Chemistry Reference
In-Depth Information
O
O
(OC)
3
Co
Co(CO)
3
Ph
Ph
Ph
H
53%
47%
Scheme 2.18
Regioselectivity of the PK reaction between phenylacetylene and propene.
With olefins bearing a functional group capable of coordinating a Co atom, this total lack
of selectivity in the olefin part completely disappears experimentally. In the computational
approach, 4-aminobutene was used as a model for this kind of system and the four possible
structures of
TS1
were considered. A partial optimization approach was used, keeping
the core reaction center frozen (Co
1
-Co
2
-C
1
-C
2
-C
3
-C
4
) in the geometry of
TS1
for the
unsubstituted model and optimizing the substituents (Figure 2.5). A significant preference
for
TS1a
, in which the chelating substituent was linked to C
4
and coordinating Co
1
,was
observed. This result was consistent with the experimental observation of the chelating
substituents occupying preferentially the C
4
position in the final product (Scheme 2.19).
O
X
O
R
(OC)
3
Co
Co(CO)
3
X
+
R
R
H
R = Ph, Bu, TMS
X = NMe
2
, SMe
X
Major
Minor
Scheme 2.19
P-K reaction of chelating olefin substrates with terminal alkynes.
CO
X
X
CO
CO
(OC)
3
Co
Co
(OC)
2
Co
Co
H
H
H
H
R
1
R
1
H
H
H
H
TS1a
TS1b
CO
(CO)
2
Co
X
X
H
H
(OC)
2
Co
(OC)
3
Co
Co
H
H
R
1
R
1
H
H
H
H
TS1c
TS1d
Figure 2.5
Schematic representation of the transition state models for the stereodetermining
alkene insertion step with chelating olefins (X
=
NH
2
).
