Amines (R2) Conv.b (%) Sel.c (%) TONd (%)
1
2
3
4
5
6
7
8
Ph–(CH2)2
Ph–CH2
Ph
CH3(CH2)5
Br–Ph
CH3O–Ph
CH3–Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Cl–Ph
Br–Ph
CH3–Ph
CH3O–Ph
C6H11
90
86
79
50
80
56
55
93
84
71
47
91
91
98
99
98
97
98
97
98
98
96
50
71
0
118
113
104
66
105
74
Fig. 1 (A) UV-Vis spectra of TiO2(B) (black line), Au/TiO2(B) (green line),
4-phenyl-1-butyne adsorbed on TiO2(B) (red line) and Au/TiO2(B) (blue line),
and free liquid 4-phenyl-1-butyne (cyan line). (B) FT-IR spectra of phenylacetylene
72
(0.1 g) adsorbed on TiO2
ÀN (0.02 g) after drying at 60 1C under vacuum
122
111
93
62
1
7
0
42
(a), TiO2 N (b), free liquid phenylacetylene (c, the intensity was divided by 4).
À
9
Fig. S4 (ESI†) shows the spectra in the wavenumber range from 600 to
4000 cmÀ1. The bands at 1650–1330 cmÀ1 arise from the characteristic vibrational
modes n(C–C) of the mono-substituted phenyl ring as well as the phenyl ring.
10
11
12
13e
14e
15e
0.5
5
0
Ph–CRC–CH3 Ph
Ph–CRC–Ph Ph
Ph
Ph–NH–CH3 32
84
by the AuNP photocatalyst on the nitrogen doped TiO2. It is known
that the oxidation state of Ti4+ can be reduced during nitrogen
doping.8 It would be expected that the Ti3+ sites on the support
surface would interact with the reactant much more strongly than
the Ti4+ sites. Indeed Ti3+, within such a defected structure, has
previously been found to be more able to adsorb and interact with
alkynes.9 Thus, the presence of Ti3+ in N-doped samples would
a
Reaction conditions: 0.38 Â 10À2 mmol of AuNPs, 0.5 mmol alkynes,
0.5 mmol anilines, 0.5 ml of toluene as solvent, reacted under visible
light at 40 1C for 25 h in an argon atmosphere. Determined by GC
b
c
analysis. The selectivity of imine was estimated from GC results.
d
Turnover number (TON) was calculated using mole content of gold.
Internal alkynes or amines.
e
Control experiments showed that very low conversion of alkynes likely enhance the conversion without sacrificing selectivity. The
was achieved in the dark at 40 1C. Given that the reaction under significant visible light absorption after adsorbing alkynes in
light illumination was maintained at the same temperature as the UV-Vis spectra (Fig. 1A and Fig. S2 and S3, ESI†), the shift of the
reaction in the dark, visible light contributes most for this reaction peaks and disappearance of the peak for RC–H in FT-IR spectra
in the system. The heat produced by the light irradiated AuNPs12 is (Fig. 1B and Fig. S4, ESI†) imply that the adsorption of alkynes on
minor as described in the ESI.† When the light intensity was the support surface affects the triple bond significantly, which may
reduced from 0.49 W cm2 to 0.43 W cm2 and 0.30 W cm2, while lead to the activation of the triple bond for reaction on the support
all the other experimental conditions remained unchanged, the surface. Indeed, Infrared Emission Spectroscopy (IES) analysis
conversion of 4-phenyl-1-butyne for 10 h decreased from 61% to indicates that the interaction of alkynes with the surface of N-doped
47% and 5%, respectively. These results further confirm that the TiO2 is much stronger than that with the undoped surface (Fig. S5a
reduction reactions were driven by visible light.
and S5b, ESI†). Furthermore, aniline adsorption on undoped
In the absence of the AuNPs (using supports only as catalysts), no (Fig. S5c, ESI†) and N-doped TiO2 surfaces (Fig. S5d, ESI†) is
reaction occurred. This demonstrated that the AuNPs are essential negligible, compared with the adsorption of phenylacetylene.
for the reaction. It is thus logical to assume that the properties of Therefore, the mechanism of the reaction is unlikely to involve
AuNPs can influence the photocatalyst performance. Catalysts with aniline activation on the surface of the support materials and is
different Au contents and TiO2ÀN support were prepared and the more likely to involve gold surface binding.
specific surface area of the AuNPs in 1 g of the photocatalysts was
The interaction between gold and the nitrogen atom of aniline
estimated (by assuming that the AuNPs are spheres) and the results was investigated using Raman spectroscopy (Fig. 2). No character-
are provided in Table S1 (ESI†). As the content of AuNPs increased istic Raman peaks of pure aniline liquid are observed for an aqueous
from 1% to 3%, the specific surface area of the AuNPs in Au/TiO2À
N
solution of 7.7 mM aniline (spectrum f). When the aqueous solution
increased from 0.7 m2 to 2.0 m2, and the conversion of 4-phenyl-1- was adsorbed on AuNP photocatalysts, most of the characteristic
butyne increased from 67% to 90%. The fact that higher conversion bands (spectrum b) can be observed. However, these bands cannot
is observed on AuNPs with a larger specific surface area implies that be detected (spectrum d) when the aqueous solution was adsorbed
the AuNPs activated at least one reactant.
on the support without the AuNPs. Thus, the AuNPs are necessary to
To explore the role of the supports in this novel photocatalytic provide enhancement of the Raman bands. Such a surface enhance-
process, reactions were examined using AuNPs loaded on a range ment effect is well known in Raman spectroscopy (the so-called
of different inorganic support materials. A very interesting finding SERS effect). The SERS effect observed may also be associated with
is that supports have much more important impact on the catalyst the enhanced reactivity of aniline, as the enhancement in Raman
activity than the size of AuNPs, sufficient to make the photocatalyst signals arises from charge transfer.10a Comparing with spectrum d,
active or inactive (Table S2 and Fig. S1, ESI†). Generally, a new band at 317 cmÀ1 is observed (in spectrum b), which can be
TiO2 supports exhibit superior performance to other supports attributed to the Au–N stretching mode.10a The shift of the C–N
(entries 1–4 in Table S2, ESI†). The highest activity was obtained stretching mode from 1279 cmÀ1 in spectrum a to 1267 cmÀ1 in
c
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terminal C atom of CRC. Ti3+ sites formed due to the N doping
have higher coordination ability than Ti4+ sites, which may also
make the terminal C atom more favourable for the H addition.
The final step would be desorption of the product molecules
from the support surface. This desorption process proceeds
because the adsorption of CQC on the surface is weaker than
CRC.13 Subsequently, the activated sites on the photocatalyst
return to the state needed to re-start the process (Fig. 3).
In conclusion, the hydroamination of alkynes with amines can
be driven with visible light using photocatalysts generated from
AuNPs supported on inorganic materials, with excellent yields
achievable at ambient temperature. During this photocatalytic
process, both AuNPs and the supports contribute to the activation
of reactants, which may lead to the discovery of new reaction
pathways for the hydroamination reactions. This approach reveals
a new class of useful catalytic processes with the potential to
utilize solar energy for cleaner, lower environmental impact
synthesis of fine chemicals for organic chemistry.
Fig. 2 Raman spectra of pure aniline liquid (a), aniline adsorbed on Au/TiO2ÀN
(b), and TiO2
À
N (d) from 7.7 mM aniline aqueous solution. The spectra of pure
N (e), and the 7.7 mM aniline aqueous solution (f) are
Au/TiO2 N (c), pure TiO2
À
À
also provided for comparison. The bands at 317 cmÀ1 and 1267 cmÀ1 are,
respectively, attributed to the stretching modes of Au–N and C–N.
Notes and references
1 (a) R. A. Widenhoefer and X. Q. Han, Eur. J. Org. Chem., 2006, 4555–4563;
(b) T. E. Mu¨ller, K. C. Hultzsch, M. Yus, F. Foubelo and M. Tada, Chem.
Rev., 2008, 108, 3795–3892; (c) M. Beller, J. Seayad, A. Tillack and
H. J. Jiao, Angew. Chem., Int. Ed., 2004, 43, 3368–3398; (d) R. Severin
and S. Doye, Chem. Soc. Rev., 2007, 36, 1407–1420.
2 (a) C. Y. Zhou, P. W. Chan and C. M. Che, Org. Lett., 2006, 8,
325–328; (b) W. B. Wang, G. B. Hammond and B. Xu, J. Am. Chem.
Soc., 2012, 134, 5697–5705; (c) Y. H. Zhang, J. P. Donahue and
´
C. J. Li, Org. Lett., 2007, 9, 627–630; (d) M. Stratakis and H. Garcıa,
Chem. Rev., 2012, 112, 4469–4506; (e) K. D. Hesp and M. Stradiotto,
J. Am. Chem. Soc., 2010, 132, 18026–18029; ( f ) X. Y. Liu, Z. Guo,
S. S. Dong, X. H. Li and C. M. Che, Chem.–Eur. J., 2011, 17, 12932–12945;
Fig. 3 The proposed mechanism for the photocatalytic hydroamination of
´
´
(g) G. Kovacs, A. Lledos and G. Ujaque, Angew. Chem., Int. Ed., 2011, 50,
11147–11151; (h) E. Mizushima, T. Hayashi and M. Tanaka, Org. Lett.,
alkynes on supported AuNPs. The interpretation is given in the text.
´
´
2003, 5, 3349–3352; (i) A. Corma, P. Concepcion, I. Domınguez,
´
V. Fornes and M. J. Sabater, J. Catal., 2007, 251, 39–47.
spectrum b further confirms that the chemical mechanism for the
charge transfer is dominant for this reaction system, where the
charge transfer is from aniline to metal atoms.10b Therefore, it is
concluded that amines are most likely to be activated on AuNPs via
the interaction between the nitrogen atoms and AuNPs, while the
alkynes would be adsorbed on the support material. The combined
effect is that the catalyst is able to activate the two reactants
simultaneously. Thus, we propose a tentative mechanism for the
photocatalytic hydroamination of alkynes on the AuNP photocata-
lysts as illustrated in Fig. 3.
3 (a) J. Seayad, A. Tillack, C. G. Hartung and M. Beller, Adv. Synth.
Catal., 2002, 344, 795–813; (b) F. Pohlki and S. Doye, Chem. Soc. Rev.,
2003, 32, 104–114.
4 (a) H. Y. Zhu, X. B. Ke, X. Z. Yang, S. Sarina and H. W. Liu, Angew.
Chem., Int. Ed., 2010, 49, 9657–9661; (b) X. Chen, H. Y. Zhu,
J. C. Zhao, Z. F. Zheng and X. P. Gao, Angew. Chem., Int. Ed., 2008,
47, 5353–5356.
5 (a) R. C. Hoft, M. J. Ford, A. M. McDonagh and M. B. Cortie, J. Phys.
Chem. C, 2007, 111, 13886–13891; (b) J. F. Li, S. Y. Ding, Z. L. Yang,
M. L. Bai, J. R. Anema, X. Wang, A. Wang, D. Y. Wu, B. Ren,
S. M. Hou, T. Wandlowski and Z. Q. Tian, J. Am. Chem. Soc., 2011,
133, 15922–15925.
6 M. Haruta, CATTECH, 2002, 6, 102–115.
The AuNPs absorb visible light due to the SPR effect. The
conducting electrons of the AuNPs gain the energy of light and
can migrate to the conduction band of the support, which would
result in the formation of positively charged AuNPs.11 The
nucleophilic aniline could interact with the positively charged
AuNPs forming N-centred radical cations, which would lead to
its activation for the electrophilic attack to the electron rich sites
7 A. Leyva and A. Corma, Adv. Synth. Catal., 2009, 351, 2876–2886.
8 (a) X. B. Chen and C. Burda, J. Phys. Chem. B, 2004, 108,
15446–15449; (b) Y. K. Kim, S. Park, K. J. Kim and B. Kim, J. Phys.
Chem. C, 2011, 115, 18618–18624.
9 M. Anpo, N. Aikawa, S. Kodama and Y. Kubokawa, J. Phys. Chem.,
1984, 88, 2569–2572.
10 (a) Z. Q. Tian, L. C. Lei and X. B. Jing, Acta Phys.-Chim. Sin., 1988, 4,
458–460; (b) G. S. Hu, Z. C. Feng, J. Li, G. Q. Jia, D. F. Han, Z. M. Liu
and C. Li, J. Phys. Chem. C, 2007, 111, 11267–11274.
of alkynes (e.g. the C atom at the position of RC–Ph). Mean- 11 (a) Z. F. Zheng, H. W. Liu, J. P. Ye, J. C. Zhao, E. R. Waclawik and
H. Y. Zhu, J. Mol. Catal. A: Chem., 2010, 316, 75–82; (b) Z. F. Zheng,
J. Teo, X. Chen, H. W. Liu, Y. Yuan, E. R. Waclawik, Z. Y. Zhong and
H. Y. Zhu, Chem.–Eur. J., 2010, 131, 17885–17893.
while, alkynes could be activated via the interaction of the C–H
bond of the terminal alkynes with active Ti sites in the support,
preferably the electron rich sites with a lower oxidation state, 12 W. Zhang, Z. P. Li, Z. Q. Guan, H. Shen, W. B. Yu, W. D. He,
such as Ti3+ sites. The next step would most likely entail the
addition of the two activated reactants: the N atom links to the C
X. P. Yan, P. Li and H. X. Xu, Chin. Sci. Bull., 2012, 57, 68–71.
13 (a) W. Huang, J. R. McCormick, R. F. Lobo and J. G. Chen, J. Catal.,
2007, 246, 40–51; (b) W. Huang, W. J. R. Pyrz, R. F. Lobo and
atom of the phenyl ring meanwhile a H atom is added onto the
J. G. Chen, Appl. Catal., A, 2007, 333, 254–263.
c
2678 Chem. Commun., 2013, 49, 2676--2678
This journal is The Royal Society of Chemistry 2013