High-Loading Single-Atom Copper Catalyst Supported on Coordinatively Unsaturated Al2O3 for Selective Synthesis of Homoallylboronates

Article abstract of DOI:10.1002/cssc.202000536

Single-atom catalysts (SACs) as a bridge between hetero- and homogeneous catalysis have attracted much attention. However, it is still challenging to generate stable single atoms with high metal loadings, and the application of SACs in traditionally homogeneous catalytic reactions is highly desirable. Herein, a Cu SAC with a high Cu loading of 8.7 wt % supported on coordinatively unsaturated Al₂O₃ was prepared and used in the amine-free synthesis of homoallylboranes. Up to 99 % conversion, 95 % 1,4-selective boration of the enals, and 48–68 % isolated yields of homoallylboranes were achieved, equaling the results of reported homogenous catalysts, and the system was more efficient and stable than nano Cu/γ-Al₂O₃. Mechanistic investigation indicated that Cu-Bpin species are the active intermediates of selective boration. The superior catalytic and recycling performance of Cu SAC paves an efficient and green path toward selective synthesis of homoallyborane fine chemicals.

Full text of DOI:10.1002/cssc.202000536

Accepted Article
Title: High Loading Single-Atom Copper Catalyst Supported with
Coordinatively Unsaturated Al2O3 for Selective Synthesis of
Homoallylboronates
Authors: Tenglong Guo, Nanfang Tang, Feng Lin, Qinghao Shang,
Shuai Chen, Haifeng Qi, Xiaoli Pan, Chuntian Wu, Guoliang
Xu, Jian Zhang, Dezhu Xu, and Yu Cong
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To be cited as: ChemSusChem 10.1002/cssc.202000536
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10.1002/cssc.202000536
ChemSusChem
COMMUNICATION
High Loading Single-Atom Copper Catalyst Supported with
Coordinatively Unsaturated Al2O3 for Selective Synthesis of
Homoallylboronates
†
†
†
Tenglong Guo,[a] Nanfang Tang,*[a] Feng Lin,[a, c] Qinghao Shang,[a, b] Shuai Chen,[a] Haifeng Qi,[a, b]
Xiaoli Pan,[a] Chuntian Wu,[a] Guoliang Xu,[a] Jian Zhang,[a] Dezhu Xu,[a] and Yu Cong*[a]
Abstract: Single-atom catalysts (SACs) as a bridge between hetero-
dispersed Zn-N-C with an ultrahigh Zn loading of 9.33 wt% by
and homogeneous catalysis have attracted much attention. However, adopting a very low annealing rate of 1 oC/min.[7] However,
it is still challenging to generate stable single atoms with high metal
loadings and the application of SACs in more traditionally
homogeneous catalytic reactions are highly desirable. Herein,
coordinatively unsaturated Al2O3 supported Cu SAC with a high Cu
loading of 8.7 wt% was prepared and firstly used in the amine-free
synthesis of homoallylboranes. Up to 99% conversion and 95% 1,4-
selective boration of the enals and 48-68% isolated yields of
homoallylboranes were achieved, equaling to those of reported
homogenous catalysts and more efficient and stable than nano Cu/γ-
Al2O3. Mechanistic investigation indicated that Cu-Bpin species are
the active intermediates of selective boration. The superior catalytic
and recycling performance of Cu SAC paves an efficient and green
path toward selective synthesis of homoallyborane fine chemicals.
SACs with high-loading metal anchored on metal oxides, the
most popular industrial catalyst support (such as Al2O3), have
yet been achieved.
On the other hand, owing to the 100% metal utilization, rich
unsaturated coordination and unique electronic properties,[8,9]
SACs are promising in realizing the homogeneous synthesis
with heterogeneous catalyst but has not been exclusively
demonstrated to date. Presently, only Lang et al. reported that
the single-atom Rh catalysts supported on ZnO nanowires
demonstrate similar efficiency (TON ≈ 40000) to homogeneous
Wilkinson’s catalyst (TON ≈ 19000).[10] Liu et al. found that the
Fe-N-C single-atom catalysts with high oxidation state of Fe
show high performance in selective oxidation of C-H bond.[11] As
a bridge connecting homogeneous and heterogeneous catalysis,
high loading SACs are expected to exhibit superior performance
in more conventional homogeneous catalytic reactions.
Organoboron compounds are of great importance in organic
synthesis, not only for their special characteristics and biological
activities,[12] but also for the key and broad utility in a variety of
reactions.[13] Conjugate addition of diboron to α,β-unsaturated
carbonyl and related compounds catalyzed by homogeneous
transition metals are the most common preparation method.[14]
As versatile intermediates, homoallyl-boronates could easily give
access to multifunctional organic compounds through additional
derivatization of alkene moiety and boron group,[15] but their
synthesis is very challenging due to the poor regioselectivity.
Using borane conjugate addition/Wittig reaction between the
α,β-unsaturated aldehydes (enals), bis(pinacolato) diboron
(B2Pin2) and 2-(triphenylphosphoranylidene) acetate esters in a
one-pot three-component system, Ibrahem et al. synthesized a
Homogeneous catalysts often exhibit high activity and selectivity
due to the well-defined nature of active sites, whereas
heterogeneous catalysts own the advantages of easier
separation from reaction system and recyclability. In recent
years, single-atom catalysts (SACs) with isolated atoms
dispersed onto the support surface, are becoming a new frontier
in catalysis field which exhibits high performances in a number
of reactions, such as CH4 conversion,[1] methanol reforming,[2]
CO2 conversion,[3] CO conversion[4] and photo-electrocatalysis.[5]
However, the metal loading amount of most synthized SACs is
relatively low (< 1 wt%) and not suitable for practical usefulness.
Currently, one major challenge in single-atom catalysis is to
ensure the loading amount high enough for practical applications
while maintaining metal species atomic dispersed. The present
synthetic methods are mainly limited to stablize metal atoms on
nitrogen-doped carbon materials with rich anchoring sites. Liu et
al. reported a Ni-N-C SAC with 7.5 wt% Ni for cellulose
conversion to polyols with high hydrogenation activity and
stability under harsh conditions.[6] Li et al. prepared an atomic
series of homoallylboronates under copper[16a] or NHC[16b]
-
catalyzed conditions as shown in (Scheme 1a). In them, amines
were employed to form iminium intermediates to ensure the high
1,4-selective boration of enals rather than the 1,2- boration, and
the separation and recyclability issues associated with efficient
homogeneous catalysts remains the major problem. Therefore,
to develop an efficient and green amine-free homoallylboronates
synthesis path is highly desirable.
[a]
Dr. T. Guo, Dr. N. Tang, Dr. F. Lin, Q. Shang, S. Chen, H, Qi, X.
Pan, Dr. C. Wu, Dr. G. Xu, Dr. J. Zhang, Prof. D. Xu, Prof. Y. Cong,
CAS Key Laboratory of Science and Technology on Applied
Catalysis, Dalian Institute of Chemical Physics, Chinese Academy
of Sciences, Dalian 116023 (China), E-mail: ycong@dicp.ac.cn,
nftang@dicp.ac.cn
Herein, Al2O3 rich in coordinatively unsaturated Al3+ sites
anchored Cu single-atom catalyst (named as Cu1/rcu-Al2O3) was
prepared through evaporation induced self-assembly (EISA)
method. Aberration-corrected high-angle annular dark-field
scanning transmission electron microscopy (AC-HAADF-STEM),
extended X-ray absorption fine structure (EXAFS) spectroscopy
[b]
[c]
Q. Shang, H. Qi, University of Chinese Academy of Sciences,
Beijing 100049 (China)
Dr. F. Lin, Dalian Nationalities University, Dalian 116600 (China)
† These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/cssc.202000536
ChemSusChem
COMMUNICATION
Scheme 1. Catalytic one-pot three-component synthesis of homoallylboranes.
Figure 1. a) HR-TEM images of Cu1/rcu-Al2O3. b, d) HAADF-STEM images of
Cu1/rcu-Al2O3. c) Normalized image intensity in the c direction of the Cu1/rcu-
Al2O3. e) EDS mapping images of copper, aluminium and oxygen. f) Extended
X-ray absorption fine structure (EXAFS) spectra of Cu1/rcu-Al2O3, CuO, Cu2O
and Cu foil. Note that the intensity of Cu foil is transformed by 0.3. The circles
in the figures represent the single Cu atoms.
and electron paramagnetic resonance (EPR) measurements
confirmed that Cu were atomically dispersed on rcu-Al2O3. 27Al
solid state MAS-NMR spectra indicated that coordinatively
unsaturated pentacoordinate Al3+ (Al3+penta) centers served as
anchor sites to stabilize Cu atoms through oxygen bridge bonds.
The Cu1/rcu-Al2O3 demonstrates remarkable activity in amine-
free 1,4-selective boration of enals and selective synthesis of
homoallylboranes, comparable to homogeneous catalysts
(Scheme 1b).
Al2O3 and Cu/γ-Al2O3 exhibit type IV isotherm shape. BET
surface area of Cu1/rcu-Al2O3 and Cu/γ-Al2O3 are 172.5 m2/g
and 126.6 m2/g, respectively, and their total pore volumes are
0.44 cm3/g and 0.36 cm3/g, respectively, and the BJH pore size
are 6.7 nm and 9.0 nm, respectively. Different from the Cu1/rcu-
Al2O3, the diffraction peaks at 35.5o, 38.7o, 48.9o and 61.7o
corresponding to crystalline CuO can be clearly observed
(Figure S6). The HAAD-STEM and EDS images further confirm
the existence of 3D CuO nanoparticles with diameter from
several nanometers to several tens nanometers in Cu/γ-Al2O3
(Figure S7). From Figure S8, there were two notable peak at
1.50 Å and 2.49 Å assigned to Cu-O and Cu-O-Cu contribution
from the EXAFS result. Cu2+ species can be clearly identified
from the XANES and XPS results (Figure S9 and S10). EPR
spectroscopy was further collected to investigate the different
morphology between Cu1/rcu-Al2O3 and Cu/γ-Al2O3. It has been
reported that atomically dispersed Cu2+ species is EPR active
due to the unpaired electron in the dx2-y2 orbital of the d9 electron
configuration. Whereas, the EPR signal may broaden or even
mute as Cu2+ species get closer.[22] As can been found in Figure
2a, the EPR intensity of Cu1/rcu-Al2O3 is much stronger than that
of Cu/γ-Al2O3, indicating isolated Cu2+ ions and nano Cu2+
species supported on rcu-Al2O3 and γ-Al2O3, respectively.
The high resolution transmission electron microscopy (HR-
TEM) image of the Cu1/rcu-Al2O3 revealed that no Cu nano
particles but only amorphous alumina was observed (Figure 1a).
This result is consistent with the result of X-ray diffraction (XRD),
in which no Cu diffraction peak except highly disordered Al3+
phase can be observed (Figure S1).[17] The AC-HAADF-STEM
and EDS images further revealed that Cu is homogeneously
distributed on alumina (Figure 1b-1e), though only a few Cu
single atoms are observed due to the weak image contrast of Cu
in rcu-Al2O3.[18] The Cu loading was determined to be 8.7 wt% by
ICP-OES and 8 wt% by EDS analysis, which is much higher
than that prepared by other techniques.[19,20] Extended X-ray
absorption fine structure (EXAFS) could verify the existence of
isolated Cu single atoms (Figure 1f). There was only one notable
peak at 1.47 Å from the Cu-O contribution, while no peaks at
2.49 Å were examined ascribed to Cu-O-Cu bonding. The best-
fit curve of Cu1/rcu-Al2O3 (Figure S2) suggested that the Cu-O
shell has an average coordination number of 3.3 ± 0.5,
indicating Cu atoms stabilized by the oxygen atoms in the rcu-
Al2O3 surface. The X-ray absorption near-edge structure
(XANES) spectra were also measured to determine the
electronic properties of Cu1/rcu-Al2O3 (Figure S3). The Cu K-
edge profile shows an absorption edge energy at 8997.1 eV and
a shoulder at 8986.9 eV, which are typical absorption of Cu2+. X-
ray photoelectron spectro-scopy (XPS) measurements testified
this result (Figure S4). The Cu 2p3/2 peak at 933.1 eV can be
assigned to Cu2+, whereas the two new peaks at 942.3 eV and
962.7 eV are characteristic Cu 2p satellites generated from the
parallel excitation (shakeup) of a ligand electron into an empty
Cu 3d orbital.[21] As a contrast, γ-Al2O3 supported Cu nano-
catalyst was prepared by incipient wetness impregnation.
Surface area and pore size distribution of these two Cu catalysts
were determined by N2 physisorption (Fig. S5). Both Cu1/rcu-
27Al solid state MAS-NMR spectra were collected to better
understand why Cu single atoms can be stabilized on rcu-Al2O3.
The spectrum of the rcu-Al2O3 exhibits three peaks centered at 6,
35 and 63 ppm chemical shift (Figure 2b). The two characteristic
27Al MAS-NMR peaks at 6 and 63 ppm can be assigned to Al3+
ions in octahedral (Al3+octa) and tetrahedral (Al3+tetra) coordination,
respectively. The NMR peak at 35 ppm represent Al3+ ions in
pentahedral (Al3+penta) coordination, which accounts for about
40% of all the Al3+ ions.[23] These large amount of coordinatively
unsaturated Al3+ sites are formed by thermal decomposition and
dihydroxylation of crown-ether-type complexes, which are
created from the hydrophilic poly(ethyleneoxide) of P123
coordinated with Al3+ ions during hydrolysis and condensation.[17]
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10.1002/cssc.202000536
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COMMUNICATION
Table 1. Catalytic performance of various catalysts for selective β-boration of
cinnamic aldehyde 2a.
Temp
[oC]
Time
[h]
Con.
[%][a]
Entry
Cat.
3a/3a‘[a]
95/5
93/7
23/77
90/10
95/5
-
Ref.
Cu1/rcu-
Al2O3
This
work
1
2
3
4
5
6
65
65
25
90
70
22
0.5
0.5
6
99
75
75
19
95
99
nano Cu/γ-
Al2O3
This
work
Figure 2. a) EPR spectra of Cu1/rcu-Al2O3 and Cu/γ-Al2O3. b) 27Al MAS NMR
spectra (solid line) and their deconvolution results (short line) of γ-Al2O3, Cu/γ-
Al2O3, rcu-Al2O3, and Cu1/rcu-Al2O3.
Pd complex
24a
24b
16b
24c
Cu
complex
1
After loading Cu species the Al3+penta sites decrease to 11 %, and
NHC[b]
1
the Al3+
sites reveal a substantial increase. This result
octa
indicates that Al3+
centers can anchor Cu atoms through
Rh
complex
penta
1
weak oxygen bridges, and coordinatively saturate these sites
from pentahedral coordination to octahedral coordination.
Whereas for γ-Al2O3, no coordinatively unsaturated Al3+penta sites
can be found. There are no anchor sites for Cu species, and
CuO nanoparticles can be clearly observed on γ-Al2O3 because
of the weak metal-support interaction.
[a] Conversion and selectivity determined by the 1HNMR spectra of the crude
reaction mixture. [b] NHC = N-heterocyclic carbene.
conversion and selectivity (Table 1, entries 3 and 4), and it
exhibits comparable reactivity to homogenous NHC[16b] and
Rh[24c] even in shorter reaction time (Table 1, entries 5 and 6).
These results indicate that the single-atom Cu1/rcu-Al2O3
catalyst is a very efficient catalyst for the selective β-boration of
cinnamic aldehyde. The origin of the high catalytic performance
could be assigned to the maximum Cu2+ active sites of Cu SACs,
which were contributed to the high dispersion of Cu atoms
stabilized through Cu-O bridge bond.
Additionally, the Cu1/rcu-Al2O3 can be recycled by centrifu-
gation and reused three times without significant loss of activity
and selectivity (Figure. S11 and Figure S12). The amount of Cu
leached into the filtrate from nano Cu/γ-Al2O3 and Cu1/rcu-Al2O3
was detected by ICP-OES, respectively (Figure. S13). 0.127 mg
Cu of nano Cu/Al2O3 leached into the solvent and 6% yield of 3a
was formed when the filtrate was used instead of the catalyst.
Comparetively, only 0.00179 mg Cu of Cu1/rcu-Al2O3 leached
into the filtrate and no product was formed when the filtrate was
used instead of the catalyst. There was no detectable CuO nano
particles in the HR-TEM and EDS images after three catalytic
cycles (Figure. S14 and Figure. S15). The EPR characterization
of the recycled Cu1/rcu-Al2O3 catalyst also revealed that the
atomically dispersed morphology and the electronic state of Cu
species remained unchanged (Figure. S16). These results
confirmed that Cu single atoms are strongly bound by the Al2O3
support under working conditions without obvious leaching or
aggregation and more stable than nano Cu/γ-Al2O3.
The catalytic β-boration of enals was firstly examined which
undergoes either 1,4-addition or 1,2-addition. High 1,4-
selectivity is an important premise and basis for the further
synthesis of homoallylboranes. In the initial experiment,
bis(pinacolato) diboron (B2pin2; 1) was reacted with cinnamic
aldehyde 2a in tAmOH at 90 oC using single-atom Cu1/rcu-Al2O3
as the catalyst, K3PO4 as the base, giving the corresponding
Michael addition products 3a and 3a’ in a ratio of 93:7 with a
high conversion (99%) (see supporting information Table S1,
entry 1). With the temperature lowered to 65 oC, the 1,4-
selectivity slightly improved (95:5) (Table S1, entry 2). However,
decreased conversions (65% or 42%) were obtained when the
o
temperature reduced to 40 C (Table S1, entry 3) or without a
base (Table S1, entry 4). Other screened bases, i.e., KOtBu,
Cs2CO3 and K2CO3, were of less advantage to the 1,4-addition
(Table S1, entries 5-7). Various solvents were also investigated
(Table S1, entries 8-11). The highest conversion and 1,4-
selectivity (99% conv. and 95:5 selectivity) were achieved in only
30 minutes with toluene and MeOH mixture (v:v = 20:1) as the
solvent (Table S1, entry 10), while the sole solvent of toluene led
to a very low conversion (14%) (Table S1, entry 11). The
reaction efficiency reduced with the Cu loading (4 wt% and 1
wt%) decreasing (Table S1, entries 12 and 13) and the reaction
hardly occurred without a catalyst (Table S1, entry 14).
When nano Cu/γ-Al2O3 (prepared by incipient wetness
impregnation as shown in the supporting information) with the
same Cu loading was used as catalyst, only 75% conversion
that partly attributed to the soluble Cu species and 93:7 1,4-
selectivity was achieved. Reaction monitoring confirmed there
are no induction periods in the selective β-boration processes
catalyzed by Cu1/rcu-Al2O3 or nano Cu/γ-Al2O3 (Table S2),
revealing the higher reactivity of single-atom Cu1/rcu-Al2O3
relative to nano Cu/γ-Al2O3 (Table 1, entry 2). Compared with
the reported homogeneous catalysts, e.g., Pd[24a] and
IPrCuOMe,[24b] Cu1/rcu-Al2O3 is also more active with regard to
With the optimal results in hand, synthesis of homoallyl-
boronates was further carried out via one-pot three-component
reaction among 1, enals 2, and Wittig reagents 4 by using the
single-atom Cu1/rcu-Al2O3 as catalyst, K3PO4 as the base, and
mixture of Toluene/MeOH (v:v = 20:1) as the solvent. Ester
substituted Wittig reagents 4 reacting with in-situ generated 3a
gave good yields
(59-62%)
of
the corresponding
homoallylboronates 5a-5c (Table 2, entries 1-3). It is noteworthy
that 63% yield (1.04 g) of 5a could also be obtained in gram-
scale synthesis, revealing the high efficiency of the β-boration
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10.1002/cssc.202000536
ChemSusChem
COMMUNICATION
Table 2. Cu1/rcu-Al2O3-catalyzed one-pot three-component synthesis of
homoallylboranes 5.[a]
Yield
Entry
1
R
R1
Product
3/3’[c]
Scheme 2. Derivations of the homoallylborane 5a.
[%][b]
62
Organoboron compounds are of great importance
intermediates in organic synthesis. To demonstrate the potential
application of this method, homoallylboronate 5a was further
derived to synthetize the valuable homoallylic alcohol 6 and
Ph
OEt
95:5
(63)[d]
2
3
Ph
Ph
OMe
OPh
60
59
95:5
95:5
homoallylic amine
7
under mild condition through the
transformation of C-B bond (Scheme 2).
To investigate reaction mechanism of the above reaction,
controlled experiments were conducted and characterized by
11B-NMR. Equivalent Cu1/rcu-Al2O3 catalyst reacting with 1 and
K3PO4 at room temperature for 30 min without substrates 2
resulted in the formation of a new signal at 21.3 ppm in the 11B
NMR spectrum (Figure 3c). Following that, the original signal of
1 at 30.5 ppm disappeared (Figure 3a). While in the absence of
Cu1/rcu-Al2O3, no new signals were detected under the same
conditions (Figure 3b). These results revealed that the B-B bond
of 1 was cleaved and new boration species coordinated with Cu
single atom was generated under the Cu1/rcu-Al2O3-catalyzed
conditions.[25] The reacted catalyst was then isolated and
characterized with FT-IR. Typical characteristic peaks of Bpin
moiety including CH3 (2981 cm-1), B-O (848 cm-1 and 542 cm-1)
were detected, confirming the formation of borylcopper species
on Cu1/rcu-Al2O3 (see Figure S17). This is in stark contrast to
the similar addition of diborons to enones catalyzed by CuOTf or
CuCl/PBu3 because there is no cleavage of the B-B bond
observed in the absence of substrates.[26] The interaction
between rcu-Al2O3 support and substrate 2a was also
investigated and characterized by FT-IR. The result shows that
4
Ph
Me
55
95:5
5
6[e]
7
4-MeC6H4
4-MeOC6H4
4-FC6H4
OEt
OEt
OEt
OEt
61
60
58
48
90:10
90:10
92:8
8
2-ClC6H4
83:17
9
nPr
OEt
OEt
65
68
93:7
95:5
10
Et
[a] Conditions: 1 (0.26 mmol), 2 (0.2 mmol), Cu1/rcu-Al2O3 (20 mg), K3PO4 (0.02
mmol), Toluene/MeOH (v/v = 20:1, 2.5 mL), 65 oC, N2, 0.5 h. Then 4 (0.3
mmol), 30 oC, 3 h. [b] Isolated product yield after column chromatography of
pure 5. [c] The ratio of 3:3’ was determined by 1H NMR analysis of the crude
reaction mixture after the initial conjugate addition step. [d] Gram-scale
reaction. [e] Reaction for 1.5 h.
reaction. Changing the R1 moieties to methyl, the yield of
desired product 5d was slightly decreased to 55% (Table 2,
entry 4). The reaction is also suitable for enals with aryl and
aliphatic substituents at β-position. Cinnamic enals bearing
electron-donating (Me-, MeO-) or withdrawing (-F) substituent on
aryl moiety proceeded smoothly to form the desired products 5e-
5g in yields of 58-61% (Table 2, entries 5-7). Increasing the
steric hindrance of the aryl moieties in 2 by introduction of -Cl
reduced the 1,4-selectivity of corresponding β-boration and yield
of 5h to 83:17 and 48%, respectively (Table 2, entry 8). To our
delight, alkyl substituted enals exhibited higher reactivities,
affording the target products 5i and 5j in 65% and 68% yields
(Table 2, entries 9-10). It should be noticed that, the
intermediates 3 could decompose to give back enals 2 and
corresponding dienes through elimination in the Wittig step, as a
result, all the final yields were in the range of 48-68%, similar to
the previously reported homogeneous catalysts.[16]
Figure 3. 11B NMR characterization of controlled reactions. a) 11B NMR
spectra of B2pin2 in CDCl3; b) 11B NMR spectra of the work-up residues without
catalyst; c) 11B NMR spectra of the work-up residues in the presence of
equivalent Cu1/rcu-Al2O3.
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10.1002/cssc.202000536
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COMMUNICATION
C=O characteristic peak of cinnamic aldehyde 2a has an 8 cm-1
red-shift in the presence of rcu-Al2O3 support, indicating there is
a weak interaction between support and substrate (see Figure
S18). This weak interaction might be beneficial for the formation
of copper enolate intermediate, which leads to good 1,4-
selectivity.
This work is supported by National Natural Science Foundation
of China (NSFC, No. 21802134 and No. 21603025), National
Science and Technology Major Project (2017-IIⅠ-0005-0030)
and DICP (DICP I201932). We also thank the BL 14 W beamline
at the Shanghai Synchrotron Radiation Facility (SSRF).
A deuterium labeling experiment with enal 2a and CD3OD
as the alcohol additive was further examined (Scheme S1a).[27]
The reaction afforded [D]-3a with deuterium incorporated at the
α-position. Comparetively, the control experiment of H/D
exchange after 3a formed was also conducted (Scheme S1b)
and no [D]-3a product was detected, proving that a copper
enolate intermediate was substantially generated in the boration
addition step of enals.
On the basis of the above experiments, a synthesis
pathway of homoallylboronates is proposed as depicted in
Scheme 3. The Cu1/rcu-Al2O3 catalyst initially reacts with B2pin2
1 in the presence of K3PO4 to generate Cu-Bpin species A.
Addition of A to enals 2 leads to the copper enolate intermediate
B/C. Subsequent protonation of B/C by MeOH affords 3 and
copperalkoxide D. The further Witting-reactions between 3 and 4
forms the target products 5, and the interaction of D with 1
regenerates the catalytically active species A to accomplish the
catalytic cycle.
Conflict of interest
The authors declare no conflict of interest.
Keywords: Coordinatively Unsaturated Al2O3 •copper • single-
atom catalyst • homoallylborane • selective boration
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Scheme 3. Proposed reaction mechanism.
In conclusion, coordinatively unsaturated Al2O3 supported
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Acknowledgements
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10.1002/cssc.202000536
ChemSusChem
COMMUNICATION
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This article is protected by copyright. All rights reserved.

10.1002/cssc.202000536
ChemSusChem
COMMUNICATION
COMMUNICATION
Coordinatively unsaturated Al2O3
supported Cu single-atom catalyst with a
high Cu loading of 8.7 wt% was
prepared and firstly used in the amine-
free synthesis of homoallylboranes. Up
to 99% conversion and 95% 1,4-
selective boration of the enals and 48-
68% isolated yields of homoallylboranes
were achieved, equaling to those of
reported homogenous catalysts and
more efficient and stable than nano
Cu/γ-Al2O3.
Tenglong Guo, Nanfang Tang, Feng Lin,
Qinghao Shang, Shuai Chen, Haifeng
Qi, Xiaoli Pan, Chuntian Wu, Guoliang
Xu, Jian Zhang, Dezhu Xu, and Yu Cong
Page No. – Page No.
High Loading Single-Atom Copper
Catalyst Supported with Coordinatively
Unsaturated Al2O3 for Selective
Synthesis of Homoallylboronates
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