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is the energetically preferred option. This chelation effect leads to an extremely efficient
chirality transfer (Scheme 5.56).
O
SiMe
3
H
O
O
SiMe
3
SiMe
3
Xc
H
O
N
S
OC
OC
H
N
Co
Co(CO)
3
Co
Co(CO)
3
O
2
S
O
Me
O
CO
CO
Me
Me
Me
Scheme 5.56
In 2002, Moyano and co-workers reported that the heterobimetallic (Mo-Co) com-
plexes of
N
-(2-alkynoyl) derivatives of chiral oxazolidinones (
Cf.
69a
) or sultams (
Cf.
74c
)
showed an unprecedented reversal of stereoselectivity in their reaction with norbornadiene,
predominantly giving rise to
endo
-fused cyclopentenone adducts in a totally regioselective
fashion.
56
These heterobimetallic complexes were easily prepared by
in situ
generation of
the dicobalt hexacarbonyl complex in tetrahydrofuran and treatment with 1.5molar equivs
of Na[MoCp(CO)
3
] (Table 5.10).
Table 5.10
O
O
Co
2
(CO)
8,
THF, rt;
Me
Xc
69a
74c
81a
,
b
82a
,
b
Me
OC
OC
Xc
Mo
Co(CO)
3
Na[CpMo(CO)
3
],
THF, reflux, 1.5 h
Alkyne
Xc-
Product
Yield (%)
Dr
O
81a,b
43
2.3:1
Ent-
69a
ON
Ph
Me
Me
82a
,
b
64
1.2:1
74c
N
O
2
S
