Chemistry Reference
In-Depth Information
contrast to a very low catalytic activity of Pd complex coordinated with a chelate
ligand, such as binap, in the hydrosilylation, chiral binap-Pd nanoparticles did catalyze
the asymmetric hydrosilylation of styrene with trichlorosilane even at 0°C to show an
excellent enantioselectivity [16]. Moderate to high enantioselectivities have been
reported with planar chiral ferrocenyl and (
6
- arene)chromium monophosphine
ligands [15g]. Theoretical investigations of the mechanism and infl uence of electronic
and steric effects on enantioselectivity in hydrosilylation of styrene with trichlorosilane
were reported for palladium complex with chiral ferrocenyl ligand
20
[17] . It was
suggested based on computational calculations that the reaction proceeds in agreement
with a classical Chalk-Harrod mechanism, where the hydropalladation step is involved,
and the extent of the chiral induction is mainly dominated by steric effects of the sub-
strate and the ligands. A very fast hydrosilylation, completed within only 15 min, with
0.1 mol % of the catalyst, was also reported as using a new type of chiral ferrocenyl-
monophosphine
21
[18] .
Recently, chiral phosphoramidites, readily available from chiral diols, were reported
to be a potent ligand in the hydrosilylation of styrene [19]. The highest enantioselectivity
of 99% ee in the hydrosilylation of styrene was realized by using the chiral phosphora-
midite ligand
22
having a (
S
) - 1,1
η
-binaphthol moiety and a bis((
R
) - 1 - phenylethyl)amino
group. A very high enantioselectivity was also reported by using another phosphorami-
dite ligand
23
derived from a spiro diol.
The hydrosilylation of styrenes has been utilized successfully in the asymmetric syn-
thesis of 1 - aryl - 1,2 - diols from arylacetylenes (Scheme 9.5 ) [20] . Platinum - catalyzed
hydrosilylation of arylacetylene (
24
) gave (
E
) - 1 - aryl - 2 - (trichlorosilyl)ethenes (
25
), and
the following palladium-catalyzed asymmetric hydrosilylation of the resulting
′
-
silylstyrene with the H-mop derivative
18
gave 1 - aryl - 1,2 - bis(trichlorosilyl)ethanes (
26
),
which was converted to the enantiomerically enriched 1,2-diols (
27
) by oxidation. Thus,
formal dihydration of arylacetylene was achieved by tandem platinum-catalyzed hydro-
silylation of the acetylene and palladium-catalyzed asymmetric hydrosilylation of the
corresponding
β
β
- silylstyrene in one pot.
[PtCl
2
(C
2
H
4
)]
2
(0.01 mol %)
[PdCl(
-C
3
H
5
)]
2
L* (0.3 mol %)
p
Ar
Ar
H
+ HSiCl
3
(excess)
SiC l
3
20 C, 24 h
20 C, 48 h
24
25
Ar
Ar
H
2
O
2
,KF,KHCO
3
SiCl
3
OH
SiCl
3
OH
26
27
Ar = Ph: 95% ee (
R
)
Ar = 4-MeC
6
H
4
:95%ee(
R
)
Ar = 4-CF
3
C
6
H
4
:96%ee(
R
)
Ar = 3-NO
2
C
6
H
4
: 98% ee (
R
)
F
3
C
H
L* =
P
2
F
3
C
(
R
)-
18
Scheme 9.5.
