Chemistry Reference
In-Depth Information
One year later, Evans reported
54
a single rhodium-catalyst to facilitate both transforma-
tions, the allylic alkylation and PK annulation reaction, in a tandem sequence, just by the
controlling the reaction temperature to modulate the catalytic activity.
9.5.1 Chiral Pd(II) Complex and Bulk Cobalt on Charcoal-Catalyzed Tandem
Asymmetric Allylic Alkylation and PK Annulation Reactions
55
A tandem reaction related to the PKR catalyzed by a combination of homogeneous and
heterogeneous catalysts was studied by Chung's group (Scheme 9.23).
55
They used a com-
bination of chiral Pd(II) complex and bulk cobalt metal on charcoal as the catalyst system.
This is a rare example of the use of a combination of homogeneous and heterogeneous
catalysts in a tandem reaction.
56
3
-allyl)PdCl]
2
catalyzed an asymmetric allylic
The [(
Me
OAc
MeO
2
C
Me
MeO
2
C
MeO
2
C
Me
Pd(II), L*
Co/C
30 atm CO
O
+
Ph
Ph
Ph
BSA, KOAc
0
MeO
2
C
MeO
2
C
MeO
2
C
°
C, THF
Ph
Ph
95%, 96% ee
PPh
2
L*=
N
O
Scheme 9.23
alkylation to generate enantioselective enynes and the heterogeneous Co/C carried out a
catalytic PKR of the
in situ
generated enantiomerically enriched enynes. Chiral diphos-
phines, phosphinooxazoline and
(R,R)-
NORPHOS were screened as an ancillary ligand and
phosphinooxazoline was the choice. High yield up to 95% and high
ee
value up to 96% were
observed. The enantioselectivity of this tandem reaction was absolutely dependent on the
Pd-catalyzed asymmetric allylic alkylation. The success of this tandem catalytic reaction
is primarily due to the use of Co/C. When Co
2
(CO)
8
instead of Co/C was used as a cata-
lyst, only 2% of the desired product was isolated. They attributed the generation of acetic
acid in the palladium-catalyzed allylic alkylation step for the background of the low yield.
The usefulness of the combined catalyst is highlighted by the synthesis of various tricyclic
enones (Scheme 9.24).
trans
-1,2-Bis[2-(diphenylphosphino)benzamido]cyclohexane
57
was
used as a chiral ligand. High yields (90-97%) were observed, but the ee values (31-94%
ee) were highly sensitive to the ring size.This study shows that the tandem action of a
homogeneous chiral Pd(II) catalyst and a heterogeneous Co/C catalyst leads to a highly
enantioselective two-step one-pot PK-type reaction.
9.5.2
Pd(II) Complex and Cobalt Nanoparticle-Catalyzed Three-Step One-Pot
Synthesis of Fenestranes
60
The fenestranes are a unique class of hydrocarbons. Major advances in the synthesis of fen-
estranes were achieved by the application of transition metal-induced transformations.
58, 59
Chung reported the use of cobalt nanoparticles in combination with a palladium complex to
develop an efficient synthetic procedure both reducing the number of steps and increasing
