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catalyst was then able to promote Heck coupling with TONs up to 100 000.
Bergbreiter et al. explored the reactivity of two polyisobutylene (PIB)-supported
Pd-NHC catalysts in the Heck reaction (48, Figure 4.23).
112
The PIB-bound
NHC complexes were found to be selectively soluble in heptane versus acet-
onitrile or DMF. This allowed the catalysts to be recovered by phase separation
and reused in a new catalytic run. Complex 48a proved active in the coupling
but could not be recycled more than three times. On the other hand, 48b was
found to be less reactive but more recyclable; indeed, it could be used 10 times
without any significant loss of reactivity.
In 2012, Pleixats and co-workers developed recyclable hybrid silica-based
precatalysts for the Heck reaction (49, Figure 4.24).
113
The systems promoted
the reaction between 4-bromoacetophenone and n-butyl acrylate with rela-
tively short reaction times and high yields (Table 4.18). They were also
successfully recycled up to five times.
PIB
NN
N
PdI
2
Pd
N
Cl
PIB
PIB
2
48a
48b
Figure 4.23 PIB-bound Pd-NHC complexes for Heck reactions.
SiO
1.5
.40SiO
2
SiO
1.5
.40SiO
2
NN
NN
Cl
Pd Cl
Pd
Cl
O
N
O
Cl
49a
49b
Figure 4.24
Structures of silica-based catalysts for Heck reactions.
Table 4.18 Activity and recyclability of 49a and 49b in the Heck reaction.
Br
COOn Bu
49a-b (0.2 mol%)
+
COOn Bu
NBu
3
DMF, 1 50°C
MeOC
MeOC
49a
49b
Cycle
Time (h)
Yield (%)
Time (h)
Yield (%)
1
3
98
3
98
2
2
99
2
99
3
3
98
3
99
4
3
96
3
98
5
3
97
3
95
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