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O
O
Ar
NO
2
NO
2
9
(2.5 mol%
)
+
NO
2
Ar
NO
2
HSBM
rt, 0.5 h
1.0 equiv
1.5 equiv
Yield 80
-
95 %
dr
94:6
-
98:2
N
ee
92
-
98 %
S
H
H
9
Scheme 4.7 Asymmetric Michael addition organocatalyzed by thiourea 9 in a ball-
mill.
37
O
O
NO
2
Ph
O
O
H
MeO
OMe
2.0 equiv
MeO
OMe
1.0 equiv
NO
2
Ph
NO
2
10
(20 mol%)
HSBM
rt, 6h
H
Ph
11
(20 mol%)
HSBM, rt, 4.5 h
K
2
CO
3
(20 %mol)
1.0 equiv
Yield 66 %
ee
Yield 29 %
ee
92 %
80 %
CF
3
CF
3
F
3
C
S
H
NMe
2
H
CF
3
S
HN
N
H
HN
MeO
11
N
10
Scheme 4.8 Asymmetric Michael addition organocatalyzed by cinchona and
L
-Binam-thioureas 10 and 11 in a ball-mill.
39
experiments in solution. In particular, evaluation was reported of several
chiral thiourea-containing cinchonas, binaphthyls, and squaramides and
bis-thioureas as organocatalysts under solvent-free conditions using LAG to
obtain enantioenriched Michael adducts (Scheme 4.8).
39
Reaction yields
were significantly improved relative to the reactions carried out in solution.
Evidently, HSBM can be an excellent strategy to carry out reactions activated
by hydrogen bonding.
40
The lower eciency observed in solution was
tentatively ascribed to heating of the containers that takes place under
traditional heating conditions and that disrupts the otherwise hydrogen-
bonded transition states.
39
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