Carbon−Carbon Coupling Reactions Catalyzed by Heterogeneous ...

Heterogeneous Pd Catalyzed C−C Coupling Reactions Chemical Reviews, 2007, Vol. 107, No. 1 149
As another technique worth mentioning, the Pd/C catalyst
is kept in a separate liquid triphase system. In isooctane/
water/Aliquat 336 (methyltrioctylammonium chloride), 142 it
was found that the rate of Heck coupling of aryl iodides with
olefins was accelerated 10-fold in the presence of Aliquat
336. Both when conducted in A336 as solvent and when
conducted in an isooctane/A336/water triphasic mixture, the
Heck reaction of aryl iodides with electron-deficient olefins
proceeded with high yields and selectivities (Table 34).
Table 34. Triphase Heck Coupling of Aryl Iodides with Ethyl
Acrylate a
entry R A336 T (h)
Pd on graphite was prepared by reduction of PdCl2 by
C8K and could be used (7 mol % catalyst) in Heck reactions
of acrylates, fumarates. and styrene with aryl or vinyl iodides
in the presence of tributylamine. 143
3.2. Pd on Metal Oxides
yield
(%, GC)
1 H no 1.5 5
20 17
2 H yes 1.5 30
20 94
3 4-NO2 no b 6.0 10
4 4-NO2 yes b 3.0 78
5 4-Ac yes 20 80
6 4-Cl yes 20 78
7 4-OMe yes 20 68
8 4-NH2 yes b 20 68
9 3-CN yes 20 81
a Reaction conditions: aryl iodide (1.0 mmol), ethyl acrylate (1.5
mmol), Et3N (1.5 mmol), Pd/C (0.05 mmol), 10 mL of isooctane, 5
mL of water, A336 (0.30 mmol). b Since the substrate is insoluble in
isooctane, toluene was used.
The first report about a heterogeneous Heck reaction using
Pd supported on metal oxide was published by Kaneda et
al. in 1990. 144 Chlorobenzene was coupled with styrene in
methanol at 150 °C using Pd/MgO as catalyst and Na2CO3
as base. Later on several metal oxides (MgO, Al2O3, SiO2,
TiO2, ZrO2, ZnO, mixed MgLaO, etc.) have been used as
supports for Pd catalysts in Heck reactions. Ar-X with X
) I, Br, Cl, OTf, COCl, SO2Cl, or N2BF4 were coupled with
acrylates, 12,19,38,55,67,145-147 acrylonitrile, 52,53,55 styrene, 20,21,55,146,147
vinyl alkyl ether, 51,55,136 terminal alkenes, 55 and cycloalkenes.
27 Biffis and co-workers reviewed palladium metal
catalysts in Heck reactions in 2001. 10
Most applications were performed under ligand-free
conditions. Detailed studies of Köhler and co-workers on
catalyst efficiency and on optimization of Heck reaction of
bromobenzene with styrene catalyzed by solid-supported Pd
(5% Pd (w/w) in DMF/NaOAc, 140 °C, 20 h) revealed the
following order of effectiveness: 20,21
Pd/C > Pd/TiO 2 > Pd/ZrO 2 > Pd/MgO > Pd/ZnO >
Pd/SiO 2
A different order was found in Heck coupling of iodobenzene
with acrylonitrile in the presence of PPh3 (5% Pd, Et3N as
base, CH3CN, 140 °C, 14 h) by Wali and co-workers: 52
Pd/MgO > Pd/γ-Al 2 O 3 > Pd(acac) 2 > Pd/C >
Pd/CaCO 3
Köhler and co-workers reported for the first time that
specifically prepared heterogeneous Pd/MOx catalysts (M )
metal center) converted even nonactivated aryl chlorides into
stilbenes in Heck reactions with styrene. Short reaction times
and remarkably low amounts of catalysts were sufficient. 85,148
The highly active palladium species were generated in situ
by dissolution from the support and stabilized against
agglomeration by reprecipitation on the surface. These
processes could be controlled by a specific choice of catalyst
and tuning the reaction conditions. Evidence was found that
the solid catalyst functions as a reservoir for molecular
palladium species in solution. Their concentration in solution
correlates with the progress of the reactions. Kinetic investigation
of the Heck reaction of bromobenzene with styrene
showed that only a little palladium leached to the solution
in the starting period. After the reaction temperature reached
140 °C, 21 about one-third of the palladium was leached from
the surface of the support. Simultaneously, the majority of
the aryl halide was converted. After the reaction was finished,
the palladium was re-deposited onto the support to a great
extent.
Analogous investigations were performed with different
aryl chlorides under modified conditions affording comparable
results (Table 35). 148
Table 35. Heck Coupling of Aryl Bromides and Chlorides with
Styrene
entry R, X catalyst
cat. conc.
[mol %] base
T (°C),
t (h)
conv
(%)
yield
(%)
1 H, Br Pd/TiO2 a 0.0011 NaOAc 140, 4 95 86
2 H, Br Pd/Al2O3 a 0.0009 NaOAc 140, 4 96 87
3 b Ac, Cl Pd/Al2O3 a 0.01 Ca(OH)2 160, 2 98 90
4 b Ac, Cl Pd/Al2O3 c 0.01 Ca(OH)2 160, 2 87 83
5 Ac, Cl Pd/NaY 0.005 Ca(OH)2 160, 2 99 95
6 b H, Cl Pd/NaY 0.05 Ca(OH)2 160, 6 49 45
7 b H, Cl Pd/NaY 0.05 Ca(OH)2 160, 6 85 83 d
8 b Me, Cl Pd/NaY 0.05 Ca(OH)2 160, 6 40 36 d
a Precipitation of Pd(OH)2 on MOx. b Addition of TBAB. c Prepared
by coprecipitation. d Under O2 atmosphere.
A recently developed Pd catalyst on Mg-La mixed oxide
turned out to be superior for Heck coupling of activated and
inactivated aryl iodides, bromides, and chlorides even at low
temperatures (80 °C) under ambient atmosphere (Scheme
24). 55 High E/Z ratios of products 95 were observed in most
cases, depending on the nature of alkene. The solvent had a
determinant effect on the yield. The catalyst could be reused
without additional activation four times without a significant
loss of activity and could be stored under air without
problems.
Scheme 24