Notes
J . Org. Chem., Vol. 65, No. 15, 2000 4771
Ta ble 1. Reu sa bility of 1a
experiment
yield using 1 (%)
1
2
3
4
5
89
91
90
88
89
a
Oxidation of 1-phenylethanol to acetophenone.
is performed for a shorter time but this is at the expense
of yield, 80% of aldehyde being formed after 1 h, the
remaining 20% being unreacted alcohol. Similar results
are obtained in the case of primary allylic alcohols, the
yield of aldehyde being much greater using the polymer-
supported complex compared to that achieved using 2.
To show that 1 can be recycled a number of times, the
oxidation of 1-phenylethanol to acetaldehyde was re-
peated five times using the same batch of supported
catalyst. As seen in Table 1, the yields remain around
F igu r e 3. Use of 1 in acid anhydride synthesis.
of asymmetrically substituted anhydrides often involves
reagents that are unstable or not easy to handle or else
requires lengthy workup. Recently, however, the use
2
CoCl as a catalyst for anhydride formation has been
9
reported. This removes the need for difficult reagents
but the sequestration of the cobalt salt at the end of a
reaction often proves a problem. Using 1, we have found
that both symmetrically and asymmetrically substituted
anhydrides can be prepared rapidly and in high yield
with the catalyst being easily removed at the end of the
reaction by filtration (Figure 3). Again the catalyst can
be reused a number of times without loss of activity.
9
0% clearly illustrating the reusability of the catalyst.
The entire crude reaction mixture in each case was
dissolved in CDCl (200 mg in 1 mL) and analyzed by
P{ H} NMR. There were no peaks observed in any of
3
3
1
1
the spectra in the range from δ +300 f -300 ppm. Also,
the UV-vis spectrum of the product mixture showed no
absorptions due to Co(II) complexes. This was taken as
a preliminary indication that there was no significant
leaching of the catalyst from the polymer support to at
least this level of detection. Elemental analysis of the
product mixture for chlorine showed only traces to be
present but varied from one reaction to another, and
hence the results were slightly inconclusive. Work is
under way to look more closely at the leaching process
in polymer-supported catalyst systems, as this is of key
importance when considering the viability of such sys-
tems in large-scale synthesis of fine chemicals where
contamination of the product with heavy metals is highly
undesirable.
In conclusion, we have shown that attachment of CoCl
2
-
(
PPh to polymer-supported triphenylphosphine leads
3 2
)
to an air stable, versatile immobilized catalyst that is as
active as its homogeneous analogue and has the advan-
tage that it can be reused numerous times. The only
reaction workup required is a filtration to remove the
polymer-supported catalyst and then isolation of the pure
product by recrystallization or passing through a short
column of silica gel. Work is currently underway to
exploit the activity of other polymer-supported organo-
metallic complexes in metal-mediated organic synthesis.
The polymer-supported catalyst 1 complements other
polymer-supported oxidation catalysts such as ion-
exchange resin-bound perruthenate, ([RuO ] ) 3. In-
4
Exp er im en ta l Section
-
6,7
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.
deed in some regards 1 has distinct advantages over 3,
the loading in 1 of 2.4 mmol/g of resin representing a
significantly higher loading than that reported for 3 (0.1
The 1H and C NMR spectra were recorded at 400 MHz and
13
7
mmol/g ). Consequently much less of the polymer-sup-
1
2
93 K, the 31P{ H} NMR spectra at 250 MHz and 293 K. IR
8
ported catalyst is required in the case of 1. Comparing
spectra of polymer-bound complexes were recorded using diffuse
reflectance spectroscopy. IR spectra of nonsupported complexes
were recorded in KBr pellets.
P r ep a r a tion of P olym er -Su p p or ted Coba lt P h osp h in e
Com p lex 1. Commercially available polymer-supported tri-
phenylphosphine (Fluka) was first washed several times with
THF and then dichloromethane before being dried in vacuuo and
the activity of 1 and 3, the two systems are complemen-
tary in their activity, primary aliphatic alcohols being
unaffected when using 1 whereas 3 catalyses the oxida-
tion of all primary alcohols but not secondary alcohols.
An advantage of 3 is that molecular oxygen can be used
as co-oxidant, and hence no peroxide is required but this
is somewhat offset by the low loading and the high
catalyst-to-substrate ratio required (10 mol % catalyst
1
2 3 2
00 mg added to a dichloromethane solution of CoCl (PPh ) (114
mg, 0.175 mmol). The resultant mixture was shaken overnight
using a mechanical stirrer during which time the originally light
brown polystyrene beads turned blue in color. The beads were
filtered off using a sintered funnel and washed five times with
dichloromethane and then twice with hexane before drying in
vacuo. Loading of the cobalt complex on the resin was found to
be 2.4 mmol/g resin by elemental analysis (comparison of P, Cl,
and Co content).
7
reported ).
Use of 1 in Acid An h yd r id e Syn th esis. To broaden
the study of the use of 1 in synthesis, we assessed its
use as a catalyst in the coupling of acid chlorides and
carboxylic acids to yield acid anhydrides. The synthesis
Gen er a l Meth od for Coba lt-Ca ta lyzed Oxid a tion Rea c-
t
(
(
(
6) Hinzen, B.; Ley, S. V. J . Chem. Soc., Perkin Trans. 1 1997, 1907.
7) Hinzen, B.; Lenz, R.; Ley, S. V. Synthesis 1998, 978.
8) Ley et al. have recently reported a new oxidation catalyst
tion s. The appropriate alcohol (3 mmol), BuOOH (70% solution
in water, 6 mmol), and 1 (25.6 mg, 1 mol % Co complex) in
dichloromethane (20 mL) were refluxed for 4 h, the reaction
mixture being agitated using a slow nitrogen bubble flow. After
containing perruthenate immobilized within MCM-41 has been pre-
pared and used in the clean oxidation of alcohols to carbonyl com-
pounds with molecular oxygen. This has a much increased loading of
catalyst. See: Bleloch, A.; J ohnson, B. F. G.; Ley, S. V.; Price, A. J .;
Shephard, D. S.; Thomas, A. W. Chem. Commun. 1999, 1907.
(9) Srivastava, R. R.; Kabalka, G. W. Tetrahedron Lett. 1992, 33,
593.