Chemistry Reference
In-Depth Information
5.3.4 Chiral Ynamines and Ynamides
The synthesis of dicobalt hexacarbonyl complexes of chiral ynamines was reported by
Pericas and co-workers in 2000.
48
The silylated complexes
65a
-
e
were prepared from a set
of chiral secondary amines by a reaction sequence involving
in situ
generation and trap-
ping of dichloroacetylene from trichloroethylene, followed by sequential treatment of the
intermediate dichloroenamine with
n
-butyl lithium and trimethylsilyl chloride.
14
Without
further purification, the resulting silylated enamines were converted into the complexes
65
by reaction with dicobalt octacarbonyl. The corresponding desilylated complexes
66a
-
e
could be accessed by exposure of
65
to potassium carbonate and methanol.
42
The green-
colored complexes
65
and
66
readily experience loss of carbon monoxide but are stable at
room temperature under a positive pressure of this gas (Scheme 5.47).
Et
2
O, -70 °C;
33 °C, 1.5-24 h
2 equiv
n
-BuLi,
-70 to -10 °C;
R*
2
N
Cl
R*
2
NH
R*
2
N
SiMe
3
Cl
H
Me
3
SiCl,
-10 °C to rt
Cl
Cl
Co
2
(CO)
8
K
2
CO
3
,
MeOH, CO,
rt, 30 min
H
Cl
R*
2
N
SiMe
3
R*
2
N
H
Cl
Cl
(OC)
3
Co
Co(CO)
3
(OC)
3
Co
Co(CO)
3
66a-e
(50-88% yield)
65a-e
(43-82% overall yield)
(
65a
,
66a
)
OMe
R*
2
NH
H
(
65d
,
66d
)
Me
Me
H
(
65b
,
66b
)
MeO
OMe
Me
H
(
65e
,
66e
)
Ph
H
Ph
(
65c
,
66c
)
BnO
OBn
H
Scheme 5.47
While complexes
65
were too hindered and gave very low yields, intermolecular PKRs
of complexes
66
with strained olefins (norbornene, norbornadiene) took place at unprece-
dented low temperatures (rt, 0
◦
C,
35
◦
C) to give the expected adducts
in poor to moderate yields and with good diastereoselectivities (see Scheme 5.48 for a
representative example).
21
◦
C, or even
−
−
