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O
O
AcO
OH
AcO
O
α, β
AcO
OAc
AcO
OAc
2,3,4,tri-
O
-acetyl-D-xylopyranose (-
α, β)
+
O
O
AcO
OH
Pd(acac)
2
/PPh
3
DMF
AcO
O
α, β
AcO
OAc
AcO
OAc
2,3,4,tri-
O
-acetyl-L-arabinose (-
α, β)
Scheme 13 Telomerization of butadiene with tri-O-acetylated pentopyranoses.
H
2
O/NaOH solvent, a DS of 0.11 was obtained with 83% butadiene con-
version after 24 h at 50 1C (TOF
=
86 h
1
).
During the years 2004-2007, telomerization of butadiene with
protected or free sugars has intensively been studied by the group of
Muzart. The first study reported on telomerization of butadiene with
tri-O-acetylated pentopyranoses as 2,3,4-tri-O-acetyl-
D
-xylopyranose and
2,3,4-tri-O-acetyl-
L
-arabinose having a free anomeric hydroxyl group
(Scheme 13).
36
The reactions were carried out in DMF with Pd(acac)
2
/
PPh
3
catalytic system at 75 1C.
In the case of xylose, the a/b ratio in the starting sugar was about 2 : 1
whereas the ratio of a/b octadienylethers was close to 1 : 1. The loss of the
stereochemistry of the starting xylopyranose could be due to a rapid
catalyzed mutarotation compared to the rate of the telomerization
reaction. Analogous results were observed starting from the
L
-arabinose
derivative.
The same free carbohydrates were also evaluated by the authors to
determine the best conditions for an ecient conversion of the sugars
into monooctadienylethers. With DMF as solvent,
37
the palladium based
catalytic system was found to be e
cient by addition of an amine as base,
optimized ratios of substrate, reactants and catalytic system: for example
with the system Pd(acac)
2
/PPh
3
/Et
3
N: 1/2/150, an arabinose conversion of
88% with a selectivity into the monooctadienylethers of 76% could be
obtained after 5 h at 75 1C. Moreover, a grafting of the octadienyl chain
occurred almost exclusively at the anomeric hydroxyl group, owing to its
more acidic character. Conditions have been developed to achieve the
telomerization reaction of the same free pentoses in aqueous medium.
38
By using a catalytic system based on Pd(acac)
2
/TPPTS and a crowded
amine such as ethyldiisopropylamine, the monoetherification of the two
pentoses occurs selectively on the anomeric hydroxyl group whereas in
the presence of the amphiphilic dimethyldodecylamine, a mixture of the
mono-, di- and triethers was obtained with the disubstituted ethers as
major compounds. It has to be noticed that interfacial properties of
octadienyl pentosides either pure or in crude mixture after telomeriza-
tion reaction have then been evaluated, showing interesting capacity of
lowering surface tension down to 30-35 mN/m at relatively low concen-
trations in the range of 1-4mmol/L.
39,40
On the other hand, a palladium-TPPTS catalyst heterogenized on KF/
alumina has been shown to be effective and recyclable for the selective
formation of monooctadienylxylopyranosides via the telomerization of
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