Chemistry Reference
In-Depth Information
8B.6.1. Fundamentals of R h - Catalyzed Allylic Alkylations
Tsuji et al. fi rst reported on the regioselectivity of Rh-catalyzed allylations of unsym-
metric allylic substrates [30]. Under neutral conditions, the branched allylic carbonate
reacted with acetylacetone to give the branched product exclusively (Table 8B.45, entry
1), while
-keto esters showed a lower regioselectivity (entry 2). Comparison of the Rh-
and the Pd-catalyzed version gave further insight into the reaction mechanism [263].
While the Pd-catalyzed reaction provided nearly the identical product ratios and yields
(entries 3 and 4) independent on the substrate used, a strong difference was observed
in the presence of the Rh complexes (entries 5 and 6). Tsuji et al. postulated that the
Rh-catalyzed reaction might proceed via a
β
- allyl
intermediate. On the other hand, the linear substrate gave nearly the identical product
ratio (entries 4 and 6) with both metal complexes, which might indicate a similar catalytic
intermediate.
A breakthrough was the observation by Evans and Nelson that phosphites are much
better ligands than phosphines, and that the reaction rate and regioselectivity toward
the branched product could be increased signifi cantly [264]. The reactions proceeded
under much milder conditions, and the Wilkinson complex was found to be a suitable
σ
-allyl-Rh complex and not via a
π
TABLE 8B.45. Rhodium - Catalyzed Allylic Alkylations with 1,3 - Dicarbonyl Compounds
as Pronucleophiles
OCO
2
Me
Nu
Nu
l
Nu
Catalyst,
react. cond.
+
+
or
Nu
OCO
2
Me
b
(
E
)-
l
(
Z
)-
l
b
Product
Entry
Substrate
Nu - H
Catalyst
React. Cond.
Yield (%)
b
:
(
E
) -
l
:
(
Z
) -
l
1
b
O
O
RhH(PPh
3
)
3
Dioxane, 100 ° C
86
100:0:0
PPh
3
2
b
O
O
RhH(PPh
3
)
3
Dioxane, 100 ° C
74
90:10:0
PPh
3
O
3
b
Pd
2
(dba)
3
· CHCl
3
THF, rt
89
27:65:8
O
O
PPh
3
4
l
Pd
2
(dba)
3
· CHCl
3
THF, rt
93
29:63:8
O
O
PPh
3
5
b
RhH(PPh
3
)
3
Dioxane, 100 ° C
81
86:12:2
O
O
PPh
3
6
l
RhH(PPh
3
)
3
Dioxane, 100 ° C
97
28:63:9
O
O
PPh
3
