Preparation of a resin-bound ruthenium phosphine complex and assessment of its use in transfer hydrogenation and hydrocarbon oxidation

Full text of DOI:10.1021/jo001497y

2168
J . Org. Chem. 2001, 66, 2168-2170
P r ep a r a tion of a Resin -Bou n d Ru th en iu m
P h osp h in e Com p lex a n d Assessm en t of Its
Use in Tr a n sfer Hyd r ogen a tion a n d
Hyd r oca r bon Oxid a tion
Nicholas E. Leadbeater
Department of Chemistry, King’s College London,
Strand, London WC2R 2LS United Kingdom
F igu r e 1. Preparation of polymer-bound catalyst 1.
nicholas.leadbeater@kcl.ac.uk
Received October 18, 2000
with that of 2 which is also black in color. The assignment
was further confirmed by elemental analysis, this also
allowing us to determine the catalyst loading at ap-
proximately 0.3 mmol per gram of resin. The polymer
bound complex formed is stable in air but for prolonged
storage is better stored under an atmosphere of nitrogen
in which case no decomposition is noted over the period
of three months at room temperature.
In tr od u ction
Although there are many advantages in using transi-
tion metal complexes in synthesis, the problems of metal
extraction and product purification make them less than
ideal for use in applications such as synthesis of fine
chemicals where contamination of the product with heavy
metals is highly undesirable. There is now increasing
interest in the development of polymer-bound metal
catalysts and reagents for organic synthesis that main-
tain high activity and selectivity.1 Advantages of attach-
ing a catalyst to a polymer support include ease of
separation from the product mixture at the end of a
reaction and the fact that attaching a metal complex to
a polymer can reduce the toxicity and air sensitivity of
the species considerably. In addition, as the catalyst is
easily removed from the reaction mixture by filtration,
there is the possibility that it can be reused in subsequent
reactions.
In this note, we report the preparation and synthetic
versatility of the polymer-supported ruthenium complex
1. This and other chloro-ruthenium complexes are used
frequently in metal-mediated organic synthesis for a
diverse range of reactions.2 In our studies we have
focused attention on transfer hydrogenation and hydro-
carbon oxidation as representative reactions for compar-
ing the activity of 1 with its nonsupported analogue
RuCl2(PPh3)3 (2). Work has also been focused on the effect
of catalyst recycling.
Use of 1 a s a Ca ta lyst for Tr a n sfer Hyd r ogen a -
tion . The ability of ruthenium complexes to dehydroge-
nate alcohols and deliver the hydrides to a ketone has
made them useful as transfer hydrogenation catalysts.3
However, the majority of methods described require
elevated temperatures (150-200 °C). Recently, Ba¨ckvall
reported that base has a profound effect in hydrogen
transfer catalysis using 2.4 The mechanism for the
reaction is thought to proceed via initial attack of an
alkoxide ion on the Ru complex followed by â-elimination
from the alkoxide leading to an anionic ruthenium
hydride complex. Following protonation of the anionic
species to give a ruthenium dihydride, the reduction of
the ketone by this dihydride results in the desired alcohol
product. With this mechanism in mind we felt that
transfer hydrogenation may be a suitable reaction for
adaptation to solid-phase conditions using 1 since the
M-P bond between the ruthenium center and the
polymer-bound phosphine moiety may stay intact through-
out the process thereby limiting the chances of catalyst
leaching by direct decomplexation.
We find that, in the presence of a catalytic amount of
1, the selective hydrogen transfer from propan-2-ol to
various ketones occurs rapidly giving the product alcohols
in yields comparative to those obtained using 2 as shown
in Table 1. For comparative purposes, reported yields for
the analogous reactions using 2 are also shown.4 The
substrate-to-catalyst ratio used in the case of the polymer-
supported complex (C/S ) 0.15%) is slightly higher than
in the case of the homogeneous analogue (C/S ) 0.10%)
in order to compensate for the fact that the polymer beads
do not swell totally in the solvent mixture used. Indeed
the choice of solvent in which the reaction is performed
proves to be very important. The homogeneous reactions
are performed using propan-2-ol as the solvent; however,
the swelling properties of the resin in the solvent are very
low. On the other hand, 1,2-dichloroethane is a good
solvent for swelling the resin, but is clearly not suitable
Resu lts a n d Discu ssion
P r ep a r a tion of P olym er -Su p p or ted Ca ta lyst 1.
The polymer support chosen for immobilization of the
ruthenium complex was commercially available “polymer-
supported triphenylphosphine” (polystyrene cross-linked
with 2% divinylbenzene; 3 mmol P/g resin). The im-
mobilized complex 1 was prepared by stirring 2 with the
functionalized resin in dichloromethane overnight during
which time the resin beads turned from light brown to
dark brown in color (Figure 1). Subsequent filtration,
washing, and drying of the polymer gave a black powder,
characterized as 1 by comparison of spectroscopic data
(1) For an introduction to the area see: (a) Hartley, F. R. Supported
Metal Complexes: A New Generation of Catalysts; Reidel: Amsterdam,
1986. (b) Ley, S. V.; Baxendale, I. R.; Bream, R. N.; J ackson, P. S.;
Leach, A. G.; Longbottom, D. A.; Nesi, M.; Scott, J . S.; Storer, I.; Taylor,
S. J . J . Chem. Soc., Perkin Trans. 1 2000, 3815.
(2) See: Naota, T.; Takaya, H.; Murahashi, S. I. Chem. Rev. 1998,
98, 2599-2660.
(3) (a) Yamakawa, M.; Ito. H.; Noyori, R. J . Am Chem. Soc. 2000,
122, 1466-1478. (b) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997,
30, 97-102
(4) Chowdhary, R. L.; Backvall, J .-E. J . Chem. Soc., Chem. Commun.
1991, 1063-1064.
10.1021/jo001497y CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/21/2001

Notes
Ta ble 1. Th e Use of 1 in Tr a n sfer Hyd r ogen a tion
J . Org. Chem., Vol. 66, No. 6, 2001 2169
Ta ble 2. Th e Use of 1 in Hyd r oca r bon Oxid a tion
for performing the reaction. Other solvents such as THF
and dioxane, when mixed with propan-2-ol, led to sig-
nificant byproduct formation, and so we find that the
reaction is best performed in a mix of 1,2-dichloroethane
and propan-2-ol (1:2).
In an attempt to show that 1 can be recycled, the
transfer hydrogenation of cyclohexanone to cyclohexanol
was repeated four times using the same batch of sup-
ported catalyst. The yields remain around 80% clearly
illustrating the reusability of the catalyst.
the 2 is chemically modified during the oxidation reac-
tion, and the resultant ruthenium complex is a catalyst
for the hydride transfer reaction but not as good as 1.
In conclusion, we have shown that attachment of
RuCl2(PPh3)3 to polymer-supported triphenylphosphine
leads to an air stable, versatile immobilized catalyst that
is as active as its homogeneous analogue and has the
advantage that it can be reused a number of times. Work
is currently underway to exploit the activity of other
polymer-supported organometallic complexes in metal-
mediated organic synthesis.
A further objective of our studies was to determine
whether the catalysis was due to 1 or to a homogeneous
ruthenium complex that comes off the support during the
reaction and then returns to the support at the end. To
test this, we focused on the transfer hydrogenation of
cyclohexanone to cyclohexanol. We filtered off the resin
after 30 min of reaction time and allowed the filtrate to
react further. The catalyst filtration was performed at
the reaction temperature (80 °C) in order to avoid
possible recoordination or precipitation of soluble ruthe-
nium upon cooling. We found that, after this hot filtra-
tion, little further reaction was observed. This suggests
that the catalyst remains on the support at elevated
temperatures during the reaction.
Exp er im en ta l Section
Gen er a l. All chemicals were reagent grade and used as
purchased including polymer-supported triphenylphosphine (Flu-
ka, 3 mmol P/g resin). All reactions were performed under an
inert atmosphere of dry nitrogen using distilled dried solvents.
P r ep a r a tion of P olym er -Su p p or ted Ru th en iu m Com -
plex 1. Commercially available polymer-supported triphenylphos-
phine was first washed several times with THF and then
dichloromethane before being dried in vacuo and 100 mg added
to a toluene solution of RuCl2(PPh3)3 (300 mg, 0.31 mmol). The
resultant mixture was agitated overnight during which time the
originally light brown polystyrene beads turned deep brown in
color. After cooling, the beads were filtered off using a sintered
funnel and washed five times with dichloromethane and then
twice with hexane before drying in vacuo to give a black powder.
Loading of the ruthenium complex on the resin was found to be
approximately 0.3 mmol/g resin by elemental analysis (compari-
son of P, Cl, and Ru content).
Use of 1 a s a Ca ta lyst for Oxid a tion Rea ction s.
The oxygenation of unsaturated C-H bonds with metal
complexes is of importance to synthetic chemists. Of the
many metal complexes studied, 2 has shown a high
catalytic activity when used in conjunction with tert-butyl
hydroperoxide.5,6 We find that similar activity is observed
when using 1 in these reactions. The oxidation of a range
of alcohols and hydrocarbons using t-BuOOH and a
catalytic amount of 1 gave the desired carbonyl products
in comparable yield to when 2 is used,5,6 as shown in
Table 2. Although the catalyst can be reused a number
of times, work is ongoing to determine whether the
ruthenium complex at the end of the first reaction (and
hence that used in subsequent reactions) is the same as
that at the beginning of the first reaction. One observa-
tion is that, if a sample of catalyst that has been used
for the oxidation reaction is subsequently used in the
hydride transfer reaction, the observed activity is lower
than when a new sample of 1 is used. This indicates that
either some of the catalytically active 2 is chemically
modified during the oxidation reaction such that it is not
active in the hydride transfer reaction. Alternatively, all
Gen er a l Meth od for Tr a n sfer Hyd r ogen a tion Ca ta lyzed
by 1. To solid 1 (50 mg) were added degassed propan-2-ol (10
mL) and 1,2-dichloroethane (5 mL). The mixture was heated to
reflux and then the ketone (10 mmol) added. The resulting
mixture was stirred at reflux for 15 min, and then a solution of
NaOH in propan-2-ol (10 mg, in 2 mL) was added dropwise. After
a further 1.5 h at reflux, the mixture was cooled, the supported
ruthenium complex filtered off, and the product analyzed.
Spectroscopic data for the products were compared with those
in the literature, showing formation of the appropriate alcohol.
Yields of products are shown in Table 1.
Assessm en t of th e Reu se of 1. The reaction of the transfer
hydrogenation of cyclohexanone to cyclohexanol catalyzed by 1
was repeated four times using the same batch of polymer-bound
catalyst. Between experiments the catalyst was washed with
dichloromethane and dried in vacuo before placing it back in
the reaction vessel. The microanalysis before and after each
(5) Tsuji, Y.; Ohta, T.; Ido, T.; Minbu, H.; Watanabe, Y. J . Orga-
nomet. Chem. 1984, 270, 333-341.
(6) Murahashi, S.-I.; Oda, Y.; Naota, T.; Kuwabara, T. Tetrahedron
Lett. 1993, 34, 1299-1302.

2170 J . Org. Chem., Vol. 66, No. 6, 2001
Notes
reaction shows that leaching of small quantities of the ruthe-
nium from the support is a problem that needs to be addressed
in future work.
Gen er a l Meth od for Hyd r oca r bon Oxid a tion Ca ta lyzed
by 1. To a stirred mixture of hydrocarbon (2 mmol) and 1 (50
mg) in 1,2-dichloroethane/ethyl acetate (7:1) was added a 30%
solution of peracetic acid in ethyl acetate (6 mmol in 4 mL)
dropwise at reflux over the period of 2 h. After 2 h at reflux, the
mixture was cooled, the supported ruthenium complex filtered
off, and the product analyzed. Data collected were compared with
those in the literature confirming formation of the carbonyl
complexes. Yields are shown in Table 2.
Ack n ow led gm en t. The Royal Society is thanked for
a University Research Fellowship. Zeneca Pharmaceu-
ticals, Pfizer, and Novartis are thanked for financial
assistance. Kathryn Scott and Lucy Scott are thanked
for assistance in initial experiments.
J O001497Y