Chemistry Reference
In-Depth Information
d
n
1
r
3
n
g
|
7
3
.
Scheme 5.17
Ligands for metal-supported gels.
construct Pd-metallogel networks in DMSO that were active catalysts for the
aerobic oxidation of benzyl alcohol to benzaldehyde (Scheme 5.17).
71
Catalytic reactions were analysed after 2 h with turnover number values
twice those obtained using Pd(OAc)
2
as the catalyst, as a consequence of the
superior stability of the catalytic gels.
More recently, Zhang and coworkers have prepared pyridine-based tripodal
ligands for Pd(II) (61) that are able to form metallogels in methanol-
chloroform mixtures and can be used as catalysts for the Suzuki-Miyaura C-C
crosscoupling between 4-bromopyridine and phenylboronic acid in high yield
after few hours (Scheme 5.17).
72
This gel-phase catalyst was also used for the
cross coupling of other aryl halides such as iodobenzene being reused up to
three times, with progressive inactivation. However, when the metallogel was
dried and used directly as a xerogel, catalytic activity was comparable to the gel
in the first run and maintained for at least five consecutive runs. More recently,
the same authors have reported the study of Fe-metallogels based on 1,3,5-
substituted benzene as ligand (62), their postmodification by diffusion of a
Pd(COD)Cl
2
solution (COD: cyclooctadienyl), and their use as catalysts for the
same organic reactions.
73
In that case, the catalyst was packed in a paper bag to
avoid mechanical degradation and easy separation from reaction medium. The
products were obtained in high yield and recycled for several runs.
