Chemistry Reference
In-Depth Information
O
TBSO
Me
TBSO
O
O
HO
Me
Me
TBSO
H
Me
H
Me
Me
Co
2
(CO)
8
Me
O
Me
Me
Heptane, 110 °C
CO (1 atm)
Me
O
Me
Me
H
H
H
OH
65%, 54%
de
OH
dl-coriolin
Scheme 3.4
trigonal pyramidal bimetallic structure. Coordination of the alkene proceeds to give a
-
complex
A
along with dissociation of one of the CO ligands. The coordination of the gener-
ated carbon monoxide induces alkene insertion, which provides complex
B
. The following
migratory insertion of carbon monoxide gives the carbonyl complex
C
. Reductive elimi-
nation affords the cyclopentenone, along with the generation of the dicobalt hexacarbonyl
complex. In the course of various mechanistic studies, complex
A
has been isolated and
characterized, but the structures of complexes
B
and
C
have not been fully characterized yet.
R
R
R
CO
R'
Co(CO)
3
Co
2
(CO)
6
Co(CO)
3
-CO
Co
( CO)
3
Co(CO)
2
R'
A
R
R
Co
2
(CO)
6
Co(CO)
3
CO
R
O
Co(CO)
3
-[Co
2
(CO)
6
]
O
R'
R'
R'
B
C
Scheme 3.5
Based upon the above mechanism, the dicobalt carbonyl complex is regenerated, and
the catalytic reaction can be achieved by the supply of carbon monoxide. However, the
development of the catalytic Pauson-Khand reaction has been difficult to achieve, proba-
bly because the coordinatively unsaturated Co
2
(CO)
6
complex is unstable and is readily
transformed into a more stable complex by oligomerization.
3.3 An Early Example of Catalytic Reaction
In the first paper on the Pauson-Khand reaction, the catalytic reaction was already
disclosed. Acetylene-dicobalt hexacarbonyl was used as a catalyst precursor, and the
reaction of norbornene was examined under the mixed gas of acetylene and carbon
monoxide (Scheme 3.6).
2
