Chemistry Reference
In-Depth Information
have been prepared by changing substituents at the 2
′
- position of the 2 - diphenylphosphino -
1,1
′
-binaphthyl and applied to the reaction. It was found that the substituents at the
2
′
-position in mop ligands strongly affected the enantioselectivities.
For styrene, (
S
) - H - mop (
13
) having only hydrogen at the 2
- position is a particularly
effective chiral ligand to give 94% ee [10]. On the other hand, (
R
) - MeO - mop (
14
), (
R
) -
CN - mop (
15
), and (
S
) - Et - mop (
16
) are not effective chiral ligands to give only 14% ee,
26% ee, and 18% ee, respectively [11]. The monophosphine (
S
) -
17
prepared through
the catalytic asymmetric cross-coupling gave 91% ee in the hydrosilylation of styrene
[12]. These results indicate that the small size of the hydrogen at the 2
′
- position in (
S
) -
H - mop and (
S
) -
17
is responsible for high enantioselectivity in the palladium-catalyzed
asymmetric hydrosilylation of styrene with trichlorosilane. Electronic tuning of
palladium/H-mop catalysts also affects the enantioselectivity in the hydrosilylation of
styrene. The enantioselectivity and catalytic activity of catalysts were greatly enhanced
by electronically tuned H-mop derivative
18
on which two trifl uoromethyl groups were
introduced onto the phenyl rings of the diphenylphosphino group [13]. Thus, the hydro-
silylation of styrene with trichlorosilane in the presence of 0.1 mol % of the palladium
complex of an electron-defi cient H-mop derivative
18
was completed within 1 h at 0°C
to afford the product of 97% ee in a quantitative yield. As well as the excellent enanti-
oselectivity and catalytic activity was achieved by ligand
18
, a plausible mechanism was
proposed in the hydrosilylation of styrene using palladium/
18
catalysts. Much faster
′
-
hydrogen elimination than reductive elimination from the intermediate
II
was revealed
by deuterium-labeling studies on the hydrosilylation of regiospecifi cally deuteriated
styrene, and a hydropalladation step, not a silylpalladation step, in the catalytic
cycle was supported by side product analyses from the reaction of
o
- allylstyrene [14]
(Scheme 9.4 ).
β
SiCl
3
Pd-L*
Ph
HSiCl
3
Ph
Reductive elimination
Oxidative addition
Hydropalladation
SiCl
3
H
SiCl
3
L*
Pd
L*
Pd
H
b
-Hydrogen elimination
Ph
Ph
II
I
Scheme 9.4.
The chirality sources of monophosphine ligands proven to be effi cient for palladium-
catalyzed asymmetric hydrosilylation of styrene with trichlorosilane so far are classifi ed
into two categories: axial and planar chirality [15]. Of those monophosphine ligands
having the binaphthyl skeleton like mops, ligand
19
with a stibano group at the 2′ -
position showed an excellent enantioselectivity comparable with H-mop [15d]. In
