Chemistry Reference
In-Depth Information
The mechanism of the PKR forwarded by Magnus in 1985
5
and depicted in Scheme
5.4, has been supported both by theoretical studies and by some experimental evidence
for the proposed intermediates.
6, 7
The first step is the reaction of the alkyne unit with
dicobalt octacarbonyl to form the isolable hexacarbonyl complex
I
.
8
Thermal, chemical,
or photochemical activation leads to the loss of a carbon monoxide ligand affording a
coordinatively unsaturated pentacarbonyl complex (
2
-alkyne)[
-Co
2
(CO)
5
](
II
), and the
resulting vacant position is occupied by an alkene (
III
). The alkene undergoes 1,2-insertion
at a carbon-cobalt bond of the tetrahedral C
2
Co
2
cluster, followed by (or concomitantly
with) coordination of carbon monoxide, to form the cobaltacycle
IV
, which experiences a
carbonyl insertion reaction (
V
) and reductive elimination of a cobalt atom to give
VI
. Finally,
reductive elimination of dicobalt hexacarbonyl affords the final cyclopentenone
VII
.Aswe
will see later in more detail, the transformation of
III
into
IV
determines the stereochemical
outcome of the reaction. Under certain conditions, the unstable dicobalt hexacarbonyl
fragment can take up two molecules of carbon monoxide (or lead directly to the formation
of
I
), rendering the whole process catalytic in cobalt.
9
Otherwise, decomposition into inert
metallic cobalt leads to the classical experimental procedure of the PKR, that calls for the
use of stoichiometric amounts of dicobalt octacarbonyl.
10
2CO
CO
(OC)
3
Co
Co(CO)
2
(OC)
3
Co
Co(CO)
3
II
I
R
1
O
Co
2
(CO)
8
R
2
*
R
1
2CO
*
R
2
Co
2
(CO)
6
VII
III
(OC)
3
Co
Co(CO)
2
R
2
R
1
CO
(CO)
3
Co
O
(OC)
3
Co
Co(CO)
3
(OC)
3
Co
O
(OC)
3
Co
R
1
Co(C
O
)
3
R
1
*
*
R
1
IV
*
*
*
R
2
R
2
*
VI
R
2
CO
V
Scheme 5.4
The chiral auxiliary strategy has not only been of paramount importance for the develop-
ment of asymmetric synthesis, but still provides in many instances the most practical and
reliable procedures for the efficient preparation of chiral compounds in highly enantiopure
fashion, both in academy and in industry.
11
This is especially true for the PKR, since the
use of chiral auxiliaries, that enabled for the first time its application to the enantioselective
