Carbon−Carbon Coupling Reactions Catalyzed by Heterogeneous ...

Heterogeneous Pd Catalyzed C−C Coupling Reactions Chemical Reviews, 2007, Vol. 107, No. 1 151
Djakovitch and Köhler studied a series of palladium
catalysts obtained by ion exchange at Na- or H-zeolites
(mordenite, Y) with Pd(NH3)4Cl2. Pd-modified zeolites
exhibited a high activity comparable to homogeneous
catalysis toward the Heck reaction of aryl bromides with
styrene for small Pd concentrations. 10,16-18,21,27 The zeolite
obviously controlled the selectivity of the reaction. 16 Reaction
temperature plays an important role. No substantial leaching
was observed in most cases. But evidence for dissolved
molecular Pd species being responsible for the catalysis was
found. 27 The catalysts could be easily separated from the
reaction mixture and reused up to five times without a
considerable loss in activity. Some of the results are
summarized in Table 39. 16,18 The catalyst could also be
applied to Heck coupling of 4-chloroacetophenone but did
not perform so well even at higher temperatures. 18
Table 39. Heck Reaction of Various Aryl Bromides with Alkenes
by Pd-Zeolite Catalysts
GLC yielda T
(%)
Pd-zeolite R R′ (°C) 101 102 103
[Pd(0)]-NaY F Ph 140 89.4 (86.0) 0.9 8.2
100 85.1 (80.1) 1.0 7.9
[Pd(NH3)4] 2+ -NaY F Ph 140 93.0 (80.9) 1.0 8.8
100 94.5 (89.5) 0.7 6.7
[Pd(OAc)2]-NaY F Ph 140 79.2 (81.2) 0.9 7.2
100 57.6 (39.9) 0.4 3.9
[Pd(NH3)4] 2+ -NaY OMe Ph 140 81.2 (75.8) 9.5 9.5
[Pd(NH3)4] 2+ -NaY
[Pd(NH3)4]
NO2 Ph 140 94.8 (89.8) 1.1 4.1
2+ -NaY
[Pd(NH3)4]
H Ph 140 84.9 (75.8) 0.7 6.5
2+ -NaY
[Pd(NH3)4]
H BuO 140 25.7 20.4 12.4
2+ -NaY H CO2Me 140 91.0 (69.4) 0.5 0.4
a Isolated yields in parenthesis.
K + - and Cs + -exchanged X-zeolites containing PdCl2
(bifunctional catalysts) developed by Garcia and co-workers
allowed implementation of the Heck reaction of iodo- and
bromobenzene with styrene in the absence of an extrinsic
base. 49 Obviously, sites of the support act as base in these
cases. No leaching was observed. The activity of the used
catalyst could be regained to a large extent by reactivation
by washing with water. As an alternative to styrenes 104,
the formation of 1,1-diphenylethene regioisomer can be
favored by high Pd loadings (Scheme 27). The authors also
investigated the effect of pore size and Pd loading of different
zeolites on the catalytic activity. When DMF was used as
solvent, the catalytic activity was mainly attributed to leached
Pd.
Scheme 27
In contrast to the conclusions of Djakovitch and Köhler
to the nature of catalysis of Pd on zeolites (molecular Pd
species in solution, Vide supra), Dams and co-workers. 61,150
and Okumura 84 et al. assumed that the heterogeneous nature
of the catalysis with Pd-zeolites in Heck reactions largely
depended on the pretreatment of the catalyst, the oxidation
state of Pd, the solvent, and the base. For a critical review
about this subject favoring homogeneous catalysis as the
general mode of action see Jones et al. 36 The excellent
performance of Pd(0)/HY was attributed to the formation of
stable Pd13 clusters kept inside the supercage of HY. 84 This
catalyst had to be generated by calcination in O2 and
reduction by H2 before it could be reused. With tributylamine
as the base in toluene, the Heck olefination with Pd-
(NH3)4 2+ -zeolites (0.4 wt % Pd; mordenite, Y, ZSM-5) and
Pd(0)-mordenite (0.4 and 4 wt % Pd) were concluded as
truly heterogeneous. 61,150 Pd leaching from the zeolites was
evaluated in a very strict filtrate activity test. It was clearly
related to the presence of oxidized Pd(II) in an all-oxygen
environment, that is, ionic Pd(II) or PdO. The heterogeneous
reactions with the zeolite-supported catalyst can be accelerated
by the addition of a quaternary ammonium salt promoter.
The catalytic activity of Pd-zeolites in Heck reaction
followed the order
2+ 2+
Pd(NH3 ) 4 -Y > Pd(NH3 ) 4 -mordenite >
2+
Pd(NH3 ) 4 -ZSM-5
Pd on porous glass served as a useful, reusable catalyst
for Heck reaction of iodobenzene and 4-bromoacetophenone
with styrene and allylic alcohol allowing the reaction to be
carried out in the presence of air. 24
3.4. Pd on Modified Silica (Organic−Inorganic
Hybrid Support)
Organochemically modified silica can also serve as solid
support for Pd catalysts. Silica was modified by various
chlorohydrosilanes (trichloro-, dichloromethyl-, chlorodimethyl-,
dichlorophenyl-, and chlorodiphenylsilane). The
resulting materials, which were modified at the surface by
methyl or aryl groups, were treated with saturated solutions
of PdCl2 in methanol to form different Pd-on-silica catalysts
with various Pd loadings (Pd/SiO2Me, Pd/SiO2Me2, Pd/SiO2-
Ph, Pd/SiO2Ph2).
Pd/SiO2Ph exhibited high catalytic activity in the Heck
reaction of aryl iodides and bromides with styrene or methyl
acrylate (Table 40). 151,152 The catalyst could be recovered
and reused.
Table 40. Catalytic Performance of 0.3% Pd-Silica Catalysts in
Heck Coupling
Pd cat. R1 ,X R2 t (h) conv (%) select. (%)
Pd/SiO2Me
Pd/SiO2Me2
Pd/SiO2Ph
Pd/SiO2Ph2
Pd/SiO2Me
Pd/SiO2Me2
Pd/SiO2Ph
Pd/SiO2Ph2
Pd/SiO2Me
Pd/SiO2Ph
Pd/SiO2Me
Pd/SiO2Ph
H, I
H, I
H, I
H, I
NO2,Br
NO2,Br
NO2,Br
NO2,Br
Ac, Br
Ac, Br
H, I
H, I
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
Ph
Ph
2
2
2
2
2
2
2
2
2
2
5
5
84
54
97
51
100
68
100
66
67
75
58
80
99
99
99
99
99
99
99
99
99
99
86
83
As an alternative to mercaptopropyl modification, also
arsanopropyl or methylselenoundecyl groups were introduced