Aerobic Oxygenation of Alkylarenes over Ultrafine Transition-Metal-Containing Manganese-Based Oxides

Article abstract of DOI:10.1002/cctc.201701587

The oxygenation of alkylarenes to the corresponding aryl ketones is an important reaction, and the development of efficient heterogeneous catalysts that can utilize O₂ as the sole oxidant is highly desired. In this study, we developed an efficient alkylarene oxygenation process catalyzed by ultrafine transition-metal-containing Mn-based oxides with spinel or spinel-like structures (M-MnO_x, M=Fe, Co, Ni, Cu). These M-MnO_x catalysts were prepared by a low-temperature reduction method in 2-propanol-based solutions using tetra-n-butyl ammonium permanganate (TBAMnO₄) as the Mn source, and they exhibited high Brunauer–Emmett–Teller surface areas (typically >400 m² g^?1). Using fluorene as the model substrate, the catalytic activities of M-MnO_x and Mn₃O₄ were compared. The catalytic activities of M-MnO_x were significantly higher than that of Mn₃O₄, which demonstrates that the incorporation of transition metals in manganese oxide was critical. Among the series of M-MnO_x catalysts examined, Ni-MnO_x exhibited the highest catalytic activity for the oxygenation. In addition, the catalytic activity of Ni-MnO_x was higher than that of a physical mixture of Mn₃O₄ and NiO. Furthermore, Ni-MnO_x exhibited a broad substrate scope with respect to various types of structurally diverse (hetero)alkylarenes (16 examples). The observed catalysis was truly heterogeneous, and the Ni-MnO_x catalyst was reusable for the oxygenation of fluorene at least three times and its high catalytic performance was preserved, for example, the reaction rate, final product yield, and product selectivity. The present Ni-MnO_x-catalyzed oxygenation process is possibly initiated by a single-electron oxidation process, and herein a plausible oxygen-transfer mechanism is proposed based on several pieces of experimental evidence.

Full text of DOI:10.1002/cctc.201701587

Accepted Article
Title: Aerobic Oxygenation of Alkylarenes over Ultrafine Transition-
Metal-Containing Manganese-Based Oxides
Authors: Satoru Nakai, Tsubasa Uematsu, Yoshiyuki Ogasawara,
Kosuke Suzuki, Kazuya Yamaguchi, and Noritaka Mizuno
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10.1002/cctc.201701587
ChemCatChem
Aerobic
Oxygenation
of
Alkylarenes
over
Ultrafine
Transition-Metal-Containing Manganese-Based Oxides
Satoru Nakai, Tsubasa Uematsu, Yoshiyuki Ogasawara, Kosuke Suzuki, Kazuya Yamaguchi,*
and Noritaka Mizuno*[a]
Abstract
Oxygenation of alkylarenes to the corresponding aryl ketones is an important reaction, and the
development of efficient heterogeneous catalysts that can utilize molecular oxygen (O2) as the sole
oxidant is highly desired. In this study, we successfully developed an efficient alkylarene oxygenation
process catalyzed by ultrafine transition-metal-containing manganese-based oxides with spinel or
spinel-like structures (M–MnOx, M = Fe, Co, Ni, Cu). These M–MnOx catalysts were prepared via a
low-temperature reduction method in 2-propanol-based solutions using tetra-n-butylammonium
permanganate (TBAMnO4) as the manganese source, and they exhibited high Brunauer–Emmett–
Teller surface areas (typically >400 m2 g−1). Using fluorene as the model substrate, the catalytic
activities of M–MnOx and Mn3O4 were compared. The catalytic activities of M–MnOx were significantly
higher than those of Mn3O4, demonstrating that the incorporation of transition metals in manganese
oxide was critical. Among the series of M–MnOx catalysts examined, Ni–MnOx exhibited the highest
catalytic activity for the oxygenation. In addition, the catalytic activity of Ni–MnOx was higher than that
of a physical mixture of Mn3O4 and NiO. Furthermore, Ni–MnOx exhibited a broad substrate scope with
respect to various types of structurally diverse di-activated (hetero)alkylarenes (16 examples). The
observed catalysis was truly heterogeneous, and the Ni–MnOx catalyst was reusable for the
oxygenation of fluorene at least three times while preserving its high catalytic performance, i.e., the
reaction rate, final product yield, and product selectivity. The present Ni–MnOx-catalyzed oxygenation
process is possibly initiated by a single-electron oxidation process, and herein a plausible
oxygen-transfer mechanism is proposed based on several pieces of experimental evidence.
[a] S. Nakai, T. Uematsu, Dr. Y. Ogasawara, Dr. K Suzuki, Prof. K Yamaguchi, Prof. N. Mizuno
Department of Applied Chemistry, School of Engineering, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: kyama@appchem.t.u-tokyo.ac.jp, tmizuno@mail.ecc.u-tokyo.ac.jp
1
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Introduction
Aryl ketones and heteroaryl ketones have been recognized as useful building blocks in organic
synthesis for versatile applications and as important structural motifs in pharmaceuticals,
agrochemicals, natural products, and biologically active molecules.[1–5] The direct oxygenation of
(hetero)alkylarenes is considered a reliable approach for the synthesis of (hetero)aryl ketones.
Because traditional metal-, peracid-, or oxoacid-based stoichiometric oxidants suffer from toxicities,
high costs, and non-sustainability, the selection of the oxidant is critical, and molecular oxygen (O2)
has been receiving attention as a more environmentally friendly oxidant. To date, several efficient
homogeneous catalytic systems have been reported for aerobic alkylarene oxygenation
(Table S1).[6–15] However, homogeneous catalysts have several drawbacks, e.g., the use of various
additives and difficulty in catalyst/product separation in exchange for their high catalytic activities.
Therefore, the development of efficient heterogeneous catalysts is more desirable, and to date, many
research groups have focused on the development of heterogeneous catalysts for alkylarene
oxygenation utilizing O2 as the oxidant (Tables S2 and S3).
Manganese oxides have attracted tremendous research interest because of their potential use
as electrode materials, adsorbents, oxidants, catalysts, and catalyst supports.[16–24] In particular, their
oxidation catalysis is of great interest, and thus numerous aerobic oxidation systems using
manganese-oxide-based catalysts have been developed to date.[25–39] As for alkylarene oxygenation,
several manganese-oxide-based catalysts have also been reported to be highly effective
(Table S1);[40–44] for example, Hara and colleagues very recently reported that nano-sized hexagonal
SrMnO3 perovskite exhibits high catalytic performance for alkylarene oxygenation under relatively
mild conditions (60–120°C, 1 atm O2, no additives).[43,44] In this study, we prepared
manganese-oxide-based heterogeneous catalysts for alkylarene oxygenation while considering the
following factors. (1) Miniaturizing the particle size is important because manganese oxide with high
specific surface area typically exhibits high catalytic performance.[44–46] (2) Incorporating additional
metals into the manganese oxide structure is important to increase the redox stability. Several
manganese oxides are susceptible to reduction; once such manganese oxides are reduced by
substrates, their reoxidization by O2 is quite difficult, leading to the formation of redox inactive
species.[21,22] We recently reported that manganese oxides with additional metals in their structures,
such as potassium-containing tunnel manganese oxide (α-MnO2, K-OMS-2) and
potassium-containing layered manganese oxide (δ-MnO2), exhibit high catalytic performance for
several aerobic oxidation reactions and maintain their crystal structures after their use for oxidation
reactions even under nonaerobic conditions, unlike manganese oxides containing no additional
metals such as β-MnO2 and γ-MnO2.[41] (3) Selecting the appropriate additional metals is also
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important. The catalytic activity of manganese oxide has frequently been improved by the
incorporation of 3d metals; however, the role of the additional metals is often unclear.[47–49] As there
has been little insight into the detailed catalytic function of manganese oxides for oxidation reactions
thus far, understanding the reaction mechanism for the manganese-oxide-catalyzed oxidation is also
very important.
Typically, small-sized manganese oxides have been synthesized using a hydrothermal
method.[50–58] In a hydrothermal method, however, the dissolution–recrystallization process limits the
reduction of the particle size and controlling the crystal structure is difficult. We recently developed a
new method to obtain various manganese oxide structures using a rational low-temperature
reduction method in organic media.[45,59] This method enables us to obtain ultrafine M–Mn binary
metal oxide crystalline nanoparticles (M = Li, Mg, Co, Ni, Zn) by suppressing the dissolution–
recrystallization process and allows us to selectively control the crystal phase by modifying the
hydration of additional metal cations.[59] Among the various crystal structures synthesized using this
method, crystalline M–Mn binary metal oxides with the spinel structure (M = Li, Co) possessed
considerably high surface areas (typically >300 m2 g−1) and exhibited high thermal and redox
stabilities.[45,59] In addition, spinel oxides (ideal formula: MMn2O4) can introduce sufficient amounts of
various additional metals. In this study, we prepared ultrafine transition-metal-containing manganese
oxides with spinel or spinel-like structures (M–MnOx, M = Fe, Co, Ni, Cu) using a slightly modified
version of our previously reported low-temperature reduction method in 2-propanol-based
solutions,[45,59] and their catalytic performance was examined for the oxygenation of fluorene. Among
the series of M–MnOx catalysts examined, Ni–MnOx exhibited the highest catalytic activity for the
oxygenation. In the presence of Ni–MnOx, various types of structurally diverse (hetero)alkylarenes
could be converted into the corresponding (hetero)aryl ketones. The observed catalysis was truly
heterogeneous, and the Ni–MnOx catalyst was reusable for the oxygenation of fluorene at least three
times while preserving its high catalytic performance. In this manuscript, a plausible reaction
mechanism is also discussed based on several pieces of experimental evidence.
Results and Discussion
Synthesis and characterization of transition-metal-containing manganese oxides M–MnOx
Four types of transition-metal-containing manganese oxides (M–MnOx, M = Fe, Co, Ni, Cu) and
manganese oxide Mn3O4 were prepared using a slightly modified version of our previously reported
low-temperature reduction method in 2-propanol-based solutions with organic-solvent-soluble
tetra-n-butylammonium permanganate (TBAMnO4) as the manganese source (see Experimental
Section).[45,59] The M/Mn molar ratios in M–MnOx were in the range of 0.46–0.67 (Table 1). As shown
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in Figure 1, the X-ray diffraction (XRD) peaks of the M–MnOx and Mn3O4 samples were very broad,
indicative of their low crystallinities. The XRD peaks of Co–MnOx were slightly sharper than those of
the other M–MnOx samples, and the diffraction peaks at approximately 36°, 44°, 58°, and 64° were
attributed to Co–Mn binary oxide with the tetragonal spinel structure, i.e., CoMn2O4 (Figure 1c). For
the other M–MnOx oxides, very broad diffraction peaks were also observed at approximately 36°, 44°,
and 61° (Figures 1b, 1d, and 1e), analogous to Co–MnOx. No additional peaks attributed to
segregated phases of Fe2O3, CoO, NiO, and CuO were detected. Broad XRD peaks were also
observed for Mn3O4 at approximately 29°, 32°, 36°, 44°, and 61° (Figure 1a), which is consistent with
those of hausmannite with the tetragonal spinel structure. Figure 2 presents the Raman scattering
spectra of M–MnOx and Mn3O4. For Co–MnOx, a relatively sharp band was observed at
approximately 660 cm−1 (Figure 2c), which can likely be attributed to the vibration of oxygen atoms
inside the octahedral MnO6 unit.[60–63] The Raman band with medium intensity located at
approximately 480 cm−1 most likely originated from tetrahedral cation movements (Figure 2c).[60,61]
These features are characteristic of manganese oxides with the spinel structure.[60,61] The Raman
bands of the other M–MnOx oxides and Mn3O4 showed broader peaks at approximately 610–
660 cm−1 and 500 cm−1, respectively (Figures 2a, 2b, 2d, and 2e), suggesting their low
crystallinities[64] and/or the formation of an inverse spinel structure,[61] where trivalent manganese
cations occupy the A site of the spinel structure. The strong Raman bands typically observed for
segregated phases of Fe2O3 (~1300 cm−1), CoO (~1060 cm−1), NiO (~1100 cm−1), and CuO
(~290 cm−1) were not detected in any of the M–MnOx samples.[65–68] These results suggest that all
four M–MnOx samples possess spinel or spinel-like structures and that no segregated phases due to
the additional metals are formed.
The transmission electron microscopy (TEM) images presented in Figure 3 reveal that M–MnOx
and Mn3O4 consisted of closely aggregated ultrafine nanoparticles. The micrograph of Co–MnOx
clearly displays lattice fringe spacings of 4.9 Å and 3.0 Å, which can be attributed to the (101) and
(112) planes of the tetragonal spinel structures, respectively (Figure 3d). For the other M–MnOx
oxides, however, lattice fringes were not clearly observed (Figures 3a, 3b, 3e, and 3f). M–MnOx and
Mn3O4 exhibited high Brunauer–Emmett–Teller (BET) surface areas; 381 m2 g−1 for Fe–MnOx,
425 m2 g−1 for Co–MnOx, 415 m2 g−1 for Ni–MnOx, 443 m2 g−1 for Cu–MnOx, and 248 m2 g−1 for
Mn3O4 (Table 1). In addition, M–MnOx and Mn3O4 possessed mesopores; the diameters of the
mesopores were in the range of 2–4 nm for the M–MnOx oxides and 4–12 nm for Mn3O4, which can
likely be attributed to the pores between nanoparticles (Figure S2). Considering the differences in the
TEM images, surface areas, and pore diameters, the particle sizes of the M–MnOx oxides were
significantly smaller than that of Mn3O4. From these XRD, Raman, TEM, and BET surface area
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analyses, we consider that Co–MnOx possesses the spinel structure and consists of closely
aggregated nanocrystals. On the other hand, the other M–MnOx oxides are randomly stacked
spinel-like amorphous structures. Mn3O4 also consists of small particles with the spinel structure;
however, its particle size was slightly larger than that of M–MnOx. These ultrafine particles are likely
formed by inhibiting the dissolution–recrystallization process by employing the present
low-temperature reduction method in organic media.
Only minimal differences were observed in the X-ray photoelectron spectroscopy (XPS) spectra
of M–MnOx and Mn3O4 in the Mn 2p region (Figure 4A). Manganese oxides generally consist of Mn2+,
Mn3+, and Mn4+ species, and it is quite difficult to determine the exact Mn2+, Mn3+, and Mn4+ contents
according to XPS analysis in the Mn 2p region because their binding energies are very close to each
other. Instead, the average oxidation states (AOSs) of manganese species can be determined based
on the splitting width (Δ) of the XPS spectra in the Mn 3s region using the following equation: AOS =
8.95 − 1.13Δ.[69] Using the Δ values, the AOSs of manganese species in Fe–MnOx and Mn3O4 were
calculated to be approximately 2.8, and those in the other M–MnOx oxides were in the range of 3.0–
3.3 (Table 1, Figure 4B). The AOS values suggest that the major manganese species in Mn3O4 and
M–MnOx is Mn3+ and that there is more Mn2+ species in Mn3O4 and Fe–MnOx than in the other M–
MnOx oxides. The XPS spectra of the incorporated metals in the 2p region are presented in
Figures S3–S6. In general, the XPS spectrum of cationic Fen+ species in the Fe 2p region can be
divided into two peaks: Fe2+ (709 eV) and Fe3+ (711 eV).[70] The main peak of Fe–MnOx was
observed at approximately 711 eV (Figure S3), suggesting that the valence of Fe is mainly +3. The
XPS spectrum of Con+ species in the Co 2p region can be classified into two peaks: Co2+ and Co3+
are observed at ~781 eV and Co0 is observed at ~778 eV,[71,72] and Co2+ and Co3+ species can be
distinguished by the presence of the satellite peak at approximately 786 eV, which is characteristic of
the Co2+ species.[72] For Co–MnOx, the main peak was detected at approximately 781 eV, and also
the satellite peak was observed at approximately 786 eV (Figure S4), suggesting that Co2+ is the
major species in Co–MnOx. The XPS spectrum of Ni2+ species in the Ni 2p region can frequently be
deconvoluted into two peaks: high-energy (856.5 eV) and low-energy (854.4 eV) peaks. The
low-energy Ni 2p peak is attributed to Ni2+ species surrounded by lattice oxygen, and the high-energy
Ni 2p peak often observed in metal-containing nickel oxides are attributed to the Ni2+ species
surrounded by specific oxygen species, e.g., OH species and lattice oxygens adjacent to other
metals.[73–77] The main peak of Ni–MnOx appeared at approximately 856 eV (Figure S5), suggesting
the presence of Ni2+ species surrounded by such specific oxygen species. The XPS spectrum in the
Cu 2p region can be separated into two peaks: Cu2+ species at approximately 934 eV, and Cu+ and
Cu0 species at approximately 932 eV.[78] The main Cu 2p peak of Cu–MnOx appeared at
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approximately 934 eV, and a satellite peak was observed at 932–935 eV (Figure S6), which is
characteristic of Cu2+ species.[78] From these XPS spectra of manganese and the incorporated
metals, the possible main species in M–MnOx were M2+ and Mn3+ (except for Fe–MnOx: Fe3+ and
Mn3+). Fe–MnOx contains more Mn2+ species because of the charge compensation caused by the
incorporation of Fe3+ species.
XPS spectra of M–MnOx and Mn3O4 in the O 1s region are presented in Figure 5. Each O 1s
spectrum can be deconvoluted into three components: saturated lattice oxygen (Osat, ~530 eV),
unsaturated oxygen species, (Ounsat, e.g., hydroxy species, ~532 eV), and surface-adsorbed water
(~534 eV).[26] Curve-fitting analyses of these XPS spectra resulted in Ounsat/(Osat + Ounsat) values of
0.30 for Mn3O4, 0.31 for Fe–MnOx, 0.36 for Co–MnOx, 0.55 for Ni–MnOx, and 0.36 for Cu–MnOx
(Table 1). Ni–MnOx prominently exhibited a higher Ounsat/(Osat + Ounsat) value than the other samples.
Considering the O 1s and Ni 2p spectra, Ni–MnOx likely possesses more surface hydroxy species
than the other M–MnOx oxides, which may contribute to its high catalytic performance.
Catalytic performance of M–MnOx, reaction mechanism, and substrate scope
The catalytic activities of M–MnOx and Mn3O4 were compared for the aerobic oxygenation of fluorene
(1a) under the conditions described in Table 2. M–MnOx exhibited significantly higher catalytic
performance than undoped manganese oxide Mn3O4, suggesting that the catalytic performance of
manganese oxide was improved by the introduction of the additional transition metals (Table 2,
entries 1–5). Among the series of M–MnOx catalysts examined, Ni–MnOx exhibited the highest
catalytic performance, i.e., the reaction rate and final product yield (Table 2, entries 2–5, Figure S7),
giving the corresponding aryl ketone (9-fluorenone, 2a) in >99% yield under the conditions described
in Table 2 in 1 atm of O2 (Table 2, entry 4). Notably, 2a was also obtained in a quantitative yield in the
presence of Ni–MnOx even under an air atmosphere (Table 2, entry 7). The difference in the catalytic
performances of M–MnOx was not simply attributable to their specific surface areas (Table 1), thus
suggesting that the incorporation of nickel is effective for the oxygenation. The yield of 2a obtained
with Ni–MnOx was significantly higher than that obtained with α-MnO2 (K-OMS-2) which is reported to
be highly active for several oxidation reactions (Table 2, entry 6).[31–39] In addition, Ni–MnOx exhibited
higher catalytic performance than NiO (bunsenite), Mn3O4, and a physical mixture of Mn3O4 and NiO
(using the same amounts of Mn and Ni as those in Ni–MnOx) for the oxygenation of 1a (Table 2,
entries 1, 9, and 10), suggesting that the introduction of nickel into manganese oxide is important for
the improvement of the catalytic activity.[79]
Although the oxygenation of an alkylarene possessing a lower ionization potential, i.e., 1a
(ca. 7.9 eV[80]), proceeded to some extent even in the presence of NiO (Table 2, entry 9), the
oxygenation of an alkylarene possessing a higher ionization potential (diphenylmethane, 1b,
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ca. 8.8 eV[80]) did not proceed at all when using NiO (Table 2, entry 14). In contrast, the oxygenation
in the presence of Mn3O4 proceeded to give the corresponding ketone (benzophenone, 2b) in 18%
yield (Table 2, entry 13). In addition, the effect of the introduction of nickel into manganese oxide was
more clearly observed when the oxygenation of 1b was carried out. A physical mixture of Mn3O4 and
NiO afforded 2b in 21% yield (Table 2, entry 15), and the yield was almost the same as that obtained
with Mn3O4 (Table 2, entry 13). The catalytic activity of Ni–MnOx for the oxygenation of 1b (61% yield
of 2b, Table 2, entry 12) was significantly higher than those of Mn3O4 and the physical mixture. From
these experimental results, we consider that manganese oxide is the indispensable active species
especially for the oxygenation of alkylarenes possessing higher ionization potentials and that the
introduction of nickel can significantly improve the catalytic activity of manganese oxide.[79]
Among the previously reported catalysts for the oxygenation of 1a, nano-sized hexagonal
SrMnO3 perovskite exhibited excellent catalytic activity under relatively mild conditions (60–120°C,
1 atm O2, no additives).[43,44] The oxygenation of 1a using Ni–MnOx was performed under the same
reaction conditions as those reported for the SrMnO3-catalyzed oxygenation;[43,44] the oxygenation of
1a using Ni–MnOx was completed more rapidly (within 1 h) compared with that using SrMnO3 (within
8 h), showing the effectiveness of Ni–MnOx. Hereafter, we mainly utilized the most active Ni–MnOx
catalyst for further detailed investigations.
To verify whether the observed catalysis was derived from solid Ni–MnOx or leached metal
species (manganese and/or nickel), Ni–MnOx was removed by hot filtration during the reaction at
10 min, and the reaction was then repeated with the filtrate under the same reaction conditions. As
observed in Figure 6A, the production of 2a was completely stopped by the removal of Ni–MnOx. In
addition, we confirmed by inductively coupled plasma atomic emission spectroscopy (ICP-AES)
analysis that neither manganese nor nickel species was present in the filtrate (below 0.01%). This
experimental evidence could rule out any contribution to the observed catalysis of metal species
leached into the reaction solution, indicating that the observed catalysis for the present oxygenation
was truly heterogeneous.
Several manganese oxides are known to function as “stoichiometric oxidants” for oxidation
reactions accompanied by structural change. Once the structure changes into a redox inactive phase
during the reaction, the generation of the product is significantly suppressed, and severe deactivation
is observed when using the recovered catalyst for reuse experiments.[22,41] To investigate whether the
formation of 2a in the presence of Ni–MnOx was due to the “aerobic oxidation catalysis” or
“stoichiometric oxidation,” the reaction was performed under nonaerobic conditions (Ar atmosphere).
Under nonaerobic conditions, the conversion of 1a was 11%, the yield of 2a was only 3% under
standard conditions (c.f., >99% yield of 2a under aerobic conditions, Table 2, entry 4), and the
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formation of 9,9'-bifluorene, which is likely produced through the radical–radical homocoupling, was
also observed albeit in a small amount (3% yield) (Table 2, entry 8). In addition, the XRD pattern of
Ni–MnOx retrieved after the oxygenation under nonaerobic conditions was almost unchanged as
compared with the freshly prepared Ni–MnOx (Figure 6B a and d). From these experimental results,
it was revealed that the formation of 2a was not derived from “stoichiometric oxidation” accompanied
by a structural change of Ni–MnOx but mostly from “aerobic oxidation catalysis” by Ni–MnOx utilizing
O2 as the terminal oxidant. The low conversion and yield under nonaerobic conditions suggest that
the activation of the catalyst by O2, e.g., the oxidation of Mn3+ species to Mn4+ species, is likely
needed to initiate the reaction effectively.
After the reaction of 1a was completed, Ni–MnOx could readily be retrieved from the reaction
mixture by simple filtration. Ni–MnOx could be reused for the same reaction at least three times
without any loss of the initial rate, final product yield, or product selectivity (Figure 6C). The XRD
pattern of the retrieved Ni–MnOx catalyst was almost the same as that of the freshly prepared Ni–
MnOx catalyst (Figure 6B a–c), suggesting that structural change was not prominent after its
repeated reuse for the oxygenation. The high redox stability of the M–MnOx oxides is attributed to
their structure being similar to that of spinel structure which is highly stable under redox conditions.
Such high stability likely contributed to the preservation of the structure during the reaction and the
maintenance of the catalytic activity when the catalyst was reused.
In general, manganese-based catalysts are known to activate substrates through single-electron
transfer (SET) mechanism or hydrogen-atom transfer (HAT) mechanism. The manganese center is
thought to effectively accept an electron via the SET mechanism from the n-orbital of heteroatoms or
π-orbital of aromatic rings.[29–31,81–83] On the other hand, active oxygen species generated on the
surface of the catalyst or oxygen atoms adjacent to the metal center are thought to abstract a
hydrogen atom from weak C–H bonds directly via the HAT mechanism.[43,44,82] To distinguish the
predominant mechanism in the Ni–MnOx-catalyzed oxygenation, the reactions of alkylarenes
4-ethylanisole and 4-isopropylanisole were conducted in the presence of Ni–MnOx. The difference in
reactivity between these two substrates has been reported to reflect the reaction mechanism;
4-ethylanisole is more reactive than 4-isopropylanisole in the case of SET mechanism, whereas
4-isopropylanisole is more reactive than 4-ethylanisole in the case of HAT mechanism.[83–85] When
using Ni–MnOx, the initial rate for the oxygenation of 4-ethylanisole was significantly higher than that
for 4-isopropylanisole (Figure S8). Therefore, the oxygenation using Ni–MnOx is suggested to be
initiated mainly by the SET process. The oxygenation of 1a was strongly suppressed by the presence
of a radical scavenger 2,6-di-tert-butyl-4-methylphenol (BHT), and a small amount of BHT-trapped
1a (m/z 384, detected by gas chromatography–mass spectrometry (GC–MS) analysis) was
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generated during the reaction (Table 2, entry 11). In addition, 9,9'-bifluorene was observed under
nonaerobic conditions, as previously mentioned (Table 2, entry 8). These results indicate that radical
intermediates are possibly involved in the present oxygenation and that direct hydride abstraction is
less likely to occur.
Next, to better understand the oxygen-transfer process, a set of 18O-labeling experiments was
conducted. Firstly, the oxygenation of 1a was performed using 18O2 under Condition A (20 mol% Mn,
total 16O content in Ni–MnOx of approximately 60 mol%, and total 18O2 content in the reactor of
approximately 100 mol% with respect to 1a). As shown in Figure 7A, 18O-labeled 2a (18O-2a) was
mainly produced, and the molar ratio of 18O-2a to the sum of 18O-2a and unlabeled 2a (16O-2a) was
approximately 0.7 throughout the reaction, thus demonstrating that the oxygen in the product mostly
originates from O2 under Condition A. Upon decreasing the amount of 1a used, i.e., increasing the
catalyst/substrate ratio (Condition B, 100 mol% Mn, total 16O content in Ni–MnOx of approximately
300 mol%, and total 18O2 in the reactor of approximately 500 mol% with respect to 1a), the 18O
contents in the product significantly decreased as compared with those for the reaction under
Condition A (Figure 7B). This result clearly indicates that oxygen-transfer from the catalyst to the
activated substrate also occurs. It should be noted that both 18O and 16O were introduced to the
product in reasonable amounts even during the initial stage of the reaction and that their molar ratio
was almost unchanged throughout the reaction (Figure 7). Therefore, these two oxygen-transfer
processes, i.e., (1) the direct introduction of O2 and (2) the oxygen-transfer from the catalyst, possibly
occur in parallel. An oxygenation mechanism that includes oxygen-transfer from the catalysts to the
substrates is called the “Mars–van Krevelen mechanism”. In this mechanism, a lattice oxygen in
metal oxide catalysts is considered to be transferred to the cationic intermediates. In addition, water
is considered to be introduced via the lattice oxygen or activation on the catalyst surface (not directly
to the cationic intermediates).[85–87] In the present Ni–MnOx-catalyzed oxygenation, 18O-2a was
obtained in a significant amount (24% yield) when the reaction of 1a was performed under the
standard conditions in the presence of H218O (100 mol% with respect to 1a) under 16O2 atmosphere.
Consequently, we consider that water is also an oxygen source via the aforementioned Mars–van
Krevelen-type mechanism.
Based on the aforementioned pieces of experimental evidence, we here propose a plausible
reaction mechanism for the present Ni–MnOx-catalyzed alkylarene oxygenation. Initially, Mn3+
species in the catalyst is likely oxidized to Mn4+ species by O2, and a cation radical species is formed
by the SET process from an alkylarene substrate to the oxidized catalyst. Then, deprotonation of the
cation radical species occurs to form an alkyl radical species, which is trapped directly by O2 (Path 1
in Scheme 1). Alternatively, the second SET and deprotonation of the cation radical species proceed
9
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to form a carbocation species, followed by nucleophilic trapping by the oxygen species in the catalyst,
e.g., lattice oxygen, hydroxy species, and/or activated water (Path 2 in Scheme 1). The reduced
catalyst is then reoxidized by O2 or possibly by peroxy species formed in Path 1. As
above-mentioned, Ni–MnOx likely possesses more hydroxy species than the other M–MnOx oxides
and Mn3O4. Therefore, we consider that such species may contribute to the high catalytic
performance via promotion of the deprotonation and/or activation of the oxygen sources (O2 and/or
water), for example, though we do not have clear evidence at present.
Finally, we investigated the substrate scope and limitation for the Ni–MnOx-catalyzed aerobic
oxygenation of (hetero)alkylarenes to the corresponding (hetero)aryl ketones (Table 3). Benzo-fused
substrates, such as fluorenes, xanthene, and dibenzosuberane, were efficiently oxygenated to afford
the corresponding aryl ketones in high yields (Table 3, entries 1‒4). The oxygenation of
dibenzosuberane occurred selectively at the di-activated 5-position (Table 3, entry 4).
Diphenylmethane and its derivatives, which possess electron-donating as well as
electron-withdrawing substituents on the phenyl rings, could be converted into the corresponding
benzophenones, and the reactions of substrates with electron-donating substituents more smoothly
proceeded as compared with those with electron-withdrawing substituents (Table 3, entries 5‒9).
Notably, various types of structurally diverse nitrogen- and sulfur-containing heteroaryl ketones,
which are important skeletons for biologically active molecules and potentially useful as synthetic
intermediates en route to various pharmaceuticals,[2–5] could be effectively synthesized from the
corresponding heteroalkylarenes (Table 3, entries 10‒14). Phthalan produced the corresponding
lactone and carboxylic acid anhydride (Table 3, entry 15), and 2-phenylacetophenone afforded benzil
(Table 3, entry 16). Therefore, the present Ni–MnOx-catalyzed system was quite effective for
substrates possessing di-activated methylene units. Unfortunately, under the conditions described in
Table 3, the oxygenation of alkylarenes possessing mono-activated methylene units, such as
ethylbenzene and n-butylbenzene, failed.
Conclusion
In this study, we synthesized transition-metal-containing manganese oxide nanoparticles catalysts
with spinel or spinel-like structures (M–MnOx) using a low-temperature reduction method in
2-propanol-based solutions. M–MnOx possessed extremely low crystallinities and high specific
surface areas. The catalytic activities of these M–MnOx for the oxygenation of fluorene were
significantly higher than that of manganese oxides, such as Mn3O4 and α-MnO2 (K-OMS-2). Among
the series of M–MnOx oxides examined, Ni–MnOx exhibited the highest activity for the oxygenation of
fluorene, and 9-fluorenone was obtained in a quantitative yield under relatively mild conditions even
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under an air atmosphere. The observed catalysis was truly heterogeneous. Ni–MnOx was not a
stoichiometric oxidant but acted as an aerobic oxidation catalyst. In addition, Ni–MnOx was reusable
at least three times while preserving its high catalytic performance. In the present Ni–MnOx-catalyzed
alkylarene oxygenation, a substrate is initially activated by the SET process to produce an alkyl
radical species and a carbocation species, and then O2 is directly introduced to the radical
intermediate, or alternatively the lattice oxygen, hydroxy species, and/or activated water are possibly
introduced to the cationic intermediate. In the presence of Ni–MnOx, various types of structurally
diverse (hetero)alkylarenes could efficiently be converted into the corresponding (hetero)arylketones.
Experimental Section
Materials
KMnO4, CoCl2, Ni(NO3)2･6H2O, concentrated HNO3, and 2-propanol were purchased from Kanto
Chemical. FeCl3, CuCl2, and naphthalene were acquired from Wako Pure Chemical Industries. NiO
(BET surface area: 109 m2 g−1) was acquired from Aldrich. Tetra-n-butylammonium bromide (TBABr),
hexylamine, and diethylene glycol dimethyl ether (diglyme) were purchased from TCI. MnSO4 was
purchased from Aldrich. 18O2 (99 atom%) was purchased from Isotec. The solvents and substrates
for
the
oxygenation
of
(hetero)alkylarenes
(except
for
benzylthiophene
and
4,4'-dimethoxydiphenylmethane) were purchased from Kanto, TCI, Wako, and Aldrich.
2-Benzylbenzimidazole hydrochloride was treated with Na2CO3 to remove HCl before its use for the
reaction. 2-Benzylthiophene and 4,4'-dimethoxydiphenylmethane were synthesized by the Wolff
Kishner reduction of the corresponding ketones. The other reagents were used as received without
further purification. Tetra-n-butylammonium permanganate (TBAMnO4) was prepared according to a
previously reported procedure.[45]
Preparation of manganese oxide catalysts
Four types of transition-metal-containing manganese oxides (M–MnOx, M = Fe, Co, Ni, Cu) and
manganese oxide (Mn3O4) were prepared using a slightly modified version of our previously reported
low-temperature method in organic media (2-propanol-based solutions) with organic solvent-soluble
tetra-n-butylammonium permanganate (TBAMnO4)[45,59] Mn3O4: A solution of TBAMnO4 (6 mmol) in
2-propanol/n-hexylamine (200 mL/10 mL) was stirred at 86°C for 6 h. The resulting brown
precipitates were collected on a membrane filter (average pore size was 0.2 μm) by suction, washed
with deionized water and acetone several times, and air-dried at 160°C to afford Mn3O4 (0.5 g). M–
MnOx: TBAMnO4 (6 mmol) was added to a 2-propanol/diglyme-based solution (100 mL/100 mL)
containing an additional metal source (3 mmol of FeCl3, CoCl2, Ni(NO3)2･6H2O, or CuCl2), and the
resulting solution was stirred at 86°C for 3 h. The brown or black precipitates formed were collected
11
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on a membrane filter (average pore size was 0.2 μm) by suction, washed with deionized water and
acetone several times, and air-dried at 120°C to obtain M–MnOx (0.7–0.8 g). α-MnO2 was prepared
according to the following procedure.[35] α-MnO2: An aqueous solution (100 mL) of KMnO4 (37 mmol)
was added to an aqueous solution (30 mL) of MnSO4･H2O (52 mmol) and concentrated HNO3 (3 mL).
The resulting mixture was refluxed at 100°C for 24 h. The dark brown solid was filtered off, washed
with a large amount of water, and air-dried at 150°C to obtain α-MnO2 (7.5 g).
Characterization of manganese oxide catalysts
XRD patterns were obtained using a Rigaku SmartLab instrument under Cu Kα radiation (λ =
1.5418 Å, 45 kV, 200 mA). Raman spectra were measured on a JASCO NRS-5100 Raman
apparatus using a laser with a wavelength of 532. TEM images were obtained using a JEOL
JEM-2000EX II instrument at an acceleration voltage of 200 kV. The TEM samples were dispersed in
ethanol and sonicated for 30 min before deposition on a carbon-coated copper grid. BET surface
areas were measured by N2 adsorption at −196°C using a Micromeritics ASAP 2010 instrument.
Elemental analyses were performed using ICP-AES with a Shimadzu ICPS-8100 apparatus. XPS
analyses were performed using a JEOL JPS-9000 instrument under monochromatized Al Kα
radiation (hν = 1486.6 eV, 12 kV, 25 mA). The XPS peak positions were calibrated using the Au 4f
7/2 peak (84.0 eV).
Catalytic reactions
GC analyses were performed using a Shimadzu GC-2014 apparatus equipped with a flame
ionization detector using an Rtx-1 capillary column. GC–MS analyses for product identification were
conducted using a Shimadzu GCMS-QP 2010 system at an ionization voltage of 70 eV using an
Inert-Cap 5 capillary column. Oxygenation of alkylarenes was typically performed as follows. The
manganese-oxide-based catalyst (20 mg), alkylarenes (0.2 mmol), solvent (2 mL), and naphthalene
(internal standard, 0.2 mmol) were placed in a Schlenk-type reactor, and the reaction mixture was
then heated at 80–150°C under 1 atm of O2 atmosphere. After the reaction was completed, the
catalyst was removed by filtration, and the filtrate was analyzed. The products were confirmed by
comparing their GC retention times and GC–MS spectra with those of authentic data. The retrieved
catalyst was washed with acetone and water and then dried under open air at 100°C for 1 h before
18
16
recycling. O-labeling experiments were performed using 18O2 (99 atom%, 1 atm). The O content
in Ni–MnOx was estimated by assuming that all the weights other than those of Ni and Mn were
derived from oxygen. The 18O2 content in the reactor (gas phase) was estimated considering the
volume of the reactor used (26 mL). The 18O content in 2a was determined using the ratio of the GC–
MS peak area at m/z = 182 to the sum of the peak areas at m/z = 180 and 182.
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Acknowledgments
This work was financially supported by JSPS KAKENHI Grant No. 15H05797 in Precisely Designed
Catalysts with Customized Scaffolding, and by The Mitsubishi Foundation.
Keywords
aerobic oxygenation; alkylarenes; aryl ketones; heterogeneous catalysis; manganese oxides
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prepared by impregnation method, and 40% yield with the catalyst prepared by
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Table 1. Properties of Mn3O4 and M–MnOx.
Catalyst
Content [wt%]
M/Mn
BET surface
AOS
Ounsat
/
molar ratio[a] area [m2 g−1]
(Osat + Ounsat
)
Mn
M
Mn3O4
61.7
37.9
40.0
36.6
43.7
–
–
248
381
425
2.8
2.8
0.30
Fe–MnOx
Co–MnOx
Ni–MnOx
Cu–MnOx
25.3
28.8
18.7
23.4
0.66
0.67
0.31
3.0–3.3 0.36
3.0–3.3 0.55
3.0–3.3 0.36
0.48 (0.52[b]) 415
0.46 443
[a] Bulk M/Mn molar ratios in M–MnOx were determined by elemental analysis using
ICP-AES. [b] The surface Ni/Mn molar ratio estimated by XPS analysis.
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Table 2. Oxygenation of 1a and 1b using various catalysts.[a]
O
Catalyst
1a
2a
Entry Catalyst
Atmosphere[b] Conv. of 1a [%]
Yield of 2a [%]
1
Mn3O4
O2
O2
O2
O2
O2
O2
air
Ar
33
69
77
>99
87
46
>99
11
33
67
70
>99
85
42
99
3[c]
28
73
19
2
Fe–MnOx
3
Co–MnOx
4
Ni–MnOx
5
Cu–MnOx
6
α-MnO2 (K-OMS-2)
Ni–MnOx
7
8
Ni–MnOx
9
NiO[d]
O2
O2
O2
30
77
24
10
11
Mn3O4[e] + NiO[d] (physical mixture)
Ni–MnOx + BHT[f]
O
Catalyst
1b
2b
Entry Catalyst
Atmosphere[b] Conv. of 1b [%]
Yield of 2b [%]
12
13
14
15
Ni–MnOx
O2
O2
O2
O2
64
24
7
61
18
<1
21
[e]
Mn3O4
NiO[d]
Mn3O4[e] + NiO[d] (physical mixture)
27
[a] Reaction conditions for 1a (entries 1–11): Catalyst (20 mg), 1a (0.2 mmol), n-octane (2 mL), 80°C, 1 h;
Reaction conditions for 1b (entries 12–15): Ni–MnOx (20 mg), 1b (0.2 mmol), o-dichlorobenzene (2 mL),
130°C, 2 h. The conversions and yields were determined by GC analysis using naphthalene as an
internal standard. See Table S4 for the solvent effect on the oxygenation of 1a. [b] 1 atm.
[c] 9,9'-Bifluorene was also detected (3%) [d] 4.6 mg. [e] 11.9 mg. [f] BHT (100 mol% with respect to 1a).
19
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10.1002/cctc.201701587
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Table 3. Oxidation of various (hetero)alkylarenes catalyzed by Ni–MnOx.[a]
Entry
Substrate
Temp. [°C]
Time [h]
Product
Yield [%]
>99
O
1
80
1
O
2
3
80
6
>99
O
80
2
6
5
6
>99
82
O
O
O
4[b]
5[b]
6[b]
120
130
130
O
O
>99
96
MeO
OMe
MeO
OMe
O
O
O
7[b]
130
130
6
6
88
67
Ph
Ph
8[b]
F
F
F
F
130
110
120
110
120
24
6
79[d]
>99
>99
>99
84
9[b]
O2N
NO2
O2N
NO2
O
10[b]
11[b]
12[b]
N
N
O
6
N
N
O
6
N
N
O
H
N
H
N
13[c]
6
N
N
O
14[b]
15
120
110
24
24
74
54
S
S
O
O
O
O
O
9
O
O
16[b]
110
24
60
O
[a] Reaction conditions: Ni–MnOx (20 mg), substrate (0.2 mmol), n-octane (2 mL), O2 (1 atm). The yields
were determined by GC analysis using naphthalene as an internal standard. [b] o-Dichlorobenzene
(2 mL), [c] DMF (2 mL). [d] NMR yield.
20
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(a)
(b)
(c)
(d)
(e)
(f)
(g)
20
40
60
80
2θ (degree)
Figure 1. XRD patterns of (a) Mn3O4, (b) Fe–MnOx, (c) Co–MnOx, (d) Ni–MnOx, and (e) Cu–MnOx.
Literature data for (f) Hausmannite (JCPDS 24-0734) and (g) CoMn2O4 (JCPDS 01-077-0471).
21
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10.1002/cctc.201701587
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(a)
(b)
(c)
(d)
(e)
200 400 600 800 1000 1200
Raman shift (cm-1)
Figure 2. Raman spectra of (a) Mn3O4, (b) Fe–MnOx, (c) Co–MnOx, (d) Ni–MnOx, and (e) Cu–MnOx.
22
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10.1002/cctc.201701587
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(a)
(b)
20 nm
20 nm
(c)
(d)
4.9 Å
3.0 Å
10 nm
20 nm
(e)
(f)
20 nm
20 nm
Figure 3. TEM images of (a) Mn3O4, (b) Fe–MnOx, (c)(d) Co–MnOx, (e) Ni–MnOx, and (f) Cu–MnOx.
23
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10.1002/cctc.201701587
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(A)
(B)
∆
(a)
∆
(a)
(b)
(c)
(b)
(c)
∆
∆
(d)
(e)
(d)
(e)
∆
92
90
88
86
84
82
660
655
650
645
640
635
Binding energy (eV)
Binding energy (eV)
Figure 4. XPS spectra of (a) Mn3O4, (b) Fe–MnOx, (c) Co–MnOx, (d) Ni–MnOx, and (e) Cu–MnOx in
the (A) Mn 2p and (B) Mn 3s regions.
24
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10.1002/cctc.201701587
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(a)
(b)
(c)
(d)
(e)
536
534
532
530
528
Binding energy (eV)
Figure 5. XPS spectra of (a) Mn3O4, (b) Fe–MnOx, (c) Co–MnOx, (d) Ni–MnOx, and (e) Cu–MnOx in
the O 1s region. The black dots represent the obtained spectra. The blue lines represent the curve
fitted spectra for saturated (530 eV) and unsaturated (532 eV) O species. The red lines represent the
sum of deconvolution.
25
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(A)
100
80
60
40
20
0
0
20
40
60
Time (min)
(B)
(a)
(b)
(c)
(d)
20 30 40 50 60 70 80 90
(degree)
2θ
(C)
fresh
2nd reuse
1st reuse
3rd reuse
100
80
60
40
20
0
0
20
40
60
Time (min)
Figure 6. (A) Oxygenation of 1a without (●) or with (○) the removal of Ni–MnOx. (B) XRD patterns of
Ni–MnOx: (a) as-prepared, (b) retrieved after the first reaction under O2 atmosphere, (c) retrieved
after the 3rd reuse reaction under O2 atmosphere, and (d) retrieved after the reaction under Ar
atmosphere. (C) Reaction profiles for the oxygenation of 1a catalyzed by fresh and recovered Ni–
MnOx. Reaction conditions: Ni–MnOx (20 mg), 1a (0.2 mmol), n-octane (2 mL), 80°C, O2 or Ar
(1 atm). The yields were determined by GC analysis using naphthalene as an internal standard.
26
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(A)
18O-2a
16O-2a
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
60
50
40
30
20
10
0
0
0
50
100
50
100
150
200
Total yield (%)
Time (min)
(B)
18O-2a
16O-2a
1
60
50
40
30
20
10
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0
0
50
100
10
20
30
40
50
60
Total yield (%)
Time (min)
Figure 7. Reaction profiles and temporal changes in the 18O content in the product for (A) Condition
A: 1a (1.0 mmol), Ni–MnOx (20 mg), n-octane (5 mL), 80°C, 18O2 (1 atm) and (B) Condition B: 1a
(0.2 mmol), Ni–MnOx (20 mg), n-octane (2 mL), 80°C, 18O2 (1 atm). The yields were determined by
GC and GC–MS analyses using naphthalene as an internal standard.
Scheme 1. Plausible reaction mechanism for the oxygenation of 1a by M–MnOx.
27
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Entry for the Table of Contents
FULL PAPER
In
the
presence
of
ultrafine
Satoru Nakai, Tsubasa Uematsu,
transition-metal-containing
Yoshiyuki Ogasawara, Kosuke Suzuki,
Kazuya Yamaguchi,*
manganese-based oxides with spinel or
spinel-like structures, especially Ni–
MnOx, various types of structurally
diverse (hetero)alkylarenes could be
converted into the corresponding
(hetero)aryl ketones. The observed
catalysis was truly heterogeneous, and
the Ni–MnOx catalyst was reusable for
the oxygenation of fluorene at least
three times while preserving its high
catalytic performance.
and Noritaka Mizuno*
Page No. – Page No.
Aerobic Oxygenation of Alkylarenes
over Ultrafine
Transition-Metal-Containing
Manganese-Based Oxides
28
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