Chemistry Reference
In-Depth Information
widely accessible from olefins but acids are advantageous if considering
that they are issuing from agro-based resources.
59
The reaction con-
ditions to obtain the ethers from the acids are very similar to those
employed with aldehydes and a large scope of aliphatic acids is suitable
for that reaction. Amberlyst 35 was used as a heterogeneous support, thus
allowing the recycling of both acid and metallic catalyst over 4 runs
without loss of reactivity. Methyl esters were also used as carbonyl
containing starting material.
60
The synthesis of glycerol ethers was
performed either starting from a mixture of a methyl ether and glycerol or
after the synthesis of a glycerol monoester obtained from a transester-
ification step between the two reactants (Scheme 21).
Di- and triglycerol were in this context also eciently converted. The
reduction of an ester to an ether is rare and appears to be favored in
that case by the formation of an intermediate five-membered cyclic
hemi-ortho ester that can be readily hydrogenated (Scheme 22). That
assumption is in accordance with the fact that 1-propanol (no possible
cyclic hemi-ortho ester) and 1,3-propanediol (six-membered cyclic
hemi-ortho ester) are not converted in ethers whereas 1,2-propanediol,
suitable to the formation of an intermediate five-membered cyclic hemi-
ortho ester, affords the corresponding monoethers (mixture of the two
regioisomers) in good yields.
Whereas aldehydes can be reacted with a wide range of alcohols, acids
are eciently used with 1,2-diols and in both cases, a wide range of alkyl
chains has been grafted. If the reaction has essentially been applied to
direct etherification
R' = H or OMe
O
40 bar H
2
,1% of 5% Pd/
C
R
R
O
OH
HO
OH
HO
OH
+
+
O
OR'
OH
OH
10% Amberlyst; 120 °C
28-75% yield
R = alkyl and cycloalkyl groups
R
BaO/Al
2
O
3
(5 wt%)
1) filtration
Transesterification
100-120 °C
R' = OMe
2) 50 bar H
2
,1% of 5% Pd/C
Amberlyst 35 (10 wt%)
OH
O
OH
R
O
Scheme 21 Synthesis of ethers from acids or esters and glycerol.
Pd/C
Amberlyst 3
5
H
2
O
no posssible cyclic
hemi-orthoester
R
no ether
O
O
Pd/C
Amberlyst 3
5
H
2
O
R
R
R
O
O
OH
O
OH
OH
very Low yield
O
O
Pd/C
Amberlyst 3
5
H
2
HO
O
R
R
R
+
O
R
O
HO
O
OH
OH
High yield of ethers
Scheme 22 Key intermediates in the synthesis of ethers from an acid and an alcohol.
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