Monodisperse amorphous CuB23 alloy short nanotubes: Novel efficient catalysts for Heck coupling of inactivated alkyl halides and alkenes

Article abstract of DOI:10.1039/c4ra08517e

Heck-type coupling of inactivated alkyl halides and alkenes, catalyzed by amorphous CuB₂₃ alloy short nanotubes, has been developed. Such couplings occur on the surfaces of nanotubes via a single-electron oxidative reaction. The results indicate that CuB₂₃ nanotubes are efficient catalysts to replace Pd and Ni complexes for such Heck-type coupling. This journal is

Full text of DOI:10.1039/c4ra08517e

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Monodisperse amorphous CuB23 alloy short
nanotubes: novel efficient catalysts for Heck
Cite this: RSC Adv., 2014, 4, 45838
coupling of inactivated alkyl halides and alkenes†
c
ab
Fan Yang,ab Shi Yan Fu,ab Wei Chu, Chun Liab and Dong Ge Tong
Received 12th August 2014
Accepted 16th September 2014
*
*
DOI: 10.1039/c4ra08517e
www.rsc.org/advances
Heck-type coupling of inactivated alkyl halides and alkenes, catalyzed
Recently, amorphous metal–boride (M–B) alloy catalysts with
by amorphous CuB23 alloy short nanotubes, has been developed. Such well-dened nanostructures have attracted much attention,
couplings occur on the surfaces of nanotubes via a single-electron because of their low cost and unusual properties such as
oxidative reaction. The results indicate that CuB23 nanotubes are isotropic structure, high concentration of coordinatively
efficient catalysts to replace Pd and Ni complexes for such Heck-type unsaturated sites, and chemical stability.5 For example, Chen
coupling.
et al. used lyotropic liquid crystals with layered structures
(formed by mixing nonionic and anionic surfactants) as
templates to obtain M–B long nanotubes that gave excellent
catalytic performances in hydrogenation reactions.5a Li, and
Tong et al. prepared mesoporous M–B materials with good
catalytic hydrogenation performances.5b,c M–B nanoowers and
hollow spheres with excellent performances in hydrolyzing
metal borohydrides to produce H2 have also been successfully
prepared.5d,e Various M–B materials with superior catalytic
performances, including yolk–shell nanostructures,5f hollow
nanospindles,5g nanospheres,5h honeycombs,5i and nanowires,5j
have also been reported. More recently, Li et al. developed Co–B
nanospheres as efficient catalysts for Heck-type reactions.6 Cu
complexes can efficiently catalyze Heck-like cyclizations of
oxime esters,1c and B-doped Cu catalysts have excellent catalytic
activities in hydrogenation7a or dehydrogenation.7b Based on
these results, we attempted to prepare an amorphous Cu–B
alloy with a well-dened nanostructure as a novel catalyst with
high activity for Heck-type reactions. To our knowledge, there
have been no previous reports of the use of amorphous Cu–B
alloys as catalysts in Heck reactions.
Introduction
Heck cross-coupling, which forms an intermolecular C–C bond
between a halide or sulfonate electrophile and an alkene under
mild conditions, is one of the most intensively studied organic
reactions, because of its applications in many areas, including
natural products and ne chemical syntheses.1 Until now, most
catalysts for Heck reactions have been based on Pd,2 e.g., Pd
complexes and Pd nanoparticles. Among these Pd catalysts,
soluble Pd compounds, generally phosphine–Pd complexes, are
the most efficient catalysts for the Heck reaction.2 However, Pd
catalysts are very expensive, and the ligands in Pd complexes are
generally toxic and therefore cannot be used. Catalysts based on
Ni, Co, Cu, and Fe or their complexes have therefore recently
been developed as novel catalysts to replace Pd catalysts.3
However, none of these can rival Pd in synthetic versatility
except Ni, especially in Heck-type reactions involving inacti-
vated alkyl halides.4 More efficient and cheaper catalysts for
Heck reactions therefore need to be developed.
In this study, amorphous CuB23 alloy short nanotubes were
prepared by a facile solution plasma method,5h–j,8 and used to
catalyze Heck reactions of inactivated alkyl halides, with good
results (Scheme 1).
Fig. 1a shows a typical low-magnication image of CuB23
short nanotubes prepared by the reaction of [Cu(NH3)4]2+ and
KBH4 in poly(ethylene glycol) solution, intrigued by a solution
aMineral Resources Chemistry Key Laboratory of Sichuan Higher Education
Institutions, College of Materials and Chemistry & Chemical Engineering, Chengdu
University of Technology, Chengdu 610059, China. E-mail: tongdongge@163.com;
Fax: +86 28 8407 9074
bState Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu
University of Technology, Chengdu 610059, China
cCollege of Chemical Engineering, Sichuan University, Chengdu 610065, China.
E-mail: chuwei1965@foxmail.com; Fax: +86 28 8540 3397
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra08517e
Scheme 1 Heck cross-couplings involving inactivated alkyl halides.
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Table 1 CuB23-short-nanotubes-catalyzed cross-coupling of cyclo-
hexyl iodide and methyl acrylate
Entry
Deviation from conditions described above
Yielda/%
Fig. 1 (a) Low magnification STEM image; (b) enlarged STEM image;
(c) SAED pattern of the as-prepared CuB23 short nanotubes. The scale
bar for (b) is 5 nm.
1
2
3
4
5
6
7
8
None
91
<2
62
74
65
53
71
66
59
26
94
67
<2
<2
<2
7
18
78
91
7
34
68
30
70
51
35
48
52
23
THF instead of NMP
PhCF3 instead of NMP
DMF instead of NMP
K3PO4 instead of Na2CO3
Na3PO4 instead of Na2CO3
K2CO3 instead of Na2CO3
NaOH instead of Na2CO3
KOH instead of Na2CO3
333 K instead of 353 K
2.0 equiv. of methyl acrylate
1.5 equiv. of cyclohexyl iodide
Commercial nano B
Nano CuO catalyst
plasma. From Fig. 1a, it can be seen that CuB23 short nanotubes
were obtained on a large scale and with a uniform size distri-
bution. The enlarged transmission electron microscopy (TEM)
image (Fig. 1b) shows that the nanotubes had an average length
of 50 nm and diameter of 10 nm. The average wall thickness was
about 2 nm. Electron diffraction analysis of the CuB23 nano-
tubes showed that they are noncrystalline, based on the
observed halos (Fig. 1c). Further evidence comes from the X-ray
diffraction (XRD) patterns of samples heated at different
temperatures in an Ar atmosphere (Fig. S1a–c†). It can be seen
that the as-prepared CuB23 nanotubes are poorly crystalline or
amorphous. Aer annealing in an Ar atmosphere at 573 K for 2
h, crystalline CuB23 (JCPDS-71-01-02) was obtained (Fig. S1c†).
The inductively coupled plasma atomic emission spectroscopy
(ICP-AES) results show that the B/Cu ratio of CuB23 nanotubes is
23.01, which is almost the same as that of conventional CuB23
(22.99). The Brunauer–Emmett–Teller surface area of the CuB23
nanotubes is 84.7 m2 gꢀ1, which is higher than that of a
conventional irregular CuB23 alloy (23.2 m2 gꢀ1; Fig. S2 and
S3†). The formation of homogeneous alloy nanoparticles was
conrmed by black cross-sectional compositional line analyses
in high-angle annular dark-eld-scanning TEM (STEM) experi-
ments (Fig. S4a and b†), and energy-dispersive X-ray spectros-
copy (EDS) performed at different points (Fig. S4c–e†). The
results showed that the short nanotubes are composed of Cu
and B. X-ray photoelectron spectroscopy (XPS; Fig. S5a–c†)
showed that the Cu and B species in the CuB23 short nanotubes
are present in the elemental state.9 The shi in the binding
energy of B species relative to pure B (187.1 eV) indicates that
electrons are partly transferred from B to the vacant d-orbital of
Cu, resulting in electron-rich Cu species in CuB23 nanotubes.
These electron-enriched Cu sites are favorable for the Heck
coupling reaction between inactivated alkyl halides and
alkenes.7,10 To the best of our knowledge, this is the rst
example of CuB23 short nanotubes prepared using a solution
plasma process (such nanostructures have not been reported
previously).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
Nano Cu2O catalyst
Commercial nano Cu catalyst
Conventional Cu–B catalyst
Reaction in the dark
1.0 equiv. of cyclohexyl bromide
1.0 equiv. of cyclohexyl chloride
Cu(2-ethylhexanonate)2 (ref. 1c)
PdCl2(dppf)4a
Co–B nanospheres6
Ni(cod)2 (ref. 12a)
Co(acac)2 (ref. 12b)
Fe(acac)3 (ref. 12c)
Pd–UiO67 (ref. 12d)
Pd@XH12e
AgNO3/NXS12f
a
Calculated by 1H NMR spectroscopy of the crude reaction mixtures
using an internal standard.
methyl-2-pyrrolidone (NMP) were used as reaction media. The
results show that THF was not effective in this reaction (Table 1,
entry 2). PhCF3 and DMF (Table 1, entries 3 and 4) gave low
yields; NMP gave the best results (Table 1, entry 1). A basic
environment was also important for this Heck reaction; almost
all our experiments conrmed this. Common bases, namely
K3PO4, Na3PO4, Na2CO3, K2CO3, NaOH, and KOH, were tested;
Na2CO3 was found to be the best, and was therefore selected as
the base for subsequent experiments (Table 1, entries 5–9). The
most suitable temperature was 80 ꢁC. Lowering the reaction
ꢁ
temperature to 60 C resulted in a signicant decrease in the
reaction efficiency (Table 1, entry 10). A slight excess of methyl
acrylate was favorable in this cross-coupling (2 equiv.; Table 1,
entry 11), but reactions using a slight excess of cyclohexyl iodide
gave lower yields (Table 1, entry 12). If the CuB23 short nano-
tubes were ltered out aer reaction for 2 h, no conversion of
cyclohexyl iodide was observed (Fig. S6†), conrming that this
reaction occurred on the CuB23 nanotube surfaces, because
The results for the catalysis of the Heck reaction with CuB23
short nanotubes are shown in Table 1. The yields of the corre-
sponding products were moderate to good. The experiments
indicated that 2 mol% of the catalyst was sufficient to catalyze
the reaction. Solvents such as triuoromethylbenzene (PhCF3),
tetrahydrofuran (THF), N,N-dimethylformamide (DMF), and N-
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c†). Furthermore, without the presence of Na2CO3 during the
reaction, Cu–R (Fig. S8d,† m/z ¼ 231, 233) and CuBI+ (m/z ¼ 200,
201, 202, 203) were formed, indicating that R groups and I
atoms bonded with Cu-active sites to form M (Scheme 2) on the
surfaces of the CuB23 short nanotubes during the reaction
(Fig. S8d and e†). Na2CO3 therefore plays a key role in forming
the nal product of the Heck reaction (Fig. S8e†). Based on
these results, we propose that the heterogeneous Heck cross-
coupling catalyzed by our CuB23 short nanotubes proceeds via
the pathway shown in Scheme 2. First, Cu0 on the surfaces of
the CuB23 nanotubes is oxidized by giving away one electron to
alkyl halides. As a result, the carbon-centered radical R1
(Scheme 2) and B–CuI–I formed. Second, alkene addition to the
carbon-centered radical R1 forms the radical R, and then R
bonds with B–CuI–I on the CuB23 nanotube surfaces to form the
alkyl Cu(II) species M. Finally, base-assisted b-hydride elimina-
tion from the alkyl Cu(II) species M gives the cross-coupling
product P (Scheme 2). However, the exact mechanism of the
CuB23-short-nanotube-catalyzed alkyl-Heck-type cross-coupling
is not clear; further studies involving theoretical calculations
are needed, and are currently underway.
Scheme 2 Plausible catalytic processes for the CuB23-short-nano-
tubes-catalyzed alkyl-Heck-type cross-coupling between cyclohexyl
iodide and methyl acrylate.
homogeneous catalysis by Cu species leached into the solution
could be ruled out. No product was formed in the presence of B,
CuO, or Cu2O (Table 1, entries 13–15), which conrms that
metallic Cu was the active site. The catalytic role of electron-
enriched Cu sites was conrmed by the Cu 2p3/2 binding
energy shis from Cu0, CuI, and CuII to Cu0 during the reaction,
shown by in situ XPS measurements (Fig. S7†). This means that
the coupling occurs on the surfaces of the CuB23 short nano-
tubes, and is a single-electron oxidative process. Time-of-ight
secondary-ion mass spectrometry analyses of the catalyst in
the initial reaction stage indicated that I atoms could bond with
the surfaces of the CuB23 short nanotubes to form B–Cu–I,
based on the appearance of CuI+ (m/z ¼ 190, 192), BI+ (m/z ¼
137, 138), and CuBI+ (m/z ¼ 200, 201, 202, 203) peaks (Fig. S8a–
The product yields obtained using Cu and conventional
CuB23 catalysts were inferior to those achieved using CuB23
short nanotubes (Table 1, entries 16 and 17). The better
performance of CuB23 relative to that of Cu is attributed to
electron-enriched Cu sites, identied using XPS, which are
favorable for oxidative addition of metallic Cu to a carbon–
halogen bond.6,10 Additionally, the performance of Cu–B short
Table 2 CuB23-short-nanotubes-catalyzed cross-coupling of cyclohexyl iodide and different alkenes
Entry
Alkenes
Alkyl iodide
Product
Yielda,b/%
65
30c
41d
1
77 (86 : 14 E : Z)
73c (29 : 71 E : Z)
72d (69 : 31 E : Z)
2
3
87
80c
80d
59
39c
44d
4
76 (33 : 66 E : Z)
66c (33 : 66 E : Z)
55d (33 : 66 E : Z)
5
a
Yields of isolated product. b Product ratios were determined by 1H NMR spectroscopy of crude reaction mixtures. TBS ¼ tert-butyldimethylsilyl.
c
d
PdCl2(dppf) catalyst.4a Ni(cod)2 catalyst.12a
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nanotubes (active surface area SCu ¼ 47.3 m2 gꢀ1) is better than butyldimethylsilyloxy)-1-iodocyclohexane with acrylonitrile
that of conventional CuB23 (SCu ¼ 10.9 m2 gꢀ1) because their (Table 2, entry 5). A 50 : 50 ratio of cis to trans coupling products
unique structure facilitates mass transfer and increases the was obtained, which conrmed the proposed single-electron
accessibility of active sites to reactant molecules during pathway for coupling over CuB23 short nanotubes (Scheme 2).4a
heterogeneous catalysis.1d,5,11 Also, the reaction proceeded in
The data in Table 3 indicate that our CuB23 short nanotubes
the dark (Table 1, entry 18). The catalyst showed high activities also had high activities towards cyclopentyl iodide, cycloheptyl
in the Heck couplings of cyclohexyl bromide and cyclohexyl iodide, and iodooctane, as well as cyclohexyl iodide (Table 3,
iodide (Table 1, entries 1 and 19). Cyclohexyl chloride had low entries 1–6). The coupling of exo-2-norbornyl iodide with
activity in the reaction (Table 1, entry 20). The catalytic perfor- alkenes exhibited high diastereoselectivities (>95 : 5 d.r.; Table
mance of our CuB23 nanotubes was also compared with that of 3, entries 7 and 8), which further conrmed the proposed
recently reported Heck-coupling catalysts (Table 1, entries 21– single-electron pathway for coupling over CuB23 short nano-
29).12 Based on the yield, the catalytic performance of our CuB23 tubes (Scheme 2).4a The reaction yields (Tables 2 and 3) show
nanotubes is superior to those catalysts.
that the catalytic performance of our CuB23 short nanotubes is
The scope of our CuB23-nanotube-catalyzed Heck cross- superior to those of the most effective catalysts: Pd4a and Ni
coupling reaction with respect to alkenes (Table 2) and inacti- complexes12a in Heck coupling of inactivated alkyl iodides and
vated alkyl iodides (Table 3) was also investigated. Cyclohexyl alkenes (Tables 2 and 3). Considering the price difference
iodide reacted with methyl vinyl ketone, acrylonitrile, styrene, among CuB23, Pd and Ni complexes, our CuB23 short nanotubes
and 2-vinylpyridine to provide the corresponding products in are more cost effective.
moderate to good yields (Table 2, entries 1–4). To investigate the
Fig. 2 shows the recycling performance of CuB23 short
electron pathway over the CuB23 short nanotubes, we studied nanotubes. Our sample could be used 10 times with only a
the diastereoselectivity of the reaction of trans-2-(tert- slight loss of activity (8%). ICP-AES analysis shows that no Cu or
Table 3 CuB23-short-nanotubes-catalyzed cross-coupling of alkene with different alkyl iodide
Entry
Alkene
Alkyl iodide
Product
Yielda,b/%
77 (79 : 21 E : Z)
57c (61 : 39 E : Z)
77d(84 : 16 E : Z)
1
73 (88 : 12 E : Z)
56c (86 : 14 E : Z)
65d (86 : 14 E : Z)
2
3
82
72c
69d
80
72c
76d
4
5
78
57c
62d
57
40c
50d
6
7
83 (86 : 14 E : Z)
72c (86 : 14 E : Z)
69d (86 : 14 E : Z)
82 (91 : 9 E : Z)
76c (88 : 12 E : Z)
67d (86 : 14 E : Z)
8
a
b
c
Yields of isolated product. Product ratios were determined by 1H NMR spectroscopy of crude reaction mixtures. PdCl2(dppf) catalyst.4a
d
Ni(cod)2 catalyst.12a
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Fig. 2 Cycling performance of CuB23 short nanotubes for cross-
coupling of cyclohexyl iodide and methyl acrylate. Reaction condi-
tions: a catalyst containing 0.01 mmol CuB23 short nanotubes,
cyclohexyl iodide (5.0 mmol), methyl acrylate (7.5 mmol), Na2CO3
(10.0 mmol), NMP (10 mL), T ¼ 353 K, t ¼ 12 h, stirring rate ¼ 800 rpm.
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Conclusions
In summary, we have prepared amorphous CuB23 alloy short
nanotubes for the rst time, using a solution plasma process,
and successfully used this catalyst in the Heck coupling of
inactivated alkyl halides and alkenes. We showed that amor-
phous CuB23 alloy short nanotubes can replace Pd and Ni
complexes for Heck coupling of inactivated alkyl iodides and
alkenes. Furthermore, alkyl bromides were tolerated. The
CuB23-short-nanotubes-catalyzed
protocol proceeds
via
coupling on the surfaces of the nanotubes, by single-electron
oxidative reactions. More importantly, the catalyst is cheaper
than Pd and Ni catalysts and no ligands are needed. Work to
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underway in our laboratory.
Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (21376033), the Sichuan Youth
Science and Technology Innovation Research Team Funding
Scheme (2013TD0005), the Cultivating programme of Middle
aged backbone teachers (HG0092), the Cultivating programme
for Excellent Innovation Team of Chengdu University of Tech-
nology (HY0084) and Innovative Experimental Items for College
Students of Sichuan Province (SZH1106CX04).
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