Nitrogen-Doped Carbon-Encapsulated Nickel/Cobalt Nanoparticle Catalysts for Olefin Migration in Allylarenes

Article abstract of DOI:10.1002/cctc.201700316

Olefin migration in allylarenes is typically performed with precious-metal-based homogeneous catalysts. In contrast, very limited progress has been made with the use of cheap, Earth-abundant base metals as heterogeneous catalysts for these transformations—in spite of the obvious economic and environmental advantages. Herein, we report on the use of an easily prepared heterogeneous catalyst material for the migration of olefins, in particular, for allylarenes. The catalyst material consists of nickel/cobalt alloy nanoparticles encapsulated in nitrogen-doped carbon shells. The encapsulated nanoparticles are stable in air and are easily collected by centrifugation, filtration, or magnetic separation. Furthermore, we demonstrate that the catalysts can be reused several times and provide continuously high yields of the olefin-migration product.

Full text of DOI:10.1002/cctc.201700316

Accepted Article
Title: Nitrogen-Doped Carbon Encapsulated Nickel/Cobalt Nanoparticle
Catalysts for Olefin Migration of Allylarenes
Authors: Søren Kramer, Jerrik Mielby, Kasper Buss, Takeshi Kasama,
and Soeren Kegnaes
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
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To be cited as: ChemCatChem 10.1002/cctc.201700316
Link to VoR: http://dx.doi.org/10.1002/cctc.201700316
A Journal of
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Nitrogen-Doped Carbon Encapsulated Nickel/Cobalt Nanoparticle
Catalysts for Olefin Migration of Allylarenes
Søren Kramer, Jerrik Mielby, Kasper Buss, Takeshi Kasama, Søren Kegnæs*
Abstract: Olefin migration of allylarenes is typically performed with
precious metal-based homogeneous catalysts. In contrast, very
limited progress has been made using cheap, earth-abundant base
metals as heterogeneous catalysts for these transformations – in
spite of the obvious economic and environmental advantages.
Herein, we report on the use of an easily prepared heterogeneous
catalyst material for the migration of olefins, in particular allylarenes.
The catalyst material consists of nickel/cobalt alloy nanoparticles
encapsulated in nitrogen-doped carbon shells. The encapsulated
nanoparticles are stable in air and easily collected by centrifugation,
filtration, or magnetic separation. Furthermore, we demonstrate that
the catalysts can be reused several times providing continuously
high yields of the olefin migration product.
stable catalysts are warranted.[10c] In this regard, the use of
encapsulated metal nanoparticles, whereby each nanoparticle is
protected by a thin shell of nitrogen-doped carbon support, could
be a promising direction to explore.[13]
OH
Me
MeO
Me
R
Me
NH
1-propenylarenes
O
O
O
MeO
O
Me
fumimycin
OMe
HO2C
Me
MeO
MeO
OMe
Me
OH
MeO
The 1-propenylarene motif is ubiquitous in Nature. This core
structure is found in many naturally occurring compounds that
exhibit desirable properties such as antibiotic, anti-inflammatory,
anti-fungal, and anti-pesticidal activity (Figure 1).[1] Furthermore,
1-propenylarenes are used as synthetic precursors for more
complex targets with pharmacological activities.[2]
anethole
()-polysphorin
Figure 1. Naturally occurring 1-propenylarenes.[1]
Herein, we report the use of an easily prepared material
consisting of nickel/cobalt alloy nanoparticles encapsulated in
nitrogen-doped carbon shells as a heterogeneous catalyst for
the migration of olefins, in particular allylarenes, to form 1-
propenylarenes. The transformation does not involve
hydrogenation and/or dehydrogenation steps and the catalyst is
robust even under thermal conditions as illustrated by catalyst
recycling.
One of the most straightforward routes to the high-value 1-
propenylarene motif is olefin migration of easily accessible
allylarenes.[1,3] While homogeneous systems applying many
different metals have been reported for this transformation,
catalysts based on precious metals are in general dominating.[4]
Replacement of these expensive catalysts with much more
abundant base metals such as nickel and cobalt is highly
advantageous from an environmental and economic perspective.
In recent years, significant advancement of olefin migration
using homogeneous nickel and cobalt catalysts have been
The encapsulated NiCo nanoparticles were prepared based on a
method recently reported by Deng et al.[13c] In this method, the
Ni and Co precursors are dissolved in methanol and precipitated
with ethylenediaminetetraacetic acid (EDTA) under autogenous
pressure in an autoclave at 200 °C. The precipitated metal-
EDTA complex is then collected by filtration and carbonized
under an inert atmosphere. In order to investigate the effect of
the Ni/Co ratio, we synthesized and characterized a series of
encapsulated nanoparticles with varied metal composition. The
resulting catalysts are denoted NixCoy@NC where x and y
indicates the molar ratio of Ni/Co and NC is an abbreviation for
nitrogen-doped carbon.[14]
achieved.[5-7]
For
sustainable
chemical
production,
heterogeneous catalysts often present several advantages over
homogeneous catalysts such as robustness, ease of separation
from reaction mixture, and recyclability.[8] However, very limited
progress has been made for the corresponding cobalt and nickel
heterogeneous systems.[9]
Recently, metals supported on nitrogen-doped carbon have
found use as very powerful heterogeneous catalysts in a number
of organic reactions, especially for transformations involving
catalyzed hydrogenation and/or dehydrogenation steps.[10,11]
A
strong interaction between metal nanoparticles and the nitrogen-
doped carbon support is believed to account for the enhanced
stability of these catalyst materials.[12] Still, in some cases, loss
of activity is observed under thermal conditions and even more
[*]
Dr. S. Kramer, Dr. J. Mielby, K. Buss, Prof. Dr. Søren Kegnæs
Department of Chemistry, Technical University of Denmark
2800 Kgs. Lyngby (Denmark)
E-mail: skk@kemi.dtu.dk
Dr. T. Kasama
Center for Electron Nanoscopy, Technical University of Denmark
2800 Kgs. Lyngby (Denmark)
Supporting information for this article is given via a link at the end of
the document.
Figure 2. a) BF-TEM image of Ni1Co1@NC. b) Magnified image showing
graphitic carbon layers covering the nanoparticles.
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Figure 2 shows a representative bright-field transmission
electron microscope (BF-TEM) images of the Ni1Co1@NC
catalyst. The images indicate that the metal nanoparticles were
completely covered by several layers of graphitic carbon
containing ~1% of nitrogen. Electron diffraction suggests that the
phase is a NiCo alloy with a FCC structure.[14] The average size
of the encapsulated nanoparticles was around 10 nm as
measured from ~250 nanoparticles, which is in good agreement
with broad diffraction peaks in X-ray diffraction (XRD).
Table 1. Influence of various reaction parameters on reaction efficiency. [a]
Entry
Change from standard conditions
Yield of 1a [%][b]
[E:Z]
1
2
3
4
5
6
7
8
9
10
none
92
5
84:16
no Ni1Co1@NC
no HSiEt3
-
7
-
0.5 equiv HSiEt3
0.5 M instead of 1.0 M
2.0 M instead of 1.0 M
PhMe, 100 °C[c]
PhOMe[c]
70
85
83
43
53
12
5
81:19
77:23
85:15
84:16
87:13
-
PhCN[c]
reaction under air
-
[a] All data are the average of two experiments. [b] GC-FID yield with the aid
of a calibrated internal standard. [c] 13 mol% Ni1Co1@NC used as catalyst.
Figure 3. a) HAADF-STEM image along with the elemental distribution of C,
Co, and Ni determined by EDX. b) Elemental profiles across
nanoparticle.
a single
Next, we examined the influence of the catalyst composition
on the conversion and product distribution (Table 2). Materials
containing either pure cobalt or predominantly cobalt core
nanoparticles were ineffective as catalysts (entries 1 and 2). In
contrast, the materials primarily consisting of nickel could
Figure 3a shows a high-angle annular dark-field scanning
transmission electron microscope (HAADF-STEM) image of the
Ni1Co1@NC catalyst and the corresponding energy dispersive
X-ray (EDX) maps for C, Co, and Ni, respectively. The elemental
mapping confirms that Ni and Co were evenly distributed, which
is also evident from Figure 3b, showing EDX elemental profiles
measured across an individual nanoparticle. EDX and electron
energy loss spectroscopy (EELS) show some surface oxidation
signals since the specimen was exposed to air prior to analysis.
Overall, the various characterizations revealed that the materials
were different from those reported by Deng et al. in terms
particle size and in particular elemental composition.[14]
catalyze the allylbenzene migration (entries
4
and 5).[16]
Interestingly, the material containing equal amounts of both
metals displayed a superior yield compared to the other metal
compositions (entry 3). That the presence of a nickel/cobalt alloy
is crucial for the reaction outcome was further highlighted in a
reaction using a mixture of the pure metal materials. Using 4
mol% Ni@NC in combination with 4 mol% Co@NC led to only
45% yield of the styrene product (entry 6). The dependence on
the metal core illustrated in Table 2 also indicates that the
nitrogen-doped carbon support is not an active catalyst by itself.
With the in-depth characterization of the material in hand, the
catalytic features of the material toward olefin migration of
allylbenzene was tested. Subjecting allylbenzene to 8 mol%
Ni1Co1@NC (4 mol% of each metal) in the presence of 1.1
equivalents of HSiEt3 at 140 °C in mesitylene afforded a high
yield of 1-propenylbenzene (1a) with good E-selectivity (Table 1,
entry 1). The influence of various reaction parameters on the
reaction outcome is shown in Table 1. Minimal conversion takes
place in the absence of either catalyst or silane (entries 2 and 3).
Decreasing the amount of silane to 0.5 equivalents as well as
using more concentrated or diluted reaction media led to
incomplete conversion after 24 hours (entries 4–6). In other
aromatic solvents than mesitylene, the reaction is less facile
(entries 7–9). When the reaction was performed under an air
atmosphere, the catalysis was suppressed (entry 10).[15] In all
cases, only small amounts (≤5%) of propylbenzene from alkene
hydrogenation were detected.
When monitoring the reaction progress over time, no
induction period was observed and the conversion was linear
until approximately 50% conversion.[14] However, an interesting
switch in selectivity was detected (Figure 4a). After an initial
burst with very high Z-selectivity, the following conversion of
allylbenzene occurs with apparent excellent E-selectivity (~95:5).
Initially, we suspected that the selectivity switch could originate
from an in-situ modification of the catalyst. However, examining
a batch of used catalyst by transmission electron microscopy did
not reveal any obvious change in the catalyst.[14] Instead, we
investigated the possibility of product isomerization under the
reaction conditions (Figure 4b). Subjecting
allylbenzene and (Z)-1-propenylbenzene ((Z)-1a) to the standard
conditions led to significantly higher yield of (E)-1-
propenylbenzene than that which could be expected from Figure
a mixture of
a
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containing HSiEt3 and allylbenzene afforded continuously high
yields and stereoselectivites of 1a: 1st run: 92% (84:16); 2nd run:
91% (86:14); 3rd run: 91% (87:13); 4th run: 95% (86:14).[21]
Table 2. Influence of catalyst composition on reaction efficiency. [a]
In order to evaluate the generality of the developed reaction,
different allylarenes were subjected to the optimized reaction
conditions (Scheme 1). Both substrates bearing either electron-
withdrawing groups (F and CF3), or an electron-donating group
(OMe) were well tolerated. Encouraged by these results, a
series of allyltoluenes were used to examine the influence of
steric effects on the reaction outcome. It was found that as the
methyl group is moved closer to the allyl moiety, a slight
decrease in reactivity is observed. Interestingly, no change in
stereoselectivity was observed across this series. In summary
from the substrate scope, electronic and steric effects have very
limited influence on the stereoselectivity, however, both factors
can affect the substrate reactivity. It should be noted that for
substrates displaying decreased reactivity, the yields based on
conversion were in still high.[22]
Entry
Catalyst
Yield of 1a [%][b]
[E:Z]
1
2
3
4
5
Co@NC
9
0:100
0:100
84:16
81:19
79:21
Ni1Co3@NC
Ni1Co1@NC
Ni3Co1@NC
Ni@NC
9
92
76
56
6
Co@NC + Ni@NC[c]
45
73:27
[a] All data are the average of two experiments. [b] GC-FID yield with the aid
of a calibrated internal standard. [c] 4 mol% of each catalyst.
Ni1Co1@NC (8 mol%)
HSiEt3 (1.1 equiv)
4a, if no isomerization of (Z)-1a takes place. Accordingly, this
experiment indicates that, for the standard reaction, once the
concentration of Z-product builds up, the rate of isomerization
equals the rate of the olefin migration from allylbenzene forming
(Z)-1-propenylbenzene.[17,18]
Me
1bg
R
R
MesH (1 M), 140 °C, 24 h
0.6 mmol
Me
Me
Me
F
F3C
1c, 56% (80:20) [a]
MeO
1d, 75% (88:12)
1b, 72% (88:12)
Me
Me
Me
1f,
61% (82:18) [b]
Me
Me
1g, 55% (85:15) [c]
1e, 76% (87:13)
Me
Scheme 1. Substrate scope of the developed reaction. All data are the
average of two experiments. Yields are based on GC-FID analysis with the aid
of a calibrated internal standard. E:Z ratios are in parenthesis. [a-c] For some
substrates incomplete conversion was observed; the yields based on
recovered starting material for these substrates are: [a] 84%, [b] 92%, [c] 81%.
In order to expand the substrate scope beyond allylarenes,
isomerizations of 4-phenyl-1-butene and 1-dodecene were
performed (Scheme 2a and b). In both cases, high conversions
to internal olefins were achieved. Interestingly, the reactions
displayed good selectivity for one position migration leading to
high yields of the corresponding 2-alkenes. The slight decrease
in E:Z selectivity when comparing to allylarenes could indicate
that there is a minor influence from steric effects after all.
b)
Ni1Co1@NC (8 mol%)
HSiEt3 (1.1 equiv)
Me
Ph
( )-1a
0.30 mmol
Ph
Ph
(E)-1a
Ph
Me
Z
(Z)-1a Me
MesH (1 M), 140 °C, 24 h
0.30 mmol
0.37 mmol 0.15 mmol
Figure 4. a) E/Z-Selectivity as a function of reaction time. b) Probing for in-situ
isomerization of the product.
Olefin migration of allyl phenyl ether, even with poor
stereoselectivity, followed by hydrolysis would allow for
deprotection of allyl-protected phenols.[23] When subjecting allyl
phenyl ether to the standard reaction conditions, the major
product was indeed the desired phenyl vinyl ether (Scheme 2c).
However, the reaction was only 60% selective for the desired
vinyl ether while hydrogenation accounted for the rest of the
mass balance.
Hot filtration of a standard reaction after 5 hours led to a
clear colorless filtrate, which was inactive toward olefin migration
of allylbenzene. X-ray fluorescence (XRF) of the filtered reaction
mixture showed no trace of cobalt and only 0-80 ppm of
nickel.[19] Furthermore, the addition of 75 equivalents mercury
after 4 hours led to a complete stop of catalytic turnover. In
combination, these data strongly support that the active catalyst
is heterogeneous and insoluble.[20] Accordingly, catalyst
recycling was attempted. Separation of the liquid phase by
magnetic filtration followed by addition of a fresh stock solution
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a)
Ni1Co1@NC (8 mol%)
HSiEt3 (1.1 equiv)
[4]
[5]
a) D. Gauthier, A. T. Lindhardt, E. P. K. Olsen, J. Overgaard, T.
Skrydstrup, J. Am. Chem. Soc. 2010, 132, 7998; b) C. R. Larsen, D. B.
Grotjahn, J. Am. Chem. Soc. 2012, 134, 10357; c) E. Larionov, H. Li, C.
Mazet, Chem. Commun. 2014, 50, 9816. Also, see reference 1.
For examples of homogeneous nickel-catalyzed olefin migrations, see:
a) A. Wille, S. Tomm, H. Frauenrath, Synthesis 1998, 305; b) J.
Breitenfeld, O. Vechorkin, C. Corminboeuf, R. Scopelliti, X: Hu,
Organometallics 2010, 29, 3686; c) L. Wang, C. Liu, R. Bai, Y. Pan, A.
Lei, Chem. Commun. 2013, 49, 7923; d) W.-C. Lee, C.-H. Wang, Y.-H.
Lin, W.-C. Shih, T.-G. Ong, Org. Lett. 2013, 15, 5358; e) F. Weber, A.
Schmidt, P. Röse, M. Fischer, O. Burghaus, G. Hilt, Org. Lett. 2015, 17,
2952.
+
Ph
Me
Ph
Me
Ph
0.6 mmol
73%
8%
( : >99:1)
E Z =
MesH (1 M), 140 °C, 24 h
(
:
64:34)
E Z =
b)
Ni1Co1@NC (8 mol%)
HSiEt3 (1.1 equiv)
10% of other
internal alkenes
+
Me
C8H17
C8H17
0.6 mmol
85%
MesH (1 M), 140 °C, 24 h
(E:Z = 70:30)
c)
Ni1Co1@NC (8 mol%)
HSiEt3 (1.1 equiv)
O
O
O
+
Ph
Me
Ph
Me
Ph
0.6 mmol
36%
20%
MesH (1 M), 140 °C, 24 h
60% conversion
(
: 55:45)
E Z =
[6]
[7]
For a seminal reference, see: L. Roos, M. Orchin, J. Am. Chem. Soc.
1965, 87, 5502.
Scheme 2. Isomerization of a) 4-phenyl-1-butene, b) 1-dodecene, and c) allyl
phenyl ether. Yields are based on 1H NMR analysis of the crude reaction
mixture relative to an internal standard.
For recent examples of homogeneous cobalt-catalyzed olefin
migrations, see: a) F. Pünner, A. Schmidt, G. Hilt, Angew. Chem. Int.
Ed. 2012, 51, 1270; b) C. Chen, T. R. Dugan, W. W. Brennessel, D. J.
Weix, P. L. Holland, J. Am. Chem. Soc. 2014, 136, 945; c) S. W. M.
Crossley, F. Barabé, R. A. Shenvi, J. Am. Chem. Soc. 2014, 136,
16788; d) A. Schmidt, A. R. Nödling, G. Hilt, Angew. Chem. Int. Ed.
2015, 54, 801.
In summary, we have demonstrated that a heterogeneous
catalyst, based on the earth-abundant metals nickel and cobalt,
can facilitate olefin migration producing highly valuable 1-
propenylarenes. The catalyst material, consisting of metal alloy
nanoparticles encapsulated in nitrogen-doped carbon, is readily
prepared from cheap and commercially available precursors.
Furthermore, the heterogeneous nature of the catalysis ensures
easy recovery and reuse of the catalyst. For the allylarene
substrates, this new heterogeneous system can match the TOF
and selectivity of previously reported homogeneous nickel and
cobalt catalysts.[5b,7b] In addition to allylarenes, the developed
catalytic system can also be applied for the selective one
position migration of aliphatic olefins. Finally, preliminary studies
on reaction progress and pathway were disclosed.
[8]
[9]
F. Wang, J. Mielby, F. H. Richter, G. Wang, G. Prieto, T. Kasama, C.
Weidenthaler, H.-J. Bongard, S. Kegnæs, A. Fürstner, F. Schüth,
Angew. Chem. Int. Ed. 2014, 53, 8648.
Recently, olefin migration of internal alkenes was observed during a
nickel nanoparticle-catalyzed alkene hydrosilylation, however, for
terminal olefins only hydrosilylation products was obtained. I. Buslov, F.
Song, X. Hu, Angew. Chem. Int. Ed. 2016, 55, 12295.
[10] For recent reviews, see: a) J. Masa, W. Xia, M. Muhler, W. Schuhmann,
Angew. Chem. Int. Ed. 2015, 54, 10102; b) Q. Wei, X. Tong, G. Zhang,
J. Qiao, Q. Gong, S. Sun, Catalysis 2015, 5, 1574; c) L. He, F. Weniger,
H. Neumann, M. Beller, Angew. Chem. Int. Ed. 2016, 55, 12582.
[11] For examples, see: a) R. V. Jagadeesh, A.-E. Surkus, H. Junge, M.-M.
Pohl, J. Radnik, J. Rabeah, H. Huan, V. Schünemann, A. Brückner, M.
Beller, Science 2013, 342, 1073; b) R. V. Jagadeesh, H. Junge, M.-M.
Pohl, J. Radnik, A. Brückner, M. Beller, J. Am. Chem. Soc. 2013, 135,
10776; c) Z. Li, J. Liu, C. Xia, F. Li, ACS Catal. 2013, 3, 2440; d) P.
Zhang, Y. Gong, H. Li, Z. Chen, Y. Wang, Nat. Commun. 2013, 4,
1593; e) A. V. Iosub, S. S. Stahl, Org. Lett. 2015, 17, 4404; f) R. V.
Jagadeesh, T. Stemmler, A.-E. Surkus, H. Junge, K. Junge, M. Beller,
Nat. Protoc. 2015, 10, 548. g) M. Zacharska, O. Y. Podyacheva, L. S.
Kibis, A. I. Boronin, B. V. Senkovskiy, E. Y. Gerasimov, O. P. Taran, A.
B. Ayusheev, V. N. Parmon, J. J. Leahy, D: A. Bulushev,
ChemCatChem 2015, 7, 2910; h) R. V. Jagadeesh, T. Stemmler, A.-E.
Surkus, M. Bauer, M.-M. Pohl, J. Radnik, K. Junge, H. Junge, A.
Brückner, M. Beller, Nat. Protoc. 2015, 10, 916.
Acknowledgements
The authors gratefully acknowledge the support of the Danish
Council for Independent Research, Grant No. 12-127580 and
6111-00237, the support of the Lundbeck Foundation
(Lundbeckfonden), Grant No. R141-2013-13244 and the support
from Villum Fonden research grant (13158).
Keywords: Olefin migration • nanoparticles • N-doped carbon •
[12] W. Xia, Catal. Sci. Technol. 2016, 6, 630.
[13] a) F. A. Westerhaus, R. V. Jagadeesh, G. Wienhöfer, M.-M. Pohl, J.
Radnik, A.-E. Surkus, J. Rabeah, K. Junge, H. Junge, M. Nielsen, A.
Brückner, M. Beller, Nature. Chem. 2013, 5, 537; b) W. Zhong, H. Liu,
C. Bai, S. Liao, Y. Li, ACS Catal. 2015, 5, 1850; c) J. Deng, P. Ren, D.
Deng, X. Bao, Angew. Chem. Int. Ed. 2015, 54, 2100.
base metals • catalysis
[1]
[2]
[3]
M. Hassam, A. Taher, G. E. Arnoth, I. R. Green, W. L. A. van Otterlo,
Chem. Rev. 2015, 115, 5462.
P. M. Dewick, Medicinal Natural Products: A Biosynthetic Approach,
Wiley: Chippenham, 2009; pp. 156-159.
[14] See supporting information for details.
[15] The catalyst can be stored at ambient conditions under air for at least
three weeks without loss of activity.
For examples of allylation of arenes, see: a) C. C. Price, Org. React.
1946, 3, 1; b) M. Kodomari, S. Nawa, T. Miyoshi, J. Chem. Soc. Chem.
Commun. 1995, 1895; c) M. A. Kacprzynski, T. L. May, S. A. Kazane, A.
H. Hoveyda, Angew. Chem. Int. Ed. 2007, 46, 4554; d) J. Norinder, K.
Bogar, L Kanupp, J.-E. Bäckvall, Org. Lett. 2007, 9, 5095; e) D. Polet, X.
Rathgeb, C. A. Falciola, J. B. Langlois, S. El Hajjaji, A. Alexakis, Chem.
Eur. J. 2009, 15, 1205; f) K. B. Selim, Y. Matsumoto, K.-I. Yamada, K.
Tamioka, Angew. Chem. Int. Ed. 2009, 48, 8733; g) A. M. Whittaker, R.
P. Rucker, G. Lalic, Org. Lett. 2010, 12, 3216; h) M. Niggemann, M. J.
Meel, Angew. Chem. Int. Ed. 2010, 49, 3684; i) T. Yao, K. Hirano, T.
Satoh, M. Miura, Angew. Chem. Int. Ed. 2011, 123, 3046.
[16] It has previously been demonstrated that Raney nickel is
a poor
catalyst for olefin migration of allylbenzene: J. Prasad, C. N. Pillai, J.
Catal. 1984, 88, 418.
[17] When olefin migration of allylbenzene was performed in the presence of
0.3 equivalents cis-stilbene, 11% isomerization to trans-stilbene was
observed. Under the reaction conditions, but in the absence of the
catalyst, <1% isomerization of cis-stilbene occurred.
[18] Olefin migration/isomerization using homogeneous cobalt catalysis has
been proposed to proceed through a radical pathway. Consistent with
this hypothesis, our standard reaction was completely suppressed by
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the addition of 20 mol% BHT. However, no radicals were observed by
EPR of a filtered aliquot of the reaction mixture.
cycle did not afford any conversion. Accordingly, the role of the silane
does not appear to be limited to a possible initial reduction of the
catalyst surface. See Supporting Information.
[19] XRF analysis was performed on the filtrate of two separate reactions
with identical conversions and yields. In both cases, the cobalt content
in the filtrate was below the detection limit (≤1 ppm). For nickel, the first
reaction had ≤1 ppm of nickel in the filtrate and the second 80 ppm.
[20] C. A. Jaska, I. Manners, J. Am. Chem. Soc. 2004, 126, 9776.
[22] For the reactions in Table 1, Table 2, and Scheme 1, good
reproducibility was observed with the yields of duplicate experiments
falling within a range of ± 2% of the reported average value.
[23] Y. Motoyama, M. Abe, K. Kamo, Y. Kosako, H. Nagashima, Chem.
Commun. 2008, 5321.
[21] Only 5-10% of the added HSiEt3 is consumed during
a reaction.
Interestingly, a recycling experiment without addition of HSiEt3 in the 2nd
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
”Het” the base: A heterogeneous
base metal catalyst is applied for
olefin migration. Specifically, this
nickel and cobalt-based nanoparticle
catalyst can be used for accessing the
ubiquitous 1-propenylarene motif by
means of stereoselective olefin
migration of easily accessible
allylarenes.
S. Kramer, J. Mielby, K. Buss, T.
Kasama, S. Kegnæs*
Page No. – Page No.
Nitrogen-Doped Carbon
Encapsulated Nickel/Cobalt
Nanoparticle Catalysts for Olefin
Migration of Allylarenes
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COMMUNICATION
S. Kramer, J. Mielby, K. Buss, T.
Kasama, S. Kegnæs*
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Nitrogen-Doped Carbon
Encapsulated Nickel/Cobalt
Nanoparticle Catalysts for Olefin
Migration of Allylarenes
”Het” the base: A heterogeneous base metal catalyst is applied for olefin
migration. Specifically, this nickel and cobalt-based nanoparticle catalyst can be
used for accessing the ubiquitous 1-propenylarene motif by means of
stereoselective olefin migration of easily accessible allylarenes.
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