Immobilization of a platinum catalyst using the polymer incarcerated (PI) method and application to catalytic reactions

Article abstract of DOI:10.1055/s-2005-863707

Immobilization of a platinum catalyst was carried out on the basis of the polymer incarcerated (PI) method. The PI platinum catalyst thus prepared showed high activity in hydrosilylation, and recovery and reuse of the catalyst were attained without loss of activity. Application of this catalyst to hydrogenation is also reported.

Full text of DOI:10.1055/s-2005-863707

LETTER
813
Immobilization of a Platinum Catalyst Using the Polymer Incarcerated (PI)
Method and Application to Catalytic Reactions
I
mmobilization of
i
a
Platin
r
umCata
o
lyst
U
sing th
y
e
Polymer
I
u
ncarcerated
M
k
ethod i Hagio, Masaharu Sugiura, Shu Kobayashi*
Graduate School of Pharmaceutical Sciences, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
Fax +81(3)56840634; E-mail: skobayas@mol.f.u-tokyo.ac.jp
Received 24 December 2004
O
Abstract: Immobilization of a platinum catalyst was carried out on
O
H
the basis of the polymer incarcerated (PI) method. The PI platinum
catalyst thus prepared showed high activity in hydrosilylation, and
recovery and reuse of the catalyst were attained without loss of ac-
tivity. Application of this catalyst to hydrogenation is also reported.
4
0.06
0.83
0.11
Pt(PPh3)4
THF, r.t.
coacervation
hexane
Key words: immobilization, platinum, polymer, hydrosilylation,
hydrogenation
O
O
2
PI Pt (1)
Hydrosilylation of alkenes and alkynes is not only an im-
portant industrial process, but also provides very useful
synthetic tools in laboratories.1 While metal catalysts
based on Rh and Mn are known in this process,2,3 Pt
complexes are the catalyst of choice since the Speier’s
pioneering discovery.4 Indeed, Pt catalysts work well with
alkyl and alkoxysilanes as well as chlorosilanes without
deactivation. On the other hand, immobilization of Pt
catalysts on either inorganic or organic supports have
been investigated.5–7
1) filtration
2) wash (hexane)
3) dry
1) filtration
2) wash (THF)
3) dry
No solvent
120 °C, 2 h
Figure 1 Polymer-incarcerated method
Pd prepared from 2 and Pd(PPh3)4 contained no phos-
phine.8 This difference might be ascribed to higher stabil-
ity of the Pt-P bonds.10 SR-MAS 31P NMR analysis11
suggested that the P atom remained as triphenylphosphine
oxide in the PI Pt (1).12 Although it was difficult to re-
move the P components from PI Pt (1) completely, we
applied this PI Pt to hydrosilylation.13 Initially, hydrosily-
lation of 4-phenyl-1-butene (3) with pentamethyldisilox-
ane (4) in several solvents was tested using PI Pt (1).
Leaching of Pt was measured by fluorocene X-ray (XRF)
analysis14 and Inductively Coupled Plasma (ICP)
analysis15 after separation of the catalyst (Table 1). It was
found that leaching of Pt was observed when THF or H2O
was used as the sole solvent, while hexane suppressed the
leaching to afford hydrosilylated product 5 in high yield
(entries 1–3). However, reactivity was decreased signifi-
cantly in the second run (entry 2). On the other hand, the
reaction in hexane–H2O (1:2) proceeded quantitatively
without leaching of Pt, and the catalyst was recovered by
simple filtration quantitatively and could be reused five
times without loss of activity (entry 4).
Whereas some active heterogeneous Pt catalysts have
been developed, their activity and selectivity are often
decreased with repeated use, presumably because most Pt
catalysts currently used are sensitive to air oxidation. To
address this issue, Pt catalysts have been immobilized on
polymer-supported phosphines and amines, etc.; how-
ever, applicability of these catalysts is limited due to the
presence of ligands.5,7 In the meanwhile, we have recently
developed a new method, the polymer incarcerated (PI)
method, to immobilize metal catalysts onto polymers.8
The method is based on microencapsulation9 and cross-
linking. Polymer incarcerated Pd catalyst (PI Pd) was ef-
fectively synthesized from Pd(PPh3)4, and remarkably
high activity of PI Pd has been demonstrated. Along this
line, we report herein immobilization of Pt onto a polymer
using the PI method. Some different properties between
PI Pt and PI Pd are also described.
Then, the catalytic activity of PI Pt (1) was compared with
that of common heterogenous or homogenous catalysts.
The effect of additives on reactivity was also investigated
(Table 2). The reaction was found to proceed using PI Pt
even within 30 minutes (entry 1). Pt on C, which was
known to be an effective catalyst for hydrosilylation of
alkyne,16 showed almost the same activity, whereas the
reaction did not proceed well when Pt(PPh3)417 was used
as a catalyst (entries 2 and 3). In addition, PtO2, which was
recently found to be an effective catalyst for hydrosilyl-
ation,18 gave an inferior result under the conditions (entry
4). While addition of PPh3 to PI Pt completely suppressed
PI Pt (1) was prepared from co-polymer 2 and Pt(PPh3)4
(Figure 1). A solution of 2 and Pt(PPh3)4 in THF was co-
acervated by addition of hexane (microencapsulation).
The resulting precipitate was washed, dried, and heated
(cross-linked) to give PI Pt.
It was revealed by elemental analysis that the P atom re-
mained in PI Pt (P/Pt = 0.88), while it was reported that PI
SYNLETT 2005, No. 5, pp 0813–0816
2
1.
0
3.
2
0
0
5
Advanced online publication: 09.03.2005
DOI: 10.1055/s-2005-863707; Art ID: U35604ST
© Georg Thieme Verlag Stuttgart · New York

814
H. Hagio et al.
LETTER
Table 1 Effect of Solvents
PI Pt
(1 mol%)
H
+
HSiMe2OSiMe3
(2.0 equiv)
Ph
R3Si
R
Ph
Solvent
40 °C, 17 h
3 = Me2OSiMe3
3
4
5
Yield (%)a leaching (%)
Entry
Solvent
First
Second
Third
Fourth
Fifth
1
2
3
4
THF
Quant. (27.4)
90 (nd)b
–
–
–
–
Hexane
42 (nd)
49 (nd)
99 (2.3)
Quant. (nd)
–
–
H2O
95 (nd)
99 (9.7)
–
–
H2O–hexane (2:1)
Quant. (nd)
Quant. (nd)
Quant. (nd)
Quant. (nd)
a Determined by 1H NMR analysis using durene as an internal standard.
b Measured by XRF analysis; nd = not detected (<2.6%).
Table 3 Hydrosilylation of Several Substrates Using PI Pt (1)a
Table 2 Comparison of Catalytic Activity
"Pt"
(1 mol%)
Entry Substrate
TimeProduct
(h)
Yield
(%)b
H
+
HSiMe2OSiMe3
(2.0 equiv)
Ph
R3Si
Ph
H2O/hexane
(2:1)
40 °C, 0.5 h
1
3
90
O
H
R3 = Me2OSiMe3
R3Si
7
4
O
5
O
7
O
Entry ‘Pt’
Additive (1 mol%) Yield (%)a leaching (%)
2
17
Quant.
Quant.
81
OBn
H
SiR3
1
2
3
4
5
6
PI Pt
Pt/C
–
99 (nd)b
Ph
Ph
OBn
–
99
3c
1
SiR3
Ph
Ph
H
Pt(PPh3)4
PtO2
–
7
Ph
Ph
R3Si
–
87
4
17
Ph
Ph
PI Pt
PPh3
Ph3P=O
0
Ph
Ph
5
17
16
83e
82
PI Pt
Quant.
N
N
SiR3
n-C5H11
a Determined by 1H NMR analysis using durene as an internal
standard.
Ph
Ph
6c,d
n-C5H11
H
H
H
n-C5H11
H
b Measured by XRF analysis; nd = not detected (<2.6%).
+
R3Si
SiR3
H
6
7
the reaction, triphenylphosphine oxide (Ph3P=O) did not a All reactions were carried out with pentamethyldisiloxane (2 equiv)
using 1 mol% of 1 (the loading level of platinum = 0.768 mmol/g) in
hexane–H2O (2:1) under Ar at 40 °C. The leaching of platinum was
measured by XRF and ICP analyses. No peaks of the platinum were
affect the reactivity (entries 5 and 6). These results also
support that the P atom in PI Pt (1) remains as Ph3P=O.
Next, we surveyed the substrate scope of PI Pt (1)-cata-
lyzed hydrosilylation (Table 3). The reactions proceeded
smoothly to afford the desired products in good to high
yields in the presence of ester, ether, and amine function-
alities (entries 1, 2 and 5). It is noteworthy that the reac-
tion was successfully carried out even with an
alkenylamine, though hydrosilylation of aminated alkenes
is generally thought to be difficult.18,19 The hydrosilyla-
tion of a symmetrical, internal alkyne gave only the cis-
isomer in high yield,20 while reactions of terminal alkynes
afforded a mixture of two regioisomers, b-adduct 6 and a-
adduct 7 (entries 3 and 6).21 A sterically hindered alkene
also worked well (entry 4). In all cases, no leaching of Pt
was detected by XRF and ICP analyses,14,15 and PI Pt (1)
could be recovered quantitatively and reused.
detected in all entries.
b Determined by 1H NMR analysis using durene as an internal
standard.
c Triethoxysilane (2 equiv) was used.
d Ratio of 6:7 = 83:17.
e Isolated yield.
PI Pt (1) was then applied to hydrogenation. Hydrogena-
tion of benzalacetone (8) was conducted smoothly to af-
ford reduced products 9 and 10 in high yield, and recovery
and reuse of the catalyst were attained without loss of ac-
tivity even after the fifth use (Table 4). No leaching of Pt
was detected by XRF analysis in all runs (entry 1). While
Pt/C gave a better yield in the 1st run than PI Pt (1),
Pt(PPh3)4 was completely ineffective (entries 2 and 3).
Synlett 2005, No. 5, 813–816 © Thieme Stuttgart · New York

LETTER
Immobilization of a Platinum Catalyst Using the Polymer Incarcerated Method
815
Table 4 Hydrogenation of Benzalacetone Using PI Pt (1)a
"Pt" (5 mol%)
H2 (1 atm)
O
O
OH
+
Ph
Ph
Ph
THF, r.t., 2 h
8
9
10
Yield (%)b (Ratio of 9:10)
Entry
‘Pt’
First
Second
Third
Fourth
Fifth
1
2
3
PI Pt
84 (95:5)
Quant.
0
Quant. (95:5)
Quant. (94:6)
Quant. (94:6)
Quant. (94:6)
Pt/C
–
–
–
–
–
–
–
–
Pt(PPh3)4
a All reactions were carried out using 5 mol% of 1 (the loading level of platinum = 0.768 mmol/g) in THF under atmospheric H2 pressure at r.t.
The leaching of Platinum was measured by XRF and ICP analysis. No peaks of the platinum were detected in all entries.
b Determined by 1H NMR analysis using durene as an internal standard.
Typical Procedure for the Hydrosilylation of 3 (Table 2,
Entry 1)
Selective hydrogenation of the C-C triple bond of benzyl-
oxyalkyne 11 was achieved by using PI Pt to give 12
quantitatively (Scheme 1), whereas Pt/C gave several by-
products. It is noteworthy that cleavage of the benzyl
group of 11 did not proceed at all, while other Pt and Pd
catalysts generally catalyze debenzylation.
PI Pt (1, 6.5 mg, 0.005 mmol), 4-phenyl-1-butene (75.1 mL, 0.50
mmol) and pentamethyldisiloxane (196 mL, 1.0 mmol) were com-
bined in hexane–H2O (1:2) co-solvent (3 mL) under argon atmo-
sphere. The mixture was stirred for 30 min at 40 °C. The catalyst
was filtered and washed with THF, and the solvents of the filtrate
were removed under reduced pressure. The yield of hydrosilylated
1
product 5 was estimated by H NMR analysis using 1,2,4,5-tetra-
PI Pt
(5 mol%)
methylbenzene as an internal standard. Recovered PI Pt (1) was
OBn
1
dried under reduced pressure and reused. H NMR (300 MHz,
Ph
OBn
quant (nd)
+
Ph
Ph
H2
12
THF, r.t.
15 h
CDCl3): d = 7.24–7.08 (m, 5 H), 2.55 (t, J = 7.8 Hz, 1 H), 1.59 (ap-
parent quint, J = 7.7 Hz, 2 H), 1.30–1.23 (m, 2 H), 0.01 (s, 9 H),
0.00 (s, 6 H). 13C NMR (75 MHz, CDCl3): d = 142.9, 128.4, 128.2,
125.5, 35.7, 35.2, 23.0, 18.2, 2.0, 0.4.
11
11
(1 atm)
Pt/C
(5 mol%)
OBn
+
H2
THF, r.t.
15 h
(1 atm)
Acknowledgment
+
OH
12 +
Ph
O
Ph
This work was partially supported by CREST, SORST, and
ERATO, Japan Science and Technology Agency (JST) and a Grant-
in-Aid for Scientific Research from Japan Society of the Promotion
of Science (JSPS). The authors are grateful to Mr. Kuniaki
Okamoto and Dr. Ryo Akiyama (The University of Tokyo and JST)
for their helpful discussion. H.H. thanks a JSPS fellowship for
Japanese Junior Scientists.
69%
16%
15%
Scheme 1 Selective hydrogenation using PI Pt (1)
In summary, we have demonstrated that Pt catalysts could
be immobilized by using the PI method. The catalyst
showed high catalytic activity not only in hydrosilylation
but also in hydrogenation. It is noted that the catalyst can
be recovered quantitatively and reused without leaching
of Pt. Further investigations to develop new immobili-
zation methods as well as to apply this catalyst to other
reactions are now in progress.
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Synlett 2005, No. 5, 813–816 © Thieme Stuttgart · New York

816
H. Hagio et al.
LETTER
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Synlett 2005, No. 5, 813–816 © Thieme Stuttgart · New York