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H2O (1:1) with 40 ppm palladium (0.004 mol% Pd) using 3,
the reaction proceeded smoothly to give 1,3-diphenylpropene
(6a) quantitatively after 4 hours (Table 1, entry 1). In the
reaction, the turnover number and frequency were 25000 and
6250 hÀ1, respectively, which are the highest numbers for
allylic arylation to date. Moreover, 3 was reused five times
without any loss of catalytic activity to give 6a quantitatively.
The coupling reaction with the reused (5th) catalyst did not
show evidence of leaching of the palladium species into the
reaction mixture (ICP-AES analysis; entry 6). SEM and TEM
observations,[8] and XPS analysis of 3 and reused 3 indicated
that the catalyst was undamaged and unchanged under the
reaction conditions (Figure 1; top, center, and bottom right
panels). The reaction in water without the use of any organic
solvents also proceeded smoothly to give 6a in 98% yield
(entry 7). The phenyl vinyl carbinol ester 4b gave 6a with a
yield of 96% (entry 8). Electron-donating and electron-
withdrawing substituents on the cinnamyl esters 4c–4g
reacted with substituted tetraaryl borates 5a-d and were
efficiently converted into the corresponding products 6b–i
with yields in the range of 93–98% (entries 9–16).
It is interesting to note that the alkyl vinyl carbinol esters
4h–j underwent palladium-catalyzed (40 ppm) allyl–aryl
coupling to yield the corresponding coupling products 6j–l
quantitatively (Table 1, entries 17–20). The reaction of alkyl
vinyl carbinol esters must proceed via the corresponding p-
allylpalladium intermediate bearing the b hydride on the sp3-
carbon center, a species that often suffers from the b-hydride
elimination under palladium-catalyzed conditions to give
undesirable 1,3-dienes.[9,10] However, no trace of 1,3-dienes
was observed in the reactions. The palladium-catalyzed
(40 ppm) reaction of 4i was also performed in water to give
6k quantitatively (entry 19).
Figure 1. SEM images of 3 before use (top left) and after the 5th reuse
(top right). TEM images of the 3 before use (center left) and after the
5th reuse (center right). XPS images of the 3 before use (bottom left)
and after 5th reuse (bottom right).
Furthermore, the coupling of aliphatic 2-alkenyl acetates
is more challenging than that of cinnamyl acetates in terms of
reactivity. However, geranyl acetate (4k), neryl acetate (4l),
prenyl acetate (4m), and 2-hexenyl acetate (4n) efficiently
led to the corresponding phenylated compounds 6m–p with
yields in the range of 96–99% (Table 1, entries 21–25).
Isomerization was not observed in the reactions, and water
was used as a reaction solvent (entry 23). Alicyclic acetates 4o
and 4p were readily converted into the corresponding
products 6q and 6r with yield of 96% (entries 26 and 27);
the reaction of cis-4p proceeded through net inversion to give
trans-5-methoxycarbonyl-3-phenyl-1-cyclohexene (6r) as a
single diastereomer (entry 27).
The heterogeneous catalyst 3 (40 ppm) also promoted
allylic substitution with aryl/alkenylboronic acids, which are
versatile and readily available boron reagents (Table 2). Thus,
the reaction of 4a with phenylboronic acid (7a) and
methanolic or aqueous KF at 708C yielded 6a quantitatively
(entries 1 and 2). The substituted arylboronic acids 7b–d
readily underwent carbon–carbon bond formation under
similar conditions to give 6h, 6s, and 6t with a yield of 98%
(entries 3–5). The alkyl vinyl carbinol esters 4i–j, neryl
acetate (4l), and cyclohexenyl acetate (4o) were converted
into the corresponding alkenes 6k, 6l, 6n, and 6q with yields
in the range of 97–99% (entries 6–9). Allylic alkenylation, the
allyl–alkenyl coupling reaction, of allylic acetates with
catalyst was a composite of the polymeric imidazole 1,
palladium complexes, and palladium nanoparticles. In the
composite, the palladium complexes should act as cross-
linkers of 1 through palladium–imidazole coordination, and
the imidazole units in 1 could serve to stabilize the palladium
nanoparticles. Elementary analysis and ICP-AES analysis of
the palladium supported the structure shown in Scheme 1.
The catalytic activity and reusability of the novel catalyst 3
were examined for allylic arylation/alkenylation of allylic
esters with tetraaryl borates and arylboronic acids. Although
there are numerous reports on aryl–aryl coupling with aryl
boron reagents (Suzuki–Miyaura coupling), little attention
has been paid to allyl–aryl coupling, which often requires a
relatively high reaction temperature with a large amount (1–
10 mol%) of catalyst.[6] We previously reported the allylic
arylation of allylic esters and tetraaryl borates using a
microchannel reactor with poly(acrylamide triarylphosphine
palladium) catalytic membrane.[7] However, both the reac-
tivity and substrate generality were insufficient. The results
led us to the idea that newly developed catalysts could be
applied to the allylic arylation to provide high catalytic
activity and reusability as well as high substrate tolerance.
When the allylic arylation was examined using cinnamyl
acetate (4a) and sodium tetraphenylborate (5a) in iPrOH/
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 9437 –9441