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OH
OH
O
OH
O
OH
OH
OH
+
+
Pd/Phosphine
OH
O
O
O
+
+
O
OH
O
O
O
OH
+ branched ethers
Scheme 16 Telomerization of butadiene with glycerol.
time, the quantity of the aqueous phase, the effect of a base addition and
the influence of the ligand have been adressed and detailed. The surface
properties of monooctyl- and octadienylethers of glycerol have been
appraised and compared showing the influence of the double bonds on
these properties.
47
Supercritical CO
2
has also been found to be an
effective medium for a selective control towards glycerol monotelomers
during heterogeneous palladium telomerization of butadiene.
48
Hetero-
geneous palladium telomerization of butadiene with glycerol has also
been performed in the presence of a microporous 4,4
0
-biphenylphos-
phine-based covalent organic framework (COF) under solvent- and base-
free conditions.
49
Remarkably, this catalytic system outperforms its
homogeneous PPh
3
-based counterpart.
Very ecient catalytic systems for butadiene telomerization with gly-
cerol were described by Palkovits et al.
50,51
They used catalytic systems
based on palladium coordinated by tris-(o-methoxytriphenyl)phosphine
(TOMPP) ligands under solvent free conditions.
Maximum activity and telomers yields were reached with the system
Pd(acac)
2
/TOMPP for high butadiene/glycerol ratios at 90 1C. The select-
ivity into monooctadienylethers or dioctadienylethers could be optimized
by combining high reaction temperatures and short reaction times with
low butadiene/glycerol ratios.
The TOMPP-based catalytic system has been extended to a large scope
of biomass-based substrates such as carbohydrates and sugar alcohols
(Scheme 17).
52
A fixed metal loading per OH group (0.023%) as well as a fixed number
of OH groups (400mmol) were used in all cases. Dimethylacetamide was
found to be the best solvent in terms of catalyst stability, substrate
conversion and product distribution. All the substrates could be
converted into telomers at the exception of cellobiose for which no
conversion occurred (Table 1).
An order of reactivity was delineated from these experiments:
simple alcoholsWsucroseWaldohexosesWaldopentosesWketohexoses. It
has been established that the participation of the anomeric hydroxyl is
a major factor of deactivation mechanism (compare glucose and
Me-glucose). Moreover, the sluggish reactivity of cellobiose, which has
been chosen as a model for cellulose, is due to the very poor solubility of
this substrate in DMAc.
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