Chemistry Reference
In-Depth Information
8B.3.2. Platinum - Catalyzed Allylations
In contrast to palladium-catalyzed reactions, reports on platinum-catalyzed allylic alkyl-
ations are extremely rare [202]. Platinum
-allyl complexes show a different reaction
behavior compared with the corresponding palladium complexes. While terminal
π
- allyl -
Pd complexes are preferentially attacked at the terminal position, the Pt complexes give
a higher ratio of branched product. Williams et al. observed a nearly complete retention
of regiochemistry in reactions of branched allyl acetates if a tetrameric (allyl)-Pt chloride
complex was used in combination with PCy
3
as ligand (Scheme 8B.49 ) [203] .
π
OAc
CH(CO
2
Me)
2
NaCH(CO
2
Me)
2
[(allyl)PtCl]
4
/PCy
3
+
R
CH(CO
2
Me)
2
R
R
15
:
1
Up to
Scheme 8B.49.
Platinum - catalyzed regioselective allylations.
Brown and McIntyre were the fi rst who studied asymmetric allylations in the pres-
ence of the bidentate (
R
,
R
) -
DIOP
ligand (
L41
) (Scheme 8B.50 ) [204] . Using linear
butenyl acetate as substrate also, the branched product was formed preferentially, unfor-
tunately with low ee.
CH(CO
2
Me)
2
NaCH(CO
2
Me)
2
+
OAc
CH(CO
2
Me)
2
Ph
2
O
P
+
11% ee
Pt
5
:
1
P
O
Ph
2
BF
−
Scheme 8B.50.
Asymmetric platinum - catalyzed regioselective allylations.
Labeling experiments and detailed NMR-spectroscopic investigations indicated that
in a solution, an interconverting mixture of all four possible isomeric
- complexes in
comparable amount exists (Fig. 8B.20). All these complexes can react with the nucleo-
phile, although in somewhat different rates. This might explain the relatively high amount
of branched product formed and the low ee.
π
O
O
O
O
O
O
O
O
Ph
2
P
P
Ph
2
Ph
2
P
P
Ph
2
Ph
2
P
P
Ph
2
Ph
2
P
P
Ph
2
Pt
Pt
Pt
Pt
Figure 8B.20.
Isomeric complexes formed in Pt-catalyzed allylations.
