Bifunctional mesoporous silica nanoparticles as cooperative catalysts for the Tsuji-Trost reaction-tuning the reactivity of silica nanoparticles

Article abstract of DOI:10.1039/c3cc00235g

Bifunctional mesoporous silica nanoparticles (MSNs) bearing Pd-complexes and additional basic sites were prepared and tested as cooperative active catalysts in the Tsuji-Trost allylation of ethyl acetoacetate. Functionalization of the MSNs was realized by postmodification using click-chemistry. The selectivity of mono versus double allylation was achieved by control of reaction temperature and the nature of the catalyst.

Full text of DOI:10.1039/c3cc00235g

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Bifunctional mesoporous silica nanoparticles as
cooperative catalysts for the Tsuji–Trost
reaction – tuning the reactivity of silica nanoparticles†
Arne T. Dickschat,a Frederik Behrends,b Sabrina Surmiak,a Mark Weiß,b
Hellmut Eckert*b and Armido Studer*a
Cite this: Chem. Commun., 2013,
49, 2195
Received 10th January 2013,
Accepted 27th January 2013
DOI: 10.1039/c3cc00235g
www.rsc.org/chemcomm
Bifunctional mesoporous silica nanoparticles (MSNs) bearing Pd-com- approach allowed for the synthesis of a small catalyst library B1Àn
plexes and additional basic sites were prepared and tested as cooperative containing organic acids and bases as cooperative active moieties
active catalysts in the Tsuji–Trost allylation of ethyl acetoacetate. Functio- in a short time from a single MSN of type A.
nalization of the MSNs was realized by postmodification using click-
Herein we present first results on the application of this
chemistry. The selectivity of mono versus double allylation was achieved concept for the preparation of MSNs bearing amines along with
by control of reaction temperature and the nature of the catalyst.
Pd-complexes as cooperative active functionalities and their use
as catalysts in the Tsuji–Trost reaction.3 For these studies we
Bi- or multifunctional mesoporous silica nanoparticles (MSNs) chose azides and alkoxyamines as orthogonal addressable
have been successfully used as catalysts in synthesis.1 The large entities. The azide–alkyne cycloaddition is the standard click
surface area and the immersive channels in mesoporous reaction4 and the alkoxyamine moiety at the MSN is readily
materials such as MCM-41 render them highly valuable as solid chemically modified by thermal nitroxide exchange.2,5
supports to conduct heterogeneous catalysis. Furthermore, spatial
We first had to identify a route for late stage immobilization of a
separation of different functionalities in these materials allows for Pd-ligand via azide modification. To this end, we co-condensed
bifunctional catalysis.1 We recently introduced a new concept for tetraethyl orthosilicate (TEOS) 1 with the triethoxysilane 2 to obtain
preparation of bifunctional MSNs by late stage chemical functio- the azide functionalized material 3 (Scheme 2). Cu-catalyzed azide–
nalization starting with one ‘mother’-particle A bearing two alkyne click-reaction (CuAAC)4 with 2-ethynylpyridine provided MSN
chemical orthogonal functionalities that are for example azides 4 bearing triazole–pyridyl moieties serving as Pd-ligands (see 5).6,7
and alkoxyamines (Scheme 1).2 The two functionalities were
The success of these transformations was readily monitored via
addressed selectively by orthogonal click-chemistry and the novel solid state 13C CP/MAS NMR, elemental analysis and IR. The 13C NMR
spectra are shown in Fig. 1 (for further characterisation of 5, see ESI†).
In comparison to the spectra of triethoxysilane 2 in solution and azide
functionalized material 3, the 13C CP/MAS NMR spectra of 4 and 5
clearly show three broad new carbon resonances in the aromatic
region comprising the seven carbons of the formed ligand. The signal
that can most likely be assigned to the carbon next to the newly
Scheme 1 Late stage functionalization of bifunctional particle A.
a
¨
¨
Westfalische Wilhelms-Universitat, Organisch-Chemisches Institut,
¨
Corrensstrasse 40, 48149 Munster, Germany. E-mail: studer@uni-muenster.de;
Fax: +49 251-833-6523
b
¨
¨
Westfalische Wilhelms-Universitat, Physikalisch-Chemisches Institut,
¨
Corrensstrasse 28/30, 48149 Munster, Germany. E-mail: eckerth@uni-muenster.de
Scheme 2 Azide–alkyne cycloaddition and complexation.
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc00235g
c
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Fig. 3 MSNs 9–12.
Fig. 1 (a) 13C NMR of 2 in CDCl3 and 13C CP/MAS NMR of (b) azide functiona-
lized material 3, (c) triazole 4 and (d) Pd-loaded particles 5. The marked signals
are due to residual solvents diethyl ether and methanol.
region, the signals of the triazole pyridine moiety and those of the
phenyl rings associated with the alkoxyamine ligand closely overlap,
precluding their individual resolution. Altogether, the comparison of
the signals of 8 with those of 6 and 4 clearly indicates the successful
synthesis of 8. MSN 8 was then subjected to nitroxide exchange
reactions using various nitroxides containing different amino groups
to provide MSNs bearing basic amino functionalities. Treatment with
PdCl2 finally afforded MSNs 9–12 (Fig. 3; for synthesis and character-
isation of the various nitroxides and MSNs, as well as their BJH plots,8
see the ESI†). Hence, four different catalysts were readily prepared
starting with one mother system 8 documenting the power of our late
stage functionalization approach.
formed triazole ring shifts from 51.4 ppm in 2 (carbon next to the
azide) to 52.3 ppm in 4 and 53.8 ppm in 5, respectively. For the MSN 5
only minor changes in the NMR spectra can be detected compared to
MSN 4 (see also the ESI†) but the broadening of the aromatic
resonances could indicate a coordination of the ligands to Pd. The
13C NMR spectra give clear evidence for the successful synthesis and
functionalization of the materials investigated. Encouraged by these
initial experiments we prepared bifunctional MSNs 7 by co-condensa-
tion of azide 2, alkoxyamine 6 and TEOS in a 1 : 1 : 8 ratio
(Scheme 3).
The MSNs 9–12 (2–5 mol%)9 were successfully applied as catalysts
in the Tsuji–Trost allylation of allyl methyl carbonate 13 with ethyl
acetoacetate 14 in THF using K2CO3 as an external base (Scheme 4
and Table 1). Near quantitative yield of monoallylated product 15
resulted with catalyst 9 (entry 1). The amino functionalities in 9
influenced the reaction outcome10 since MSN 10 bearing aminoamide
groups instead showed a higher activity and bisallylated product 16
was obtained in 96% yield (entry 2).
CuAAC reaction of 7 with 2-ethynylpyridine provided 8 which
was analysed using 13C CP/MAS NMR spectroscopy (Fig. 2).
Resonances that can be clearly attributed to the alkoxyamine
moiety in the solid material are the peak near 91 ppm, assigned to
the a-carbon of the alkoxyamine heterocycle and the signal near
70 ppm attributed to the two oxygen-bonded C-atoms. In the aromatic
Like MSN 9, catalysts 11 and 12 afforded exclusively the mono-
allylated product 15 in high yields (entries 3 and 4) further document-
ing the role of the amine as a cooperative active unit. Reaction with 10
using 1 equiv. of K2CO3 did not change the outcome (compare entries
2 and 5). However, in the absence of K2CO3 the allylation did not
proceed (entry 6). Interestingly, clean monoallylation was also achieved
with the most active catalyst 10 upon running the allylation at 30 1C
(compare entries 2 and 7). Hence, product control was possible by
simple temperature adjustment. Even with the less reactive catalysts 9
and 11 reactions could be conducted at lower temperature, but
reaction time had to be increased and yields were lower (compare
entries 1 and 3 with entries 8 and 9). In both cases the reaction only
proceeded to give the monoallylated product in good yields. To
evaluate the effect of the additional amine (cooperativity) on the
reaction we tested catalyst 5 lacking the amine moiety and noted a
significantly lower degree of conversion to the monoallylation product
15 at longer reaction time under otherwise identical conditions
Scheme 3 Co-condensation of 2, 6 and TEOS to give 7 and formation of 8 via
azide–alkyne reaction.
Fig. 2 (a) 13C NMR of 6 in CDCl3 and (b) 13C CP/MAS MAS NMR of 8.
2196 Chem. Commun., 2013, 49, 2195--2197
Scheme 4 Tsuji–Trost allylation.
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Table 1 Results of Pd-catalyzed Tsuji–Trost allylation
Table 2 Pd-catalyzed Tsuji–Trost allylation with MSNs 17–19
Yield [%]
Yielda [%]
Entry Catalyst Temperature [1C] Reaction time [h] 15
16
Entry Catalyst Temperature [1C] Reaction time [h] 15
16
1
2
3
4
5
6
7
8
9
10
11
12
10
10
10
9
70
70
70
70
70
70
30
rt
18
18
12
18
18
18
18
24
24
24
24
18
18
99
0
99
93
0
0
96
0
1
2
3
4
5
6
17
18
19
17
18
19
70
70
70
rt
rt
rt
24
24
24
24
24
24
99
93
33
0
0
0
0
0
66
0
0
0
0
93a
0
0b
91
62
52
77
77
0
0
a
0c
GC-conversion.
9
11
5
rt
70
0c
10
11
12
13
0c
with the triazole ligand gave the most active catalyst 10, showed
a slightly lower activity in combination with the bipyridyl ligand
(entry 2). The most active system in this series was found to be
19 which provided a 1 : 2 mixture of mono- and bisallylated
product (entry 3). In contrast to the pyridyltriazole based
catalysts, 17–19 were not active at room temperature.
5, NEt3 70
9
9
0c
70
70
99c,d
15c,e
85
a
b
c
1 eq. K2CO3 was used. 0 eq. K2CO3 was used. GC-conversion.
Pd(OAc)2 was used as a Pd-source. Pd2(dba)3 was used as a Pd-
d
e
source.
In conclusion, we prepared various bifunctional mesoporous
nanoparticles by co-condensation of TEOS and triethoxysilanes
bearing azides and alkoxyamines at their termini. Late stage
functionalization by orthogonal click chemistry and subsequent
palladation provided different catalysts which were successfully used
in a Tsuji–Trost allylation. The successful synthesis of these MSNs
was ensured by solid state NMR. We showed that the structure of the
amine functionalities heavily influences reaction which shows that
they act as cooperative active entities.
(entry 10). Addition of external NEt3 to 5 did not improve the result
(entry 11) which clearly shows the beneficial effect of immobilized
amine bases on the allylation. To exclude that reactions are catalysed
by Pd-compounds that leached out of the MSN, we ran the reaction
with MSN 9 to obtain around 75% conversion and carefully filtered off
the particles. The filtrate was then kept at 70 1C for another 18 h and
no further conversion was noted which gives evidence that catalysis
occurs exclusively at the MSN particles. We tested different Pd-sources
and noted a similar reactivity upon starting with Pd(OAc)2 (entry 12),
whereas particles derived from Pd2(dba)3 were significantly less
reactive (entry 13). In addition, we also tested catalyst 9 in a recycling
experiment and the reactivity remained high for 4 runs. However, in
the 5th run high conversion was observed but a mixture of mono- and
bisallylated product was formed.11 Finally, we also tested the effect of
the structure of the Pd-ligand on the allylation. To this end, we
prepared a bipyridyl type ligand, which was then attached to the
surface via CuAAC chemistry. Nitroxide exchange reaction provided
MSNs 17, 18 and 19 (Fig. 4).
This work was funded by the DFG via project SFB858. F.B. thanks
the Fonds der Chemischen Industrie for a doctoral fellowship.
Notes and references
1 For recent development see: (a) K. Motokura, M. Tomita, M. Tada and
Y. Iwasawa, Chem.–Eur. J., 2008, 14, 4017–4027; (b) Y. Huang, S. Xu and
V. S. Y. Lin, Angew. Chem., 2011, 123, 687–690; (c) E. L. Margelefsky,
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G. Rothenberg, Angew. Chem., 2011, 123, 9789–9793; (e) S. Shylesh,
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Angew. Chem., 2010, 122, 188–191; (g) H. Noda, K. Motokura, A. Miyaji
and T. Baba, Angew. Chem., 2012, 124, 8141–8144; (h) K. K. Sharma,
A. V. Biradar, S. Das and T. Asefa, Eur. J. Inorg. Chem., 2011, 3174–3182.
2 A. T. Dickschat, F. Behrends, M. Bu¨hner, J. Ren, M. Weiß, H. Eckert
and A. Studer, Chem.–Eur. J., 2012, 18, 16689–16697.
The results of the allylations with catalysts 17–19 are pre-
sented in Table 2. With 17 the reaction proceeded smoothly and
the monoallylated product 15 was obtained in quantitative yield
(entry 1). The aminoamide functionality, which in combination
3 B. M. Trost, Chem. Rev., 1996, 96, 395–422.
4 (a) H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem., 2001,
113, 2056–2075; (b) C. Barner-Kowollik, F. E. Du Prez, P. Espeel,
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¨
5 (a) H. Wagner, M. K. Brinks, M. Hirtz, A. Schafer, L. Chi and
A. Studer, Chem.–Eur. J., 2011, 17, 9107–9112; (b) B. Schulte,
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6 ‘‘Click to chelate’’ approach: (a) H. Struthers, T. L. Mindt and R. Schibli,
Dalton Trans., 2010, 39, 675–696, see also: (b) G. Zhang, Y. Wang, X. Wen,
C. Ding and Y. Li, Chem. Commun., 2012, 48, 2979–2981.
7 Pd-catalysts immobilized into MSNs: (a) K. Komura, H. Nakamura and
Y. Sugi, J. Mol. Catal. A: Chem., 2008, 293, 72–78; (b) G. Lv, W. Mai, R. Jin
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S. Das and T. Asefa, Eur. J. Inorg. Chem., 2011, 3174–3182.
8 The surface area found for this type of material is around 20 m2 gÀ1
9 Catalyst loading was estimated based on elemental analysis.
.
10 Similar observation: H. Noda, K. Motokura, A. Miyaji and T. Baba,
Angew. Chem., Int. Ed., 2012, 51, 8017–8020.
11 Due to the rather harsh conditions, some decomposition of the
particles was observed. This is likely the reason for the loss of activity.
Fig. 4 MSNs 17–19 bearing bipyridyl complexed Pd.
c
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Chem. Commun., 2013, 49, 2195--2197 2197