C O M M U N I C A T I O N S
Table 2. Allylation Reactions Using PI Pd 4ca
PI method, a normally labile species such as ligand-free Pd(0) is
stabilized and can be stored at room temperature. (iii) PI Pd is suc-
cessfully used in reduction, carbon-carbon bond-forming reactions,
and carbon-oxygen bond-forming reactions. (iv) The catalyst is
recovered quantitatively by simple filtration and reused several times
without loss of activity, and no leaching of Pd is observed. (v) The
higher activity of PI Pd as compared to even homogeneous Pd
catalysts has been demonstrated. (vi) Ignition has never occurred
in all of the PI Pd-catalyzed reactions we tested. PI Pd is regarded
as a safe catalyst. (vii) The PI method has been shown to be effec-
tive in immobilizing Pd(0) species onto polymers, and this method
is expected to be useful to immobilize other metal species onto
polymers directly. Further studies to develop other PI catalysts are
now in progress.
Acknowledgment. This work was partially supported by
CREST and SORST, Japan Science Technology Corp., and a Grant-
in-Aid for Scientific Research from Japan Society of the Promotion
of Sciences. R.A. thanks the JSPS fellowship for Japanese Junior
Scientist.
a All reactions were carried out using PI Pd 4c (5 mol %) and PPh3 (5
mol %) in THF under reflux for 2 h.
Supporting Information Available: A table of details in the
reaction of 7 with 8 and experimental details (PDF). This material is
smoothly to afford the desired products in high yields (eqs 1-6).
The activity of PI Pd was comparable to that of 5% Pd/C (entry
4), but no ignition occurred using PI Pd. It is also noteworthy that
PI Pd was much more active than the original catalyst, Pd(PPh3)4.
The reaction did not proceed at all using Pd(PPh3)4 under the same
reaction conditions (entry 5). Furthermore, it was revealed that the
reduction also proceeded smoothly using <0.1 mol % of PI Pd.
PI Pd (4a-c) were then applied to allylic substitution reactions,
one of the representative carbon-carbon bond-forming reactions.12
We chose the reaction of allyl methyl carbonate (7) with dimethyl
phenylmalonate (8) as a model, and several reaction conditions were
examined.13 It was found that the allylation reaction proceeded in
the presence of PI Pd (5 mol %) and PPh3 (5 mol %) in THF under
reflux conditions. While 4a gave lower yields, the activity of 4b
and 4c was high, and the desired allylation adducts were obtained
in excellent yields. The catalysts were recovered by simple filtration
and reused several times. In the case of 4b, however, leaching of
palladium from 4b occurred in the fourth and the fifth use. On the
other hand, no leaching of palladium was observed in the reactions
using 4c even after the fifth use, and the reactions proceeded
smoothly to afford the desired adducts in high yields in all cases.
Several examples of allylic substitution reactions using the PI
Pd 4c are summarized in Table 2. Malonate and â-ketoester
smoothly reacted under these conditions to afford the corresponding
allylation products in high yields. Moreover, PI Pd 4c was
successfully applied to carbon-oxygen bond-forming reactions.
That is, phenol or naphthol derivatives smoothly reacted with 7 to
afford the corresponding allyl phenyl or allyl naphthyl ethers in
high yields. It should be noted that 4-nitrophenol also worked well
to produce allyl 4-nitrophenyl ether quantitatively. It was already
reported that the reactivity of phenol or naphthol derivatives having
electron-withdrawing groups such as a nitro group decreased
significantly even using very reactive homogeneous palladium
systems.14 In this reaction, it was exciting to demonstrate that PI
Pd 4c was more reactive than the homogeneous Pd catalysts.
Thus, we have developed an efficient method for immobilizing
a palladium catalyst onto polymer. The method named “the polymer
incarcerated (PI) method” is novel, and the following characteristic
features are noted. (i) The method is operationally simple, and the
desired immobilized catalyst is readily prepared. (ii) By using the
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