Chemistry Reference
In-Depth Information
MeO
OMe
O
10 equiv
N
H
N
NMO 6 equiv
CH
2
Cl
2,
-30 ºC,
5 h
MeO
(OC)
3
Co
Co(CO)
3
MeO
18%, 15.6:1 dr
66b
Scheme 5.48
The diastereomeric adducts were not separable, so that from a practical point of view
chiral ynamines are much less convenient than chiral alkoxy acetylenes for asymmetric
intermolecular PKRs. The high reactivity of complexes
66
is associated to the unusually
facile dissociative loss of carbon monoxide that they undergo, a phenomenon that was
studied by means of DFT calculations and was found to be triggered by the delocalization
of the amine lone pair in the cobalt-carbon cluster.
48
On the other hand, the dicobalt hexacarbonyl complexes of alkynyl amides appear to
be much more stable, due to the reduced availability of the nitrogen lone pair, and they
have been shown to undergo efficient intermolecular PKRs.
49
An asymmetric variant of
this process was reported by Shen and Hsung in 2003.
50
These authors found that the
dicobalt hexacarbonyl complexes of 2-substituted ynamides derived from 4-phenyl-1,3-
oxazolidin-2-ones, formed in moderate yields (40-50%) by treatment of the acetylenes
with dicobalt octacarbonyl, reacted with norbornadiene under NMO-promoted conditions
to give mixtures of a standard
exo
-adduct with an unusual
endo
-adduct (Scheme 5.49).
Interestingly enough, in all instances only the 2-amido regioisomers were obtained. The
exo
-adducts were obtained with no diastereoselectivity (1:1 dr), while the
endo
-adducts
were isolated as single diastereomers. The absolute configuration of the
endo
-adduct was
ascertained in one case (R
=
Ph) by X-ray diffraction analysis.
Ph
Ph
O
O
Ph
H
H
O
N
R
+
N
N
O
O
O
NMO (6 equiv)
CH
2
Cl
2
, -10 ºC
(OC)
3
Co
Co(CO)
3
H
H
O
R
R
O
R = alkyl, phenyl
exo
16-30%, 1:1 dr
endo
13-31%,
one diastereomer
Scheme 5.49
H), its reaction with
norbornadiene afforded the
exo
-2-amido regioisomer in 16% yield and with a 2:1 dr.
These results can be interpreted by assuming that the oxazolidinone carbonyl acts as a
ligand to the metal, displacing a CO and coordinating preferentially to the pro-(
R
) cobalt.
When an unsubstituted
N
-alkynyloxazolidinone was used (R
=
