Oxygenation of styrenes catalyzed by N-doped carbon incarcerated cobalt nanoparticles

Article abstract of DOI:10.1246/bcsj.20190251

NCI-Co catalyzed olefin oxygenation reactions were investigated. Among the metals examined, including noble metals, the reaction proceeded specifically on Co catalysts, and nitrogen dopant was crucial for the catalytic activity. The presence of NaBH₄ as a hydride source, the corresponding alcohols were obtained in high yields. The substrates bearing a reductant-sensitive functional group were made tolerant by changing the reductant and using an additive, and furthermore, the corresponding ketones were accessed by changing reaction conditions. A preliminary examination of other SOMOphiles suggested that the heterogeneous catalyst systems have the potential to be applied to more general hydrofunctionalization of olefins to form various kinds of bonds. Several mechanistic studies suggested that the reaction proceeded in a heterogeneous manner and formed a radical intermediate on cobalt nanoparticle species.

Full text of DOI:10.1246/bcsj.20190251

Advance Publication Cover Page
Oxygenation of Styrenes Catalyzed by N-Doped Carbon Incarcerated Cobalt Nanoparticles
Tomohiro Yasukawa and Shū Kobayashi*
Advance Publication on the web October 2, 2019
doi:10.1246/bcsj.20190251
© 2019 The Chemical Society of Japan
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Publication manuscripts.

Oxygenation of Styrenes Catalyzed by N-Doped Carbon Incarcerated Cobalt
Nanoparticles
Tomohiro Yasukawa, Shū Kobayashi*
Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
E-mail: < shu_kobayshi@chem.s.u-tokyo.ac.jp>
Shū Kobayashi
Shū Kobayashi studied at the University of Tokyo, receiving his PhD in 1988 working under the direction of Professor T.
Mukaiyama. Following an initial period as an assistant professor, he was promoted to lecturer then associate professor at
the Science University of Tokyo. In 1998, he moved to the Graduate School of Pharmaceutical Sciences, The University
of Tokyo, as a full professor. In April 2007, he was appointed to his current position as professor of organic chemistry in
the Department of Chemistry, within the Faculty of Science of The University of Tokyo.
Abstract
We found that cobalt nanoparticle catalysts supported on
The use of nitrogen-doped carbon as a support is a
promising strategy for constructing active cobalt NP catalysts
for several organic transformations,10 such as oxidative
esterification of alcohols and aldehydes,11 oxidation of
hydrocarbons,12 and hydrogenation reactions.13 However, these
reactions can be performed by other metal NPs and the
catalytic activity is often not comparable with that of precious
metal-based heterogeneous catalysts. Bond forming reactions
specifically catalyzed by nitrogen-doped carbon supported
cobalt NPs are not well explored,14 although cobalt has been
utilized in a wide range of unique organic reactions in the field
of homogeneous metal complex catalysis and is not a mere
substitute for precious metals.15
nitrogen-doped carbon could facilitate oxygenation of styrenes
in a heterogeneous manner. Both the nitrogen dopant and cobalt
species were essential to promote the reactions. Based on
several mechanistic studies, the formation of radical
intermediates on cobalt nanoparticles is proposed.
Keywords: Cobalt nanoparticles, Nitrogen-doped
carbon, Olefin oxygenation
1. Introduction
We previously developed methods based on
a
Metal nanoparticle (NP) catalysts have great potential to
construct ideal heterogeneous catalysts for organic synthesis
due to their high activity, ease of immobilization on a solid
support, and high stability. However, their applications to
organic synthesis have been limited to systems based on noble
metals such as Rh, Pd, Pt, and Au.1 Although it is desirable to
use inexpensive, earth-abundant transition metals such as
cobalt, applications of these catalysts have been limited to
simple gas phase reactions until recently.2
polymer-incarceration strategy16 to prepare nitrogen-doped
carbon supported NP catalysts from poly(4-vinylpyridine) as a
nitrogen source.11a The process included NP formation by
reduction of metal salts in
a polymer solution and
microencapsulation of NPs by polymer before pyrolysis,
expecting efficient stabilization of small NP species. We then
found that bimetallic Co/Cu NP catalysts prepared by this
method achieved high catalytic performance for oxidative
esterification of alcohols and aldehydes that was comparable
with that of Au NP catalysts. Motivated by these results, we
hypothesized that a nitrogen dopant could work as a good solid
ligand to construct catalytically active Co NP species for olefin
oxygenation reactions. Herein, we describe these types of
heterogeneous reactions using Co NP catalysts supported on
nitrogen-doped carbon.
Hydrofunctionalizations of olefins with a metal hydride
are useful reactions that enable various types of bond formation
including C–C, C–O, C–N, C–halogen, C–S, and C–Se bonds.3
Among them, C–O bond forming reactions with molecular
oxygen are important because they are applied frequently to a
wide range of natural product syntheses.3–4 Inspired by
metalloenzymatic systems with cofactors, metalloporphyrin
catalyst systems with hydride sources, such as borohydrides
and alcohols, were pioneered in the 1980s.5 Later, Mukaiyama
developed simpler cobalt diketonate complex catalyst systems
using silanes, which became a valuable tool in organic
synthesis.6 In spite of the usefulness and long history of these
reactions,7 to our knowledge, there is no report of a
corresponding heterogeneous catalyst system. In the cases of
reported metal complex catalysts, electronic states of metal
centers have been tuned by changing counter anions8 or using
porphyrin-type ligands5,7a,7d to design suitable catalysts.
However, anchoring such anions or ligands on a solid support
would be difficult or tedious. Alternatively, decoration of metal
NPs by ligand molecules can be considered;9 however, whether
such modification is possible for first-row metal NPs is
questionable, as they may form an inert oxide species on the
surface.2a
2. Results and Discussion
2.1. Development of catalysts. Nitrogen-doped carbon
incarcerated NP catalysts (NCI-M) were prepared from
poly(4-vinylpyridine) based on a previously reported procedure
(Scheme 1). Their catalytic activity was tested on the
oxygenation of styrene 1a in the presence of phenylsilane as a
hydride source under an atmosphere of oxygen (Table 1).
Gratifyingly, 1a was fully consumed to produce the oxygenated
products as a mixture of alcohol 2a and ketone 3a in the
presence of NCI-Co (entry 1). The catalyst prepared without
the reduction treatment significantly decreased the activity
(entry 2), and X-ray diffraction (XRD) and scanning
transmission electron microscopy (STEM) analysis of this
catalyst revealed that aggregation of NPs occurred.17 As a
control, other metal NPs, including a precious metal, were
examined and neither NCI-Cu nor NCI-Au catalyzed this type

of reaction (entries 3 and 4). The reaction barely proceeded in
the absence of any catalysts or reductants (entries 5 and 6).
These findings suggested that this reaction was specifically
enhanced by the cobalt species.
excellent yields. In the reaction with a-phenyl
vinylcyclopropane 1j, the corresponding tertiary alcohol 2j was
obtained in good yield without generating a ring-opening
adduct. We then examined several b-substituted styrenes.
Indene 1k could be converted to the corresponding secondary
alcohol 2k in moderate yield. Cinnamyl alcohol 1l was also
afforded the diol 2l in good yield. Although trans-anethole 1m
was a good substrate for this reaction system, an additional
methoxy group decreased reactivity and higher reaction
temperature was required to afford the desired product 2n. As
trans-stilbene was also less reactive, probably as the result of a
stable conjugated system, we screened catalysts at elevated
temperature and found that bimetallic Co/Cu catalyst
(NCI-Co/Cu) slightly improved the result to afford the alcohol
2o in good yield. Although cyclooctene as an aliphatic
substrate had much less reactivity, the product 2p could be
observed in low yield with higher catalyst loading. Not only
styrene derivatives, but also an unsaturated carbonyl compound
was tested. Because NaBH4 was a sufficiently strong enough
reductant to reduce carbonyl groups under the optimized
condition, PhSiH3 was used for the reaction with
methacrylamide derivative 1q to afford the corresponding
a-hydroxy carbonyl compound 2q in moderate yield. We
examined the reusability of our catalyst in the reaction with 1b
and found that high yields could be maintained until the third
run. Since full conversion of 1b could not be achieved in the
third run, we treated the recovered catalyst at 500 ˚C for 2 h
under vacuum to reactivate the catalyst.11a However, further
decrease of the yield in fourth and fifth runs was observed
(63% and 67%, respectively).
To obtain one of the products selectively, the effect of
hydride sources was examined. When NaBH4 was used instead
of PhSiH3, only alcohol adduct 2a was obtained in excellent
yield (entry 7). The reaction could proceed even under air
without loss of the yield (entry 8). To confirm the importance
of the nitrogen dopant on the support, several catalysts were
prepared as controls and examined under these conditions.
Polymer-microencapsulated Co catalyst (N-MC-Co), that is a
precursor before pyrolysis to produce NCI-Co, barely promoted
the reaction (entry 9). Moreover, carbon black (Ketjen Black
EC300J)-supported Co NP catalysts (Co/C and Co/C-800),
prepared by the same procedure as for N-MC-Co or NCI-Co
respectively without using the polymer, barely catalyzed the
reaction either regardless of whether pyrolysis was performed
during the catalyst preparation or not (entries 10 and 11). These
results clearly showed that the presence of nitrogen-doped
carbon enables Co NPs to catalyze olefin oxygenation
reactions.
Scheme 1. Preparation of NCI Co catalysts.
We were then interested in controlling alcohol/ketone
selectivity in the reaction with silane. Such milder conditions
are important in the reaction with a substrate bearing strong
reductant-sensitive functional groups, such as an ester.
4-Acetoxy styrene 1r was chosen as a model for this purpose;
however, almost no selectivity between 2r and 3r was observed
in the absence of any additives (Scheme 3). We then screened
several additives, and it was found that catalytic amounts of
lithium halide and lithium alkoxide changed the selectivity to
produce 2r as a major product.17 Finally, 1 mol% of lithium
ethoxide was determined as the best additive, and the alcohol
adduct 2r was obtained in good yield without decomposition of
the acetoxy group. Considering that lithium triflate and
perchlorate did not affect the selectivity, a coordinative anion
might work as a strong electron donor to the metal to tune the
reactivity.18
Table 1. Effect of catalysts in oxygenation of styrenes.
Entry
Catalyst
Reductant
2a (%)a 3a (%)a
1
2
3
4
5
NCI-Co
NCI-Cob
NCI-Cu
NCI-Au
no catalyst
NCI-Co
NCI-Co
NCI-Co
N-MC-Co
Co/Cd
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
no reductant
NaBH4
NaBH4
NaBH4
NaBH4
37
22
1
trace
0
57
40
4
trace
2
6
0
0
0
0
0
0
0
7
8c
9
10
11
89
89
1
4
3
Co/C-800e
NaBH4
a
b
Determined by GC analysis. The catalyst was prepared
c
without reduction treatment. The reaction was conducted
under air. d The catalyst was prepared by following the method
to access N-MC-Co without using poly(4-vinylpyridine). e The
catalyst was prepared by following the method to access
NCI-Co without using poly(4-vinylpyridine).
2.2. Substrate scope. With the optimized conditions
for alcohol selective olefin oxygenation in hand, substrate
generality was surveyed (Scheme 2). Styrene derivatives
bearing an electron-withdrawing group (1b), electron-donating
group (1c–1e), or naphthyl ring (1f) were smoothly converted
into the corresponding secondary alcohols (2a–2f). Styrene
derivatives 1g–1i, with a substituent at an a-position, were also
tolerant to afford the corresponding tertiary alcohols 2g–2i in
a
b
d
e
2nd run. 3rd run. c 60 ˚C. NCI-Co/Cu was used. NCI-Co

(2 mol%) was used. f PhSiH3 was used instead of NaBH4.
presence of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as a
radical trapping agent, the formation of the TEMPO inserted
adduct 4a was observed; however, alcohol product 2a was also
obtained, probably because insertion of molecular oxygen and
TEMPO to the benzyl radical intermediate remained
competitive (Scheme 6). The reaction in the presence of
TEMPO under air could further suppress the formation of 2a,
and the yield of 4a was increased. A radical clock experiment
using the cyclopropylbenzene substituted styrene 1s was
performed (Scheme 7). Compared with a-cyclopropylstyrene
Scheme 2. Substrate scope in hydration of olefins.
Scheme 3. Hydration of 4-acetoxystyrene.
1j,19 no cyclopropyl substituted alcohol adducts 2s and 2s
¢
were
We then attempted to obtain a ketone adduct selectively
(Scheme 4). Slow addition of silane was crucial to lead the
selective production of a ketone. Moreover, the use of a
bimetallic catalyst (NCI-Co/Cu), that is the better catalyst for
oxidation than NCI-Co, improved the result. Several
acetophenone derivatives 3a–3d could be produced in moderate
to good yields.
observed and several ring opening products, such as 5s and 6s,
were obtained instead.
Other singly occupied molecular orbital-philic reagents
(SOMOphiles), instead of molecular oxygen, were tested
(Scheme 8). We found that nitrite20 or tosyl chloride7b could
insert to the benzylic position to form the corresponding oxime
7a or benzyl chloride 8a in moderate yield. These experiments
supported a reaction pathway via radical intermediates and
showed the possibility that the catalyst systems could be
applied to various bond formation reactions.
Scheme 4. Substrate scope in oxygenation of olefins to ketones
Scheme 6. Radical trapping experiments.
2.3. Mechanistic insights. We were interested in how
the reaction was promoted by the nitrogen-doped carbon
supported cobalt NP catalyst. At first, we confirmed the
heterogeneity of the current catalyst system. No metal leaching
into the reaction solution was detected by an ICP analysis,
whose detection limit was 0.083% of cobalt. Moreover, a hot
leaching test was conducted (Scheme 5). The solid materials
were removed by filtration in the middle of the reaction (5 h)
and the reaction was continued in the filtrate. As a part of the
NaBH4 was also removed, so a new portion of NaBH4 was
added to the filtrate. Even though an excess amount of the
reducing agent existed, the reaction did not proceed any further,
indicating that the catalysis occurred in a heterogeneous
manner.
Scheme 7. Radical clock experiments.
Scheme 8. Hydrofunctionalization of styrene with other
SOMOphiles.
NCI-Co was characterized to elucidate the role of the
nitrogen dopant and active species. STEM analysis of NCI-Co
and Co/C-800 were conducted to compare the size of Co NPs
(Figure 1). Regardless of the presence of the nitrogen dopant,
Co NPs were well distributed all over carbon black and were
kept to small size. This indicated that the role of nitrogen
dopant was not just as a stabilizer of NPs to prevent them from
aggregation.
XPS analysis of these two catalysts was then performed
(Figure 2). To check the possibility of a reaction between the
Co surface and the reductant, the Co 2p3/2 spectra of the pristine
catalysts and the catalyst treated with PhSiH3 at 60 °C were
Scheme 5. Hot leaching test.
Because conventional oxygenation of olefins was
suggested to proceed via radical intermediates,3 we examined
this possibility in our heterogeneous catalyst system. In the

recorded. In NCI-Co before the treatment with silane, two
major peaks assigned to Co3+ (779.6 eV) and Co2+ (781.9 eV)
and a satellite peak corresponding to Co2+ were observed
(Figure 2a).21 On the other hands, no peak corresponds to Co(0)
(~778 eV) was observed indicating that cobalt oxide species
mainly existed on the surface of the catalyst.22 After the
treatment with silane, the satellite peak diminished and only
one peak corresponding to Co3+ (779.7 eV) was observed
(Figure 2b). This indicated that PhSiH3 reacted with the Co
surface to change its oxidation state. We assumed that silane
could reduce Co3+ species to Co2+ species. Because the treated
catalyst was shortly exposed to air during work-up and analysis,
Co2+ species might react with absorbed O2 and silane to give
Co3+ species.23 By contrast, no change of the Co 2p3/2 spectra
was observed before or after the treatment in the case of
Co/C-800 (Figures 2c and 2d). These results suggested that the
nitrogen dopant facilitated the reaction between a hydride
source and Co surface to form an active Co intermediate.
Based on these investigations, a similar mechanistic
scenario with a homogeneous cobalt complex system (Scheme
9)6a,23 can be assumed for our heterogeneous NCI-Co system.
By the aid of nitrogen-dopant, Co(III)-H species form from a
hydride source on the surface of NPs and insert to an olefin
substrate to generate a radical intermediate. After molecular
oxygen reacts with this intermediate, further reaction with a
hydride source affords products and regenerates the Co(III)-H
species.
Figure 2. XPS analysis of (a) NCI-Co, (b) NCI-Co after
treatment with silane, (c) Co/C-800, and (d) Co/C-800 after
treatment with silane.
Scheme 9. Plausible mechanism for olefin oxygenation.
3. Conclusion
In summary, we developed NCI-Co catalyzed olefin
oxygenation reactions. Among the metals examined, including
noble metals, the reaction proceeded specifically on cobalt
catalysts, and nitrogen dopant was crucial for the catalytic
activity. In the presence of NaBH4 as a hydride source, the
corresponding alcohols were obtained in high yields. The
substrate bearing reductant-sensitive functional group was
made tolerant by changing the reductant and using an additive,
and furthermore, we could access the corresponding ketones by
changing reaction conditions. A preliminary examination of
other SOMOphiles suggested that our heterogeneous catalyst
systems have the potential to be applied to more general
hydrofunctionalization of olefins to form various kinds of
bonds. Several mechanistic studies suggested that the reaction
proceeded in a heterogeneous manner and formed a radical
intermediate on cobalt NP species. To our knowledge, this is
the first example of heterogeneous catalyst systems for
oxygenation of olefins. This work provides not only synthetic
utility, but also new insights into nitrogen-doped carbon
supported NP species.
Figure 1. STEM images of (a) NCI-Co and (b) Co/C-800.
4. Experimental
Preparation of NCI-Co. To a stirring solution of
poly(4-vinylpyridine) (200 mg) in ethanol (4 mL), a solution of
Co(OAc)2‧4H2O (27.8 mg, 0.112 mmol) in ethanol (1 mL) was
added dropwise at room temperature under air. To this solution
was added ketjen black (200 mg), and the mixture was stirred
for 1 h at room temperature under air. To this mixture, a
solution of NaBH4 (21.2 mg, 0.56 mmol) in ethanol (1 mL) was
slowly added, and the mixture was continued to stir for 2 h at
room temperature under air. To this mixture was added ethyl
acetate (50 mL) at room temperature and stirred for 30 min.
The catalyst was filtered and washed in water (50 mL). The
catalyst was filtered and washed in acetone (50 mL). After
filtration, the catalyst was grinded and dried in vacuo. The

catalyst was treated by pyrolysis at 800 ˚C for 2 h under Ar
atmosphere. After cooling to room temperature, the catalyst
was washed in ethanol (30 mL) under air. The catalyst was
filtered, washed with ethanol, water and acetone and dried in
vacuo to afford NCI-Co (280 mg). NCI-Co (10 mg) was heated
in a mixture of sulfuric acid and nitric acid at 200 ˚C, and the
mixture was cooled to room temperature. The amount of Co in
the resulting solution was measured by ICP analysis to
determine the loading of Co (0.312 mmol/g).
with PhSiH3. NCI-Co/Cu (Co: 0.5 mol%) was added in a
CarouselTM tube and dried with heat gun in vacuo. Olefin
substrate 1 (0.3 mmol) and THF (0.6 mL) were added to the
Carousel tube. The mixture was stirred under 1 bar of O2
atmosphere at 60 ˚C and PhSiH3 (32.5 mg, 0.3 mmol) in THF
(0.3 mL) was added by a syringe pump over 10 h. After
completion of the addition, the reaction mixture was further
stirred for 6 h. If necessary, anisole (25~30 mg) as an internal
standard was added to the mixture, and an aliquot of the
reaction mixture (~0.02 mL) was filtered through a silica gel
packed disposable Pasteur pipette and washed with ethyl
acetate to inject to the GC analysis. Diethyl ether was added to
the mixture and the solid catalyst was removed by filtration.
The organic phase was washed with water and dried over
Preparation of NCI-Co/Cu. To a stirring solution of
poly(4-vinylpyridine) (200 mg) in ethanol (4 mL), a solution of
Co(OAc)2‧4H2O (13.9 mg, 0.056 mmol) in ethanol (1 mL) and
Cu(OAc)2‧H2O (11.2 mg, 0.056 mmol) in ethanol (1 mL) was
added dropwise at room temperature successively under air. To
this solution was added ketjen black (200 mg), and the mixture
was stirred for 1 h at 50 ˚C under air. To this mixture, a solution
of NaBH4 (21.2 mg, 0.56 mmol) in ethanol (1 mL) was slowly
added, and the mixture was continued to stir for 2 h at 50 ˚C
under air. To this mixture was added ethyl acetate (50 mL) at
room temperature and stirred for 30 min. The catalyst was
filtered and washed in water (50 mL). The catalyst was filtered
and washed in acetone (50 mL). After filtration, the catalyst
was grinded and dried in vacuo. The catalyst was treated by
pyrolysis at 800 ˚C for 2 h under Ar atmosphere. After cooling
to room temperature, the catalyst was washed in ethanol (30
mL) under air. The catalyst was filtered, washed with ethanol,
water, THF and acetone and dried in vacuo to afford
NCI-Co/Cu (280 mg). NCI-Co/Cu catalyst (10 mg) was heated
in a mixture of sulfuric acid and nitric acid at 200 ˚C, and the
mixture was cooled to room temperature. The amount of Co
and Cu in the resulting solution was measured by ICP analysis
to determine the loading of Co and Cu (Co: 0.159 mmol/g, Cu:
0.157 mmol/g).
1
Na2SO4. The solvents were removed in vacuo and H NMR
analysis of the crude mixture was conducted with
tetrachloroethane as an internal standard. After solvents were
removed, the crude mixture was dissolved in diethyl ether and
was purified by preparative TLC to afford the corresponding
ketone 3.
Acknowledgement
This work was supported in part by a Grant-in-Aid for
Scientific Research from JSPS, the University of Tokyo, and
MEXT (Japan), JST. We thank Dr. Tei Maki (The University of
Tokyo) for STEM and EDS analyses.
Supporting Information
Experimental procedures, characterization data for
1
products and copies of H and 13C NMR spectra of products.
This material is available on https://
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the mixture and the solid catalyst was removed by filtration.
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A typical procedure of olefin hydration with PhSiH3
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Graphical Abstract
<Title>
Oxygenation of Olefins Catalyzed by N-Doped Carbon Incarcerated Cobalt Nanoparticles
<Authors' names>
Tomohiro Yasukawa, Shū Kobayashi*
<Summary>
We found that cobalt nanoparticle catalysts supported on nitrogen-doped carbon could facilitate oxygenation of olefins in a
heterogeneous manner. Both the nitrogen dopant and cobalt species were essential to promote the reactions. Based on several
mechanistic studies, the formation of radical intermediates on cobalt nanoparticles is proposed.
<Diagram>