Chemistry Reference
In-Depth Information
Ph
Ph
Ph
Ph
Ph
Ph Ph
Ph
d
n
4
r
4
n
g
|
0
Si
Si
Si
Si
O
O
O
O
O
O
O
O
Si
Si
MCM-41
X
X
X
X
X = NH
3
+
NH
3
+
RuO
4
-
Br
NMe
3
+
RuO
4
-
NEt
3
+
RuO
4
-
Si
Si
Si
Si
O
O
O
O
O
O
O
O
O
Figure 3.9
Structure of TPAP-tethered MCM-41.
x
y
RuCl
2
(PPh
3
)
3
1) Al(O
i
Pr)
3
THF
2) aq. NaOH and
i
PrOH
Si(OMe)
3
90
o
C, 1 h, then
HB-Ru
removal of solvent at 120
o
C
Scheme 3.4 Preparation of HB-Ru.
.
Another variation on this theme involves straightforward doping of me-
thyl-modified silica, referred to as ormosil, with tetrapropylammonium
perruthenate (TPAP) via the sol-gel process.
83
A serious disadvantage of
this system is the low TOF (1.0 and 1.8 h
1
) observed for primary
aliphatic alcohols and allylic alcohols, respectively. Sol-gel-encapsulated
TPAP (SG-TPAP) was developed and it was found that the surface hydro-
phobicity and the amount of water and methanol used were key to achieving
high catalytic performance. The optimized catalyst showed higher activity
than the unsupported TPAP and it could be reused at least six times.
To broaden the application range of SG-TPAP, other reaction conditions
were investigated, such as conditions of supercritical carbon dioxide (scCO
2
)
and the introduction of ionic liquids or a fluoroalkyl chain to the silica
matrix of SG-TPAP.
84
A reusable heterogeneous ruthenium catalyst, organic-
inorganic hybrid ruthenium (HB-Ru), was prepared from RuCl
2
(PPh
3
)
3
as the
metal source and a copolymer bearing a trimethoxysilyl functionality for the
heterogenization of the catalyst (Scheme 3.4). The sol-gel process was
employed to initiate hydrolysis by adding aqueous NaOH, followed by
polycondensation (cross-linking)
to give HB-Ru.
85
The treatment of
4-methylbenzyl alcohol with HB-Ru catalyst
(5 mol%) under an O
2
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