Chemistry Reference
In-Depth Information
Rh precatalyst as catalytic system gave the bicyclic cyclopentenones in up to 96% ee
(Scheme 7.13).
25
O
R
R
3 mol% [Rh(cod)Cl]
2
6 mol% L*
O
O
O
PPh
2
PPh
2
R'
Cinnamylaldehyde
tert-
amyl alcohol
100
*
X
R'
X
°
C, 36 h
O
R = aryl, alkyl
R' = H, Me
X = O, NTs, C(COOEt)
2
Up to 96%
ee
L* =
97% yield
(
S
)-BisbenzodioxanPhos
Scheme 7.13
Asymmetric Rh-catalyzed PKR in an alcoholic medium.
The electronic effect in asymmetric catalysis is one of the most important parameters to
control the stereo-outcome of the desired product.
26
In fact, this effect may be attributed to
both substrates and/or catalysts (ligand) which can significantly alter the extent of product
enantioselectivity. Recently, Kwong/Chan first disclosed the substrate electronic effect of
1,6-aromatic enynes which could alter the level of enantioselectivity of the desired product,
such as bicyclic cyclopentenone (Scheme 7.12).
24
A higher enantioselectivity was obtained
when the electron-rich substituent was attached to the aromatic ring. The Hammett study
showed that a linear free energy relationship was found (Figure 7.1).
27
Moreover, they proposed the structure of the metal complex leading to the desired
cycloadducts. The electron-rich
O
-tethered 1,6-enynes were suggested to bind the Rh metal
center more closely and thus have a better stereoinduction, whereas electron-poor enynes
were loosely coordinated (Figure 7.2).
28
3% [Rh(COD)Cl]
2
6% (
S
)-P-Phos
Cinnamylaldehyde
Water,
100
°
C
Log (
S/R
)
O
O
1. 5
R
R
p
-OMe
*
1. 4
H
O
1. 3
p
-Me
1. 2
p
-H
1. 1
p
-F
m
-OMe
1
σ
R
0.9
p
-Cl
p
-OMe
p
-Me
p
-H
p
-F
m
-OMe
p
-Cl
-0.27
-0.17
0.00
0.06
0.12
0.23
0.8
R
2
= 0.95
0.7
0.6
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
Hammett constant
σ
Figure 7.1
Hammett plot of Rh-catalyzed asymmetric PKR in water.
