Chemistry Reference
In-Depth Information
O
O
Me
H
2
NCO
TsOH (5 mol%)
HN
HN
i
Pr
+
+
O
toluene
reflux, 24 h
i
Pr
Me
O
O
HO
i
Pr
2 : 1
Me
89%
100%
14 : 1
DBU (
1
) (0.1 equiv)
N
-methypyrrolidine
Scheme 3.39
DBU (1) catalysed isomerization of spiro system
5% DBU (
1
)
DCM
CO
2
Me
CO
2
Me
N
N
MeO
MeO
O
O
Ph
Ph
Scheme 3.40
DBU (1) catalysed epimerization
99% ee) is also epimerized to trans-oxazole by treatment with a catalytic
amount of DBU (1)(cis:trans
cis-Oxazole (
>
¼
5 : 95,
99% ee) [66] (Scheme 3.40).
>
3.3.14 Metal-Mediated Reaction
3.3.14.1 Cobalt
Ketoiminatocobalt complexes catalysed enantioselective Henry reaction in the presence of
organobase to give
-nitro alcohols have been reported. Although
i
Pr
2
NEt was the most
suitable amine for this reaction, the use of DBU (1) as base accelerated the reaction but no
enantioselection was observed [67]. Bis(triphenylphophoranylidene)ammonium fluoride
(PPNF) was found to be an effective base co-catalyst in the cobalt (III)-salen complex
catalytic asymmetric addition of carbon dioxide (CO
2
) to propylene oxide giving propylene
carbonate [68] (Scheme 3.41). DBU (1) or the N-methyl analogue also afford good
enantioselectivities (75 and 72% ee, respectively). Thus, these strong and sterically
hindered organobases can act as co-catalysts in this metal-mediated reaction system.
b
catalyst
O
X = OCOCF
3
O
CO
2
(1 atm)
OO
N
N
Co
Me
catalyst (0.1 mol%)
base (0.2 mol%)
t
Bu
t
Bu
Me
O
O
X
-20 ˚C, 18 h
PPNF: 40% (80% ee)
DBU (
1
): 22% (75% ee)
t
Bu
t
Bu
Scheme 3.41
Asymmetric addition of CO
2
to propylene oxide
