Chemistry Reference
In-Depth Information
The Pd/TPPTS catalyzed telomerization reaction eciently allows the
synthesis of mono- and diethers with good yields in water or in an or-
ganic solvent for each substrate. The use of water/oil phase biphasic
conditions allowed the selective synthesis of monoethers (94% selectivity
for isosorbide). As isosorbide is a non-symmetric diol, the reactivities of
the two hydroxyl groups are different according to the reaction con-
ditions. In water, 2-O-monooctadienyl ethers are the major products
whereas in DMF, the 5-O-monooctadienyl ethers are the major products.
3.2 Palladium catalyzed reductive alkylation of polyols with carbonyl
compounds
Carbonyl containing groups can be hydrogenated in the presence of a
heterogeneous catalyst to eciently access ethers from alcohols.
54
In 1995, Lemaire reported the palladium catalyzed synthesis of
symmetrical or unsymmetrical ethers from a ketone or an aldehyde and
an alcohol. The reaction involves the hydrogenolysis promoted by
palladium over charcoal of a transient hemiacetal obtained from
the reaction between the carbonylated compound and the alcohol
(Scheme 19).
55
However, the intermediate formation of a vinylether
obtained from the dehydration of the hemiacetal followed by a hydro-
genation step of the double bond cannot be completely discarded.
This reaction has been recently applied to the synthesis of 1-O-alkyl
glycerol and diglycerol ethers from an aldehyde
56
or a ketone
57
(Scheme 20). In order to minimize the amounts of side products, the
reaction was performed with 0.5% Pd/C catalyst and a Brønsted acid as
co-catalyst. Glycerol or diglycerol were used as both solvent and reactant,
and the products were eciently extracted from unreacted materials with
toluene at the end of the reaction.
Interestingly, glycerol, diglycerol and triglycerol ethers are also
accessible from acids in place of aldehydes.
58
Aldehydes are industrially
OH
H
2
,Pd/C
O
R'
R
R'
R
R
R'
O
+
HO
O
-H
2
O
H
H
2
,Pd/C
-H
2
O
R
R'
O
Scheme 19 The palladium catalyzed reductive alkylation of alcohols with an aldehyde.
O
10 bar H
2
, 0.5% Pd
/C
R
R
O
OH
HO
OH
+
H
OH
OH
10% CSA; 140 °C
R = propyl, hexyl, decyl, 2-pentyl
O
10 bar H
2
, 0.5% Pd/C
R
R
OH
+
HO
O
O
O
OH
H
OH
OH
10% CSA; 140 °C
OH
OH
R = propyl, hexyl, decyl
Scheme 20 Glycerol and diglycerol monoethers from glycerol, diglycerol and an
aldehyde.
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