Chemistry Reference
In-Depth Information
Ru tetrakis (2,6-dichlorophenyl) porphyrin, via an
N
...
Ru interaction (Fig. 7.24).
This catalyst was capable of the epoxidation of
several alkenes using 2,6-dichloropyridine
N
-oxide
as the terminal oxidant. Alternative, greener oxi-
dants were not used and some leaching of the metal
centre in the catalyst was noted. Nonetheless, the
reactions proceeded very well and with excellent
turnover numbers. Although the combination of
poor atom efficiency in the oxidant and the loss of
metal from the catalyst mean that work remains to
be done on this system to make it more robust, the
high turnover numbers are impressive, especially for
a porphyrin-based system. Additionally, selectivity
changes were noted, which indicate that steric
factors may be at play that are absent in homoge-
neous systems, thus indicating a significant role for
the pore walls—something that may be of impor-
tance in the design of heterogeneous catalysts.
They attached the guanidine by first grafting a gly-
cidyl silane onto the surface of the support, fol-
lowed by ring-opening of the epoxide ring with
1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) (Fig. 7.25,
catalyst A).
Guanidines are of great interest in the field of het-
erogeneous base catalysis because they are powerful
enough bases to act as replacements for hydroxide
in synthetic applications. In addition to their poten-
tial in typical base-catalysed reactions (see below)
they are also excellent catalysts for the base-
catalysed epoxidation of electron-deficient alkenes.
This provides a nucleophilic route to these epoxides
and complements the metal-centred routes.
The materials produced in this way were tested
in the epoxidation of enones, leading to epoxides
that are particularly labile towards attack by water.
Nonetheless, they showed that high selectivities
could be obtained at low to medium conversion of
substrate. The results obtained with this catalyst and
those of the following catalyst are shown in
Fig. 7.25.
Further work by Brunel & Macquarrie [93] utilis-
ing variations on a different strategy developed by
Brunel
et al
. [111] has led to a different type of
guanidine catalyst (Fig. 7.25, catalyst B). In this case
the authors have shown that silylation of the surface
had a major positive influence on the efficiency of
the reaction. They achieved very high conversions
Metal-free epoxidation catalysts
Jacobs
et al
. published the first example of a guani-
dine unit supported on the surface of a micelle-
templated silica material in 1997 [110] (Fig. 7.25).
Fig. 7.24
Epoxidations catalysed by immobilised
Ru-porphyrins.
Cl
Cl
Cl
Cl
O
N
H
N
Si
N
H
CO
=
Ru
O
OR
N
H
N
OH
Cl
Cl
Cl
Cl
Cl
2
pyNO
O
yields 55 - 95%
turnover numbers 1410 - 4550
R
R
