Chemistry Reference
In-Depth Information
4.3.1.1.7 Development of Recyclable Catalytic Systems. Another ongoing
challenge to solve in cross-coupling chemistry is the development of re-
cyclable systems. One strategy to recycle a catalyst is to anchor it on a
solid support.
65
Nevertheless, this is not always an easy task owing to vari-
ous problems, including catalyst deactivation, catalyst leaching and the
fact that anchored catalysts regularly appear to be less active than their
non-immobilized congeners. Yang's group have made important contri-
butions in this area. In 2010, they developed mesoporous ethane-silicas
functionalized with IMes-type ligands (28a, Figure 4.16).
66
The support
was able to coordinate Pd(OAc)
2
and the supported catalyst was sub-
sequently used in Suzuki-Miyaura cross-coupling. The system could be
used eight times without any significant decrease in the catalytic activity.
The high recyclability was attributed to the functionalized and stable
nanopores that prevented aggregation of the Pd nanoparticles into larger
particles. A year later, they extended the utility of the support by an-
choring an IPr-type ligand (28b, Figure 4.16).
67
The resulting materials
were palladated using various palladium sources. Amongst them, [Pd(ES-
IPr)(acac)Cl] (29) was found to be the most active for Suzuki coupling. In
this case, the system could be recycled up to 10 times without any signifi-
cant decrease in reactivity (Table 4.7). Notably, the supported catalysts are
recovered by centrifugation between two catalytic runs. Yang and co-work-
ers also developed three-dimensional cubic mesoporous materials that
R
R
NN
NN
O
O
O
O
O
R
R
O
Si
Si
Si
Si
O
O
O
O
O
Pd
O
Cl
O
R=Me:ES-IMes
(
28a
)
R=
i
Pr: ES-IPr
(
28b
)
O
[Pd(ES-IPr)(acac)Cl]
29
Magnetic hybrid
silica nanosphere
O
O
Si
NN
O
OH
OH
Cl
Pd
OA c
O
O
Si
N
N
O
NN
O
O
MCM41
31
O
O
O
O
Si
Si
Cl
MSN-IPr
30
Figure 4.16
Structures of some solid supported NHC and Pd-NHC systems used in
Suzuki coupling.
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