Use of pyridine-coated star-shaped ROMP polymer as the supporting ligand for ruthenium-catalyzed chemoselective hydrogen transfer reduction of ketones

Article abstract of DOI:10.1021/om300417v

"Soluble" star-shaped polymers containing a pyridine ligand at the chain ends, prepared by adopting sequential living ring-opening metathesis polymerizations (ROMP) of norbornene and a cross-linking reagent using Mo(CHCMe₂Ph)(N-2,6-ⁱPr₂C₆H ₃)(O^tBu)₂ via a "core-first" approach, have been employed as ligands for Ru-catalyzed chemoselective hydrogen transfer reductions of various ketones (cyclohexanone, 5-hexen-2-one, 2-allylhexanone, 5-isopropenyl-2-methylcyclohexanone). The activity increased upon addition of the above polymer as the ligand: the prepared catalyst could be recovered quantitatively and reused without decreasing in both activity and selectivity.

Full text of DOI:10.1021/om300417v

Article
pubs.acs.org/Organometallics
Use of Pyridine-Coated Star-Shaped ROMP Polymer As the
Supporting Ligand for Ruthenium-Catalyzed Chemoselective
Hydrogen Transfer Reduction of Ketones
Kotohiro Nomura,*,†,‡ Kotaro Tanaka,† and Sakiko Fujita†
†Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara
630-0101, Japan
‡Department of Chemistry, Tokyo Metropolitan University, 1-1 Minami Osawa, Hachioji, Tokyo 192-0397, Japan
S
* Supporting Information
ABSTRACT: “Soluble” star-shaped polymers containing a pyridine
ligand at the chain ends, prepared by adopting sequential living
ring-opening metathesis polymerizations (ROMP) of norbornene
and a cross-linking reagent using Mo(CHCMe2Ph)(N-
2,6-iPr2C6H3)(OtBu)2 via a “core-first” approach, have been
employed as ligands for Ru-catalyzed chemoselective hydrogen
transfer reductions of various ketones (cyclohexanone, 5-hexen-2-
one, 2-allylhexanone, 5-isopropenyl-2-methylcyclohexanone). The
activity increased upon addition of the above polymer as the ligand:
the prepared catalyst could be recovered quantitatively and reused
without decreasing in both activity and selectivity.
polymer-attached” ligands for catalytic hydrogen transfer
reduction of cyclohexanone in the presence of ruthenium
compounds.15−17 Exclusive reductions of the carbonyl group in
various ketones (for example, acetophenone, 5-hexen-2-one, 2-
allylhexanone) could be achieved in this catalysis, and the
recovered catalyst could be reused without decreases in both
the activity and selectivity (although, as described below, the
recovered yields from the mixture were not perfect).15
Moreover, we also demonstrated that a half-titanocene
containing an aryloxide ligand immobilized on the chain end
of ring-opened poly(NBE)s exhibited similar catalyst perform-
ances (activity, monomer reactivity) to the nonsupported,
Cp*TiCl2(O-2,6-iPr2C6H3) for ethylene/1-hexene copolymer-
ization in the presence of methylaluminoxane (MAO).18 Since
we believe that the approach should be promising for
immobilization of homogeneous catalysts, in this paper, we
thus present the synthesis of pyridine-modified star-shaped
ring-opened poly(NBE) and its application as the polymer-
supported ligand for ruthenium-catalyzed chemoselective
hydrogen transfer reduction of various ketones.19−21
INTRODUCTION
■
Precise control over macromolecular structure is a fascinating
goal in polymer synthesis, and unique characteristics of living
polymerizations (absence of chain transfer and termination,
etc.), such as ring-opening metathesis polymerization (ROMP),
controlled radical polymerization, and anionic polymerization,
present techniques for accomplishing this purpose.1 Star
polymers containing multiple linear arms connected at a
central branched core represent one of the simplest nonlinear
polymers,1,2 and ROMP3,4 has also been used for the synthesis
of cross-linked polymers.5−8 We recently demonstrated a
controlled synthesis of “soluble” star polymers by the living
ROMP technique using a Mo-alkylidene initiator by simple
sequential additions of norbornene (NBE) and the cross-linker
(CL).7 An exclusive introduction of functionality into the
ROMP polymer chain end can be easily achieved through a
cleavage of the ROMP polymer−metal double bonds (of
Schrock-type Mo-alkylidene) with aldehyde, yielding a carbon−
carbon double bond via a Wittig-like reaction.4,6e,7,9
Use of soluble polymer-supported ligands (instead of
insoluble polymer resins such as divinylbenzene, cross-linked
polystyrene10) for preparation of supported transition metal
complex catalysts that maintain unique characteristics in the
homogeneous analogues has been recognized as one of the
most attractive subjects in the field of catalysis and synthetic
organic chemistry.11−14 We previously reported that ring-
opened poly(NBE) containing pyridine or 2,2′-bipyridyl
moieties at the chain end, prepared by living ROMP using
Mo(CHCMe2Ph)(N-2,6-iPr2C6H3)(OtBu)2 terminated with
the corresponding aldehyde(s), can be used as “ROMP
RESULTS AND DISCUSSION
■
1. Synthesis of Pyridine-Modified Star-Shaped Ring-
Opened Poly(norbornene) and Measurement of Spher-
ical Aggregates by Transmission Electron Microscopy.
According to our previous communication (Scheme 1),7a high
molecular weight ring-opened star-shaped polymers, defined as
Received: May 15, 2012
Published: July 2, 2012
© 2012 American Chemical Society
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covered with ROMP polymers (Scheme 1). 4-Pyridine
carboxaldehyde was chosen to terminate the living ROMP as
well as to introduce the pyridine moiety at the polymer chain
end, and the resultant polymers are highly soluble in ordinary
organic solvents such as toluene, THF, dichloromethane, and
chloroform. The Mn values in the resultant polymers increased
upon increasing the amount of NBE in the third polymerization
(25 → 50 equiv to Mo, runs 1−4). The observed increase in
the Mn value [ca. 39 500 (by GPC vs polystyrene standards)
from runs 1−4] was much higher than those in the linear
poly(NBE) [increased 25 NBE repeating units, 2354 by
molecular weight], suggesting that the resultant ROMP
polymers are star-shaped polymers consisting of a core and
NBE branching (first and third polymerization).24
Scheme 1. Synthesis of Soluble star-shaped ROMP polymers
employed in the present study
Figure 1 shows selected transmission electron microscopy
(TEM) micrographs of thin films prepared by casting the
resultant ROMP polymers [star-poly(NBE)n-Py, run 3 (left
and middle, n = 25) and run 4 (right, n = 50)] on a plastic
(perforated polymer, polyvinyl formal) coated Cu grid.23 The
resulting micrographs depict formation of uniform spherical
images with average diameters of 58 nm (run 3) or 106 nm
(run 4).23 The diameter is similar to that reported previously
(52 nm),7a and seems to correspond to those consisting of the
arm and a core.23 The results thus suggest that the observed
micrographs should be ascribed to a single star (ball)-shaped
polymers prepared by adopting the present ROMP method-
ology.
2. Hydrogen Transfer Reduction of Cyclohexanone
Using RuCl2(PPh3)4 −Polymer Ligand Catalysts. On the
basis of our previous report,15 hydrogen transfer reduction of
cyclohexanone using RuCl2(PPh3)4 was conducted to explore
the performance of star-poly(NBE)25-Py as the ligand in this
catalysis (Scheme 2). A linear ROMP polymer containing
pyridine at the chain end, poly(NBE)100-Py,15,23 was also used
for comparison. The results are summarized in Table 2 (and
Tables S6 and S7 in the Supporting Information (SI)).25 As
reported previously (and shown in Table S5 in the SI),15,25 the
activity was affected by NaOiPr concentration and also
increased upon addition of a small amount of pyridine (1.0
equiv to Ru, run 6; poly(NBE)100-Py, run 7).26
star-poly(NBE)25-Py, with unimodal molecular weight dis-
tributions (Mn = (8.03−8.46) × 104, Mw/Mn = 1.21−1.34, runs
1−3, Table 1) could be prepared by adopting the sequential
Table 1. Synthesis of Pyridine Modified Star-Shaped ROMP
a
Polymers, star-poly(NBE)n-Py
third reaction
b
c
c
d
run
NBE/Mo (n)
time/min
Mn × 10−4
Mw/Mn
yield /%
1
2
3
4
25
25
25
50
15
15
15
20
8.45
8.46
8.03
12.4
1.23
1.21
1.34
1.39
99
97
99
99
a
Conditions: toluene (total 10.0 g) at 25 °C, Mo cat. 1.0 × 10−2
mmol, 1st NBE 0.25 mmol, CL 0.10 mmol (detailed conditions, see
Supporting Information).23 Starting feedstock ratio (n in Scheme 1).
b
c
d
GPC data in THF vs polystyrene standards. Isolated yields.
living ring-opening metathesis polymerizations of norbornene
and a cross-linking reagent (1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo-
endo-dimethanonaphthalene) (CL, exo:endo = 0.17:1.00)22
using Mo(CHCMe2Ph)(N-2,6-iPr2C6H3)(OtBu)2.23 As re-
ported previously, the present approach enables synthesis of
star-shaped polymers that are soluble in common organic
solvents, because the molybdenum catalyst shows markedly
higher reactivity toward strained cyclic olefins than the internal
olefins (in the ring-opened polymers),9c,d and the core is
Note that the activity increased upon addition of the star-
ROMP polymer, star-poly(NBE)25-Py, and that the activities
were not affected by the chain length employed (runs 8−10 in
Table 2, the NBE/Mo molar ratio in the third reaction), which
corresponds to the size of the star ROMP polymer shown in
Figure 1. The observed activities were close to those in the
Figure 1. TEM micrographs of thin films prepared by casting star-poly(NBE)25-Py (left and middle, run 3, Table 1) and star-poly(NBE)50-Py
(right, run 4, Table 1) on a plastic-coated copper grid at a concentration of 10−5 mg/mL at varying magnification.23
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(NBE)25-Py at low NBE repeating units was used as the ligand
due to the improved solubility in methanol.
Scheme 2
We conducted the recycled experiments using the recovered
catalyst without further purification (removal of methanol in
vacuo after filtration), because we assumed that the pyridine
moiety on the star polymer would be strongly coordinated to
Ru on the basis of our previous results.15 The results are
summarized in Table 3.25 Several reaction runs were performed
Table 3. Hydrogen Transfer Reduction of Cyclohexanone by
RuCl2(PPh3)4/star-poly(NBE)25-Py or poly(NBE)100-Py
Catalysts: Selected Results for the Catalyst Recycle
a
Experiments
b
c
catalyst
time/h
TON (yield in %)
RuCl2(PPh3)4 + star-poly(NBE)25-Py
recycled runs (first recycle)
recycled runs (second recycle)
recycled runs (third recycle)
recycled runs (fourth recycle)
recycled runs (first recycle)
recycled runs (second recycle)
recycled runs (fourth recycle)
RuCl2(PPh3)4 + poly(NBE)100-Py
recycled runs (first recycle)
recycled runs (second recycle)
recycled runs (third recycle)
recycled runs (fourth recycle)
recycled runs (fourth recycle)
5
5
249 (83)
240 (80)
255 (85)
246 (82)
249 (83)
291 (97)
282 (94)
288 (96)
243 (81)
246 (82)
252 (84)
255 (85)
243 (81)
282 (94)
Table 2. Hydrogen Transfer Reduction of Cyclohexanone by
RuCl2(PPh3)4 star-poly(NBE)25-Py and poly(NBE)100-Py
Catalysts
5
a
5
b
c
d
5
run
ligand (L/Ru)
Py (1)
TON (yield in %)
10
10
10
5
5
177 (59)
249 (83)
243 (81)
248 (83)
252 (84)
234 (78)
33 (11)
18 (6)
6
7
poly(NBE)100-Py (1)
8
star-(polyNBE)25-Py (1)
star-(polyNBE)50-Py (1)
star-(polyNBE)50-Py (1)
poly(4-vinylpyridine) (1)
poly(4-vinylpyridine) (50)
poly(4-vinylpyridine) (100)
5
9
5
10
11
12
13
5
5
10
6 (2)
a
a
Conditions: 50 °C, cyclohexanone 3.0 mmol, RuCl2(PPh3)4 0.01
Conditions: 50 °C, 5 h, cyclohexanone 3.0 mmol, RuCl2(PPh3)4 0.01
mmol, NaOiPr 0.8 mmol, iPrOH 3.0 mL, toluene 2.0 mL, 1.0 equiv of
b
mmol, NaOiPr 0.8 mmol, PrOH 3.0 mL, toluene 2.0 mL. Molar
ratios of L/Ru (L = pyridine). TON = (product in mmol)/Ru
(mmol). Yield estimated by GC using internal standard.
i
b
c
pyridine moiety was added to Ru. TON = (product in mmol)/Ru
c
d
(mmol). Yield estimated by GC using internal standard. Recycle
yields >99% [star-poly(NBE)25-Py]; 89−95% [poly(NBE)100-Py].
More data are shown in the SI.23
linear polymer, poly(NBE)100-Py (run 7, Table 2) and free
pyridine (run 6). These results clearly indicate that use of the
present “star polymer-attached” pyridine ligand is potentially
effective in this ruthenium catalysis. In contrast, the activity
decreased upon addition of poly(4-vinylpyridine): the activity
decreased by further addition (runs 11−13).
The catalyst supported on star-(polyNBE)25-Py could be
separated from the reaction products and recovered by
filtration as the precipitate by pouring the reaction mixture
into a methanol solution (Scheme 3). The recovered yields
were quantitative in all cases. This is an interesting contrast to
the fact that the recovered yields were 89−95% in the linear
polymers, poly(NBE)100-Py,25 due to loss of a trace amount of
polymer during the filtration/precipitation process: the
recovered yields decreased (ca. 70−80% yields) when poly-
to check the reproducibility, and the results in detail are
summarized in Table S6 in the SI.25 The experiments using
poly(NBE)100-Py were also conducted for comparison. Figure
2 shows plots of the TON values in each of the experimental
runs.25
It is clear that no significant decreases in the TON values
were observed by reusing the catalyst in both cases [catalysts
consisting of RuCl2(PPh3)4 and linear poly(NBE)100-Py or
star-poly(NBE)25-Py]. The reaction yields reached completion
when the reactions were conducted for 10 h, and the trend did
not change after several recycle runs (Table 3). As described
above, the star polymer-supported catalyst could be recovered
quantitatively, whereas the recovered yields were 89−95% in
the case of poly(NBE)100-Py. Taking these results into account,
the present Ru catalyst can be easily reused without
deactivation. We believe that the catalyst system presented
here is a promising candidate that possesses unique character-
istics for designing efficient recyclable catalytic systems.
1H and 31P NMR Spectra for a CDCl3 Solution Containing
the Recovered Catalyst . 31P NMR spectra of the CDCl3
solution containing recycled catalysts [RuCl2(PPh3)4 and
poly(NBE)100-Py or star-poly(NBE)25-Py, after precipitation
from MeOH] showed a sharp resonance at 28.0 ppm, and the
observed resonance was different from those observed in the
CDCl3 solution containing RuCl2(PPh3)4 (Figure S7 in the
SI).28 The results thus suggest formation of another PPh3-
coordinated species, as previously demonstrated for exclusive
Scheme 3
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Figure 2. Recycling experiments for hydrogen transfer reduction of cyclohexanone catalyzed by RuCl2(PPh3)4/poly(NBE)100-Py or star-
poly(NBE)25-Py (Table 3 and Tables S6, S7).25
Scheme 4
formations of RuCl2(PPh3)2(L)2 [L = norbornadiene, MeCN]
from RuCl2(PPh3)3.29,30 The catalyst could be easily recovered
by filtration, suggesting that pyridine in star-poly(NBE)25-Py
strongly coordinates to Ru.
catalyst concentration (Table 4). Moreover, as demonstrated
above, the catalyst can be easily recycled according to a
procedure shown in Scheme 3: the catalyst can be reused
without decrease in activity (Figure 3). These results clearly
indicate that the present catalyst [RuCl2(PPh3)4/star-poly-
(NBE)25-Py] can be used as an efficient recyclable catalyst for
exclusive reduction of the carbonyl group in various ketones.
We have shown that pyridine-modified, “soluble”, star-shaped
polymers have been prepared in a precisely controlled manner
by adopting the living ROMP technique using the molybde-
num-alkylidene initiator. The resultant polymer, star-poly-
(NBE)n-Py (n = 25, 50), can be used as an effective ligand for
ruthenium-catalyzed chemoselective hydrogen transfer reduc-
tion of various ketones. The activity of RuCl2(PPh3)4 increased
upon addition of the star polymer ligand, and exclusive
reduction of the carbonyl groups in cyclohexanone, 5-hexen-2-
one, 2-allylcyclohexanone, and 5-isopropenyl-2-methylcyclo-
hexanone (dihydrocarvone) has been demonstrated in this
catalysis. The catalyst can be quantitatively recovered by
pouring the reaction mixture into methanol and can be reused
without further purification. Moreover, both activity and
selectivity did not decrease in several recycle runs. The present
method for synthesis of modified star-shaped ROMP polymers
should be facile, and many applications (syntheses of various
ligands, applications for catalytic reactions) can be thus
considered. Therefore, we highly believe that the present
approach of using recyclable spherical supported catalysts for
efficient organic transformations should thus introduce a new
insight to study polymer-supported catalysis chemistry.
1
Moreover, the H NMR spectrum for the CDCl3 solution
containing recovered catalyst was very close to that of the
original star-shaped ROMP polymer, and the intensity of the
olefinic protons was unchanged before/after the reaction
(Figure S8 in the SI).28 This suggests that the present
reduction took place by hydrogen transfer reduction (not by
hydrogenation with dihydrogen once formed) generally
proposed under these conditions.16,17 Since the olefinic double
bonds in the ROMP polymer were not hydrogenated under
these conditions, the result strongly suggests the possibility of
exclusive reduction of the carbonyl group in this catalysis.15
3. Chemoselective Hydrogen Transfer Reduction of
Various Ketones by RuCl2(PPh3)4/Star Polymer-Sup-
ported Ligand Catalysts. As described above, a possibility
for exclusive reduction of the carbonyl group was suggested in
this catalysis. The reductions of various ketones using the
RuCl2(PPh3)4/star-poly(NBE)25-Py catalyst were thus ex-
plored under the same conditions (Scheme 4). The results
are summarized in Table 4.25
Note that the reaction products from 5-hexen-2-one, 2-
allylcyclohexanone, and 5-isopropenyl-2-methylcyclohexanone
(dihydrocarvone) were 5-hexen-2-ol, 2-allyl-cyclohexanol, and
5-isopropenyl-2-methylcyclohexanol (dihydrocarveol), respec-
tively, as the sole products (Scheme 4): the exclusive reduction
of the carbonyl group has thus been achieved in this catalysis.
High product yields were attained if these reactions were
conducted for long reaction times and/or under rather high
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detector (Shimadzu Co. Ltd.) in THF (containing 0.03 wt % 2,6-di-
tert-butyl-p-cresol, flow rate 1.0 mL/min). GPC columns (ShimPAC
GPC-806, -804, and -802, 30 cm × 8.0 mm ⦶) were calibrated versus
polystyrene standard samples.
Table 4. Chemoselective Hydrogen Transfer Reduction of
Various Ketones (Selected Results25) Catalyzed by
RuCl2(PPh3)4 and star-poly(NBE)25-Py
a
All chemicals used were of reagent grade and were purified by
standard purification procedures. Polymerization grade toluene was
stored over sodium/potassium alloy in a drybox and was then passed
through a short alumina column prior to use. Anhydrous grade toluene
(Kanto Kagaku Co. Ltd.) for polymerization (before storage in the
presence of sodium/potassium alloy) was transferred into a bottle
yield
b
c
d
catalyst/ligand
substrate
5-hexen-2-one
time/h /% TON
RuCl2(PPh3)4 + star-
5
58
64
60
61
59
90
42
41
42
93
69
69
70
90
174
192
180
183
177
270
126
123
126
140
138
138
140
180
poly(NBE)25-Py
recycled runs (first
recycle)
5-hexen-2-one
5
containing molecular sieves (mixture of 3A, 4A 1/16, and 13X) in the
recycled runs (second
recycle)
5-hexen-2-one
5
31
drybox. Mo(CHCMe2Ph)(N-2,6-iPr2C6H3)(OtBu)2
and
1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo-endo-dimethanonaphtalene (CL,
recycled runs (third
recycle)
5-hexen-2-one
5
exo:endo = 0.17:1.00)22 were prepared according to the literature.
i
NaOiPr was prepared by treating PrOH with Na under a nitrogen
recycled runs (fourth
recycle)
5-hexen-2-one
5
atmosphere and isolated as a white solid. RuCl2(PPh3)4 (STREM) and
4-pyridinecarboxaldehyde (Aldrich) were used as received.
recycled runs (fourth
5-hexen-2-one
15
5
e
Transmission electron microscopy measurements were conducted
on thin films, prepared by evaporating a THF solution of the polymer
samples on an Okenshoji Co. Ltd. copper grid, which was covered
with a perforated polymer (polyvinyl formal) film and coated with
carbon on all sides (diameter 3 mm), at concentrations of 10−5 or 10−6
mg/mL (THF). They were then analyzed with a JEOL JEM-3100FEF
electron microscope (gun type: field emission), at an accelerate voltage
of 300 kV, a column vacuum of 1.0 × 10−5 Pascals, and at a
magnification of 20 000×. All pictures were energy-filtered TEM
images, which were taken with an Omega energy filter for zero-loss
images (cut inelastic scattered electrons). The temperature was set to
22 °C, and the relative humidity was set to 50%.
recycle)
RuCl2(PPh3)4 + star-
2-allylcyclohexanone
2-allylcyclohexanone
2-allylcyclohexanone
2-allylcyclohexanone
poly(NBE)25-Py
recycled runs (first
recycle)
5
recycled runs (second
recycle)
5
recycled runs (second
15
5
d
recycle)
RuCl2(PPh3)4 + star-
5-isopropenyl-2-
methylcyclohexanone
poly(NBE)25-Py
recycled runs (first
recycle)
5-isopropenyl-2-
methylcyclohexanone
5
General Polymerization Procedure. A toluene solution (0.5 g)
containing Mo(CHCMe2Ph)(N-2,6-iPr2C6H3)(OtBu)2 (1.82 × 10−5
mol) was added in one portion to a rapidly stirred toluene solution
(9.5 g) containing the norbornene (25 equiv to Mo) at room
temperature (25 °C), and the solution was stirred for 4 min. The
second monomer, 10 equiv of cross-linker, 1,4,4a,5,8,8a-hexahydro-
1,4,5,8-exo-endo-dimethanonaphtalene (CL, exo:endo = 0.17:1.00), was
then added into the solution, and the mixture was stirred for 50 min.
The third monomer (norbornene) in toluene (1.0 g) was then added
in one portion, and the reaction mixture was further stirred for
prescribed time. The polymerization was quenched by adding pyridine
carboxaldehyde (ca. >10 mg, excess), and the solution was stirred for 1
h until completion. The mixture was then removed in vacuo, and the
resultant solid was dissolved in the minimum amount of THF. The
solution was poured dropwise into methanol to afford off-white
precipitates. The polymer was then collected by filtration and dried in
recycled runs (second
recycle)
5-isopropenyl-2-
methylcyclohexanone
5
recycled runs (second
recycle)
5-isopropenyl-2-
methylcyclohexanone
15
a
Conditions: RuCl2(PPh3)4 0.01 mmol, substrate 3.0 mml (dihy-
drocarvone 2.0 mmol), NaOiPr 0.8 mmol, iPrOH 3.0 mL, toluene 2.0
b
c
mL. 1.0 equiv of pyridine (moiety) to Ru. Yield estimated by GC
d
e
using internal standard. TON = product (mmol)/Ru (mmol). Ru
0.02 mmol. Recycle yields >99% [star-poly(NBE)25-Py], and more
data are shown in the SI.23
EXPERIMENTAL SECTION
■
General Procedures. All experiments were carried out under a
nitrogen atmosphere in a Vacuum Atmospheres drybox or using
standard Schlenk techniques. All 1H and 13C NMR spectra were
recorded on a Bruker AV500 spectrometer (500.13 MHz for 1H,
125.77 MHz for 13C), and all chemical shifts are given in ppm and are
referenced to SiMe4. Obvious multiplicities and routine coupling
constants are usually not listed, and all spectra were obtained in the
solvent indicated at 25 °C. HPLC grade THF (Wako Pure Chemical
Ind., Inc.) was used for GPC and was degassed prior to use. GPC was
performed at 40 °C on a Shimadzu SCL-10A using a RID-10A
1
vacuo. H NMR (CDCl3): δ 5.19 and 5.32 (br m, 2H olefinic), 2.76
and 2.40 (br s, 2H), 1.85 and 1.04 (m, 2H), 1.77 and 1.33 (m, 4H),
8.52 and 8.55 (d, pyridine).
Catalytic Hydrogen Transfer Reduction of Ketones. The
typical procedure is as follows:15 into a Schlenk tube (50 mL),
i
RuCl2(PPh3)4, ligand, toluene, PrOH, NaOiPr, and cyclohexanone
(substrate) were charged under a nitrogen atmosphere. The reaction
mixture was stirred under N2 at 50 °C for the prescribed time (5 h).
Figure 3. Recycling experiments for hydrogen transfer reduction of 5-hexen-2-one (left), 2-allylcyclohexanone (middle), and 5-isopropenyl-2-
methylcyclohexanone (dihydrocarvone, right) catalyzed by RuCl2(PPh3)4/star-poly(NBE)25-Py (Table 4, and Tables S8−10). More data are shown
in the SI.25
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The reaction product was then determined by GLC using an internal
standard and was identified by using GLC by co-injection with the
authentic samples under different conditions (column: DB-1 30 m,
0.25 mm ⦶ × 0.25 μm). After the reaction, the solution was poured
into cold methanol (50 mL), the resultant precipitate was collected by
filtration, and the recovered catalyst was dried in vacuo and was reused
without further purification. Recycled experiments were carried out by
using the recovered catalyst mixed with independent runs to compare
the catalyst performance.
(5) Saunders, R. S.; Cohen, R. E.; Wong, S. J.; Schrock, R. R.
Macromolecules 1992, 25, 2055 (arm first approach by addition of
cross-linked reagents at the final stage).
(6) For selected examples, see: (a) Buchmeiser, M. R.; Atzl, N.;
Bonn, G. K. J. Am. Chem. Soc. 1997, 119, 9166. (b) Sinner, F.;
Buchmeiser, M. R.; Tessadri, R.; Mupa, M.; Wurst, K.; Bonn, G, K, J.
Am. Chem. Soc. 1998, 120, 2790. (c) Buchmeiser, M. R.; Seeber, G.;
Mupa, M.; Bonn, G. K. Chem. Mater. 1999, 11, 1533. (d) Sinner, F.;
Buchmeiser, M. R. Macromolecules 2000, 33, 5777. (e) Lubbad, S.;
Buchmeiser, M. R. Macromol. Rapid Commun. 2002, 23, 617.
ASSOCIATED CONTENT
(f) Bandari, R.; Prager-Duschke, A.; Kuhnel, C.; Decker, U.;
̈
■
Schlemmer, B.; Buchmeiser, M. R. Macromolecules 2006, 39, 5222.
(g) Eder, K.; Huber, C. G.; Buchmeiser, M. R. Macromol. Rapid
S
* Supporting Information
Text giving additional data for polymer synthesis (including 1H
NMR spectra), measurement of spherical aggregates (shape
polymers) by TEM, additional (recycled) results for chemo-
selective hydrogen transfer reduction of various ketones by
Commun. 2007, 28, 2029. (h) Lober, A.; Verch, A.; Schlemmer, B.;
̈
Hofer, S.; Frerich, B.; Buchmeiser, M. R. Angew. Chem., Int. Ed. 2008,
̈
47, 9138 (syntheses of insoluble polymers by copolymerization in the
presence of cross-linkers).
1
ruthenium catalysts, and H and 31P NMR spectra of CDCl3
(7) (a) Nomura, K.; Watanabe, Y.; Fujita, S.; Fujiki, M.; Otani, H.
Macromolecules 2009, 42, 899. (b) Otani, H.; Fujita, S.; Watanabe, Y.;
Fujiki, M.; Nomura, K. Macromol. Symp. 2010, 293, 53. (c) Takamizu,
K.; Nomura, K. J. Am. Chem. Soc. 2012, 134, 7892.
solutions containing recycled catalyst. This information is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Tel: +81-42-677-2547. Fax: +81-42-677-2547. E-mail:
ktnomura@tmu.ac.jp.
(8) Examples (reviews), see: (a) Lubbad, S.; Buchmeiser, M. R.
Macromol. Rapid Commun. 2003, 24, 580. (b) Mayr, M.; Wang, D.;
■
Kroll, R.; Schuler, N.; Pruhs, S.; Furstner, A.; Buchmeiser, M. R. Adv.
̈
̈
̈
Synth. Catal. 2005, 347, 484. (c) Buchmeiser, M. R. In Handbook of
Metathesis; Grubbs, R. H., Ed.; Wiley-VCH: Weinheim, Germany,
2003; Vol. 3, p 226. (d) Buchmeiser, M. R. Chem. Rev. 2009, 109, 303.
(e) Nomura, K.; Abdellatif, M. M. Polymer 2010, 51, 1861.
Notes
The authors declare no competing financial interest.
(9) For examples, see: (a) Nomura, K.; Takahashi, S.; Imanishi, Y.
Macromolecules 2001, 34, 4712. (b) Murphy, J. J.; Kawasaki, T.; Fujiki,
M.; Nomura, K. Macromolecules 2005, 38, 1075. (c) Murphy, J. J.;
Nomura, K. Chem. Commun. 2005, 4080. (d) Murphy, J. J.; Furusho,
H.; Paton, R. M.; Nomura, K. Chem.Eur. J. 2007, 13, 8985.
(10) (a) Ford, T. W. Polymeric Reagents and Catalysts; ACS
Symposium Series 308; American Chemical Society: Washington,
DC, 1986; p 1. (b) Merrifield, R. B. Angew. Chem. 1985, 97, 801.
(11) For example (review articles): (a) Hlatky, G. G. Chem. Rev.
ACKNOWLEDGMENTS
■
The project was partly supported by a Grant-in-Aid for
Challenging Exploratory Research (No. 21656209). K.N. and
K.T. express their thanks to Prof. Michiya Fujiki (NAIST) for
discussion and to Mr. Shohei Katao (NAIST) for help in TEM
measurements with S.F.
REFERENCES
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(g) Matyjaszewski, K.; Tsarevsky, N, V. Nat. Chem. 2009, 1, 276.
(3) Reviews and books for olefin metathesis: (a) Fogg, D. E.;
Foucault, H. M. In Comprehensive Organometallic Chemistry III;
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11, p 623. (b) Buchmeiser, M. R., Ed. Metathesis Polymerization;
Springer: Berlin, 2005. (c) Grubbs, R. H., Ed. Handbook of Metathesis;
Wiley-VCH: Weinheim, 2003; Vols. 1−3. (d) Buchmeiser, M. R.
(12) The Power of Functional Resins in Organic Synthesis; Tulla-Puche,
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(13) For example: (a) Toy, P. H.; Janda, K. D. Acc. Chem. Res. 2000,
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̈
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(14) (a) Bergbreiter, D. E.; Kobayashi, S. Special issue on Facilitated
Synthesis, Chem. Rev. 2009, 109, number 2: for examples, see:
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(c) Lu, J.; Toy, P. H. Chem. Rev. 2009, 109, 815.
(15) Nomura, K.; Kuromatsu, Y. J. Mol. Catal. A 2006, 245, 152.
(16) Selected review articles for (asymmetric) transfer hydro-
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1992, 92, 1051. (b) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997,
30, 97. (c) Naota, T.; Takaya, H.; Murahashi, S.-I. Chem. Rev. 1998,
Chem. Rev. 2000, 100, 1565. (e) Furstner, A., Ed. Alkene Metathesis in
̈
98, 2599. (d) Backvall, J.-E. J. Organomet. Chem. 2002, 652, 105.
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Organic Synthesis; Springer-Verlag: Berlin, 1998.
(e) Everaere, K.; Mortreux, A.; Carpentier, J. F. Adv. Synth. Catal.
2003, 345, 67. (f) Blaser, H.-U.; Malan, C.; Pugin, B.; Spindler, F.;
Steiner, H.; Studer, M. Adv. Synth. Catal. 2003, 345, 103. (g) Gladiali,
S.; Alberico, E. Chem. Soc. Rev. 2006, 35, 226. (h) Samec, J. S. M.;
(4) (a) Schrock, R. R. In Handbook of Metathesis; Grubbs, R. H., Ed.;
Wiley-VCH: Weinheim, 2003; Vol. 1, p 8. (b) Schrock, R. R.;
Hoveyda, A. H. Angew. Chem., Int. Ed. 2003, 42, 4592. (c) Schrock, R.
R. In Metathesis Polymerization of Olefins and Polymerization of Alkynes;
Imamoglu, Y., Ed.; NATO ASI Series; Kluwer Academic Publishers:
Dordrecht, The Netherlands, 1998; pp 1 and 357. (d) Schrock, R. R.
Backvall, J.-E.; Andersson, P. G.; Brandt, P. Chem. Soc. Rev. 2006, 35,
̈
237. (i) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300.
(j) Morris, R. H. Chem. Soc. Rev. 2009, 38, 2282.
In Alkene Metathesis in Organic Synthesis; Furstner, A, Ed.; Springer-
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Verlag: Berlin, 1998; p 1. (e) Feldman, J.; Schrock, R. R. Prog. Inorg.
(17) Selected examples for transfer hydrogenation of ketones:
Chem. 1991, 39, 1. (f) Schrock, R. R. Acc. Chem. Res. 1990, 23, 158.
(a) Doucet, H.; Ohkuma, T.; Murata, K.; Yokozawa, T.; Kozawa, M.;
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Organometallics
Article
Katayama, E.; England, A. F.; Ikariya, T.; Noyori, R. Angew. Chem., Int.
Ed. 1998, 37, 1703. (b) Yamakawa, M.; Ito, H.; Noyori, R. J. Am.
Chem. Soc. 2000, 122, 1466. (c) Abdur-Rashid, K.; Clapham, S. E.;
Hadzovic, A.; Harvey, J. N.; Lough, A. J.; Morris, R. H. J. Am. Chem.
Soc. 2002, 124, 15104.
(18) Kitiyanan, B.; Nomura, K. Organometallics 2007, 26, 3461.
(19) Selected examples for use of ROMP gel polymer as the ligand
(supported in the side chain):8b−d (a) Buchmeiser, M. R.; Wurst, K. J.
Am. Chem. Soc. 1999, 121, 11101. (b) Buchmeiser, M. R. Chem. Rev.
2000, 100, 1565. (c) Barrett, A. G. M.; Hopkins, B. T.; Kobberling, J.
Chem. Rev. 2002, 102, 3301. (d) Årstad, E.; Barrett, A. G. M.; Hopkins,
B. T.; Kobberling, J. Org. Lett. 2002, 4, 1975. (e) Barrett, A. G. M.;
Love, A. C.; Tedeschi, L. Org. Lett. 2004, 6, 3377. (f) Harned, A. M.;
Zhang, M.; Vedantham, P.; Mukherjee, S.; Herpel, R. H.; Flynn, D. L.;
Hanson, P. R. Aldrichim. Acta 2005, 38, 3. (g) Barrett, A. G. M.; Bibal,
B.; Hopkins, B. T.; Kobberling, J.; Love, A. C.; Tedeschi, L.
̈
Tetrahedron 2005, 61, 12033. (h) Fuchter, M. J.; Hoffman, B. M.;
Barrett, A. G. M. J. Org. Chem. 2006, 71, 724.
(20) Related review article for synthesis of end-functionalized
polymers by ROMP with Ru catalysts: Hilf, S.; Kilbinger, A. F. M.
Nat. Chem. 2009, 1, 537.
(21) Other examples for transfer hydrogenation using star polymer-
supported catalysis (supported into the star): (a) Terashima, T.;
Ouchi, M.; Ando, T.; Sawamoto, M. J. Polym. Sci., Part A: Polym. Chem.
2010, 48, 373. (b) Terashima, T.; Ouchi, M.; Ando, T.; Sawamoto, M.
Polym. J. 2011, 43, 770.
(22) Stille, J. K.; Frey, D. A. J. Am. Chem. Soc. 1959, 81, 4273.
(23) Detailed experimental procedures, additional polymerization
results, additional micrographs for the resultant polymers by TEM, and
their data analyses are shown in the SI.
(24) As described in the text, the number of polymer chains (arms
after third polymerization) may be simply assumed on the basis of the
increase in the Mn values [ca. 39 500 (by GPC vs polystyrene
standards) from runs 1−4 (estimated from average Mn values)] and
the Mn value of linear poly(NBE) [increasing 25 NBE repeating units,
2354 by molecular weight, and ca. 4000 (by GPC vs polystyrene
standards)].7a The value (39 500/4000 = 9.9, total number of arms
from first and third polymerization was assumed as ca. 20) suggests
that the resultant polymers are star polymers consisting of a core and
NBE branching.
(25) Detailed results in the transfer hydrogenation of cyclohexanone
and various ketones including their recycled experiments are shown in
the SI.
(26) As reported previously,15 the yield by using the Ru(acac)3/
poly(NBE)100-Py catalyst increased when conducted for longer times,
and a relatively linear first-order relationship between substrate
concentration and the reduction rate was observed, indicating that
the reduction proceeded without catalyst decomposition.
(27) For example: Gestwicki, J. E.; Cairo, C. W.; Strong, L. E.;
Oetjen, K. A.; Kiessling, L. L. J. Am. Chem. Soc. 2002, 124, 14922.
1
(28) Detailed H and 31P NMR spectra are shown in the SI.
(29) For example: Komiya, S.; Hirano, M. In Komiya, S., Ed.
Synthesis of Organometallic Compounds: Practical Guide; John Wiley &
Sons Ltd.: Baffins Lane, England, 1997; p 159.
(30) Reactions with norbornadiene, MeCN: (a) Robinson, S. D.;
Wilkinson, G. J. Chem. Soc., A 1965, 843. (b) Cole-Hamilton, D. J.;
Wilkinson, G. J. Chem. Soc., Dalton Trans. 1979, 1283.
(31) Schrock, R. R.; Murdzek, J. S.; Bazan, G. C.; Robbins, J.;
Dimare, M.; O’Regan, M. B. J. Am. Chem. Soc. 1990, 112, 3875.
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