Highly efficient heterogeneous aqueous Kharasch reaction with an amphiphilic resin-supported ruthenium catalyst

Article abstract of DOI:10.1002/adsc.200800359

An amphiphilic polystyrene-polyethylene glycol (PS-PEG) resin-supported ruthenium complex was designed and prepared. The polymeric Ru complex was found to promote the transition metal-catalyzed atom transfer radical addition of halogenated compounds to olefins, the Kharasch reaction, in water under heterogeneous as well as AIBN-free conditions with a high level of atom economy to meet green chemical requirements.

Full text of DOI:10.1002/adsc.200800359

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DOI: 10.1002/adsc.200800359
Highly Efficient Heterogeneous Aqueous Kharasch Reaction with
an Amphiphilic Resin-Supported Ruthenium Catalyst
Yohei Oea and Yasuhiro Uozumia,*
a
Institute for Molecular Science (IMS) and CREST, Higashi-yama, Myodaiji, Okazaki 444-8787, Japan
Fax : (+81)-564-59-5574; e-mail: uo@ims.ac.jp
Received: May 5, 2008; Revised: June 10, 2008; Published online: July 10, 2008
Supportinginformation for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: An amphiphilic polystyrene-polyethylene
glycol (PS-PEG) resin-supported ruthenium com-
plex was designed and prepared. The polymeric Ru
complex was found to promote the transition
metal-catalyzed atom transfer radical addition of
halogenated compounds to olefins, the Kharasch re-
action, in water under heterogeneous as well as
AIBN-free conditions with a high level of atom
economy to meet green chemical requirements.
sults prompted us to develop an amphiphilic resin-
supported phosphine-ruthenium complex[8] to realize
a heterogeneous aquacatalytic Kharasch reaction.
Here we report the amphiphilic polystyrene-poly(eth-
ylene glycol) (PS-PEG) resin-supported ruthenium
complex PS-PEG-NHCOC6H4PPh2-RuCp*Cl2 (1)
which exhibited high aquacatalytic activity and recy-
clability for the Kharasch reaction without radical ini-
tiators (Scheme 1).
A polymeric ruthenium complex was readily pre-
pared by mixingPS-PEG resin-supported triarylphos-
phine and [RuCp*Cl2]2 (Scheme 1) accordingto the
preparation of RuCp*Cl2PPh3.[9] Thus, a ruthenium-
monophosphine complex PS-PEG-NHCOC6H4PPh2-
RuCp*Cl2 (1)[10] was obtained by treatment of PS-
PEG-phosphine (phosphorus loading=0.26 mmolgÀ1;
average diameter of polymeric beads=100 mm; 1%
Keywords: allylic substitution; aqueous media;
asymmetric catalysis; combinatorial chemistry; pal-
ladium; polymer-supported catalyst
The transition metal-catalyzed atom transfer radical DVB cross-linking) with an excess amount of
addition of halogenated compounds to olefins, the [RuCp*Cl2]2 (1.4 mol equiv to phosphorus) in di-
Kharasch reaction, offers a powerful tool to function- chloromethane at room temperature for 5 h followed
alize olefins with a high level of atom economy. by removal of non-immobilized ruthenium species by
Ruthenium complexes are amongthe most active cat-
washingseveral times with dichloromethane to give
alysts for the Kharasch reaction, where AIBN, a radi- the polymeric ruthenium complex 1 as reddish beads.
cal initiator, is often used as a co-catalyst to promote The ICP-AES analysis of the resultingpolymeric
the reaction with synthetically useful efficiency. While beads exhibited the loadingvalues of ruthenium and
only homogeneous transition metal catalysts have phosphorus to be 0.23 mmol Ru/gand 0.24 mmol P/g,
been developed thus far,[1] the aqueous[2] and hetero- respectively, demonstratingthat its structure is
geneous switching[3] of this catalysis still remains a RuCp*Cl2(phosphine).
major challenge in terms of chemical safety and green
The aquacatalytic potential of the novel polymeric
chemistry.[4] If it took place in water with a readily re- ruthenium complex 1 was examined for the Kharasch
cyclable immobilized catalyst[5] without any additional reaction of styrene with carbon tetrachloride
radical initiator, the Kharasch reaction would repre- (Scheme 2). Thus, the reaction of styrene (2) and
sent an ideal olefin functionalization.
CCl4 (2/CCl4 =1/4) was carried out in water as a
We have previously developed amphiphilic polymer single reaction medium in the presence of 0.05 mol%
resin-supported transition metal complexes (Pd, Rh) ruthenium of the polymeric complex 1 (S/C=2,000)
and nanoparticles (Pd, Pt) that catalyze various or- at 608C to give 1,1,1,3-tetrachloro-3-phenylpropane 3.
ganic transformations in water under heterogeneous We were pleased to find that a high TOF value of
conditions.[6] Very recently, Severin et al. have report- 1450 hÀ1 (duringthe initial 15 min period) was
ed that RuCp*Cl2PPh3-AIBN (Cp*=pentamethylcy-
A
clopentadienyl) acts as a highly efficient catalyst AIBN, and was comparable to the highest TOF value
system to promote the Kharasch reaction.[7] Their re- hitherto reported under homogeneous AIBN-free
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Yohei Oe and Yasuhiro Uozumi
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Scheme 1. Preparation of an amphiphilic resin-supported Ru complex.
with an excess amount of a chlorinated compound.[1]
Considering the high toxicity of chlorinated organic
materials, a highly efficient, heterogeneous aquacata-
lytic Kharasch reaction with an equimolar mixing
ratio would offer a safe, clean, and highly atom-eco-
nomical alternative to more conventional Kharasch
reaction systems.
A mixture of an equimolar ratio of styrene and
CCl4 and 0.33 mol% ruthenium of the PS-PEG resin-
supported ruthenium complex 1 in water was shaken
Scheme 2. Amphiphilic polymeric Ru-catalyzed Kharasch
reaction (water vs. toluene).
conditions.[11] It is noteworthy that the reaction did at 608C for 9 h, and the Kharasch adduct 3 was ob-
not proceed at all in toluene even with 1 mol% of tained in 91% isolated yield (Table 1, entry 1). Repre-
ruthenium under similar conditions. The hydrophobic sentative results are summarized in Table 1. Styrenes
organic substrates must diffuse into the hydrophobic bearing ortho-, meta- and para-chloro groups, 4, 6,
polystyrene matrix in water to provide a highly self- and 8, underwent the Kharasch reaction with CCl4
concentrated reaction sphere where the addition-type under similar conditions to give the corresponding
reaction is significantly accelerated to reduce the ini- 1,1,1,3-tetrachloro-3-arylpropanes 5, 7, and 9 in 92%,
tial molar concentration.
87% and 86% isolated yield, respectively (entries 5, 6
With the efficient catalytic system in hand, we next and 7). The reaction of para-trifluoromethylstyrene
examined the Kharasch reaction of various olefins (10) gave an 80% isolated yield of 11 (entry 8). para-
and chlorinated compounds in an exact equimolar Methylstyrene (12) bearingan electron-rich aromatic
ratio. The Kharasch reaction generally is carried out ringrequired a higher reaction temperature (70 8C) to
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Highly Efficient Heterogeneous Aqueous Kharasch Reaction
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Table 1. The Kharasch reaction of various olefins with PS-PEG-PPh2-Cp*RuCl2 (1).[a]
Entry
Substrate
Chlorine Species
Product
Yield [%][b]
91
1
CCl4
2
3
4
Recyclingexperiment: 1 st run
94
2nd run
3rd run
96[c]
98[c]
5
6
7
8
9
CCl4
92
87
86
80
84
CCl4
CCl4
CCl4
CCl4
10
11
12
CCl4
CCl4
CCl4
83[c]
72
36
13
14
2
2
2
CBr4
89
89
92
Cl3CCOOEt
TsCl
15
[a]
All reactions were carried out with 4 at 60–708C in water under N2. The ratio of the alkene (mol)/halo reagent (mol)/Ru
(mol)=1.0/1.0/0.0033–0.01.
Isolated yield.
NMR yield usingbibenzyl as an internal standard.
[b]
[c]
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Yohei Oe and Yasuhiro Uozumi
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beads were washed with CH2Cl2 (5 times of 5 mL) and dried
under reduced pressure overnight to give PS-PEG-PPh2-
Cp*RuCl2 (1) as reddish beads (0.23 mmol Ru/g, 0.24 mmol
P/g).
afford a high yield of the Kharasch adduct 13 (84%)
(entry 9). The reactions of 1,1- and 1,2-disubstituted
alkene substrates took place at 608C with 1.0 mol%
Ru of the complex 1 (entries 10 and 11). Thus, methyl
methacrylate (14) reacted with CCl4 to afford 15
havinga quaternary chloride in 83% yield, and nor-
bornene 16 afforded 2-chloro-3-trichloromethylnor-
bornane (17) in 72% yield. It is noteworthy that the
General Procedure for Kharasch Reaction with
Polymeric 1 in Water
A
mixture of PS-PEG-PPh2-Cp*RuCl2 (1) (7.2 mg,
Kharasch reaction of the non-activated alkene sub- 0.0017 mmol Ru), alkene (0.5 mmol), CCl4 (76 mg,
0.5 mmol) and 1.5 mL of water was shaken at 608C for 9 h
under nitrogen atmosphere. The reaction mixture was fil-
tered, and the resin was rinsed three times with 5 mL of an
appropriate organic solvent or super critical CO2 (flow ex-
traction system).[12] The crude product was purified by silica
gel column chromatography.
strate, n-1-octene (18) also took place with the poly-
meric ruthenium complex 1 in water without AIBN
to give 1,1,1,3-tetrachlorononane (19) with exclusive
regioselectivity although the chemical yield was only
36% after 24 h of reaction time (entry 12). The atom
transfer addition reaction was also examined with
CBr4, Cl3CCOOEt, and p-ClSO2C6H4CH3 under the
aquacatalytic conditions. Thus, the reaction of styrene
with CBr4 was smoothly catalyzed in water with the
polymeric ruthenium species 1 to give an 89% yield
Acknowledgements
of
1,1,1,3-tetrabromo-3-phenylpropane
(20)
This work was supported by the CREST program, sponsored
by the JST. We also thank the MEXT (Scientific Research on
Priority Areas, no. 460) for partial financial support of this
work.
(entry 13). Trichloroacetate and tosyl chloride added
to styrene in the atom transfer fashion under similar
aquacatalytic conditions to give ethyl 2,2-dichloro-1-
chloro-1-phenylbutyrate (21) and (2-chloro-2-phenyl)-
ethyl tolyl sulfone in 89% and 92% yield, respectively
(entries 14 and 15).
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and successively subjected to a second and third run
of the reaction under the same conditions to afford 3
in 96% and 98% yield, respectively (entries 3 and 4).
In conclusion, an amphiphilic PS-PEG resin-sup-
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