C(sp3)-H selective benzylic borylation by in situ reduced ultrasmall Ni species on CeO2

Article abstract of DOI:10.1021/acscatal.1c00185

Herein, we report that highly dispersed Ni hydroxide species supported on CeO2 act as an efficient heterogeneous catalyst for the selective borylation of benzylic C(sp3)-H bonds of alkylarenes including secondary derivatives, using pinacolborane as the bo

Full text of DOI:10.1021/acscatal.1c00185

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Letter
C(sp3)−H Selective Benzylic Borylation by In Situ Reduced Ultrasmall
Ni Species on CeO2
Daichi Yoshii, Takafumi Yatabe, Tomohiro Yabe, and Kazuya Yamaguchi*
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ABSTRACT: Herein, we report that highly dispersed Ni hydroxide species supported on CeO2 act as an efficient heterogeneous
catalyst for the selective borylation of benzylic C(sp3)−H bonds of alkylarenes including secondary derivatives, using pinacolborane
as the borylating agent. A thorough physicochemical analysis reveals that in situ reduced ultrasmall Ni species on CeO2 during the
reaction are the actual active species responsible for the borylation.
KEYWORDS: benzylic borylation, ceria, C−H borylation, heterogeneous catalysis, nickel
atalytic C−H borylation offers a simple route to valuable
Corganoboron compounds from readily available hydro-
carbons. To date, a number of effective catalysts have been
reported for various types of C−H borylation reactions.1
However, the direct transformation of alkylarenes into the
corresponding benzylic boronate esters2 is uncommon and
challenging because C(sp3)−H bonds need to be selectively
borylated in the presence of aryl C(sp2)−H bonds. Although
significant progress on the borylation of primary benzylic
C(sp3)−H bonds of methylarenes has been made,3 reports on
the selective borylation of secondary benzylic C(sp3)−H
bonds to produce synthetically valuable secondary benzylic
boronate esters are still scarce.4
The development of heterogeneous catalysts for liquid-phase
organic transformations is becoming increasingly important.5
However, heterogeneously catalyzed C−H functionalization
has been less extensively investigated.6 Particularly for benzylic
C(sp3)−H borylation, only few examples using heterogeneous
catalysts have been reported (Scheme 1a).7,8 In these systems,
however, large excess amounts of alkylarenes with respect to
the borylating agents were required, and the applicability to
secondary benzylic borylation was very limited.7,8 Meanwhile,
Chatani and co-workers reported a C(sp2)−H borylation of
arenes in the presence of Ni(cod)2 and NHC ligands (Scheme
1b).9 In this system, when using methylarenes, the borylation
was not selective, resulting in the formation of mixtures of
C(sp2)−H and C(sp3)−H borylation products. Subsequently,
Mandal and co-workers revealed by performing a series of
controlled reactions and detailed spectroscopic studies that, in
the Ni(cod)2/NHC system, Ni nanoparticles formed during
the reaction are the actual active species.10
In this study, we found that highly dispersed Ni hydroxide
species supported on CeO2 (Ni(OH)x/CeO2) act as an
excellent heterogeneous catalyst (precursor) for the C(sp3)−H
selective benzylic borylation of alkylarenes using pinacolborane
(HBpin) as the borylating agent (Scheme 1c). Various
physicochemical characterizations revealed that ultrasmall Ni
species formed on CeO2 during the reaction are the actual
active species. As mentioned above, previously reported Ni
nanoparticle catalysts were unsuccessful in the selective
benzylic borylation due to the concomitant progression of
C(sp3)−H and C(sp2)−H bond borylation. In contrast,
C(sp3)−H bonds were preferentially borylated even in the
presence of C(sp2)−H bonds using the in situ reduced
ultrasmall Ni species catalyst newly developed in this study.
We initially examined the reaction of heptylbenzene (1a)
using various catalysts under the conditions shown in Table 1.
Ni(OH)x/CeO2, which was prepared by a deposition−
precipitation method, gave the desired benzylic boronate
ester (2a) in 79% yield (Table 1, entry 1, Table S1) together
Received: January 14, 2021
Revised: January 26, 2021
Published: February 3, 2021
https://dx.doi.org/10.1021/acscatal.1c00185
© 2021 American Chemical Society
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Letter
previously reported borylation using Ni nanoparticles.9,10 The
generation of H2 gas during the reaction was confirmed by
GC−MS analysis. No reaction proceeded when other supports
were used (Table 1, entries 2−4). In addition, the physical
mixture of Ni(OH)x/Al2O3 and CeO2, a supported NiO
catalyst, and Ni compounds barely showed catalytic activity
(Table 1, entries 5−11). Meanwhile, other 3d transition metal
hydroxide species were not effective (Table S2). The observed
catalysis of Ni(OH)x/CeO2 was determined to be heteroge-
neous by the hot filtration test (see SI). The reusability of
Ni(OH)x/CeO2 was also examined, but a significant loss of its
catalytic activity was observed in the reuse experiment (see SI).
Immediately after the reaction started, the color of
Ni(OH)x/CeO2 turned from pale yellow to black (Figure
S1). In a separate experiment, we obtained a similar black color
by treating the catalyst with HBpin under nearly identical
conditions to those of the reaction (henceforth referred to as
Ni(OH)x/CeO2−HBpin). Ni(OH)x/CeO2−HBpin also pro-
moted the borylation of 1a (Table 1, entry 13). Therefore,
Ni(OH)x/CeO2−HBpin was subjected to detailed character-
izations to elucidate the active species generated during the
reaction. The Ni K-edge XANES spectrum of Ni(OH)x/CeO2
was similar to that of Ni(OH)2 (Figure 1a, Figure S2). The
partial reduction of the Ni(II) species in Ni(OH)x/CeO2 upon
HBpin treatment was evidenced by comparing the XANES
spectra of Ni(OH)x/CeO2 and Ni(OH)x/CeO2−HBpin
(Figure 1a).11 The XPS spectrum of Ni(OH)x/CeO2−HBpin
displayed two Ni 2p3/2 peaks at 855.4 and 852.4 eV ascribable
to Ni(OH)2 and Ni(0) species, respectively (Figure 1b, Figure
S3). The k3-weighted Fourier-transformed Ni K-edge EXAFS
spectra are shown in Figure 1c (Figure S4, Table S3).
Ni(OH)x/CeO2 exhibited strong Ni−O scattering in Ni-
(OH)2, whereas Ni−Ni scattering in Ni(OH)2 was hardly
observed. Meanwhile, Ni(OH)x/CeO2−HBpin exhibited weak
Ni−Ni scattering in Ni metal as well as strong Ni−O
scattering. The Ni−Ni scattering of Ni(OH)x/CeO2−HBpin
appeared at a slightly longer distance compared with that of Ni
metal. This was possibly caused by the contribution of Ni−Ce
scattering at the interface between Ni species and CeO2.12
The direct observation of Ni species by HAADF-STEM was
quite difficult (Figure 1d,e); however, EDS mapping showed
that they were highly dispersed on CeO2 (Figure 1f,g). In
addition, the XRD patterns of Ni(OH)x/CeO2 and Ni(OH)x/
CeO2−HBpin were the same as that of CeO2; no signals due
to Ni metal, oxides, or hydroxides were observed (Figure 1h,
Figure S5). These results indicate that highly dispersed
ultrasmall Ni species were formed on CeO2 by HBpin
treatment.
Scheme 1. C−H Borylation of Alkylarenes by
Heterogeneous Catalysts7−10
Table 1. Effect of Catalyst on the Borylation of
a
Heptylbenzene (1a)
yield (%)
entry
catalyst
2a
2a′
2a′′
1
2
3
4
5
6
7
8
9
10
11
12
Ni(OH)x/CeO2
Ni(OH)x/Al2O3
Ni(OH)x/TiO2
Ni(OH)x/HAP
Ni(OH)x/Al2O3 + CeO2
NiO/CeO2
Ni(OH)2
NiO
NiCl2
Ni(OAc)2·4H2O
Ni(cod)2
79
<1
<1
<1
<1
<1
<1
<1
<1
2
5
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
6
2
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
1
b
Meanwhile, the XANES and XPS spectra of Ni(OH)x/Al2O3
were almost unchanged even after HBpin treatment, indicating
that Ni(II) species in Ni(OH)x/Al2O3 was hardly reduced by
HBpin (Figure 1a,b). As can be extracted from Table 1, only
Ni(OH)x/CeO2 served as the effective catalyst. Considering
that the occurrence of a charge transfer from CeO2 to Ni
species, which facilitates the reduction of Ni species, has been
reported,13 it seems reasonable to assume that there is a
metal−support interaction in the Ni(OH)x/CeO2 catalyst, in
which CeO2 enables the reduction of Ni(II) hydroxide species
to Ni(0) by HBpin under mild reaction conditions using
HBpin. In addition, Ni−Ni scattering in Ni(OH)2 appeared
prominently in the EXAFS spectrum of Ni(OH)x/Al2O3 in
contrast with that of Ni(OH)x/CeO2 (Figure 1c), indicating
<1
<1
65
1
c
CeO2
13
14
Ni(OH)x/CeO2−HBpin
Ni(OH)x/CeO2−H2
<1
<1
a
Conditions: 1a (0.2 mmol), HBpin (0.8 mmol), catalyst (Ni: 3.6
mol %), methylcyclohexane (1 mL), 120 °C, Ar (1 atm), 4 h. Yields
b
were determined by GC. Ni(OH)x/Al2O3 (Ni: 3.6 mol %) and CeO2
c
(32 mg). CeO2 (32 mg). HAP = hydroxyapatite.
with the homobenzylic borylation product 2a′ (5%). The total
yield of aryl C(sp2)−H borylation products 2a′′ was only 2%.
Thus, the selectivity to the C(sp3)−H borylation (benzyl +
homobenzyl) reached up to 98%, which contrasts with the
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Letter
CeO2−HBpin, both Ni−Ni scatterings due to Ni metal and
Ni(OH)2 appeared prominently in the EXAFS spectrum of
Ni(OH)x/CeO2−H2 (Figure S4). Therefore, the transforma-
tion of Ni(II) hydroxide species on CeO2 into highly dispersed
reduced Ni species was more efficient with HBpin than with
H2.
Next, the substrate scope for the proposed Ni(OH)x/CeO2-
catalyzed borylation was investigated (Scheme 2a,b, Figure
S6). Alkylbenzenes (1a−1d) and diphenylmethanes (1e−1g)
were converted to the corresponding benzylic boronate esters.
The reaction of toluene (1h) gave the corresponding gem-
diborylatedproduct as the major product (Figure S7). Xylenes
(1i−1k) and mesitylene (1l) also gave the corresponding gem-
diborylated products as the major products, whereas the yields
of borylated compounds with the two methyl groups bearing
boron substituents were very low (<10%). These results
indicated that the benzylic Bpin substituent activated the
remaining benzylic C−H bonds to give gem-diborylated
products preferentially, as previously reported.3a,f The
conversion of para-tert-butyl toluene (1m) was low. Toluenes
with methoxy (1n), dimethylamino (1o, 1p), and Bpin (1q,
1r) groups were also applicable to the gem-diborylation.
Meanwhile, the reactions of alkylarenes substituted with
carbonyl groups or halogens, heteroaromatic compounds,
and methylnaphthalene were unsuccessful (Scheme S1). The
reaction of 4-propyltoluene (1s) gave the compound with the
diborylated methyl group as the major product, indicating that
the catalytic system preferred primary C−H bonds over
secondary ones (see SI).
When using a large excess of methylarene substrates, the
corresponding monoborylated products were obtained as the
major products (Scheme 2c). To demonstrate the synthetic
utility of the present catalytic system, we conducted the
synthesis at larger scale, followed by functionalization of the
borylation product (Scheme 2d). The larger-scale reaction of
1a (4.0 mmol) was successful, and 70% yield of 2a was
isolated. Then, the synthesized product was employed in the
Suzuki−Miyaura cross-coupling with iodobenzene, giving the
arylated product 4a. Using NaOH and H2O2, 2a could be
converted to the corresponding benzylic alcohol 5a. The one-
carbon homologation of 2a using BrCH2Cl and n-BuLi was
also successful, affording the corresponding homobenzylic
boronate ester 6a.
Several experiments were performed to clarify the
mechanism of the proposed borylation. First, a deuterium-
labeling experiment was conducted. The initial rates of 1a and
dideuterated heptylbenzene at the benzylic position (1a-d2)
were almost the same (kH/kD = 1.0; Figure S8), revealing that
the benzylic C−H cleavage was not the rate-determining step.
The changes in the GC−MS spectra of the remaining 1a-d2
during the borylation with HBpin showed a rapid decrease in
the deuterium content at the benzylic position (Figure S9),
indicating that the benzylic C−D cleavage and the
incorporation of a H atom from the external hydrogen source
(i.e., HBpin) were very fast in the presence of Ni(OH)x/CeO2.
To get further information, the reaction of 1a with D2
instead of HBpin was also performed. When 1a was stirred in
methylcyclohexane at 120 °C under D2 atmosphere for 4 h in
the presence of Ni(OH)x/CeO2−HBpin, deuterium-labeling
was observed at the benzylic (20%) and homobenzylic (3.5%)
positions of 1a without aromatic C(sp2)−H deuteration
(Scheme S2). It can be assumed that a Ni−H species was
formed in the presence of an external hydrogen source such as
Figure 1. Characterization of catalysts: (a) Ni K-edge XANES
spectra; (b) Ni 2p XPS spectra; (c) k3-weighted Fourier-transformed
Ni K-edge EXAFS spectra; (d,e) HAADF-STEM images of Ni(OH)x/
CeO2−HBpin; (f,g) STEM-EDS mappings of Ni(OH)x/CeO2−
HBpin, showing the distributions of Ni in yellow (panel (f)) and
Ce in cyan (panel (g)); (h) XRD patterns.
that CeO2 had a higher dispersion of Ni(II) hydroxide species
than Al2O3.14
We also prepared Ni(OH)x/CeO2−H2 by treating Ni-
(OH)x/CeO2 with H2 at 400 °C, and its performance in the
borylation of 1a was evaluated. However, despite having a
XANES spectrum similar to that of Ni(OH)x/CeO2−HBpin
(Figure S2), the reaction with Ni(OH)x/CeO2−H2 hardly
proceeded (Table 1, entry 14). At a difference with Ni(OH)x/
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†
Scheme 2. Substrate Scope and Application
†
Yields were determined by GC (averages of 2−4 runs for 2a−2g, 3h−3s, 2h, 2o, and 2q), and the values in parentheses are the isolated yields.
a
b
Conditions: 1 (0.2 mmol), HBpin (0.8 mmol), Ni(OH)x/CeO2 (Ni: 3.6 mol %), methylcyclohexane (1 mL), Ar (1 atm), 120 °C, 4 h. Isolated
c
d
with the homobenzylic boronate ester impurity. Ni(OH)x/CeO2 (Ni: 11 mol %). Isolated with 8% yield of the regioisomer impurity.
e
f
Cyclopentyl methyl ether (CPME) was used instead of methylcyclohexane. The values in parentheses are the isolated yields of the corresponding
g
alcohols or aldehydes. Conditions: 1 (0.2 mmol), HBpin (1.0 mmol), Ni(OH)x/CeO2 (Ni: 3.6 mol %), methylcyclohexane (1 mL), Ar (1 atm),
h
i
j
100 °C, 8 h. Borylated compounds with the two methyl groups bearing boron substituents were also formed (<10% GC yields). 120 °C. HBpin
k
(0.8 mmol), 120 °C, 4 h. Other borylated byproducts were also formed (see SI). Conditions: 1 (2 mL), HBpin (0.4 mmol), Ni(OH)x/CeO2−
l
HBpin (Ni: 1.8 mol %, Ni(OH)x/CeO2 treated with HBpin before use), Ar (1 atm), 100 °C, 20 h. Isolated with 4% yield of the regioisomer
m
n
o
impurity. Using 4.0 mmol of 1 and 1 mL of CPME. Using 2.0 mmol of 1 and 1 mL of CPME. Experimental details are described in SI. Isolated
yields are shown here.
HBpin and H2. Moreover, when the reaction of 1a was
performed under H2 atmosphere, the 2a production was
suppressed (37% yield, Table S4, entry 2), suggesting that a H2
elimination step from Ni−H species was involved in the
borylation. In addition, when 1a-d2 was treated with Ni(OH)x/
CeO2−HBpin even in the absence of HBpin, rapid H−D
exchange between the benzylic C−D and homobenzylic C−H
bonds was confirmed by GC−MS and NMR (Figure S10),
suggesting that the Ni species catalyzed the benzylic C−H
activation. According to previous reports,15 the hydrogen
migration likely occurs via oxidative addition of 1a to Ni(0)
species followed by β-hydride elimination and migratory
insertion on Ni(II) species. It seems reasonable to conclude
that the aforementioned homobenzylic borylation with HBpin
and deuteration with D2 proceeds via a similar mechanism. In
addition, when HBpin was stirred under a D2 atmosphere in
the presence of Ni(OH)x/CeO2−HBpin, DBpin was obtained
(Figure S11), suggesting that the Ni species catalyzed the B−H
cleavage independently of the benzylic C−H activation.
Finally, a plausible reaction mechanism may be proposed as
follows (Scheme S3):3b,10 The oxidative addition of benzylic
C−H bonds occurs over the Ni species to give Ni benzyl
intermediates and Ni−H species. At the same time, the
reaction of the Ni species and HBpin produces Ni−H and Ni−
Bpin species. Then, the desired borylated products and H2 are
produced by successive reductive eliminations, and the
catalytically active Ni species are regenerated. Meanwhile, it
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Letter
́
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The present results show that the catalyst possesses high
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preferentially over C(sp2)−H bond cleavage, possibly as a
result of the formation of highly dispersed ultrasmall Ni species
as the active catalytic species. However, further studies are
needed to gain more insight into the origin of the observed
interesting selectivity.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c00185.
■
sı
Experimental details, additional experimental results,
characterization data, and NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Kazuya Yamaguchi − Department of Applied Chemistry,
School of Engineering, The University of Tokyo, Tokyo 113-
8656, Japan; orcid.org/0000-0002-7661-4936;
Email: kyama@appchem.t.u-tokyo.ac.jp
Authors
Daichi Yoshii − Department of Applied Chemistry, School of
Engineering, The University of Tokyo, Tokyo 113-8656,
Japan
Takafumi Yatabe − Department of Applied Chemistry, School
of Engineering, The University of Tokyo, Tokyo 113-8656,
Japan; orcid.org/0000-0001-5504-4762
Tomohiro Yabe − Department of Applied Chemistry, School
of Engineering, The University of Tokyo, Tokyo 113-8656,
Japan; orcid.org/0000-0003-1697-9711
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.1c00185
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by JSPS KAKENHI Grant
No. 19H02509. D.Y. was supported by the JSPS through the
Research Fellowship for Young Scientists (Grant No.
18J21337). We greatly appreciate Dr. Hironori Ofuchi
(Japan Synchrotron Radiation Research Institute, SPring-8)
for giving great support for XAFS measurements in BL14B2
(proposal no. 2019B1820). A part of this work was conducted
at the Advanced Characterization Nanotechnology Platform of
the University of Tokyo, supported by “Nanotechnology
Platform” of the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), Japan. We also thank Mr.
Hiroyuki Oshikawa (The University of Tokyo) for his
assistance with the HAADF-STEM and EDS analyses.
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Metal and Metal Oxide Nanoparticles: A Lever for C−H
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Fairlamb, I. J. S. Catalytic C−H bond functionalisation chemistry: the
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ACS Catal. 2021, 11, 2150−2155