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O
O
0.5 equiv. DABCO
PBM
500 min
-1
, 48 h
+
O
OH
O
O
Abs
O
+
O
+
0.5 equiv. DABCO or NEt
3
O
OH
PBM
500 min
-1
, 48 h
OH
O
O
Scheme 2.25 DABCO catalyzed oxa-Michael-aldol reaction.
O
OH
O
O
20 mol% DABCO
MBM
60 s
-1
, 0.5 h
+
OCH
3
OCH
3
R
R
R = NO
2
,
Br, Cl, H, OCH
3
Scheme 2.26
Synthesis of a-hydroxy alkylated vinyl derivatives by Baylis-Hillman
reaction.
The reaction of salicylaldehyde and prenal led to a mixture of two products
since either oxa-Michael-aldol reaction took place or a vinylogous aldol re-
action occurred followed by oxa-Michael addition giving dihydrobenzopyran.
With NEt
3
or DABCO as base comparable yields were achieved for reaction in
ball mills and solution. In contrast, on milling reactants with Na
2
CO
3
, only
traces of the product could be detected while in solution 19% and 65% of the
coupling products were formed.
2.5.4 Baylis-Hillman Reaction
a-Hydroxy alkylated vinyl derivatives can be obtained by the Morita-
Baylis-Hillman reaction and are interesting molecules for synthesis of
drugs, e.g. for antimalarial agents. Mack et al. reported high yields within
0.5 h by milling methyl acrylate, 4-substituted aromatic aldehydes and
DABCO as catalyst (Scheme 2.26).
47
They observed a significant rate en-
hancement of the reaction in comparison to other solvent-free methods
where reaction times of days are necessary.
2.5.5 Wittig Reaction
The Wittig reaction is of great interest and an industrially important method
for the formation of double bonds, e.g. in the synthesis of vitamin A.
48
Shearouse et al. investigated the Wittig reaction under ball milling con-
ditions (Scheme 2.27).
49
By reacting a premilled PPh
3
-functionalized poly-
styrene with various aldehydes and ethyl bromoacetate or benzyl bromide,
stilbenes could be obtained in high yields. Referring to the concept of liquid
assisted grinding (LAG, see Chapter 7.3.3) the addition of small amounts of
solvents was beneficial to the reaction. While without additional solvents a
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