Specific Inhibition of the Hydrogenolysis of Benzylic C?O Bonds Using Palladium Nanoparticles Supported on Nitrogen-Doped Carbon Nanofibers

Article abstract of DOI:10.1002/cctc.201701326

Palladium nanoparticles supported on 5 %-nitrogen-doped, herringbone-type carbon nanofibers (Pd/N-CNF-H), which are prepared by thermally decomposing [Pd₂(dba)₃?CHCl₃] (dba=dibenzylideneacetone) in toluene in the presence of N-CNF-H, were found to be an efficient catalyst for the chemoselective hydrogenation of alkenyl and nitro moieties in benzyl-protected alcohols and carboxylic acid derivatives with high turnover frequencies: the hydrogenation reactions of these functional groups proceeded smoothly even at ambient temperature under atmospheric H₂ pressure, and the benzyl protecting groups in the molecules remained intact. Moreover, the recovered Pd/N-CNF-H catalyst could be reused without loss of its catalytic activity or chemoselectivity. The Pd/N-CNF-H catalyst also acted as an effective hydrogenation catalyst for the reduction of aromatic ketones to the corresponding benzyl alcohol derivatives with good to high product selectivity.

Full text of DOI:10.1002/cctc.201701326

Accepted Article
Title: Specific Inhibition of the Hydrogenolysis of Benzylic C-O Bond
Using Palladium Nanoparticles Supported on Nitrogen-Doped
Carbon Nanofiber
Authors: Yukihiro Motoyama, Koshi Morii, Shoya Ishizuka, Sou
Inomoto, Zhenzhong Zhang, and Seong-Ho Yoon
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201701326
Link to VoR: http://dx.doi.org/10.1002/cctc.201701326
A Journal of
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10.1002/cctc.201701326
ChemCatChem
Specific Inhibition of the Hydrogenolysis of Benzylic C–O Bond Using Palladium
Nanoparticles Supported on Nitrogen-Doped Carbon Nanofiber
Yukihiro Motoyama,*[a] Koshi Morii,[a] Shoya Ishizuka,[a] Sou Inomoto,[b] Zhenzhong
Zhang,[a] and Seong-Ho Yoon[c]
[a] Prof. Dr. Y. Motoyama, K. Morii, S. Ishizuka, Dr. Z. Zhang
Department of Advanced Science and Technology
Toyota Technological Institute
Nagoya, Aichi 468-8511, Japan
E-mail: motoyama@toyota-ti.ac.jp
[b] S. Inomoto
Graduate School of Engineering Sciences, Kyushu University
Kasuga, Fukuoka 816-8580, Japan
[c] Prof. Dr. S.-H. Yoon
Institute for Materials Chemistry and Engineering
Kyushu University
Kasuga, Fukuoka 816-8580 (Japan)
Supporting information for this article is given via a link at the end of the
document.
Keywords: heterogeneous catalysis • hydrogenation • hydrogenolysis • palladium •
reduction
Abstract: Palladium nanoparticles supported on 5%-nitrogen-doped, herringbone-type
carbon nanofiber (Pd/N-CNF-H), which are prepared by thermally decomposing
[Pd2(dba)3•CHCl3] (dba = dibenzylideneacetone) in toluene in the presence of N-CNF-H,
are found to be an efficient catalyst for the chemo-selective hydrogenation of alkenyl-
and nitro moieties in the benzyl-protected alcohols and carboxylic acid derivatives in
high turnover frequencies: the hydrogenation reactions towards these functional groups
proceed smoothly even at ambient temperature under 1 atm of H2 pressure with benzyl
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10.1002/cctc.201701326
ChemCatChem
protecting groups in the molecules remaining intact, and the recovered Pd/N-CNF-H
catalyst can be reusable without loss of its catalytic activity and chemo-selectivity. The
Pd/N-CNF-H also acts as an effective hydrogenation catalyst for the reduction of
aromatic ketones to the corresponding benzyl alcohol derivatives with good to high
product selectivity.
Benzyl moiety is widely used as a protecting group for the hydroxyl, carboxyl, and
amino functions in organic synthesis because of its stability under a variety of reaction
conditions.[1] These benzyl protecting groups are generally removed (deprotected) by
commercially available palladium on carbon (Pd/C)-catalyzed hydrogenolysis under
mild conditions.[1] Therefore, much attention has been recently paid to the development
of highly chemo-selective hydrogenation methods using heterogeneous Pd catalysts for
the transformation of not only benzyl-protected organic molecules but also other
multi-functionalized compounds.[2] One of the solutions of this subject is the chemical
modification of the Pd catalysts by using a catalyst poison. For examples, Sajiki and
Hirota reported the effectiveness of nitrogen-containing compounds as an inhibitor
towards Pd/C-catalyzed hydrogenolysis of benzyl ethers.[3] They also developed the
efficient catalyst systems, Pd–fibroin and Pd/C–SPh2, for the chemo-selective
hydrogenation of functionalized organic molecules bearing benzyl ethers and esters.[4,5]
The product selectivity in the alkene-hydrogenation of alkenyl benzyl esters and ethers
over Pd–fibroin and Pd/C–SPh2 catalysts is quite high, however, the catalytic activity of
these catalyst is not enough at room temperature under ambient H2 pressure: the
turnover frequency (TOF) of these reactions is bellow 70 (mol substrate)•(mol
Pd)-1•h-1.[4,5] In some cases, furthermore, the reactions require 3−10 atm pressure of
H2.[4]
The key to success in developing a novel chemo-selective hydrogenation method
using heterogeneous catalysts is the choice of the catalyst support and preparation
method of metal nanoparticles. We previously reported that carbon nanofibers (CNFs)[6]
are the good supports for immobilization of metal nanoparticles with narrow size
distribution, and the metal-supported CNFs (M/CNFs), which are prepared by pyrolysis
of zero-valent organometallic complexes in the presence of CNFs, act as efficient
catalysts for some hydrogenation reactions.[7] Recently, we also reported the platinum
and palladium nanoparticles supported on nitrogen-doped, herringbone-type CNF[8]
(M/N-CNFs: M = Pt, Pd); the nitrogen atoms in the CNF-H effectively act as a catalyst
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poison and the catalytic activity can be controlled by an appropriate choice of the
metal/nitrogen ratio leading to the chemo-selective and partial hydrogenation of
nitroarenes and internal alkynes in high TOFs.[9] We were interested in the application
of Pd/N-CNF-H as a hydrogenation catalyst without hydrogenolysis of the benzylic
carbon-oxygen (C–O) bonds.[3-5,10] In this paper, we report the Pd/N-CNF-H-catalyzed
selective hydrogenation of alkenes and nitroarenes without affecting the benzyl
protecting groups in the molecules.
The catalytic activity of Pd/N-CNF-H[11] towards the hydrogenolysis of benzylic
C–O bond was compared with the commercially available Pd/C using benzyl benzoate
(1a) and benzyl phenyl ether (1b) as a substrate. The reaction was performed by using
1a or 1b (0.5 mmol) and Pd catalyst (substrate/catalyst molar ratio; S/C = 250) in
toluene (1.5 mL) at 25 °C under H2 (balloon). As shown in Figure 1, the 5%
Pd/C-catalyzed hydrogenolysis of benzylic C–O bond smoothly proceeded to afford the
corresponding carboxylic acid 2a and alcohol 2b: 2a was formed in 57.6% yield after 4
h and the debenzylation reaction of 1a was almost completed after 8 h (>97% yield). In
the case of the reaction of 1b, phenol (2b) was obtained in 51.7% yield after 14 h and in
81.6% after 24 h, respectively. In sharp contrast, treatment of 1a with 2.5%
Pd/N-CNF-H under these conditions gave only 2.8% of 2a after 4 h. Similarly, 0.7% of
2b was obtained after 8 h, and only 2.6% of 1b was converted to 2b after 24 h. When
using the normal CNF-H-immobilized Pd nanoparticles (2.5% Pd/CNF-H) as a catalyst,
hydrogenolysis of the benzyl-protecting group on 1b proceeded in 2.2% for 8 h and
5.3% for 24 h, respectively. These results clearly show that the catalytic activity of
palladium nanoparticles on N-CNF-H towards the cleavage reaction of benzylic C–O
bond under hydrogen atmosphere is eliminated effectively.
< Figure 1>
The usage of Pd/N-CNF-H as a chemo-selective hydrogenation catalyst was
demonstrated by the reaction of benzyl esters and ethers, which contain other reducible
functional groups. It is well known that benzyl esters are easily converted to the
corresponding carboxylic acids under Pd-catalyzed hydrogenation conditions.[1]
Therefore, it is extremely difficult to achieve the selective transformation of other
reducible functional groups of substrates in the presence of a benzyl-protected
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10.1002/cctc.201701326
ChemCatChem
carboxylic acid under these conditions with benzyl ester moiety remaining intact.[4,5,10]
So we first investigated the Pd/N-CNF-H-catalyzed hydrogenation of alkenyl benzyl
esters (3a–c). The hydrogenation of mono-substituted alkene, benzyl 10-undecenoate
(3a), over 2.5% Pd/N-CNF-H (S/C = 250) in ethyl acetate proceeded smoothly at 25 °C
under hydrogen atmosphere (balloon) to afford benzyl undecanoate (6a) in almost
quantitative yield (97% isolated yield by silica gel column chromatography) within 30
min: only trace amount of undecanoic acid, which formed via hydrogenation of alkene
moiety followed by hydogenolysis of benzyl protecting group, was detected by GC
analysis, and TOF of this reaction was 500 (mol 3a)•(mol Pd)-1•h-1 (Table 1, Entry 1). It
is well known that the reactivity of alkenes decreases in the order mono-substituted
alkenes > 1,2-disubstituted ones > trisubstituted ones. For examples, the hydrogenation
of mono- and 1,2-disubstituted alkenes over Pd-fibroin catalyst smoothly proceeded at
room temperature under ambient H2 pressure, but the reaction of trisubstituted
(E)-benzyl tiglate, CH3CH=C(CH3)CO2Bn, needed 5 atm pressure of H2 to afford
benzyl 2-methylbutyrate, CH3CH2CH(CH3)CO2Bn, as a sole product in 77% yield after
18 h: TOF of this reaction was ca. 18.[3] In the Pd/N-CNF-H-catalyzed hydrogenation,
however, 1,2-di- and tri-substituted alkenes 3b and 3c were smoothly converted to the
corresponding benzyl esters 6b and 6c in good yields with 97–99% selectivity within 1
or 2 h even under ambient pressure of H2 (balloon); TOFs of these reactions were 118
and 270, respectively (Entries 2 and 4). In these cases, the hydrogenolytic cleavage of
the benzylic C–O bond was inhibited by appropriate choice of the catalyst, solvent, and
reaction temperature: the rate of the hydrogenolysis in protic or polar solvents, such as
MeOH, THF, and ethyl acetate, is known to be faster than that in non-polar solvents
such like toluene.[12] In addition, catalytic activity of metal particles on N-CNF-H is
highly dependent on the metal/nitrogen ratio.[9] The hydrogenation of 3b over 1%
Pd/N-CNF-H in toluene gave 6b in 99% yield with over 99% selectivity, and TOF of
this reaction was 333 (Entry 3). While in the reaction of trisubstituted alkene 3c, the
reaction proceeded in ethyl acetate even at 0 °C and the product 6c was obtained with
over 99% selectivity (Entry 5). In the hydrogenation of a benzyl ester bearing aromatic
nitro group 3d over 2.5% Pd/N-CNF-H, however, debenzylation reaction proceeded to
give 4-aminobenzoic acid in 70% and 39% yields, respectively (Entries 6 and 7). In this
case, the desired benzyl 4-aminobenzoate (6d) was obtained in high yield by using 1%
Pd/N-CNF-H in toluene (Entries 8 and 9). The hydrogenation of alkenyl group in benzyl
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ether derivative 4a over 1% Pd/N-CNF-H (S/C = 1,000) in toluene at ambient
temperature for 2 h gave benzyl 3-phenylpropyl ether (7a) in high yield with over 99%
selectivity: the calculated TOF of this reaction was 485 (Entry 11). Similar to the
reaction of 3d, some amounts of debenzylated product was formed in the reaction of
4-benzyloxynitrobenzene (4b), but the corresponding aniline derivative 7b was obtained
with 93% selectivity by using 1% Pd/N-CNF-H in toluene at 0 °C (Entries 12–15). In
addition, hydrogenolytic cleavage of the benzylic N–O bond did not proceed under
these conditions: 2.5% Pd/N-CNF-H-catalyzed hydrogenation of N-benzylcinnamamide
(5) in ethyl acetate afforded N-benzyl-3-phenylpropanamide (8) as a sole product in
quantitative yield (Entry 16).
< Table 1 >
When developing a heterogeneous catalyst system, it is important for the reusability
of the catalyst. Therefore, we next examined the durability of the Pd/N-CNF-H in the
present hydrogenation. After the reaction of benzyl cinnamate (3b) in ethyl acetate
under H2 (balloon) as described in Table 1, Entry 2 (S/C = 250, 25 °C for 1 h), the 2.5%
Pd/N-CNF-H catalyst was recovered by decantation and subjected to the further
catalytic runs. As shown in Table 2, the recovered 2.5% Pd/N-CNF-H proved to be
useful for five successive catalyst-recycling experiments: benzyl dihydrocinnamate (6b)
was obtained in quantitative yields with high selectivity (97–99%). Microwave
plasma-atomic emission spectroscopy (MP-AES) analysis of the crude product showed
the existence of trace amounts of palladium species (288 ng, 1.20 ppm); the calculated
catalyst leaching was 0.127% of the charged palladium (227 µg).
< Table 2; Scheme >
< Table 2 >
The above-described results clearly demonstrate that Pd/N-CNF-Hs are an efficient
and recyclable catalyst for the chemo-selective hydrogenation of alkenyl- and nitro
groups in various benzyl esters, ethers, and an amide. Another application of the present
Pd/N-CNF-Hs as a hydrogenation catalyst is the selective conversion of aromatic
ketones to the corresponding secondary alcohols. Catalytic hydrogenation of ketones
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10.1002/cctc.201701326
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over heterogeneous catalysts is a simple method for the production of secondary
alcohols.[13] However, the application of conventional Pd/C systems to the
hydrogenation of aromatic ketones is problematic because the hydrogenolytic cleavage
of benzylic C–O bond of alcohols obtained is often accompanied resulting in the
formation of the corresponding methylene compounds as a final product.[14] For
example, the hydrogenation of acetophenone (9a) over 5% Pd/C (S/C = 270) in ethyl
acetate at 25 °C for 5 h (99% conversion) gave 1-phenylethyl alcohol (10a) in 38%
yield along with the formation of the hydrogenolysis product, ethylbenzene (11a), in
61% yield. In sharp contrast, 2.5% Pd/N-CNF-H–catalyzed hydrogenation of 9a under
the same reaction conditions afforded the desired alcohol 10a with 98% selectivity
(Table 3, Entry 1). Comparing with the hydrogenation of alkenes and nitroarenes
bearing benzyloxy groups as shown in Table 1, the product selectivity decreased to 90%
by using toluene as a solvent in the present reaction (Entry 2). In the hydrogenation over
1% Pd/N-CNF-H, the product selectivity was high (99%) but the reaction was very slow
(41% conversion after 10 h) (Entry 3). Reactivities of the sterically hindered ketones 9b
and 9c, which have β-naphthyl- or isopropyl substituents on the carbonyl moiety, were
relatively lower than that of phenyl methyl ketone (9a), although application of slightly
higher temperature (40 °C) and/or use of an increased amount of the catalyst (S/C =
50–100) resulted in satisfactory yields within 2–3 h with 98% and 99% selectivity
(Entries 4 and 5). Similarly, the hydrogenation of keto ester 9d over 2.5% Pd/N-CNF-H
(S/C = 100) at ambient temperature for 2 h gave methyl 2-hydroxy-2-phenylacetate
(10d) in quantitative yield with 99% selectivity (Entry 6). Comparing with the alcohols
derived from aryl alkyl ketones, those from diaryl ketones are well known to be easily
converted to the corresponding alkanes via hydrogenolytic cleavage of the C–O bond.
Although the hydrogenation of diphenylketone (9e) over 2.5% Pd/N-CNF-H afforded a
small amount of diphenylmethane after 11 h (97% conversion), the desired
diphenylmethanol (10e) was obtained with 96% selectivity (Entry 7).
< Table 3; Scheme >
< Table 3 >
In summary, we have developed a new hydrogenation procedure for benzyl esters
and ethers, which contain other reducible functional groups such as alkenyl- and nitro
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10.1002/cctc.201701326
ChemCatChem
moieties. The Pd/N-CNF-H-catalyzed hydrogenation reactions smoothly proceeded at
ambient temperature under 1 atm of H2 pressure in high turnover frequencies with
benzyl protecting groups in the molecules remaining intact. The present catalyst system
was also effective for the hydrogenation of aromatic ketones by minimizing the
hydrogenolysis of the benzylic C–O bond of the benzyl alcohol derivatives formed. We
believe that these new results using Pd/N-CNF-H as a hydrogenation catalyst provide
unique and practical protocols in organic synthesis. We are now investigating the
further use of the N-CNF-H as an effective support for preparing the chemo-selective
hydrogenation catalysts.
Experimental Section
Hydrogenation of 3a (Table 1, Entry 1)
The hydrogenation was performed in a two-necked, 20 mL round-bottomed flask
equipped with a stopcock in the presence of 3a (137.2 mg, 0.5 mmol), ethyl acetate (1.5
mL) and 2.5% Pd/N-CNF-H (S/C = 250 [mol (3a)/mol (Pd)]) at 25 °C under H2
(balloon). After it was stirred for 30 min, the insoluble catalyst was removed by
filtration, and the filtrate was concentrated under reduced pressure to give the crude
material. The conversion of the substrate and the product selectivity were determined by
1
using capillary GC and/or H NMR analysis of the crude material. The product 6a was
isolated by silica gel column chromatography (n-hexane/ether = 10:1) in 97% yield
(268.1 mg).
Recycling Experiments (Table 2)
After the hydrogenation of benzyl cinnamate (3b) (1 h), the catalyst was separated by
decantation and the supernatant was removed by using membrane filter
(DISMIC®-13HP) under nitrogen atmosphere. The 2.5% Pd/N-CNF-H recovered was
washed three rimes with ethyl acetate (totally 6 mL). After it was dried under reduced
pressure, the catalyst was subjected to a further run of hydrogenation of 3b.
Acknowledgements
A part of this work was supported by a Grants-in-Aid for Scientific Research
(26390031) from Japan Society for the Promotion of Science, and for the Strategic
Research Foundation at Private Universities (S1511022) from Ministry of Education,
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Culture, Sports, Science and Technology, Japan. We are grateful to Prof. Hideo
Nagashima (Kyusyu University) for his helpful discussion. Help for the synthesis and
characterization of N-CNF by Dr. Jin Miyawaki and Mr. Kazunari Teshima (Kyushu
University) is also acknowledged.
[1] P. G. M. Wuts, T. W. Green in Protective Groups in Organic Synthesis, 4th Ed.,
John Wiley & Sons, Inc., New Jersey, 2007, pp. 102-120, 396-402, 598-602,
814-818.
[2] Review: a) Y. Monguchi, T. Ichikawa, H. Sajiki, Chem. Pharm. Bull. 2017, 65, 2-9.
Recent representative papers: b) R. Singuru, K. Dhanalaxmi, S. C. Shit, B. M.
Reddy, J. Mondal, ChemCatChem 2017, 9, 2550-2564; c) Z.-C. Ding, C.-Y. Li, J.-J.
Chen, J.-H. Zeng, H.-T. Tang, Y.-J. Ding, Z.-P. Zhang, Adv. Synth. Catal. 2017,
359, 2280-2287; d) C. Zhang, Y. Leng, P. Jiang, J. Li, S. Du, ChemistrySelect 2017,
2, 5469-5474.
[3] a) H. Sajiki, H. Kuno, K. Hirota, Tetrahedron Lett. 1998, 39, 7127-7130; b) H.
Sajiki, K. Hirota, Tetrahedron 1998, 54, 13981-13996; c) H. Sajiki, K. Hattori, K.
Hirota, J. Org. Chem. 1998, 63, 7990-7992. Pd/C–ethylenediamine catalyst,
Pd/C(en), shows high chemo-selectivity in the hydrogenation of aromatic ketones
to the corresponding benzyl alcohols: d) K. Hattori, H. Sajiki, K. Hirota,
Tetrahedron, 2001, 57, 4817-4824.
[4] Pd-Fibroin catalyst: a) H. Sajiki, T. Ikawa, K. Hirota, Tetrahedron Lett. 2003, 44,
8437-8439; b) T. Ikawa, H. Sajiki, K. Hirota, Tetrahedron 2005, 61, 2217-2231.
[5] Pd/C–SPh2 catalyst: A. Mori, Y. Miyakawa, E. Ohashi, T. Haga, T. Maegawa, H.
Sajiki, Org. Lett. 2006, 8, 3279-3280.
[6] CNFs are classified into three types, where graphite layers are either perpendicular
(platelet: CNF-P), parallel (tubular: CNF-T), or stacked obliquely (herringbone:
CNF-H); see: a) H. Murayama, T. Maeda, Nature 1990, 345, 791-793; b) N. M,
Rodriguez, J. Mater. Res. 1993, 8, 3233-3250; c) A, Tanaka, S.-H. Yoon, I.
Mochida, Carbon 2004, 42, 591-597; d) A, Tanaka, S.-H. Yoon, I. Mochida,
Carbon 2004, 42, 1291-1298.
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10.1002/cctc.201701326
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[7] a) M. Takasaki, Y. Motoyama, K. Higashi, S.-H. Yoon, I. Mochida, H. Nagashima,
Chem. Asian J. 2007, 2, 1524-1533; b) M. Takasaki, Y. Motoyama, S.-H. Yoon, I.
Mochida, H. Nagashima, J. Org. Chem. 2007, 72, 10291-10293; c) M. Takasaki, Y.
Motoyama, K. Higashi, S.-H. Yoon, I. Mochida, H. Nagashima, Org. Lett. 2008, 10,
1601-1604; d) Y. Motoyama, M. Takasaki, S.-H. Yoon, I. Mochida, H. Nagashima,
Org. Lett. 2009, 11, 5042-5045; e) Y. Motoyama, M. Taguchi, N. Desmira, S.-H.
Yoon, I. Mochida, H. Nagashima, Chem. Asian J. 2014, 9, 71-74.
[8] N-CNF-H was synthesized by chemical vapor deposition over MgO-supported
Ni–Fe catalyst using acetonitrile and hydrogen at 530 °C. Elemental analysis of
the N-CNF-H revealed the amount of nitrogen incorporated into the N-CNF-H to be
ca. 5 wt %, and two N 1s peaks mainly appeared at binding energies of 398.6
(pyridinic units) and 400.1 eV (pyrrolic units) in the XPS spectrum of N-CNF-H
with the area ratio of ca. 2:1, see: S. Lim, S.-H. Yoon, I. Mochida, D.-H. Jung,
Langmur 2009, 25, 8268-8273.
[9] a) Y. Motoyama, Y. Lee, K. Tsuji, S.-H. Yoon, I. Mochida, H. Nagashima,
ChemCatChem 2011, 3, 1578-1581; b) Y. Lee, Y. Motoyama, K. Tsuji, S.-H. Yoon,
I. Mochida, H. Nagashima, ChemCatChem 2012, 4, 7789-781.
[10]Other representative papers for the Pd-catalyzed chemo-selective hydrogenation of
alkenes in the presence of benzyloxy groups: a) J. L. Bras, D. K. Mukherjee, S.
González, M. Tristany, B. Ganchegui, M. Moreno-Mañas, R. Pleixats, F. Hénin, J.
Muzart, New J. Chem. 2004, 28, 1550-1553; b) N. A. Soom, T. Thiemann, Int. J.
Org. Chem. 2016, 6, 1-11.
[11]1%- and 2.5% Pd/N-CNF-Hs were synthesized by the method reported previously.
Actual Pd amounts were 0.9–1.0 wt% for 1% Pd/N-CNF-H (average diameter; dav =
3.7 nm), and 2.4–2.6 wt% for 2.5% Pd/N-CNF-H (dav = 3.6 nm), respectively. From
the Pd 3d region of the XPS spectrum, the intensive doublet attributed to metallic
Pd appears near 336 (Pd 3d5/2) and 341 eV (Pd 3d3/2). The shoulder region observed
at 343–345 eV can be assigned to the Pd(II) chemical state; Pd(0)/Pd(II) = ca. 3:1,
see Supporting Information.
[12]A. P. G. Kieboom, F. van Rantwijk, Hydrogenation and Hydrogenolysis in
Synthetic Organic Chemistry, Springer, Netherlands, 1977.
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[13]a) P. N. Rylander, The Catalytic Hydrogenation in Organic Syntheses, Academic
Press, New York, 1979, pp. 82-112; b) S. Nishimura, Handbook of Heterogeneous
Catalytic Hydrogenation for Organic Synthesis, John Wiley & Sons, New York,
2001, pp. 170-225.
[14]S. Nishimura, Handbook of Heterogeneous Catalytic Hydrogenation for Organic
Synthesis, John Wiley & Sons, New York, 2001, pp. 572-663.
A)
B)
Ph OBn
Ph CO2Bn
Ph CO2H
Ph OH
1a
2a
1b
2b
100
100
80
80
Pd/C
Pd/C
60
40
20
60
40
20
Pd/N-CNF-H
Pd/N-CNF-H
0
1
2
3
4
0
4
8
12
16
time (h)
time (h)
Figure 1. The plots of the products formed (%) versus time (h) in the hydrogenolysis
over 5% Pd/C (●) and 2.5% Pd/N-CNF-H (○): A) reaction of benzyl benzoate (1a); B)
reaction of benzyl phenyl ether (1b).
Table 1. Selective hydrogenation of alkenyl- and nitro groups in the benzyl esters 3a-d,
ethers 4a,b, and amide 5 using Pd/N-CNF-H as a catalyst.[a]
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10.1002/cctc.201701326
ChemCatChem
Time
[h]
Conversion
[%][b]
Selectivity
[%][b]
Entry
Substrate
1
0.5
>99
>99 (97)
2
1
3
2
9
>99
>99
94
99 (99)
>99
3[c,d,e]
4
5[f]
97
98
>99 (91)
30
6
7[e]
8[c,d,e]
9[c,d,e,f]
10
0.75
1.25
1.75
6
>99
>99
99
61
99(96)
98
>99
>99
97
1
98
11[c,d,e]
2
>99 (91)
34
12
3
>99
>99
86
13[e]
14[c,d,e,f]
15[c,d,e,f]
16
4
29
12
16
1
>99
>99
>99
93 (91)
>99
[a] Unless stated otherwise, all reactions were performed by using substrate (0.5 mmol)
and 2.5% Pd/N-CNF-H (S/C = 250–270) in ethyl acetate (1.5 mL) at 25 °C under H2
(balloon). [b] Determined by using GC and 1H NMR of the crude product. The number
in parentheses was the isolated yield. [c] 1% Pd/N-CNF-H was used. [d] S/C = 1,000.
[e] In toluene. [f] At 0 °C.
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10.1002/cctc.201701326
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Table 2. Recycle experiments in the reaction of benzyl cinnamate (3b).[a]
Run
1st
Conversion [%][b]
Selectivity of 6b [%][b]
>99
>99
>99
>99
>99
99
99
97
99
99
2nd
3rd
4th
5th
[a] All reactions were performed by using 3b (0.5 mmol) and 2.5% Pd/N-CNF-H (S/C =
250) in ethyl acetate (1.5 mL) at 25 °C for 1 h under H2 (balloon). [b] Determined by
using GC and 1H NMR of the crude product.
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10.1002/cctc.201701326
ChemCatChem
Table 3. Hydrogenation of aromatic ketones (9a–e) over Pd/N-CNF-H.[a]
Entry
Substrate
Time Product
Conv.
[%][b]
>99
Selectivity
[%][b,c]
[h]
5
1
98
2[d]
3[e]
4[f]
24
10
2
>99
90
41
99
>99
98
5[g]
6[g]
7
3
>99
>99
97
99
99
96
2
11
[a] Unless stated otherwise, all reactions were performed using substrate and 2.5%
Pd/N-CNF-H (S/C = 270) in ethyl acetate (1 mL) at 25 °C under H2 (balloon). [b]
1
Determined by using GC and H NMR of the crude product. [c] Product selectivity
(10/11). [d] In toluene. [e] 1% Pd/N-CNF-H was used. [f] S/C = 50 at 40 °C. [g] S/C =
100.
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10.1002/cctc.201701326
ChemCatChem
Table of Contents (Layout 2)
Yukihiro Motoyama,* Koshi Morii, Shoya Ishizuka, Sou Inomoto, Zhenzhong Zhang,
and Seong-Ho Yoon
Specific Inhibition of the Hydrogenolysis of Benzylic C–O Bond Using Palladium
Nanoparticles Supported on Nitrogen-Doped Carbon Nanofiber
Catalytic activity of palladium nanoparticles towards hydrogenolytic cleavage of
benzylic C–O bond is eliminated effectively by immobilizing on the nitrogen-doped,
herringbone-type carbon nanofiber leading to the chemo-selective catalyst for the
hydrogenation of alkenyl- and nitro functionalities highly tolerant to the benzyl
protecting groups in the molecules.
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